Monte-Carlo simulations of the phase transitions in the Rb2ZnCl4 crystal

The model of the order- disorder phase transitions in Rb₂ZnCl₄ is studied. In this model BX₄, group has a four equlibrium orientations in a disordered hexagonal phase. The constants of the interaction between orderinq BX₄ groups are calculated in the framework of the electrostatic model. It is shown that these constants have a competitive nature. The Monte-Carlo method is applied to study the successive phase transitions hexagonal ⇒ orthorombic ⇒ incommensurate ⇒ commensurate ferroelecrtic phases.     

Structural and kinetic study of phase transitions in LaYSi2O7

Phase transitions in LaYSi₂O₇ have been investigated as a function of temperature using XRD, NMR and TEM. Previously described empirical crystal structure guidelines based on average cation radius in rare-earth RE₂Si₂O₇-type disilicates predict a stable tetragonal A-LaYSi₂O₇ polymorph at temperatures below 1500 °C. This study demonstrates that A-LaYSi₂O₇ is not thermodynamically stable at these temperatures and suggests that guidelines based on average cation size do not accurately describe the equilibrium behaviour of this silicate system. The A to G-type polymorph transition is extremely sluggish; complete transformation requires ～250 h at 1200 °C, and more than 3 weeks of calcination at 1100 °C. This observation is important when this type of material is used as environmental barrier coating (EBC) of advanced ceramics. Analysis of XRD and TEM data reveal complete substitution of Y and La on the rare-earth cation sites in both LaYSi₂O₇ polymorphs, but indicate preferential site occupancies in the G-type polymorph.     

Effect of Sc substitution and pressure on phase transition in Rb2KGaF6 elpasolite

Solid solutions Rb₂KGa_xSc_1-xF₆ (x = 1.0; 0.95; 0.9; 0.8; 0.6) were studied by X-ray, neutron, calorimetric and DTA under hydrostatic pressure methods. Both heat capacity and entropy are strongly influenced by x concentration. According to (T vs. x) phase diagram suggested the phase transition temperatures of compounds with x from 1.0 to 0.8 belong to the same phase boundary.     

Dilatometric studies on structural phase transitions in (NH4)2 SnBr6 crystal

Thermal expansion along several directions was measured in the temperature range including the two transition temperatures. The orientation of the domain walls in two lower temperature phases was examined with a polarizing microscope. The results, together with the experiments so far reported, suggest the possibility that the symmetry of the phase between -116 °C and -129 °C is trigonal, in contrast to being tetragonal as previously conjectured.     

Raman scatteribg and phase transitions in Cd2Nb2O7

Raman scattering and electric permittivity of Cd₂Nb₂O₇ were studied between 4 and 230 K. Phase transitions at T₆ = 44 K and T₇ = 19 K have been discovered. It is hggested that T₆ = 80 K and T = 44 K correspond to normal-incohmensurate and ihcommensurate-commensurate phase transition points respectively.     

Structural phase transitions of methane+ethane mixed-gas hydrate induced by 1,1-dimethylcyclohexane

Methane+ethane+1,1-dimethylcyclohexane+water system was investigated by using Raman spectroscopy and isothermal phase equilibrium measurements under four-phase (gas+aqueous+large guest species+hydrate phases) equilibrium conditions at 288.15 K. The results suggest that three kinds of hydrate structures emerge at 288.15 K in the methane+ethane+1,1-dimethylcyclohexane+water system. The hydrate structure for this system changed from structure-H to structure-I via structure-II with increase in the mole ratio of ethane to methane.     

X-ray diffraction study of phase transitions in Sr0.39Ba0.61Nb2O6 between 20 and 500 K

X-ray powder diffraction patterns of the ferroelectric mixed crystal Sr_0.39,Ba_0.61Nb₂O₆ (SBN-39) are recorded in Guinier geometry in a temperature range between 20 K and 500 K by means of the film-lift technique. The lattice parameters were refined for a single tetragonal structure in the whole temperature range ignoring the incommensurate (IC) modulation of the structure as well as a temperature dependent small splitting of reflections, most clearly observed for the (001)-type. These values show anomalies in the region of diffuse ferroelectric phase transition as well as below room temperature indicating the three other phase transitions. The splitting may be caused by a two-phase state of a relaxor-like ferroelectric system due to coexistence of polarized regions (clusters) and untransformed remnants of the para-phase.     

Structural and phase transitions in TiN-AlN and TiN-AlN-(Ni-Cr-Al) composites produced by focused solar radiation

Results are given on the effects of concentrated solar radiation on the structure and phase transitions of composite materials (CM) based on TiN-AlN. Oxidation in the TiN-AlN system leads to the formation of β-Al₂TiO₅, which prevents the diffusion of titanium and aluminum to the surface of the specimen and correspondingly is a protective barrier for the CM in air up to temperatures above 1500°C. A CM based on TiN-AlN with Ni-Cr-Al bonding has high resistance to oxidation above 1500°C. The introduction of this metal alloy into the CM favors the formation of the spinel NiCr₂O₄, which reacts with titanium and aluminum oxides to form a dense oxide film providing protection of the CM from oxidation. Then CM with compositions TiN-AlN and TiN-AlN-30% (Ni-Cr-Al) have elevated corrosion resistance above 1500°C and can be used as corrosion-resistant CM and as coatings. Translated from Novye Ogneupory, No. 9, pp. 51–53, September, 2008.     

X-ray diffraction study of phase transitions in Sr0.75Ba0.25Nb2O6 between 20 K and 400 K

X-ray powder diffraction patterns of the ferroelectric mixed crystal Sr_0.75Ba_0.25Nb₂O₆ are recorded in Guinier geometry in a temperature range between 20 K and 400 K by means of the film-lift technique. The lattice parameters were refined for a single tetragonal structure in the whole temperature range ignoring the incommensurate modulation of the structure. The lattice parameter c exhibit strong anomalous temperature dependence at 200 KT350 K, indicating the diffuse ferroelectric phase transition (DPT). The proceeding of the DPT is also detectable by an increase of the diffuse scattering around the (002)-reflection. The temperature dependence of the unit cell volume indicates further PTs below room temperature in this relaxor material.     

Structural characterization of (Sm,Tb)PO4 solid solutions and pressure-induced phase transitions

Sm_1-xTb_xPO₄ solid solutions were synthesized and extensively characterized by powder X-ray diffraction, vibrational spectroscopy, and X-ray absorption spectroscopy. At ambient conditions solid solutions up to x?=?0.75 crystallize in the monazite structure, whereas TbPO₄ is isostructural to xenotime. For x?=?0.8 a mixture of both polymorphs was obtained. Moreover, a phase with anhydrite structure was observed coexisting with xenotime, which was formed due to mechanical stress. Selected solid solutions were investigated at pressures up to ～40?GPa using in situ high pressure synchrotron X-ray diffraction and in situ high pressure Raman spectroscopy. SmPO₄ and Sm_0.5Tb_0.5PO₄ monazites are (meta)stable up to the highest pressures studied here. TbPO₄ xenotime was found to transform into the monazite structure at a pressure of about 10?GPa. The transformation of Sm_0.2Tb_0.8PO₄ xenotime into the monazite polymorph commences already at about 3?GPa. This study describes the reversibility of the pressure-induced (Sm,Tb)PO₄ xenotime-monazite transformation.     

Crystal Structure of Rb2CoCl4 and Rb3Mn2Cl7

Previously published cell dimensions of Rb₃Mn₂Cl₇ are confirmed and atomic positions are given. The crystal structure of low temperature Rb₂CoCl₄ is reported. The material was found to belong to the orthorhombic system, space group Pnma with a = 9.272 ± 0.006 Å, b = 7.283 ± 0.004, c = 12.723 ± 0.006 Å and Z = 2.     

Structural phase transitions to the state with anomalously high-ionic conductivity in some perroelectric and ferroelastic crystals of the bisulpate group

The temperature dependences of the conductivity CsHSO₄, CsHSeO₄, RbHSO₄, RbHSEO₄, and NH₄HSO₄ have been measured and optical investigation was also made. It was found that the high temperature phase transitions in CsHSO₄, CsHSeO₄, and RbHSeO₄ at 413, 400 and 446 K, respectively are the transitions to the superionic state with 6'7 10^-30hm^-1.cm^-1. Some pecularities of these phase transitions are reported.     

New type of incommensurate phase transitions

Specific features of incommensurate phase transitions are considered when the Lifshitz invariant does not exist in the parent phase but is present in the commensurate phase.     

工业级五氧化二铌粉体物相转变技术

针对不同工艺条件下制备的五氧化二铌粉体产品,详细分析了其不同的相结构及微观形貌,指出了制备技术中存在的纯相问题。针对性地提出了解决五氧化二铌粉体产品纯相问题的方法,即物相转变技术的化学转变法:①将非纯相五氧化二铌粉体与双氧水在耐压容器中混合蒸煮;②将非纯相五氧化二铌与尿素混合研磨。结果表明,第一种方法较简便,但是对反应器要求高;第二种方法较廉价,但是工序较多。可以根据实际情况选择使用。     

Super-high field-induced phase transitions in NaNbO3 crystals

Optical and dielectric studies of sodium niobate crystals have been carried out in an extended electric field (up to 2.2·10⁸ V/m) at temperatures from 8 0 K to 530 K. It has been found that apart from the known ferroelectric Palma phase, a ferroelectric phase of tetragonal symmetry is induced in NaNbO₃ crystals by a super-high field. Using the obtained results, an extended phase T, E-diagram of sodium niobate has been constructed.     

Electronic processes and phase transitions in transparent PLZT ferroelectric ceramics

The ferroelectric phase transition in PLZT is qualitatively explained on the basis of the theory of highly doped and compensated ferroelectrics.     

Low-frequency acoustic study of ferroelastic phase transitions in Ba2NaNb5O15

The peculiarities of low-frequency internal friction near the first order low-temperature (110 K) and second order high-temperature (573 K) phase transitions in improper Ba₂NaNb₅O₁₅ ferroelastic were studied. A large and wide internal friction peak and diffused step-like change in the temperature dependence of shear modulus were also observed within an incommensurate phase between these two phase transitions.     

Isochoric lines and determination of phase transitions in supercritical reactors

The problem of finding the conditions for homogeneous operation of supercritical chemical reactors is studied experimentally using an isochoric method. Several reacting mixtures related to the transesterification of vegetable oils with supercritical ethanol are studied. The isochoric method provides an indirect way of determining the conditions of phase transitions from the pressure vs. temperature variation of the isochoric lines. The linear variation found both in the homogeneous and the vapor-liquid region of the reacting mixtures allows a clear determination of the transition points at different mixture densities and compositions. The linearity in the vapor-liquid region is also verified by flash computations of the reaction products mixture. The procedure can also be applied to the selection of cosolvents needed to carry out reactions under homogeneous conditions. For this purpose the binaries of methyl oleate, n-octadecane and naphthalene with methanol are also investigated. The colinearity of isochores at the criocondentherm point that has been reported in previous works was also found in the present study.     

Ferroelectric phase transitions in materials embedded in porous media

The preliminary dielectric and neutron scattering results obtained on two granular ferroelectrics (with particles diameters ∼ 7 nm) - sodium nitrite and Rochelle salt embedded into microporous glass matrix are presented.