Régis Gautier

23Na multiple-quantum MAS NMR of the perovskites NaNbO3 and NaTaO3

The distorted perovskites NaTaO3 and NaNbO3 have been studied using 23Na multiple-quantum (MQ) MAS NMR. NaTaO3 was prepared by high temperature solid state synthesis and the NMR spectra are consistent with the expected room temperature structure of the material (space group Pbnm), with a single crystallographic sodium site. Two samples of NaNbO3 were studied. The first, a commercially available sample which was annealed at 900 °C, showed two crystallographic sodium sites, as expected for the room temperature structure of the material (space group Pbcm). The second sample, prepared by a low temperature hydrothermal method, showed the presence of four sodium sites, two of which match the expected room temperature structure and the second pair, another polymorph of the material (space group P21ma). This is consistent with powder X-ray diffraction data which showed weak extra peaks which can be accounted for by the presence of this second polymorph. Density functional theory (DFT) calculations support our conclusions, and aid assignment of the NMR spectra. Finally, we discuss the measured NMR parameters in relation to other studies of sodium in high coordination sites in the solid state.

DFT calculations of quadrupolar solid‐state NMR properties: Some examples in solid‐state inorganic chemistry

This article presents results of first‐principles calculations of quadrupolar parameters measured by solid‐state nuclear magnetic measurement (NMR) spectroscopy. Different computational methods based on density functional theory were used to calculate the quadrupolar parameters. Through a series of illustrations from different areas of solid state inorganic chemistry, it is shown how quadrupolar solid‐state NMR properties can be tackled by a theoretical approach and can yield structural information.

The cluster dication [H6Ru4(C6H6)4]2+ revisited: the first cluster complex containing an intact dihydrogen ligand?

Abstract A low-temperature 1H-NMR study suggests the tetranuclear cluster dication [H6Ru4(C6H6)4]2+ (1) to contain an H2 ligand that undergoes, upon warming of the solution, an intramolecular exchange with the four hydride ligands at the Ru4 framework. Whereas two of the three NMR signals at −120°C in the hydride region show T1 values in the range 200–300 ms, the least deshielded resonance at δ=−17.33 ppm exhibits a T1 value of only 34 ms, characteristic of an H2 ligand. A re-examination of the single-crystal X-ray structure analysis of the chloride salt of 1 supports this interpretation by a short distance of 1.14(0.15) A between two hydrogen atoms coordinated as a HH ligand in a side-on fashion to one of the triangular faces of the Ru4 tetrahedron. The distance between one of the two hydrogen atoms of the H2 ligand and one of the four hydride ligands is also very short [1.33(0.15) A], suggesting an additional H2⋯H interaction. The presence of this H3 unit over one of the three Ru3 faces in 1 may explain the deformation of the Ru4 skeleton from the expected tetrahedral symmetry. Density functional theory (DFT) calculations on 1 indicate a very soft potential energy surface associated with the respective displacement of the three interacting cofacial hydrogen atoms. In accordance with these results, the cluster dication 1 tends to loose molecular hydrogen to form the cluster dication [H4Ru4(C6H6)4]2+ (2). The equilibrium between 1 and 2 can be used for catalytic hydrogenation reactions.

Synthesis, crystal and electronic structures, and electrical properties of Rb2Mo12Se14 containing trioctahedral Mo12 clusters

Abstract The synthesis of the new ternary molybdenum chalcogenide Rb 2 Mo 12 Se 14 is reported. It crystallizes in the trigonal space group R 3 with hexagonal lattice parameters a = 9.538 (2) and c = 24.66 (1) A. The main structural building block of the crystal structure of Rb 2 Mo 12 Se 14 is the Mo 12 Se 14 cluster unit, which is also found in Cs 2 Mo 12 Se 14 . Electrical resistivity measurements performed on a single crystal indicate that Rb 2 Mo 12 Se 14 is a metallic conductor with an unusual temperature dependence of its resistivity. Moreover, a superconducting transition was observed at 2.5 K. Extended Huckel tight-binding (EHTB) calculations have been made and are discussed.

X-ray Characterization, Electronic Band Structure, and Thermoelectric Properties of the Cluster Compound Ag2Tl2Mo9Se11

We report on a detailed investigation of the crystal and electronic band structures and of the transport and thermodynamic properties of the Mo-based cluster compound Ag2Tl2Mo9Se11. This novel structure type crystallizes in the trigonal space group R3̅c and is built of a three-dimensional network of interconnected Mo9Se11 units. Single-crystal X-ray diffraction indicates that the Ag and Tl atoms are distributed in the voids of the cluster framework, both of which show unusually large anisotropic thermal ellipsoids indicative of strong local disorder. First-principles calculations show a weakly dispersive band structure around the Fermi level as well as a semiconducting ground state. The former feature naturally explains the presence of both hole-like and electron-like signals observed in Hall effect. Of particular interest is the very low thermal conductivity that remains quasi-constant between 150 and 800 K at a value of approximately 0.6 W·m(-1)·K(-1). The lattice thermal conductivity is close to its minimum possible value, that is, in a regime where the phonon mean free path nears the mean interatomic distance. Such extremely low values likely originate from the disorder induced by the Ag and Tl atoms giving rise to strong anharmonicity of the lattice vibrations. The strongly limited ability of this compound to transport heat is the key feature that leads to a dimensionless thermoelectric figure of merit ZT of 0.6 at 800 K.

Thermotropic luminescent clustomesogen showing a nematic phase: a combination of experimental and molecular simulation studies.

Octahedral Mo6 nanoclusters are functionalized with two organic ligands containing cyanobiphenyl (CB) units, giving luminescent hybrid liquid crystals (LC). Although the mesogenic density around the bulky inorganic core is constant, the two hybrids show different LC properties. Interestingly, one of them shows a nematic phase, which is particularly rare for this kind of supermolecular system. This surprising result is explained by using large-scale molecular dynamic simulations.

Multinuclear NMR as a tool for studying local order and dynamics in CH3NH3PbX3 (X = Cl, Br, I) hybrid perovskites

We report on 207Pb, 79Br, 14N, 1H, 13C and 2H NMR experiments for studying the local order and dynamics in hybrid perovskite lattices. 207Pb NMR experiments conducted at room temperature on a series of MAPbX3 compounds (MA = CH3NH3+; X = Cl, Br and I) showed that the isotropic 207Pb NMR shift is strongly dependent on the nature of the halogen ions. Therefore 207Pb NMR appears to be a very promising tool for the characterisation of local order in mixed halogen hybrid perovskites. 207Pb NMR on MAPbBr2I served as a proof of concept. Proton, 13C and 14N NMR experiments confirmed the results previously reported in the literature. Low temperature deuterium NMR measurements, down to 25 K, were carried out to investigate the structural phase transitions of MAPbBr3. Spectral lineshapes allow following the successive phase transitions of MAPbBr3. Finally, quadrupolar NMR lineshapes recorded in the orthorhombic phase were compared with simulated spectra, using DFT calculated electric field gradients (EFG). Computed data do not take into account any temperature effect. Thus, the discrepancy between the calculated and experimental EFG evidences the fact that MA cations are still subject to significant dynamics, even at 25 K.

New experimental and theoretical studies on condensed molybdenum chalcogenide cluster compounds

Abstract New theoretical investigations using density functional theory calculations on condensed molybdenum chalcogenide cluster compounds A n − 2 Mo 3n X 3n + 2 (A = alkali metal) suggest that it should be possible to reduce them with several electrons without changing their structural arrangement. Experimental work on several examples such as Cs 2 Mo 12 Se 14 confirms these conclusions.

Density Functional Theory Calculations of 95Mo NMR Parameters in Solid‐State Compounds

The application of periodic density functional theory-based methods to the calculation of (95)Mo electric field gradient (EFG) and chemical shift (CS) tensors in solid-state molybdenum compounds is presented. Calculations of EFG tensors are performed using the projector augmented-wave (PAW) method. Comparison of the results with those obtained using the augmented plane wave + local orbitals (APW+lo) method and with available experimental values shows the reliability of the approach for (95)Mo EFG tensor calculation. CS tensors are calculated using the recently developed gauge-including projector augmented-wave (GIPAW) method. This work is the first application of the GIPAW method to a 4d transition-metal nucleus. The effects of ultra-soft pseudo-potential parameters, exchange-correlation functionals and structural parameters are precisely examined. Comparison with experimental results allows the validation of this computational formalism.

Octahedral niobium cluster-based solid state halides and oxyhalides: effects of the cluster condensation via an oxygen ligand on electronic and magnetic properties

The influences of an oxygen ligand on the structural, magnetic and electronic properties of octahedral niobium cluster-based oxides and oxychlorides are reported. The Nb6 metal cluster is edge-bridged by twelve inner ligands and additionally bonded to six apical ligands to form Nb6Li12La6 units (L = Cl, O) wherein oxygen and chlorine are perfectly ordered. Oxygen favours the interconnection of clusters via double Oi–a/Oa–i bridges in a similar way to the double Si–a/Sa–i bridges found in Chevrel phases based on face capped Mo6Li8La6 units. Periodic density functional theory (DFT) calculations confirm that increasing the number of inner oxygen ligands at the expense of chlorine atoms favours the 14 metal-electron (ME) count per octahedral cluster unit. It is also shown that weak interactions occur between neighbouring clusters. Indeed, magnetic measurements performed on AxNb6Cl12O2 (A = Rb, x = 0.816(8); A = Cs, x = 1) series containing 15-ME species evidence antiferromagnetic interactions at low temperatures. Broken-symmetry DFT calculations of exchange parameters within spin dimer analysis confirm the experimental results.