Lattice distortion and magnetolattice coupling in CuO

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=230K and T_N2=213K, 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^-] and [101] directions. The Cu-Cu distance along [101^-] hardly changes in the temperature range below TS, which is similar to the temperature dependence of the Cu-Cu distance in the CuO₂ plane of the high-TC cuprate La_2-xSr_xCuO₄. We suggest that the structural phase transition is caused by the softening of A¹_g Raman mode at T_S. The present study elucidates a strong spin-lattice coupling in CuO below TS, indicating persistence of magnetic interaction up to 3.5 times higher-temperature region than T_N1.