Maroua Ben

Haj Salah

Nicolas Mercier

Magali Allain

Antonin Leblanc

Jens Dittmer

Chiara Botta

Claudio Quarti

Claudine Katan

Synthesis and Characterization of (FA)3(HEA)2Pb3I11: A Rare Example of -Oriented Multilayered Halide Perovskites

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INTRODUCTION

Layered halide perovskites that have attracted much attention in the 1990s and the beginning of the 2000s, [1][START_REF] Mitzi | Conducting Tin Halides with a Layered Organic-Based Perovskite Structure[END_REF][START_REF] Mitzi | Conducting Layered Organicinorganic Halides Containing ⟨110⟩-Oriented Perovskite Sheets[END_REF][START_REF] Kagan | Organic-Inorganic Hybrid Materials as Semiconducting Channels in Thin-Film Field-Effect Transistors[END_REF][START_REF] Mitzi | Templating and Structural Engineering in Organic-Inorganic Perovskites[END_REF][START_REF] Calabrese | Preparation and Characterization of Layered Lead Halide Compounds[END_REF][START_REF] Ishihara | Exciton State in Two-Dimensional Perovskite Semiconductor (C10H21NH3)2PbI4[END_REF][START_REF] Era | Organic-inorganic heterostructure electroluminescent device using a layered perovskite semiconductor (C6H5C2H4NH3)2PbI4[END_REF][9][10][11] most effort being focused on chemistry, structural and optical characterizations, have also revealed these recent years great potentials for various applications including photovoltaics. 12- 19 Since the discovery of a photovoltaic effect using the three-dimensional (3D) metal halide perovskite (CH3NH3)PbI3 as absorber in 2009, [START_REF] Kojima | Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells[END_REF] 3D organic-inorganic materials attracted wide attention for photovoltaic applications (PSC technology). [21][START_REF] Gratzel | The Rise of Highly Efficient and Stable Perovskite Solar Cells[END_REF][START_REF] Jena | Halide Perovskite Photovoltaics: Background, Status, and Future Prospects[END_REF][START_REF] Snaith | Present status and future prospects of perovskite photovoltaics[END_REF] However, the limited stability of 3D perovskite materials and their degradation under operative conditions imposed a critical limit for the exploitation of these materials. [START_REF] Aristidou | Insights into the Increased Degradation Rate of CH3NH3PbI3 Solar Cells in Combined Water and O2 Environments[END_REF][START_REF] Manser | Making and Breaking of Lead Halide Perovskites[END_REF][START_REF] Kaezmi | Molecular-Level Insight into Correlation between Surface Defects and Stability of Methylammonium Lead Halide Perovskite Under Controlled Humidity[END_REF] This remotivated the interest for layered systems that, thanks to the incorporation of bulky hydrophobic organic cations, present much improved stability compared to their 3D counterpart. [12][13][14][15] Since the 1990s, layered perovskites were also known to exhibit photoluminescence properties with narrow emission, whose energy depends on the nature of the halides, 1,9 while electroluminescence was demonstrated at low temperature. [START_REF] Era | Organic-inorganic heterostructure electroluminescent device using a layered perovskite semiconductor (C6H5C2H4NH3)2PbI4[END_REF] More recently, it has been discovered that some lead-halide layered perovskites display broad band emission (white emission), [START_REF] Dohner | Self-Assembly of Broadband White-Light Emitters[END_REF][START_REF] Dohner | Intrinsic White-Light Emission from Layered Hybrid Perovskites[END_REF][START_REF] Smith | White-Light Emission from Layered Halide Perovskites[END_REF][START_REF] Booker | Formation of Long-Lived Color Centers for Broadband Visible Light Emission in Low-Dimensional Layered Perovskites[END_REF] while electroluminescence at room temperature with high quantum efficiencies were demonstrated, particularly for 2D/3D systems [START_REF] Liu | Organic-Inorganic Hybrid Ruddlesden-Popper Perovskites: An Emerging Paradigm for High-Performance Light-Emitting Diodes[END_REF][START_REF] Wang | Perovskite light-emitting diodes based on solution-processed self-organized multiple quantum wells[END_REF][START_REF] Yang | Efficient green light-emitting diodes based on quasi-two-dimensional composition and phase engineered perovskite with surface passivation[END_REF] (mixtures of layered and 3D perovskites).

Two main families of low-dimensional perovskites are known, called <1 0 0>-and <1 1 0>oriented layered perovskites, as proposed by D.B. Mitzi in 1999. 1 The different perovskite networks of each of this family can be obtained by cutting the ABX3 (A + is small cation, B is metal and X is Halogen) mother structure along the (1 0 0) and (1 1 0) reticular planes every n or m layers, leading to BnX3n+1 and BmX3m+2 anionic frameworks for the <1 0 0>-(n series) and <1 1 0>-oriented (m series) layered perovskites, respectively. By far the most common layered compounds (butylammonium and phenylethylammonium lead halide perovskites at the forefront) belong to the <1 0 0>-oriented family. [START_REF] Mercier | Hybrid Halide Perovskites: Discussions on Terminology and Materials[END_REF] Their general formula is (A')2(A)n-1BnX3n+1 or (A'')(A)n-1BnX3n+1 (A' + and A'' 2+ are cations in the interlayer space). If most of known perovskites belonging to this family have a single layer (n= 1) of corner-sharing octahedra sandwiched between templating cations, more and more multilayered materials (n >1) have been recently reported due to their potentialities in PSCs. [START_REF] Smith | A Layered Hybrid Perovskite Solar-Cell Absorber with Enhanced Moisture Stability[END_REF][START_REF] Tsai | High-efficiency twodimensional Ruddlesden-Popper perovskite solar cells[END_REF] The general formula of corrugated <1 1 0>-oriented compounds instead is either (A')2(A)mBmX3m+2, (A)m+2BmX3m+2 or (A'')2(A)m-2BmX3m+2. For the two last categories, mostly m= 2 type compounds are known: (A)4B2X8 (A + = guanidinium, B= Pb, Sn, X= I [START_REF] Szafranski | Phase Transitions in the Layered Structure of Diguanidinium Tetraiodoplumbate[END_REF][START_REF] Stoumpos | Structure-Band Gap Relationships in Hexagonal Polytypes and Low-Dimensional Structures of Hybrid Tin Iodide Perovskites[END_REF] ; A + = acetamidinium, B= Pb, X= Br) [START_REF] Guo | Structural Diversity in Layered Hybrid Perovskites, A2PbBr4 or AA'PbBr4, Templated by Small Disc-Shaped Amines[END_REF] and several (A'')2B2X8 materials. [START_REF] Dohner | Self-Assembly of Broadband White-Light Emitters[END_REF][START_REF] Dohner | Intrinsic White-Light Emission from Layered Hybrid Perovskites[END_REF][START_REF] Li | Structure, and Optical Properties of a Contorted -Oriented Layered Hybrid Perovskite: C3H11SN3PbBr4[END_REF][START_REF] Li | Novel ⟨110⟩-Oriented Organic-Inorganic Perovskite Compound Stabilized by N -(3-Aminopropyl)Imidazole with Improved Optical Properties[END_REF][43][START_REF] Guo | Structure-Directing Effects in (110)-Layered Hybrid Perovskites Containing Two Distinct Organic Moieties[END_REF][START_REF] Salah | Mechanochromic and Electroluminescence Properties of a Layered Hybrid Perovskite Belonging to the ⟨110⟩ Series[END_REF][START_REF] Mcnulty | Unprecedented Tin Iodide Perovskite-Like Structures Featuring Ordering of Organic Moieties[END_REF][START_REF] Li | Small Cyclic Diammonium Cation Templated (110)-Oriented 2D Halide (X = I, Br, Cl) Perovskites with White-Light Emission[END_REF] This last type of perovskite became famous since (EDBE)PbBr4 ((EDBE)2Pb2Br8 (m=2), EDBE 2+ = 2,2'-(ethylendioxy)bis(ethylammonium)) was one of the first halide perovskite exhibiting broad band emission, [START_REF] Dohner | Intrinsic White-Light Emission from Layered Hybrid Perovskites[END_REF] which leads to the emerging domain of halide perovskites for white light emission application. 16,[START_REF] Smith | White-Light Emission from Layered Halide Perovskites[END_REF] Till now, the only series of such multilayered perovskites has been reported by D. B. Mitzi in 1995 for the iodoformamidinium/methylammonium/Sn 2+ /I -system: (NH2C(I)=NH2)2(CH3NH3)mSnmI3m+2 (m= 2-4). [START_REF] Mitzi | Conducting Layered Organicinorganic Halides Containing ⟨110⟩-Oriented Perovskite Sheets[END_REF] The electrical properties of these halide perovskites are compared to that of <1 0 0>-oriented perovskites (n series) (C4H9NH3)2(CH3NH3)n-1SnnI3n+1 (n= 1-5), [START_REF] Mitzi | Conducting Tin Halides with a Layered Organic-Based Perovskite Structure[END_REF] revealing "a consistent exponential drop in low temperature resistivity with increasing perovskite layer thickness, and approximately independent of layer orientation". This highlights the potentiality of such rarer <1 1 0>-oriented multilayered perovskites (m= 3, 4,…) in all fields already covered by the <1 0 0>-oriented perovskites, including photovoltaics. Moreover, only crystal structures for m= 2 perovskites, including (NH2C(I)=NH2)2(CH3NH3)2Sn2I8, [START_REF] Mitzi | Conducting Layered Organicinorganic Halides Containing ⟨110⟩-Oriented Perovskite Sheets[END_REF] have been reported till now.

We report here the synthesis and, to the best of our knowledge, the first successful XRD structural characterization of a m=3 <1 1 0>-oriented hybrid perovskites, in the form of (FA)3(HEA)2Pb3I11 perovskite (FA + : formamidinium, HEA + : hydroxyethylammonium). Structural investigation is complemented by solid state NMR measurements and first principles calculations. The simulations are further used to highlight the potential of NQR spectroscopy to provide information on the local structure. We show that the small size cation FA + is located both within perovskite sheets (cubic site) and in the interlayer space. The HEA + cation is found in the outer cavities, playing a templating role and leading to the formation of the specific <1 1 0>-oriented perovskite network.

The optical and electronic properties are studied via optical spectroscopy (absorption and PL) and periodic Density Functional Theory simulations, respectively. The material shows an absorption onset at 1.96 eV, and the computed band structure has direct character. Furthermore, the short distance between consecutive inorganic perovskite sheets is shown to result in effective masses close to me (that is, comparable to promising organic molecular semiconductors), suggesting improved out of plane charge and energy transport as compared to the well-known alkylammonium (butylammonium, hexylammonium) based < 1 0 0 >-oriented layered lead iodide perovskites. These characteristics are comparable to those of (4AMP)(MA)2Pb3I10 that has recently been demonstrated to achieve a solar cell efficiency of 18,3%, boosted by light-induced interlayer contraction. [START_REF] Li | Light-activated interlayer contraction in two-dimensional perovskites for high-efficiency solar cells[END_REF] The present work suggests similar potential exists for the < 1 1 0 >-oriented family.

EXPERIMENTAL SECTION

Synthesis and characterizations. The (FA)3(HEA)2Pb3I11 material has been prepared as a pure phase, in great quantity and high yield, by room-temperature solution techniques. Iodide salts (PbI2, FAI and HEAI) were first dissolved in a hydriodic acid solution (see S.I. for details). The resulting saturated solution was placed into a sutured ethyl acetate vapor atmosphere leading through a liquid-gas diffusion process to crystals of (FA)3(HEA)2Pb3I11 after one day. The dark red plate-like crystals were filtered off (Figure S1), washed several times with ethyl acetate and dried at 40°C for 30 minutes. X-Ray powder diffraction (XRPD) analysis which was performed on a D8 Bruker diffractometer (Cu Kα) equipped with a linear Vantec super speed detector, confirmed the phase purity of the title compound, showing that all observed reflections could be indexed in the unit cell obtained from single crystal X-ray diffraction experiments (Figure S2). S3) allowed us to determine the FA + /HEA + ratio of 1.51, which is good agreement with the expected formula of (FA)3(HEA)2Pb3I11.

Thermogravimetric analysis shows that (FA)3(HEA)2Pb3I11 is stable up to 200 °C while no signature of phase transition in the RT-200°C range is detected by means of differential scanning calorimetry analysis (Figure S4).

Single crystal X-ray crystallography. X-ray diffraction data were collected at T = 280.0(1) K, on a Rigaku Oxford Diffraction diffractometer equipped with Atlas CCD detector and micro-focus Cu-Kα radiation (λ = 1.54184 Å). Intensities were corrected for Lorentz-polarization effects, as well as for absorption effect (gaussian method using CrysAlisPro program -CrysAlisPro, Rigaku Oxford Diffraction, V1.171. 40.45a, 2019). The structures were solved using SHELXT program, and refined by full matrix least-squares routines against F 2 using SHELX programs (G. M. Sheldrick 2018, SHELXT 2018/2 and SHELXL 2018/3). The hydrogen atoms were treated with a riding model except for the H atom on the hydroxyl group which were first placed at calculated position taking into account the local electron density and then fixed with HFIX constraint for the convergence of the refinement. The refinements of positions and anisotropic thermal motion parameters of the non-H atoms, converge to R(F) = 0.0532 (3069 independent reflections (Rint= 0.0378), 142 parameters), wR2(F2) = 0.1443 (all data), GOF on F 2 is 1.052. Many restraints on the distances and angles of the three organic molecules were applied to be in agreement with the geometry of such molecules. This suggests possible dynamic disorder of the organic cations. A summary of crystallographic data and refinement results are listed in Tables S1 andS2. A complete list of crystallographic data, along with the atomic coordinates, the anisotropic displacement parameters and bond distances and angles are given as CIF file. CCDC number 2098405.

Electronic band structure calculations. Electronic properties of the material are investigated for the newly reported crystal structure of (FA)3(HEA)2Pb3I11, assumed as pure phase. We adopted periodic Density Functional Theory (DFT) calculations, within the plane-wave/pseudopotential formalism, as implemented in the pwscf program from the Quantum Espresso suite software. [START_REF] Giannozzi | QUANTUM ESPRESSO: a Modular and Open-Source Software Project for Quantum Simulations of Materials[END_REF] We adopted norm-conserving pseudopotentials, as available from the pseudodojo dataset, [START_REF] Van Setten | The PseudoDojo: Training and grading a 85 element optimized normconserving pseudopotential table[END_REF] and set kinetic energy cutoff to 50 Ry and 200 Ry for the plane-wave expansion of the electron wavefunction and electron density, which provides accurate results for lead-iodide perovskite networks. [START_REF] Quarti | Interplay of Orientational Order and Electronic Structure in Methylammonium Lead Iodide: Implications for Solar Cell Operation[END_REF] Electronic structures are based on PBE exchange-correlation functional, [START_REF] Perdew | Generalized Gradient Approximation Made Simple[END_REF] with Spin-Orbit-Coupling explicitly accounted. [START_REF] Even | Importance of Spin-Orbit Coupling in Hybrid Organic/Inorganic Perovskites for Photovoltaic Applications[END_REF] A homogeneous 4x6x2 sampling of the first Brillouin zone in the Monkhorst Pack scheme, [START_REF] Monkhorst | Special Points for Brillouin-Zone Integrations[END_REF] with different sampling reflecting the anisotropy in the lattice parameters (shorter parameters are associated to denser k-point sampling). In light of the presence of a molecular component, we addressed the potential impact of van der Waals interactions. Namely, we performed partial optimization of the organic component both adopting DFT-D2 semi-empirical corrections [START_REF] Grimme | Semiempirical GGA-type density functional constructed with a long-range dispersion correction[END_REF] and fully ab-initio VDW-DF2 functional [START_REF] Thonhauser | Van der Waals density functional: Self-consistent potential and the nature of the van der Waals bond[END_REF] for the description of the exchange and correlation. Corresponding band structures are reported in the Supplementary information (Figures S12 andS13) and clearly point out towards a very limited role of the van der Waals interactions in affecting the electronic structure of the material.

First-principle prediction of solid state NMR and NQR signals. This is based on periodic, plane-wave/pseudopotential DFT simulations, as implemented in the CASTEP suite code. [START_REF] Clark | First principles methods using CASTEP[END_REF] This code adopts a perturbative approach to evaluate the chemical shielding tensor of the atoms composing the investigated crystal model. The present method also takes advantage of the GIPAW (Gauge Including Projector Augmented Wave) formalism to map back the non-local information from the plane wave basis set to the local information associated to the electron shielding close to the nuclei, meanwhile solving the Gauge problem, typical of finiteness of the basis set. [START_REF] Van De Walle | First-principles calculations of hyperfine parameters[END_REF][START_REF] Pickard | All-electron magnetic response with pseudopotentials: NMR chemical shifts[END_REF] Kinetic energy cutoff of 500 eV and 0.03 Å -1 automatic sampling of the first Brillouin zone have been employed, as best trade-off between accuracy and computational cost. [START_REF] Quarti | DFT Simulations as Valuable Tool to Support NMR Characterization of Halide Perovskites: the Case of Pure and Mixed Halide Perovskites[END_REF] Although crucial for a proper description of the NMR signals of 207 Pb nuclei, [START_REF] Alkan | Chemical-shift tensors of heavy nuclei in network solids: a DFT/ZORA investigation of 207 Pb chemical-shift tensors using the bond-valence method[END_REF] the present calculations do not include Spin-Orbit-Coupling, as this is not currently available in CASTEP. The same computational formalism and details are adopted for the estimate of the Electric Field Gradients and corresponding NQR parameters (namely, quadrupolar coupling constants, Cq, and asymmetry parameters, ).

Experimental solid-state NMR. Solid-state NMR spectra have been acquired on a Bruker

Avance III 300 MHz WB spectrometer equipped with a 4 mm X/H MAS probe used for 13 C and a 1.3 mm X/H MAS probe used for 207 Pb. 13 C direct excitation experiments were done under 5 kHz MAS by means of the DEPTH sequence for background suppression and with proton decoupling.

1 K scans were accumulated with a recycle delay of 40 s. 207 Pb experiments were done with a spinning frequency of 40 kHz. The ARING sequence was used for background suppression. 500 K scans were accumulated with a recycling delay of 240 ms.

Optical properties.

Optical absorption is performed with a UV-Vis Lambda9 spectrometer. PL and PLE spectra are obtained with a NanoLog composed by a iH320 spectrograph equipped with a Synapse QExtra charge-coupled device and a PPD-850 single photon detector by exciting with a monochromated 450W Xe lamp. The spectra are corrected for the instrument response. Timeresolved TCSPC measurements are obtained with PPD-850 single photon detector module and DeltaTime serie DD-405L DeltaDiode Laser. Analysis of the emission decays are performed with the instrument Software DAS6. The average emission lifetime is obtained as  = ∑Aiti by multiexponential fits of the decays I= ∑Aiexp(t/ti).

RESULTS AND DISCUSSION

Structural aspects.

In the HEA + /Pb 2+ /I -system, the <1 0 0>-oriented layered perovskite (HEA)2PbI4 (n= 1) has been obtained by our group in a previous work. 10 An interesting structural feature was the unique hydrogen-bond network connecting adjacent layers owing to OH…I, NH3 + …Iapical and intermolecular NH3 + …OH interactions. In contrast, only weak NH3 + …Iequatorial hydrogen interactions were counted. As a result, a significant reduction of the band gap was correlated to weak distortions of perovskite layers. More recently, working with the MA + or FA + /HEA + /Pb 2+ /I - system in order to prepare multilayered Ruddlesden-Popper phases, we discovered a new family of 3D materials, called lead-deficient hybrid perovskites (d-HP) (HEA,A)1+xPb1-xI3-x (A: MA + , FA + ). The single-crystal X-ray study led to solve the inorganic 3D network but, unfortunately, organic cations were not localized. [START_REF] Leblanc | Lead and iodide deficient (CH3NH3)PbI3, d-MAPI: the bridge between 2D and 3D hybrid perovskites Angew[END_REF][START_REF] Leblanc | Enhanced Stability and Band Gap Tuning of α-[HC(NH2)2]PbI3 Hybrid Perovskite by Large Cation Integration[END_REF] In the present work, the same reagents exposed to different experimental conditions brought to a pure phase of the <1 1 0>-oriented multilayered m= 3 perovskite (FA)3(HEA)2Pb3I11 in the form of plate like crystals.

The crystal structure was solved in the P2 monoclinic space group (see general view, Figure 1a, Tables S1 andS2). Let us notice that the inorganic framework can be correctly described either in a twice greater orthorhombic unit cell or in a corresponding centrosymmetrical monoclinic space group. However, organic components could be correctly located only within the P2 space group, and considering constraints on bond lengths and angles (see experimental section). An interesting structural feature is that a nearly undistorted perovskite sheet is obtained (Figure 1a). The inner Pb(1)I6 octahedra exhibit six similar Pb-I bond lengths (3.18 Å-3.20 Å range) whereas for the outer Pb(2)I6 octahedra, the Pb-Iterminal bond lengths appear only slightly shorter (3.093 Å) than the Pb-I ones involving shared iodides (3.167 Å-3.303 Å range). As regards bond angles, the I-Pb-I ones are close to 90° (adjacent iodides) or 180 ° (opposite iodides), and more important, the Pb-I-Pb ones, revealing the degree of distortion of perovskite layers, are almost linear (177.7°). As underlined in a recent review, 17 <1 1 0>-oriented multilayered perovskites with m= 2 very often exhibit highly distorted inorganic networks, especially those based on dications (A'' 2+ ) linking two adjacent sheets. Here, the combined effect of HEA + located in outer cavities and FA + placed in the interlayer space leads to this unusual undistorted perovskite sheets. The HEA + cation, which has also afforded the m= 2 hybrid perovskite in the lead/bromide system, [START_REF] Salah | Mechanochromic and Electroluminescence Properties of a Layered Hybrid Perovskite Belonging to the ⟨110⟩ Series[END_REF] has surely a templating role in the formation of such network due to its ability both to fill the rectangle parallelepiped space and to interact with iodides through H…I bonding and with FA + cations located in the interlayer space through NH2…OH bonding (Figure 1b). FA + cations are therefore localized both in the inner layer of the perovskite sheet (cubic site) and in the interlayer space, so that a formula of (FA)2[(HEA)2(FA)Pb3I11] is better suited to account for the specific structural arrangement.

Another structural feature observed in such <1 1 0>-oriented perovskites consists in short I…I interlayer distances. In (FA)3(HEA)2Pb3I11, the I…I contact is roughly twice the van der Waals radii (d(I…I)= 4.547 Å). This interlayer distance is much smaller than the one of known <1 0 0>oriented structures adopting a staggered interlayer arrangement with shifts of half a crystal unit cell in the two in-plane directions, also named Ruddlesden-Popper phases. While remaining slightly larger than those of the eclipsed (4AMP)(MA)2Pb3I10 compound, [START_REF] Mao | Hybrid Dion-Jacobson 2D Lead Iodide Perovskites[END_REF] so called Dion-Jacobson phase, it is comparable to that of (GA)(MA)3Pb3I10 that shows a similar pattern with the layer shifted by half a unit cell vector only in one of the in-plane directions. [START_REF] Myae Soe | New Type of 2D Perovskites with Alternating Cations in the Interlayer Space, (C(NH2)3)(CH3NH3)nPbnI3n+1: Structure, Properties, and Photovoltaic Performance[END_REF] The presence of FA + in a cubic site of a iodoplumbate perovskite at room temperature is quite unusual. In fact, as well known, the 3D α-FAPbI3 compound is stable above 150 °C at ambient pressure, while the room temperature stable phase is the -FAPbI3 (yellow color) having 1D inorganic PbI3 -chains of face-sharing octahedra. [START_REF] Stoumpos | Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and near-Infrared Photoluminescent Properties[END_REF] Layered iodoplumbate perovskites (n series) including FA + are also rare, even if organic dications in the interlayer space have successfully stabilized such layered perovskites (n=2,…). [START_REF] Niu | Advances in Dion-Jacobson phase two-dimensional metal halide perovskite solar cells[END_REF] Given these observations, we performed investigations to assess the stability of (FA)3(HEA)2Pb3I11. When kept in inert atmosphere at room temperature (RT), (FA)3(HEA)2Pb3I11 is stable (Figure S5a). In addition, TGA and DSC experiments (Figure S4) reveal no thermal accident in the RT-150°C temperature range before the decomposition occurs. However, it appears that this compound is unstable under physical strain or in humid atmosphere at RT, leading to a decomposition into -(FA)PbI3 and certainly into (FA)2(HEA)2Pb2I8, the m= 2 member of the (FA)m(HEA)2PbmI3m+2 series (<1 1 0> series). This is clear from PXRD analysis of samples previously ground one to two minutes in inert atmosphere, or under exposure in air atmosphere (humidity rate of 40%), with decreasing intensity of the X-ray lines associated to (FA)3(HEA)2Pb3I11 together with the appearance of specific X-ray lines of both -(FA)PbI3 and (FA)2(HEA)2Pb2I8 (Figures S5b andS6). Notice that the synthesis and crystal structure determination of this last compound (Figure S7, Table S3) allowed us to get its corresponding calculated PXRD pattern. Overall, the novel (FA)3(HEA)2Pb3I11 shows improved stability to moisture, as compared to reference MAPbI3 3D halide perovskite. The latter in fact degrades in PbI2 already within 24 hours under humidity rate of 40%, while the m=3 compound does not evidence signatures of m=2 layered phase and/or -FAPI for one week under same conditions (Figure S5). suited for hybrid perovskites, as the organic cations can be very mobile in terms of reorientations, partly averaging out the necessary dipolar coupling. [START_REF] Dahlman | Dynamic Motion of Organic Spacer Cations in Ruddlesden-Popper Lead Iodide Perovskites Probed by Solid-State NMR Spectroscopy[END_REF][START_REF] Dahlman | Interfaces in metal halide perovskites probed by solid-state NMR spectroscopy[END_REF] The relatively narrow width of the 1 H signals shows that the cations are indeed mobile, which is furthermore an indication of good and regular insertion (Figure S10). Therefore, the experiments have been done by direct excitation, which is moreover quantitative. There are three different cation sites, i.e. interlayer, inner layer and outer cavity (Figure 1). The 13 C spectrum (Figure 2a) shows the two HEA signals at 61.8 (…-CH2-OH) and 45.8 (…-CH2-NH3 + ) ppm with only minor dispersion (in form of small signal feet), indicating that they overall occupy one of the three potential sites, consistent with the particular role attributed to the HEA + cations on the outer cavity of the perovskite layers (vide infra). Periodic DFT calculations of the chemical shielding tensor performed on the model in Figure 1, where HEA + are placed in the outer cavities, yield to chemical shifts for 13 C nuclei of HEA + within few ppm with respect to the experimental data (see Figure 2), namely at 65.2 ppm (…-CH2-OH) and 44.7 ppm (…-CH2-NH3 + ). These computed chemical shifts and the existence of only one signal per carbon also indicate that HEA + cations are likely to seat in the outer cavities of the inorganic sheet. In contrast, the formamidinium cation shows a group of three signals. This might on the first glance indicate an occupation of all three possible sites, however, one of them turns out to be an experimental artefact. The central signal at 156.3 ppm corresponds to the non-perovskite -phase of formamidinium-PbI3 (FAPI), product of degradation upon physical strain to which this material is sensitive as revealed by a DRX pattern of a ground sample (vide supra and Figure S6). [START_REF] Raval | Understanding Instability in Formamidinium Lead Halide Perovskites: Kinetics of Transformative Reactions at Grain and Subgrain Boundaries[END_REF] In fact, also previous NMR studies in the literature pointed towards material degradation due to the measuring procedure itself. [START_REF] Roiland | Multinuclear NMR as a tool for studying local order and dynamics in CH3NH3PbX3 (X = Cl, Br, I) hybrid perovskites[END_REF][START_REF] Kubicki | Dynamics in Mixed-Cation (MA)x(FA)1-xPbI3 Hybrid Perovskites from Solid-State NMR[END_REF][START_REF] Van Gompel | Degradation of the Formamidinium Cation and the Quantification of the Formamidinium-Methylammonium Ratio in Lead Iodide Hybrid Perovskites by Nuclear Magnetic Resonance Spectroscopy[END_REF] Degradation therefore turns out to be inevitable when performing NMR, even with very careful filling of the NMR rotor, as the magic angle spinning with at least 3 kHz necessary for spectral resolution applies much physical strain to at least a part of the sample powder. XRD of the material after MAS NMR confirms the formation of this phase in previously pure samples (data not shown). Humidity enhances the effect: Figure S8a shows the formamidinium 13 C NMR region of a less (sample 1) and very (sample 2) carefully stored and prepared sample, and a sample deliberately degraded by keeping it in 100% humidity, compared to a sample of FAPI mainly in -phase. The center signal is increasing, representing already 75% in sample 1, and corresponds to -FAPI. On the side of HEA + , a progressed degradation (sample 2) is not manifested by additional distinct signals, but line broadening and dispersion around the initial m = 3 signals (Figure S8b), in particular for …-CH2-OH (65.2 ppm). This is consistent with the proposed formation of m = 2 structures, in which the HEA + cations have essentially the same local environment (Figure S7). The effect is better visible by cross-polarization (CP) experiments in which the carbons in less perfect structures are enhanced, as there are more constraints to cation reorientational motions. [START_REF] Leblanc | Enhanced Stability and Band Gap Tuning of α-[HC(NH2)2]PbI3 Hybrid Perovskite by Large Cation Integration[END_REF] In the deliberately humidity degraded sample the HEA + ions are already in a sort of solution state, as seen from the strong shift, very narrow signals (experimentally limited by the acquisition time) and the absence of signals in the CP spectrum.

There are thus two distinct FA signals of the (FA)3(HEA)2Pb3I11 phase itself, at 157.6 and 155.6 ppm, with an intensity ratio of 0.55:0.45. This matches, within the experimental uncertainty, the picture of formamidinium occupying the interlayer site and the inner perovskite intralayer site with a stoichiometric ratio of 2:1 (Figure 1). This assignment is further confirmed by DFT calculations (Figure 2b), which predict 13 C at 152.5 ppm and 149.8 ppm for the two FA + cations in the interlayer cavity and the one in the intralayer cavity, respectively. Similarly to HEA + , the computed absolute values are shifted with respect to the experimental ones, but the distance between the two signals matches relatively well.

The 207 Pb spectrum (Figure 2c) is consistent with the 13 C findings: there are two signals of intensity ratio of roughly 1:2, which we therefore tentatively assign to the inner (1240 ppm) and outer (1500 ppm) layer lead atoms. However, both signals do not have a perfect shape, which is probably due to smaller contributions of degradation products as discussed above. While m = 2 is expected to have a similar chemical shift as the outer layer, the 1240 ppm signal probably contains a contribution of the -FAPI phase upfield. The latter is illustrated in Figure S9: the (FA)3(HEA)2Pb3I11 sample is compared with a -FAPI (which still contains a residual  phase) and the further degraded sample 1 with very high (75%) degree of -FAPI formation. Theoretical calculations of the NMR frequencies of 207 Pb confirm the experimental data; they predict 838 ppm for the inner and 1038 ppm for the outer layer lead. Besides the general 400 ppm underestimation, which is often observed in chemical shift calculations, the order is consistent and the difference of Nuclear Quadrupole Resonance (NQR) technique was also employed in the past for the structural characterization of halide perovskite materials, [START_REF] Sharma | Phase transition in CsSnCl3 and CsPbBr3 and NMR and NQR study[END_REF][START_REF] Xu | Molecular Motions and Phase Transitions in Solid CH3NH3PbX3 (X = C1, Br, I) as Studied by NMR and NQR[END_REF][START_REF] Senocrate | Short-range ion dynamics in methylammonium lead iodide by multinuclear solid state NMR and 127 I NQR[END_REF][START_REF] Franssen | Dynamics, and Defects in Methylammonium Lead Halide Perovskites[END_REF][START_REF] Piveteau | Solid-State NMR and NQR Spectroscopy of Lead-Halide Perovskite Materials[END_REF][START_REF] Piveteau | Bulk and Nanocrystalline Cesium Lead-Halide Perovskites as Seen by Halide Magnetic Resonance[END_REF][START_REF] Aebli | Local Structure of Multinary Hybrid Lead Halide Perovskites Investigated by Nuclear Quadrupole Resonance Spectroscopy[END_REF] especially in relation to the local chemical environment probed by the halide, that, presenting nuclear spin I>1/2, is subject to quadrupolar interaction with the electric field gradient. However, we did not observe any 127 I signal for the m = 3 compound likely because of extreme line broadening caused by inherent disorder associated to the organic component, which in particular has been observed for mixed cation halide perovskites. [START_REF] Piveteau | Bulk and Nanocrystalline Cesium Lead-Halide Perovskites as Seen by Halide Magnetic Resonance[END_REF][START_REF] Aebli | Local Structure of Multinary Hybrid Lead Halide Perovskites Investigated by Nuclear Quadrupole Resonance Spectroscopy[END_REF][START_REF] Mozur | Cesium Substitution Disrupts Concerted Cation Dynamics in Formamidinium Hybrid Perovskites[END_REF] Nevertheless, DFT may provide useful feedback about the expected response of iodine in (FA)3(HEA)2Pb3I11, so to extend the overall limited amount of results reported in the literature, [START_REF] Sharma | Phase transition in CsSnCl3 and CsPbBr3 and NMR and NQR study[END_REF][START_REF] Xu | Molecular Motions and Phase Transitions in Solid CH3NH3PbX3 (X = C1, Br, I) as Studied by NMR and NQR[END_REF][START_REF] Senocrate | Short-range ion dynamics in methylammonium lead iodide by multinuclear solid state NMR and 127 I NQR[END_REF][START_REF] Franssen | Dynamics, and Defects in Methylammonium Lead Halide Perovskites[END_REF][START_REF] Piveteau | Solid-State NMR and NQR Spectroscopy of Lead-Halide Perovskite Materials[END_REF][START_REF] Piveteau | Bulk and Nanocrystalline Cesium Lead-Halide Perovskites as Seen by Halide Magnetic Resonance[END_REF][START_REF] Aebli | Local Structure of Multinary Hybrid Lead Halide Perovskites Investigated by Nuclear Quadrupole Resonance Spectroscopy[END_REF] and narrow the field for future experimental investigation on this or similar compounds. In Table 1, we report the values of the computed quadrupolar interaction constant (Cq), asymmetry parameter () and corresponding spin-transition frequencies for the 127 I nuclei in two cases of interest, namely the γ-phase of CsPbI3, taken as reference, and our m=3 layered halide perovskite. Since 127 I has a 5/2 nuclear spin, it presents two times two allowed transitions, ±1/2 → ±3/2 and ±3/2 → ±5/2. For γ-CsPbI3, our theoretical data nicely match both previous DFT estimates and the experimental data available from the literature, [START_REF] Piveteau | Bulk and Nanocrystalline Cesium Lead-Halide Perovskites as Seen by Halide Magnetic Resonance[END_REF] [START_REF] Sutton | Cubic or Orthorhombic? Revealing the Crystal Structure of Metastable Black-Phase CsPbI3 by Theory and Experiment[END_REF] For the (FA)3(HEA)2Pb3I11 compound, the situation is more involved, as the poor symmetry evidenced from XRD measurements results in several non-equivalent crystalline sites for the 127 I nuclei, each with different NQR features. Still, a chemically-driven classification emerges when distinguishing between bulk (Ibulk) and terminal iodines (Iterm), bounded to two and one lead atoms, respectively. NQR response of bulk iodines in the multilayered (FA)3(HEA)2Pb3I11 perovskite is close to that of iodines in the 3D γ-CsPbI3 phase, showing signatures for the 1/2→3/2 and 3/2→5/2 transitions in the range of the 80-84 MHz and 160-180 MHz, respectively. In contrast, terminal iodines present a spin-transitions at comparably smaller frequencies, 62-65 MHz and 71-78 MHz for the 1/2→3/2 and 3/2→5/2, respectively. The smaller splitting between the 1/2→3/2 and 3/2→5/2 transitions compared to bulk iodines is related to the very large asymmetry parameter  for terminal iodines. All findings clearly confirm the undeniable sensitivity of the NQR technique in probing the perovskite local structure [START_REF] Sharma | Phase transition in CsSnCl3 and CsPbBr3 and NMR and NQR study[END_REF][START_REF] Xu | Molecular Motions and Phase Transitions in Solid CH3NH3PbX3 (X = C1, Br, I) as Studied by NMR and NQR[END_REF][START_REF] Senocrate | Short-range ion dynamics in methylammonium lead iodide by multinuclear solid state NMR and 127 I NQR[END_REF][START_REF] Franssen | Dynamics, and Defects in Methylammonium Lead Halide Perovskites[END_REF][START_REF] Piveteau | Solid-State NMR and NQR Spectroscopy of Lead-Halide Perovskite Materials[END_REF][START_REF] Piveteau | Bulk and Nanocrystalline Cesium Lead-Halide Perovskites as Seen by Halide Magnetic Resonance[END_REF][START_REF] Aebli | Local Structure of Multinary Hybrid Lead Halide Perovskites Investigated by Nuclear Quadrupole Resonance Spectroscopy[END_REF] and shall raise further attempts in this direction, especially whenever less disorder is present. Table 1. DFT computed NQR response for 127 I nuclei in reference γ-phase of CsPbI3 perovskite and in the investigated m=3 layered (FA)3(HEA)2Pb3I11 perovskite. Cq and  are the computed quadrupolar coupling constant and the asymmetry parameter, respectively. Corresponding NQR resonances for the 1/2→3/2 and 3/2→5/2 transitions are reported as well and compared to experimental results (when available).

Cq (MHz

Optical absorption and photoluminescence

The optical absorption of (FA)3(HEA)2Pb3I11 is reported in Figure 3a together with the Tauc Plot (inset) from which an optical gap of 1.96 eV (632 nm) is estimated. The emission properties are characterized by a sharp PL peak with maximum at 735 nm, whose excitation profile well corresponds to the absorption (Figure 3b). The PL band displays a FWHM of 130 meV and is characterized by an average lifetime of = 2.96 ns, obtained by a three-exponential fit of the decay (see inset of Figure 3b). The large energy separation between the absorption onset and the PL peak is likely related to emission from layer edge states, located at the crystal edges, after migration of the photogenerated excitons towards lower energy states, as already reported for 2D Ruddlesden-Popper phases with different n values. [START_REF] Blancon | Extremely efficient internal exciton dissociation through edge states in layered 2D perovskites[END_REF][START_REF] Shang | Unveiling Structurally Engineered Carrier Dynamics in Hybrid Quasi-Two-Dimensional Perovskite Thin Films toward Controllable Emission[END_REF]

Electronic structure

Electronic properties of the newly synthesized (FA)3(HEA)2Pb3I11 halide perovskite are elucidated via DFT simulations. These are performed as for the case of the NMR/NQR calculations, relying on the model structure retrieved from XRD measurements at 280 K, so to stay close to the conditions probed experimentally, hence accounting for the role of thermal effects on structural details like the octahedral rotations. In addition, to better clarify the role of the organic spacer on the frontier electronic levels of the material and to avoid spurious effects due to the more difficult resolution of the position of the organic atoms via XRD, we computed the band structure both for model where i) the organic moieties are substituted by Cs ions and ii) the positions of the organic atoms are relaxed at zero K (with the inorganic lattice frozen at 280 K). In light of the important role of relativistic effects in presence of heavy lead element, [START_REF] Even | Importance of Spin-Orbit Coupling in Hybrid Organic/Inorganic Perovskites for Photovoltaic Applications[END_REF] hybridization pattern (Figure 4c), peculiar of the frontier orbitals of metal-halide perovskites, at the center of the Brillouin zone. [START_REF] Quarti | Physical properties of bulk, defective, 2D and 0D metal halide perovskite semiconductors from a symmetry perspective[END_REF] DFT-SOC single-particle band gap is estimated to ~ 1 eV. Still, one should refrain to compare this with the optical absorption edge, because of the well-known tendency of standard GGA calculations to underestimate the single particle band gaps, 86 and of the very likely presence of excitonic effects in these dimensionally confined systems, as induced by confinement effects, [START_REF] Katan | Quantum and Dielectric Confinement Effects in Lower-Dimensional Hybrid Perovskite Semiconductors[END_REF] which further stabilize the optical band gap, compared to direct singleparticle band-to-band transition. Additional calculations are performed by relaxing all the atomic positions in the crystalline cell (both organic and inorganic), so to roughly estimate the potential impact of structural modifications induced by temperature. Resulting band structure in the Supporting Information (Figure S11) clearly show that thermal effects do impact on the electronic properties but only to a minor extent, e.g. band gap opens by ~60 meV. Interestingly, a small displacement of the conduction band minimum from the high symmetry point D is found, which may be ascribed to Rashba effect, as induced by structural distortion. 87 Threshold for the isosurface is set to 3 10 -4 e/bohr 3 ; d) structure and band dispersions along the stacking direction for the currently investigated <1 1 0 >-oriented m=3 layered perovskites and for reference <1 0 0>-oriented n=3 layered perovskites. The latter structure is taken from Ref. [START_REF] Mao | Hybrid Dion-Jacobson 2D Lead Iodide Perovskites[END_REF] As for the case of <1 0 0>-oriented hybrid perovskites build from non-conjugated cations, [START_REF] Katan | Quantum and Dielectric Confinement Effects in Lower-Dimensional Hybrid Perovskite Semiconductors[END_REF] the organic component does not contribute to the frontier orbitals of (FA)3(HEA)2Pb3I11. This clearly emerges when comparing the results of the calculations performed in presence of the specific FA and HEA molecules and of the Cs atoms at their place, showing almost equivalent band dispersion along the C→Z→D→E0 path. Most notably, small but non-negligible dispersion is found along the D→Z direction in Figure 4a, both for the valence and conduction band edges, hence indicating that consecutive layers are not fully decoupled but they maintain the electronic communication.

This is very important from an application viewpoint, as the weak inter-planar electronic communication and reduced charge transport along the plane stacking direction is usually considered one of the main culprits for the lower efficiencies of layered perovskite based optoelectronic devices, compared to those constituted by their 3D analogues. [START_REF] Tsai | High-efficiency twodimensional Ruddlesden-Popper perovskite solar cells[END_REF][START_REF] Cao | 2D Homologous Perovskites as Light-Absorbing Materials for Solar Cell Applications[END_REF][START_REF] Tsai | Stable Light-Emitting Diodes Using Phase-Pure Ruddlesden-Popper Layered Perovskites[END_REF] In the case of the (FA)3(HEA)2Pb3I11 layered perovskite, our band structure calculations indicate 36 meV/41 meV band dispersion along the D→Z direction for the valence and conduction band edges (Figure 4d), respectively. Corresponding hole and electron effective masses are estimated here to 1.26 and 1.07 me (see Table 2), respectively, which are surprisingly small for disconnected PbI6 octahedra and which are likely related to the short I…I interlayer distance of 4.55 Å (vide infra and Figure 4d). For the sake of reference, flat band dispersion and infinite hole and electron effective masses were found in the literature for the monoclinic and orthorhombic phases of hexylammonium lead iodide, in correspondence with significantly larger I…I interlayer distances (ca 8.35 Å). [START_REF] Quarti | Tuning the Optoelectronic Properties of Two-Dimensional Hybrid Perovskite Semiconductors with Alkyl Chain Spacers[END_REF][START_REF] Billing | Synthesis, characterization and phase transitions in the inorganic-organic layered perovskite-type hybrids [(CnH2n+1NH3)2PbI4], n = 4, 5 and 6[END_REF] In this sense, the larger band dispersion/smaller effective masses in correspondence of shorted interlayer distance is fully expected, as this allows for more effective hybridization of the atomic orbitals. One may therefore wonder whether the specific < 1 1 0 > termination can have an impact on the interlayer electronic communication. [START_REF] Marchal | Cation Engineering for Resonant Energy Level Alignment in Two-Dimensional Lead Halide Perovskites[END_REF] To address this point, we have performed additional calculations on the (4AMP)(MA)2Pb3I10 layered perovskite, an analogue with <1 0 0 > termination with thickness of the inorganic frame of 3 octahedra (n=3), reported in the recent literature. [START_REF] Li | Light-activated interlayer contraction in two-dimensional perovskites for high-efficiency solar cells[END_REF][START_REF] Mao | Hybrid Dion-Jacobson 2D Lead Iodide Perovskites[END_REF] Corresponding I…I distance is even shorter for this n=3 (4AMP)(MA)2Pb3I10 layered perovskite when compared to the m=3 (FA)3(HEA)2Pb3I11 structure, with correspondingly increasing band dispersion along the stacking direction (see Figure 4d) and reduced hole and electron effective masses of 0.35 and 0.35, respectively. In this sense, the reduced effective mass when going from m=3 to n=3 is ascribable both to the shorter I…I distance but also to the absence of shift between subsequent perovskite layers (eclipsed configuration).

Table 2. Diagonal component of the effective mass tensor for hole (mh) and electron (me) carriers, computed for the newly reported (FA)3(HEA)2Pb3I11 <1 1 0>-oriented layered perovskites and for (4AMP)(MA)2Pb3I10, as reference for <1 0 0>-oriented layered perovskite with comparable Pb/I ratio. Effective masses are evaluated from band dispersion along the crystallographic direction corresponding to the Pb-Iequatorial-Pb bonds (in-plane) and along the plane stacking direction (outof-plane).

(FA)3(HEA)2Pb3I11 In order to further assess the potential of (FA)3(HEA)2Pb3I11 for optoelectronic applications that require attractive carrier transport properties, we computed in-plane effective masses. From the band edge dispersion with respect to the crystallographic direction corresponding to the Pb-Iequatorial-Pb bond, we found hole effective masses of ca. 0.17 mh both for (FA)3(HEA)2Pb3I11 and for (4AMP)(MA)2Pb3I10 layered perovskites, that is, slightly smaller than corresponding masses in n=1 hexylammonium and dodecylammonium halide perovskites. [START_REF] Quarti | Tuning the Optoelectronic Properties of Two-Dimensional Hybrid Perovskite Semiconductors with Alkyl Chain Spacers[END_REF] In plane electron effective masses me are also small both for (FA)3(HEA)2Pb3I11 and (4AMP)(MA)2Pb3I10 (see Table 2), with slightly smaller value for the former likely due to the almost linear Pb-I-Pb bonds, as retrieved from XRD measurements (vide infra). Effective masses from Table 2 overall suggest the suitability of the present <1 1 0>-oriented perovskites as active material for optoelectronic devices.

(4AMP)(MA)2Pb3I10 (m=3)

CONCLUSION

Combining the excellent semiconducting properties of halide perovskite frames together with improved chemical resistance against moisture, layered halide perovskites have great potential for efficient and stable optoelectronic devices. Multilayered compounds, for which the 2D perovskite network is not confined to only one octahedron thickness but spans over several octahedra, are of particular interest, with the <1 0 0>-oriented family currently widely investigated. On the other hand, also <1 1 0>-oriented layered perovskites can be designed, whose properties have been much less explored, thus far. Here, a new member of the multilayered corrugated <1 1 0>-oriented hybrid metal halide perovskite, (FA)3(HEA)2Pb3I11, has been synthesized and characterized. In particular, by combining X-Ray diffraction on crystals and NMR on powders, we successfully resolved its structure, which represents the only crystalline model for multilayered <1 1 0>-oriented compounds reported so far. The newly reported compound shows optical absorption edge around 2 eV and in plane effective masses comparable to those of (4AMP)(MA)2Pb3I10 <1 0 0>-oriented layered perovskite, that was successfully exploited as photoactive materials in photovoltaic devices. [START_REF] Li | Light-activated interlayer contraction in two-dimensional perovskites for high-efficiency solar cells[END_REF] The overall picture is hence that <1 1 0>-oriented multilayered halide perovskites may be successfully used in technological fields where <1 0 0>-analogues are currently under scrutiny.

In particular, (FA)3(HEA)2Pb3I11 has small effective masses along the plane stacking direction, as a result of short distance between iodine atoms belonging to consecutive inorganic sheets, hence imparting more isotropic transport properties to this natively structurally confined material.

HPC resources of TGCC/CINES/IDRIS under the allocation 2020-A0090907682 made by The multilayered hybrid perovskite (FA) 3 (HEA) 2 Pb 3 I 11 based on the hydroxyethylammonium cation (HEA + ), FA + and <1 1 0>-oriented perovskite network of three octahedra thick has been synthesized and characterized by X-ray diffraction and solid state NMR, supported by DFT calculations. Electronic band structure shows a direct band gap and evidence sizeable inter layer electronic coupling related to short I…I distances.

Figure 1 .

 1 Figure 1. Structure of the <1 1 0>-oriented hybrid perovskite (FA)3(HEA)2Pb3I11: General view along the b direction (a) and partial view showing FA + cations localized both in the inner cavity of the perovskite sheet and in the interlayer space and HEA + cations localized in outer perovskite cavities (b) (dashed lines: H…I and H…O contacts < 3.10 Å).

  200 ppm between the two signals is not too far from the experimental value of 260 ppm. Previous simulations performed on other mixed halides perovskites provided a similar shift of 207 Pb signals compared to experiment.[START_REF] Quarti | DFT Simulations as Valuable Tool to Support NMR Characterization of Halide Perovskites: the Case of Pure and Mixed Halide Perovskites[END_REF]

Figure 2 .

 2 Figure 2. a) Direct excitation 13 C spectrum of (FA)3(HEA)2Pb3I11 (red) and deconvolution of the FA + signal group with three lorentzians (blue). b) Simulated 13 C spectrum based on the chemical shieldings extracted from a DFT calculated electronic structure. c) 207 Pb spectrum.

  hence supporting the reliability of our methodology. Most notably, DFT calculations predict the presence of two distinct signals, both for the 1/2 →3/2 (81.54 and 86.73 MHz) and the 3/2→5/2 transition (163.03 and 169.32 MHz), in agreement with experiment. The presence of these doublets reflects the different environment experienced by the non-equivalent 127 I nuclei in the γ-phase of CsPbI3, which lie at the c (apical; Ic) and d (equatorial; Id) Wyckoff positions, respectively, of the corresponding Pnam space group.

Figure 3 .

 3 Figure 3. a) Optical absorption, Tauc Plot (inset) b) PL (solid line) and PLE spectra (dashed line) of (FA)3(HEA)2Pb3I11. Excitation wavelength 540nm, emission 740nm. In the inset the emission decay (solid points) recorded at 740nm with three-exponential fit (red solid line) are reported. Fitting parameters: (0.2845) 3.707ns; (0.6694) 0.803ns; (0.0460) 29.76ns; χ 2 =1.31081.

  DFT electronic structure calculations are carried out including SOC. The resulting band structure shows direct band gap at the D point of the Brillouin zone (Figure 4a, b). The shift of the valence band/conduction band (VB/CB) edges from  reflects the presence of only one PbI6 octahedron per unit cell along the bcrystallographic axis, which does not allow to fold back the anti-bonding I(p)-Pb(s) and I(s)-Pb(p)

Figure 4 .

 4 Figure 4. a) DFT-SOC band structure computed for (FA)3(HEA)2Pb3I11 both considering the Cs atoms in place of the FA and HEA (Cs) and considering the organic moieties explicitly (FA+HEA); b) corresponding first Brillouin zone with high symmetry points; c) isosurfaces computed for the wave-functions associated to the valence (VBE) and conduction band edges (CBE).Threshold for the isosurface is set to 3 10 -4 e/bohr 3 ; d) structure and band dispersions along the stacking direction for the currently investigated <1 1 0 >-oriented m=3 layered perovskites and for reference <1 0 0>-oriented n=3 layered perovskites. The latter structure is taken from Ref.[START_REF] Mao | Hybrid Dion-Jacobson 2D Lead Iodide Perovskites[END_REF]

For Table of Contents Use Only

  GENCI. C.Q. is FNRS research associate. (9) Papavassiliou, G. C. Three-and Low-Dimensional Inorganic Semiconductors. Prog. Solid State Chem. 1997, 25, 125-270. (10) Mercier, N.; Poiroux, S.; Riou, A.; Batail, P. Unique Hydrogen Bonding Correlating with a Reduced Band Gap and Phase Transition in the Hybrid Perovskites (HO(CH2)2NH3)2PbX4 (X = I, Br). Inorg. Chem. 2004, 43, 8361-8366 (11) Lemmerer, A; Billing, D. G. Lead halide inorganic-organic hybrids incorporating diammonium cations. CrystEngComm, 2012, 14, 1954-1966. (12) Shao, S.; Loi, M. A. Advances and Prospective in Metal Halide Ruddlesen-Popper Perovskite Solar Cells. Adv. Energy Mater. 2021, 2003907. Smith, M. D.; Connor, B. A.; Karunadasa, H. I. Tuning the Luminescence of Layered Halide Perovskites. Chem. Rev. 2019, 119, 3104-3139 (17) Li, X.; Hoffman, J. M.; Kanatzidis, M. G. The 2D Halide Perovskite Rulebook: How the Spacer Influences Everything from the Structure to Optoelectronic Device Efficiency. Chem. Saparov, B.; Mitzi, D. B. Organic-Inorganic Perovskites: Structural Versatility for Functional Materials Design. Chem. Rev. 2016, 116, 4558-4596 Authors Maroua Ben Haj Salah, Nicolas Mercier, Magali Allain, Antonin Leblanc, Jens Dittmer, Chiara Botta, Claudio Quarti and Claudine Katan

	Rev. 2021, 121, 2230-2291. Synthesis and characterization of (FA)3(HEA)2Pb3I11: a rare example of <1 1 0>-oriented multilayered halide perovskite. TOC graphic (18) Manuscript title Synopsis

(13) Zhang, F.; Lu, H.; Tong, J.; Berry, J. J.; Beard, M. C.; Zhu, K. Advances in Two-Dimensional Organic-Inorganic Hybrid Perovskites. Energy Environ. Sci. 2020, 13, 1154-1186. (14) Grancini, G.; Nazeeruddin, M. K. Dimensional Tailoring of Hybrid Perovskites for Photovoltaics. Nat. Rev. Mater. 2019, 4, 4-22 (15) Mao, L.; Stoumpos, C. C.; Kanatzidis, M. G. Two-Dimensional Hybrid Halide Perovskites: Principles and Promises. J. Am. Chem. Soc. 2019, 141, 1171-1190 (16)

C NMR was used to obtain further structural information, especially related to the location of the organic cations. While 13 C NMR is usually done as cross-polarization, this technique is less

Unfortunately, this new m=3 member faces stability issues under pressure and humidity, although its stability against moisture is improved compared to MAPbI3. All these findings pave the path towards new efforts to design/synthesize novel high m-value members with increased stability and improved carrier mobilities, namely with eclipsed interlayer packing.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge.

Experimental details for synthesis, additional structural, NMR and calorimetry data.

X-ray crystallographic data of (FA)2[(HEA)2(FA)Pb3I11].

AUTHOR INFORMATION

Corresponding Author *nicolas.mercier@univ-angers.fr ; * claudine.katan@univ-rennes1.fr

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT

At ISCR and MOLTECH-Anjou the work was mainly supported by the Agence Nationale pour la Recherche (MORELESS project). For DFT calculations, this work was granted access to the