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 $T_{N1\mathrm{}}=230\mathrm{}\mathrm{K}$ and $T_{N2\mathrm{}}=213\mathrm{}\mathrm{K},$ 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 ${(T}_S).\mathrm{}$ Below 800 K, anisotropic behaviors in the Cu-Cu distances are observed along the $\left[101¯\right]$ and $\left[101\right]$ directions. The Cu-Cu distance along $\left[101¯\right]$ 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 ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4.$ 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 $T_{N1\mathrm{}}.$

• Received 24 February 2003

DOI:https://doi.org/10.1103/PhysRevB.69.104104