Xin Wang

The high-pressure structural evolution of Gd₂Ce₂O₇ has been studied by synchrotron X-ray diffraction (up to 31.8 GPa) and Raman spectroscopy (up to 38.9 GPa). A pressure-induced phase transition from cubic (space group Ia-3) to monoclinic (C2/m) crystal structure was observed to initiate at 11.3 GPa. This transition was primarily driven by the compression-induced cation rearrangement, which significantly altered the local coordination environment. The structural transformation proceeded via a gradual, continuous pathway and persisted up to the maximum experimental pressure of 31.8 GPa. Notably, the high-pressure phase remains stable upon complete decompression, confirming an irreversible structural transformation of Gd₂Ce₂O₇.

The structural evolution of Ho₂Ce₂O₇ under high pressure was systematically investigated using synchrotron X-ray diffraction (up to 31.5 GPa) and Raman spectroscopy (up to 41.7 GPa). At ambient pressure, the compound adopts a common C-type cubic rare earth oxide structure (space group Ia-3). A pressure-induced phase transition was observed to commence at 23.8 GPa, characterized by a gradual structural evolution that persisted through the maximum experimental pressure of 31.5 GPa. This transition involves cation disordering accompanied by coordination environment modifications. High-pressure X-ray diffraction analysis reveals the coexistence of two distinct phases above the transition threshold: the parent cubic phase (Ia-3) and a metastable hexagonal phase (R-3c). Notably, the high-pressure phase configuration persists upon complete decompression to ambient conditions, demonstrating the irreversible nature of this pressure-induced structural transition.

Transition metal borides (TMBs) are promising catalysts for hydrogen evolution reaction (HER). While the commercially available TMBs indicate poor HER performance due to powder electrode and low activity sites density, optimizing commercial TMBs for better HER performance is urgent. To break through the challenge, a new strategy is proposed to compose integral bulk electrodes with needle surfaces in TMBs. The integral bulk electrodes in TiB₂, ZrB₂, and HfB₂ are formed under high pressure and high temperature (HPHT), and the nanoneedle morphology is constructed by chemical etching. In the three materials, the smallest overpotential is 346 mV at 10 mA cm⁻² in the HCl etched bulk TiB₂ electrode, which is about 61.9% higher than commercial TiB₂ powder. Better performance arises from better conductivity of the integral bulk electrode, and the nano morphology exposes the edge sides of the structure which have high activity site density. This work is significant for developing new kinds of bulk TMBs catalysts.

The pyrochlore manganate In₂Mn₂O₇ is a very promising ferromagnetic semiconductor material, which has a good application prospect in spin transport due to its very low electron effective mass, high Curie temperature, and structural stability. In this paper, In₂Mn₂O₇ with pyrochlore structure was successfully prepared by high temperature and high pressure combined with the sol–gel method, and the in situ high-pressure X-ray diffraction experiment was carried out on it. The results showed that the structure of In₂Mn₂O₇ was very stable in the pressure range of 0–29.0 GPa, and its bulk modulus was given. This lays a foundation for the application of In₂Mn₂O₇ in extreme environments.

Two polymorphs of Ho₂Ge₂O₇ were synthesized using different methods. The structural stabilities were investigated by ADXRD at high pressures.

Amorphous carbon (a-C) films are attracting considerable attention to due their large optical band gap (E_opt) range of 1–4 eV. But the hopping conducting mechanism of boron doping a-C (a-C:B) is still mysterious. To exploring the intrinsic reasons behind the semiconductor properties of a-C:B, in this work, the boron doping a-C (a-C:B) nano-film was prepared, and the growth rate and E_opt changing were analyzed by controlling the different experimental conditions of magnetron sputtering. A rapid deposition rate of 10.55 nm/min was obtained. The E_opt is reduced from 3.19 eV to 2.78 eV by improving the substrate temperature and sputtering power. The proportion of sp²/sp³ increasing was uncovered with narrowing the E_opt. The shrinking E_opt can be attributed to the fact that boron atoms act as a fluxing agent to promote carbon atoms to form sp² hybridization at low energy. Furthermore, boron atoms can impede the formation of σ bonds in carbon atom sp³ hybridization by forming B–C bonds with high energy, and induce the sp³ hybridization transfer to sp² hybridization. This work is significant to further study of amorphous semiconductor films.

La₂Sn₂O₇ is a transparent conducting oxide (TCO) material and shows a strong near‐infrared fluorescent at ambient pressure and room temperature. By in situ high‐pressure research, pressure‐induced visible photoluminescence (PL) above 2 GPa near 2 eV is observed. The emergence of unusual visible PL behavior is associated with the seriously trigonal lattice distortion of the SnO₆ octehedra, under which the Sn–O1–Sn exchange angle θ is decreased below 22.1 GPa, thus enhancing the PL quantum yield leading to Sn ³P₁ → ¹S₀ photons transition. Besides, bandgap closing followed by bandgap opening and the visible PL appearing at the point of the gap reversal, which is consistent with high‐pressure phase decomposition, are discovered. The high‐pressure PL results demonstrate a well‐defined pressure window (7–17 GPa) with flat maximum PL yielding and sharp edges at both ends, which may provide a great calibration tool for pressure sensors for operation in the deep sea or at extreme conditions.

The structural properties of pyrochlore Eu₂Zr₂O₇ under high pressure have been studied by using Raman spectroscopy and in situ angle-dispersive X-ray diffraction (ADXRD). The results of Raman spectra indicate that Eu₂Zr₂O₇ undergoes a reversible structural change around 21.2 GPa. The results of Rietveld refinements from in situ ADXRD data indicate that the ordered pyrochlore structure (Fd-3m) transforms to the defect-cotunnite structure (Pnma) at 26.5 GPa. The phase transition is irreversible and the transformation process is mainly induced by the accumulations of anti-site defects of the cation sublattice and Frenkel defects on the anion sublattice. Besides, the bonds should play a more important role than the bonds in the process of the phase transformation.

Polycrystalline BiCuSeO oxides are prepared by solid-state reaction followed by pressureless sintering (PLS) and high pressure sintering (HPS) methods. Both the experimental results and the density functional theory calculations indicate that the crystal defect concentrations of BiCuSeO can be reduced under the effect of high pressure. By comparing with the PLS sample, a larger power factor and a lower thermal conductivity can be obtained for the HPS sample. The maximum figure of merit ZT∼0.4 @ 800 K was obtained for the HPS sample, which is about 3 times higher than the PLS sample. These results indicate that the effect of high pressure is beneficial to modifying the microstructure and improving the thermoelectric performance of BiCuSeO oxyselenide.

Raman spectroscopy, synchrotron angle-dispersive X-ray diffraction (ADXRD), first-principles calculations, and electrical resistivity measurements were carried out under high pressure to investigate the structural stability and electrical transport properties of metavanadate MgV₂O₆. The results have revealed the coordination change of vanadium ions (from 5+1 to 6) at around 4 GPa. In addition, a pressure-induced structure transformation from the C2/m phase to the C2 phase in MgV₂O₆ was detected above 20 GPa, and both phases coexisted up to the highest pressure. This structural phase transition was induced by the enhanced distortions of MgO₆ octahedra and VO₆ octahedra under high pressure. Furthermore, the electrical resistivity decreased with pressure but exhibited different slope for these two phases, indicating that the pressure-induced structural phase transitions of MgV₂O₆ was also accompanied by the obvious changes in its electrical transport behavior.