Metallization without Charge Transfer in CuReO₄ Perrhenate under Pressure

Using high-pressure synchrotron X-ray diffraction combined with Raman spectroscopy and density-functional calculations, we determined the sequence of the pressure-induced transformations in CuReO₄. At 1.5 GPa, the lattice symmetry changes from I4₁cd to I4₁/a with the transformation of isolated ReO₄-tetrahedra into infinite chains of ReO₆-octahedra. The second, isosymmetric transition at 7 GPa leads to the formation of a NbO₂-type structure with the octahedral oxygen coordination for both Cu¹⁺ and Re⁷⁺ cations. Both transitions are of the first order and accompanied by discontinuities in the unit-cell volume of 7 and 14%, respectively. Density-functional calculations predict the metallic state of the high-pressure NbO₂-type phase of CuReO₄, and this prediction is in-line with the disappearance of the Raman signal above 7 GPa and visual observations (darkness/reflection of the sample). This metallization is caused by the increased bandwidth of both Cu 3d and Re 5d bands without any significant charge transfer between Cu and Re. At ambient pressure, the crystal structure of CuReO₄ is retained between 4 and 700 K (melting point), showing a negative thermal expansion along the c-axis and a positive expansion along the a-axis within the entire temperature range.