Structural insight into the cooperativity of spin crossover compounds.

Spin-crossover (SCO) compounds are promising materials for a wide variety of industrial applications. However, the fundamental understanding of their nature of transition and its effect on the physical properties are still being fervently explored; the microscopic knowledge of their transition is essential for tailoring their properties. Here an attempt is made to correlate the changes in macroscopic physical properties with microscopic structural changes in the orthorhombic and monoclinic polymorphs of the SCO compound Fe(PM-Bia)2(NCS)2 (PM = N-2'-pyridylmethylene and Bia = 4-aminobiphenyl) by employing single-crystal X-ray diffraction, magnetization and DSC measurements. The dependence of macroscopic properties on cooperativity, highlighting the role of hydrogen bonding, π-π and van der Waals interactions is discussed. Values of entropy, enthalpy and cooperativity are calculated numerically based on the Slichter-Drickamer model. The particle size dependence of the magnetic properties is probed along with the thermal exchange and the kinetic behavior of the two polymorphs based on the dependence of magnetization on temperature scan rate and a theoretical model is proposed for the calculation of the non-equilibrium spin-phase fraction. Also a scan-rate-dependent two-step behavior observed for the orthorhombic polymorph, which is absent for the monoclinic polymorph, is reported. Moreover, it is found that the radiation dose from synchrotron radiation affects the spin-crossover process and shifts the transition region to lower temperatures, implying that the spin crossover can be tuned with radiation damage.

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

The crystal structure of ß-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 ß-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 ß-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)].

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

Piezoelectric fluorides of the composition BaMF(4) (M = Mg, Zn, Mn) have been studied in situ at high pressures in diamond anvil cells with single-crystal x-ray diffraction and Raman spectroscopy. All three compounds crystallize in the acentric space group Cmc 2(1) at ambient pressure. BaMgF(4) undergoes a reversible second order phase transition to the paraelectric phase (space group Cmcm) at pressures between 5 and 6 GPa. BaZnF(4) undergoes a reversible first order phase transition to a monoclinic phase (space group P 11n). Both high- and low-pressure polymorphs coexist in the pressure range 5-7 GPa. BaMnF(4) maintains the Cmc 2(1) structure up to pressures of 4 GPa. Above this pressure the diffraction signal decreases rapidly and at 6 GPa no diffraction signal could be detected in our experiment. The compound does not recover its crystallinity on decompression. A comparison of the effects of external and chemical pressure is presented.

(NH4)2WTe2O8 at 5.09 GPa: a single-crystal study using synchrotron radiation.

The polar crystal structure of diammonium [octaoxidoditellurato(IV)]tungstate, (NH(4))(2)WTe(2)O(8), was studied at high pressures using single-crystal X-ray diffraction in a diamond-anvil cell at the HASYLAB synchrotron (DESY, Hamburg, Germany). No phase transition was observed up to 7.16 GPa. However, a full structure determination at 5.09 GPa shows that the coordination number of one of the two non-equivalent Te atoms has decreased from four to three.

Crystal structure and stability of Tl2CO3 at high pressures.

The crystal structure of dithallium carbonate, Tl(2)CO(3) (C2/m, Z = 4), was investigated at pressures of up to 7.4 GPa using single-crystal X-ray diffraction in a diamond anvil cell. It is stable to at least 5.82 GPa. All atoms except for one of the O atoms lie on crystallographic mirror planes. At higher pressures, the material undergoes a phase transition that destroys the single crystal.

Tl(2)CO(3) at 3.56 GPa.

The crystal structure of thallium carbonate, Tl(2)CO(3) (C2/m, Z = 4), is stable at least up to 3.56 GPa, as demonstrated by hydrostatic single-crystal X-ray diffraction measurements in a diamond anvil cell at room temperature. Our results contradict earlier observations from the literature, which found a structural phase transition for this compound at about 2 GPa. Under atmospheric conditions, all atoms except for one O atom reside on the mirror plane in the high-pressure structure. The compression mainly affects the part of the structure where the nonbonded electron lone pairs on the Tl(+) cations are located.

Advances in data reduction of high-pressure x-ray powder diffraction data from two-dimensional detectors: a case study of schafarzikite (FeSb(2)O(4)).

Methods have been developed to facilitate the data analysis of multiple two-dimensional powder diffraction images. These include, among others, automatic detection and calibration of Debye-Scherrer ellipses using pattern recognition techniques, and signal filtering employing established statistical procedures like fractile statistics.All algorithms are implemented in the freely available program package Powder3D developed for the evaluation and graphical presentation of large powder diffraction data sets.As a case study, we report the pressure dependence of the crystal structure of iron antimony oxide FeSb(2)O(4) (p≤21 GPa, T = 298 K) using high-resolution angle dispersive x-ray powder diffraction. FeSb(2)O(4) shows two phase transitions in the measured pressure range. The crystal structures of all modifications consist of frameworks of Fe(2+)O(6) octahedra and irregular Sb(3+)O(4) polyhedra. At ambient conditions, FeSb(2)O(4) crystallizes in space group P4(2)/mbc (phase I). Between p = 3.2 GPa and 4.1 GPa it exhibits a displacive second order phase transition to a structure of space group P 2(1)/c (phase II, a = 5.7792(4) Å, b = 8.3134(9) Å, c = 8.4545(11) Å, ß = 91.879(10)°, at p = 4.2 GPa). A second phase transition occurs between p = 6.4 GPa and 7.4 GPa to a structure of space group P4(2)/m (phase III, a = 7.8498(4) Å, c = 5.7452(5) Å, at p = 10.5 GPa). A nonlinear compression behaviour over the entire pressure range is observed, which can be described by three Vinet equations in the ranges from p = 0.52 GPa to p = 3.12 GPa, p = 4.2 GPa to p = 6.3 GPa and from p = 7.5 GPa to p = 19.8 GPa. The extrapolated bulk moduli of the high-pressure phases were determined to K(0) = 49(2) GPa for phase I, K(0) = 27(3) GPa for phase II and K(0) = 45(2) GPa for phase III. The crystal structures of all phases are refined against x-ray powder data measured at several pressures between p = 0.52 GPa, and 10.5 GPa.

Breakdown of intermediate-range order in liquid GeSe(2) at high pressure.

Studies of liquids with tetrahedral coordination, particularly during compression or quenching, have indicated the existence of distinct phases in the liquid state, distinguishable by density and local structure. In systems that exhibit critical phenomena in the supercooled state, anomalous behaviour of the compressibility is also anticipated above the critical point, as revealed by simulations of water. Liquid GeSe(2) is a potentially attractive system for studying both types of phenomena, given its two-dimensional tetrahedral structure and anomalous physical properties (including a density minimum near its melting point). Here we report in situ X-ray diffraction measurements of solid and liquid GeSe(2) at high temperature and high pressure, revealing that the structure of the liquid is sensitive to pressure and that anomalous compressibility is expected. During compression of liquid GeSe(2), the connectivity of the liquid changes from two- to three-dimensional, leading to a breakdown of the intermediate-range order. The gradual change in structure above the melting line may develop to a first-order liquid-liquid transition in the supercooled regime.

Pressure-induced quenching of the Jahn-Teller distortion and insulator-to-metal transition in LaMnO(3).

LaMnO(3) was studied by synchrotron x-ray diffraction, optical spectroscopies, and transport measurements under pressures up to 40 GPa. The cooperative Jahn-Teller (JT) distortion is continuously reduced with increasing pressure. There is strong indication that the JT effect and the concomitant orbital order are completely suppressed above 18 GPa. The system, however, retains its insulating state to approximately 32 GPa, where it undergoes a bandwidth-driven insulator-metal transition. Delocalization of electron states, which suppresses the JT effect but is insufficient to make the system metallic, appears to be a key feature of LaMnO(3) at 20-30 GPa.

Reversible phase transitions in Na2S under pressure: a comparison with the cation array in Na2SO4.

The structural behavior of the antifluorite Na(2)S, disodium sulfide, has been studied under pressure up to 22 GPa by in situ synchrotron X-ray diffraction experiments in a diamond anvil cell at room temperature. At approximately 7 GPa, Na(2)S undergoes a first phase transition to the orthorhombic anticotunnite (PbCl(2)) structure (Pnma, Z = 4). The lattice parameters at 8.2 GPa are a = 6.707 (5), b = 4.120 (3), c = 8.025 (4) A. At approximately 16 GPa, Na(2)S undergoes a second transition adopting the structure of the Ni(2)In-type (P6(3)/mmc, Z = 2). The lattice parameters at 16.6 GPa are a = 4.376 (18), c = 5.856 (9) A. Both pressure-induced phases have been confirmed by full Rietveld refinements. An inspection of the cation array of Na(2)SO(4) reveals that its Na(2)S subarray is also of the Ni(2)In-type. This feature represents a new example of how the cation arrangements in ternary oxides correspond to the topology of the respective binary compounds. We discuss analogies between the insertion of oxygen and the application of pressure.