The crystal structure of tetragonal KH 2 PO 4 and KD 2 PO 4 as a function of temperature and pressure

Within the four-particle cluster approximation for the proton ordering model, we study the effects of external pressures (which do not lower the crystals' symmetry) upon static and dynamic dielectric properties of deuterated ferroelectrics and antiferroelectrics of KH 2 PO 4 family. The theory provides a satisfactory description of the available experimental data. The importance of the pressure induced changes in the D-site distance in the dielectric response of the hydrogen bonded crystals is shown. Key words: KDP, pressure, spontaneous polarization, dielectric permittivity PACS: 77.84.FaIn spite of serious doubts regarding the validity of the proton ordering model raised by Raman scattering experiments [1,2], this model still remains the most elaborated and widely used approach to the description of the phase transitions in the KDP family crystals. According to the proton ordering model, these phase transitions are triggered by ordering of protons on hydrogen bonds, which leads to displacements of heavy ions, hence a spontaneous polarization arises. Therefore, the hydrogen subsystem and, particularly, the geometry of hydrogen bonds seem to play an important role in dipole moment formation in these H-bonded crystals.High pressure studies, being the only means of continually varying the parameters of hydrogen bonds, are the best tool of exploring the role of hydrogen bonds subsystem as well as the H-bond geometry in the physics involved. Amongst the examples of important insights into the problem provided by high pressure studies, there is the universality of the transition temperature vs. H-site dependence [3] observed by means of neutron scattering techniques in KDP, KD 2 PO 4 , NH 4 H 2 PO 4 , and ND 4 D 2 PO 4 strained by hydrostatic pressure. Our theoretical calculations have shown [4][5][6] that this universality is obeyed under the hydrostatic pressure by all six deuterated crystals of KH 2 PO 4 family MeD 2 XO 4 (Me = K, Rb, ND 4 , X = P, As) with a three dimensional network of hydrogen bonds as well as by K(H 0.13 D 0.87 ) 2 PO 4 c I.V.Stasyuk, R.R.Levitskii, A.P.Moina 731

Section: The Fitting Proceduresmentioning

confidence: 99%

Pressure Effect Upon the Dielectric Response of KDP Family Crystals

Stasyuk¹,

Levitskii²,

Moina³

1999

“…It is known (see [2,3]) that both KDP and DKDP show a large negative pressure dependence of T c , and T c of KDP falls to 0K at 17 kbar. According to Nelmes and co-authors [4][5][6][7][8][9], above 17 kbar, at 0K, the along-bond (zero-point) thermal motion of the H atom about each site (i. e. the two minima of the potential well with the distance δ between them), U 22 (H) is larger than (δ/2) 2 . Strictly, U 22 (H) is a mean-square amplitude along the y-axis; but this axis is very close to the line joining the two H sites.…”

Section: Introductionmentioning

confidence: 96%

“…There then seems to be a simple structural explanation for the loss of the ordered phase that does not necessarily entail tunnelling effects: above 17 kbar, the thermal amplitude of the H atom is always greater than δ/2, the distance from each of the sites to their midpoint, and it seems reasonable to expect that the H atom cannot be localised on one site only. The experimental data of [4][5][6][7][8][9] describe the changes of the KDP structure with pressure in the pressure region 1 bar p 17 kbar. In paper [12], similar investigations for the pressures 1 bar p 54 kbar were carried out, but the proton positions were not determined.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Band Electron Spectrum and Optical Properties of KDP-Crystals Under the External Hydrostatic Pressure

Stetsiv¹

1999

The electron spectrum of the KDP-type crystals has been investigated as a function of the external hydrostatic pressure using the tight-binding approximation. The joint density of electron states, real and imaginary parts of the dielectric permeability, refraction indices, the gyration coefficient, absorption and reflection coefficients for different polarizations of light are determined. Their pressure and frequency dependencies are investigated. The results are discussed by comparing them with the experimental data on piezooptic coefficients. The anomalous behaviour of the optical constants at the pressure p ≃ 17 kbar is due to the transformation of the hydrogen bond potential from a double minimum one to a single minimum one, where the proton is localised at the midpoint of the hydrogen bond.

“…This occurs if either the particles are light and/or the energy levels of the one-particle Hamiltonian of the corresponding scalar model (see below) are sufficiently separated from each other. Both these mechanisms are observed experimentally as isotopic effects (light particles) [1,38] and external hydrostatic pressure effects (big separation) [13,37]. The proof of the second theorem is performed in the approach which employs functional integrals, briefly described in section 3.…”

Section: Introductionmentioning

confidence: 99%

Quantum Effects in an Anharmonic Crystal

Kozitsky¹

2002

A model of quantum particles performing D -dimensional anharmonic oscillations around their equilibrium positions which form the d -dimensional simple cubic lattice Z d is considered. The model undergoes a structural phase transition when the fluctuations of displacements of particles become macroscopic. This phenomenon is described by susceptibilities depending on Matsubara frequencies ω n , n ∈ Z . We prove two theorems concerning the thermodynamic limits of these susceptibilities. The first theorem states that the susceptibilities with nonzero ω n remain bounded at all temperatures, which means that the macroscopic fluctuations in the model are always non-quantum. The second theorem gives a sufficient condition for the static susceptibility (i.e. corresponding to ω n = 0 ) to be bounded at all temperatures. This condition involves the particle mass, the anharmonicity parameters and the interaction intensity. The physical meaning of this result is that, for all D and all values of the temperature, strong quantum effects suppress critical points and the long range order. The proof is performed in the approach where the susceptibilities are represented as functional integrals. A brief description of the main features of this approach is delivered.