Three‐dimensional H‐bonding and ferroelectric transition in KDP. Quantum‐chemical study

Abstract

On basis of the nonempirical methods (SCF, B3LYP, MP2-MP4) and several cluster models, the ferroelectric KH 2 PO 4 (KDP) and its deuteroanalogue are studied. The tunneling integrals ⍀ and the parameters of the effective coupling of protons/deuterons U,V (the Ising parameters) are calculated for these materials with the 3d network of H/D-bonds. Using the obtained U,V and ⍀ values in the frames of molecular field approximation, it is found that the structural phase (ferroelectric) transition occurs for both crystals within the lowering of temperature. Such low-temperature behavior differs the 3d KDP-family materials from 0d systems, where the low-temperature phase transition takes place only upon deuteration. It is demonstrated that this difference is associated with an abrupt Ising parameters growth for KDP if compared with the nondeuterated 0d materials.

On the possibility of quantum‐chemical cluster approach to the description of structural phase transition in H‐bonded materials on the example of KDP

“…In precedent papers, we presented an approach to estimate independently the parameters of the Ising pseudospin Hamiltonian (PSH) (with tunneling effects) on the basis of ab initio calculations. The algorithm proposed includes four main steps: (1) the choice, relying on diffraction data, of a model cluster that correctly reproduces the character of nearest neighbors interactions in the lattice of H‐bonded crystal; (2) the calculation of double‐well potential profile describing the proton motion along H‐bond and the estimate of the tunneling integral; (3) the calculation of energies for all pseudospin configurations and the evaluation of the coupling constants for the Ising model including the estimates of long‐range interaction influence; and (4) the interpretation of the obtained information within the frames of the molecular field model or/and the Bethe cluster approximation (BCA) method …”

“…As our technique uses the quantum chemistry to obtain the values of the PSH's parameters, the question of the sensibility of the results on the level of quantum‐chemical calculations naturally arises. It was found that for the estimates of tunneling integrals and potential profiles (DWP) made for KDP‐dimers and for Zundel‐ions H 5 , H 3 one needs the high‐level methods at least MP4/SDQ or CCSD(QCISD) with basis set at least 6‐31+G**, while to obtain reasonable estimates for Slater parameters the MP2 level is enough with the same basis sets.…”

“…This study presents a simple (from the viewpoint of structural chemistry) theoretical analysis of the influence of PLC effects on the above indicated properties of SPT in ferroelectric KDP crystals and their analogs. This quantum‐chemical approach is based on derivations from the vibronic theory of heteroligand systems (VTHS) in analytical form and on the results of the numerical calculations of the main parameters of PSH …”

“…The analysis performed was aimed to obtain a theoretical rationale for the procedure for quantum‐chemical modeling of PSH parameters suggested in Refs. based on the calculations of the energies of various cluster models using their total energies (but not the MO parameters) including the experimental averaged geometry of the structure forming units of the crystal in the paraphase (HT phase). The calculation procedure itself using pseudomodes of Z P , X P , Y P types and their linear combinations that describe the offsets of the central P atoms along the indicated axes in various pseudospin configurations was shown to be correct.…”

Quantum‐chemical modeling of squaric acid ferroelectric behavior

“…In the case of H 2 SQ/D 2 SQ, as for KDP/DKDP 13 and some other H‐bonded ferroelectric materials that we have studied earlier, several types of model clusters were used for the evaluation of the Hamiltonian (1) parameters. Different quantum‐chemical computational methods are used in these calculations: Hartree–Fock (RHF), density functional theory (B3LYP), and many‐particle perturbation theory (MP2‐MP4) with various basis sets of 6‐31G type up to 6‐311++G(2d,2p) and cc‐pvnz ( n = 2,3).…”