Anomalous lattice compression and magnetic ordering in CuO at high pressures: A structural study and first-principles calculations

The structural and magnetic properties of multiferroic CuO have been studied by means of neutron and x-raypowder 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 GPa, followed by its reduction at higher pressures. The lattice distortion of the monoclinic structure at highpressures is accompanied by a progressive change of the oxygen coordination around Cu atoms from the squarefourfold towards the octahedral sixfold coordination. The pressure-induced evolution of the structural propertiesand electronic structure of CuO was successfully elucidated in the framework of full-electronic density functionaltheory calculations with range-separated HSE06, and meta–generalized gradient approximation hybrid M06functionals. The antiferromagnetic (AFM) ground state with a propagation vector q = (0.5,0, − 0.5) remainsstable in the studied pressure range. From the obtained structural parameters, the pressure dependencies of theprincipal superexchange magnetic interactions were analyzed, and the pressure behavior of the N´eel temperatureas well as the magnetic transition temperature from the intermediate incommensurate AFM multiferroic stateto the commensurate AFM ground state were evaluated. The estimated upper limit of the N´eel temperature atP = 38 GPa is about 260 K, not supporting the previously predicted existence of the multiferroic phase at roomtemperature and high pressure