Dilatometric studies of phase transition in NaNH4SeO4·2H2O single crystal

Thermal expansion of NaNH₄SeO₄SeO₄2H₂O crystal was measured by means of dilatometric method along three crystallographic axes in the temperature range of 300-140 K. Anomalies of relative expansion and thermal expansion coefficients related to ferroelectric phase transition were observed and confirmed its continuous character.     

Phase transition of Eu2Ti2O7 under high pressure and a new ferroelectric phase with perovskite‐like layered structure

Ferroelectrics with perovskite‐like layered (PL) structure are well‐known for their high T_c and the application prospect of high‐temperature‐piezoelectric sensing. In this study, the PL‐structure Eu₂Ti₂O₇ was prepared by 1‐step high‐pressure sintering, which show the pyrochlore structure of Eu₂Ti₂O₇ would change into PL structure at 11 GPa, 1300°C. The PL‐structure Eu₂Ti₂O₇ is metastable, which will change back to pyrochlore structure at about 900°C in the air. The PL‐structure Eu₂Ti₂O₇ was confirmed as a high‐temperature ferroelectric material for the first time. The ferroelectric domain switching was directly observed using piezoelectric force microscope. The piezoelectric constant of the PL Eu₂Ti₂O₇ ceramic was measured as 0.7‐0.9 pC/N and its thermal depoling temperature (T_d) was determined as 800°C, which is associated with the PL‐pyrochlore transition.     

High‐pressure synthesis of a 12CaO·7Al2O3–12SrO·7Al2O3 solid solution

Solid solutions of 12CaO·7Al₂O₃ (C12A7) and 12SrO·7Al₂O₃ (S12A7) crystals were synthesized under high pressure. X‐ray diffraction patterns revealed that the lattice constants of the synthesized samples depend linearly on the compositional ratio of C12A7 and S12A7. Electron‐probe X‐ray microanalyses show that the chemical compositions of the crystals are represented by xC12A7·(1?x)S12A7 (0<x<1). These results indicate that the variation in the lattice constants is originated from a difference in the ionic radii of Ca²⁺ and Sr²⁺ ions. From impedance measurements, it was found that S12A7 has the highest conductivity (~1 × 10^?3 Scm^?1 at 550°C) among the solid solutions in the C12A7–S12A7 system.     

Mechanistic in situ High‐Pressure NMR Studies of Benzene Hydrogenation by Supramolecular Cluster Catalysis with [(η6‐C6H6)(η6‐C6Me6)2Ru3(μ3‐O)(μ2‐OH)(μ2‐H)2][BF4]

In situ high‐pressure NMR spectroscopy of the hydrogenation of benzene to give cyclohexane, catalysed by the cluster cation [(η⁶‐C₆H₆) (η⁶‐C₆Me₆)₂Ru₃(μ₃‐O)(μ₂‐OH)(μ₂‐H)₂]⁺ 2 , supports a mechanism involving a supramolecular host‐guest complex of the substrate molecule in the hydrophobic pocket of the intact cluster molecule.     

High‐pressure behavior of (Cs,K)Al4Be5B11O28 (londonite): A single‐crystal synchrotron diffraction study up to 26 GPa

The compressional behavior and the P‐induced deformation mechanisms at the atomic scale of (Cs,K)Al₄Be₅B₁₁O₂₈ (londonite, a ~7.31 Å and space group P3m) were investigated by in situ single‐crystal synchrotron X‐ray diffraction with a diamond anvil cell up to 26 GPa. No phase transition was observed within the P‐range investigated: this material exhibits isotropic compression (i.e., with cubic symmetry) in response to the applied pressure. Fitting the P–V data with a Birch‐Murnaghan isothermal equation of state, we obtained: V₀=390.8(3) ų, K_P0=212(7) GPa (β₀=1/K_P0=0.0047(1) GPa^?1) and K′=4.6(6). A series of structural refinements, based on the high‐pressure intensity data, were performed. The stiffness of londonite (similar to that of carbides) is governed by its close‐packing structure, and in particular by the very low compressibility of B‐ and Be‐tetrahedra and the modest compressibility of the Al‐octahedra. The Cs‐polyhedra are the most compressible units of the structure. The effects of pressure can be accommodated by intrapolyhedral compression or deformation, leading to a modest bulk compression. The high amount of boron in londonite (B₂O₃ ~50 wt%) makes its synthetic counterpart a potential neutron absorber. In addition, the high content of Cs makes londonite‐type materials as potential hosts for nuclear waste.     

Phase transition behavior of Pb(Hf,Sn, Ti,Nb)O3 ceramics at morphotropic phase boundary

The phase transition and dielectric properties of Pb_0.988(Hf_0.945Sn_xTi_0.03-xNb_0.025)O₃ ceramics (0 ≤ x ≤ 0.03, correspondingly abbreviated as H1, H2, H3, and H4) at the morphotropic phase boundary were systematically investigated. X-ray diffraction results and P-E hysteresis loops show that the dominate orthorhombic antiferroelectric phase and a small amount of the tetragonal FE phase coexist in Pb_0.988(Hf_0.945Sn_xTi_0.03-xNb_0.025)O₃ ceramics. As the Sn content increases, the antiferroelectricity is significantly enhanced, accompanied with an increased Curie temperature and sharply reduced peak dielectric constant. H1 and H2 experience an irreversible field-induced AFE-FE phase transition at the ambient temperature, and the transition from a metastable FE phase to the original AFE phase is observed in H2 when heated to 60°C. H3 and H4 experience an invertible AFE-FE phase transition, along with an enhanced forward phase switching field E_F. Moreover a decreased backward phase switching field E_A for H4 is detected as the electric field increases due to the AFE/FE coexistence. These results reveal the unique phase transition characteristics of AFE materials near the phase boundary, which is helpful for better understanding of AFE/FE materials.     

Structural phase transition and high temperature phase structure of Friedels salt, 3CaO · Al2O3 · CaCl2 · 10H2O

Friedels salt, the chlorinated compound 3CaO · Al₂O₃ · CaCl₂ · 10H₂O (AFm phase), presents a structural phase transition at about 30°C from a monoclinic to a rhombohedral phase. It has been studied by X-ray powder diffraction and optical microscopy in transmitted light with crossed polarisers on single crystals prepared by hydrothermal synthesis. The high temperature phase was determined at 37°C from X-ray single crystal diffraction data. The compound crystallises in the space group R c with lattice parameters of a = 5.7358(6)Å and c = 46.849(9)Å (Z = 3 and D_x = 2.111 g/cm³). The refinement of 498 independent reflections with I > 2σ(I) led to a residual factor of 7.1%. The Friedels salt can be described as a layered structure with positively charged main layers of composition [Ca₂Al(OH)₆]⁺ and negatively charged layers of composition [Cl⁻,2H₂O]. The chloride anions are surrounded by 10 hydrogen atoms, of which six belong to hydroxyl groups and four to water molecules. The structural phase transition may be related to the size of the chloride anions, which are not adapted to the octahedral cavity formed by bonded water molecules.     

Influence of Nb2O5 addition on dielectric properties and diffuse phase transition behavior of BaZr0.2Ti0.8O3 ceramics

Nb₂O₅ doped Ba(Zr_0.2Ti_0.8)O₃ (short as BZT20) ceramics were prepared by a mixed-oxide method using a high-energy planetary ball mill and the influence of Nb₂O₅ addition on microstructure, dielectric properties and diffuse phase transition behavior of BZT20 ceramics were investigated. It was demonstrated that Nb⁵⁺entered the B-site of BZT20 ceramic and substituted for Ti⁴⁺, which caused the expansion and distortion of crystal lattice. BZT20 ceramics doped with 0.2 mol% Nb₂O₅ showed excellent dielectric property and lower diffusivity with ε_m=37,823 and γ=1.49. We supposed that the increase of dielectric constant and decrease of diffuseness parameter with increasing Nb₂O₅ content were caused by lattice disorder and unbalancing of cations induced by the substitution of Ti⁴⁺ by Nb⁵⁺ in the B sites of BZT20 ceramics. The Curie temperature decreased with the increase of Nb₂O₅ content, which can be attributed to enlarged distortion energy of the Nb doped BZT20 structure. Besides, grain size effect on the dielectric property and diffuse phase transition behavior of Nb₂O₅ doped BZT20 ceramics was also investigated.     

State diagram of phase transition temperatures and solvent‐induced recovery behavior of shape‐memory polymer

In this study we analyzed the phase and state transitions of shape‐memory polymers (SMPs)/solvent mixtures using the Flory–Huggins (FH) theory by extension of Vrentas and the Couchman–Karasz theory for glass transition, as well as Clausius–Clapeyron relation for melting transition. Using scaling relations of model parameters, we have obtained a theoretical prediction of state diagrams of the phase transition temperature and solvent‐induced recovery in SMPs. The inductive decrease in transition temperature is identified as the driving force for the solvent‐induced shape‐memory effect in SMPs Consequently, the thermodynamics of the polymer solution and the relaxation theory were employed to characterize the dependencies of shape recovery time on the FH parameter and the ratio of the molar volume of solute to solvent. With the estimated model parameters, we constructed the state diagram for SMP, which provides a powerful tool for design and analysis of phase transition temperatures and solvent‐induced recovery. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

An unprecedented 2D isopolymolybdate with (3·4)(3·5·8)(3·4·5·6·8)(3·4·5·6) topology

A novel two-dimensional molybdenum oxide polymer, [Mo₄O₁₃]_n · 2nH₃O, has been prepared under specific hydrothermal conditions and characterized by IR spectroscopy and TG analysis. Single-crystal X-ray diffraction analysis reveals that compound 1 exhibits a (3, 4, 5, 6)-connected 2D layer structure with (3²·4)(3²·5³·8)(3·4²·5⁴·6·8²)(3⁴·4³·5⁴·6⁴) topology, which is constructed by two parallel molybdenum oxide dimeric chains (Mo₂O₁₀)_n bound together by the dimer of Mo₂O₉ subunits via the sharing of corners. Additionally, the electrochemistry activity of compound 1 is also reported.     

Dielectric and dilatometric studies of (CH3)2NH2Ga(SO4)2 · 6H2O (DMAGaS)

The results of dielectric, pyroelectric, spontaneous polarization and dilatometric investigations in the temparature interval 110 K-295 K are presented for DMAGaS crystal. At both phase transition temperature, 136 K and 116 K, discontinuous changes of dielectric permittivity, spontaneous polarization and length of bars are observed. Temperature dependences of alongation along the ferroelectric direction and of spontaneous polarization are quite similar.     

Enhanced thermal stability of piezoelectricity in lead-free (Ba,Ca)(Ti,Zr)O3 systems through tailoring phase transition behavior

Good thermal stability in lead-free BaTiO₃ ceramics is important for their applications above room temperature. In this study, thermal stable piezoelectricity in lead-free (Ba,Ca)(Ti,Zr)O₃ ceramics was enhanced by tailoring their phase transition behaviors. Comparison between (1-x)Ba(Ti_0.8Zr_0.2)O₃-x(Ba_0.65Ca_0.35)TiO₃ and (1-y)Ba(Ti_0.8Zr_0.2)O₃-y(Ba_0.95Ca_0.05)TiO₃ revealed that latter system at y?=?0.80 had much better thermal stable piezoelectric coefficient than the former at x?=?0.45. Both systems crystalized in tetragonal to orthorhombic phase boundary at room temperature. The phase transition temperature and degree of diffusion were adjusted by Ca and Zr ions contents and demonstrated great influence on temperature dependent dielectric permittivity, hysteresis loops, and in-situ domain structures. The improved thermal stability of (1-y)Ba(Ti_0.8Zr_0.2)O₃-y(Ba_0.95Ca_0.05)TiO₃ prepared at y?=?0.80 was linked to its higher paraelectric to ferroelectric phase transition temperature (T_m?=?115.7?°C) and less degree of diffusion (degree of diffusion constant γ?=?1.35). By comparison, (1-x)Ba(Ti_0.8Zr_0.2)O₃-x(Ba_0.65Ca_0.35)TiO₃ prepared at x?=?0.45 revealed T_m?=?81.3?°C and γ?=?1.65. Overall, these findings look promising for future stimulation of phase transition behaviors and design of piezoelectric materials with good thermal stabilities.     

A green process for recovery of H2SO4 and Fe2O3 from FeSO4·7H2O by modeling phase equilibrium of the Fe(П)– –H+–Cl– system

Ferrous sulfate heptahydrate FeSO₄·7H₂O is a major waste produced in titanium dioxide industry by the sulfate process and has caused heavy environmental problem. A new green process for the treatment of FeSO₄·7H₂O was proposed to make use of iron source and recycle sulfate source as H₂SO₄. It was found that by adding concentrated HCl to the FeSO₄ solution, FeCl₂·4H₂O was crystallized out, which was subsequently calcined to produce Fe₂O₃ and HCl. Concentrated H₂SO₄ solution (about 65 wt %) was obtained by evaporating the FeCl₂·4H₂O‐saturated filtrate. To facilitate the process development and design, the solubilities of FeCl₂·4H₂O in HCl, H₂SO₄, and HCl + H₂SO₄ solutions were measured and the experimental data were regressed with both the mixed‐solvent electrolyte model and the electrolyte NRTL model. On the basis of the prediction of the optimum conditions for the crystallization of FeCl₂·4H₂O, material balance of the new process was calculated. FeCl₂·4H₂O and Fe₂O₃ were obtained from a laboratory‐scale test with about 70% recovery of ferrous source for a single cycle, indicating the feasibility of the process. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4549–4563, 2017     

Thermosensitive phase transition kinetics of poly(N‐isopropylacryl amide‐co‐acrylamide) microgel aqueous dispersions

Thermosensitive poly(N‐isopropylacrylamide‐co‐acrylamide) microgel particles were prepared through precipitation polymerization. The diameters of the microgel particles were in the range of 220–270 nm and showed a monodispersion. The lower critical solution temperatures (LCST) of the microgel dispersions were measured by dynamic light scattering and turbidimetric analysis. The results indicated that the LCST increased with an increase of acrylamide (AAm) content in the copolymer composition. The kinetics of the thermosensitive phase transitions of the microgel particles were investigated by time‐course UV–vis spectroscopy. The results indicated that the higher the content of AAm in copolymer composition, the more time is required for equilibrium deswelling and the less time required for equilibrium swelling. In addition, the time required for equilibrium deswelling decreased with an increase of the content of the microgel particles in dispersions. By contrast, the time required for equilibrium swelling increased slightly. Thus, a suitable LCST and time required for equilibrium of phase transition can be achieved by adjusting the molar ratio of the comonomers in the microgels and the content of the microgel particles in dispersions. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

The influence of phase transition on electrocaloric effect in lead‐free (Ba0.9Ca0.1)(Ti1−xZrx)O3 ceramics

In this paper, the influence of phase evolution on polarization change and electrocaloric response in lead‐free (Ba_0.9Ca_0.1)(Ti_1?xZr_x)O₃ ceramics (BCTZ) was systematically investigated. With increasing Zr/Ti ratio, the phase structure and phase transition behavior were greatly changed, resulting in various temperature and electric field dependence of electrocaloric responses. For x=0.05, a peak electrocaloric temperature change 1.64 K (at 130°C) and corresponding entropy change 1.78 J·kg^?1·K^?1 were obtained for 0‐7 kV·mm^?1 electric field. Negative electrocaloric temperature change in ?0.1 K was obtained below Curie temperature (T_c), which may be induced by the orthorhombic‐tetragonal ferroelectric phase transition. With the increase in x, the peak value of the electrocaloric response decreased but much better temperature stability was observed. Simultaneously the negative electrocaloric response gradually disappeared with the disappearance of the low temperature ferroelectric‐ferroelectric phase transition. For x=0.2, electrocaloric response showed good temperature stability ranging from room temperature to 130°C, attributing to the relaxor ferroelectric feature.     

Global phase behavior of imidazolium ionic liquids and compressed 1,1,1,2‐tetrafluoroethane (R‐134a)

Novel processes involving ionic liquids with refrigerant gases have recently been developed. Here, the complete global phase behavior has been measured for the refrigerant gas, 1,1,1,2‐tetrafluoroethane (R‐134a) and 1‐n‐alkyl‐3‐methyl‐imidazolium ionic liquids with the anions hexafluorophosphate [PF₆], tetrafluoroborate [BF₄] and bis(trifluoromethylsulfonyl)imide [Tf₂N] from ～0°C to 105°C and to 33 MPa. All of the systems studied were Type V from the classification scheme of Scott‐van Konynenburg with regions of vapor‐liquid equilibrium, miscible/critical regions, vapor‐liquid‐liquid equilibrium, and upper and lower critical endpoints (UCEP and LCEP). The effect of the alkyl chain length has been investigated, for ethyl‐([EMIm]), n‐butyl‐([BMIm]), and n‐hexyl‐([HMIm]). With increasing chain length, the temperature of the lower critical end points increases and pressure at the mixture critical points decrease. With a common cation, the temperature of the LCEP increased and the mixture critical point pressures decreased in the order of [BF₄], [PF₆], and [Tf₂N]. © 2008 American Institute of Chemical Engineers AIChE J, 2009     

Optical-vibration properties and pressure-induced phase transition in (In,Sc)2Ge2O7 pyrogermanates

Thortveitite-type (T-type) scandium and indium pyrogermanates were obtained through a solid-state reaction route. The crystal structures, morphologies, and chemical compositions of these compounds were investigated by X-ray diffraction and transmission electron microscopy techniques. The optical-vibration properties were evaluated by Raman scattering and infrared spectroscopy. Reflectivity far-infrared data were obtained for sintered Sc₂Ge₂O₇ samples, which exhibited good surfaces after polishing, allowing the determination of the infrared optical functions, the complete polar phonon characteristics, the intrinsic dielectric constant and the unloaded quality factor. Because of the poor surface reflectivity of In₂Ge₂O₇ samples, only mid-infrared absorbance data could be collected by using loose powders in an attenuated total reflectance accessory. By using polarized Raman spectroscopy on both ceramic materials, at room temperature and ambient pressure, all the 15 active Raman modes predicted by group theory for the monoclinic C2/m T-type structure were identified and assigned. Further, the phase stabilities of In₂Ge₂O₇ and Sc₂Ge₂O₇ were studied by high-pressure Raman spectroscopy. In the limit of pressures studied here (up to 10 GPa), a reversible structural phase transition (SPT) for In₂Ge₂O₇ and Sc₂Ge₂O₇ was induced by pressures of 4.2 GPa and 2.4 GPa, respectively. These would correspond to the reported C2/m (#12) ↔ P2₁/c (#14) SPT. Representative Raman spectra of the high-pressure phase for both materials were fitted and 24 first-order Raman modes were depicted for each material, a number greater than compatible with a proposed distorted and oxygen-deficient fluorite-like structure, but explained by an analysis of the gtroup-subgroup constraints of this SPT.     

Surface modification of ZrW2O8 and ZrW2O7(OH)·2H2O by in situ polymerization: Enhanced filler particles for use in composites

To explore the effect of negative thermal expansion on the reduction of the coefficient of thermal expansion of polymer with respect to that of chain stiffening, the preparation of polycarbonate composites with ZrW₂O₈ and ZrW₂O₇(OH)₂·2H₂O are attempted. In the process, the dispersion of the filler particles in the polymer needs to be optimized, and the transfer of properties at the interface between the two phases. Therefore, surface modification by in situ polymerization is performed on these two particles. Several parameters of the reaction are optimized to achieve higher surface coverage of oligomer, including reaction time and the amount of monomers and particles. On the basis of the optimized conditions, two modification steps are then used to maximize both recovered amount of modified particles and surface coverage. The composites prepared with modified particles show enhanced dispersion of the particles and interaction at the interfaces. POLYM. COMPOS., 37:1359–1368, 2016. © 2014 Society of Plastics Engineers