First-principles study of paraelectric and ferroelectricCsH2PO4including 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 ${\mathrm{CsH}}₂{\mathrm{PO}}₄$, a ferroelectric material at low temperature. This material cannot be described properly by the usual (semi)local approximations within DFT. The long-range ${e}^{\ensuremath{-}}\ensuremath{-}{e}^{\ensuremath{-}}$ 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 (${\mathrm{KH}}₂{\mathrm{PO}}₄$, ${\mathrm{RbH}}₂{\mathrm{PO}}₄$, ${\mathrm{CsH}}₂{\mathrm{PO}}₄$), 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 ${\mathrm{CsH}}₂{\mathrm{PO}}₄$ 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 ${\mathrm{CsH}}₂{\mathrm{PO}}₄$, not yet reported experimentally. Finally, we report the implementation of DFT-D contributions to elastic constants within DFPT.