Ab initiostudy of the electronic and structural properties of the ferroelectric transition inKH2PO4

First-principles electronic structure calculations were carried out for the paraelectric and ferroelectric phases of ${\mathrm{KH}}_2{\mathrm{PO}}_4$ using the pseudopotential method. The calculated structures are in good agreement with experiment. The calculations reveal that the distance δ between the two equilibrium positions of H along the O-O bond in the paraelectric phase depends on both the O-O bond length and the coordinated motion of the heavier P and K atoms. The results reconcile the previously proposed tunneling and geometric phenomenological models. The spontaneous polarization is found to arise from the redistribution of charge density caused by the displacement of the P atoms relative to the O atoms along the ferroelectric axis. The critical pressure for the transition from the orthorhombic to the tetragonal structure at zero temperature is found to be 30 kbar.