Anharmonic effects in potassium-dihydrogen-phosphate-type ferroelectrics

Evidence for phase transitions and pseudospin phonon coupling in K0.9(NH4)0.1H2AsO4

“…Appearance of mode K11 below T c * clearly suggest different site symmetry of AsO 4 tetrahedra in the orthorhombic and tetrahedral phase rather than same site symmetry in both phases as suggested earlier. 30 We note that low temperature splitting of mode v 1 (Raman band around ∼ 800 cm ) is also reported for similar systems such as K 1-x (NH 4 ) x H 2 PO 4 , CDA and CADA. 16,21,23 Splitting of symmetric stretching mode (v 1 ) may be due to different combinations of surrounding ions NH 4 + and K + with AsO 4 tetrahedra, which may give non-zero perturbation to the v 1 frequency due to slightly different molecular interaction.…”

“…Simultaneously, the anharmonic interaction decreases with decreasing temperature and phonon lifetime increases (which is inversely proportional to the phonon damping coefficient). Wesselinowa et al, 30 considered the pseudospin-phonon and phonon-phonon interaction within the pseudospin Ising model in potassium dihydrogen phosphate type ferroelectrics and found that contribution to phonon self-energy above phase transition is coming from anharmonic interaction. However, contribution at low temperature is dominated by pseudospin-phonon interactions.…”

Theory of related to shear strain u6 physical properties of ferroelectrics and antiferroelectrics of the KH2PO4 family

“…It indicates an important role played at these temperatures by the lattice anharmonicity (see also [19]) which we took into account phenomenologically. The temperature curve of the "seed" elastic constant C E0 66 ðTÞ is almost parallel to the curve of the elastic constant c TP 66 and lies slightly above it.…”

“…Second, the used in [9] model [8] does not contain any possible manifestation of high-temperature lattice anharmonicity and this negatively affects (as will be illustrated in the present paper) an agreement between theory and experiment for, first of all, the elastic constant of a KH 2 PO 4 crystal at temperatures essentially higher than T c . The importance of the lattice anharmonicity above T c has been noted in paper [19] as well.…”

Hydrous Phases and Hydrogen Bonding at High Pressure

“…Amongst the more studied of the H-bond ordered ferroelectrics is the family of structures based upon potassium dihydrogen phosphate (KHP) (Ramakrishnan and Tanaka 1977, Kumar et al 1992, Rakvin and Dalal 1992, Wesselinowa et al 1994, Gallardo et al 1997. The transition from the ferro-to para-electric state in this class of materials is dramatically affected by substitution of deuterium for hydrogen.…”