Abstract
The structural and magnetic properties of multiferroic CuO have been studied by means of neutron and x-ray powder diffraction at pressures up to 11 and 38 GPa, respectively, and by first-principles theoretical calculations. Anomalous lattice compression is observed, with enlargement of the lattice parameter $a$, reaching a maximum at $P=13\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, followed by its reduction at higher pressures. The lattice distortion of the monoclinic structure at high pressures is accompanied by a progressive change of the oxygen coordination around Cu atoms from the square fourfold towards the octahedral sixfold coordination. The pressure-induced evolution of the structural properties and electronic structure of CuO was successfully elucidated in the framework of full-electronic density functional theory calculations with range-separated HSE06, and meta--generalized gradient approximation hybrid M06 functionals. The antiferromagnetic (AFM) ground state with a propagation vector $q=(0.5,0,\ensuremath{-}0.5)$ remains stable in the studied pressure range. From the obtained structural parameters, the pressure dependencies of the principal superexchange magnetic interactions were analyzed, and the pressure behavior of the N\'eel temperature as well as the magnetic transition temperature from the intermediate incommensurate AFM multiferroic state to the commensurate AFM ground state were evaluated. The estimated upper limit of the N\'eel temperature at $P=38\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ is about 260 K, not supporting the previously predicted existence of the multiferroic phase at room temperature and high pressure.
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