The high-pressure crystal structure evolution of CH3NH3PbBr3 (MAPbBr3) perovskite has been investigated by single-crystal X-ray diffraction and synchrotron-based powder X-ray diffraction. Single-crystal X-ray diffraction reveals that the crystal structure of MAPbBr3 undergoes two phase transitions following the space-group sequence: Pm3̅m → Im3̅ → Pmn21, unveiling the occurrence of a nonpolar/polar transition (Im3̅ → Pmn21). The transitions take place at around 0.8 and 1.8 GPa, respectively. This result contradicts the previously reported phase transition sequence: Pm3̅m → Im3̅ → Pnma . In this work, the crystal structures of each of the three phases are determined from single-crystal X-ray diffraction analysis, which is later supported by Rietveld refinement of powder X-ray diffraction patterns. The pressure dependence of the crystal lattice parameters and unit-cell volumes are determined from the two aforementioned techniques, as well as the bulk moduli for each phase. The bandgap behavior of MAPbBr3 has been studied up to around 4 GPa, by means of single-crystal optical absorption experiments. The evolution of the bandgap has been well explained using the pressure dependence of the Pb–Br bond distance and Pb–Br–Pb angles as determined from single-crystal X-ray diffraction experiments.
High-pressure synchrotron X-ray diffraction studies of cobalt iodate, Co(IO3)2, reveal a counterintuitive pressure-induced expansion along certain crystallographic directions. High-pressure Raman and infrared spectroscopy, combined with density-functional theory calculations, reveal that with increasing pressure, it becomes energetically favorable for certain I–O bonds to increase in length over the full range of pressure studied up to 28 GPa. This phenomenon is driven by the high-pressure behavior of iodate ion lone electron pairs. Two pressure-induced isosymmetric monoclinic–monoclinic phase transitions are observed at around 3.0 and 9.0 GPa, which are characterized by increasing oxygen coordination of the iodine atoms and the probable formation of pressure-induced metavalent bonds. Pressure–volume equations of state are presented, as well as a detailed discussion of the pressure dependences of the observed Raman- and infrared-active modes, which clarifies previous inconsistencies in the literature.
In the present study, the high-pressure high-temperature equation of the state of iridium has been determined through a combination of in situ synchrotron X-ray diffraction experiments using laser-heating diamond-anvil cells (up to 48 GPa and 3100 K) and density-functional theory calculations (up to 80 GPa and 3000 K). The melting temperature of iridium at 40 GPa was also determined experimentally as being 4260 (200) K. The results obtained with the two different methods are fully consistent and agree with previous thermal expansion studies performed at ambient pressure. The resulting thermal equation of state can be described using a third-order Birch–Murnaghan formalism with a Berman thermal-expansion model. The present equation of the state of iridium can be used as a reliable primary pressure standard for static experiments up to 80 GPa and 3100 K. A comparison with gold, copper, platinum, niobium, rhenium, tantalum, and osmium is also presented. On top of that, the radial-distribution function of liquid iridium has been determined from experiments and calculations.
We report on high-pressure synchrotron X-ray diffraction measurements on Ni3V2O8 at room-temperature up to 23 GPa. According to this study, the ambient-pressure orthorhombic structure remains stable up to the highest pressure reached in the experiments. We have also obtained the pressure dependence of the unit-cell parameters, which reveals an anisotropic compression behavior. In addition, a room-temperature pressure–volume third-order Birch–Murnaghan equation of state has been obtained with parameters: V0 = 555.7(2) Å3, K0 = 139(3) GPa, and K0′ = 4.4(3). According to this result, Ni3V2O8 is the least compressible kagome-type vanadate. The changes of the crystal structure under compression have been related to the presence of a chain of edge-sharing NiO6 octahedral units forming kagome staircases interconnected by VO4 rigid tetrahedral units. The reported results are discussed in comparison with high-pressure X-ray diffraction results from isostructural Zn3V2O8 and density-functional theory calculations on several isostructural vanadates.
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