Lattice distortion and magnetolattice coupling in CuO

Abstract

High-resolution powder x-ray-diffraction measurements on cupric oxide CuO are carried out in an extensive temperature range from 100 K to 1000 K. Anomalies in the lattice constants appear at the known antiferromagnetic phase transitions at ${T}_{N1}=230\mathrm{K}$ and ${T}_{N2}=213\mathrm{K},$ respectively, suggesting strong spin-lattice coupling in CuO. The Rietveld analysis with precise x-ray-diffraction data between 300 K and 1000 K clarifies a different structural phase transition at 800 K ${(T}_{S}).$ Below 800 K, anisotropic behaviors in the Cu-Cu distances are observed along the $[101\ifmmode\bar\else\textasciimacron\fi{}]$ and $[101]$ directions. The Cu-Cu distance along $[101\ifmmode\bar\else\textasciimacron\fi{}]$ hardly changes in the temperature range below ${T}_{S},$ which is similar to the temperature dependence of the Cu-Cu distance in the ${\mathrm{CuO}}_{2}$ plane of the high-${T}_{C}$ cuprate ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}.$ We suggest that the structural phase transition is caused by the softening of ${A}_{g}^{1}$ Raman mode at ${T}_{S}.$ The present study elucidates a strong spin-lattice coupling in CuO below ${T}_{S},$ indicating persistence of magnetic interaction up to 3.5 times higher-temperature region than ${T}_{N1}.$