Structural stability of BaMF4 (M = Mg, Zn and Mn) at high pressures

High-Pressure Structural Phase Transformation of Ferroelectric Bis-benzylammonium Lead Tetrachloride Studied by Raman Spectroscopy and X-ray Diffraction

Hybrid organic-inorganic 2-D perovskite bis-benzylammonium lead tetrachloride (BALC) is a room-temperature ferroelectric semiconductor. A structural phase transformation from the ambient Cmc2₁ structure is evident at 1.8 GPa from the Raman spectra, and this is confirmed by our high-pressure X-ray diffraction studies that point to a centrosymmetric structure Cmcm at 1.7 GPa. The ambient phase is recoverable on decompression. Using density functional theory calculations, we have studied the intermolecular and intramolecular vibrations to get an idea of the structural changes as a function of pressure. The high-pressure transition is identified to be due to a distortion in the PbCl₆ octahedra and a conformation change in the molecule. There are several discontinuities, broadening, and splitting of the Raman bands, corresponding to NH₃ units above 1.8 GPa that point to rearrangements in the hydrogen bond network in the new phase. The ambient structure shows anisotropic compressibility, with a bulk modulus of 14.5 ± 0.33 GPa. As the new phase is a centrosymmetric structure, BALC is expected to lose its ferroelectricity above ∼1.8 GPa.

Pressure-induced non-superconducting phase of β-Na0.33V2O5 and the mechanism of high-pressure phase transitions in β-Na0.33V2O5 and β-Li0.33V2O5 at room temperature

The crystal structure of β-Na0.33V2O5 (C2/m, Z  =  6) has been studied on compression to 19 GPa at room temperature using synchrotron single-crystal diffraction in a diamond anvil cell. The vanadate bronze undergoes a phase transition to a non-superconducting phase at about 12 GPa due to changes of polyhedral connectivities in the vanadate framework and due to ordering of the Na(+) cations. This novel structure (Cm, Z  =  6) is interpreted as an intermediate stage in the sequence of pressure-induced transformations in the β-A0.33V2O5 bronzes (A: Li, Na) at room temperature. This study reveals the close relation between the loss of the two-leg ladder V-V system and non-superconducting state of the β-A0.33V2O5 materials.

A high-pressure single-crystal synchrotron diffraction study of NH4RbTe4O9·2H2O: stability of three different TeO(x) coordination polyhedra

The crystal structure of ammonium rubidium nonaoxotetratellurate(IV) dihydrate has been studied as a function of pressure up to 7.40 GPa. The ambient-pressure structure is characterized by the co-existence of three different Te-O polyhedra (TeO(3), TeO(4) and TeO(5)), which are connected to form layers. NH(4)(+), H(2)O and Rb(+) are incorporated between the layers. Both the Rb1 position, which is located on a twofold axis, and the Rb2 position are partially occupied. The three different types of coordination polyhedra around Te(4+) are stable up to at least 5.05 GPa. No phase transition is observed. The fit of the unit-cell volume as a function of pressure gives a zero-pressure bulk modulus of 34 (1) GPa with a zero-pressure volume of V(0) = 2620 (4) Å(3) [B' = 1.4 (2)].