Ab-initio calculation of band structure and linear optical properties of SbSI in para- and ferroelectric phases

An ab-initio pseudopotential calculation has been performed by using density functional methods within the local density approximation (LDA) to investigate the band structure and optical properties of the ferroelectric-semiconductor SbSI in the para- and ferroelectric phases. It has been shown that SbSI has an indirect gap in both phases (1.45 eV and 1.49 eV in the para- and ferroelectric phases respectively) and that the smallest direct gap is at the S point of the Brillouin zone (1.56 eV and 1.58 eV in the para- and ferroelectric phases respectively). Furthermore, it is shown that first-order phase transition, from the paraelectric phase to the ferroelectric phase (the transiton temperature is about 22 °C), does not change the nature of the band gap. Moreover, the linear frequency dependent dielectric function, including self-energy effects, has been calculated along the c-polar axis in the para- and ferroelectric phases.      

Interfaces and ripple states in ferroelastic crystals—A simple model

A simple atomistic model with two harmonic potentials between layers of atoms describes surface relaxations equivalent to those found as kink solitary waves in ferroelastic crystals. The parameters of the model are expressed in terms of the entropy term in a Landau potential and the Ginzburg gradient energy. We discuss the possibility that metastable ripple states exist in which a modulation is superimposed to an underlying uniform order parameter. Such ripple states can occur at temperatures close to the transition point between a ferroelastic phase and an incommensurate phase. In the ripple state, domain walls consist of kinks with modulations on either side of the kink.     

The domain structure of ferroelastic BiVO4 crystal studied by Gd EPR

The domain structure of ferroelastic BiVO₄ single crystal has been investigated using the electron paramagnetic resonance (EPR) of the Gd³⁺ ions existing as an impurity in the crystal. Two sets of Gd³⁺ EPR signals were obtained in the crystallographic ca-plane. These two sets of signals originated not from the two kinds of substitutional sites but from the twin-domain structure in the host crystal. It is found that the BiVO₄ crystal investigated with Gd³⁺ EPR has the prominent (W-plane) domain wall. The domain structure is stable in contrast with a previous report by Baran et al. From the observed W-plane of the domain wall, it is suggested that a ferroelastic transition in BiVO₄ is 4/mmm F 2/m instead of 4/m F 2/m. The model of twinning mechanism improved in a previous report by Mn²⁺ EPR is confirmed by Gd³⁺ EPR.     

The structure and ferroelastic phase transition of BiVO4

The structure of BiVO₄ has been determined by the Rietveld method at 300, 573 and 898 K and confirms the space group of the ferroelastic phase as 12/a and that of the paraelastic phase as 14₁/a. The displacement vectors of the atoms involved in the switching of ferroelastic states are calculated and indicate that the Bi displacements are the controlling factors in the process. An analysis is given of the eigenvectors of the B_g mode responsible for the transition.     

Ferroelastic domain structure of (CH3)4N CdCl3 (TMCC) crystal

The ferroelastic domain structure and the phase boundaries of TMCC have been studied in the temperature range 114-90 K by direct observation under polarised light. By applying an external, compressive and unidirectional mechanical stress the ferroelastic character of the domain structure has been confirmed. The orientation of the domain walls and phase boundaries are analysed. To characterise quantitatively the observed domain wall distribution the classical symmetry approach, based on the criterion of spontaneous strain compatibility, has to be extended to allow small rotations of the domain walls with respect to their ideal orientation. The observed switching process among the different domains can be understood as a mechanism that minimises the elastic energy. Received 21 July 2000     

Analysis of the ferroelastic domains and phase transition in the K2SO4 crystal

Abstract

The domain structures of the β-K₂SO₄ crystal were analyzed by group theory. We obtained the permissible kinds of domain association and domain walls from the results of the group theory. It is suggested from the analysis that the (110) and (130) planes are W_mb walls. The value of the spontaneous strain of K₂SO₄ wast = 6.32 × 10^?3 at room temperature and also its temperature dependence was observed.     

Observation of87Rb NMR satellite transitions in the commensurate and incommensurate phases of Rb2ZnCl4

It is shown that the first order quadrupole split NMR satellite transition frequencies of the⁸⁷Rb nucleus can be detected in the paraelectric, incommensurate and ferroelectric phases of Rb₂ZnCl₄. From rotation patterns the electric field gradient tensor at the Rb sites is determined for the paraelectric phase. The data demonstrate a considerable influence of the structural changes in the incommensurate and ferroelectric phases on the observed NMR transition frequencies. For some crystal orientations the satellite transitions are followed through the incommensurate into the ferroelectric phase. Whereas in the former typical quasi continuous spectra are observed in the latter several sharp lines appear. The results are discussed in relation to the structural changes at the phase transitions.     

The crystal structure of Fe2In2Se5, a FeIn2Se4-related polytype

Fe₂In₂Se₅, a polytype of FeIn₂Se₄ (a material belonging to the II □ III₂ VI₄ family of semiconducting compounds) has been synthesized by conventional solid-state reaction of their constituent elements. The product of the reaction was sequentially used as starting material in the crystal growth process carried out by chemical transport using I₂ as transporting agent. The crystal structure of a new polytype of this compound was determined using single crystal techniques with data collected with a CCD-based diffractometer. The successful indexing of the data and refinement of the structure led to an hexagonal unit cell with a=4.0371(4) and c=32.746(4) Å. Although the absorption effect in the data was quite noticeable, because of the layered morphology exhibited by the material, a good agreement was obtained for a structural model similar to the structure reported for a related polytype belonging to the ZnIn₂S₄ system.     

Acoustic study of the ferroelastic phase transition in LiCsSO4 crystal

The paper reports on an acoustic study of the temperature dependences of the ultrasonic-wave velocity and attenuation in a LiCsSO₄ crystal within the 190–295 K temperature region, which includes the interval of the pseudoproper second-order ferroelastic phase transition (202 K). The velocity of the transverse xy acoustic mode is found to decrease by more than six times at the phase transition. The possibility of performing ultrasonic studies both in the region of the ferroelastic phase transition temperature and below it is demonstrated. The results are treated in terms of Landau’s theory. Waves not associated with the soft mode are shown to exhibit anomalies which are supposedly due to an intermediate phase, whose existence was suggested in a number of publications.     

Investigation of the ferroelastic phase transition in the SrMgF4 pyroelectric crystal

A SrMgF₄ compound has been synthesized and a high optical-quality crystal has been grown. Optical-polarization observations, X-ray diffraction analysis, and the measurement of the birefringence Δn _i (t) in the SrMgF₄ crystal have been carried out in the temperature range of 90–1200 K. A second-order improper ferroelastic phase transition accompanied by birefringence anomalies and the symmetry change P112₁ (Z = 12) ? Cmc2₁ (Z = 4) has been discovered at T ₀ = 478 ± 1 K. The crystal remains pyroelectric in both phases. Considerable contributions of the fluctuations of the order parameter have been observed in the temperature ranges of (T ₀ ? T) < 15 K and (T ? T ₀) < 60 K.     

Electronic structure of non-centrosymmetric AgCd2GaS4 and AgCd2GaSe4 single crystals

X-ray photoelectron core-level and valence-band spectra for pristine and Ar⁺-ion irradiated (0 0 1) surfaces of AgCd₂GaS₄ and AgCd₂GaSe₄ single crystals grown, respectively, by the Bridgman method and the method of direct crystallization have been measured in the present work. The X-ray photoelectron spectroscopy (XPS) results reveal high chemical stability of (0 0 1) surfaces of AgCd₂GaS₄ and AgCd₂GaSe₄ single crystals. Electronic structure of AgCd₂GaS₄ has been calculated employing the full potential linearized augmented plane wave method. For the AgCd₂GaS₄ compound, the X-ray emission bands representing the energy distribution of the valence Ag d-, Cd d-, Ga p- and S p-like states were recorded and compared on a common energy scale with the XPS valence-band spectrum. The theoretical and experimental data regarding the occupation of the valence band of AgCd₂GaS₄ were found to be in excellent agreement to each other. Second harmonic generation (SHG) efficiency of AgCd₂GaS₄ by using the 320 ns CO laser at 5.5 μm has been recorded within the temperature range 80–300 K. Substantial increase of the photoinduced SHG which in turn is substantially dependent on the temperature has been detected for the AgCd₂GaS₄ compound.     

Analysis of incommensurate structure in [N(CH3)4]2ZnCL4 crystal

We analyzed the temperature dependence of the amplitude of modulated structures of [N(CH₃)₄]₂ZnCl₄ crystal in the incommensurate (INC) phase by measuring the EPR spectra for Mn²⁺ doped single crystal. The amplitude of the vibrational modulation linearly increased with decreasing temperature. Applying Frank and Van der Merwe model, we simulated the EPR spectra in the INC phase. The simulated spectra agreed with the observed one very well. We, therefore, revealed that the vibrational modulation in this INC phase is due to the interaction between the harmonic chain of inter-particles and the modulation due to underlying potential which comes from the commensurate structure.     

High Tc superconductors with different Cu2+/Cu3+ ratios in the Y–Ba–Cu–O system

Studies on different phases in the Y-Ba-Cu-O system have revealed that T_C around 90 K is obtainable with different Cu /Cu ratios provided the correct oxygen proportion is maintained.     

The crystal structure, magnetism, and colossal magnetoresistance of Y2CrS4

Ternary yttrium chromium sulfide,Y2CrS4,prepared by the solid-state reaction of Y2S3,Cr,and S,was found to exhibit an antiferromagnetic transition at about 64 K.The X-ray diffraction pattern at 300 K was refined with space group Pca2 1,and the structure parameters were determined to be a = 12.51 Ab = 7.53A,and c = 12.49A,We investigated the magnetotransport properties,and observed negative colossal magnetoresistance reaching up to 2.5 × 10^ 4 % in the semiconducting compound of Y2CrS4.     

High-temperature crystal structure and electronic properties of cesium niobate Cs2Nb4O11

The antiferroelectric material Cs₂Nb₄O₁₁ transforms at 165 °C from a low-temperature, antiferroelectric phase in space group Pnna to a high-temperature, paraelectric phase in space group Imma; the latter structure has been determined by single-crystal X-ray diffraction. The high-temperature lattice is comprised of niobium-centered tetrahedra and octahedra connected through shared vertices and edges; cesium atoms occupy channels afforded by the three-dimensional polyhedral network. Calculated band structures for both phases predict a bandgap of 3.1-3.2 eV, which is similar to that found experimentally through photoluminescence. The calculated band structure is also conducive to its observed photocatalytic properties.