High-pressure X-ray Powder Diffraction Research Articles

High-pressure structural behavior and elastic properties of Ce2In and Ce3In: A combined in situ synchrotron X-ray diffraction and density functional theory study

Across the cerium (Ce)-indium (In) binary phase diagram, there are five line compounds including Ce3In, Ce2In, Ce3In5, CeIn2 and CeIn3. So far, CeIn3 is the only compound whose high-pressure structural behavior and elastic properties have been studied in detail. In this work, we investigated the compression behavior of Ce3In and Ce2In at pressures up to 26.2 GPa using in situ high-pressure angle dispersive synchrotron powder X-ray diffraction (XRD). Our results indicate that Ce3In retains its Cu3Au-type structure in the pressure range investigated, whereas Ce2In undergoes a phase transformation from the Ni2In-type structure to an unidentified phase at ∼4.6 GPa. Unit-cell parameters as a function of pressure were obtained by Rietveld analysis of XRD data. Unit cell volumes were fitted to a second order Birch-Murnaghan equation of state (EOS). The experimentally determined bulk moduli (K0) of the two cerium indides are 50 ± 3 GPa for Ce3In and 39.2 ± 1.6 GPa for Ce2In. The zero-pressure compressibilites of the a-axis (βa,0) for Ce3In is 6.67 (±0.36) × 10–3 GPa–1 and of the a- and c-axis for Ce2In are βa,0 = 8.85 (±0.39) × 10−3 GPa−1 and βc,0 = 9.26 (±0.51) × 10−3 GPa−1, respectively. These results are in general agreement with those calculated by density functional theory (DFT + U) and open-source machine learning algorithms.

Pressure-Induced Structural Phase Transition of Co-Doped SnO2 Nanocrystals

Co-doped SnO2 nanocrystals (with a particle size of 10 nm) with a tetragonal rutile-type (space group P42/mnm) structure have been investigated for their use in in situ high-pressure synchrotron angle dispersive powder X-ray diffraction up to 20.9 GPa and at an ambient temperature. An analysis of experimental results based on Rietveld refinements suggests that rutile-type Co-doped SnO2 undergoes a structural phase transition at 14.2 GPa to an orthorhombic CaCl2-type phase (space group Pnnm), with no phase coexistence during the phase transition. No further phase transition is observed until 20.9 GPa, which is the highest pressure covered by the experiments. The low-pressure and high-pressure phases are related via a group/subgroup relationship. However, a discontinuous change in the unit-cell volume is detected at the phase transition; thus, the phase transition can be classified as a first-order type. Upon decompression, the transition has been found to be reversible. The results are compared with previous high-pressure studies on doped and un-doped SnO2. The compressibility of different phases will be discussed.

High-Pressure Structural and Thermodynamic Properties of Cerium Orthosilicates (CeSiO4)

Pressure-induced phase transitions from the zircon structure-type (I41/amd) to the scheelite structure type (I41/a) are known for many ternary oxides systems (ABO4). In this work, we present the first high-pressure study on synthetic stetindite (CeSiO4) by a combination of in situ high-pressure synchrotron powder X-ray diffraction up to 36 GPa, implemented with and without dual sided laser heating, and in situ high-pressure Raman spectroscopy up to 43 GPa. Two phase transitions were identified: zircon to a high-pressure low-symmetry (HPLS) phase at 15 GPa and then to a scheelite at 18 GPa. The latter from HPLS scheelite phase was found irreversible; i.e., scheelite is fully quenchable at ambient conditions, as in other zircon-type phases. The bulk moduli (K0) of stetindite, HPLS, and high-pressure scheelite phases were determined, respectively, as 171(5), 105(4), and 221(40) GPa by fitting to a second-order Birch–Murnaghan equation of state. The pressure derivatives of vibrational modes and Grüneisen parameters of the zircon-structured polymorph are similar to those of other orthosilicate minerals. Due to the larger ionic radii of Ce4+, with respect to Zr4+, stetindite was found to possess a softer bulk modulus and undergo the phase transitions at a lower pressure than zircon (ZrSiO4), such observations are consistent with what were found in coffinite (USiO4).

Pressure effect on the magnetism and crystal structure of magnetoelectric metal-organic framework [CH3NH3][Co(HCOO)3

[CH3NH3][Co(HCOO)3] is the first perovskite-like metal-organic framework exhibiting spin-driven magnetoelectric effects. However, the high-pressure tuning effects on the magnetic properties and crystal structure of [CH3NH3][Co(HCOO)3] have not been studied. In this work, alongside ac magnetic susceptibility measurements, we investigate the magnetic transition temperature evolution under high pressure. Upon increasing the pressure from atmospheric pressure to 0.5 GPa, TN (15.2 K) remains almost unchanged. Continuing to compress the sample results in TN gradually decreasing to 14.8 K at 1.5 GPa. This may be due to pressure induced changes in the bond distance and bond angle of the O-C-O superexchange pathway. In addition, by using high pressure powder X-ray diffraction and Raman spectroscopy, we conducted in-depth research on the pressure dependence of the lattice parameters and Raman modes of [CH3NH3][Co(HCOO)3]. The increase in pressure gives rise to a phase transition from the orthorhombic Pnma to a monoclinic phase at approximately 6.13 GPa. Our study indicates that high pressure can profoundly alter the crystal structure and magnetic properties of perovskite type MOF materials, which could inspire new endeavors in exploring novel phenomena in compressed metal-organic frameworks.

Tuning the mechanical properties of dicyanamide-based molecular perovskites

By combining lattice dynamic calculations and high-pressure powder X-ray diffraction, the mechanical properties of a series of dicyanamide-based molecular perovskites are studied.

57Fe Mössbauer spectroscopy and high-pressure structural analysis for the mechanism of pressure-induced unique magnetic behaviour in (cation)[FeIIFeIII(dto)3] (cation = Ph4P and nPrPh3P; dto = 1,2-dithiooxalato).

A mixed-valence iron(II,III) coordination polymer, (Ph4P)[FeIIFeIII(dto)3] (2; Ph4P = tetraphenylphosphonium, dto = 1,2-dithiooxalato), exhibits a thermal hysteresis loop and a low temperature shift of the ferromagnetic phase transition temperature, with increasing pressure. The latter magnetic behaviour can also be observed in a novel compound (nPrPh3P)[FeIIFeIII(dto)3] (3; nPrPh3P = n-propyltriphenylphosphonium). To understand the structural information under pressure, we performed high-pressure powder X-ray diffraction, and the result suggests that there was no structural phase transition for either compound. Considering the 57Fe Mössbauer spectroscopy studies, both 2 and 3 may have a high transition entropy, and this finding is caused by pressure-induced unique magnetic behaviours.

Pressure-induced metallization in the absence of a structural transition in the layered ferromagnetic insulator Cr2Ge2Te6

We report the crystallographic and electrical transport properties of single crystals of the ferromagnetic two-dimensional (2D) material ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ under high pressure. In contrast to previous studies, our high-pressure single-crystal x-ray diffraction under hydrostatic conditions shows prominent anisotropic compressibility in the layered structure of the crystalline $R\overline{3}$ phase of ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ without any structural phase transitions up to 20 GPa. Our data confirm a distinct and irreversible crystalline-amorphous transformation in ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$. The loss of crystallinity starts at 20 GPa; however, the crystalline phase and amorphous state coexist even at the maximum pressure of 31.2 GPa. High-pressure powder x-ray diffraction data and electrical resistivity measurements of ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ using NaCl as the pressure-transmitting medium reveal an insulator-to-metal transition in the absence of a phase transition at \ensuremath{\sim}3.9 GPa; at a considerably lower pressure than the previously reported (7--14 GPa). Density functional theory calculations demonstrate the density of states around the Fermi level are primarily dominated by Cr $3d$ and Te $5p$ states. Hence the large reduction of Cr-Te bond lengths within the ${\mathrm{CrTe}}_{6}$ octahedra under compression is most likely responsible for the band-gap closure. This study clarifies that the phase stability and onset of metallization pressure in the ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ sample are sensitive to the hydrostatic environments and demonstrates how pressure can be used to tune the physical properties of 2D ferromagnetic compounds.

Phase stability and dense polymorph of the BaCa(CO3)2 barytocalcite carbonate.

The double carbonate BaCa(CO3)2 holds potential as host compound for carbon in the Earth’s crust and mantle. Here, we report the crystal structure determination of a high-pressure BaCa(CO3)2 phase characterized by single-crystal X-ray diffraction. This phase, named post-barytocalcite, was obtained at 5.7 GPa and can be described by a monoclinic Pm space group. The barytocalcite to post-baritocalcite phase transition involves a significant discontinuous 1.4% decrease of the unit-cell volume, and the increase of the coordination number of 1/4 and 1/2 of the Ba and Ca atoms, respectively. High-pressure powder X-ray diffraction measurements at room- and high-temperatures using synchrotron radiation and DFT calculations yield the thermal expansion of barytocalcite and, together with single-crystal data, the compressibility and anisotropy of both the low- and high-pressure phases. The calculated enthalpy differences between different BaCa(CO3)2 polymorphs confirm that barytocalcite is the thermodynamically stable phase at ambient conditions and that it undergoes the phase transition to the experimentally observed post-barytocalcite phase. The double carbonate is significantly less stable than a mixture of the CaCO3 and BaCO3 end-members above 10 GPa. The experimental observation of the high-pressure phase up to 15 GPa and 300 ºC suggests that the decomposition into its single carbonate components is kinetically hindered.

Abstimmung des Hochdruckphasenverhaltens hochkomprimierbarer zeolithischer Imidazolatgerüstverbindungen: Vom diskontinuierlichen zum kontinuierlichen Porenverschluss durch Linkersubstitution

AbstractDas Hochdruckverhalten von flexiblen Zeolitic Imidazolate Frameworks (ZIFs) des Typs ZIF‐62 und der chemischen Zusammensetzung M(im)2−x(bim)xwird vorgestellt (M2+=Zn2+, Co2+; im−=Imidazolat; bim−=Benzimidazolat, 0.02≤x≤0.37). Mittels Hochdruck‐Pulverröntgendiffraktion wird gezeigt, dass die Materialien unter einem hydrostatischen Druck von bis zu 4000 bar reversibel von einer offenporigen (open pore,op) in eine geschlossenporige (closed pore,cp) Phase kontrahieren. Eine sukzessive Erhöhung des bim−‐Anteils (x) verstärkt das Netzwerk und führt zu einem höheren Schwellendruck für den Übergang von derop‐ zurcp‐Phase, wohingegen die Volumenkontraktion während des Übergangs abnimmt. Bemerkenswert ist, dass der typische diskontinuierliche Übergang von derop‐ zurcp‐Phase (erster Ordnung) fürx≥0.35 ein ungewöhnlicher kontinuierlicher Übergang wird (zweiter Ordnung). Dies ermöglicht eine kontinuierliche Feinabstimmung des Hohlraumvolumens und der Porengröße des Materials durch Anpassung des Drucks und eröffnet somit neue Möglichkeiten für den Einsatz von MOFs in druckschaltbaren Systemen, Membranen und Aktuatoren.

Tuning the High-Pressure Phase Behaviour of Highly Compressible Zeolitic Imidazolate Frameworks: From Discontinuous to Continuous Pore Closure by Linker Substitution.

The high‐pressure behaviour of flexible zeolitic imidazolate frameworks (ZIFs) of the ZIF‐62 family with the chemical composition M(im)2−x (bim)x is presented (M2+=Zn2+, Co2+; im−=imidazolate; bim−=benzimidazolate, 0.02≤x≤0.37). High‐pressure powder X‐ray diffraction shows that the materials contract reversibly from an open pore ( op ) to a closed pore ( cp ) phase under a hydrostatic pressure of up to 4000 bar. Sequentially increasing the bim− fraction (x) reinforces the framework, leading to an increased threshold pressure for the op ‐to‐ cp phase transition, while the total volume contraction across the transition decreases. Most importantly, the typical discontinuous op ‐to‐ cp transition (first order) changes to an unusual continuous transition (second order) for x≥0.35. This allows finetuning of the void volume and the pore size of the material continuously by adjusting the pressure, thus opening new possibilities for MOFs in pressure‐switchable devices, membranes, and actuators.

High Pressure Brillouin Spectroscopy and X-ray Diffraction of Cerium Dioxide

Simultaneous high-pressure Brillouin spectroscopy and powder X-ray diffraction of cerium dioxide powders are presented at room temperature to a pressure of 45 GPa. Micro- and nanocrystalline powders are studied and the density, acoustic velocities and elastic moduli determined. In contrast to recent reports of anomalous compressibility and strength in nanocrystalline cerium dioxide, the acoustic velocities are found to be insensitive to grain size and enhanced strength is not observed in nanocrystalline CeO. Discrepancies in the bulk moduli derived from Brillouin and powder X-ray diffraction studies suggest that the properties of CeO are sensitive to the hydrostaticity of its environment. Our Brillouin data give the shear modulus, = 63 (3) GPa, and adiabatic bulk modulus, = 142 (9) GPa, which is considerably lower than the isothermal bulk modulus, 230 GPa, determined by high-pressure X-ray diffraction experiments.

Configurational Entropy Driven High-Pressure Behaviour of a Flexible Metal-Organic Framework (MOF).

Flexible metal–organic frameworks (MOFs) show large structural flexibility as a function of temperature or (gas)pressure variation, a fascinating property of high technological and scientific relevance. The targeted design of flexible MOFs demands control over the macroscopic thermodynamics as determined by microscopic chemical interactions and remains an open challenge. Herein we apply high‐pressure powder X‐ray diffraction and molecular dynamics simulations to gain insight into the microscopic chemical factors that determine the high‐pressure macroscopic thermodynamics of two flexible pillared‐layer MOFs. For the first time we identify configurational entropy that originates from side‐chain modifications of the linker as the key factor determining the thermodynamics in a flexible MOF. The study shows that configurational entropy is an important yet largely overlooked parameter, providing an intriguing perspective of how to chemically access the underlying free energy landscape in MOFs.

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.

Suppression of magnetic ordering in Fe-deficient Fe3−xGeTe2 from application of pressure

Two-dimensional van der Waals magnets with multiple functionalities are becoming increasingly important for emerging technologies in spintronics and valleytronics. Application of external pressure is one method to cleanly explore the underlying physical mechanisms of the intrinsic magnetism. In this paper, the magnetic, electronic, and structural properties of van der Waals-layered, Fe-deficient ${\mathrm{Fe}}_{3\ensuremath{-}x}\mathrm{Ge}{\mathrm{Te}}_{2}$ are investigated. Magnetotransport measurements show a monotonic decrease in the Curie temperature $({T}_{C})$ and the magnetic moment with increasing pressure up to 13.9 GPa. The electrical resistance of ${\mathrm{Fe}}_{3\ensuremath{-}x}\mathrm{Ge}{\mathrm{Te}}_{2}$ shows a change from metallic to a seemingly nonmetallic behavior with increasing pressure. High-pressure angle dispersive powder x-ray diffraction shows a monotonic compression of the unit cell and a reduction of the volume by $\ensuremath{\sim}25%$ with no evidence of structural phase changes up to 29.4(4) GPa. We suggest that the decrease in the ${T}_{C}$ due to pressure results from increased intralayer coupling and delocalization that leads to a change in the exchange interaction.

Charting the Metal-Dependent High-Pressure Stability of Bimetallic UiO-66 Materials.

In theory, bimetallic UiO-66(Zr:Ce) and UiO-66(Zr:Hf) metal-organic frameworks (MOFs) are extremely versatile and attractive nanoporous materials as they combine the high catalytic activity of UiO-66(Ce) or UiO-66(Hf) with the outstanding stability of UiO-66(Zr). Using in situ high-pressure powder X-ray diffraction, however, we observe that this expected mechanical stability is not achieved when incorporating cerium or hafnium in UiO-66(Zr). This observation is akin to the earlier observed reduced thermal stability of UiO-66(Zr:Ce) compounds. To elucidate the atomic origin of this phenomenon, we chart the loss-of-crystallinity pressures of 22 monometallic and bimetallic UiO-66 materials and systematically isolate their intrinsic mechanical stability from their defect-induced weakening. This complementary experimental/computational approach reveals that the intrinsic mechanical stability of these bimetallic MOFs decreases nonlinearly upon cerium incorporation but remains unaffected by the zirconium: hafnium ratio. Additionally, all experimental samples suffer from defect-induced weakening, a synthesis-controlled effect that is observed to be independent of their intrinsic stability.

Praseodymium polyhydrides synthesized at high temperatures and pressures

Rare earth element polyhydrides have been predicted to exhibit high-${T}_{c}$ superconductivity at extreme compressions. Through a series of in situ high-pressure high-temperature x-ray powder diffraction experiments combined with density functional theory calculations, we report the emergence of polyhydride species in the praseodymium-hydrogen system. We initially observe the formation of ${\mathrm{PrH}}_{3}$, which continuously increases in hydrogen content on compression towards ${\mathrm{PrH}}_{4}$. Laser heating ${\mathrm{PrH}}_{4}$ in a hydrogen medium at pressures of 85 GPa leads to the synthesis of both ${\mathrm{PrH}}_{9}$ and ${\mathrm{PrH}}_{7}$. Both structures are characterized by hexagonal arrays of praseodymium atoms surrounded by hydrogen clathrate cages.

Synthesis and characterization, by high-pressure neutron and X-ray powder diffraction, of hybrid perovskites containing helium as a structural component

Synthesis and characterization, by high-pressure neutron and X-ray powder diffraction, of hybrid perovskites containing helium as a structural component

Using high-pressure X-ray powder diffraction to explore plastic and elastic deformations in ultra-hard transition metal borides

Using high-pressure X-ray powder diffraction to explore plastic and elastic deformations in ultra-hard transition metal borides

Pressure-induced symmetry changes in body-centred cubic zeolites.

Previous work has shown a strong correlation between zeolite framework flexibility and the nature of structural symmetry and phase transitions. However, there is little experimental data regarding this relationship, in addition to how flexibility can be connected to the synthesis of these open-framework materials. This is of interest for the synthesis of novel zeolites, which require organic additives to permutate the resulting geometry and symmetry of the framework. Here, we have used high-pressure powder X-ray diffraction to study the three zeolites: Na-X, RHO and ZK-5, which can all be prepared using 18-crown-6 ether as an organic additive. We observe significant differences in how the occluded 18-crown-6 ether influences the framework flexibility—this being dependent on the geometry of the framework. We use these differences as an indicator to define the role of 18-crown-6 ether during zeolite crystallization. Furthermore, in conjunction with previous work, we predict that pressure-induced symmetry transitions are intrinsic to body-centred cubic zeolites. The high symmetry yields fewer degrees of freedom, meaning it is energetically favourable to lower the symmetry to facilitate further compression.