Mathias Da Rocha

Bruce Dunn

Aline Rougier
email: aline.rougier@icmcb.cnrs.fr

Faradaic and/or capacitive: Which contribution for electrochromism in NiO thin films cycled in various electrolytes?

Keywords:

This study compares the electrochromic performance of NiO thin films deposited by RF magnetron sputtering cycled in lithium, sodium and small cations free based electrolytes, namely 1:9 LiTFSI in EMITFSI, 1:9 NaTFSI in EMITFSI and EMITFSI respectively. Regardless of the electrolyte nature, NiO thin films show similar electrochromic properties associated with an optical switch from colorless to brownish upon oxidation correlated to a modulation of transmittance close to 40 % associated with a good electrochemical stability. Interestingly, depending on the electrolyte nature, the EC behavior is correlated with various CV shapes raising the question of the mechanism involved. In particular, the reversible coloration mechanism cannot be only described by a single insertion/extraction of lithium ions, as observed for tungsten oxide in lithium media. The rectangular shape of the CV curves of NiO thin films in neat electrolyte and the behavior of NiO with various scan rates suggest that the EC properties result from a combination of both faradaic and capacitive contributions, with redox reactions largely occurring at the surface. Integration of NiO thin films in a full device is illustrated in double sided PANI/ white electrolyte-EMITFSI/NiO EC devices showing simultaneous progressive color changes from blue to green and white to brown, on PANI and NiO sides, respectively.

Introduction

Modern architecture for both houses and buildings makes extensive use of fully glazed exteriors, thus providing numerous opportunities for smart windows. Smart windows offer indoor temperature regulation which increases the comfort of the occupant [START_REF] Sbar | Electrochromic dynamic windows for office buildings[END_REF][START_REF] Wesoff | View Has Raised More than $500 Million for Smart Adaptive Windows[END_REF]. Among the various smart window approaches, electrochromic windows have received considerable attention, however, their widespread commercialization still suffers from critical issues including cycle life, pin holes in large panels, and cost [START_REF] Gillaspie | Metal-oxide films for electrochromic applications: present technology and future directions[END_REF]. This smart window technology uses electro chromic materials that display a reversible change in optical properties under an applied voltage. The operating principle is based on the re versible exchange of ions (via the electrolyte) and electrons between the two electrodes (via the external circuit); this resembles the electro chemical processes occurring in ion insertion batteries, such as lithium ion batteries. Currently, the majority of smart windows are designed to combine WO 3 , the most commonly used inorganic electrochromic ma terial, with either IrO 2 leading to blue coloration or to NiO. NiO has the advantage of being less expensive than IrO 2 and, when combined with the cathodically colored blue of WO 3 , brings a neutral color (gray) which is often more suitable for commercial applications. However, the mechanism of NiO coloration, especially in lithiated electrolytes, has not been well established. The initially proposed mechanism based on Li ion insertion/desinsertion has been questioned as recent investiga tions have shown that Li free electrolytes also produced coloration [START_REF] Mihelčič | Comparison of electrochromic properties of Ni 1 x O in lithium and lithium-free aprotic electrolytes : from Ni 1 x O pigment coatings to flexible electrochromic devices[END_REF].

Passerini and co workers first reported the coloration mechanism in NiO as arising from the insertion/desinsertion of lithium ions. NiO as an anodic electrochromic material, switches from a bleached reduced state to a colored oxidized one. The EC mechanism was often expressed [5 9] by the following equations Ni 1 x O + yLi + +ye -↔ Li y Ni 1 x y O [START_REF] Sbar | Electrochromic dynamic windows for office buildings[END_REF] or Li y NiO x + zLi + +ze -↔ Li y+z NiO x [START_REF] Wesoff | View Has Raised More than $500 Million for Smart Adaptive Windows[END_REF] More recently, Moulki et al. [START_REF] Moulki | Electrochromic performances of nonstoichiometric NiO thin films[END_REF], reported the successful coloration of nickel oxide thin films through oxidation without pre insertion of lithium ions. This process suggests that anions, namely TFSI -in that case, were involved in the coloration process. Further cycling showed a reversible bleaching/coloration process suggesting that both anions and cations participated in the mechanism. Wen et al. also indicated that both cations and anions contributed to the charge exchange, favoring surface reactions rather than bulk processes [START_REF] Wen | Anodic electrochromism for energy-efficient windows: cation/anion-based surface processes and effects of crystal facets in nickel oxide thin films[END_REF].

In this study we consider the different electrochromic responses of NiO using ionic liquid electrolytes which either contain lithium salts, sodium salts or are salt free. The coloration properties are determined using cyclic voltammetry at various scan rates in combination with transmittance modulation, which enables us to obtain greater insight regarding differences in electrochemical kinetics. Opening the range of suitable electrolytes in which NiO thin films exhibit good EC perfor mance allows to consider NiO thin films in devices exhibiting new configurations aiming at applications ranging from smart windows to displays. Indeed, the combination of EC materials is no more limited to insertion compounds such as WO 3 but materials of which EC properties are reported in salt free electrolytes become good candidates. Conductive polymers belong to those categories of materials switching their optical properties in salt free electrolytes. For instance, we earlier reported PEDOT NiO devices showing good compatibility in this type of electrolyte for both electrodes [START_REF] Mihelčič | Comparison of electrochromic properties of Ni 1 x O in lithium and lithium-free aprotic electrolytes : from Ni 1 x O pigment coatings to flexible electrochromic devices[END_REF]. More recently, our group has fo cused on suggesting non common devices configuration leading to hy dride device combining oxide and polymers [START_REF] Mjejri | Enhanced coloration for hydrid niobium based electrochromic devices[END_REF] or to devices of which properties lie on the modulation of colors [START_REF] Mjejri | Double-sided electrochromic device based on metal-organic frameworks[END_REF]. Further step was raised by adding a white electrolyte membrane in between two EC layers leading to devices showing a simultaneous color change on both sides of the device. Herein, further investigation of hybrid device is carried out through the integration of NiO combined with a conductive polymer, namely polyaniline, separated by a white membrane electro lyte.

Experimental details

Deposition of NiO thin films by RF magnetron sputtering

NiO thin films were deposited on 2.5 × 2.5 cm 2 In 2 O 3 :Sn coated glasses (SOLEM ITO glass, ~25Ω/□) by reactive RF magnetron sput tering from 75 mm diameter nickel (99.99 %) targets (Neyco). During the deposition process, the distance from the substrate to the target was fixed at 8 cm. Such conditions led to the production of uniform thin films. The gas flow ratio of Ar (99.99 %) to O 2 (99.99 %) was fixed by mass flow controllers at 98:2 (sccm), and a working pressure of 4 Pa. The power was maintained at 90 W. The thickness of the films was found to be about 330 nm, as measured by a profilometer (Dektak), leading to a deposition rate of 17 18 nm/min.

Characterization methods

The film structure was determined by X ray diffraction (XRD) using a PANanalytical X'pert MPD diffractometer with Cu Kα incident ra diation. The morphology of the films was characterized by Scanning Electron Microscopy (SEM) (Hitachi Tabletop microscope TM 100). Cyclic voltammograms (CV) and chrono amperograms (CA) were ob tained using a three electrode glass cell consisting of Glass/ITO/NiO as the working electrode, a Pt sheet as counter electrode and a Saturated Calomel Electrode SCE (0.234 V/ENH) reference electrode. The ionic liquid electrolytes (Solvionic; purity > 99.99%) consisted of 1:9 mol ratio of lithium or sodium bis(trifluoromethanesulfonyl)imide (LiTFSI, NaTFSI) dissolved in 1 ethyl 3 methylimidazolium bis(tri fluoromethanesulfonyl)imide (EMITFSI). Neat EMITFSI electrolytes were also investigated. The CV and CA measurements were made using a Biologic potentiostat. In situ optical transmittance measurements of the Glass/ITO/NiO electrode at a wavelength of 550 nm were carried out using a Varian Cary UV Visible NIR Spectrophotometer.

Results and discussion

Structural characterization

The XRD pattern of the NiO (Fig. 1a) consists of a major peak located at 2Ɵ = 37.28°. The NiO thin film crystallizes in the cubic rocksalt structure (Fm 3m) with a strong (111) preferred orientation. The lattice parameter of 4.177 Å is close to that of bulk NiO (4.164 Å) (JCPDS 00 001 1239). A crystallite size of about 30 35 nm is calcu lated using the Scherrer formula. SEM micrographs indicate a homo geneous film with significant porosity (Fig. 1b). A TEM image (Fig. 1c) indicates a columnar growth morphology as expected from deposition conditions combining a high pressure in the sputtered plasma and room temperature substrate [START_REF] Anders | A structure zone diagram including plasma-based deposition and ion etching[END_REF].

Electrochromic behavior

Comparison of electrolyte nature

Fig. 2a, b, c show a series of CVs at a 40 mV/s scan rate for NiO thin films cycled in 1:9 LiTFSI EMITFSI, 1:9 NaTFSI EMITFSI and EMITFSI. While the electrochromic behavior is reversible, in order to fully bleach or color NiO, a small adjustment of the voltage window was required. The evolution of the in situ transmittance recorded at specific voltages maintained for various durations from 1 s to 45 s is displayed in Fig. 3a andb, c. Regardless of the electrolyte, a reversible process is observed where there is a decrease in transmittance upon oxidation and an in crease in transmittance upon reduction whatever the time. An activa tion occurs in the very first cycles leading to an increase in capacity. In lithium and sodium based electrolytes, some similarities in the CV curves are observed as there are two distinct peaks in both the anodic and cathodic sweeps. The separation between these peaks is further apart in the Na based electrolyte requiring the oxidation potential to be increased from 1.2 to 1.4 V in order to reach similar transmittance modulation. With the neat EMITFSI electrolyte, NiO exhibits a rectan gular CV shape between -0.5 V and 1.3 V. This shape generally arises from capacitive behavior. For all of the electrolytes, NiO films re versibly switch from a colorless to a brownish state which is correlated with modulation of the transmittance. Interestingly, for a duration time of 45 s, whatever the electrolyte, the transmittance decreases from 88 % down to about 48 % corresponding to a contrast of 40 %. The main differences in the transmittance evolution occur for very short dura tions mostly in the bleached state with a transmittance that reaches only 70 % for 1 s in salt free electrolyte, and required 5 s for a stable value of about 88 %. In Li, or Na based electrolytes, the transmittance in the bleached states is close to the maximum value as earlier as 2s.

The transmittance as a function of time identifies a memory effect, which is described as the ability to remain in the colored oxidized state when no potential is applied. This behavior was characterized using a 4 phase protocol (Fig. 4 and Table 1). In, phase 1, a negative voltage is applied for a few seconds until reaching the full reduction state (i.e. maximum in transmittance). Then, in phase 2, no voltage is applied for few minutes. The phases 3 and 4 are similar to 1 and 2 but now involve the oxidation state (i.e. minimum in transmittance). In these experi ments, the voltages used to bleach or color are the extreme values used in the CVs for each electrolyte, namely -1.2 1.2 V for Li; -1.0 1.4 V for Na; and -1 V 1.3 V for EMITFSI. To a first approximation, NiO behavior seems comparable for all three electrolytes. After phase 1 (i.e. reduced state), the transmittance sharply decreases by 2 3 % prior to stabilizing after 5 10 min during phase 2. In contrast, after the oxida tion state (phase 3), the transmittance continuously increases and does not stabilize even after 30 min.

Table 1 lists the characteristic features of the memory effect and the switching times for NiO cycled in the three different electrolytes. The former corresponds to the modulation in transmittance after 1000 s in the bleached state and 1500 s in the colored state while the switching time corresponds to the time required for reaching 90 % of full trans mittance. In all electrolytes, NiO tends to bleach more easily in open circuit. The faster bleaching time than the coloration one was already reported [START_REF] Rocha | Influence of cycling temperature on the electrochromic properties of WO 3 //NiO devices built with various thicknesses[END_REF] and attributed to the transition from a conductive colored state composed of Ni 3+ and Ni 2+ to a less conductive one in the bleached state NiO (Ni 2+ ). Although sodium ions are larger than li thium ions, the switching time for the NaTFSI EMITFSI electrolyte is shorter than the one for the LiTFSI EMITFSI electrolyte. In general, the electrochromic properties of NiO are almost the same for all three electrolytes, including the neat EMITFSI electrolyte.

Comparison of WO 3 and NiO in lithium based electrolyte

The faculty of NiO thin films to cycle in salt free electrolytes raises the question of the mechanism. Commonly, EC properties in oxides are associated with insertion desinsertion phenomenon while cycling in protonic, lithium or sodium based electrolytes. To provide greater in sight of the EC behavior for NiO, its EC properties were compared to those of the state of the art EC oxide, namely WO 3 , a material for which the insertion mechanism in lithium and sodium based electrolytes is well established.

In lithium based electrolytes [14 18], WO 3 undergoes the following reaction:

WO 3 + xLi + + xe -↔Li x WO 3 (3)
Upon reduction, cathodically colored WO 3 switches from a colorless to a blue state. NiO and WO 3 thin films were cycled in 1:9 LiTFSI EMITFSI at various scan rates and the in situ transmittance was re corded. During all measurements, the voltage windows were kept constant, between -0.8 and 1.3 V for WO 3 (Fig. 5a) and -0.7 and 1.4 V for NiO (Fig. 5b). As the scan rate increases from 2 to 100 mV/s, the current density for both NiO and WO 3 increases. However, the re spective bleaching behavior is different for the two materials. The voltage needed to fully bleach WO 3 increases from 0.4 V to 1.3 V while the voltage needed to bleach NiO decreases from -0.3 V to -0.7 V. For WO 3 , the decrease in the ΔT as the scan rate increases illustrates the difficulties of inserting lithium ions into the material at high scan rates (Fig. 5c). In contrast, the delta in transmittance for NiO appears quite stable, independent of the scan rate (Fig. 5d) suggesting that the col oration mechanism is more complex than simple lithium insertion. Another interesting difference is observed in the sweep rate dependence of the capacity ratio, which is defined as the charge, Q, at a given sweep rate divided by Q at 2 mV/s. For WO 3 , there is a continuous decrease in the capacity ratio with increasing sweep rate (Fig. 6) while this value tends to stabilize above a scan rate of 20 mV/s for NiO. This flat re sponse at high sweep rates is consistent with having reactions localized at the surface as occurs with double layer capacitance. Thus, NiO can support fast switching without altering its electrochromic properties. The comparison of WO 3 and NiO EC behaviors allows to highlight differences in the « depth» of cations diffusion combined to the redox reaction raising several questions. Is the mechanism taking place in the bulk or at the surface? Is the coloration associated with a single faradic process or do we have a major contribution of a capacitive phenom enon? Besides, cyclability associated with color switch was earlier re ported for NiO thin films in our group [START_REF] Moulki | Electrochromic performances of nonstoichiometric NiO thin films[END_REF] while oxidation of WO 3 cannot take place.

Distinction between capacitive and faradaic contributions

The distinction between the faradaic and capacitive contributions in battery or electrochromic systems, has been recently discussed for dif ferent materials including TiO 2 [START_REF] V. Augustyn | Pseudocapacitive oxide materials for high-rate electrochemical energy storage[END_REF][START_REF] Wang | Pseudocapacitive contributions to electrochemical energy storage in TiO 2 (anatase) nanoparticles[END_REF] and WO 3 [START_REF] Giannuzzi | Enhanced efficiency of the honeycomb-structured film WO3 composed of nanorods for electrochromic properties[END_REF]. Indeed, the current (i) at a fixed potential (V) can be quantitatively separated into a diffusion contribution and a capacitive contribution as shown by the following equation:

i(V) = k 1 v + k 2 v 1/2 (4)
For analytical purposes, this equation is slightly rearranged to

i(V)/v 1/2 = k 1 v 1/2 +k 2 (5)
In eq [START_REF] Sbar | Electrochromic dynamic windows for office buildings[END_REF], k 1 v and k 2 v 1/2 correspond to the current contributions from the surface capacitive phenomena and the diffusion controlled insertion process, respectively. Thus the determination of k 1 and k 2 , allows, at specific potentials, one to quantify the fraction of the current due to each of the two contributions.

The comparison of the capacitive current calculated from k1 (black line) to the experimental current (red line) depending on the scan rate shows that a significant fraction of the current is attributed to a capacitive contribution (Fig. 7). At higher sweep rates (100 mV/s) dif fusion controlled currents are unable to contribute to the total current because of their slower time response. Therefore, it is not surprising that at 100 mV/s, both curves are very close. Moreover, for all of the electrolytes, the capacitive contribution to the total current will in crease with sweep rate as there is progressively less contribution from diffusion controlled processes (Fig. 8). Regardless of the electrolyte, the charge storage associated with capacitive processes is in the range of 9 mC/cm 2 (Fig. 8). This capacity can be attributed to surface adsorption effects linked to EMI + and TFSI -ions. For this reason, the charge storage and charge transfer at the surface are favored as the sweep rate increases. Thus, even with salt free electrolytes such as EMITFSI, the capacitive contribution to the total current remains constant as a function of sweep rate because there is no significant diffusion expected for EMI or TFSI. Noneless to remind that, the contribution of both anions and cations was earlier suggested by EC behavior with departure in oxidation. In addition, the absence of ion insertion was confirmed by the fact that no shift of the 111 peak was observed.

V] regions

The cycling of NiO films in different voltage regimes is able to provide additional insights regarding the EC mechanism. NiO thin films were cycled in two different voltage regions, namely between and 0.4 V and 1.2 V and in between -0.7 V and 0.4 V, considering a three elec trode cell using SCE electrode as reference electrode. Fig. 9 shows the electrochemical and optical behavior of NiO thin films cycled between -0.7 V and 0.4 V after an initial cycle to 0.6 V. As expected, the de crease in the upper voltage from 0.6 V to 0.4 V initially corresponds to an increase in the transmittance value (shown as the change from the blue to green dot in Fig. 8b). Upon cycling, the transmittance pro gressively decreases from 70 to 65 % confirming a greater level of oxidation as indicated by the increase in current density. The latter should be correlated to a gradual lithium insertion. In contrast, a limit in the reduction potential by cycling in the 0.4 V 1.2 V window leads to a slightly higher transmittance value from 42 to 47 % in oxidation while a rather constant value close to 60 % is reached at 0.4 V (Fig. 10). An optical contrast of 12.5 % is observed in this potential window. In addition, in this voltage range, the rectangular shape which is observed is characteristic of either double layer capacitance effects or pseudo capacitance.

PANI/NiO device

Investigations of EC behavior of NiO thin films in lithium, sodium and cation free electrolytes conclude on two competing phenomena leading to consider a pseudocapacitive behavior for NiO thin films. Such identification of a more complex EC mechanism offers opportunity to consider EC devices with unusual architecture. Indeed in our group, we recently develop new device architectures by using a white elec trolyte aiming at displays applications. A further step forward was achieved by modifying the electrochromic layer by mixing oxides with commercial polymer inks for higher performance hybrid electrodes [START_REF] Yogi | Organic nanostructures on inorganic ones : an efficient electrochromic display by design[END_REF]. The concept of hybrid device can be extended not only to single electrode but to the combination of a polymer electrode and an oxide one. Herein, the possibility of cycling in NiO in cations free electrolyte is further extended to hybrid architecture based on a white membrane. Besides, in respect of current research in the field of batteries, avoiding Li for resources purpose, the ability to cycle in lithium free electrolyte was evaluated in a full device using a EMITFSI PMMA membrane and polyaniline (PANI), which is one of the commonly used conductive polymers, as second electrode (Fig. 11) [START_REF] Wang | Polyaniline (PANi) based electrode materials for energy storage and conversion[END_REF]. The PANI film was elec trodeposited from a solution of H 2 SO 4 1 M and 0.1 M of aniline using a potential of 1 V for 3 min. The excess material was removed by ethanol. The good cyclibility of the PANI layer in EMITFSI electrolyte was confirmed in three electrodes cell configuration. For building the de vice, the electrolyte consists of a white membrane soaked over one night in EMITFSI [START_REF] Goujon | Flexible solid polymer electrolytes based on nitrile butadiene rubber : poly(ethylene oxide) interpenetrating polymer networks containing either LiTFSI or EMITFSI[END_REF]. The association of NiO and PANI led to the following device: ITO/PANI/EMITFSI white membrane/NiO/ITO Prior to device assembly, each layer was activated in the 0.8 V 1.2 V window, 50 times for NiO and 6 times for PANI. The PANI/NiO device has been cycled one hundred times between -0.7V and 0.8V showing a simultaneous color change from transparent (appearing whitish due to the electrolyte membrane) to brownish on the NiO side and from dark to light green on the PANI side (Fig. 11). The CVs from 2 to 100 cycles show a rather featureless shape with a limited decrease in capacity. The optical contrasts of the two faces were evaluated using the L*a*b* color system. It is worth recalling that in CIE colorimetric space, the color is represented by three parameters: a luminance axis (L*) and two hue axes (a*) and (b*), which can be used to define and compare quanti tatively the colors. The optical contrast ΔE* is presented by this equa tion

ΔE* = [(L* c -L* b ) 2 + (a* c -a* b ) 2 + (b* c -b* b ) 2 ] ½ (6)
Close ΔE* values were determined for both sides, namely 14.6 for NiO and 14.4 for PANI at the inception of cycling. With more extended cycling, the value slighty decreases to 12.1 after 50 cycles for NiO and remains constant for PANI. Chronoamperograms, recorded for 30 s at 0.8 V and 30 s at -0.7 V for 20 times (Fig. 12), show a fast switching time of 1.3 s.

Conclusion

Despite, numerous years of study and their integration in commer cialized devices, the electrochromic mechanism occurring in NiO thin films is still the subject of debate. In this paper, RF sputtered NiO thin films were cycled in various ionic liquid electrolytes based on EMITFSI that contained LiTFSI, NaTFSI or were neat. At first glance, the EC properties of NiO, that is, the electrochemical behavior and optical contrast appear only slightly affected by the electrolyte nature with a transmittance modulation between 40 (EMITFSI) to 56 % (LiTFSI) and a colorless to brownish switch. To gain greater insight of the EC me chanism, we used an analytical approach to separate the current into contributions which were diffusion controlled or capacitor like in nature. The results suggest that a pseudocapacitive mechanism is con tributing to the electrochromic response of NiO. Redox reactions oc curring at the surface or near surface of NiO in response to the ad sorption or slight insertion of Li + , Na + , EMI + or TFSI -are responsible for the charge transfer reactions that produce the coloration effects. Integration of NiO thin films in a full device is illustrated in double sided PANI/white electrolyte/NiO EC devices which exhibit simulta neous progressive color changes from dark to light green and white to brown.

Fig. 1 .

 1 Fig. 1. a) X-Ray diffraction pattern of 250 nm NiO thin films deposited on glass substrate, b) SEM morphology, c) TEM morphology.

Fig. 2 .

 2 Fig. 2. Voltammetry cyclic of (a) NiO/1:9 LiTFSI-EMITFSI/Pt vs SCE using a 40 mV scan rate and 1.2V-1.2V voltage window, (b) NiO/1:9 NaTFSI-EMITFSI/Pt vs SCE using a 40 mV scan rate and -1V-1.4V voltage window, (c) NiO/EMITFSI/Pt vs SCE using a 40 mV scan rate and -1V-1.3V voltage window.

Fig. 3 .

 3 Fig. 3. in-situ transmittance vs time of chronoamperometry at the wavelength of 550 nm of (a) NiO/1:9 LiTFSI-EMITFSI/Pt, (b) NiO/1:9 NaTFSI-EMITFSI/Pt, (c) NiO/ EMITFSI/Pt. The voltage range for chronoamperometry has been chosen on the basis of CV at 100 mV/s.

Fig. 4 .

 4 Fig. 4. Memory effect of (a) NiO/1:9 LiTFSI-EMITFSI/Pt, (b) NiO/1:9 NaTFSI-EMITFSI/Pt, (c) NiO/EMITFSI/Pt. 1-voltage to reach the reduce state is applied, 2-voltage release (open circuit), 3-voltage to reach the oxidation state, 4-voltage release (open circuit).

3. 2 . 4 .

 24 Cycling of NiO thin films in [ 0.7 to 0.4 V] and [0.4 V 1.

Fig. 5 .

 5 Fig. 5. Voltammetry cyclic of (a) WO 3 /LiTFSI-EMITFSI/Pt and (b) NiO/LiTFSI-EMITFSI/Pt at different scan rate and the corresponding In-situ transmittance vs time at the wavelengths of 550 nm, (c) WO 3 /LiTFSI-EMITFSI/Pt (d) NiO/LiTFSI-EMITFSI/Pt.

Fig. 6 .

 6 Fig. 6. ratio of capacity (x mV/s /2 mV/s ) at various scan rate of NiO and WO 3 thin films cycled in 1:9 LiTFSI-EMITFSI.

Fig. 7 .Fig. 8 .

 78 Fig. 7. Voltammetric response (2 mV/s and 100 mV/s) for NiO/LiTFSI-EMITFSI/Pt vs SCE. The total current (red line) is obtained experimentally. The capacitive currents (black line) are determined from the eq. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 9 .

 9 Fig. 9. NiO thin films cycled in NiO/1:9 LiTFSI-EMITFSI/Pt vs SCE using a 10 mV/s scan rate and 0.7V-0.4V voltage window and (b) in-situ transmittance vs time at the wavelengths of 550 nm.

Fig. 10 .

 10 Fig. 10. NiO thin films cycled in NiO/1:9 LiTFSI-EMITFSI/Pt vs SCE using a 10 mV/s scan rate and 0.4V-1.2V voltage window and (b) in-situ transmittance vs time at the wavelengths of 550 nm.

Fig. 11 .

 11 Fig. 11. Voltammetry cyclic of the device NiO/white membrane-EMITFSI/ PANI at 20 mV/s.

Fig. 12 .

 12 Fig. 12. Chronoamperometry of the device NiO/white membrane-EMITFSI/ PANI (+0.8V (30s)/-0.7V (30s)).

Table 1

 1 summary of some electrochromic properties of NiO in various electrolytes.

	Electrolyte		LiTFSI-EMITFSI	NaTFSI-EMITFSI	EMITFSI
	Delta Optical transmittance	56	52	40
	Memory effect (%)	bleaching	7	8	6
		coloration	3	2	3
	Switching time (s)	T bleach	1.5	1.5	2
		T colored	5	2	5

Acknowledgments

The authors wish to thank the University of Bordeaux for funding this research and in particular the Ph. D. grant of M. Da Rocha. Bruce Dunn wishes to thank the LabEx AMADEus (ANR 10 LABX 42) in the framework of IdEx Bordeaux (ANR 10 IDEX 03 02), i.e. the Investissements d'Avenir programme of the French government managed by the Agence Nationale de la Recherch, for supporting his invited professor fellowship.


Olivia Munoz

Donatella Usai

Wadi Shab (GAS-1, Oman): the Neolithic (4 th /3 rd mill. BC) cemetery and the settlement

Although heavily eroded and disturbed, Wadi Shab-GAS1 produced relevant evidence of a 4 th and 3 rd millennium BC prehistoric occupation. Packed in less than forty centimetres anthropogenic deposit, well-preserved stone structures, including huts and fireplaces, and a small group of burials, the site has provided a possibility to infer on the exploitation of a rich eco-system located on the north-eastern coast of Oman, near a permanent source of freshwater and close to key resources, by a prehistoric population of fishers living on a cliff facing the sea.

Introduction

Located north-west of Sur, on a cliff on the northeastern Oman Coast (Figure 1), and known since the early eighties [START_REF] Tosi | Reconnaissance Activity[END_REF], Wadi Shab GAS1 is a 4 th /3 rd millennium BC site whose investigation has been part of the Joint Hadd Project [START_REF] Tosi | Preliminary report on the excavations at Wadi Shab, Area 1[END_REF][START_REF] Gaultier M | Living structures and death rituals at Wadi Shab-GAS1, Sultanate of Oman. Report on 2002 and 2003 field seasons[END_REF][START_REF] Usai | A IV millennium BC Oman site and its context: Wadi Shab-GAS1[END_REF]. After a brief exploration in 1992 by Paolo [START_REF] Biagi | A radiocarbon chronology for the aceramic shell-middens of coastal Oman[END_REF]2004: 39)

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Additional operations were made in the southern part of the site, corresponding more or less to the southern limit of the cliff, where some burials and another stone structure were noticed, brought to light by erosion. While the settlement structures located in the 2005 fieldseason (Figure 3) have been already the subject of a publication [START_REF] Usai | A IV millennium BC Oman site and its context: Wadi Shab-GAS1[END_REF][START_REF] Usai | The lithic industry of Wadi Shab, a Middle Holocene site on the coast of Oman[END_REF], this report will mainly concentrate on the description of the graves' group excavated in the southern part of the site, to provide a complete picture of the burial record. For the description of the first group of graves excavated in 2003 we refer to [START_REF] Gaultier M | Living structures and death rituals at Wadi Shab-GAS1, Sultanate of Oman. Report on 2002 and 2003 field seasons[END_REF], but a summary of the main characteristics of all the graves is presented in Table 2. The anthropological close examination, instead, as well as the synthesis on funerary practices, concerns all the burials.

The burial phase and the settlements': the sequence.

Excluding the Islamic evidence identified almost on the surface of Wadi Shab GAS1 site (Figure 3), constituted by the foundation of two orthogonal walls associated to wheel-made and hand-made pottery (Figure 4) and vitreous materials, the best preserved settlement traces located at the sites were recovered under a deflational deposit made of small to medium size rubble and archaeological artefacts. They are made by circular stone structures with post-holes and associated fireplaces whose floors were paved by poligonal fire-cracked stones [START_REF] Usai | A IV millennium BC Oman site and its context: Wadi Shab-GAS1[END_REF]. A sample of charcoal from one of this fireplaces provided a date in the 3 rd millennium BC (Table 1, Beta-206089). Among the artefacts associated to this phase worth mentioning are the numerous fragments of copper (?) objects, quite few ascribable to anything definite except for a hook, and a steatite bead production that is similar to that of Ra's al-Hadd HD6 (ZAROS 2004), a site also dating at nearly the same period [START_REF] Cleuziou | The Shadow of the Ancestors. The Prehistoric Foundations of the Early Arabian Civilization in Oman[END_REF]. The burial phase recorded at the site, according to stratigraphic observations, should preceed this 3 rd millennium BC settlement phase and post-date another settlement phase producing more ephemeral structures, like wind-shelter (TOSI and USAI 2003: Fig. 3.b). Among artefacts associated to this phase are softstone jewels, earrings and beads [START_REF] Tosi | Preliminary report on the excavations at Wadi Shab, Area 1[END_REF], also in an unfinished stage that suggests that Wadi Shab GAS1 was a manufacturing place (Figures 5 and6; [START_REF] Usai | Softstone. Approaches to the study of Chlorite and Calcite vessels in the Middle East and Central Asia from prehistory to the present[END_REF].

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The most ancient structures recovered at the site were incising into the limestone bedrock of the cliff, likely similar to those identified at Ra's al-Hamra RH5 by P. [START_REF] Biagi | Gli scavi nell'insediamento preistorico e nella necropoli di Ra's al-Hamra 5 (Sultanato dell'Oman)[END_REF][START_REF] Marcucci | Recent investigations at the prehistoric site RH-5 (Ra's al-Hamra, Muscat, Sultanate of Oman)[END_REF]. Typical of other sites along the Oman and Gulf coast [START_REF] Charpentier | Archaeology of the Erythraean Sea: craft specialization and resources optimization as part of the coastal economy on eastern coastlands of Oman during the 4 th and 3 rd millennia BC[END_REF]MÉRY et al. 2008;MÉRY and CHARPENTIER 2013) shell hooks, finished and in different stages of manufacture, netsinkers, and an interesting lithic industry were recovered in all phases and also in some burials [START_REF] Usai | The lithic industry of Wadi Shab, a Middle Holocene site on the coast of Oman[END_REF].

Unfortunately, the faunal remains, within which fish is prominent, were never studied in detail although preliminary observations confirm the presence also of ovicaprines [START_REF] Tosi | Preliminary report on the excavations at Wadi Shab, Area 1[END_REF].

The burial phase

The first batch of graves were discovered at Wadi Shab GAS1 during 2002 and 2003 field seasons and a detailed report is provided in [START_REF] Gaultier M | Living structures and death rituals at Wadi Shab-GAS1, Sultanate of Oman. Report on 2002 and 2003 field seasons[END_REF]. These were seven graves, located most of them (except Grave 7) inside or close to Area 1/100 (Figure 2). It is in this context that it was possible to document the stratigraphic relationship between the burial and the settlement phases, placing the first chronologically as ante-dating the 3 rd millennium BC structures. Conversely, it was not possible to figure out any relationship with the archaeological deposits for some of the graves excavated in 2005 in the southern part of the site, including Grave 7 that was the first of this group to be documented in the end of 2003 field season. These graves were emerging on the sloping part of the cliff and while some of them were still in good preservation condition, probably due to the elaborated covering stone superstructure, some were rather badly preserved. The typology of the graves however suggests their possible contemporaneity to the others.

Among the six graves excavated in the 2005 field season two, Grave 12 and 14, were located near Area 1/100 and the Islamic walls, not far from the first group (Figure 2). Altogether seven individuals were recovered in these graves.

A stone structure, originally also supposed to be a grave and therefore labelled Grave 10, turned out to have a different function, not yet fully understood (Figure 7).

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The graves

Grave 8

This grave did not present any stone covering. It is highly likely that flat stones, like those documented in the other graves discovered at Wadi Shab GAS1, were present but had fallen down the scarp or had been removed by successive occupants of the site. In fact, the deceased's head rested against a dark grey stone, which could constitute the only vestige of a superstructure subsequently dismantled. The individual buried in this grave (Figure 8) was covered by a light brown sandy soil mixed with small white pebbles, was lying on the left side, in a strong contracted position, oriented NS, with the head to the N, facing SE. The arms were flexed at acute angle with the hands very close to the face. Whether poorly preserved all the bones of the skeleton were present except those of the left leg, maybe destroyed by further disturbance. They were largely covered by a whitish deposit, which could suggest the presence of clothes or a perishable covering, made with vegetal materials, that would have been mineralized, as observed at Ra's al-Hamra 5 (MUNOZ 2014: 169-70; MUNOZ 2018: Fig. 7.2).

A flint flake was found near the left hand, and a fragment of a shell and another flint flake were found on the right pelvis (Figure 8b). They may be grave goods but it cannot be definitely excluded that are part of grave refilling.

Grave 9sup

This grave had been discovered during the 2003 field season together with Grave 7. It soon revealed quite an elaborated superstructure that turned out to be made of two levels of flat stones (Figure 9a,b). In between the two levels of stones, a lot of animal bones, mainly fish, flint, some charcoal and two ochre pebbles were found. Furthermore, one of the stones of the second level was a grinding base. Under the second level of stones the pit appeared filled with small white pebbles (Figure 9c), these were covering the skeleton and often adhering to it so that removing could damage the bones.

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The individual was lying tightly flexed (Figure 9d), on the left side, oriented WE with the head to the W. The arms were flexed at an acute angle with the hands close to the face. Whether poorly preserved all the bones of the skeleton were present except those of the right foot, which probably had been eroded away.

Grave 9inf

When removing the skeleton of Grave 9sup another long bone pertaining to a second individual appeared positioned between the two legs and the arms. It was clear that Grave 9 had disturbed a previous inhumation that was only partially left in place. Only the left part of the second individual buried in the same pit-grave was present but without the skull, the left arm and the left foot (Figure 10). Furthermore, other disturbance to the grave came from post-depositional animal burrow. Although disturbed it was possible to record the position of this individual that was lying flexed on the left side, oriented NS with the head to the N, facing E.

Grave 11

This grave was covered by a round concentration of polygonal stones, not flat as usual (Figure 11). The concentration was partly lying on the bedrock and this indicated, from the very beginning, that the pit grave could not have been very deep. The conservation of the grave was actually worse than expected as the filling of the pit had eroded almost completely together with the human bones of which very tiny pieces were preserved (fragments of skull, ribs, long bones, metacarpals and phalanges; Figure 11b). Nothing else could be recorded of this grave.

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Grave 12 of big flat limestone slab and other polygonal stones at the edge signalled the presence of a grave (Figure 12). This was disturbed by a small fireplace, full of charcoal chunks and flakes, which had cut through it on one side. The original preparation of the grave must have included also the small white beach pebbles that were still present under the stone covering but confined to one side of the grave (Figure 12b). This filling was then removed to check if part of the grave was hidden underneath but nothing else was preserved apart from few rib bones (Figure 12c) and other scanty part of the skeleton that had appeared soon after removing the limestone slabs. They were lying very near to the limestone bedrock. As for Grave 9 (and for Grave 4, GAULTIER et al. 2005) one of the stone forming the covering superstructure was a grinding stone (Figure 13).

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Grave 13

A single surface stone slab (Figure 14), probably what was left of a more elaborate superstructure, was covering this grave. In fact, the bones of a skull pertaining to an individual here buried were eroding on the surface. After a careful brushing the other bones pertaining to the inhumation of an adult individual were brought to light; they were only partially covered by the flat stone slab. Preserved only in one part, a filling made of small white beach pebbles, as documented in other graves, mixed to a black powdery soil was covering the bones. These appeared coated by a hardened gangue with part of the small pebbles 'glued' to them, making the excavation and cleaning very difficult, indeed the preservation condition of the skeletal remains resulted very bad.

The few skull fragments found almost on the surface, thought to pertain to a young individual (Ind.

A; Figure 14a), were lying at an higher level compared to the rest of skeleton found in this grave and positioned more or less on top of its chest. The more complete skeleton (Ind. B; Figure 14b) was buried in a flexed position, on the right side. It was oriented NE-SW, with the head at NE, facing NW. The arms were flexed at an acute angle with the hands close to the face.

Although the skeleton is completely represented, the bones were very poorly preserved: only the right half of the mandible and the right part of the skull were recovered for the study; also the right foot was missing, probably due to animal disturbance.

Grave 14

Grave 14 was located north of the west-east wall of the Islamic structure (Figure 15), near a well preserved fireplace made of burnt polygonal cracked stones. Here a round concentration of small white beach pebble suggested the possible presence of a prehistoric grave (Figure 15a). In spite of the time-shortage, Grave 14 was excavated in order to ascertain its chronological position in relation to the circular stone structure. Some of the stone pertaining to the ellipsoidal structure were visible in the wall of the pit grave and few bones were recovered under them. Therefore it is confirmed that the grave is antedating the stone structure. Any upper covering had been clearly removed and the grave appeared completely disturbed.

The 'pit-grave' contained the inhumation of a small child. A heavily disturbed skull, the mandible, some vertebrae and an undetermined long bone were recognised (Figure 15b,c). Teeth were also recovered but, unfortunately, they didn't preserve. It was not possible to record the position and the orientation of the deceased.

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The anthropological study

We present here the data relating to all the individuals discovered at GAS-1, in order to give a complete overview of the results of the analysis of the osteological collection.

Due to the very poor overall preservation of the human remains from GAS-1, and despite a rigorous examination, it was difficult to systematically consider each of the characteristics usually recorded in an anthropological study. The absence of remarks in some cases is therefore often linked to the condition of bone and tooth material, which in most cases has reached a very fragmentary form.

Place of storage

The human remains are currently kept partly at the Maison René Ginouvès 1 (Nanterre, France) and partly in the storage of the Ministry of Heritage and Culture of Oman (Muscat).

State of preservation

The general condition of the bones is very poor, most of them preserved in the form of pulverulent fragments. On average, for all individuals whose bones have been studied in the laboratory, the average Anatomical Preservation Index (API, or ICA in [START_REF] Bello | Quantification de l'état de conservation des collections ostéoarchéologiques et ses champs d'application en anthropologie[END_REF]) is 12% and the extent of fragmentation reaches 94% (see However, for several graves, it was found that the skeleton was quite well represented, with each of the anatomical parts represented by at least one fragment. The poor preservation of the remains can be attributed to natural (erosion, climate, burrowers) and anthropogenic factors (deposit mode, trampling, disturbance, looting). In some graves, skeletons are very poorly represented, which can be attributed to intense post-depositional alterations.

Several fragments of long bone diaphysis had gnawing stigmas due to the action of animals. In cases where these traces are found to affect several anatomic parts (Graves 1, 2, 3B, 3C, 8, 9sup, 9inf) they reflect a fairly systematic nibbling that indicates that access to the remains was possible. On the basis of these elements, it is possible that some bodies were not buried in the strict sense of the term, as the decomposition took place in a confined space, subsequently filled by sedimentary input (DUDAY 1990: 193-194). The possibility of subsequent intervention by burrowing and rodent animals cannot be ruled out, as a burrow has been identified in Grave 2. However, if it is monitored elsewhere, it has not caused bone migration outside the volume of the corpse in graves 1, 8, and 9sup, suggesting that the action of the animals occurred before the complete dislocation of the various elements of the skeleton.

In addition, the bones were sometimes covered with an indurated grey gangue -whose formation process was not identified -preventing observation of their surface.

In spite of these limitations, the remains observed in situ -when the anthropological team 2 was present during the removal of the skeletons-made it possible to record certain metric or morphological characters. In the laboratory, the preserved dental remains were observed after identification of the fragments, reassembly and restoration, when it was possible.

Age and sex

In rare cases, sex could be assessed in the field based on the morphology of the coxal bones [START_REF] Bruzek | A Method for Visual Determination of Sex, Using the Human Hip Bone[END_REF], and in one case measurements were made using the metric method [START_REF] Murail P | DSP: a probabilistic sex diagnosis tool using worldwide variability in hip bone measurements[END_REF]. Skull morphology was also observed [START_REF] Ferembach | Recommandations pour déterminer l'âge et le sexe sur le squelette[END_REF]. Where applicable, we mention in the Table 3 the sex and age of the individuals, the results being detailed in MUNOZ 2014 (Tables A3.4-5). Age estimation was often restricted to the distinction between osteologically adult or sub-adult individuals, except when teeth were present. We can therefore mention the presence of children, women and men. According to the observations made during the excavation, and those resulting from the anthropological study, at least four sub-adults would be represented, out of a total of 17 individuals. We could only reliably estimate in the laboratory the age of two sub-adult individuals whose teeth were preserved (7-15 years and 0.6-3.7 years). The age of a third (Grave 14: 3-5 years) was estimated from the stages of dental formation on the basis of a field photograph (estimate to be reviewed if the remains from this grave are found). Apart from the child from Grave 14, the youngest subjects are buried with an adult, in contexts that are difficult to specify from the available documentation.

We note the absence of individuals less than one year old, which we do not know if it is due to differential conservation, practices excluding the youngest children from this sepulcral space, and/or partial recovery due to limits of excavated area.

Stature

Some long bones could be measured in the field, and allowed an estimation of the size stature (CLEUVENOT and HOUËT 1993) of three individuals (Table 3). The stature estimated from the femoral lengths of three individuals is 157.3 ± 4.1 cm on average for all sexes combined.

Anatomical variations, discrete traits

Some traits have been noticed several times. However, given the state of preservation of the skeletons, it was not possible to systematically check the presence or absence of these traits for the entire collection. We mention them here for the record in decreasing order of occurrence: Olecranon perforation of the humerus (3 present/5 observable); Lateral notch on the patella (3/3); Allen's pit (2/2); Spina bifida (1/2); Distal tibia squatting facet (1/1); Vestigial metope suture (1/1). The small number of observations and their non-exhaustive nature do not allow for interpretation.

Pathologies

Degenerative diseases

Five individuals showed signs of degenerative pathologies (osteophytes, subchondral cysts, eburnation) that were moderate to severe (Graves 1, 2, 3C, 8, 9sup). Given the conservation of the remains, it was not possible to systematically check the presence of such signs throughout the skeleton. It should be noted that when they were observed, they were more often located on the upper limbs. That being said, it is very likely that differential conservation could have biased this observation.

Fractures

In five individuals (Graves 2, 3C, 8, 9sup, 13), bone calluses were found (Table 4), associated with periosteal changes on some long bone fragments, interpreted as consolidated fractures; radiographs would probably be required to confirm this. This frequency seems quite high if we reduce it to the number of adults, especially since not all skeletons are equally represented, and other traumas may have been missed due to the poor preservation of the bones.

It should be noted that the fractures involving the two forearm bones (radio-ulnary), observed for the subjects of Grave 2 and 13 are supposed to be characteristic of a trauma caused in case of defence against an assailant (parade fracture) (CHARLIER 2008: 299). In this case, they would likely indicate a violent episode during the individual's lifetime. However, since the parry is an instinctive protective reaction, it can be mobilized in the face of dangers other than aggression. Traumas involving the lower limbs may have been disabling and have probably required the rest of the group to assume the care of the individual during convalescence.

Oral conditions

The retained dental fragments, once reassembled, have enabled the recording of several pathologies, summarized in the Table 5; recording and scoring methods are detailed in MUNOZ 2017.

Wear is average to severe (average degree = 4.1), and more pronounced on the anterior teeth. All teeth have light to moderate calculus (average degree = 0.98, with 88.5% having calculus). No carious lesions were observed. The observable alveoli (n=42) show a slight to severe resorption of the alveolar bone (mean degree=1.8).

Abscesses are visible on 11.8% of the alveoli (Figure 16). One is associated with the loss of an upper central incisor (Grave 8), and others, periapical, probably result from intra-vitam fractures of the underlying teeth (Graves 8 and 9sup).

Three cases of ante-mortem loss of teeth were observed, two of which were on the same individual (Grave 9sup), which represents 11.1% of the total number of observable alveoli. Given the state of preservation of the material, their occurrence is probably underestimated. Finally, 37% of the anterior teeth observed have linear enamel hypoplasia (LEH), which is rather a moderate rate compared to other coastal neolithic groups (MUNOZ 2017).

Many teeth had micro-fractures (41.9% of teeth) or fractures (21%), that

Figure 16. Abscesses on a right upper central incisor in Grave 8 (A) and left upper molars in Grave 9sup (B). Figure 17. Chipping on an upper central incisor in Grave 9sup (A), fractures on the upper dentition in the same grave (B), fractures and wear on right lower molars in Grave 1 (C, D), and polished fractures, grooving and notches on upper canines and premolars in Grave 1 (E).

occurred intra-vitam (patinated, calculus-covered, or polished, Figure 17, C-E). Some microfractures (Figure 17,A), such as those located on the interproximal margins of teeth, could be attributed to abrasion caused by the introduction of hard particles into the food (sand, shellfish) or as a result of extra-masticating activities such as food preparation (fracturing of bones to remove marrow, seeds or nuts). According to several studies, dental chipping is more frequent in populations that consume meat than in populations whose livelihood depends on intensive agriculture [START_REF] Turner | Dental chipping in Aeuts, Eskimos and Indians[END_REF][START_REF] Scott | Dental Chipping: Contrasting Patterns of Microtrauma in Inuit and European Populations[END_REF].

Other stigmas can testify to the use of teeth as tools (Graves 1, 8, 9sup). This is probably the case for occlusal notches or grooves characterized by enamel loss and having a polished appearance (21% of the observed teeth). Indeed, these modifications can be produced by a flexible or rigid cylindrical object, simply held between the teeth, which are then used as pliers or whose repeated passage in an identical orientation alters the enamel [START_REF] Consiglio | Non masticatory dental lesions in the study of biology and behaviour of ancient populations: the contribution of the stereomicroscopy ans scanning electron microscopy[END_REF]. Interproximal grooving (Figure 18) are visible on 11.3% of the teeth at the base of the collar.

On the other hand, paradoxically, the poor conservation of the alveolar bones has been an advantage: the roots are often observable, as are cementicles that would have gone unnoticed without X-rays if the teeth had remained in place in their wellpreserved alveoli. We were thus able to observe several pathologies rarely mentioned in the archaeological literature: a cases of severe hypercementosis [START_REF] Tang N | Dentine and Cementum Structure and Properties[END_REF] was remarked for the individual from Grave 1, and cementicles were found on two individuals (Figure 19; Graves 8 and 13). As both formations are due to chronic periodontal inflammations that can be caused by repeated mechanical stress (occlusion forces), they could result of an intense use of teeth both for alimentary and occupational purposes. The intra-vitam loss of two teeth, observed for the individual buried in Grave 9sup, may also result from significant stress.

All these elements suggest that the population of Wadi Shab -or at least the individuals whose dental remains we observed -also used their teeth as tools. These include the crushing of hard materials for food preparations (bone, shell, nuts, seeds, etc.) that may have caused fractures of the posterior teeth, but also the preparation or treatment of fibres, which could be seen in some occlusal and labial grooves on the anterior teeth, although they are much less obvious than some published cases (e.g.: [START_REF] Bocquentin F | Abrasion dentaire et travail spécialisé dans la population natoufienne de Mallaha (Israël)[END_REF]; on this subject see also [START_REF] Molnar | Tooth wear and culture: a survey of tooth functions among some prehistoric populations[END_REF]. The significant abrasion of some teeth, with a rounded vestibular edge, may be related to skin treatment (SCOTT and TURNER 1988).

The interproximal grooving observed in several individuals could attest to the recurrent use of a toothpick for palliative purposes. They are very commonly observed in all regions of the world, dating

Figure 19. Cementicle between the left upper second premolar and the first molar in grave 8 (A) and hypercementosis on the left upper first molar in grave 1 (B).

[AUTHOR VERSION] MUNOZ, O. & D. USAI. 2020. Wadi Shab (GAS-1, Oman): the Neolithic (4 th /3 rd mill. BC) cemetery and the settlement, in D. Usai, S. Tuzzato & M. Vidale (ed.) Tales of Tree Worlds. Archaeology and beyond: Asia, Italy, Africa. A Tribute to Sandro Salvatori, Archeopress Archaeology: 65-84. Oxford, UK: Archeopress.

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back to the early Paleolithic, and seem closely related to the human lineage (for a review, see [START_REF] Ungar | A review of interproximal wear grooves on fossil hominin teeth with new evidence from Olduvai Gorge[END_REF]. Several etiologies have been proposed, ranging from dental hygiene to cultural or idiopathic habits (FORMICOLA 1988;FRAYER 1991;BONFIGLIOLI et al. 2004). However, in California, Australia and Pakistan, interproximal grooving have sometimes been interpreted as being related to activities such as intertooth fibre treatment [START_REF] Schulz | Task activity and anterior tooth grooving in prehistoric California Indians[END_REF][START_REF] Brown | Interproximal grooving and task activity in Australia[END_REF][START_REF] Lukacs | Activity induced patterns of dental abrasion in prehistoric Pakistan[END_REF], a possibility that should not be excluded for GAS1 individuals. On this site, they are more marked on the neck of the posterior teeth, but grooving also appear on the occlusal surfaces of observed at al-Buhais BHS-18 (KIESEWETTER 2006: 218-219), at Ra's al-Hamra RH-5 (MUNOZ 2014), or on the other side of the Persian Gulf, at Mehrgarh (LUKACS andPASTOR 1988, 1990).

However, the macroscopic observations we were able to make do not allow us to go any further in the interpretation of these stigmas (materials worked or gestures made) and would require further indepth analyses (optical and electron microscopy, experimentation).

Synthesis on funerary practices and biological data

Burial structures

The 14 graves excavated at Wadi Shab GAS1 consist ofsemi-circular pits, about one meter in diameter, most of which are covered with a more or less elaborate assemblage of stones. Most of the time, the deceased are covered with medium sized stones (flat pebbles and polygonal limestones) or large flat slabs. Some coverings seem more elaborate than others and include several layers of carefully arranged stones; this is the case for Graves 6 and 9. In half of the graves, small beach pebbles were included in the covering.

Treatment, position and orientation of the deceased

Many of the graves have experienced disturbances that may have led to the alteration of the human remains they contained, damaging both the study of the funerary context and the study of the bones. When the bone remains were still in the primary position, it could be seen that the deceased were placed in a flexed position on the side, with variations in the side on which they rest, and a more or less significant constriction of the body. In four cases, the skeletons were in an extremely crouched position, suggesting that the bodies had originally been artificially constrained. Hands are almost always placed close to the face. However, one of the individuals (Grave 4) had both arms placed under the legs and the one in Grave 8 had the right hand under the right thigh. In both graves, there is ample evidence to suggest that the body was restrained by ties or a flexible envelope. Grave 4 in particular raises questions about the practices related to the burial of the deceased: why does the body, artificially forced, occupy only half of the grave? The probable presence of basketry objects, as detected from excavation documentation and/or gypsum remains still present on the bones (Graves 7, 8, 9sup, 13), would confirm the hypothesis of the existence of an envelope, clothing, or vegetable covering.

In the documented cases, there is a preference for deposition on the left side (n=7), but some deceased were deposited on their right side (n=3). This does not seem to be sex-related since two individuals whose sex could be determined in the field -probably two women -each rested on a different side.

For seven of the individuals, it was not possible to determine the initial position of the body due to disturbances to the skeleton.

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The orientation of the bodies as it could be inferred for 11 cases is variable and it is difficult to draw interpretations on a possible link with the solar trajectory, as proposed for Ra's al-Hamra 5 (SALVATORI 2007). The most frequent position is the deposition on the left side with the head to the north but it only concerns three cases; it can then be assumed that the face was facing the East. But in two cases, the body deposited on the right side follows a NE/SW orientation, and then the face is turned towards the NW, while in another case, the body follows a W/E axis, the face towards the south. It should be added that when you are on site, the sea is visible at an angle of about 180° (from NW to SE). In any case, given the small size and variability of the orientations observed, it is not possible to discuss this further.

Burial types

As we have seen, many graves are disturbed. Some disturbances may be due to post-depositional alterations of natural or anthropogenic origin (burrows, looting, overlapping...), while others seem to be related to mortuary practices in several stages. Some graves have undergone post-depositional interventions: Grave 2 could be the result of a reduction (rearrangement with secondary deposit of selected defleshed remains), while Grave 6 could be the site of the primary deposition of a corpse, from which certain bones would then have been removed. This variety of burial methods certainly reflects the complexity of mortuary practices, that could be accomplished over a fairly long period of time, as also observed at al-Buhais BHS-18 (KIESEWETTER 2006), Ra's al-Hamra RH5 (SALVATORI 2007;MUNOZ 2014: 299).

Most of the graves are individual but two of them contained the remains of several individuals: Grave 3 provided the remains of at least three individuals, two children and an adult; Grave 13 contained the remains of an adult and a child. In both cases, the disturbed context prevent a clear reading of the primary or secondary nature of the deposits. In Grave 3, some skeletal parts still had anatomical connections, and many small bones are present, suggesting that it contains highly disturbed primary deposits; however, it is also possible that it is the result of time-deferred deposits, the latter of which would have disturbed the previous one(s).

In any case, this example shows the intention to group the three deceased together in the same grave.

In Grave 13, a completely altered child's skull is present, which appeared under the covering, and above an adult in the primary position. Again, the state of preservation of the remains make it impossible to know whether the tomb contained the entire bodies of these two individuals, one of whom suffered intense post-depositional destruction, or whether a child's skull was deposited, alone, on the adult.

It should be noted, however, that documented graves containing the remains of more than one subject contain an adult associated with one or more children. However, Grave 14, contains the remains of a child (3-5 years old), alone. This suggests that young subjects could, in some cases, be treated individually without being systematically associated with an older individual.

Grave goods and faunal remains

Furniture is rare, only a few graves have delivered objects, and no ornaments were clearly associated with the deceased notwithstanding the large quantity of chlorite beads and earrings that were recovered at the site (TOSI and USAI 2003; USAI 2018). Similarly, as ISETTI and BIAGI (1989) also pointed out, in RH5, where a large number of graves have been excavated, out of 308 items found so far, only a very small proportion of earrings (6.8%) have been found whole associated with the deceased in the graves, 3.2% being present as a fragment in the filling, and the overwhelming majority (89.9%) in the settlement layers (MUNOZ 2014: 179).

The few associated objects can be described more as utilitarian (bone points, flint tools, mother-ofpearl hooks), although the meaning of their presence in the graves remains unknown. The function of other objects, such as the large gastropod fragment from Grave 8, remains difficult to interpret. We notice the presence of remains of articulated fish in some graves, which could testify to the deposition of food with the deceased. Were they a viaticum, intended to accompany the deceased in death and beyond? Are they the reliefs of a meal celebrated around the grave in homage to the deceased? Or do they represent a symbolic gesture accompanying the funeral ceremony whose meaning escapes us?

Other faunal remains are frequent in the filling (bone fragments of marine turtles and land mammals) without being it possible to know whether their presence is accidental or deliberate. A grave also produced a valve of Pinctada sp. lying between the levels of covering stones, whose presence is probably not accidental. In several cases (Graves 4, 7 and 9sup) the excavation revealed the probable presence of a fireplace containing faunal remains in the pit, between the deceased and the stone roof that sealed the grave.

Biological data

Given the very poor preservation of the bones, the sex and age of the burials could only be estimated in rare cases, and no one can pronounce on the recruitment of this burial area, except for the presence of men, women and children. It is noted that no remains belonging to children under one year of age have been identified.

Five adults had bone calluses or swellings of the diaphyseal shaft on some long bones, which we interpreted as consolidated fractures. X-rays would undoubtedly make it possible to refine the description of these pathologies. Some of them, located on the lower limb, must have been disabling; this suggests that the group was able to manage for a while the convalescent subjects affected by these traumas. Other fractures could reveal violent episodes in people's lives.

The study of dental remains, where possible, has been informative. The population buried at Wadi Shab GAS1 is globally characterized by a high dental wear, more important on anterior teeth, combined with moderate calculus rates, moderate LEH frequency, and frequent periodontal diseases as demonstrated by the high rates of alveolar resorption. On the other hand, they show rather low abscesses and AMTL frequencies, and a total absence of carious lesions. It denotes a diet relying basically on protein rich, unprocessed food requiring intensive mastication, with an abrasive consistency. In addition to the severe attrition related to the abrasive nature of the food, the high frequency of chipping and fractures affecting the teeth in this group indicates the widespread and intense solicitation of the teeth for processing hard components. Such heavy use of teeth is also attested by cases of severe hypercementosis, as well as some cases of cementicles. These observations indicate a food probably rich in hard particles, and a probable use of teeth for paramasticatory tasks, sometimes causing tooth fractures, and possibly causing tooth loss and/or periodontal disease (abscesses, cementicles, hypercementosis).

Conclusion

The Wadi Shab GAS1 graveyard is of particular interest for the knowledge of the mortuary practices and lifestyles of Omani coastal Neolithic groups, as it contains a fairly large number of tombs, which provide a representative overview. Indeed, apart from the funeral complexes of Ra's al-Hamra (RH4, RH5, RH6 and RH10), the other Omani Neolithic sites excavated have so far delivered only a few graves or the partial remains of individuals (2 at Ra's al-Khabbah KHB1, [START_REF] Munoz | The burials of the middle Holocene settlement of KHB-1 (Ra's al-Khabbah, Sultanate of Oman)[END_REF]4 at Suwayh SWY1, CHARPENTIER et al. 2003[START_REF] Munoz | Pratiques funéraires et paramètres biologiques dans la Péninsule d'Oman du Néolithique à la fin de l'âge du Bronze ancien[END_REF]3 at Khuwaymah KHU2, CHARPENTIER et al. 2012:69-70;6 at Ra's al-Hadd HD5, BORGI and MAINI 2018;1 at Wadi Wuddaya WD58, Cleuziou and TOSI 2000:15;4 at Ra's al-Jinz RJ1, MONCHABLON et al. 2003:36;5 at Ruways RWY1, BERGER et al. in press).

The graves have suffered many disturbances that have affected both the reading we could have of the modalities of the mortuary deposits, but also the state of preservation of human remains, which makes it more difficult to study them. However, it was possible to obtain several important information about the population buried in Wadi Shab. First, there is a certain diversity in the treatment of the dead, with the presence of mortuary practices in several stages (reopening of graves and removal of remains, secondary repository of selected remains), although individual primary burial seems to be the most common practice. Complex practices of this type have also been recorded at other sites on the Oman Peninsula, such as Ra's al-Hamra RH5 (SALVATORI 2007; MUNOZ 2014), Ra's al-Khabbah KHB1 [START_REF] Munoz | The burials of the middle Holocene settlement of KHB-1 (Ra's al-Khabbah, Sultanate of Oman)[END_REF] or al Buhais BHS18 [START_REF] Kiesewetter | Analysis of the Human Remains from the Neolithic Cemetery at al-Buhais 18 (excavations 1996-2000)[END_REF]. There is also a certain variety in the funerary arrangements and treatment of the deceased (complexity of the cover, side, position and orientation of the deceased) that does not seem to be related to the sex of the individuals and which may also find parallels in the other two major Neolithic cemeteries known. The material culture of GAS1 is similar to that of Ra's al-Hamra, especially net-sinkers, fish hooks and stone earrings (SALVATORI 2007; TOSI and USAI 2003; USAI 2018) but substantial differences may be noticed between the lithic industries produced at the two sites (USAI 2006) part of which may have been determined by the more favourable location of GAS1 and its vicinity to good quality flint sources.

Finally, the biological data collected made it possible to determine the lifestyle of the group living in Wadi Shab, both on dietary practices and on the violent episodes that may have marked the life of the group.

  , four field seasons were carried out at Wadi Shab GAS1, from 2001 to 2003 and a last one in 2005. While badly affected, after its discovery, by road paving and house constructions, the site produced relevant data concerning the Middle Holocene human occupation of the area with structures relating to a settlement and a small group of burials. A first 5x5 m area was explored in 2001 (Area 1; TOSI and USAI 2003) and a northern extension opened in 2002 whose exploration was finished in 2003 (Area 100; GAULTIER et al. 2005); a further and last field season was forwarded at the site in 2005 (USAI 2006) when other 50 m2 were excavated to the north-west of the Area1/Area100 (Figure 2).

Figure 1 :

 1 Figure 1: Map of the area.

Figure 2 .

 2 Figure 2. Distribution of the main areas and features recorded at Wadi Shab GAS1.

Figure 4 .

 4 Figure 4. Handmade pottery from the Islamic phase at Wadi Shab.

Figure

  Figure 5. A softstone plaquette found at Wadi Shab-GAS1.

Figure 3 .

 3 Figure 3. Islamic walls (dark grey) and 3 rd millennium BC structures at Wadi Shab GAS1.

Figure 8 .

 8 Figure 8. Grave 8 with a highligth (b) of the shell and flint deposited near the right pelvis.

Figure 6 .

 6 Figure 6. An unfinished chlorite earring with initial perforation (left) and a broken unfinished specimen.

Figure 7 .

 7 Figure 7. Structure 10, originally supposed to be a grave.

Figure 9 .

 9 Figure 9. Grave 9sup; different stages of excavation of the superstructure (a, b), the filling of small rubble (c) and the skeleton emerging under the filling (d).

[ 19 Figure 10 .

 1910 Figure 10. The partial skeleton of Grave 9inf.

Figure 11 .

 11 Figure 11. Grave 11 superstructure (a) and the few human remains recovered underneath (b).

[

  AUTHOR VERSION] MUNOZ, O. & D. USAI. 2020. Wadi Shab (GAS-1, Oman): the Neolithic (4 th /3 rd mill. BC) cemetery and the settlement, in D. Usai, S. Tuzzato & M. Vidale (ed.) Tales of Tree Worlds. Archaeology and beyond: Asia, Italy, Africa. A Tribute to Sandro Salvatori, Archeopress Archaeology: 65-84. Oxford, UK: Archeopress.

Figure 12 .

 12 Figure 12. Grave 12: the apparently well preserved superstructure (a), the filling of small rubble (b) and the few human remains recovered underneath (c).

Figure 13 .

 13 Figure 13. A grinding stone that was part of the superstructure of Grave 12.

Figure 14 .

 14 Figure 14. Skull remains of a first individual (a) lying on top of the second individual recovered in Grave 13 (b).

Figure 15 .

 15 Figure 15. Remains of the individual buried in Grave 14 (b) in spite of an apparently untouched filling (a).

1

  Since January 2020 the former Maison René-Ginouvès de l'Archéologie et de l'Ethnologie has been renamed Maison des Sciences de l'Homme « Mondes » 2 During the field season 2003, the anthropological team included one of the authors (OM), as well as Matthieu Gaultier and Hervé Guy.

Figure 18 .

 18 Figure 18. Interproximal grooving on left upper molars (A, B), a molar fragment (C) and a left lower canine (D) in Grave 1.

Table 1 . Dates available for site Wadi Shab-GAS1.

 1 Calibrated with Calib7.0.4 software, with IntCal13 curve* and Marine13 curve**, with ΔR=215±15 (SALIÈGE et al. 2005) and considering 0-100% marine input into the diet (see MUNOZ 2014 : 39-40 for more details)

	Context	Material	Lab. No.	Date bp	Cal. BC 1σ	Cal. BC 2σ
	Unknown	Sediment	GX-17881	5127±80	4030-3800*	4220-3710*
	Grave 4/fill	Charcoal	Beta-201729	4860±70	3710-3530*	3800-3530*
	US114	Charcoal	Beta-206089	4070±60	2850-2490*	2870-2470*
	Grave 3/indiv. A	Enamel	AA-90259/P-974	5046±38	3940-3110**	3960-3040**
	Grave 2	Bioapatite	SacA-11377	5070±30	3940-3140**	3950-3090**

Table 2 .

 2 Summary of the characteristics of the graves from Wadi Shab-GAS1.

	Grave n°	Covering		Burial type	Side	Orientation	Gravegoods	Faunal remains
	1	2 flat stones and	Primary, individual	Left	W-NW/S-SE	-	-
		stone circle
	2	Several slabs	Secondary		-	NW/SE	-	-
	3	Several large flat	Primary, perturbed,				Turtle bones
		pebbles and small	multiple (3
		white pebbles	individuals)
	4	2 layers of large	Primary, individual	Right	NE/SW	2	grinding	Fish and turtle
		stones and half-					stones in the	bones
		circle of small					covering
		stones
	5	Small white pebbles Primary, individual	Right	W/E	Flint	blade,	-
								stone tool
	6	3 layers of flat	Primary	perturbed?	Left?	E/W	Flint tools in the	Fish and turtle
		stones		individual				covering	bones
	7	Large slabs and	Primary, individual	Left	E/W	Small	oval	Fish and turtle
		small white pebbles					pebble under the	bones, Pinctada
								right hand,	margaritifera
									valve in the pit
	8	No	covering	Primary individual	Left	N/S	Flint tool in the	-
		preserved, a large					right hand, big
		pebble close to the					gastropod
		skull
	9sup	2 layers of flat	Primary, individual	Left	W/E	Flint tools in the	Fish bones
		stones, small white					filling, 2 balls of
		pebbles						ochre
	9inf	See 9sup		Primary, perturbed,	Left	N/S	-	-
				individual
	11	Several	irregular	Perturbed, individual	-	-	-	-
		stones
	12	Several slabs in	Perturbed, individual	-	-	-	Fish and other
		limestone and small						faunal bones
		white pebbles
	13	1 slab and small	Primary, perturbed,	Right	NE/SW (ind. B)	Stone	earring	-
		white pebbles	multiple (2 indiv.)	(ind.B)		fragment
	14	Irregular stones and	Perturbed, individual	-	-	-	-
		small pebbles

Left (Ind. C) N/S (ind.A)

Mother-of-pearl hook

[

  AUTHOR VERSION] MUNOZ, O. & D. USAI. 2020. Wadi Shab (GAS-1, Oman): the Neolithic (4 th /3 rd mill. BC) cemetery and the settlement, in D. Usai, S. Tuzzato & M. Vidale (ed.) Tales of Tree Worlds. Archaeology and beyond: Asia, Italy, Africa. A Tribute to Sandro Salvatori, Archeopress Archaeology: 65-84. Oxford, UK: Archeopress.

Table 3 ,

 3 and preservation forms in MUNOZ 2014: figs. A4.92-107).

Table 3 . Anatomical Preservation Index, fragmentation rate, age, sex and stature of the individuals recovered at Wadi Shab

 3

	Grave/Individual Anatomical	Fragmentation	rate	Age (yrs)	Sex Stature (cm)
		Preservation Index	(%)
		(%)
	1	8.9	91.4		>20	F
	2	4.4	98.6		>20	-
	3/A	6.6	91.4		7-15	-
	3/B	1.1	95.5		0.6-3.7	-
	3/C	17.2	94.1		>20	-
	4	-	-		>20	-	158 ± 4.1
	5	-	-		>20	M	162.9 ± 4.1
	6	16.9	97.7		>20	-
	7	-	-		20-30	F?	150.9 ± 4.1
	8	-	81.4		>20	M?
	9Sup	29.3	86.9		>20	F
	9Inf	5.2	96.4		>20	-
	11	4.8	97.3		>17	-
	12	0.3	100		>15	-
	13/A	-	-		subadult	-
	13/B	26.7	84.2		>20	M
	14	0.6 %	100		3-5 ?	-

-GAS1 (for further details, see MUNOZ 2014)

Table 4 . Occurrence and localisation of bone calluses interpreted as consolidated fractures.

 4

	For further details, see MUNOZ 2014.
	Localisation	Occurrence (Grave n°)
	Skull	-
	Vertebra	-
	Rib	-
	Humerus	-
	Radius/Ulna	3 (G.2 ;13)
	Hand	1 (G.9sup)
	Femur	1(G.3C)
	Tibia/fibula	2(G.8, 9sup)
	Foot	-
	Total	7

Table 5 . Rate of dental pathologies observed on permanent dentition in the study sample (number of observed teeth/alveoli

 5 ). (For further details on the recording methods and raw data, see MUNOZ 2014; 2017).

	Wear mean	Heavy wear	Calculus mean	Calculus	Caries	Alveolar resorption	Alveolar	Abscesses	Ante-mortem
	grade (1-8)	(%)	grade (1-3)	(%)	(%)	mean grade (1-3)	Resorption (%)	(%)	Tooth Loss
									(%)
	4.1 (64)	46.9 (64)	0. 98 (61)	88.5 (61) 0.0 (63)	1.81 (42)	71.4 (42)	11.8 (34)	11.1 (45)

  [AUTHOR VERSION] MUNOZ, O. & D. USAI. 2020. Wadi Shab (GAS-1, Oman): the Neolithic (4 th /3 rd mill. BC) cemetery and the settlement, in D. Usai, S. Tuzzato & M. Vidale (ed.) Tales of Tree Worlds. Archaeology and beyond: Asia, Italy, Africa. A Tribute to Sandro Salvatori, Archeopress Archaeology: 65-84. Oxford, UK: Archeopress.

Sandro Salvatori, Archeopress Archaeology: 65-84. Oxford, UK: Archeopress.

Acknowlegement

I would like to thank Olivia for accepting to collaborate in the making of this contribution to dedicate to Sandro. To Sandro, my work and life partner, I owe to have introduced me to the archaeology of this region and made me a wiser researcher and better woman.