Temperature dependence of the quadratic electrooptic effect and estimation of antipolarization of ADP

Measurements of the temperature dependence of the quadratic electrooptic coefficient |g₁₁₁₁‐g₂₂₁₁| of ADP were made. The coefficient is found to decrease with an increase in temperature. The results show that the intrinsic quadratic electrooptic coefficient |M₁₁₁₁‐M₂₂₁₁| defined in terms of polarization also dependes on temperature. Employing the temperature dependence of |M₁₁₁₁‐M₂₂₁₁| estimation of antipolarization in lowtemperature antiferroelectric phase of ADP is made. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Domain structure and birefringence studies on a 0.91Pb(Zn1/3Nb2/3)O3 ‐0.09PbTiO3 single crystal

Domain structure and phase transition sequence of 0.91PZN‐0.09PT single crystal, grown from high temperature solution, have been analysed using polarised light microscopy. The domain structure of (001)_cub cut single crystal exhibit co‐existence of rhombohedral and tetragonal phases. The sequence and phase transition temperatures have been determined from temperature dependence of birefringence. Birefringence measurements during heating and cooling reveal a first order nature of phase transition between rhombohedral (R3m) and tetragonal (P4mm). The birefringence was measured with accuracy of 10^‐3. However, dielectric measurement does not provide any evidence of R3m ‐ P4mm phase transition. It is shown that in‐situ analysis of domain structure and phase transition can be used as non‐destructive analytical tool for determination of local composition and phase transition sequence. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Barrier to crystal nucleation in anorthite

The kinetics of crystal nucleation in anorthite (CaAl₂Si₂O₈) have been determined over the range of undercooling between 475 and 600°C. The plot of In(I_vη) versus (ΔT_|staggered|r²T_|staggered|r³)^?1 is a straight line of negative slope over the temperature range investigated. The slope of this relation indicates a free energy barrier to crystal nucleation of about 82 kT1 (where $\text{T1 = 0.8T}{}_{\mathrm{E}}$). The intercept at (ΔT_|staggered|r²T_staggered|r³)^?1 = 0 is approximately 10²⁷ cm^?3 s^?1P. Samples of anorthite glass were also analyzed by differential thermal analysis, with the temperature of maximum crystallization rate being determined as a function of heating rate. From this variation, a nucleation barrier of about 80 kT1 was estimated. The combined results are in good agreement with predictions of the classical theory of homogeneous nucleation.     

Structural properties of Zn1–xMgxO nanomaterials prepared by sol‐gel method

The mixed oxides Zn_1‐xMg_xO (ZMO) were prepared as nano‐polycrystalline powders and thin films by a simple sol–gel process and dip coating method. Thermogravimetric (TG) and differential thermal analysis (DTA) were used to study the thermal chemistry properties of dried gel. Structural and microstructural analysis was carried out applying x‐ray diffraction (XRD) and Rietveld method. Analysis showed that for x < 0.25, Mg replaces Zn substitutionally yielding ZMO single phase, while for x ≥ 0.25 two phases are identified ZMO and MgO. Replacing Zn²⁺ by Mg²⁺ distorts the cation tetrahedrons and decreases the lattice constants ratio c/a of the wurtzite ZMO which deviate the lattice gradually from the hexagonal structure as Mg⁺² increases. These distortions are attributed to the difference in electronic configuration of the two cations which suppress the paraelectric‐ferroelectric phase transition in the ZMO wurtzite. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth,structure and thermal properties of CuAlxGa1‐xSe2 alloys

Ingots of the CuAl_xGa_1‐xSe₂ (0 ≤ x ≤ 1) alloys system were prepared by direct fusion of the stoichiometric mixture of the elements. The analysis of X‐ray Powder Diffraction data showed the presence of one single phase with chalcopyrite tetragonal structure at room temperature for all the studied compositions. The lattice parameters, a and c, and the bond lengths were calculated. The phase transition temperatures were obtained by the onset method from Differential Thermal Analysis measurements performed on samples sealed in evacuated quartz ampoules. Fusion or transition enthalpies were determined from the area of the corresponding DTA peak. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High resolution neutron and X‐ray diffraction studies as a function of temperature and electric field of the ferroelectric phase transition of RDP

Neutron and high resolution X‐ray diffraction investigations on perfect single crystals of RbH₂PO₄ (RDP), a hydrogen bonded ferroelectric of KDP type are reported. The results of crystal structure analysis from diffraction data, below and above the paraelectric ‐ ferroelectric phase transition, support a disorder ‐ order character of [PO₄H₂]^‐‐groups. The tetragonal symmetry of the paraelectric phase with the double well potential of the hydrogen atoms obtained by diffraction, results simply from a time‐space average of orthorhombic symmetry. According to the group ‐ subgroup relation between the tetragonal space group I42d and the orthorhombic Fdd2 a short range order of ferroelectric clusters in the tetragonal phase is observed. With decreasing temperature the ferroelectric clusters increase and the long range interaction between their local polarisation vectors leads to the formation of lamellar ferroelectric domains with alternating polarisation directions at T_C = 147 K. From the high resolution X‐ray data it is concluded that below T_C the ferroelastic strain in the (a,b)‐plane leads to micro‐angle grain boundaries at the domain walls. The tilt angle is enhanced by an applied electric field parallel to the ferroelectric axis. The resulting dislocations at the domain walls persist in the paraelectric phase leading to a memory effect for the arrangement of twin lamellae. With increased electric field the phase transition temperature T_C is decreased. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural Characterization of Y2Pd14B5

Y₂Pd₁₄B₅ is the major phase in as‐cast and annealed multiphase alloys with nominal compositions near YPd₅B₃C_0.3. Its crystal structure determined the first time here by single crystal X‐ray diffraction is body‐centered tetragonal (space group I4₁/amd). Transmission electron microscopy (TEM) reveals that in as‐cast specimens the tetragonal phase has a modulated structure and is oriented intergrown with a face‐centered cubic phase of similar composition, namely YPd₇B₂. According to Rietveld analyses of the multiphase system the structure of this phase can be well‐described by space group Fm m. Annealing the sample for 150 hrs at 973 K results in a coarsening and enrichment of the tetragonal phase as well as a disappearance of the modulations allowing a detailed structure analysis by single crystal diffractometry.     

Soft‐Mode Spectroscopy in Fe:KTa1‐xNbxO3 Crystals

Fe‐doped potassium tantalite niobate, Fe:KTa_1‐xNb_xO₃ (Fe:KTN), crystallizes with x = 0.48 and a perovskite‐type structure in tetragonal system with point group 4mm, conforming space group P4mm. The paraelectric‐ferroelectric structural transition of the Fe:KTN is studied by Raman scattering investigations. A condensed soft lattice vibrational mode at the phase transition has been analyzed. It originates from the symmetric O2/O3‐Nb/Ta‐O3/O2 in‐plane bending of the Nb/TaO₆ group. The soft optical phonon mode concerns the extraordinary transverse optical phonons propagating along the [110] direction. The Raman spectra measured reflect the crystal disorder. Curie temperature measured by two methods is within 353 and 356.5 K.     

A Structural Study of Aluminate Sodalite Ca8[Al12O24](CrO4)2(CACr)

The structure of the cubic high temperature phase of Ca₈[Al₁₂O₂₄] (CrO₄)₂, CACr, has been determined at 700 K by Rietveld analysis of neutron powder diffraction data. The structure is discussed with respect of those of its homologues. The phase transitions of the title compound have been investigated by means of synchrotron x-ray powder diffraction and differential thermo analysis. High precision lattice parameters have been determined in the temperature range between 350 and 750 K. Three phase transitions occurring at 610 K, 453 K, and 432 K could be verified. Observed anomalies in the lattice parameters versus temperature might indicate a yet unknown phase transition at about 555 K. This is supported by results of differential thermo analysis.     

Dielectric characteristics of Ce3+ doped Sr0.61Ba0.39Nb2O6 with Cole‐Cole plots technique

In this paper, we have investigated two‐relaxator mechanism and dielectric characteristics of Ce³⁺ doped Sr_0.61Ba_0.39Nb₂O₆ with dielectric spectroscopy measurements. The crystal undergoes a ferroelectric phase transition at 340 K. The temperature dependence of the real and imaginer part of the complex dielectric susceptibility in vicinity of ferroelectric‐paraelectric phase transition has been studied in the frequency region 0.1 kHz–10 MHz. The measurements of the dielectric constant of the real and imaginer parts show strongly frequency dependence. The investigations of the dielectric constant revealed a non‐Debye type dielectric relaxation for Ce⁺³ doped SBN61 by using Cole‐Cole plots. It reveals the coexistence of the two dielectric relaxators in vicinity of the phase transition. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal structure and thermal behaviour of (Rb,Cs)BSi2O6 solid solutions

The crystal chemistry of Rb‐Cs boroleucites has been studied by means of X‐ray powder diffraction at room and elevated temperatures. The cubic I‐43d → cubic Ia3d phase transition was investigated using a series of samples prepared by solid‐state reaction along the pseudobinary system RbBSi₂O₆ ‐ CsBSi₂O₆. The Rietveld refinement of the structures of Rb_1‐xCs_xBSi₂O₆ solid solutions (x = 0.2, 0.4, 0.6, 0.8) demonstrates that the solutions with a high Rb content crystallise in the cubic I‐43d space group, and the boroleucites with a considerable Cs content have Ia3d symmetry. Rb can substitute Cs in a wide range of compositions. Within a narrow range of x = 0.5 ‐ 0.6 immiscibility was revealed. Under Rb‐Cs substitution the cubic lattice parameter, the (Rb,Cs)‐O distances, and the angles between tetrahedra of the I‐43d phase change clearly, while those of the Ia3d phase change slightly. The HTXRD data shows that the I‐43d phase transforms into a Ia3d phase on heating analogously to a change of the composition. As the Cs content increases the transition temperature decreases. The low temperature I‐43d phase shows a higher thermal expansion than the high temperature Ia3d phase. © 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim     

Crystal structure of non-stoichiometric copper selenides studied by neutron scattering and X-ray diffraction

Structural characteristics of non-stoichiometric copper selenides were studied by the elastic neutron and X-ray scattering techniques. Rietveld analysis was used to refine the structure of the high-temperature β-phase of the Cu_1.75Se, Cu_1.78Se, and Cu_1.83Se samples. The homogeneity ranges of the cubic phase were determined. The modification of the crystal structure accompanying the β-α phase transition was studied for Cu_1.75Se and Cu_1.98Se compounds within the 443-10 K temperature range. It was shown that the phase transition is accompanied by distortions of the fcc lattice and the ordering of copper ions.     

Dielectric constant,loss factor and ac conductivity of Ni2+‐doped K2ZnCl4 crystals in the ferroelectric‐commensurate,incommensurate and normal phases

The dielectric constant (ϵ), dielectric loss (tanδ) and ac conductivity (σ_ac) of virgin and thermally cycled undoped and Ni²⁺‐doped K₂ZnCl₄ (KZC) crystals in the ferroelectric‐commensurate (FC), incommensurate (IC) and normal phases (N) have been studied. Anomalous behaviour at the two‐phase transition points was observed while measuring ϵ along the polar a‐axis for the undoped sample. With increasing Ni²⁺concentration a systematic shift of the phase transition temperature towards lower values and a continuous inhibition of the peak height were detected. ϵ changed linearly with lnf up to f =10⁵ Hz. tanδ along the a‐axis declared the phase transitions by peak changes. After Ni²⁺‐doping this behaviour was preserved at the FC‐IC phase transition point while the IC‐N phase transition was manifested by a change in the slope of the straight line representing the tanδ‐T dependence. The ac conductivity changed lineally with frequency according to a relation of the form σ_ac = σ_o f ^β where 0>β>1.9. σ_ac increased monotonically with increasing temperature and doping concentration in the low‐temperature phases tending to merge in one straight‐line with high activation energy that might be due to superionic dc conduction in the high temperature N‐phase. Doping with Ni²⁺ pinned stripples, decreased the soliton‐soliton interaction, weakened discommensuration effects, shortened the IC‐phase and strongly affected the ‘chaotic' behaviour at the T_C‐IC. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Raman spectra and phase transitions in Rb<Subscript>2</Subscript>KInF<Subscript>6</Subscript> elpasolite

The Raman spectra of Rb₂KInF₆ elpasolite crystal have been studied in a wide temperature range, including two phase transitions: from the cubic phase to the tetragonal phase and then to the monoclinic phase. Several anomalies of internal modes of InF₆ octahedra and low-frequency lattice vibrations, which are related to the structural changes at the transition points, have been found and quantitatively analyzed. The results of a quantitative analysis of the temperature dependences of the parameters of spectral lines are in good agreement with the thermodynamic data on the phase transitions.     

Doping‐induced‐effects on conduction mechanisms in incommensurate ammonium zinc chloride crystals

The dc conductivity (σ) along the polar b‐axis of ammonium zinc chloride (AZC) crystals in its four high‐temperature phases has been measured as a function of temperature. Doping with Mn²⁺ in different concentrations changed strongly both values of σ at all temperatures and the dependence of ln σ_dc on 1/T in the phase transition regions. The activation energy of conduction was calculated from the linear portions of this dependence in each phase. The results were discussed in the light of the decomposition of (NH₄)₂ in the high‐temperature normal phase, the discommensuaration (DC) formation/annihilation in the incommensurate phase and domain wall motion and stripples nucleation in the commensurate‐ and antiferroelectric‐phases. Pinning of DC's in the crystal lattice and/or by the structural defects and the possibility of dislocation formation was also discussed. The bulk‐ and the electrode‐limited conduction mechanisms were also considered. The current density‐voltage gradient relationship according to the usual Richardson‐Schottky (R‐S) equation shows disagreement between extracted parameters and experimentally measured ones. A modified equation was used to solve this difficulty which, in addition, facilitated the calculation of the electronic mobility (μ), the barrier height (φ) at the electrode‐dielectric interface and the R‐S constant (β_RS). The effect of Mn²⁺‐content on values of μ, φ and β_RS in different phases of AZC was also considered. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High temperature x‐ray diffraction studies of zirconia thin films prepared by reactive pulsed laser deposition

Zirconium oxide thin films have been deposited on Si (100) substrates at room temperature at an optimized oxygen partial pressure of 3x10^‐2 mbar by reactive pulsed laser deposition. High temperature x‐ray diffraction (HTXRD) studies of the film in the temperature range room temperature‐1473 K revealed that the film contained only monoclinic phase at temperatures ≤ 673 K and both monoclinic and tetragonal phases were present at temperatures ≥ 773 K. The tetragonal phase content was significantly dominating over monoclinic phase with the increase of temperature. The phase evolution was accompanied with the increase in the crystallite size from 20 to 40 nm for the tetragonal phase. The mean thermal expansion coefficients for the tetragonal phase have been found to be 10.58x10^‐6 K^‐1 and 20.92x10^‐6K^‐1 along a and c‐axes, respectively. The mean volume thermal expansion coefficient is 42.34x10^‐6 K^‐1 in the temperature range 773‐1473 K. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal growth,dielectric and FIR reflectivity studies on PZN(0.91)‐PT(0.09) single crystals

Crystal growth of PZN‐PT single crystals using slow cooling flux technique with PbO flux is reported in this communication. Optimum growth conditions to maximize the amount of perovskite are also suggested. The grown crystals are characterized by dielectric and FIR spectroscopy. Temperature dependence of ε′ very close to the transition temperature shows a first order phase transition. Diffused phase transition and strong frequency dependence of ε′ around transition temperature are also observed. The real part of ε′ was found to obey the relation ε′ – ε_∞ = χ′(T‐T_o)². Dispersion in the ferroelectric phase is suggested to originate from ordering of domains. Competition in the B‐site occupancy by Zn, Nb and Ti ions is suggested to be the origin for the additional modes in the FIR reflectivity at room temperature. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal structure refinement of CuxAg1‐xInTe2 bulk material determined from X‐ray powder diffraction data using the Rietveld method

The influence of the Cu‐content in the quaternary compounds Cu_xAg_1‐xInTe₂ (0 ≤ x ≤1) on the structural properties of the bulk material was discussed. Bulk ingot materials of Cu_xAg_1‐xInTe₂ solid solutions (x = 0.0, 0.25, 0.50, 0.75 and 1.0) have been synthesized by fusion of the constituent elements in the stoichiometric ratios in vacuum‐sealed silica tubes. The materials compositions were confirmed by using energy dispersive analysis of X‐rays (EDAX). X‐ray powder diffraction measurements were performed for all the prepared samples at 300 K in step scanning mode. The analysis of X‐ray data has indicated that the crystal structure of the prepared materials with different compositions is single‐phase polycrystalline materials corresponding to the tetragonal chalcopyrite structure with space group I 2d. The crystal structural parameters were refined by Rietveld method using the Full Prof program. The refined lattice constants (a and c), anion positional parameter, u, and the determined bond distances and angles were found to vary with composition, x, attaining zero tetragonal distortion at x ≈ 0.75, which corresponds to an ideal tetragonal unit cell. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

X‐ray structural phase analysis of CdTe semiconductor annealed in air

X‐ray structural phase analysis of CdTe semiconductor, thermally annealed in air at temperature range 373‐773 K and annealing time 24 hrs, is investigated applying Rietveld method. The results showed that at low temperatures < 523 K, oxygen diffuses into the (1/21/21/2) interstitial sites of the CdTe lattice and its relative occupancy increases with the annealing temperature. For higher temperatures ≥ 523 K, the thermally grown oxide CdTeO₃ phase is developed on expense of CdTe phase. The percentage phase abundance of each phase is determined at each temperature applying a standardless method. The rate of oxidation with temperature is found to be non‐linear.     

Growth,composition, ferroelectric and magnetic properties of new multiferroic Pb3.3Mn4.8Ni1.1Ti0.56O15.3 single crystals

Multiferroic single crystals in the novel system Pb‐Mn‐Ni‐Ti‐O have been grown by the high temperature solution growth method. At room temperature the crystals are indexed in the hexagonal space group P6₃cm. The dielectric and magnetic properties along with the temperature dependence of the c‐lattice parameter have been studied in the temperature range 2 K ‐ 500 К. The magnetic measurements reveal a paramagnetic to antiferromagnetic phase transition around 48 K. The dielectric permittivity exhibits a maximum at 430 K, indicating ferroelectric to paraelectric phase transition. The temperature dependent Raman and XRD measurements around 430 K reveal an anomaly and abrupt change of the lattice parameter along the z‐axis respectively, thus confirming the ferroelectric‐to‐paraelectric phase transition.