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=230 K and T-N2=213 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 [10 (1) over bar] and [101] directions. The Cu-Cu distance along [10 (1) over bar] hardly changes in the temperature range below T-S, which is similar to the temperature dependence of the Cu-Cu distance in the CuO2 plane of the high-T-C cuprate La2-xSrxCuO4. 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.