Cd2Re2O7 under high pressure: Pyrochlore lattice distortion-driven metal-to-nonmetal transition.

Pyrochlore Cd2Re2O7 has been investigated under high pressure by x-ray diffraction, resistivity and, midIR reflectance measurements. At ∼14 GPa, the high-temperature cubic pyrochlore phase [space group (SG): Fd3m] undergoes a second-order transition to a rhombohedral structure (SG: R3m) with trigonal distortion in its pyrochlore lattice. Temperature-dependent resistivity p(T ) reveals a large number of exotic electronic phases (including superconducting and unconventional metallic phases) in its P-T phase diagram. At the metalnonmetal 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 (dp/dT < 0), the nondivergent and quadratic power-law resistivity as T → 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. [ABSTRACT FROM AUTHOR]