Properties of porous glasses with embedded ferroelectric materials

Methods of introducing two ferroelectric materials, viz. NaNO₂ and (CH₃NH₃)₅Bi₂Cl₁₁ (PMACB), into porous glass matrices and the results of dielectric, pyroelectric, thermal expansion and SEM investigations are presented. The best filling degree was achieved for NaNO₂ introduced from the melt, whereas a significantly worse filling effect was achieved for a water solution as the introducing agent. A splitting was observed in the temperature dependence of dielectric permittivity related to the incommensurate phase of NaNO₂ embedded in porous glass from a water solution, whereas no incommensurate phase was observed in the case of glass filled with NaNO₂ from the melt. The confined materials turned out to possess ferroelectric properties and exhibited phase transitions. The negative size effect was observed for porous glasses filled with NaNO₂ and PMACB. The above conclusions are based on the anomaly of dielectric, pyroelectric and dilatometric properties observed in the phase transition regions.     

Problems in crystal chemistry of ferroelectric and antiferroelectric perovskites PbB

The most interesting problems in the crystal chemistry of ferroelectric and antiferroelectric perovskites PbB _0.5 ^′ B _0.5 ^″ O₃ are considered. The properties and specific features of perovskites (such as the smearing of ferroelectric phase transitions, relaxor state, ferroelectric-antiferroelectric transformations, and spontaneous phase transitions to a macrodomain state) depend on the distribution of B′ and B″ ions over octahedral positions of the crystal lattice, i.e., on the order and disorder of these ions. __________ Translated from Kristallografiya, Vol. 49, No. 5, 2004, pp. 806–810. Original Russian Text Copyright ? 2004 by Isupov.     

Phase transitions in ammonium pyroselenite crystals

The character of the phase transitions in ammonium pyroselenite crystals, (NH₄)₂Se₂O₅ and (ND₄)₂Se₂O₅, in the high-temperature phase has been investigated. The temperature dependences of the permittivity and the angular dependences of the high-resolution ⁷⁷Se spectra in the phase-transition range have been measured. The parameters of the chemical shift tensor have been determined for ⁷⁷Se nuclei in the Se₂O₅ pyroselenite ion in the high-temperature phase and compared with the known parameters of this tensor in the low-temperature phase. Some conclusions are drawn on the character of transitions and the structure of the (Se₂O₅)²⁻ ion.     

Crystal structure and phase transitions in the new crystals of [(CH3)2NH2]2CuCl4[(CH3)2NH2]Cl

Crystals grown from a solution of dimethylammonium and copper chlorides are studied using electron paramagnetic resonance (EPR) and X-ray diffraction. The dielectric properties of the crystals grown are measured. It is established that the crystals have the composition [(CH₃)₂NH₂]₂CuCl₄[(CH₃)₂NH₂]Cl and, in phase I at room temperature, are described by the orthorhombic space group Pna2₁ with the unit cell parameters a = 11.338 Å, b = 9.981 Å, and c = 15.675 Å. At temperatures of 279 K and 253 K, the crystals undergo jumpwise phase transitions into the incommensurate modulated ferroelectric phase II and commensurate modulated phase III, respectively. __________ Translated from Kristallografiya, Vol. 49, No. 1, 2004, pp. 92–100. Original Russian Text Copyright ? 2004 by Kirpichnikova, Pietraszko, Bednarski, Waplak, Sheleg. Dedicated to the 80th Birthday of L.A. Shuvalov     

Synthesis and characterization of dihydroxylammonium tetrachlorocuprate dihydrate crystals

A new hydroxylammonium compound, [(NH₃OH)₂CuCl₄], was synthesized and its crystals were grown at room temperature by slow evaporation of aqueous solutions. The crystals were characterized through powder XRD, thermogravimetry (TG), differential thermal analysis (DTA), low temperature differential scanning calorimetric (DSC) and FTIR spectra. The X‐ray powder diffraction confirms the crystallinity of the compound. A fitting decomposition pattern of the compound was formulated based on the TG and thereby confirming the formation of the compound in the stoichiometric ratio. The thermal anomalies occurring in the low temperature DSC indicate successive phase transitions. The low temperature phase transitions are attributed to the ordering of [NH₃OH]⁺ ions. While most of the phase transitions are of first order type, the one occurring at –126 °C is of second order. Two glass transitions occur when the compound was cooled between –157.9 and –136.9 °C. The characteristic vibration bands due to [NH₃OH]⁺ and CuClequation/tex2gif-stack-1.gif ions are observed in the IR spectra. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

X-ray diffraction study of phase transitions in Na3Sc2(PO4)3 and Ag3Sc2(PO4)3 single crystals in temperature range from 160 to 500 K

Two phase transitions are revealed for the first time in Ag₃Sc₂(PO₄)₃ single crystals in the vicinity of the temperatures 303 and 165–180 K. It is established that the phase transition at 303 K corresponds to the well-known phase transition to the superionic state in Na₃Sc₂(PO₄)₃ single crystals in the temperature range 423–433 K, whereas the phase transition observed in the temperature range 170–180 K corresponds to the phase transition from the rhombohedral to monoclinic phase at about 320 K in the monoclinic Na₃Sc₂(PO₄)₃ single crystals. It is also established that rhombohedral Na₃Sc₂(PO₄)₃ single crystals undergo the second phase transition. __________ Translated from Kristallografiya, Vol. 50, No. 1, 2005, pp. 122–126. Original Russian Text Copyright ? 2005 by Shilov, Atovmyan, Kovalenko.     

Phase Transition Studies of the Liquid Crystal Pentyloxybenzylidene Phenylazoaniline

Phase transition studies of the liquid crystal pentyloxybenzylidene phenylazoaniline are investigated with density and ultrasonic velocity measurements. The density variation with temperature confirms that the isotropic liquid-nematic, nematic-smectic A and smectic A-smectic B transitions are of first order. The temperature variation of ultrasonic velocity confirms all the phase transitions. The adiabatic compressibility (β_ad), Rao number (R_n) and Molar compressibility (B) are estimated from density and ultrasonic velocity.     

The Metabolemeter III: Phase Behaviour Under Pressure of bis-(4-4′-n-Heptyloxybenzylidene)- 1, 4-Phenylenediamine and Terephthalylidene-bis-(4-n-Decylaniline)

By using a metabolemeter, the phase transitions of bis-[4, 4′-n-heptyloxybenzylidene]-1, 4 phenylenediamine (HBPD) and terephthalylidene-bis-[4-n-decylaniline] (TBDA) have been studied. The weakly first order phase transitions (S_G-S_F and S_F-S_I) and the narrowly separated phase transitions (S_C-S_A and S_A-I) of TBDA are detectable. For HBPD, both transitions S_G-S_I and S_I-S_C are individually observed and the enthalpy changes are deduced from pressure increments at the tranformations: two triple points are detected, one directly by a change of slope in the drawing of the equilibrium curves. The pressure-temperature phase diagrams are given; for each transiton, the volume increases on increasing the temperture.     

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.     

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.     

X-ray diffraction study of (TlInSe<Subscript>2</Subscript>)<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>(TlGaTe<Subscript>2</Subscript>)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> crystal system

The crystallographic and dynamic characteristics of TlInSe₂ and TlGaTe₂ crystals have been studied by X-ray diffraction in the temperature range of 85–320 K. The temperature dependences of the unit-cell parameters a of TlInSe₂ and TlGaTe₂ crystals, as well as their coefficients of thermal expansion along the [100] direction, are determined. The concentration dependences of the unit-cell parameters a and c for (TlInSe₂)_{1 − x} (TlGaTe₂) _x crystals are measured. Anomalies are found in the temperature dependences of the unit-cell parameters a and, correspondingly, the coefficient of thermal expansion, indicating the existence of phase transitions in TlInSe₂ and TlGaTe₂ crystals.     

Density and Ultrasonic Velocity Measurements in Hexyloxybenzylidene Phenylazoaniline

The temperature variation of density and ultrasonic velocity of the liquid crystal hexyloxybenzylidene phenylazoaniline are reported. The density across the smectic A—smectic B transition is more predominant than the other transitions. The density variation with temperature and the calculated thermal expansion coefficients suggest that the transitions isotropic liquid—nematic, nematic—smectic A and smectic A—smectic B are of first order. Anomalous behaviour of ultrasonic velocity is observed across the isotropic liquid—nematic transition and prominent dips in velocity are observed at the nematic—smectic A and smectic A—smectic B transitions. The adiabatic compressibility (β _ad ) Rao number (R _a ) and molar compressibility (B) are estimated using the experimental density and ultrasonic velocity.     

Study of phase transitions and the incommensurate phase in ACBX 4 crystals by the Monte Carlo method

The statistical model of successive phase transitions in the family of crystals described by the general formula ACBX ₄ has been studied. It is assumed that tetrahedral BX ₄ groups in the disordered tetragonal phase can be located in four equally probable equilibrium positions. Five representatives with different sequences of phase transitions in this family of crystals are considered. The effective equilibrium constants of interactions between tetrahedral groups are calculated within the framework of the electrostatic model. The corresponding phase diagrams and the thermodynamic characteristics of the phase transitions are studied by the Monte Carlo method. The experimental data provided the establishment of the sequence of phase transitions in Rb₂ZnCl₄ and K₂SeO₄ crystals with the formation of an intermediate incommensurate phase. The character of the structure modulation in this phase is established. The calculated thermodynamic characteristics of the phase transitions agree quite well with the experimental data for all the crystals under consideration.     

Calorimetric Study of Crystalline and Liquid-Crystalline Polymorphism in the Homologues of Terephthal-bis-n-alkylaniline

The phase sequences of the homologues with n (the number of carbon atoms in the terminal alkyl chains) from 2 to 8 are obtained. At low temperatures, i) several crystalline modifications are found in each of the members with n = 4, 5, and 6 and ii) the monotropic transitions between crystal and smectic phase are observed in almost all the members. At intermediate temperature, iii) between the smectic H and C phases, the smectic F phase is found in the homologues with n ≥ = 5 by morphological observation or by a rheological method. The constancy of the temperature of the S_H(or S_F) – S_c transition with respect to “and broadening of the temperature range of S_H with increasing n in the expense of nematic phase are worthy of note. The even-odd effect of the end chain length n is observed very distinctively on the crystal melting and clearing point. Proton NMR measurement reveals a large mobility in the terminal alkyl chains in the crystal of the member with n = 6.     

Spectroscopic and Transport Properties of (NH4)2CuCl4 · 2 H2O Single Crystal

Electron paramagnetic resonance (EPR), Optical absorption and Transport properties of (NH₄)₂CuCl₄ 2 H₂O single crystals have been studied. An anisotropic g tensor was observed with gl = 2.241 and g∥ = 2.081 by EPR method. The spin orbit coupling constant is found to be 540 cm⁻¹. The optical absorption in UV region are characterized by charge transfer band, in the visible and near infrared region at 13,333, 4,480, 4,336, and 3,998 cm⁻¹ attributed to the transitions between the (d-d) stark energy levels of the copper (II) ion in an extended octahedral crystal field. Dc electrical conductivity measurements with temperature reveal an anisotropic characteristic of a two-dimensional layered structure, and exhibits two first order structural phase transitions at about 383 K and 413 K. These transitions are attributed to loss of the two water inolecules of hydration and free rotation of NH⁺₄ ion from a state of torsional oscillation.     

Optical,X-ray,and calorimetric studies of phase transitions in ND4BeF3

Phase transitions in ND₄BeF₃ crystals at T₁ = 330 and T₂ = 245 K were studied by specific heat (DSC) and X-ray lattice parameters measurements in function of temperature. These ferroelastic transitions are accompanied by anomalous heat absorption, sudden changes in the unit cell parameters and crystal symmetry. Formation of domain structure in particular phases examined under a polarizing microscope is also described. Domain walls of the type (100), (010), and (001) observable at room temperature indicate on the triclinic symmetry of the Phase II. Above T₁ the crystal becomes orthorhombic. First-order phase transition at T₂ causes the symmetry of the crystal in the Phase III to be monoclinic with the c-axis unique. The phase transition diagram is proposed and compared with the phase situation in nondeuterated NH₄BeF₃ crystals.     

Phase transitions in compounds with the Ca<Subscript>3</Subscript>Ga<Subscript>2</Subscript>Ge<Subscript>4</Subscript>O<Subscript>14</Subscript> structure

Possible structural changes described by the group-subgroup relationships in the Ca₃Ga₂Ge₄O₁₄-type structure (sp. gr. P321) are considered. The most probable phase transitions seem to be those accompanied by lowering of the symmetry to the maximal non-isomorphic subgroups P3 and C2. It is shown that only destructive phase transitions accompanied by symmetry rise up to the minimal non-isomorphic supergroups for the given structure type can take place. The change of the trigonal symmetry to monoclinic is revealed in La₃SbZn₃Ge₂O₁₄, whose crystal structure is refined as a derivative structure of the Ca₃Ga₂Ge₄O₁₄ structure type within the sp. gr. A2 (C2). At ～250°C, La₃SbZn₃Ge₂O₁₄ undergoes a reversible phase transition accompanied by symmetry rise, A2 ? P321. Similar phase transitions, P321 ? A2, are also observed in La₃Nb_0.5Ga_5.5O₁₄ and La₃Ta_0.5Ga_5.5O₁₄ under the hydrostatic pressures 12.4(3) and 11.7(3) GPa, respectively. The mechanisms of compression and phase transition are based on the anisotropic compressibility of a layer structure. With the attainment of the critical stress level in the structure, the elevated compressibility in the (ab) plane gives rise to a phase transition accompanied by the loss of the threefold axis. Attempts to reveal low-temperature phase transitions in a number of representatives of the langasite family have failed.     

Theory of the formation of the ordered LiZn0.5Mn1.5O4 phase

A theory of the structural phase transition in LiZn_0.5Mn_1.5O₄ cathode material is proposed. The symmetry of the order parameter, thermodynamics, and mechanisms of formation of the atomic structure of low-symmetry ordered cubic lithium-zinc-manganese oxide spinel have been studied. The critical order parameter inducing the phase transition has been found. It is shown that the calculated LiZn_0.5Mn_1.5O₄ structure is formed due to displacements and orderings of lithium, zinc, manganese, and oxygen atoms. Within the Landau theory of phase transitions, it is shown that the phase states may change from high-symmetry cubic disordered Fd3m phase to the low-symmetry ordered cubic P2₁3 phase as a result of first-order phase transitions.     

DSC characterisation of various phase transitions in mesomorphic benzylidene anilines

To analyse the order of the phase transitions in a number of mesomorphic benzylidene anilines, the DSC (Differential Scanning Calorimetry) method proposed by NAVARD and HAUDIN is used. This method involves the measurement of the ratio (N) of the DSC peak heights H₂/H₁ for a given transition, performed with two different heating rates (T₂/T₁). N is a function of the ratio r₂/r₁(= R) and the magnitude of N provides a simple way to characterize the order of a phase transition.     

Structural Analysis of the Phase Transitions of (NH<Subscript>4</Subscript>)<Subscript>4</Subscript>HgBr<Subscript>6</Subscript>

Abstract Single crystal diffraction and differential scanning calorimetry DSC techniques have been used to investigate the different phases of (NH₄)₄HgBr₆, tetrammonium mercury hexabromide, from room temperature to 120 K. Two anomalies in thermal behaviour were detected for this compound at 190 and 268 K, by DSC experiment. X-ray diffraction measurements confirm these anomalies. At room temperature the structure is tetragonal P4/mnc (No. 128) with lattice parameters a = b = 9.25560(8) ?; c = 8.8657(11) ?; V = 759.49(9) ?³ and Z = 2. At T = 250 K the structure is orthorhombic Pnnm with lattice parameters a = 8.8436(8) ?; b = 9.2191(8) ?; c = 9.2232(7) ?; V = 751.97(11) ?³ and Z = 2. Below approximately 200 K the structure is monoclinic P2₁/n (No. 14) with: a = 8.8080(9) ?; b = 9.1608(8) ?; c = 9.1498(8) ?; β = 90.230(7)°; V = 738.28(12) ?³ and Z = 2 (T = 120 K). The structure of (NH₄)₄HgBr₆ consists of isolated HgBr₆-octahedra in the whole temperature range which are slightly compressed in c-direction. The ammonium groups are located between the octahedra ensuring the stability of the structure by hydrogen bonding contacts: N–H···Br. The structural phase transformations are described by a rotation of the [HgBr₆]²⁻ octahedra around the c-axis, and this behaviour is attributed to an orientational disorder of ammonium groups. Index abstract Structural analysis of the phase transitions of (NH₄)₄HgBr₆; M. Loukil, A. Kabadou, I. Svoboda, A. Ben Salah and H. Fuess; The phase transformations in (NH₄)₄HgBr₆ are explained by large rotation of [HgBr₆]²⁻ octahedra around the c-axis.