High Pressure Raman, Optical Absorption, and Resistivity Study of SrCrO4

Characterization and Decomposition of the Natural van der Waals SnSb2Te4 under Compression

High pressure X-ray diffraction, Raman scattering, and electrical measurements, together with theoretical calculations, which include the analysis of the topological electron density and electronic localization function, evidence the presence of an isostructural phase transition around 2 GPa, a Fermi resonance around 3.5 GPa, and a pressure-induced decomposition of SnSb₂Te₄ into the high-pressure phases of its parent binary compounds (α-Sb₂Te₃ and SnTe) above 7 GPa. The internal polyhedral compressibility, the behavior of the Raman-active modes, the electrical behavior, and the nature of its different bonds under compression have been discussed and compared with their parent binary compounds and with related ternary materials. In this context, the Raman spectrum of SnSb₂Te₄ exhibits vibrational modes that are associated but forbidden in rocksalt-type SnTe; thus showing a novel way to experimentally observe the forbidden vibrational modes of some compounds. Here, some of the bonds are identified with metavalent bonding, which were already observed in their parent binary compounds. The behavior of SnSb₂Te₄ is framed within the extended orbital radii map of BA₂Te₄ compounds, so our results pave the way to understand the pressure behavior and stability ranges of other "natural van der Waals" compounds with similar stoichiometry.

Accurate Control of VS2 Nanosheets for Coexisting High Photoluminescence and Photothermal Conversion Efficiency

In two-dimensional (2D) amorphous nanosheets, the electron-phonon coupling triggered by localization of the electronic state as well as multiple-scattering feature make it exhibit excellent performance in optical science. VS₂ nanosheets, especially single-layer nanosheets with controllable electronic structure and intrinsic optical properties, have rarely been reported owing to the limited preparation methods. Now, a controllable and feasible switching method is used to fabricate 2D amorphous VS₂ and partial crystallized 2D VO₂ (D) nanosheets by altering the pressure and temperature of supercritical CO₂ precisely. Thanks to the strong carrier localization and the quantum confinement, the unique 2D amorphous structures exhibit full band absorption, strong photoluminescence, and outstanding photothermal conversion efficiency.

Putting the Squeeze on Lead Chromate Nanorods

We have studied by means of X-ray diffraction and Raman spectroscopy the high-pressure behavior of PbCrO₄ nanorods. We have found that these nanorods follow a distinctive structural sequence that differs from that of bulk PbCrO₄. In particular, a phase transition from a monoclinic monazite-type PbCrO₄ to a novel monoclinic AgMnO₄-type polymorph has been discovered at 8.5 GPa. The crystal structure, Raman-active phonons, and compressibility of this novel high-pressure phase are reported for the first time. The experimental findings are supported by ab initio calculations that provide information not only on structural and vibrational properties of AgMnO₄-type PbCrO₄ but also on the electronic properties. The discovered phase transition triggers a band gap collapse and a subsequent metallization at 44.2 GPa, which has not been observed in bulk PbCrO₄. This suggests that nanoengineering can be a useful strategy to drive metallization under compression.

High-Pressure Single-Crystal X-ray Diffraction of Lead Chromate: Structural Determination and Reinterpretation of Electronic and Vibrational Properties

We have investigated the high-pressure behavior of PbCrO₄. In particular, we have probed the existence of structural transitions under high pressure (at 4.5 GPa) by single-crystal X-ray diffraction and density functional theory calculations. The structural sequence of PbCrO₄ is different than previously determined. Specifically, we have established that PbCrO₄, under pressure, displays a monoclinic-tetragonal phase transition, with no intermediate phases between the low-pressure monoclinic monazite structure (space group P2₁/ n) and the high-pressure tetragonal structure. The crystal structure of the high-pressure polymorph is, for the first time, undoubtedly determined to a tetragonal scheelite-type structure (space group I4₁/ a) with unit-cell parameters a = 5.1102(3) Å and c = 12.213(3) Å. These findings have been used for a reinterpretation of previously published Raman and optical-absorption results. Information of calculated infrared-active phonons will be also provided. In addition, the pressure dependence of the unit-cell parameters, atomic positions, bond distances, and polyhedral coordination are discussed. The softest and stiffest direction of compression for monazite-type PbCrO₄ are also reported. Finally, the theoretical pressure dependence of infrared-active modes is given, for the first time, for both polymorphs.

Pressure Effects on the Optical Properties of NdVO4

We report on optical spectroscopic measurements in pure NdVO4 crystals at pressures up to 12 GPa. The influence of pressure on the fundamental absorption band gap and Nd3+ absorption bands has been correlated with structural changes in the crystal. The experiments indicate that a phase transition takes place between 4.7 and 5.4 GPa. We have also determined the pressure dependence of the band-gap and discussed the behavior of the Nd3+ absorption lines under compression. Important changes in the optical properties of NdVO4 occur at the phase transition, which, according to Raman measurements, corresponds to a zircon to monazite phase change. In particular, in these conditions a collapse of the band gap occurs, changing the color of the crystal. The changes are not reversible. The results are analyzed in comparison with those deriving from previous studies on NdVO4 and related vanadates.

Electric discharge machine for preparation of diamond anvil cell sample chambers

We have designed and constructed a novel electric discharge machine designed primarily for the preparation of sample chambers in rhenium and stainless steel gaskets for diamond anvil cell experiments. Our design combines automatic stage movement with relatively low voltage (100 V) operation and routinely achieves a drilling/erosion speed of approximately 0.4 μm s^-1. The machine is used for preparing 100 μm diameter sample chambers for diamond anvil cell experiments with 250 μm culets and has also been used to prepare 50 μm diameter sample chambers for diamond anvil cell experiments with 100 μm culets to access a pressure of 165 GPa.