Novel Valence Transition in Elemental Metal Europium around 80 GPa.

Valence transition could induce structural, insulator-metal, nonmagnetic-magnetic and superconducting transitions in rare-earth metals and compounds, while the underlying physics remains unclear due to the complex interaction of localized 4f electrons as well as their coupling with itinerant electrons. The valence transition in the elemental metal europium (Eu) still has remained as a matter of debate. Using resonant x-ray emission scattering and x-ray diffraction, we pressurize the states of 4f electrons in Eu and study its valence and structure transitions up to 160 GPa. We provide compelling evidence for a valence transition around 80 GPa, which coincides with a structural transition from a monoclinic (C2/c) to an orthorhombic phase (Pnma). We show that the valence transition occurs when the pressure-dependent energy gap between 4f and 5d electrons approaches the Coulomb interaction. Our discovery is critical for understanding the electrodynamics of Eu, including magnetism and high-pressure superconductivity.

Comparative Study on the Magnetic and Transport Properties of B-Site Ordered and Disordered CaCu3Fe2Os2O12.

The B-site Fe/Os ordered and disordered quadruple perovskite oxides CaCu3Fe2Os2O12 were synthesized under different high-pressure and high-temperature conditions. The B-site ordered CaCu3Fe2Os2O12 is a system with a very high ferrimagnetic ordering temperature of 580 K having the Cu2+(↑)Fe3+(↑)Os5+(↓) charge and spin arrangement. In comparison, the highly disordered CaCu3Fe2Os2O12 has a reduced magnetic transition temperature of about 350 K. The Cu2+Fe3+Os5+ charge combination remains the same without any sign of changes in the valence state of the constituent ions. Although the average net moments of each sublattice are reduced, the average ferrimagnetic spin arrangement is unaltered. The robustness of the basic magnetic properties of CaCu3Fe2Os2O12 against site disorder may be taken as an indication of the tendency to maintain the short-range order of the atomic constituents.

High-Pressure Synthesis and Ferrimagnetic Ordering of the B-Site-Ordered Cubic Perovskite Pb2FeOsO6.

Pb2FeOsO6 was prepared for the first time by using high-pressure and high-temperature synthesis techniques. This compound crystallizes into a B-site-ordered double-perovskite structure with cubic symmetry Fm3̅m, where the Fe and Os atoms are orderly distributed with a rock-salt-type manner. Structure refinement shows an Fe-Os antisite occupancy of about 16.6%. Structural analysis and X-ray absorption spectroscopy both demonstrate the charge combination to be Pb2Fe3+Os5+O6. A long-range ferrimagnetic transition is found to occur at about 280 K due to antiferromagnetic interactions between the adjacent Fe3+ and Os5+ spins with a straight (180°) Fe-O-Os bond angle, as confirmed by X-ray magnetic circular-dichroism measurements. First-principles theoretical calculations reveal the semiconducting behavior as well as the Fe3+(↑)Os5+(↓) antiferromagnetic coupling originating from the superexchange interactions between the half-filled 3d orbitals of Fe and t2g orbitals of Os.

Emergent physical properties of perovskite-type oxides prepared under high pressure.

The perovskite ABO3 family demonstrates a wide variety of structural evolutions and physical properties and is arguably the most important family of complex oxides. Chemical substitutions of the A- and/or B-site and modulation of oxygen content can effectively regulate their electronic behaviors and multifunctional performances. In general, the BO6 octahedron represents the main unit controlling the electronic and magnetic properties while the A-site ion is often not involved. However, a series of unconventional perovskite materials have been recently synthesized under high pressure, such as the s-d level controlled Pb-based perovskite family and quadruple perovskite oxides containing transition metal ions at the A-site. In these compounds, the intersite A-B correlations play an important role in electronic behaviors and further induce many emergent physical properties.

Metallization of Quantum Material GaTa4Se8 at High Pressure.

Pressure is a unique thermodynamic variable to explore the phase competitions and novel phases inaccessible at ambient conditions. The resistive switching material GaTa4Se8 displays several quantum phases under pressure, such as a Jeff = 3/2 Mott insulator, a correlated quantum magnetic metal, and d-wave topological superconductivity, which has recently drawn considerable interest. Using high-pressure Raman spectroscopy, X-ray diffraction, extended X-ray absorption, transport measurements, and theoretical calculations, we reveal a complex phase diagram for GaTa4Se8 at pressures exceeding 50 GPa. In this previously unattained pressure regime, GaTa4Se8 ranges from a Mott insulator to a metallic phase and exhibits superconducting phases. In contrast to previous studies, we unveil a hidden correlation between the structural distortion and band gap prior to the insulator-to-metal transition, and the metallic phase shows superconductivity with structural and magnetic properties that are distinctive from the lower-pressure phase. These discoveries highlight that GaTa4Se8 is a unique material to probe novel quantum phases from a structural, metallicity, magnetism, and superconductivity perspective.

Probing Cerium 4f States across the Volume Collapse Transition by X-ray Raman Scattering.

Understanding the volume collapse phenomena in rare-earth materials remains an important challenge due to a lack of information on 4f electronic structures at different pressures. Here, we report the first high-pressure inelastic X-ray scattering measurement on elemental cerium (Ce) metal. By overcoming the ultralow signal issue in the X-ray measurement at the Ce N4,5-edge, we observe the changes of unoccupied 4f states across the volume collapse transition around 0.8 GPa. To help resolve the longstanding debate on the Anderson-Kondo and Mott-Hubbard models, we further compare the experiments with extended multiplet calculations that treat both screening channels on equal footing. The results indicate that a modest change in the 4f-5d Kondo coupling can well describe the spectral redistribution across the volume collapse, whereas the hybridization between neighboring atoms in the Hubbard model appears to play a minor role. Our study helps to constrain the theoretical models and opens a promising new route for systematic investigation of volume collapse phenomena in rare-earth materials.