Deuteron tunneling and phase transition in M₃D(AO₄)₂ antiferroelectrics

Abstract

An effect of the AO 4 tetrahedron reorganization vibrational modes on the deuteron tunneling splitting ⌬ D is treated for fully deuterated M 3 D(AO 4 ) 2 materials (M alkali metal; A ϭ S, Se) of the TKHS family using various quantum chemical models and computational techniques. It is found that the reorganization of the nonhydrogen framework of the D(AO 4 ) 2 3Ϫ dimers in a crystal results in the reduction of ⌬ D and favors the transition of a material in the ordered (antiferroelectric) phase. Numerical estimation of this effect with the use of the available diffraction data leads to only minor reduction of ⌬ D . At the same time, substantial pronounced contributions to ⌬ D reduction is due to the corrections of potential energy profiles, calculated by different quantum chemical techniques.

Pseudospin Hamiltonian parameters evaluated with regard to alkali metal ions in K3(H/D)(SO4)2 materials

“…It differs somewhat from the previously used (‘model I’) in Ref. 3–5. The main difference is connected to the interatomic distance R (O h …O h′ ), where O h , O h′ are the oxygens participating in the intradimer H‐bond.…”

“…The calculations are fulfilled at the Hartree–Fock (HF) levels within the 6‐31‐type basis sets. As earlier 4, 5, more economic pseudopotential method (PP‐SBK, GAMESS codes 14, 15) is also applied, because it has an accuracy comparable to HF procedure in this case 3. At the same time the energies of the two‐spin clusters are expressed in terms of J ij parameters within the limits of the static Ising model (Ω = 0) that results in system of the equations yielding the J ij values.…”

“…The 1D PES calculations are performed at the same ab initio level as the basis sets mentioned in Refs. 3–5. As earlier, the procedures mainly used are HF and MP2.…”

“…Nowadays this family is investigated actively by different experimental methods due to its unusual ferroelectric properties. Earlier we treated the ferroelectric behavior of these materials from a quantum‐chemistry viewpoint 3–5. However, the counter ions (outer‐of‐sphere alkali metal ions) are not taken into account at all in the structural models used in our investigations.…”

“…7, 8. Earlier 3–5, the available neutron diffraction data 7 for the Na 3 H(SO 4 ) 2 crystal was used in the theoretical analysis of the S‐containing TKHS materials (although its space group is P2 1 /c rather than A2/a or A2 that are typical for the TKHS family). The reason for this is that the H(SO 4 ) 2 ‐dimer has an asymmetrical form in this crystal that can be employed in deciding on appropriate structural model of the dimer in the ferroactive phase of the M 3 (H/D)(SO 4 ) 2 materials.…”