Dielectric relaxation of mixed crystals of Rb1-x(NH4)xH2PO4 at microwave frequencies

Measurements of the clamped dielectric constant on mixed single crystals of Rb_1-x(NH₄)_xH₂PO₄,x=0.35, are reported up to 11 GHz. Between 24 and 60 K, the dielectric dispersion can be fitted to a Cole-Cole relaxation, the parameters of which indicate a temperature-dependent distribution of relaxation times consistent with Vogel-Fulcher freezing. Both the audio and the microwave measurements can be scaled up to 100 MHz, with a freezing temperature ofT _o8.4 K. In the GHz range, a relaxation process in addition to the low-frequency freezing mode contributes to the dielectric response.     

The structure of the glass phase of Rb1−x(ND4)xD2PO4

X-ray scattering techniques have been used to study the diffuse scattering from a single crystal of Rb_1–x(ND₄)_xD₂PO₄ withx=0.65. This system has a structural glass phase at low temperatures resulting from the competing ferroelectric interactions of RbD₂PO₄ and antiferroelectric interactions of (ND₄)D₂PO₄. The diffuse scattering shows a broad peak with a maximum occurring at a wavevector of about 0.3a ^*, and the intensity of these peaks is surprisingly different for wavevectorsq on opposite sides of the Bragg reflections. A model of the D bonding is developed which suggests that the diffuse scattering arises from the interaction between ferroelectric displacements alongc, ferroelectric displacements alongb, and transverse acoustic modes polarized alongb andc. The model accounts for the incommensurate wavevector and, qualitatively, for the intensity of the diffuse scattering around different Bragg reflections. The temperature dependence of the scattering is also measured.     

Molecular field study of the isotopic effect in KH2x D2(1−x)PO4 mixed crystals

Theory of KDP and isomorphous crystals is developed, which takes into consideration possible difference in ferroelectric interactions between pairs of H-bonds with deuteration. It is shown, why the interaction should differ for pairs of H-H, D-D, and H-D isotopes. Expressions for the transition temperature and static susceptibility above it are derived by approximating the molecular field acting on a proton (deuteron) by the field of the average surrounding. The theory results in a linear dependence of the asymptotical transition temperature and the Curie-Weiss constant on concentration. For the actualT _c(x) also a linear dependence was found under the condition that tunneling is small, the distribution of H, D isotopes over H-bonds is quite random, and that the interaction between isotopes is of purely dipolar character. Modifications of the linear dependence due to ordering and deviations of the interaction from dipolar are found. It is further suggested, as a result of comparison with recently performed measurements of pressure dependence of the transition point, that the main part of the isotopic shift of the transition point is due to the difference in H-H and D-D coupling, whereas the tunneling contributes by a relatively small amount.The author has the pleasure to express his sincere thanks to Dr. V. Janovec and Dr. V. Dvoák for many valuable discussions and to Dr. A. Fousková for information about unpublished results.     

Vibrational study of the superprotonic phase transition in the mixed crystal Rb2(HSeO4)(H2PO4)

Raman spectra (10–1200 cm⁻¹) of polycrystalline samples of Rb₂(HSeO₄)(H₂PO₄) were studied at temperatures ranging from 300 to 423 K. An assignment of most of the observed bands is proposed. The first‐order phase transition previously detected at 382 K was characterized as: This superionic‐protonic transition is believed to be governed by librations of the HSe/PO₄²⁻ ion and the A OH (A = Se, P) stretching mode. It corresponds to the weakening of  Se(P) O H˙˙˙ H O Se(P) hydrogen bonds and to the melting of the proton sublattice into a quasi‐liquid state in which the protons and the HSe/PO₄²⁻ ions contribute to the unusually high conductivity. The activation energy that was determined from the plot Δν_1/2 versus temperature for the ν (A OH) band has the same order of magnitude as that determined from conductivity measurements. Copyright © 2006 John Wiley & Sons, Ltd.     

Renormalization phenomena in Ba-diluted ferroelastic lead phosphate, (Pb1-x Ba x )3(PO4)2

Abstract

In the improper ferroelastic palmierite-type lead phosphate order parameter coupling with a defect induced conjugate field leads to the renormalization of the two different critical temperatures of three order parameter components {Q ₃} and {Q ₁ Q ₂}. The influence of the lead dilution by barium on the ferroelastic domain pattern, the critical temperature of the ferroelastic transformation R m–C2/c and the development of the intermediate regime in (Pb_1–x Ba _x )₃(PO₄)₂ is studied using optical birefringence measurements, Raman and infrared spectroscopy. At the ferroelastic transition temperature T _c the orientational contribution of the three-states Potts model becomes critical. T _c is reduced from 453 K (x = 0) to zero K at x? 0.12. Modifications of the shape of zigzag needle domains as well as the angle between the monoclinic binary axis and the W walls along [031] with temperature and increasing Ba-content are reported. Above the ferroelastic transition point the component Q ₃, which corresponds to the displacive part in the Gibbs free energy, leads to dynamic short-range monoclinic deformation in the trigonal matrix. The temperature where Q ₃ shows critical behaviour is renormalized to 720 K (x = 0.12) as compared with 563 K in pure lead phosphate. For x>0.13 no monoclinic precursor clusters were found.     

Nonlinear dielectric response in the mixed K0.91(NH4)0.09H2PO4 crystal

The influence of bias and variable electric fields on the dielectric response of the K_0.91(NH₄)_0.09H₂PO₄ single crystal has been studied in the vicinity of the ferroelectric phase transition temperature T _C. Below T _C, the nonlinear response is caused mainly by the domain mechanism. This is confirmed, in particular, by observations of chaotic oscillations in a series RLC-circuit containing the studied sample as a capacitor C and excited by a sinusoidal voltage. Peculiarities of the behavior of the dielectric nonlinearity are found near the Curie temperature, which are explained by the emergence of an intermediate heterophase state.     

X-ray determination of mean Debye-Waller factors and Debye temperatures of the K x Rb(1−x)Br mixed crystal system

The integrated intensities of Bragg reflections have been measured for mixed crystals in the K _x Rb_(1−x)Br system with an x-ray powder diffractometer. From the intensities, the mean Debye-Waller factors are determined. The Debye-Waller factors are corrected for static contribution and Debye temperature values are determined for the entire composition range. The x-ray Debye temperatures follow the Kopp-Neumann equation closely.