Dielectric properties of TlH₂PO₄and TlD₂PO₄under hydrostatic pressure

Abstract

The pressure (0 to 7 kbar) and temperature (-120 to +80 degrees C) dependence of the dielectric properties for TlH2PO4 and TlD2PO4 were investigated. The large isotope effects on the transition temperature (Tc) and the pressure derivative of Tc were observed. From these facts and the negative pressure coefficient of Tc, the phase transition mechanism for TlH2PO4 is microscopically discussed on the basis of the tunnelling model of the proton on the O-H---O bond. The temperature dependence of the permittivity of TlH2PO4 under constant pressure is interpreted in terms of antiferroelectric ordering of the proton by making use of the pseudospin model. It is suggested from the present dielectric study that the phase transition for TlH2PO4 and TlD2PO4 is not ferroelectric but antiferroelectric.

Phase Diagram of TlH2(P1-xAsx)O4Mixed Crystals

“…4,8,13,17,19) This implies that the II-III phase transition of TlH 2 (P 1Àx As x )O 4 is close to a tricritical point. On the other hand, isotope effect and the pressure dependence of the dielectric constant reveal a change of the II-III transition from first to second on deuteration 5,13,20) and vice versa with increasing pressure. 5) The imaginary part of the dielectric constant of TlH 2 (P 0:60 As 0:40 )O 4 pellet, " 00 , shows an anomaly similar to that of " 0 .…”

“…[4][5][6][7][8][9][10][11][12][13] The I-II transition is a ferroelastic one, [1][2][3] while the II-III transition is believed to be an antiferroelectric one 5,6) but its ferrielectric 7) or nonferroic 8,9) transition was suggested. The crystal system of the phase III was proposed as presumably monoclinic (C-centered cell with 2a  2b  c with respect to the lattice constants a, b, and c in phase II), 6,9) but was assessed as triclinic with the space group P 1 1-C 1 i .…”

“…13). [5][6][7][10][11][12][13] Upon heating TlH 2 PO 4 above 357 K (T c1 ), the high-temperature dielectric anomaly was observed at about 425 K (T p ) and was interpreted as an onset of thermal decomposition (condensation polymerization) rather than a polymorphic transition. [14][15][16] Thallium dihydrogen arsenate (TlH 2 AsO 4 ) belongs to TlH 2 PO 4 family, with transition temperatures of 391 K (T c1 ), 251 K (T c2 ), and onset of thermal decomposition 413 K (T p ).…”

“…II phase transition near T h c2 (230 K for x ¼ 0:00 and 251 K for x ¼ 1:00) on heating with a small discontinuous change of the " 0 . 4,5,8,13,17,19,20) The smearing of the discontinuity of " 0 due to the random mixing of PO 4 3À and AsO 4 3À indicates that the III ! II transition of TlH 2 (P 0:60 As 0:40 )O 4 is of the second order, while the cases of TlH 2 PO 4 (x ¼ 0:00) and TlH 2 AsO 4 (x ¼ 1:00) are of first order.…”

Phonon dynamics simulation of TlH2PO4 crystal

“…Careful structural research was carried out by different investigators in all temperature phases [1][2][3][4][5]. An exhaustive set of Brillouin scattering [6], elastic [7], resonance [8][9][10][11][12], dielectric [13][14][15][16] and spectroscopic [17,18] measurements have been performed over the last three decades. However, despite the extensive bibliography devoted to the investigation of TDP crystal, one cannot claim that all the aspects of physical properties of this crystal are properly understood.…”