Manuel Gaudon

Guillaume Salek

Malang Kande

Ines Andron

Christine Frayret

Etienne Durand

Nicolas Penin

Mathieu Duttine

Alain Wattiaux

Veronique Jubera
email: veronique.jubera@u-bordeaux.fr

Véronique Jubera

Cationic environment impact on Pr3+ doped compounds luminescence

Keywords:

published or not. The documents may come

Introduction

Because of their wide range of applications, perovskite-type alkaline earth metal stannates of formula MSnO3 have been deeply investigated. Surprisingly, the luminescent properties of rare earth doped perovskite related structure are not largely reported yet [START_REF] Ueda | Luminescence and valence of Tb ions in alkaline earth stannates and zirconates examined by X-ray absorption fine structures[END_REF][START_REF] Shi | Preparation and characterization of a new long afterglow phosphor CaSnO3: Yb 3+[END_REF][START_REF] Vieira | Photoluminescence in SrSnO3:Fe 3+ Perovskite[END_REF][START_REF] Kamimura | Strong reddish-orange light emission from stress-activated Srn+1SnnO3n+1:Sm 3+ (n = 1, 2, ∞) with perovskite-related structures[END_REF][START_REF] Nakamura | Synthesis of Pr 3+ doped or Tb 3+ -Mg codoped CaSnO3 perovskite phosphor by the polymerized complex method[END_REF][START_REF] Goto | Photoluminescence properties of Pr doped and Tb-Mg codoped CaSnO3 with perovskite structure[END_REF][START_REF] Lu | Preparation and luminescence properties of Eu 3+ -doped MSnO3 (M = Ca, Sr and Ba) Perovskite materials[END_REF][START_REF] Endo | Europium (2+) luminescence in Srn+1SnnO3n+1 with layered perovskite structure[END_REF][START_REF] Macke | Luminescence and energy transfer in the ordered perovskite system lanthanum magnesium tin titanium oxide (La2MgSn1-xTixO6)[END_REF]. However, promising results describe the photoluminescence of praseodymium doped perovskite as novel materials for multifunctional devices as the absorption lines of this rare earth overlaps the emission of commercial blue diode [START_REF] Shen | Red-emitting enhancement of Bi4Si3O12:Sm 3+ phosphor by Pr3+ co-doping for White LEDs application[END_REF][START_REF] Ma | KSr4(BO3)3:Pr 3+ : A new red-emitting phosphor for blue-pumped white light-emitting diodes[END_REF][START_REF] Tang | Emission characteristics of Pr 3+ -doped InNbO4 used as a red-emitting phosphor for white LEDs J. Optoelectron[END_REF][START_REF] Zhang | Pr 3+ /Ho 3+ co-doped glass phosphors for application in warm white light-emitting diodes[END_REF][START_REF] Zhou | Hydrothermal synthesis and photoluminescent properties of Li2Sr0.996SiO4:Pr 3+ 0.004 phosphors for white-LED lightings[END_REF][START_REF] Xiong | Luminescent properties of deep red light-emitting phosphors NaGd(WO4)2:Pr 3+ for blue LED Opt[END_REF][START_REF] Marin | Photoluminescence properties of YAG:Ce 3+ , Pr 3+ phosphors synthesized via the Pechini method for white LEDs[END_REF][START_REF] Wang | Enriching red emission of Y3Al5O12: Ce 3+ by codoping Pr 3+ and Cr 3+ for improving color rendering of white LEDs Opt[END_REF][START_REF] Qi | Pr 3+ doped LaTiNbO6 as a single phosphor for white[END_REF][START_REF] Zhou | A Promising Deep Red Phosphor AgLaMo2O8:Pr3+ with Blue Excitation for White LED Application[END_REF].

The CaSn(OH)6, CaSnO3 and CaSnF6 exhibit obvious structural filiation. Indeed, these compounds have a structural network built on corner-sharing octahedra constituting the covalent skeleton. CaSn(OH)6 and CaSnF6 can be described as "double -ReO3" structure in which Ca 2+ and Sn 4+ are strictly alternately distributed to form the octahedral chains. The octahedral sites of the double hydroxide are anisotropic, with low point group symmetry. Historically, CaSn(OH)6 crystal was described as an orthorhombic perovskite structure with a P n -3 space group, where the Ca(OH)6 and Sn(OH)6 octahedra share corners. More important, this framework can facilitate the mobility of the charge carriers, and then enhance the photocatalytic activity of CaSn(OH)6 under light irradiation [START_REF] Moshtaghi | Characterization of CaSn(OH)6 and CaSnO3 Nanostructures Synthesized by a New Precursor[END_REF]. Nonetheless, the twist of the octahedral sites can also occur in such a way that a cubic unit-cell is maintained with P n -3 m space group [START_REF] Meng | Sonochemical synthesis, characterization and photocatalytic properties of a novel cube-shaped CaSn(OH)6[END_REF], as for the Ca-Sn hydroxides (burtite) prepared in our work. The double fluoride is constituted of non-twisted isotropic octahedral sites that lead to a cubic unit-cell with F m -3 m space group. Regarding CaSnO3, the octahedral skeleton is built on [SnO6] tilted octahedra (P b n m space group), the Ca 2+ ions occupying the interstitial icosahedral sites. A scheme emphasizing the structural similarities and differences between the three studied compounds is reported (Fig. 1.). Even if a common skeleton is clearly visible and although cationic local environment is quite similar in the three matrices, the anionic group or ligands deeply change the nature of the chemical bonding. Because of their low phonon energy, their high transparency in the infrared and their stability under an exciting beam, fluoride hosts are of great interest for laser, up-converting or up-conversion materials [START_REF] Biswas | Enhanced near-infrared to green upconversion from Er3+doped oxyfluoride glass and glass ceramics containing BaGdF5 nanocrystals[END_REF][START_REF] Kaminskii | Cascade stimulated emission from praseodymium(3+) and holmium(3+) ions in fluorineand oxygen-containing crystals Kvantovaya Elektronika[END_REF][START_REF] Yang | Energy transfer and 1.8 μm emission in Tm3+/Yb3+ co-doped LiYF4 crystal[END_REF][START_REF] Yu | Enhanced photoluminescence and initial red laser operation in Pr:CaF2 crystal via co-doping Gd3+ ions[END_REF][START_REF] Soulard | 1.87 μm laser operation by 980 nm pumping in Yb,Tm:CaF2[END_REF][START_REF] Petit | Thermomechanical properties of Yb3+ doped laser crystals: Experiments and modeling[END_REF]. Nevertheless, the increase of the chemical bonding ionicity is generally associated to an UV/blue shift of the spectral distribution, which can reduce drastically the efficiency of the absorption process regarding the matching with excitation sources.

Synthesis flow charts of CaSn(OH)6 were recently described in literature [START_REF] Liu | Highly enhanced photocatalytic activity of CaSn(OH)6 through tuning CaSn(OH)6/SnO2 heterostructural interaction and optimizing Fe 3+ doping concentration[END_REF][START_REF] Wang | Hydrothermal synthesis and catalytic performances of a new photocatalyst CaSnO3 with microcube morphology[END_REF]. CaSnO3 perovskite compounds are typically synthetized at high temperature (over 1200°C) from traditional solid state reaction between CaO and SnO2 oxide [START_REF] Liu | Synthesis and luminescent properties of a new green afterglow phosphor CaSnO3[END_REF][START_REF] Canimoglu | Catholuminescence properties of rare earth doped CaSnO3 phosphor[END_REF][START_REF] Zhao | Synthesis and luminescent properties of a new cyan afterglow phosphor CaSnO3: Gd 3+[END_REF] Nevertheless, a two-step method was already employed for CaSnO3 sample preparation including the synthesis at room temperature of the precursor CaSn(OH)6 and then its conversion into CaSnO3 by calcination [START_REF] Wang | Hydrothermal synthesis and catalytic performances of a new photocatalyst CaSnO3 with microcube morphology[END_REF][START_REF] Lei | Preparation and luminescence properties of CaSnO3:Sm 3+ phosphor emitting in the reddish orange region Opt[END_REF][START_REF] Chen | Novel porous CaSnO3:Eu 3+ and Ca2SnO4:Eu 3+ phosphors by co-precipitation synthesis and postannealing approach: A general route to alkaline-earth stannates[END_REF][START_REF] Cheng | Synthesis and gas-sensing properties of CaSnO3 microcubes[END_REF]. CaSnF6 compounds were typically made by solid state synthesis in a sealed tube [START_REF] Mayer | Struktur und Bindung in Übergangsmetall-Fluoriden MIIMeIVF6. Neutronenbeugungs-Strukturuntersuchungen an CaSnF6, FeZrF6[END_REF]. By consequence, it represents a high challenge to find new synthesis routes of doped CaSnF6 phases, cost-effective and with industrial feasibilities.

The first part describes the synthesis and the structural characterizations of the CaSn(OH)6, CaSnF6 and CaSnO3 compounds. Especially, the description of a specific synthesis process based on the fluorination of CaSn(OH)6 precursor is detailed. In a second part, the balance of the ioniccovalent bonds surrounding the luminescent ions is compared. We propose here to follow the evolution of the luminescent properties of Pr 3+ doped compounds as a function of the chemical bonding nature. Crystal field effect will be highlighted through the 4f-4f absorption and emission lines of trivalent praseodymium. These three compounds issued from a single synthesis batch and which present a strong structural filiation constitute school-case examples for studying the impact of the iono-covalence of the bonds around the luminescent ion, on the optical properties. This constitutes a first step to control and drive the emission properties to a targeted range. a)

Ca 2+ Sn 4+ OH - a b c Ca 2+ Sn 4+ O 2- Ca 2+ Sn 4+ F - a b c b) c) a b c

Materials and Methods

Synthesis.

A co-precipitation process in aqueous solution allows obtaining the polycrystalline hydroxide compounds with CaSn(OH)6 or Pr-doped CaSn(OH)6 target formulae. CaCl2 and SnCl4.5H2O with eventually Pr(NO3)3, are firstly introduced into stoichiometric concentration into 25 mL of water. NaOH aqueous solution is added to the cationic solution in order to adjust the final pH to 10, 11.5 or 13.

Once the precipitation has occurred, a maturation time of the precipitate under magnetic stirring is made during 2 hrs; the precipitate is filtered and washed with demineralized water. The solid residue is then crushed into an agate mortar. The as-prepared powders are then placed into a tubular furnace and treated at 1200°C -12 hours under air, or between 100 and 250°C -2 hours under HF gas atmosphere in order to obtain the mixed oxides and mixed fluorides, respectively. It can be noted that HF gas requires necessary warming: an inductive heater allows the heating avoiding HF and resistance contacts and the HF outgas is neutralized through KOH traps in order to be free of toxic exhaust gas production.

The CaSnO3 and CaSnF6 matrices and the corresponding 2 mol% Pr-doped oxide or fluoride (CaSn0.98Pr0.02O3 and CaSn0.98Pr0.02F6) are crushed before characterization.

X-ray diffraction analysis.

Powder X Ray Diffraction (XRD) patterns were collected on a Philips X'Pert MPD diffractometer operating in Bragg-Brentano geometry with a Cu Kα1,2 radiation (10 < 2θ < 130°, step 0.02° and counting time of 30 s). Some of the diffraction patterns were refined using the full-pattern matching method (Le Bail) with the conventional reliability factors (Fullprof program package). Unit-cell parameters, zero-shift and peak profile parameters were refined.

ICP titration

ICP/OES (Inductively Coupled Plasma / Optical Emission Spectrometry) was conducted on Varian ICP/OES 720 ES apparatus.

SEM images

The Scanning Electron Microscopy (SEM) analyses were performed with a JEOL Field Emissive Gun -Scanning Electron Microscope (FEG-SEM) in order to characterize the crystallite (shape, size and special rearrangement) with a spatial resolution of about 1 nm.

Mossbauer investigation

119 Sn Mossbauer measurements were carried out using a constant-acceleration Halder-type spectrometer operating in transmission geometry with a room temperature 119m Sn source (370 MBq, CaSnO3 matrix). Thin absorbers containing about 10 mg.cm -2 of Sn ( 119 Sn natural abundance 8.59 %) were placed into a liquid helium bath cryostat. In order to evaluate Sn 4+ and/or Sn 2+ local environments of the studied compounds, spectra were recorded from room temperature down to 4.2 K and the refinement of Mossbauer hyperfine parameters (δ isomer shift, Δ quadrupole splitting, Γ Lorentzian linewidth and relative areas) was performed using both homemade programs and the WinNormos® software (Wissenschaftliche Elektronik GmbH). The 119 Sn isomer shifts are referenced to BaSnO3 at room temperature.

Photoluminescence analysis.

The photoluminescent properties were analyzed using a spectrofluorimeter Horiba Fluorolog3. Excitation spectra were corrected based on the variation of the incident flux. The emission spectra were corrected based on the transmission of the monochromator and the response of the photomultiplier. The equipment used is composed of a 450 W xenon lamp, an excitation double monochromator, an emission double monochromator and a thermoelectrically cooled photomultiplier tube.

Results and discussion

Un-doped matrices

Synthesis and XRD characterization

The compound was obtained by the co-precipitation produced by the addition of NaOH into a Ca:Sn aqueous solution with 1:1 molar ratio and was analysed by XRD after drying and grinding steps (Fig. 2.). The compound obtained by co-precipitation produced at pH =10 is different from the two other ones, obtained at pH=11.5 and 13. The first one is nearly amorphous. For the two latter, all the diffraction peaks can be clearly indexed as CaSn(OH)6 compound with P n -3 m cubic space-group. A refinement of the unit-cell parameters was performed on the powder obtained at pH=11.5, using Fullprof method leading to unit-cell dimension: a = 8.1598(1) Å. The peak profile was refined using isotropic peak profile function (Caglioti function with u, v, w and shape as refined parameters). The quality of the refinement with a very flat difference signal (between the experimental and the calculated diffraction pattern) shows the presence of a pure phase constituted by isotropic crystals.

CaSn(OH)6 double-hydroxide prepared by the co-precipitation at pH=11.5 was treated in air or HF atmosphere in order to obtain the CaSnO3 perovskite and the double-fluoride, respectively. ). SnO2 traces constitute less than 2 weight% of the total mass. Interestingly, one can note that the SnO2 traces tend to increase while the synthesis is performed from the double hydroxide obtained at pH = 13. Unfortunately, ICP titration is not enough accurate to be conclusive on a slight deviation of the Ca:Sn stoichiometric proportion. Anyway, the stoichiometric ratio between calcium and tin seems very sensible to the pH, without any impact of a slight stoichiometric deviation on the obtaining of a pure double hydroxide but with a drastic impact on the purity of the mixed CaSnO3 oxide issued from air calcination of the double hydroxide.

SEM investigations on CaSn(OH)6, CaSnO3, CaSnF6 matrices

The morphological investigations made using SEM technique allowed the comparison of the double-hydroxide, the perovskite oxide and the double-fluoride (Fig. The raw double-hydroxide, obtained from a direct co-precipitation process, shows nanometric crystallite size, with isotropic agglomerates of about 1 micron diameter. The oxide obtained after high thermal treatment shows large crystallites (100 nm -1 µm) with a quite large size distribution. The crystallites are strongly aggregated to form coherent grains of about 3-5 µm. Finally, the morphology of CaSnF6 can be described as constituted by well-defined geometrical micrometric crystallites with intra-grown cubic polyhedra. It is quite surprising that the thermal treatment under HF at only 200°C can produce such an important crystallite growth. Some fine and long needles are also visible and have been characterized, as the CaF2 impurities by EDX measurements which were previously observed on the X-ray diffraction patterns.

119 Sn Mossbauer spectroscopy: CaSn(OH)6, CaSnO3, CaSnF6 matrices

The three reference samples: CaSnO3, CaSn(OH)6 and CaSnF6 were analyzed by 119 Sn Mossbauer spectroscopy at room temperature (Fig. 6.). The observed apparent singlets may be considered as paramagnetic quadrupole doublets with low quadrupole splitting (Δ) values (Table 1). For the three compounds, the calculated hyperfine parameters, isomer shift (δ) and quadrupole splitting, are characteristic of six-fold coordinated Sn 4+ ions. The isomer shift is related to the charge density at the tin nucleus which is assumed to be a function of the number of 5s and 5p electrons (the contribution of d electrons is neglected). For tetravalent tin in octahedral site, the bonds may be considered as sp 3 d 2 hybrid orbitals and the ratio of 5s/5p electrons is 1/3 [START_REF] Lees | Mössbauer Effect in Tin Compounds : Interpretation of Isomer Shifts and Determination of the Nuclear Radius Change in 119 Sn[END_REF]. Then, from the observed isomer shift values we can estimate the number of s-like electrons in the tin valence shell and the difference in electronegativity between tin and its ligands (∆χ) by using the empirical relations proposed by Lees & Flinn [START_REF] Lees | Mössbauer Effect in Tin Compounds : Interpretation of Isomer Shifts and Determination of the Nuclear Radius Change in 119 Sn[END_REF] and Baggio & Sonnino [START_REF] Baggio | Isomer Shift and Electronegativity in Compounds of Tin and Group-VI Elements[END_REF], respectively. Thus, the differences observed in isomer shift values are consistent with a decrease of the s-electron density at the tin nucleus due to an increase of the ionic character of the Sn-X bond (ns decreases) along with a higher electronegativity of X (∆χ increases).

Moreover, considering that the quadrupole splitting is mainly determined by distortions of the electronic charge around the Sn atom, slight variations in electric-field gradient and indirectly, in Table 1. 119 Sn Mossbauer hyperfine parameters determined from the fitted room temperature spectra of the Ca-Sn mixed oxide, hydroxide and fluoride. δ isomer shift (relative to BaSnO3 at room temperature), Δ quadrupole splitting and Γ Lorentzian linewidth. ns number of s-like electrons in the tin valence shell and ∆χ the electronegativity difference between tin and its ligand, estimated from the isomer shift values [START_REF] Lees | Mössbauer Effect in Tin Compounds : Interpretation of Isomer Shifts and Determination of the Nuclear Radius Change in 119 Sn[END_REF][START_REF] Baggio | Isomer Shift and Electronegativity in Compounds of Tin and Group-VI Elements[END_REF].

T =293 K δ (mm.s -1 )
∆ (mm.s -1 )

Γ (mm.s -

Synthesis and XRD Diffraction characterization

In a second step, Pr-doped CaSn(OH)6, Pr-doped CaSnO3 (from a 1200°C treatment under air) and Pr-doped CaSnF6 (using a 200°C HF treatment) were investigated. X-ray diffraction patterns show a pure Pr-doped CaSn(OH)6 hydroxide phase, a Pr-doped CaSnO3 with traces of SnO2 (same relative amount than for the non-doped matrix) and almost a pure Pr-doped CaSnF6 compound along with a minor CaF2 phase. The doping with Pr element does not seem to affect the results in comparison with the ones previously recorded for the non-doped matrices. Anyway, similar XRD patterns do not constitute a proof of the efficient incorporation of the doping ion within the crystalline structure. Indeed, all or part of the Pr 3+ ions might remain in solution during the coprecipitation process and/or incorporated in a secondary phase which could be undetectable by XRD due to its too low content or to an amorphous state.

It can be noted that annealing treatment in air leads to the formation of a nearly pure Pr-doped perovskite (traces of SnO2 being still detected with the same relative amount than for the matrix). Finally, a treatment at 200°C under HF leads to a Pr-doped CaSnF6 fluoride phase.

Chemical analyses

The efficient introduction of 2 mol% of Praseodymium in substitution for Sn (i.e. corresponding to target composition: CaSn0.98Pr0.02(OH)6) was investigated by ICP titration in order to check the Sn/Pr ratio in the hydroxide precipitate. The ICP results reported in Table 2, show clearly that the doping element are introduced with the adequate concentration into the precipitate. A blank measurement was made on the un-doped hydroxide for comparison. Without any doubt, from ICP measurement, the Pr 3+ is in substitution of Sn 4+ ion. It is well an aliovalent doping and the electroneutrality can be reached in such cases by anionic vacancies. Especially, ReO3 and perovskite structural network are known, from the octahedral site with corner sharing as skeleton, to accept the occurrence of a quite large quantity of anionic vacancies. This could well explain the positioning of praseodymium in place of tin ions. We suppose that this Sn/Pr ratio remains unchanged in the oxide and fluoride stabilized by heat treatments of this initial powder.

Table 2. ICP titration results

Sample

Sn (mg.L -1 ) Pr (mg.L -

Luminescence characterization

First of all, it has to be noted that all the three samples are white, i.e. they do not exhibit any absorption phenomenon in the visible range.

The luminescence of Pr 3+ -doped CaSn(OH)6 could not be detected even after heat-treatment processes in reducing conditions. Praseodymium might be stabilized at the 4+ oxidation state which would explain the total lack of radiative transition in this compound.

The luminescence of Pr 3+ doped CaSnO3 and CaSnF6 is illustrated in Fig. 7. Because of the high thermal annealing under oxidizing atmosphere, praseodymium ions introduced in the SnO2 impurity praseodymium ions must be stabilized at the tetravalent oxidation state. By consequence, they should not present visible luminescent properties. Concerning the trace of CaF2 detected on the X-Ray diffraction patterns, we also eliminated the possibility of its contribution in the optical properties observed because the response of our sample clearly does not match with the luminescence of Pr 3+ in the fluorite CaF2 host lattice as a strong shift of the emission lines exists [START_REF] Yu | Enhanced photoluminescence and initial red laser operation in Pr:CaF2 crystal via co-doping Gd 3+ ions[END_REF][START_REF] Boiaryntseva | The influence of praseodymium concentration on the luminescence properties of Ca1 -x Pr x F2 + x (0.002 ≤ x ≤0.35)[END_REF].

The absorption of the CaSnO3 is detected below 300 nm as reported by Goto et al [START_REF] Goto | Photoluminescence properties of Pr doped and Tb-Mg codoped CaSnO3 with perovskite structure[END_REF]. The emission curves are dominated by radiative de-excitation from the 3 P0 level. They consist in four main groups of lines corresponding to the intraconfigurational 4f-4f transitions i.e. 3 P0→ 3 H4, 3 P0→ 3 H5, 3 P0→ 3 H6 and 3 P0→ 3 F2 [START_REF] Carnall | Electronic energy levels in the trivalent lanthanide aquo ions. I. Pr 3+ , Nd 3+ , Pm 3+ , Sm 3+ , Dy 3+ , Ho 3+ , Er 3+ , and Tm 3+[END_REF]. The corresponding energy levels are listed in Table 3. The energy level of 3 P1 and 1 D2 levels was determined using the semi-empirical method described by Antic-Fidancev [START_REF] Antic-Fidancev | Simple way to test the validity of 2S+1 LJ barycenters of rare earth ions (e.g. 4f 2 , 4f 3 and 4f 6 configurations)[END_REF] and based on the nephelauxetic scale (cloud expansion) of the rare earth ions. The Nephelauxetic effect reflects not only changes in the inter-electronic repulsion but also the spin-orbit interaction (depending on the number of 4f electrons) of a central metal ion which impacts the position of the energetic levels. The nature of the ligand in the Pr 3+ coordination polyhedron and its influence on the covalent bonding character notably impacts the energetic position and the intensity of emission ). However, in none of the two emission curves, radiative transition has been observed from the 1 D2 as announced by Andrius Stanulis et al. [START_REF] Boiaryntseva | The influence of praseodymium concentration on the luminescence properties of Ca1 -x Pr x F2 + x (0.002 ≤ x ≤0.35)[END_REF] and Bingfu Lei et al. [START_REF] Manu-Lei | Synthesis and luminescence properties of cubestructured CaSnO3/Re 3+ (RE=Pr,Tb) long-lasting Phosphors[END_REF] respectively. # Energy position corresponding to the barycentre position deduced from Antic-Financev semiempirical approach [START_REF] Antic-Fidancev | Simple way to test the validity of 2S+1 LJ barycenters of rare earth ions (e.g. 4f 2 , 4f 3 and 4f 6 configurations)[END_REF] Taking into account the energetic position listed in Table 3, the 1 D2→ 3 H4 transition is expected to peak at 600 nm in the oxide matrix but it is no detected here. Because of the low phonon energy of the fluoride matrix, non-radiative multiphonon depopulation of the 3 P0 level to the lower 1 D2 level is excluded in fluoride matrices. Thus no radiative desexcitation is neither expected from this level in Pr 3+ -CaSnF6. The contribution observed at 590 nm is by consequence part of the 3 P0-3 H6 groups of lines. But surprisingly, no emission from the 3 P1 level is observed as the 3 P1→ 3 H5,6 should be detected at 520 nm and 598 nm respectively in Pr 3+ -CaSnO3 and at 506 nm and 572 nm respectively in Pr 3+ -CaSnF6.

The excitation curves are constituted by the 3 H4→ 3 Pj 4f-4f transitions. In addition a large band corresponding to the host lattice absorption is detected in the oxide matrix [START_REF] Goto | Photoluminescence properties of Pr doped and Tb-Mg codoped CaSnO3 with perovskite structure[END_REF]. One cannot exclude a small contribution of a metal-metal charge transfer between Pr 3+ and Sn 4+ band as observed by Weiyi Jia et al. in Bi 3+ , Pr 3+ co-doped CaTiO3 [START_REF] Jia | Energy transfer between Bi 3+ and Pr 3+ in doped CaTiO3[END_REF].

The effect of the crystal field on the 3 Hj level can be compared in both matrices (Table 3-Δ( 3 P0→ 2S+1 LJ) value). The highest values observed in the oxide can be correlated to the strongest octahedral tilting, which leads to significant changes in the local environment of the rare earth element substituting the A-site of the perovskite (Sn 4+ ). The distortion in the nonlinear Sn-O-Sn is expected to have a great influence of the 4f-4f transition probabilities, which is consistent with a higher emission intensity observed in the Pr 3+ doped CaSnO3. As a consequence of the high sensitivity of the rare earth element to their cationic environment, the barycentre of the different Pr 3+ energy levels is significantly lowered for the oxide compared to the fluoride matrices. The 3 P0 energy level of the studied matrices is reported in Fig. 8. among several other host lattices to clearly show the influence of the so-called nephelauxetic effect. It indicates that the octahedral environment of Pr 3+ placed both matrices among those for which this effect is maximum. (2).

Conclusions

The synthetic route based on the calcination and/or fluorination of calcium hydroxystannate allow obtaining Pr 3+ -doped CaSnO3 and CaSnF6 Pr 3+ compounds. The change in metal-ligand bond covalence / ionicity (evidenced by 119 Sn Mössbauer spectroscopy) in the oxide, hydroxide, fluoride host matrices clearly influence the luminescence properties of the rare earth doped compounds. Indeed, the high resolution of luminescence spectroscopy made possible to observe a significant shift and also a higher splitting of Pr 3+ energy levels in the doped oxide host lattice in comparison with the fluoride one. Even if the coordination polyhedra of the rare earth element and their connection by common corner with other octahedra are similar in both host lattices, the nephelauxetic effect induces a noticeable reddening of the emitted color.

Supplementary Materials:

The following are available online at www.mdpi.com/link, Fig. SI-1. and SI-2.

Fig. 1 .

 1 Fig. 1. a) Structural description of a) CaSn(OH)6; b) CaSnO3; c) CaSnF6 crystals.

Fig. 2 .

 2 Fig. 2. (a) X-ray diffraction pattern of the CaSn(OH)6 compound obtained by Ca-Sn coprecipitation at pH equal to 13, 11.5 and 10. (b) Full pattern matching analysis of CaSn(OH)6 (pH= 11.5) diffractogram. Red and black lines are experimental and calculated spectra, respectively and the blue line represents the difference I(θ) = I exp -I cal.

  The compound heat-treated at 1200°C under air is mainly CaSnO3 phase. Traces of SnO2 were observed (Fig 3.a.) Indeed, all the main peaks are indexed in the CaSnO3 phase by considering an orthorhombic unit-cell with P b n m space-group. Unit-cell refinements lead to a = 5.6520(2) Å, b = 7.8825(3) Å and c = 5.5162(2) Å (Fig. 3.b.

Fig. 3 .

 3 Fig. 3. (a) X-ray diffraction pattern of the CaSnO3 compound obtained by annealing treatment at 1200°C under air of the CaSn(OH)6 precursor. (b) Full pattern matching analysis of the corresponding compound diffractogram.

Fig. 4 .

 4 Fig. 4. (a) X-ray diffraction pattern of the CaSnF6 compounds obtained by annealing treatment at 100, 150, 200, 250°C under HF atmosphere from the as-prepared CaSn(OH)6 precursor. Red arrows indicate CaF2 impurity. (b) Full pattern matching analysis of CaSnF6 (HF 200°C) diffractogram.

Fig. 5 .

 5 Fig. 5. SEM photographs of CaSn(OH)6 (a), CaSnO3 (b) and CaSnF6 (c) compounds.

  may occur between the mixed Ca-Sn oxide, hydroxide and fluoride. Indeed, the lower value of quadrupole splitting (and Lorentzian linewidth) was observed for the CaSnF6 compound, indicating that the octahedral site and local environment of tin are more regular in the fluoride than in the hydroxide or oxide.Finally, from low-temperature (4.2 K) measurements (Fig.SI-1), no Sn 2+ signature was evidenced for the three analyzed samples.

Fig. 6 .

 6 Fig. 6. Room temperature experimental (black dots) and calculated (red line) 119 Sn Mossbauer spectra of the CaSnO3, CaSn(OH)6 and CaSnF6 compounds.

  lines. A clear red shift of the Pr 3+ -CaSnO3 is detected in comparison with the Pr 3+ -CaSnF6 lines reflecting the increased covalent character of the chemical bonding in the oxide (Fig. 7. and Fig. SI-2.

Fig. 7 .

 7 Fig. 7. Excitation and emission spectra of Pr-doped CaSnO3 (a) and Pr-doped CaSnF6 (b) samples.

2 Fig. 8 .Fig. 9 .

 289 Fig. 8. Energy value of 3 P0 Pr 3+ levels in several fluorine and oxygenized matrices. Each symbol is associated to data reported in the cited references [42-46; 50] (a) and energetic levels of Pr 3+ in the oxide and fluoride studied compounds. Dotted lines are positioned for reading whereas full lines are deduced from experimental data (b).

1 ) Exp. Composition 1

	CaSn(OH)6	123.7(1)	0	CaSn(OH)x
	CaSn0.98Pr0.02(OH)6	110.3(1)	2.02(7)	CaSn0.981Pr0.019(OH)x

1

Experimental composition calculated considering 2 cations per unit-formulae.

Table 3 .

 3 Energy levels of the Pr 3+ in several matrices

Energy levels , Energy splitting of some transitions in cm -1 (in italic) And corresponding wavelengths range in nm (in parenthesis)

	System	3 P2	3 P1 #	3 P0	1 D2 #	3 F2/	3 H6/	3 H5/	3 H4/
	(coordinatio n number of (Pr 3+ )					ΔE(3P0→3F2) (Δλ range)	ΔE(3P0→3H6) (Δλ range)	ΔE(3P0→3H5) (Δλ range)	ΔE(3P0→3H4) (Δλ range)
	[ref]
	CaSnO3	21978	20967	20366	16700	5099/57	4119/755	1622/855	0/681
	(6)	(455.5)	(480.5)	(491)	(599)	(655-657.5)	(615.5-	(533.5-559)	(491-508)
	[this work]						645.5)
	CaSnO3			20161			-	-	0/-
	(6)			(496)			(622.5)	(530)	(488)
	[42]
	Pb/Ge glass	22222	21141	20576	16835				-
	(-)								(486)
	[43]
	Sb oxide	22436	21173	20606	16849	-	-	-	-
	glass	(445.7)	(472.3)	(485.3)	(593.5)	(652)	(607)
	(-)
	[44]
	CaSnF6	22497	21385	20790	16943	5201	3898/552	1633/808	0/445
	(6)	(444.5)	(468.5)	(481)	(590)	(641.5)	(592-612)	(522-545)	(481-491.5)
	[this work]
	LiF	22580	21351	20886	16990
	(6)	(443)	(468)	(479)	(588)
	[45]
	CsF	24210	-	20833	17089
	(6)	(413)		(480)	(585)
	[45]
	LiLuF4	22576	21377	20817	16835	4917/447	4316/73	2282/151	0/81
	(8)	(443)	(468)	(478.3)	17126	(628-647)	(606-612)	(539-541)	(480-482)
	[45]	22727	21452		17384
		(440)	(466)

Acknowledgments:

The authors thank the CNRS, the Nouvelle Aquitaine region. This study was carried out with financial support from the French State, managed by the French National Research Agency (ANR) in the frame of program PRIDE (n° ANR-16-CE08-0029). The author would like to associate Alexandre Fargues, ICMCB for his contribution in the technical aspect of luminescence spectroscopy (calibration and correction file validation).

Conflicts of Interest: "The authors declare no conflict of interest."


Olivier Minck

Nicolas Binder

Olivier Cherrier

Lucille Lamotte

Valérie Budinger

FAN NOISE ANALYSIS USING A MICROPHONE ARRAY

The purpose of this paper is to show the capabilities of MicrodB's acoustic imagery algorithm to characterize rotating sources. First part will explain the main specificity of the treatment, in the second part, Possibilities of analysis and validations on simple tests will be demonstrated. In the last part, an industrial application will be studied, a Technofan extract fan with a 10000 RPM rotational speed will be analysed. Both fixed and rotating sources will be separated, allowing the localization of rotating sources on the blades.

INTRODUCTION

For many years now, acoustic arrays are successfully used for all types of sound sources localization. Most of applications concern stationary sources with a fixed radiating surface. Fans present both, stationary and rotating sources, that make current algorithm non useable for localization. To properly study all rotating sources, MicrodB has developed an algorithm as part of its standard array software that takes into account the rotational characteristics of the sources. It allows the engineers to study industrial fans, and understand where the noise is generated.

BEAMFORMING FOR ROTATIVE SOURCES

Beamforming equations

For stationary sources, beamforming in temporal domain is basically given by the expression: This formulation has the advantage to take to the rotation of the source. This algorithm is suitable for general moving objet like pass by applications.

∑ = ∆ + = M m m m m s t P w t s P 1 )) ( ( ) , ( (1)

Data re-sampling

From equation 3, the signal reconstructed at source location will lead to irregular sampled temporal signal on microphone. The best way to data, and to use a sampling frequency more than

Autospectra exclusion

+ × =

In practical implementation, before summation over microphone shift to frequency domain, and the summation is done following the equation : This formulation has the advantage to take into account the time delays, and the to the rotation of the source. This algorithm is suitable for general moving objet like pass by From equation 3, the signal reconstructed at source location will lead to irregular sampled temporal hone. The best way to re-sample signal with good results is to interpolate linearly data, and to use a sampling frequency more than twice the analysis frequency.

∑ = = M m m t s A t s P 1 ) , ( ) , ( ) ) , ( c t s
In practical implementation, before summation over microphone is achieved, the temporal signal is shift to frequency domain, and the summation is done following the equation :

Moving calculation point d 1 (t) d 1 (t+dt) 2
e capacity of temporal approach to signal is processed by calculating reception date for time dependency :

(2)

(3) the weighting applied on microphones proportional to the distance. the time delays, and the Doppler effect due to the rotation of the source. This algorithm is suitable for general moving objet like pass by From equation 3, the signal reconstructed at source location will lead to irregular sampled temporal signal with good results is to interpolate linearly the analysis frequency.

(4)

, the temporal signal is shift to frequency domain, and the summation is done following the equation :

) ( ) ( 2 ) , ²( 0 1 * f A f A f s P M m M m n n m ∑ ∑ = + = × =
(5)

In equation 5 the autospectra terms are removed from the summation, it result in the cancelation of microphone self noise which can be electric noise or some real noise like wind noise on the microphone itself.

VALIDATION FOR BOTH MOVING AND STATIONNARY NOISE

Tonal source

First validation has been done for a tonal source at 3000Hz, this source consists in a small speaker plugged on a signal generator. It is fixed on a motor with a speed of 1000 rpm In the figure 2, the typical spectrum recorded in front of the source is given. When rotating, a frequency shift is clearly visible, it is due to the Doppler effect. Fixed and rotating measurements have been done for the same source with the same level. Figure 3 shows the spectrum backpropagated on the source, both levels at 3000 Hz are equivalent witch show the capability of the algorithm to estimate correctly the sound pressure level. Outside the sound source frequency, the global spectrum has more level, this is the aerodynamic noise emitted by the system itself (motor and rotating axis). The reconstructed level for rotating source shows that the frequency of the source is now correctly estimated.

Motor

Tonal source

Tacho

Figure 3:

In addition, the figure 3 present the spectrum of the rotating tonal source treated with the stationary algorithm, it appears that the level over the whole frequency band is lower than with the rotating algorithm. It makes possible by comparing the level given by the two algorithm the sound source is rotating or fixed.

Broadband rotating source with

In real fan noise applications, tonal noise often radiates from the blades, fixing elements). To be more close to this case consists in two broadband noise fixed on th be stationary (figure 4). The goal of this both rotating and fixed sources. Rotating source, rotating processing Fixed source, fixed processing

Microphone array

: Sound pressure estimation at the source location addition, the figure 3 present the spectrum of the rotating tonal source treated with the stationary it appears that the level over the whole frequency band is lower than with the rotating algorithm. It makes possible by comparing the level given by the two algorithm the sound source is rotating or fixed.

Broadband rotating source with stationary tonal

In real fan noise applications, tonal noise often radiates from the fixed parts of the fan (counter blades, fixing elements). To be more close to this case, a second experiment in two broadband noise fixed on the rotating axis, and two other tonal sources e goal of this approach is to show capability of the algorithm to

Test setup with fixed and rotating sound source in front of the array. As previously mentioned, by considering that the correct algorithm will give a higher pressure level, it appears that tonal sources are some stationary sources, and broadband noise is rotating noise. The noise around 10 kHz, comes from the electric motor itself, so it is also stationary noise. Rotating Proc.

Our case concerns an industrial fan use axial fan with 11 blades on the rotor, and 31 blades on the stator. Rotor diameter is around 200 mm, and rotational speed around 10000 RPM.

Figure

Validation of the algorithm for high rotational speed

The goal of the first measurement is (counter clockwise), it consists in two o assumed to be noisy.

m for high rotational speed

The goal of the first measurement is to validate the processing for a 10000 RPM rotational speed in two obstacles fixed on two blades, which produce

Obstacles on blades(left), and sound source localization (right) are shown in figure 8, they are fixed just after the leading edge of the blade. Angle between the two blades has been chosen to avoid amplifying reflection or seco Hologram is presented in figure 8, for the two obstacles, it is clear that the source is created on the following blade at its tip area. To confirm this behavior which seems possible, a second measurement at 5000 RPM has been done and gives the same localization.

Standard fan analysis

In [2] Alain Guédel gives some rules about the noise sound generation on fans. Considering that in our case, we may find the blade pass frequency and its harmonics as stationary sources radiating near the stator blades, and we may also find some broadband noise as rotating source on the trailing edge Considering that the maximum level is found on the correct algorithm, it is possible to know if sources are rotating or not. A nice improvement of this approach would be to filter stationary tonal signal on microphone before doing the rotating treatment. 96.

  at the source location s, ) (s m ∆ the time delay between the focused point s and microphone m, m w the weighting applied on microphones, and t P m ( ) the temporal signal recorded on microphone m. For rotating sources, previous work [1] has demonstrate easily locate sound sources. The each position of the source (figure .1).In the equation 1, the rotating motion will change the Equation 1 becomes : on microphones proportional to the distance.

  has demonstrated the capacity of temporal approach to beamformed signal is processed by calculating reception date for (figure .1).

Figure 1 :

 1 Figure 1: Near field signal delay equation 1, the rotating motion will change the ) (s m ∆ term with time dependenc

  calculation

Figure 2 :

 2 Figure 2: Tonal source test setup (left); typical pressure spectrum for rotating and stationary source (right)

Figure 4 :

 4 Figure 4: Test setup with fixed and rotating sound source in front of the array.

  figure 3 present the spectrum of the rotating tonal source treated with the stationary it appears that the level over the whole frequency band is lower than with the rotating algorithm. It makes possible by comparing the level given by the two algorithms to understand if parts of the fan (counter experiment has been done. It e rotating axis, and two other tonal sources which will to show capability of the algorithm to separateTest setup with fixed and rotating sound source in front of the array. been tuned to 1800 and 3000 Hz, broadband sources are fixed on the rotating axis, speed for this measurement will be 1500 rpm. Next figure (N°5) shows the average pressure level on the calculation grid.

Figure 5 :

 5 Figure 5: Average sound pressure level on the calculation grid.

Figure 6 :

 6 Figure 6: Acoustic hologram fixed tonal source 1800 Hz (top left), tonal 3000 Hz (top right), and rotating broadband noise (bottom).

Figure 8 :

 8 Figure 8: Obstacles on blades

Figure 7 :

 7 Figure 7: Test setup at the ISAE test bench.

  in aeronautics industry for air extraction. It consists in an axial fan with 11 blades on the rotor, and 31 blades on the stator. Rotor diameter is around 200 mm, for a 10000 RPM rotational speed , which produce a stall and are (left), and sound source localization (right) are shown in figure 8, they are fixed just after the leading edge of the blade. Angle reflection or secondary lobe of the , it is clear that the source is created To confirm this behavior which seems possible, a second

Figure 9 :

 9 Figure 9: Pressure level on calculation grid with and without rotating processing

Figure 9

 9 Figure9present the spectrum for both stationary and rotating processing. Considering that the maximum level is found on the correct algorithm, it is possible to know if sources are rotating or not. A nice improvement of this approach would be to filter stationary tonal signal on microphone before doing the rotating treatment.

Figure 10 :

 10 Figure 10: Hologram with rotating processing, in frequency range 4500-5500Hz Figure 10 is the rotating processing done around 5000 Hz, it clearly shows the trailing edge of each blade.

CONCLUSION

Validations measurements has shown the possibility of our algorithm to analyze rotating sources, and to separate stationary of rotating ones. The test case on a "in duct fan", has shown its capability to correctly track a high speed fan. For further analysis on such device, green functions (in free field condition in our application) has to be estimated closer to the real environment.


Rodrigo Cataldo

Ramon Fernandes

Kevin J M Martin

Johanna Sepúlveda

Altamiro Susin

Cesar Marcon

Jean-Philippe Diguet

Johanna Sepulveda

César Marcon

Subutai: Distributed Synchronization Primitives in NoC Interfaces for Legacy Parallel-Applications

come   L'archive ouverte pluridisciplinaire

Introduction

Parallel applications have become ubiquitous with the huge adoption of MPSoCs in the electronics industry. Besides, the tendency is to continue to deliver high-performance computing by increasing the number of processors, which requires efficient communication architectures, such as a NoC, lead-ing to the NoC-based MPSoC architectures and their challenges. From the parallel application point-of-view, the delay effect is felt on the synchronization primitives used to order global events among communicating processors and memories. The time spent to synchronize such events is one of the most significant parts of the execution time as such events are primarily done sequentially, delaying the application execution. Besides, a huge part of the sequential latency of parallel applications is due to interprocessor communication latency, including cache updates of shared data, and the software implementation of synchronization mechanisms. Unfortunately, Attiya et al. [START_REF] Attiya | Laws of order: expensive synchronization in concurrent algorithms cannot be eliminated[END_REF] have already formally proved that deterministic structures, such as queues and mutual exclusion, cannot eliminate the use of expensive synchronization in software.

Although it is feasible to relax the specification of such structures, the consequence is a significant source code redesign. The majority of recent works requires source code meddling, which is notoriously a hard task [START_REF] Mckenney | Introducing technology into the linux kernel: A case study[END_REF] [START_REF] Patel | A hardware implementation of the MCAS synchronization primitive[END_REF]. We developed Subutai, a hardware-software solution that tackles the synchronization overhead by offloading its processing to a new low-latency NI enhanced with dedicated resources. Our approach extends the NI architecture implementing, in a distributed way, the following synchronization primitives: mutex, barrier, and condition. Moreover, we create new types of packets to transport the synchronization primitives among NIs through the NoC. We developed an Application Programming Interface (API) that replaces the underlying synchronization API with the same function interface, and a driver for the Operating System (OS). Therefore, our approach keeps

Related Work

Accelerating parallel applications using hardware has been studied at least since the '90s. Sivaram et al. [START_REF] Sivaram | A reliable hardware barrier synchronization scheme[END_REF] propose a fault tolerant HW-based barrier synchronization. Their design uses a tree structure to sum up intermediate values for decreasing the number of packets injected into the network. Their work is complementary to our solution. Abellán et al. [START_REF] Abellán | Design of a collective communication infrastructure for barrier synchronization in cluster-based nanoscale MPSoCs[END_REF] explore three HW-based barrier architectures and integrate them on the OpenMP programming model. Unfortunately, their results are for synthetic applications only. Stoif et al. [START_REF] Stoif | Hardware synchronization for embedded multi-core processors[END_REF] implement an arbiter on FPGA that guarantees mutual exclusion to a portion of the shared memory area and a HW-based barrier that provides sound results; however, their work does not implement full barriers and conditions, and it is limited to simple test cases instead of real applications.

Patel et al. [START_REF] Patel | A hardware implementation of the MCAS synchronization primitive[END_REF] propose a special HW instruction to change multiple memory positions atomically, optimizing the synchronization process. This instruction is an extension of the currently widely available compare-and-swap instruction, which is similar to the hardware transactional memory approach [START_REF] Diegues | Virtues and limitations of commodity hardware transactional memory[END_REF] where the developer delimits atomic blocks of codes. For both cases, if the code cannot be executed atomically, the developer has to re-execute the code. Unfortunately, this procedure requires lock-free parallel applications that are difficult to design and debug. Software-oriented solutions, like the ones presented in [START_REF] Kirsch | Fast and scalable, lock-free k-fifo queues[END_REF] and [START_REF] Desnoyers | User-level implementations of read-copy update[END_REF], focus on relaxing the constraints that force the use of expansive synchronization. Kirsch et al. [START_REF] Kirsch | Fast and scalable, lock-free k-fifo queues[END_REF] propose k-FIFO, which is a lock-free queue that removes up to k -1 out-of-order elements from the queue. Desnoyers et al. [START_REF] Desnoyers | User-level implementations of read-copy update[END_REF] describe a synchronization technique based on the publish-subscribe mechanism called Read-Copy Update (RCU). Parallel applications that rely on RCU have to deal with stale data. These approaches suffer from the same problem described earlier: the burden of adapting the code to these solutions is passed on to the developer. Finally, Martin et al. [START_REF] Martin | Notifying memories: A case-study on dataflow applications with NoC interfaces implementation[END_REF] propose the Notifying Memories concept to reduce communication latencies introduced in the NoC by pruning useless memory accesses. This concept is applied to dataflow applications only. Our work is also based on NI, but targeting shared-memory systems. Tasks share memory space, but the target architecture inhibits excessive caching by employing a distributed approach to the OS. This OS replication allows us to avoid expensive memory barrier into the scheduler [START_REF] Howells | Linux kernel memory barriers[END_REF], which is crucial for our solution. A faster scheduling implies that threads can wake up more efficiently, as shown in Sec. 6.2.

API Implementation and HW/SW Communication

As a proof-of-concept, we chose to overwrite the implementation of the POSIX Threads (PThreads) library, as it is one of the most well-known interfaces used in parallel applications. We focused on three of the four main groups of PThreads operations: mutex, barrier, and condition handling. Thread events (create, exit, join) are not on the critical path and so can be achieved at the OS level. An extensive description of PThreads operations is out of our scope. We limit the discussion to the essentials of the three focused groups.

The mutex group contains locking and unlocking functions. Locking is a blocking function that exclusively locks a variable. If the variable is already locked, the calling thread is blocked. Otherwise, this operation returns with the variable locked by the calling thread. Unlocking is a non-blocking function that changes the variable state and, if there are any waiting threads, wakes up previously blocked functions. The barrier group contains a single blocking function, called wait, which synchronizes participating threads at a user-specified code point. A barrier has a fixed number of threads decided at allocation time. When all participating threads reach the barrier, all threads are woken up. The condition group contains three functions: wait, signal and broadcast. Wait is an unconditionally blocking function that puts the thread on a waiting list for a condition event. It requires that designers previously lock a mutex variable and pass a reference to it. Then, the wait function unlocks the mutex once it finished its work. Next, when the thread is woken up, it reacquires the mutex. The mutex dependency is due to race conditions. The signal and broadcast are non-blocking functions that wake up one and all threads respectively waiting for a condition event. Mutex, in these cases, is optional.

For all groups, one or more queues are required to record blocked threads. Condition functions need to handle two queues because of the mutex dependency. Barrier and mutex functions deal with only one queue. Besides, they have non-blocking functions that allocate and deallocate variables. This work replaces the handling of these operations from an entire SW solution to a HW/SW approach. Fig. 3 depicts the communicating flow of our system. First, the application makes a PThreads interface request; the Subutai library identifies the unique ID for this primitive and passes it on to the driver, along with the interface request. Depending on the the driver writes to either registers or a memory that NI has direct access to (DMA). Next, the driver writes in a control register to inform the NI of a command and waits for an interrupt to receive the remote response. The local NI injects a packet into the NoC targeting the remote Subutai-HW, which handles the request and responds to the local NI with a new packet. There are two complementary scenarios for Fig. 3. One when there is no response packet and no backward procedure; thus, the driver returns immediately after writing in the control register. The other one happens when the driver accesses the local Subutai-HW. The same procedure is followed, but without injecting packets into the NoC. An incremental counter per core determines the NI that hosts a synchronization primitive. The first primitive is hosted in NI 0 ; the second one in NI 1 , and so on following a fairness method. Other dynamic allocation strategies can be further studied, but this is out of the scope of the paper.

Subutai-HW

Subutai-HW extends the NI for handling synchronization primitives. As shown in Fig. 2, its main components are: (i) a Finite State Machine (FSM); (ii) a set of registers; and (iii) a local ScratchPad Memory (SPM), which is entirely controlled in HW by the FSM except for memory initialization. We have implemented and validated the HW architecture by RTL simulation. Subutai-HW employs double-linked queues to record events, as shown on the left-hand side of Fig. 2. As an alternative to a garbage collector, the double-linked queues allow Subutai-HW to employ a dynamic allocator to reduce memory consumption to the minimum at the cost of 33 additional sub-states on its FSM. The queue manipulation is based on the futex implementation of the Linux Kernel [START_REF] Drepper | Futexes are tricky[END_REF]. Subutai-HW operates using two record information structures. The first one, shown in Fig. 4a, records the synchronization primitives' metadata. The first 32-bit field is the only one known by software and is employed as a unique ID of this primitive. However, for Subutai-HW, the first bit F is employed to allocate/deallocate this structure. The next 7-bit field is the unique ID for the NI on the system NI ID. Lastly, the furthest 24-bit is used as a pointer to itself; we employ this technique to avoid the cost of searching for an entry every time a new request has arrived. The next 32-bit field is the head and tail of the double-linked queue implemented in the second structure. The queue records threads waiting for new events on the primitive. Finally, the last 32-bit field record values used for some of the primitives. The first 16-bit is employed to (i) record the thread and CPU that owns a mutex; and (ii) store the current number of threads waiting on a barrier. The furthest 16-bit is applied only for the barrier primitive to record the maximum number of threads allowed in a barrier. Fig. 4b shows the entry to the double-linked queue composed of six fields. The first bit is employed to allocate or deallocate the entry. The prev and next fields are pointers to the previous and next entries, respectively, or nil if they do not exist. The 16th bit R is reserved and can be used for memory alignment. The last 32-bit field identifies the requesting thread. The Processor ID field is padded with zeroes because the NoC packet uses only 8-bit to identify the processor.

The bare minimum memory requirement for SPM is one control entry and 63 queue entries, re-garding a 64-processor architecture as described in Sec. 3. Since we have to record up to p -1 processors, the minimum SPM size is (1×96+63×64)/8 = 516 bytes. Note that Subutai-HW is incorporated into every NI; consequently, we handle up to 64 primitives even with minimum sizing. For our experimentation, we use an SPM of 1 KiB (4 control entries and 122 queue entries) that handle up to 256 primitives. A double-linked list of events is employed to allocate dynamically queue entries, allowing Subutai to consume memory on demand. A static allocator, on the other hand, would not be able to handle more than one control entry with only 122 queues (< 2 × 63) -since the worst case scenario is 63 queues per entry, as shown earlier. Thus, a static solution would be either too limited or a waste of memory resource.

Table 2 shows the latencies of the states as dependent on the Subutai-HW cycle c, SPM latency m, number of synchronization primitives handled n, and maximum number of threads on a barrier max. Each memory operation can either be a write or read in SPM in a given cycle. The table is organized as follows. The first column identifies the Subutai-HW state. The second and third columns identify the fastest and slowest latencies for the state, respectively. Finally, the last column shows when the packet is ready to be injected into the NoC -as, for some states, we can inject packets before we have finalized processing the requests. Additionally, some primitives (e.g., Deallocation) do not need to generate packets at all.

To illustrate the best and worst response time of Table 2, we describe the modeling of Mutex Lock state, which is responsible for modeling the pthread mutex lock operation. The fastest scenario, whose latency is 2m+1c, happens when the control variable is unlocked. Hence, it requires two memory operations: (i) fetch the control structure (field Value from Fig. 4a) to check the owner of the mutex (latency = 1m); and (ii) rewrite this field with the requesting thread (latency = 1m). Finally, NI is notified that a new packet can be injected (latency = 1c). The injected packet is the same as the requesting packet except for the header. The worst scenario takes more time (latency = 11m) because Mutex Lock state requires dealing with two queues. It starts with the same memory operation that reads the control structure for this primitive. Thus, the circuit realizes there is already an owner, which demands to queue up the request. First, Subutai-HW allocates a free queue entry and updates the empty queue pointers (takes up to 4 memory operations); then, it writes the requesting thread information into it and the tail information in the primitive (six more memory operations), performing 11 memory operations in total. We do not describe the latency models for the other Subutai-HW states in detail; however, they follow a similar procedure.

Table 3 shows the resulting latency model used for this work. We clocked Subutai-HW at the same frequency as the NI (1 GHz). SPM employs the previously discussed 1 KiB single-port SRAM-based implementation with a uniform access taking 2 cycles [START_REF] Wang | Designing scratchpad memory architecture with emerging stt-ram memory technologies[END_REF], 4 control structures and 122 queue positions. We also considered 4 ns for an entry latency (3 flits of 32 bits and 1 cycle to take a decision) and 1 ns for an exit latency (1 cycle to set a flag) to reach any state shown in Table 3. The latency required to release threads on a barrier exceeds one thousand ns. However, this latency is due to the queue size of threads waiting on the barrier and does not represent the packet injection Figure 5: Fields of Subutai packet latency. Therefore, some of the threads execute much earlier than the total value. As shown in the last column, the packets are injected periodically at every 25 ns, except the first packet, which is injected in 7 ns. Thus, the total number of cycles is 1583 ns, which is composed by the following parameters: entry + exit + 1m + 1c + max × (11m + 3c).

The Condition Broadcast and Condition Signal states present interesting latency results. At first glance, it would seem more reasonable that releasing one thread (Signal ) would be faster than releasing all (Broadcast). However, it does not happen due to the following reasons. First, by releasing all threads, the state has to deal with only one queue (mutex ) instead of two queues (mutex and condition). Second, due to the way condition works, only one thread is truly released since a mutex is associated with it. Therefore, the broadcast state avoids the scenario previously described for the barrier state only the owner of the mutex will be released.

Subutai-HW includes six 32-bit and three 1-bit registers; three are used for the packet fields (see Fig. 5) and six more to: (i) handle empty queue entries; (ii) memory swapping operations; and (iii) control flags to receive and send packets. For receiving and sending packets, Subutai-HW reuses the already available registers of the NI. The packet structure is combined with the recorded information on the two control structures described earlier (Fig. 4) to handle any request.

The synthesis of Subutai-HW was achieved using Synopsis DC with an ST SOI 28 nm technology with 1 GHz clock frequency. The SPM area was calculated by CACTI 6.5 [START_REF] Muralimanohar | Cacti 6.0: A tool to understand large caches[END_REF] using a 32 nm SRAM technology. Subutai-HW comprises of a registerbased NI, an FSM that controls synchronization and manipulates linked pointers, and a 1 KiB SPM to store metadata and events. We use a basic NI with 32 bits links, packing and unpacking logic, no virtual channel and 2 I/O buffers of 16 × 32 bits. Besides, it is worth noting that using HW synchronization operations releases valuable memory and cache space that would otherwise be required. Additionally, the memory requirement is negligible when compared to a typical processor cache (less than 10%, if cache size is 16 KiB). Table 4 summarizes the synthesis results showing that our solution increases by 36% the basic NI area including the local SPM. However, the overhead can be negligible compared to the chip area; for a 400 mm 2 chip [START_REF] Patel | A hardware implementation of the MCAS synchronization primitive[END_REF], 64 Enhanced NI have an overhead of 0.03%.

Experimental Results

Validation Methodology and Exp. Setup

Considering the prohibitive simulation time when using Gem5 for simulating complex applications (billions of ns) with a large number of processors and distributed OS, we choose an approach that extracts only the useful information of the application and uses a high-level model to measure the speedup provided by our solution. Fig. 6 shows the methodology flow of our experimental setup in four steps. The first step is the application execution using Gem5 in full-system simulation mode to provide an accurate characterization of the applications. The second step is the application execution trace, from start to finish, as the sequential portions of code can hinder the real speedup of a parallel application [START_REF] Southern | Deconstructing PARSEC scalability[END_REF]. The application trace, which was based on [START_REF] Butko | A trace-driven approach for fast and accurate simulation of manycore architectures[END_REF], provides the execution times among synchronization calls, and the number and execution time of each synchronization call. The trace is further annotated with every synchronization primitives -so that we can simulate these functions accurately. Moreover, we make sure to employ synced POSIX clocks for recording each thread start. The third step is the execution of our NoC-based MP-SoC simulator called Subutai-simulator (modeled in SystemC). The simulator reads the traces to generate tasks in the OS. Then, these tasks mimic the of the applications, considering execution times from the traces, and execute the synchronization functions. We reproduce NoC communication, queues and hardware latencies in our SystemC environment. Additionally, we adapted an existing NoC implementation to be event-based; otherwise, the simulation would be far too slow. The NoC was set up to the configuration of Gem5's NoC (Garnet) [START_REF] Agarwal | Garnet: A detailed on-chip network model inside a fullsystem simulator[END_REF]. The OS latencies were extracted from FreeRTOS [START_REF] Howells | Freertos[END_REF]. The processing cores are clocked at 1 and are kept the same for both Gem5 and our simulation. Thus, our solution does not speed up any application computation portion. The results from our simulation tool are clustered in the fourth step of Fig. 6 and will be discussed in the following section.

Results Evaluation

Fig. 7 shows the results obtained on Subutaisimulator for two data processing applications: (i) Bodytrack, which is a computer-vision application that tracks a 3D pose of a mark-less body, and (ii) Streamcluster, which is a data-mining application that solves the online clustering problem for a stream of input points [START_REF] Bienia | The parsec benchmark suite: Characterization and architectural implications[END_REF]. We have tested these applications for three system configurations [START_REF] Sivaram | A reliable hardware barrier synchronization scheme[END_REF]32, and 64 cores) to visualize the scalability of our solution. For fair comparisons, we consider the same scheduler (round robin) for both. We plot the results for two threads for each application: the master thread (T0), responsible for global synchronization, and a worker thread instance (T7). Fig. 7 illustrates that our solution reduces the application total time by handling synchronization faster. From the designer point-of-view, the master thread (T0) shows the effective speedup, as it is responsible for initializing and finalizing the application. Bodytrack achieved a speedup of 1.71, 1.78, and 1.77 for 16, 32, and 64 cores, respectively. Streamcluster achieved a speedup of 2.11, 2.71, and 2.20 for the same core set. Therefore, our solution achieved a speedup of 2.05, on average. Table 5 shows the number of synchronization calls, explaining the speedup difference; Streamcluster requires, roughly, 18, 23, and 31 times the equivalent of Bodytrack for 16, 32, and 64 cores, respectively. Thus, we can better optimize worker threads, as they are the ones using these primitives. Table 6 shows for all cases the worker threads of Streamcluster achieve a higher speedup when compared to the worker threads of Bodytrack. The results also show that both applications are not scalable to 64, or even, 32 cores. Southern et al. [START_REF] Southern | Deconstructing PARSEC scalability[END_REF] have independently corroborated this limitation as well.

Our solution works the same regardless of the application scaling -as will be shown with a producerconsumer application.

The application results give us a systemic view of Subutai, but it does not convey the optimization on the synchronization itself. The lack of a microcosm view happens because our applications use at least tens of thousands of synchronization primitives during its execution. Consequently, we employ a one producer many consumers application encompassing the three synchronization primitives (mutex, barrier, and condition) using six threads. Table 7 shows the average absolute time of Subutai and SW for these primitives.

Subutai significantly speeds up every synchronization primitive compared to the SW implementation (Linux Kernel 2.6). The comparison is made from the application perspective. For instance, the Condition Broadcast and Mutex Unlock scenarios have no response packet; consequently, Subutai can return to the application immediately after the request packet is sent. Thus, the processing is offloaded to the HW, and the primitive is handled faster from the caller perspective. The SW implementation depends on the following costs to handle synchronization primitives: (i) context switching; (ii) synchronization for queue operations; and (iii) kernel space switching. Item (i) is reduced in Subutai by using a distributed OS. As stated in Sec. 3, we can use a faster context switch with a distributed OS. The faster OS is useful for functions that are blocking, and, as discussed in Sec. 4, every group handled by Subutai has these functions. Item (ii) is reduced by offloading all queue operations to hardware. Finally, item (iii) is not present in our OS. Subutai adds the cost of I/O operations to deal with Subutai-HW, which is not present in the SW solution. Nonetheless, these factors explain the gains shown in Table 7.

Conclusion

This paper presents Subutai, an HW/SW approach to accelerate parallel applications with no source code modification. Subutai is composed of an enhanced NI with an efficient local memory management. It includes an OS driver and a library to overwrite the underlying synchronization API. We provide an in-house tool to simulate fast MPSoCs using applications traces. Results show an average application speedup of 2.05 compared to the same architecture running an entire SW solution. Finally, the HW implementation shows a limited overhead of 36% compared to a basic NI, including a 1 KiB SPM that relieves the processor memory and cache from synchronization data. In the next steps, we will take benefit from the saved time to maximize the processor usage in HPC/Cloud domains. It means (i) Subutai-aware cooperative schedulers running multiple applications; and (ii) strategies for dynamic allocation of Subutai resources.

Figure 1 :

 1 Figure 1: Subutai solution, including library, driver, hardware part and the communication procedures

Figure 2 :

 2 Figure 2: Schematic representation of the NoCbased MPSoC architecture and the Subutai-HW

Figure 3 :

 3 Figure 3: The main message path chart of Subutai

  (a) Control structure. (b) Queue structure.

Figure 4 :

 4 Figure 4: Subutai-HW

Figure 6 :

 6 Figure 6: Validation methodologyof the applications, considering execution times from the traces, and execute the synchronization functions. We reproduce NoC communication, queues and hardware latencies in our SystemC environment. Additionally, we adapted an existing NoC implementation to be event-based; otherwise, the simulation would be far too slow. The NoC was set up to the configuration of Gem5's NoC (Garnet)[START_REF] Agarwal | Garnet: A detailed on-chip network model inside a fullsystem simulator[END_REF]. The OS latencies were extracted from FreeRTOS[START_REF] Howells | Freertos[END_REF]. The processing cores are clocked at 1 and are kept the same for both Gem5 and our simulation. Thus, our solution does not speed up any application computation portion. The results from our simulation tool are clustered in the fourth step of Fig.6and will be discussed in the following section.

Figure 7 :

 7 Figure 7: Execution times in seconds of Bodytrack and Streamcluster on SW and Subutai architectures Table 6 depicts the detailed execution values and the speedup achieved by our solution. These values are (i) Processing time; (ii) Waiting time for all synchronization primitives; (iii) NoC time for Subutai communication; and (iv) Subutai-HW time. For the SW solution, both (iii) and (iv) are handled by software; consequently, their values are computed together with (i) and (ii). The comparison SW and Subutai solutions shows a significant speedup for our solution in all examined cases.

Table 1

 1

	positions our work in the current state
	of the art. To the best of the authors' knowledge,
	this is the first work for dealing with multiple syn-
	chronization primitives in hardware while retaining
	the source code application.

Table 1 :

 1 Related work comparison.

	Work	HW/SW Solution	Legacy code compatible	Primitive		Application
	[16]	HW	Not addressed	Fault-tolerant barrier		Synthetic
	[1]	HW	Not addressed		Barrier		Synthetic
	[18]	HW	Not addressed	Locking		Synthetic
	[15]	Both	No	Atomic operations		Synthetic
	[7]	Both	No	Atomic operations	STAMP suite +Synthetic
	[11]	SW		No	Locking	Synthetic + Mandelbrot
	[6]	SW		No	Locking	OS Kernel + Synthetic
	[12]	HW	Yes	FIFO events		MPEG4-SP
	This	Both	Yes	Locking, barrier and Condition	Synthetic + PARSEC Benchmark
								Core
	Control (Fig. 4a)	Synch ID Task Data Synch ID	FSM	Registers	FSM	L1d	NI Bus L1i L2 Bus L2
	Queue
	(Fig 4b)							Core +
		nil	DATA					Caches
								NI
				ScratchPad Memory	I/O FIFO		R
				(SPM)			Core +
								Caches
								NI
								R

Table 2 :

 2 Latency of Subutai-HW states. c = cycle latency, m = memory latency, n = variables handled by Subutai-HW, max = threads on a barrier.

	State	Best response time	Worst response time	Packet Injection
	Allocation	4m + 1c	(n 1m) + 3m + 1c	(n × 1m) + 1m + 1c
	Deallocation	3m	3m	None
	Mutex Lock	2m + 1c	11m	2m + 1c
	Mutex Unlock	2m	10m + 1c	2m + 1c
	Barrier Wait	7m	max × (11m + 3c) + 1m + 1c	1m1c + 12m4c + 23m7c... = 1m+1c+max×(11m+3c)
	Condition Wait	5m + 1c+ Mutex Unlock	10m + 1c+ Mutex Unlock	None
	Condition Broadcast	1m	18m + 1c	11m + 1c
	Condition Signal	1m	29m + 2c	11m + 1c

Table 3 :

 3 Subutai-HW latencies with parameters set to

	c=1ns, m=2ns, n=4, max=63, entry=4ns, exit=1ns
	State	Best response time (empty queue)	Worst response time (queued)	Packet Injection Best Worst
	Allocation	14 ns	20 ns	10 ns	15 ns
	Deallocation	11 ns	11 ns		None
	Mutex Lock	10 ns	27 ns	None	10 ns
	Mutex Unlock	9 ns	26 ns	None	12 ns
	Barrier Wait	19 ns	1583 ns	None	7, 32, 57, ... ns
	Condition Wait	20 ns	47 ns		None
	Condition Broadcast	7 ns	42 ns	None	27 ns
	Condition Signal	7 ns	65 ns	None	27 ns

Table 4 :

 4 Synthesis results for Subutai-HW using 28 nm SOI (logic) and 32 nm CACTI ITRS LSTP (memory) technologies.

	Component	Area (µm 2 )	Technology Overhead
	Basic NI	13539.23	28 nm	-
	Subutai FSM	2626.21	28 nm	19 %
	SPM	2269.97	32 nm	17 %
	Enhanced NI (Basic NI + Subutai-HW)	18434.60	28/32 nm	36 %

Table 5 :

 5 Number of synchronization-primitives events in Bodytrack and Streamcluster executions

	Application	Type	Events per number of threads
			16	32	64
		Barrier	2,101	4,293	13,416
	Bodytrack	Condition	447	750	1,529
		Mutex	9,000	10,472	8,677
		Barrier	208,048	364,480	728,960
	Streamcluster	Condition	381	802	1,274
		Mutex	510	1,054	2,142