Cd2Re2O7under high pressure: Pyrochlore lattice distortion-driven metal-to-nonmetal transition

Pyrochlore ${\mathrm{Cd}}₂{\mathrm{Re}}₂{\mathrm{O}}₇$ has been investigated under high pressure by x-ray diffraction, resistivity and, mid-IR reflectance measurements. At $\ensuremath{\sim}14$ GPa, the high-temperature cubic pyrochlore phase [space group (SG): $Fd\overline{3}m$] undergoes a second-order transition to a rhombohedral structure (SG: $R\overline{3}m$) with trigonal distortion in its pyrochlore lattice. Temperature-dependent resistivity $\ensuremath{\rho}(T)$ reveals a large number of exotic electronic phases (including superconducting and unconventional metallic phases) in its $P\text{\ensuremath{-}}T$ phase diagram. At the metal-nonmetal boundary (at 21 GPa) the resistivity displays quadratic power-law behavior over a wide $T$ range with possible formation of heavy mass coherent quasiparticles. In the nonmetallic state $(d\ensuremath{\rho}/dT<0)$, the nondivergent and quadratic power-law resistivity as $T\ensuremath{\rightarrow}0$ reveals the novel character of the ground state. High-pressure optical conductivity measurement evidences electronic correlation playing the vital role in the emergence of the nonmetallic state as the electron-lattice coupling exceeds spin-orbit interaction due to large structural distortion.