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.