First-principles study of paraelectric and ferroelectric CsH2PO4 including dispersion forces: Stability and related vibrational, dielectric, and elastic properties

Using density functional theory (DFT) and density functional perturbation theory (DFPT), we investigate the stability and response functions of CsH2PO4, a ferroelectric material at low temperature. This material cannot be described properly by the usual (semi)local approximations within DFT. The long-range e−-e− correlation needs to be properly taken into account, using, for instance, Grimme’s DFT-D methods, as investigated in this work. We find that DFT-D3(BJ) performs the best for the members of the dihydrogenated alkali phosphate family (KH2PO4, RbH2PO4, CsH2PO4), leading to experimental lattice parameters reproduced with an average deviation of 0.5%. With these DFT-D methods, the structural, dielectric, vibrational, and mechanical properties of CsH2PO4 are globally in excellent agreement with the available experiments (<2% MAPE for Raman-active phonons). Our study suggests the possible existence of a new low-temperature phase of CsH2PO4, not yet reported experimentally. Finally, we report the implementation of DFT-D contributions to elastic constants within DFPT.