Optical spectra of ammonium sulfate in the paraelectric phase

The optical spectra of ammonium sulfate in the paraelectric phase are investigated in the visible and near-IR ranges. The temperature dependences of the parameter σ of the Urbach rule and the degree of depolarization of the luminescence spectrum exhibit a minimum in the temperature range 120–140°C. This minimum can be associated with the rearrangement of the electronic subsystem and the corresponding isostructural transformations of the lattice. In the near-IR range, the absorption spectrum contains two bands with maxima at energies of 0.50 and 0.75 eV. The shape of the bands and the temperature dependence of the conductivity allow us to interpret these bands as the result of dissociation of polarons of a large radius with binding energies of 0.12 and 0.17 eV.     

Nitrogen quadrupole coupling in paraelectric ammonium sulfate

The quadrupole coupling parameters for ^pre14N in the two chemically inequivalent sites in paraelectric ammonium sulfate have been measured by proton-nitrogen double resonance for the interval 225 ≤T≤365 K. Transition frequencies are represented to within ca. 0.2 kHz in this interval by v₊ (I) = 242.43 - 0.3380T; v₋(I) = 144.03 - 0.1855 T; v(II)=147.08 - 0.1294 T; v₋ (II) = 73.39 - 0.0281 T kHz, where T is the temperature in degrees Kelvin. These data are discussed in the context of neutron diffraction thermal parameters and an STO-3G calculation of the field gradients at the two sites. Although the temperature dependence is qualitatively in agreement with the structure data, the discrepancy between the calculated and observed asymmetry parameters, particularly for site I, suggests that hydrogen bonding to sulfate is an important feature in determining the ammonium ion charge distribution and hence the field gradient tensor at nitrogen. At 296.1 K the coupling parameters are e²gQ/h (I) = 154.53 kHz, η(I) = 0.684; e²gQ/h (II) = 115.71 kHz, η1(II) = 0.749.     

Relaxation processes in the paraelectric phase of ceramic oxides with a perovskite structure

Solid solutions of barium, strontium, and lead titanate ceramics with a perovskite structure are studied using impedance spectroscopy in the frequency range 10²–10⁶ Hz at temperatures from 20 to 450°C. The experimental data represented by the dispersion of the electric modulus are compared with the results of calculations performed for different mechanisms of the non-Debye relaxation in terms of the Havrilyak-Negami empirical formulas and the Jonscher universal dielectric response with the aim of determining a more adequate approach. The activation energy of relaxation processes in the paraelectric phase of the samples under investigation are calculated.     

Dielectric and proton relaxation from ammonium ion motion in alum

Dielectric losses and proton spin lattice relaxation T₁ and T_1? give identical correlation times in NH₄Al(SO₄)₂ · 12H₂O from 75 to 200 K. This is explained as hopping of NH⁺₄ between two positions with different orientations and electric dipole moments.     

Microscopic model of NaNO2 in the paraelectric phase

Starting from a sterical hindrance potential for the motion of the NO₂-molecular group in the deformable cage of neighbouring Na-ions, we derive a microscopic model for the NaNO₂ crystal in the paraelectric phase. The dynamical variables are the translational displacements of both the NO₂-groups and the Na-ions, and the reorientations of the NO₂-groups. Reorientations are described by means of symmetry adapted functions. From a numerical study of the model, we conclude that reorientations of the NO₂-groups take place essentially through rotations about the crystallographicc-axis. The model explains why optical experiments have led to the incorrect conclusion of reorientations about thea-axis. By studying the symmetry properties of the bilinear coupling of translations and rotations, we separate optical and acoustical displacements. Only the former couple to the order parameter in the long wavelength limit. Therefore there is no acoustical soft mode at the ferroelectric phase transitions. The bilinear coupling leads to an effective lattice mediated interaction among reorienting NO₂-groups.     

Low-temperature electrolytic coloration and spectral property of ammonium alum crystals

Ammonium alum crystals are colored electrolytically using a pointed cathode and a flat anode at low temperatures and under various voltages. SO₃^?, SO₂^? and O₃^? hole-trapped centers are produced in colored ammonium alum crystals. Characteristic absorption bands of SO₃^?, SO₂^? and O₃^? hole-trapped centers are observed in absorption spectra of colored ammonium alum crystals. Production and conversion of hole-trapped centers are explained. Current–time curves for electrolytic coloration of ammonium alum crystal and their relationship with electrolytic coloration process are given.     

Temperature variation of the optical absorption edge for ammonium aluminum alum

We have studied the absorption spectra of NH₄Al(SO₄)₂·12H₂O crystal hydrates in the visible region of the spectrum, near the low-temperature phase transition at T_c = 76 K. We have shown that the absorption edge in these materials has an exponential shape, following the empirical Urbach’s rule as a result of exciton-phonon interaction. The relatively strong exciton-phonon interaction is associated with a band formed by self-localized (self-trapped) excitons. The experimental data obtained support the presence of a phase transition in the studied ammonium alum at 76 K. We have shown that ammonium aluminum sulfate dodecahydrate crystals are promising materials for fabricating laser frequency converters. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 2, pp. 258–262, March–April, 2007.     

Mössbauer effect study of relaxation mechanism in57Fe-doped ammonium alum

The Mössbauer spectra of the ammonium alum doped with approximately 1% of⁵⁷Fe for T. 80K have been considered. The characteristics of the spectra have been reproduced by taking into account that the parameter λ of the spin Hamiltonian is different from zero and that the spin-spin interaction couples the states of each one of the Kramers doublets corresponding to S_z+1/2 and ·S_z±3/2.     

Dynamics of NaNO2 in the paraelectric and in the incommensurate phase

On the basis of a microscopic model with translation-rotation coupling, a set of coupled dynamic equations is derived. The corresponding resonances are studied in the paraelectric and in the incommensurate phase. The theory gives a qualitative explanation of inelastic neutron scattering results.     

Thermally stimulated electron emission from chromium-doped triglycine sulfate in the paraelectric phase

Thermally stimulated electron emission from a ferroelectric chromium-doped triglycine sulfate (TGS) crystal was experimentally observed to occur in a temperature range 6 K above the Curie point from samples heated at a relatively high rate. Increasing the heating rate q was shown to cause the emission current density to increase throughout the temperature range studied. The emission onset temperature in chromium-doped TGS depends only weakly on the rate q and is close to that for pure TGS, and the emission cutoff temperature grows monotonically with q at comparatively low heating rates and stabilizes at high q. At the same time, the interval of emission extension into the paraelectric phase here is about one half that for pure TGS heated at the same rate. The specific features of emission observed for this crystal can be assigned to relaxation of the charges screening the spontaneous polarization. The lower emission cutoff temperature for the chromium-doped TGS compared to that for pure TGS is accounted for by the shorter Maxwellian relaxation time in the doped crystal.     

Indication for a novel phase in the quantum paraelectric regime of SrTiO3

Electron paramagnetic resonance of Fe³⁺ gives evidence for a phase-transition-like feature in SrTiO₃ belowT _q =37±1 K, both in the tetragonal and the pressure-induced trigonal phase. In the latter,T _q shifts to lower temperatures as a function of uniaxial stressp ₁₁₁. The new phenomenon is very tentatively discussed in terms of a possible transition to a novel coherent quantum state.     

Second harmonic generation in the paraelectric phase in powders and ceramics of BaTiO3

A second harmonic signal has been clearly observed in powders and ceramics of BaTiO₃ in the high-temperature phase. The effect manifests itself at temperatures 500 K above the temperature of the phase transition from the tetragonal phase to the cubic phase, which is also significantly higher than the Burns temperature of BaTiO₃. The temperature dependence of the second harmonic generation signal I _2?? in the paraelectric phase becomes a straight line when plotted in the Arrhenius coordinates lnI _2??(1/T). The experimental results obtained indicate the presence of noncentrosymmetric local regions in the paraelectric phase over a wide range of temperatures. The contribution from these polar regions to the second harmonic generation increases as the phase transition temperature is approached. The temperature dependence of the second harmonic generation in barium titanate ceramics is significantly affected by the technique used for pressing and annealing of the samples.     

Defect induced fluctuations in the paraelectric phase of KTa0.94Nb0.06O3

First order Raman lines observed in the paraelectric phase of KTa_0.94Nb_0.06O₃ are shown to be the result of odd symmetry microdistortions or fluctuations with a correlation length that extends to several unit cells. The possibilities that the lines result from impurity induced or disorder induced Raman scattering are considered and discarded. From simple theoretical considerations it is shown that the integrated intensity of distortion induced first order Raman lines, normalized by the corresponding phonon occupation numbers, is proportional to the average distortion intensity. This proportionality is independent of the correlation length of the distortion field provided it is larger than a certain minimum.Using this relation and the experimentally measured integrated intensity of the TO₂ line we find that the average distortion intensity is proportional toT/(T _c—T). This result is compared to two models which qualitatively predict the existence of microdistortions or fluctuations in the paraelectric phase. The experimentally observed temperature dependence is found to be consistent with the defect induced fluctuations model and inconsistent with continuous phase transitions models.     

Phase transition in ammonium aluminium and ammonium chromium alums— A high pressure EPR study

An investigation of the phase transitions at high pressures in the alums mentioned in the title has been carried out using EPR of the Cr³⁺ ion (at the trivalent metal ion site). It is observed that at ambient as well as at high pressures there is a change of slope in the linear variations of the zero field splitting with temperature and that the low temperature phase is characterised by a large number of lines in the EPR spectra. The transition temperature shows a large positive shift with pressure, for both the alums. All these facts are explained in terms of our model of the origin of the trigonal field at the trivalent metal ion site as well as the details of the motion of NH₄⁺ ion.     

Fluctuation processes in the surface region of indium antimonide

Amplitudes of fluctuations in the surface potential in MOS structures are studied as a function of the state of the semiconductor substrate surface, the measurement frequency, and the intensity of background illumination. It is established that the dominant noise in the depletion-weak inversion region is generation-recombination noise produced by recharging of a deep impurity in the indium antimonide surface layer. In the strong inversion region explosive noise may be dominant. The relationship of the latter to structural defects of the semiconductor substrate is clarified. A model is proposed for generation of explosive noise in MOS-structures, based on the assumption of microplasma breakdown of the space charge region with an impurity defect atmosphere and phase readjustments of the matrix and oxide material. It is concluded that it is possible to reduce explosive noise in MOS-structures.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 60–65, September, 1988.The authors express their gratitude to I. I. Fefelova and B. G. Plomip for preparation of the MOS-structures.     

Isotope effects in paraelectric relaxation

Through measurements of the complex dielectric susceptibility in the temperature range between 0.3°K and a few °K we have studied the influence of the isotopic substitutions H→D and¹⁶O→¹⁸O on the relaxation time of the hydroxyl dipole in weakly doped KBr and RbCl. The H→D effect is present in both host lattices. The oxygen effect, however, is only for RbCl outside experimental error, indicating that in this host lattice the reorientation of the hydroxyl ion involves a considerable displacement of its center of mass.     

Effect of electric field on the dielectric permittivity of betaine phosphite crystals in the paraelectric phase

Temperature dependences of the dielectric permittivity of betaine phosphite crystals are studied both without and under application of an electric bias. It is shown that, in view of the fact that the high-temperature improper ferroelastic (antiferrodistorsive) phase transition at T _c1=355 K is nearly tricritical, the nonlinear temperature dependence of inverse dielectric permittivity in the paraelectric phase and the effect of the field on the dielectric permittivity can be described within a phenomenological model containing two coupled (polar and nonpolar) order parameters with a negative coupling coefficient. An analysis of the model revealed that, in the case where two phase transitions, a nonpolar and a ferroelectric one, can occur in the crystal, all of its dielectric properties, including the polarization response in a field, can be described by one dimensionless parameter a. For the crystal under study, we have a=?2.5. This value of the parameter corresponds to a second-order ferroelectric transition far from the tricritical point, at which a=?1. It is shown that the polarization response in the paraelectric phase in an electric field calculated within this model differs radically from that in the ferroelectric phase-transition model for which the Curie-Weiss law holds in the paraelectric phase.     

Polar regions in the paraelectric phase in a PbZr0.992Ti0.008O3 single crystal

Anomalous behaviour of ε and tan δ in the paraelectric phase was found for PbZr0.992Ti0.008O₃ single crystal. In the vicinity of temperatures 280–290°C (T _c = 236°C) the pyroelectric effect was also observed. Using the data obtained, the existence of an additional polarization (polar regions oriented in an external electric field) in the paraelectric phase is discussed.