High-pressure X-ray Powder Diffraction Research Articles

Equation of state of the high-pressure polymorph of FeSi to 67 GPa

The compressibility of high-pressure phase FeSi was investigated by in situ high-pressure X-ray powder diffraction. Pressure up to 67 GPa was generated using the diamond anvil cell technique. FeSi transformed to a B2-type structure during laser heating, and remained stable up to the maximum pressure. This first-order phase transformation showed a volume reduction of 4 % at 25 GPa. The fit of a Birch-Murnaghan equation-of-state to the pressure-volume data resulted in V 0 = 21.32 (3) A 3 and K 0 = 225 (2) GPa when K 0 ’ was fixed at 4. Our results are in good agreement with those of previous numerical studies using a GGA method employing ab initio calculations. This indicates that a small amount of B2-type FeSi phase may contribute to the decrease in seismic velocity at the base of the lower mantle.

Comparative high-pressure study and chemical bonding analysis of Rh 3Bi 14 and isostructural Rh 3Bi 12Br 2

The binary compound Rh 3Bi 14 was synthesized from the elements. The compound is isostructural with Rh 3Bi 12Br 2, crystallizes with the orthorhombic space group Fddd (no. 70) and lattice parameters a=6.8959(15) Å, b=17.379(3) Å, c=31.758(6) Å. The crystal structure consists of a three-dimensional (3D) framework of edge-sharing cubes and square antiprisms (RhBi 8/2). It is closely related to the intermetallic compound RhBi 4, in which two Y-like frameworks of antiprisms interpenetrate. In Rh 3Bi 14 and Rh 3Bi 12Br 2, additional bismuth and bromine anions, respectively, fill the channels of the 3D polyhedral framework formed by covalently bonded rhodium and bismuth atoms. High-pressure X-ray powder diffraction data from synchrotron measurements of Rh 3Bi 14 and Rh 3Bi 12Br 2 indicate a high stability of both compounds in the investigated range from ambient pressure to ca. 30 GPa at ambient temperature.

Pressure-induced phase transformations in theBa8Si46clathrate

The nature of isostructural transformations of a type-I ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{46}$ clathrate has been studied by in situ high-pressure angle-dispersive x-ray powder diffraction using liquid He as pressure transmitting medium. The good quality of the diffraction data permitted refinement of structural and thermal parameters from Rietveld analysis. The results show that the first transition at $7\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ is caused by the displacement of the Ba atoms in the ${\mathrm{Si}}_{24}$ cages. The cause of the second transition at $15\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, characterized by a dramatic reduction of cell volume, is not as clear. Theoretical calculations predicted an electronic topological transition of Fermi surface at this pressure. Analysis of the anomalously large Si thermal parameters suggested a highly disordered Si framework. This disordering is probably static and may be due to the presence of Si vacancies. The latter hypothesis is supported by electronic calculations on model disordered systems.

Electronic structure and magnetic properties of cubic and hexagonalSrMnO3

$\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$ is a rare example of a compound having both a cubic (high-temperature) and a hexagonal (low-temperature) perovskite polymorph. While the former is built from corner-sharing $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedra only, the latter contains corner-sharing confacial bioctahedral ${\mathrm{Mn}}_{2}{\mathrm{O}}_{9}$ entities along the $c$ axis. The electronic and magnetic structures of both polymorphs are investigated by density functional theory. Both the cubic and the hexagonal polymorphs are insulators at $0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ but with quite different band gaps (0.3 vs $1.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$). The hexagonal ground state shows antiferromagnetic coupling both within the ${\mathrm{Mn}}_{2}{\mathrm{O}}_{9}$ entities and between the Mn ions in the corner-sharing octahedra. The lowest energy cubic configuration is found to be $G$-type antiferromagnetic and is $260\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ per formula unit higher in energy than the hexagonal ground-state structure. While the bonding interactions involving Sr are found to be mainly ionic, there is a significant covalent contribution to the Mn-O bond. This covalency is very important for the stabilization of the hexagonal structure compared to the cubic polymorph. Two additional factors that stabilize hexagonal $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$ relative to the cubic polymorph are identified. (i) the Mn atoms in the face-sharing octahedra are displaced along the $c$ axis by about $0.012\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ from the center of the octahedra. (ii) The charge transfer giving lower charges for the oxygen in the face-sharing triangle compared to the corner-sharing oxygen. The latter effect results in a contraction of the oxygen triangle in the shared face. This negatively charged oxygen triangle effectively shields the repulsive interaction between the manganese atoms in the ${\mathrm{Mn}}_{2}{\mathrm{O}}_{9}$ dimer and facilitates the short Mn-Mn distance present in hexagonal $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$. Hexagonal $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$ is more compressible than cubic $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$, owing to the more open structure. The calculated bulk modulus for hexagonal $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$ is in good agreement with the high-pressure powder x-ray diffraction measurements also reported in the present paper.

STRUCTURAL TRANSFORMATION IN PbS AND HgS NANOCRYSTALS UNDER HIGH PRESSURE

The phase transition induced by pressure in the PbS and HgS nanocrystals was studied by in situ high pressure X-ray powder diffraction measurements. PbS underwent a phase transition from B1-B16 at 7 GPa whereas no transition was observed in HgS up to 15 GPa. The transition remained unaltered on the release of pressure. The volume decreased with an increase in pressure. There is a shift in transition pressure towards the higher side when compared with the bulk materials.

High-pressure equation of state for Nb with a helium-pressure medium: Powder x-ray diffraction experiments

High-pressure powder x-ray diffraction experiments have been carried out on Nb at room temperature up to $134\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ with a He-pressure medium and $145\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ with an alcohol-water pressure medium. Nb remains in the bcc structure to the highest pressure investigated. The bulk modulus and its pressure derivative have been determined from the data with the He medium as ${B}_{0}=168(4)\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and ${B}_{0}^{\ensuremath{'}}=3.4(3)$. No anomaly is found in the equation of state of Nb to be connected with the electronic topological transitions, which were inferred from the anomalies in the superconducting transition temperature under pressure. The origin of the asymmetric broadening of the diffraction peaks is discussed.

The high-pressure behaviour of the 10 Å phase: A spectroscopic and diffractometric study up to 42 GPa

High-pressure Raman and X-ray powder diffraction data of the 10 Å phase are collected at several pressures up to 42 GPa by using a diamond anvil cell. The 10 Å phase remains stable throughout this pressure range at room temperature. Indeed, in the quasi-hydrostatic experiments with compression and decompression paths, the behaviour is completely reversible. Fitting with a third-order Birch-Murnghan equation of state the P– V data yields values of V 0 = 492.9(3), K 0 = 39(3) GPa and K′ = 12.5(8). No significant differences are obtained if the Vinet EoS is used instead. The linear compressibility coefficients of the a, b and β parameters are 1.20(16) 10 − 3 GPa − 1 , 1.72(9) 10 − 3 GPa − 1 and 3.6(7) 10 − 4 GPa − 1 , respectively. For the c lattice parameter, an exponential decay is used to describe the evolution with P: c/ c 0 = 0.876(2) + 0.116e − P/6.7(5) . The Raman frequencies of all lattice modes (low frequency region < 1200 cm − 1) are observed to increase monotonically with increasing pressure and decrease with decompression as well as the OH stretching vibration modes of hydroxyls, at 3626 cm − 1. On the other hand, the stretching bands of the water molecules in the 10 Å phase are found to shift in opposite directions as a result of the different degree of intermolecular hydrogen-bonding between non-equivalent water H atoms and the oxygen atoms of the tetrahedral layer. The different evolution of the two OH stretching bands of the water molecules together with the lattice parameters compressibility is interpreted by a rotation of the water molecules with pressure that leads to a hydrogen bond formation due to the tetrahedral distortion. This distortion is a consequence of the larger compressibility of the octahedral layer with respect to tetrahedral layer, as observed in several phyllosilicates when pressure increases. This mechanism could explain how the pressure may stabilise this phase and give a rationale to the rarity of the natural occurrence at ambient conditions when compared with its production in the laboratory under high P/ T gradients.

Compressibility of the nitridosilicate SrYb[Si4N7] and the oxonitridoaluminosilicates MYb[Si4−x Al x O x N7−x] (x = 2; M = Sr, Ba)

The compressibilities of the nitridosilicate SrYb[Si(4)N(7)] and the oxonitridoaluminosilicates MYb[Si(4-x)Al(x)O(x)N(7-x)] (x = 2; M = Sr, Ba) were investigated by in situ high-pressure X-ray powder diffraction. Pressures up to 42 GPa were generated using the diamond-anvil cell technique. The title compounds are structurally stable to the highest pressure obtained. A fit of a third-order Birch-Murnaghan equation-of-state to the p-V data results in V(0) = 302.91 (6) A(3), B(0) = 176 (2) GPa and B' = 4.4 (2) for SrYb[Si(4)N(7)]; V(0) = 310.4 (1) A(3), B(0) = 161 (2) GPa and B' = 4.6 (2) for SrYb[Si(4-x)Al(x)O(x)N(7-x)]; and V(0) = 317.3 (5) A(3), B(0) = 168 (2) GPa and B' = 4.7 (2) for BaYb[Si(4-x)Al(x)O(x)N(7-x)]. While the linear compressibilities of the a and c axes of BaYb[Si(4-x)Al(x)O(x)N(7-x)] are very similar up to 30 GPa, distinct differences were observed for SrYb[Si(4)N(7)] and SrYb[Si(4-x)Al(x)O(x)N(7-x)], with the c axis being the most compressible axis. In all of the investigated compounds the bulk compressibility is dominated by the compression behaviour of the tetrahedral network, while the size of the substituted cation plays a minor role.

Pressure-Induced Phase Transition in BaTiO3 Nanocrystals

The crystal structure and electric properties of BaTiO3nanocrystals are studied by in situ high-pressure synchrotronradiation x-ray powder diffraction. The phase transition takes placenot only in the samples of BaTiO3 nanocrystals that aretetragonal phase with grain sizes more than 100 nm, but also in thesamples of BaTiO3 nanocrystals that are cubic phase with grainsizes less than 100 nm. The pressures of phase transition are found toincrease with decrease of the grain size from about 4 to 10 GPa forcrystallites ranging from 200 to 10 nm in radius. The bulk moduli arecalculated according to Birch–Murnaghan state equation before andafter the phase transition.

Powder X-ray diffraction study of a new polymorph of Al2(WO4)3

A new polymorph of Al2(WO4)3 is observed from in situ high pressure powder X-ray diffraction (ADXRD) measurements at 3.4 GPa. The ADXRD pattern at 3.4 GPa could be explained based on a monoclinic lattice (space group P21) with unit cell parameters: a=9.5884(24), b=12.5204(38), c=7.8463(33) Å, and β=91.98(2)°.

High-pressure x-ray and neutron powder diffraction study ofPbWO4 and BaWO4 scheelites

The room-temperature high-pressure behaviour ofPbWO4 andBaWO4 scheelites(I41/a,Z = 4) has been studied with synchrotron angle-dispersive x-ray powder diffractionin a diamond anvil cell and time-of-flight neutron powder diffraction in alarge-volume Paris–Edinburgh cell. Full profile Rietveld refinements of theneutron data provide evidence for distinct compressibility mechanisms in thesematerials. The distortion in the anionic array from the ideal fluorite packing inBaWO4 increases upon compression while it decreases inPbWO4.

Structural property of CsCl-type sodium chloride under pressure

The structure and equation of state of CsCl-type sodium chloride have been determined using high-pressure powder X-ray diffraction from 32 to 134 GPa. The CsCl-type phase remains stable over this entire pressure range. Pressure–volume data can be fitted with a Vinet equation of state with K 30 GPa=135.1 GPa, K′ 30 GPa=3.9, and V 30 GPa=27.70 Å 3. The nearest-neighbour distance between sodium and chlorine atoms decreased as pressure increased. Significant discrepancies of nearest-neighbour distance between previous theoretical predictions and this study were observed at pressures higher than 70 GPa.

Equation of state of CaIrO3-type MgSiO3 up to 144 GPa

The structure and equation of state for CaIrO 3 -type MgSiO 3 were determined using high-pressure powder X-ray diffraction from 116 to 144 Gpa. The CaIrO 3 -type phase remained stable over this entire pressure range. At each pressure increment, the sample was heated with a laser to relax the deviatoric stress in the sample. Pressure-volume data could be fitted to the Birch-Murnaghan equation of state with K 0 = 237(1) Gpa using Anderson's gold pressure standard, when K 0 ’ and V 0 were set to 4 and 162.86 Å 3 , respectively. This indicates that the bulk modulus of MgSiO 3 decreases when the phase transition occurs, because the bulk modulus of perovskite-type MgSiO 3 is 250-260 Gpa. The b axis of the unit-cell parameter was more compressive than the a and c axes. The unit-cell volumes at high pressures, observed in high-pressure experiments, were slightly smaller than those predicted by theoretical studies.

Pressure effect on the crystallization behavior of Zr 46.7Ti 8.3Cu 7.5Ni 10Be 27.5 bulk metallic glass

The pressure effect on the crystallization behavior of the bulk metallic glass Zr 46.7Ti 8.3Cu 7.5Ni 10Be 27.5 (Vit4) with a wide supercooled liquid region has been investigated by in situ high-pressure and high-temperature X-ray powder diffraction measurements using synchrotron radiation. It was found that the primary and the final crystallization phases are pressure-dependent. In the studied pressure range of 0 to 4.5 GPa, the crystallization temperature increases little with pressure having a slope of 1.6 K/GPa, whereas, a much higher rate of 19 K/GPa was found for the neighboring alloy, Zr 41Ti 14Cu 12.5Ni 10Be 22.5 (Vit1) by J.Z. Jiang et al. The different crystallization behaviour of Vit4 and Vit1 under pressure could be attributed to the different crystallization mechanisms and pressure-induced phase selections. As the important data of Vit4, the time–temperature transformation (TTT) diagram without and with a pressure of 1.75 GPa was also presented. It was found that thermal stability can be much enhanced by applying pressure due to pressure-induced suppression of atomic mobility.

Mn effect on wurtzite-to-cubic phase transformation in ZnO

Mn doping effect on a wurtzite-to-cubic phase transformation in ZnO has been investigated by in situ high pressure X-ray powder diffraction using synchrotron radiation. Unit cell expansion is clearly observed in Mn-doped ZnO samples. Mn ions sit at Zn site in the wurtzite structure. The onset transition pressure for the wurtzite-to-cubic phase transformation decreases from about 9.5 GPa for pure ZnO to 6 GPa for sintered 2at.% Mn-doped ZnO while the compressibility and volume collapse at transition pressures are not sensitive to the Mn doping in the wurtzite phase. The doping of Mn ions in ZnO increases the onset transition pressure for the cubic-to-wurtzite phase transformation. The results could be explained by a reduction of phase transformation barriers for both transition paths by the Mn doping. The observation of reduction of the wurtzite-to-cubic phase transformation pressure might point out a new direction to synthesize cubic wurtzite phase of ZnO by doping transition element(s).

Low-temperature and high-pressure structural behaviour of NaBi(MoO 4) 2—an X-ray diffraction study

NaBi(MoO 4) 2 has been characterized by single-crystal and powder X-ray diffraction in the temperature and pressure ranges 13–297 K and 0–25 GPa, respectively. The domain structure developing below T c = 241 K proves that NaBi(MoO 4) 2 undergoes a ferroelastic phase transition associated with tetragonal I 4 1 / a to monoclinic I 2 / a symmetry change. The character of the unit cell evolution as a function of temperature indicates a continuous transition with the spontaneous strain as an order parameter. The structural distortion, due to small displacements of Bi 3+ and Na + ions, develops slowly. Therefore the overall changes, as measured in single-crystal diffraction at 110 and 13 K, appear to be subtle. High-pressure powder X-ray diffraction shows that the elastic behaviour is anisotropic, the linear compressibility along the a- and c-axes of the tetragonal unit cell being β a = 2.75 ( 10 ) × 1 0 – 3 and β c = 4.30 ( 10 ) × 1 0 – 3 GPa – 1 , respectively. The cell contraction, stronger along the c-axis, causes the distances between the MoO 4 layers to be shortened. Consequently, the cation migration in the channels formed by MoO 4 tetrahedra becomes hindered, and any symmetry lowering phase transition is not observed up to 25 GPa. The zero-pressure bulk modulus is B 0 = 76 ( 5 ) GPa , and its pressure derivative B 0 ′ = 5.1 ( 5 ) .

Shear Strain inNd0.5Ca0.5MnO3at High Pressures

High-pressure x-ray powder diffraction has been measured on the half doped rare earth manganite Nd0.5Ca0.5MnO3 up to a pressure of 15 GPa. We report the presence of a quantifiable amount of shear distortion of the MnO6 octahedra in Nd0.5Ca0.5MnO3 at high pressures. The lattice strain of Nd0.5Ca0.5MnO3 is minimal at a crossover pressure of p* approximately 7 GPa, with the same lattice strain above and below this pressure achieved by shear and Jahn-Teller-type distortions, respectively. The increase in shear strain with increasing pressure provides a mechanism for the insulating behavior of manganites at high pressures that has not been considered before.

Pressure-induced crystallization in nano-arivillius oxides

The high-pressure X-ray powder diffraction studies were carried out on nano-phase Arivillius oxides Bi 2VO 5 and Bi 2VO 5.5. These oxides are synthesized using the mechanochemical activation method. The oxides prepared by this method are amorphous in nature. High-pressure X-ray powder diffraction was carried out using Mao–Bell-type diamond anvil cell up to 15 GPa. At pressures 5.2 and 7 GPa, the amorphous nanooxides transform into crystals.

The crystal structure of CeRhIn 5 under pressure

High-pressure X-ray (synchrotron) powder diffraction experiments have shown that the quasi-two-dimensional heavy fermion antiferromagnet CeRhIn 5 retains its tetragonal HoCoGa 5-type structure up to 13 GPa both at room ( T=295 K) and low ( T=10 K) temperature. Refined structural parameters reveal unusual behavior at pressures around 4–5 GPa.

Compressibility anomaly and amorphization in the anisotropic negative thermal expansion material [formula omitted] under pressure

We report the results of the high-pressure angle dispersive powder X-ray diffraction measurements obtained up to 31 GPa on the monoclinic and the tetragonal phases of NbOPO 4 . The tetrgonal phase was obtained by quenching the monoclinic phase from 5 GPa and 833 K. The lattice parameters of the monoclinic phase show an anomaly at 0.6 GPa, but the unit cell volume follow an almost linear compressional behavior. The bulk modulus B 0 and its pressure derivative B 0 ′ are estimated to be 66(3) GPa and 1.6(8), respectively, for the monoclinic phase. The irreversible pressure induced amorphization observed at ambient temperature in the monoclinic phase is due to the kinetic hindrance to bond reconstruction that is required for transformation form the monoclinic to the tetragonal phase. In the case of tetragonal NbOPO 4 , the compression is found to be anisotropic.