High-pressure structural and electronic properties of CuMO2 (M=Cr, Mn) delafossite-type oxides

We report high-pressure x-ray diffraction, x-ray absorption spectroscopy, and electrical transport measurements on $\mathrm{Cu}M{\mathrm{O}}_{2}$ ($M=\mathrm{Cr}$, Mn) delafossitelike oxides in an attempt to study their structural and electronic evolution with pressure. Recent studies of the similar $\mathrm{CuFe}{\mathrm{O}}_{2}$ 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 $\mathrm{CuMn}{\mathrm{O}}_{2}$, the O-Cu-O dumbbells tilt with respect to the $c$ axis at $Pg13$ GPa, but in contrast to $\mathrm{CuFe}{\mathrm{O}}_{2}$, the tilting is continuous with pressure increase, justifying a second-order phase transition within the C2/m structure. Meanwhile in $\mathrm{CuCr}{\mathrm{O}}_{2}$ ($R\overline{3}m$) the first-order structural phase transition to the monoclinic structure ($P{2}_{1}/m$) 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 $\mathrm{CuFe}{\mathrm{O}}_{2}$ and $\mathrm{CuMn}{\mathrm{O}}_{2}$. In both studied systems, we did not find clear evidence of valence transformations, similar to that observed in $\mathrm{CuFe}{\mathrm{O}}_{2}$.