The geometrically frustrated pyrochlore Eu2Sn2O7 is an insulator with slight trigonal lattice distortion at ambient condition. High pressure is applied to this system to investigate the responses of structural evolution, optical emission and electrical transport properties. In situ high pressure synchrotron X-ray diffraction, Raman spectroscopy, and photoluminescence studies are performed in Eu2Sn2O7 up to 31.2 and 34.1 GPa, respectively. The abrupt change of the oxygen atomic position without breaking the crystal symmetry is accompanied by disappearing of Raman mode involving SnO6 octahedron distortion around 17.8 GPa. It indicates a pressure-induced second-order iso-structural transition, which suppresses the trigonal distortion in the SnO6 octahedron but enhances the local symmetry distortion of EuO8 hexahedron. Anomalous luminescence of the Eu3+ 4f–4f transition is observed, which confirms the enhancement of EuO8 hexahedral distortion at high pressure region. In situ high-pressure electrical transport property is measured by alternating current (AC) impedance spectroscopy up to 32.5 GPa. A rapid increase in resistance with gain of 4 orders of magnitude by applied pressure is observed until 16.6 GPa, and it is followed by a slight decreasing to the highest pressure measured here. All these observations indicate a pressure-enhanced trigonal lattice distortion before the transition pressure, and thus it will enlarge an opening gap at the Fermi energy, followed by releasing distortion at higher pressures.

The pressure effects on the optical and structural properties of NiWO4 have been studied experimentally and theoretically. The fundamental bandgap decreases with a pressure coefficient of −12.0 ± 0.2 meV/GPa. Meanwhile, the Ni2+ d–d transition energies increase at a rate of 7.4–14.8 meV/GPa. Therefore, the energy differences between the fundamental band and the Ni2+ d–d transition bands gradually decrease under pressure, which is beneficial to improve its optical performance. These optical phenomena are associated with structural variations. The shrinkage of the WO6 octahedron enhances the hybridization between the W 5d and O 2p orbitals, resulting in bandgap reduction. The pressure-induced enhancement of the NiO6 octahedral symmetry increases the crystal field splitting, thereby yielding increases in the Ni2+ d–d intraband transition energies. Besides, a pressure-induced structural phase transition is also observed around 20.0 GPa by both angle-dispersive synchrotron X-ray diffraction (ADXRD) and Raman experiments. This study provides valuable insight into the electron–lattice coupling of NiWO4 under compression and an effective way to modulate the electronic structure and optical properties of isomorphic wolframite materials.

H2O ice becomes a superionic phase under the high pressure and temperature conditions of deep planetary interiors of ice planets such as Neptune and Uranus, which affects interior structures and generates magnetic fields. The solid Earth, however, contains only hydrous minerals with negligible amount of ice. Here we combine high pressure and temperature electrical conductivity experiments, Raman spectroscopy, and first-principles simulations, to investigate the state of hydrogen in the pyrite type FeO2Hx (x ≤ 1) which is a potential H-bearing phase near the coremantle boundary. We find that when the pressure increases beyond 73 GPa at room temperature, symmetric hydroxyl bonds are softened and the H + (or proton) become diffusive within the vicinity of its crystallographic site. Increasing temperature under pressure, the diffusivity of hydrogen is extended beyond individual unit cell to cover the entire solid, and the electrical conductivity soars, indicating a transition to the superionic state which is characterized by freely-moving proton and solid FeO2 lattice. The highly diffusive hydrogen provides fresh transport mechanisms for charge and mass, which dictate the geophysical behaviors of electrical conductivity and magnetism, as well as geochemical processes of redox, hydrogen circulation, and hydrogen isotopic mixing in Earth's deep mantle.Hydrogen plays an important role in the deep interior of the Earth 1,2 , where its mobility and bonding properties are altered dramatically from localized to globally itinerant with increasing depth. At shallower depths, hydrogen bonds with oxygen, the most abundant element in Earth, to form hydroxyls which modulate the electrical 3,4 , thermal 5 , and elastic 6 properties of the host minerals, and dictate redox, melting, and isotope partitioning 7 . Properties of hydroxyl groups have been extensively studied during the past half century as a means to locate deep water reservoirs and to monitor water circulation for a broad range of applications in interpretation of large geophysical and geochemical features in depth [8][9][10] . Hydroxyl starts with an asymmetric configuration O-H⋯O in which the hydrogen atom between

The structural and electrical properties of ZnV 2 O 6 under high pressure have been studied using Raman spectroscopy, in situ angle dispersive X-ray diffraction (ADXRD), and alternating current (AC) impedance spectroscopy. The results of Raman spectra indicate that ZnV 2 O 6 undergoes a reversible structural change around 16.6 GPa, as evidenced by the appearance of new peaks. The results of Rietveld refinements from in situ ADXRD data indicate that the monoclinic symmetry (C2/m) is retained up to 16.0 GPa and the C2 phase comes to coexist between 16.0 and 16.9 GPa. Above 16.9 GPa, the high-pressure phase can be distinguished only as the C2 structure. The transformation process from the C2/m phase to the C2 phase is mainly caused by the more distorted ZnO 6 octahedra and VO 6 octahedra at higher pressures. The equal bond distances Zn−O2 and V−O3 in the C2/m phase become unequal in the C2 phase. Furthermore, the measurements of the AC impedance spectroscopy of ZnV 2 O 6 reveal obvious changes in its electrical transport properties at 14.1 GPa which could correspond to the observed phase transition in the Raman and ADXRD measurements. The combined analyses of experimental results suggest the occurrence of a reversible structural phase transition of ZnV 2 O 6 around 16.0 GPa.