High-pressure structural and electronic properties of CuM O₂ (M=Cr, Mn) delafossite-type oxides

We report high-pressure x-ray diffraction, x-ray absorption spectroscopy, and electrical transport measurements on CuMO₂ (M=Cr, Mn) delafossitelike oxides in an attempt to study their structural and electronic evolution with pressure. Recent studies of the similar CuFeO₂ delafossite has revealed a pressure-induced breaking of the unusual high axial anisotropy resulting in a structural phase transition coinciding with the metal-metal intervalence charge-transfer phenomenon. The present study revealed other possible scenarios responsible for the collapse of the high axial anisotropy and evolution of the O-Cu-O bonds in delafossitelike materials under pressure. Thus in CuMnO₂, the O-Cu-O dumbbells tilt with respect to the c axis at P>13 GPa, but in contrast to CuFeO₂, the tilting is continuous with pressure increase, justifying a second-order phase transition within the C2/m structure. Meanwhile in CuCrO₂ (R¯3m) the first-order structural phase transition to the monoclinic structure (P21/m) is observed at about 26 GPa characterized by the discontinuous bending of the O-Cu-O bond in contrast to the tilting in the case of CuFeO₂ and CuMnO₂. In both studied systems, we did not find clear evidence of valence transformations, similar to that observed in CuFeO₂.