High-pressure Powder X-ray Diffraction Experiments Research Articles

Elastic softening of bulk modulus of monoclinic HfO2 under high pressure

In this study, the lattice compression properties of monoclinic HfO2 (m-HfO2) at room temperature are investigated by performing high-pressure powder x-ray diffraction experiments. The lattice constants are precisely determined from the Rietveld analysis. The linear compressibility of each crystal axis a, b, and c is estimated as 10.0 × 10−3, 3.8 × 10−3, and 12.7 × 10−3/GPa, respectively, indicating a strong anisotropy. The equation of state for m-HfO2 is obtained using the third-order Birch–Murnaghan equation based on the volume and pressure data. The obtained unit cell volume (V0), bulk modulus (B0), and its pressure derivative (B0′) at ambient pressure are 138.3(1) Å, 195(2) GPa, and –5.4(5), respectively. The negative value of B0′ implies the bulk modulus softening with an increase in pressure. The fact that the elastic softening has also been observed in m-ZrO2 suggests that the abnormal behavior is a characteristic of the monoclinic structure (P21/c). The transition to the orthorhombic phase is observed at approximately 6.4 GPa. The transition is sluggish, and two phases coexist at a pressure of up to 10 GPa.

Mechanical and acoustic properties of a hybrid organic–inorganic perovskite, TMCM-CdCl3, with large piezoelectricity

Piezoelectric hybrid organic–inorganic perovskites have attracted significant attention due to their outstanding piezoelectricity comparable to perovskite oxides. However, their elastic properties, which determine the piezoelectric performance, have not yet been explored. Here, the elastic properties of a perovskite ferroelectric TMCM-CdCl3 (TMCM = trimethylchloromethyl ammonium) with a large piezoelectric effect are comprehensively investigated. High-pressure synchrotron powder x-ray diffraction experiments reveal that it exhibits a low bulk modulus of 14.58 GPa, which is an order of magnitude lower than those of perovskite oxides. In addition, Young’s moduli and shear moduli were calculated using density functional theory, and their values are significantly smaller than perovskite piezoceramics. Furthermore, the acoustic properties of TMCM-CdCl3 were calculated. Its longitudinal wave velocity is 3.86 km s−1–4.94 km s−1, and the corresponding specific acoustic impedance is 8.28 MRayl–10.21 MRayl. These maximum values are, respectively, 31.5%, 74.4%, and 8.4%, 73.1% lower than those from BaTiO3 and PbTiO3. Our findings disclose the soft nature and low acoustic impedance of hybrid perovskite piezoelectrics, which are critical to their applications in energy conversion devices.

The Zinc Story under High Pressure

A historical review is presented on the experimental and theoretical studies on Zn under high pressure. Based on our high-pressure powder x-ray diffraction experiments that have been done for nearly a decade, we describe the structural change of Zn up to 126 GPa at room temperature. Although several experimental and theoretical studies indicated an anomalous change of the c/a axial ratio with pressure, we found no such an anomaly within our experimental uncertainty. Our high-pressure low-temperature experiments up to 18 GPa at 40 K also gave no evidence of the c/a anomaly. We suspect that the pressure-transmitting media played an important role in producing the anomaly. The structural anisotropy of Zn is drastically reduced at high pressures, which would be a general trend for hexagonal close-packed (hcp) metals.

Tuning the Mechanical Response of Metal-Organic Frameworks by Defect Engineering.

The incorporation of defects into crystalline materials provides an important tool to fine-tune properties throughout various fields of materials science. We performed high-pressure powder X-ray diffraction experiments, varying pressures from ambient to 0.4 GPa in 0.025 GPa increments to probe the response of defective UiO-66 to hydrostatic pressure for the first time. We observe an onset of amorphization in defective UiO-66 samples around 0.2 GPa and decreasing bulk modulus as a function of defects. Intriguingly, the observed bulk moduli of defective UiO-66(Zr) samples do not correlate with defect concentration, highlighting the complexity of how defects are spatially incorporated into the framework. Our results demonstrate the large impact of point defects on the structural stability of metal-organic frameworks (MOFs) and pave the way for experiment-guided computational studies on defect engineered MOFs.

Elastic properties and thermal expansion of lead-free halide double perovskite Cs2AgBiBr6

The elastic properties and thermal expansion behavior of lead-free halide double perovskite, Cs2AgBiBr6, were studied first with the aid of first principles calculations, which were followed by experimental characterization. The calculated full elastic constants enable the complete mapping of Young’s and shear moduli, and Poisson’s ratios of Cs2AgBiBr6 along all crystallographic orientations. Results show that significant anisotropy in them despite the fact that Cs2AgBiBr6 possesses cubic symmetry. Detailed structural analysis and a spring model were utilized to rationalize the observed anisotropy. Further, the theoretical results were validated by recourse to nanoindentation and high-pressure synchrotron powder X-ray diffraction experiments. The superior mechanical behavior of Cs2AgBiBr6 over the hybrid bromide analogues, MAPbBr3 (MA=CH3NH3+), were attributed to the higher framework stiffness of the former, which is a result of the presence of relatively stronger Ag-Br and Bi-Br bonds in it. Variable temperature single crystal X-ray diffraction reveal linear and volumetric thermal expansion coefficients to be about 20% lower than those of MAPbBr3, again suggesting its higher framework rigidity. The superior stiffness, thermal expansion behavior, moisture stability, and nontoxicity of Cs2AgBiBr6 make it a strong candidate for potential applications in optoelectronics and photovoltaics.

High-pressure structural study of solid mercury up to 200 GPa

High-pressure powder x-ray diffraction experiments have been carried out on solid mercury up to 196 GPa at room temperature. The hexagonal-close-packed δ phase, which exists above 36 GPa, is found to remain stable up to the highest pressure investigated. The c/a axial ratio monotonously decreases under pressure in a manner similar to that of zinc and cadmium. There is no anomaly in the pressure dependence of the c/a axial ratio. We present complete structural data of β, γ, and δ phases of solid mercury, which constrain the PV equation of state at 300 K.

Crystal structures and stabilities of cristobalite-helium phases at high pressures

First-principles calculations were used to study the structural and energetic properties of cristobalite-He I and II at high pressures, both of which were recently found in high-pressure powder X-ray diffraction experiments of α-cristobalite with helium pressure-medium at room temperature. These calculations have revealed that both cristobalite-He I and II contain one helium atom per SiO 2 with the formula SiO 2 He. It has also been revealed that cristobalite-He I is energetically favored above 6.4 GPa, cristobalite-He II is the stable phase at pressures between 1.7 and 6.4 GPa, and the mixture of cristobalite II and crystalline He is more stable than either cristobalite-He I or II below 1.7 GPa, in general agreement with the observation. Cristobalite-He I and II have been predicted to be monoclinic with space group P 2 1 / c , and rhombohedral with space group R 3 c , respectively. The unit-cell parameters of both cristobalite-He I and II were re-determined from the previously measured high-pressure X-ray diffraction data on the basis of these predicted cells. There is an excellent agreement between the observed (re-determined) and calculated pressure dependence of the cell parameters for the both phases. The calculated X-ray diffraction patterns for both cristobalite-He I and II are also consistent with the observed data. Cristobalite-He I and II have been predicted to have molar volumes 21% larger at 10 GPa and 23% larger at 4 GPa than cristobalite II due to the penetration of helium atoms into large voids of the structure.

Na-Au intermetallic compounds formed under high pressure at room temperature

High-pressure powder x-ray diffraction experiments have revealed that sodium and gold react at room temperature and form new Na-Au intermetallic compounds under high pressure. We have identified four intermetallic phases up to 60 GPa. The first phase (phase I) is the known Na2Au with the tetragonal CuAl2-type structure. It changed to the second phase (phase II) at about 0.8 GPa, which has the composition Na3Au with the trigonal Cu3As-type or hexagonal Cu3P-type structure. Phase II further transformed to phase III at 3.6 GPa. Phase III has the same composition Na3Au with the cubic BiF3-type structure. Finally, phase III changed to phase IV at around 54 GPa. Phase IV gives broad diffraction peaks, indicating large structural disorder.

High-pressure structural study of yttrium monochalcogenides from experiment and theory

High-pressure powder x-ray diffraction experiments using synchrotron radiation are performed on the yttrium monochalcogenides YS, YSe, and YTe up to a maximum pressure of 23 GPa. The ambient NaCl structure is stable throughout the pressure range covered. The bulk moduli are determined to be 93, 82, and 67 GPa for YS, YSe, and YTe, respectively. First-principles total energy calculations are carried out using the full-potential linear muffin-tin orbital method. The calculated and measured lattice constants and bulk moduli are in good agrement. Under applied pressure, the yttrium monochalcogenides are predicted to undergo a structural transition. Assuming that the high-pressure phase corresponds to the CsCl crystal structure, transition pressures of 53, 36, and 14 GPa are found for YS, YSe, and YTe, respectively.

High-pressure structural study of fluoro-perovskiteCsCdF3up to 60 GPa: A combined experimental and theoretical study

The structural behaviour of CsCdF3 under pressure is investigated by means of theory and experiment. High-pressure powder x-ray diffraction experiments were performed up to a maximum pressure of 60 GPa using synchrotron radiation. The cubic $Pm\bar{3}m$ crystal symmetry persists throughout this pressure range. Theoretical calculations were carried out using the full-potential linear muffin-tin orbital method within the local density approximation and the generalized gradient approximation for exchange and correlation effects. The calculated ground state properties -- the equilibrium lattice constant, bulk modulus and elastic constants -- are in good agreement with experimental results. Under ambient conditions, CsCdF3 is an indirect gap insulator with the gap increasing under pressure.

Pressure induced phase transformation of Ba8Ga16Ge30 clathrate studied by x-ray diffraction and Raman spectroscopy

The phase transition and the vibrational properties of Ba8Ga16Ge30 have been investigated at high pressures up to 40 GPa at room temperature. The combined study of the high-pressure Raman and synchrotron powder x-ray diffraction (XRD) experiments revealed the occurrence of a first-order phase transition at 33 GPa, on which a volume decrease of about 3% was found. Rietveld refinements of the XRD data demonstrated the atomic displacements precursory to the phase transition, allowing us to discuss the mechanism of the phase transition. In the Raman experimental data, anomalies were observed in the spectral feature and the guest vibration around 17 GPa. By combining the Raman results with the XRD ones, the vibrational frequency of the guest Ba was investigated as a function of the host cage size. As a result, a linear relation between guest vibrational frequency and the guest-host distance was identified.

High-pressure powder x-ray diffraction experiments andab initiocalculation of Ti3AlC2

The structural stability of the layered ternary carbide Ti3AlC2 was studied up to 35 GPa using x-ray diffraction with a Merrill–Basset-type diamond anvil cell and ab initio calculations. The structure (P63/mmc) was stable in the present pressure range without any phase transition. The Birch–Murnaghan equation of state was employed to fit the experimental pressure-volume date, from which the isothermal bulk modulus of Ti3AlC2 was determined as 156±5 GPa, which was also supported by theoretical results. In addition, theoretical calculations described anisotropic pressure dependences of the lattice parameters, electronic structure, and bonding properties of Ti3AlC2.

High-pressure structural, elastic, and electronic properties of the scintillator host materialKMgF3

The high-pressure structural behaviour of the fluoroperovskite KMgF_3 is investigated by theory and experiment. Density functional calculations were performed within the local density approximation and the generalized gradient approximation for exchange and correlation effects, as implemented within the full-potential linear muffin-tin orbital method. In situ high-pressure powder x-ray diffraction experiments were performed up to a maximum pressure of 40 GPa using synchrotron radiation. We find that the cubic Pm\bar{3}m crystal symmetry persists throughout the pressure range studied. The calculated ground state properties -- the equilibrium lattice constant, bulk modulus and elastic constants -- are in good agreement with experimental results. By analyzing the ratio between the bulk and shear modulii, we conclude that KMgF_3 is brittle in nature. Under ambient conditions, KMgF_3 is found to be an indirect gap insulator with the gap increasing under pressure.

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.

Bulk modulus of osmium: High-pressure powder x-ray diffraction experiments under quasihydrostatic conditions

Bulk modulus of osmium: High-pressure powder x-ray diffraction experiments under quasihydrostatic conditions

Incommensurately Modulated Phase of Iodine Under High Pressure

High-pressure powder x-ray diffraction experiments have been carried out on iodine at room temperature with a helium-pressure medium. A new high-pressure phase has been found in the pressure range 24–28 GPa, in the close vicinity of the molecular dissociation. The structure is incommensurately modulated, which we interpret as a transient state between molecular and monatomic ones.

Pressure-induced structural phase transition in fullerides doped with rare-earth metals

Rare-earth-metal-doped fullerides with nominal composition of ${R}_{3}{\mathrm{C}}_{70}$ $(R=\mathrm{Sm},$ Eu) adopt a pseudomonoclinic structure in which ${\mathrm{C}}_{70}$ dimers glued with rare-earth ions are involved. High-pressure powder x-ray diffraction experiments revealed that these compounds undergo a reversible first-order structural phase transition at 1.5 GPa, associated with 2.7%--2.9% reduction of the unit cell volume. Structural analyses showed that the rare-earth ions, which are located close to the edge of tetrahedral sites at ambient pressure, move back to the center of the tetrahedral sites. Simultaneously, ${\mathrm{C}}_{70}$ molecules are realigned so that the fivefold (long) axes are perpendicular to the $(101\ifmmode\bar\else\textasciimacron\fi{})$ or $(111\ifmmode\bar\else\textasciimacron\fi{}{)}_{\mathrm{fcc}}$ plane at high pressure. The derived charge density map indicates that the transition is regarded as a structural change from dimers to three-dimensional polymers of fullerenes. These features are ascribed to the unique bonding nature in rare-earth ${\mathrm{C}}_{70}$ compounds.

High-pressure powder x-ray diffraction experiments on Zn at low temperature

High-pressure powder x-ray diffraction experiments have been performedon Zn with a He-pressure medium at low temperature. When thesample was compressed in the He medium at low temperature, largenonhydrostaticity developed, yielding erroneous lattice parameters. On theother hand, when the pressure was changed at high temperatures, goodhydrostaticity was maintained. No anomaly in the volume dependence of thec/aaxial ratio has been found.

低温高圧下での物性測定技術 ヘリウム圧力媒体を用いた高圧力Ｘ線回折実験

Helium is the best hydrostatic pressure-transmitting medium. A gas-loading system has been constructed to load diamond-anvil cells with high-pressure helium gas at room temperature. In this article, the gas-loading system and high-pressure powder x-ray diffraction experiments with a helium-pressure medium are reviewed.

High-Pressure X-ray Studies of Zn at Room and Low Temperatures with a He-Pressure Medium

High-pressure powder X-ray diffraction experiments have been done on Zn with a He-pressure medium at room and low temperatures. No anomaly in the c / a axial ratio has been found even at low temperatures. This implies that the c / a anomaly, which is predicted to appear associated with the electronic topological transitions, would be extremely small.