Theoretical study of structural phase transition and soft mode behavior in moganite

40 cording to the data obtained in that study, the structure of its high-temperature G0 phase ~space group D4h 1 , Z51) consists of a series of perovskite-like square layers of ScF6 octahedrons separated by Cs ions ~Fig. 1!. Lowering the temperature causes a transition to the G1 phase ~space group D4h 5 , Z52) at T15490 K and then to the G2 phase (D2h 13 , Z54) at T25317 K ~the temperatures cited are based on the data obtained in the present work and differ somewhat from the data in Ref. 1; the existence of such a spread of values was noted in Ref. 1!. Such a sequence is not typical of cesium-containing perovskites, but is similar to that previously observed in RbAlF4, 2,3 in which reorganization of the structure occurs as a result of displacive phase transitions associated with turning of the AlF6 octahedra. This similarity, as well as the closeness of several macroscopic characteristics of the phase transitions in these crystals, provided Aleksandrov et al.. with some basis to postulate that the phase transitions in CsScF6 are of the same type. In that case the phase transitions should be accompanied by the condensation of soft phonon modes, which can be observed in the Raman spectra. The present work was devoted to testing this hypothesis. Investigations were carried out in the vicinity of the lowtemperature D4h 1 2D4h 5 phase transition at T15490, below which condensation of the Raman-active totally symmetric soft mode should be observed according to the selection rules. Samples measuring 23334 mm, which were taken from the same crystallization batch as in Ref. 1 were cut so that they would be oriented along the crystallographic axes in the G1 phase. The crystals were optically transparent and did not contain microscopically visible defects or inclusions. The spectra were obtained on a DFS-24-based automated spectrometer and a Jobin and Yvon U-1000 Raman spectrometer. The spectral slit width was 2 cm, and the scan step was 1 cm. The precision of the stabilization of the sample temperature during the recording of a spectrum was better than 60.2 K. In accordance with the selection rules and the structural data, the spectra of the high-temperature G0 phase displayed two intense lines, at 160 cm (Eg) and 495 cm 21 (A1g), which correspond to vibrations of the axial fluorine atoms in the xy plane and along the z axis, respectively. The frequency of the latter is close to the frequency of the longitu-

Influence of Zr content on phase formation, transition and mechanical behavior of Ni-Ti-Hf-Zr high temperature shape memory alloys

Phase transitions of superionic conductors are studied extensively at present. Here the lattice dynamical aspect is considered of both the temperature and pressure induced phase transition of such crystals. A study of the lattice dynamics of superionic Cu halides in the quasi‐harmonic approximation shows that it is possible to predict the onset of a phase transition in these crystals. This onset is indicated by an instability in the equation of state and by the softening of one or more phonon modes.

Preface: phys. stat. sol. (b) 245/11

Phase Transition and Soft Mode Behaviour in Superionic Copper Halides

Temperature dependent structural studies and phase transition behavior of K2Th(PO4)2

Structural phase transition behavior between orthorhombic and tetragonal in Ln2Ni1−xCuxO4+δ (Ln = Nd, Pr), which attracts interest as new cathode material for solid oxide fuel cells, has been investigated with thermal analysis under controlled oxygen partial pressures (P(O2)). For Nd2Ni1−xCuxO4+δ with 0.0 ≤ x ≤ 0.1, temperature, enthalpy change (ΔH) and change in excess oxygen content (Δδ) at the phase transition decreased with increasing Cu content. The phase transition temperature decreased with decreasing P(O2), whereas little variation was observed in ΔH and Δδ. For Nd2Ni1−xCuxO4+δ with x ≥ 0.15, crystal structure was tetragonal and no phase transition was detected below 750 °C. For Pr2Ni1−xCuxO4+δ with x = 0.0 and 0.1, phase transition was observed by DSC, showing similar dependence of phase transition temperature and ΔH on Cu content and P(O2) with that of Nd2Ni1−xCuxO4+δ. Δδ was not detected in TG curve of Pr2Ni0.9Cu0.1O4+δ, which could be attributed to too small Δδ. From Ellingham diagram prepared using temperature and P(O2) at the phase transition, variation of standard enthalpy (ΔH°) and standard entropy (ΔS°) was evaluated. It was revealed that variation of phase transition temperature by Cu content and difference of the phase transition temperature between Nd2Ni1−xCuxO4+δ and Pr2Ni1−xCuxO4+δ showed correspondence with variation of ΔH°. Excess oxygen content, δ, in Nd2Ni1−xCuxO4+δ, evaluated with reduction in TG apparatus, decreased with increasing Cu content. It was suggested that some amount of δ was required for stabilization of orthorhombic phase and that low δ by Cu substitution stabilized the tetragonal structure of the specimens with x ≥ 0.15.

Motivated by recent experiments, the dynamics of the ferroelectric soft mode and the ferroelectric phase transition mechanism in 18O isotope exchanged systems SrTi(16O1−x18Ox)3 (abbreviated as STO18-x) are reinvestigated as a function of the 18O isotope exchange rate x, within a quasiharmonic model (QHM) for quantum ferroelectric modes in double-Morse local potential with mean-field approximation interactions between modes. The approach was realized within the framework of the variational principle method at finite temperature through the quantum mean-field approximation and by taking into account the effect of isotope replacement through the predominant mass effect, the cell volume effect, homogeneity of the composition throughout the material and the concentration-dependent ferroelectric mode distortion effect. The dynamics of the lowest-frequency soft phonon mode clearly presents an increased softening phenomenon with increasing x and a complete one at the corresponding phase transition temperature Tc, demonstrating the perfect soft-mode-type quantum ferroelectric phase transition for x ⩾ xc. Also, a ferroelectric–paraelectric phase coexistence state has been found near the quantum critical point xc and its origin is discussed. The ferroelectric phase transition mechanism is analyzed and its nature discussed, where a second-order phase transition close to the tricritical point is predicted. In addition, the effect of quantum fluctuations on the soft mode dynamics is discussed which reveals its reduction with increasing x and the crossover of the soft mode dynamics from the quantum to the classic one at the full 18O exchange limit x = 1, for which the origin seems to lie in the new homogeneity associated with the direct reduction of quantum fluctuations effects on the soft mode behavior. Within the QHM, consistent agreement with some of the previous experimental results and theoretical predictions of quantum ferroelectricity throughout the full range of x are achieved.

Negative thermal expansion coefficient of Sc-doped indium tungstate ceramics synthesized by co-precipitation

The structural characteristics and ferroelectric phase transition behaviour of chemically derived lanthanum-substituted lead titanate powders have been investigated by high-temperature X-ray diffraction and differential scanning calorimetry. Using X-ray line profile analyses and precise lattice parameter determinations, the important influence of strain coupling through lanthanum/vacancy-induced defect fields on the first-order character of the ferroelectric phase transition was demonstrated. The relaxation of the lattice to the defects as observed in the X-ray measurements was correlated with the onset of diffuse phase transition behaviour revealed by the calorimetry experiments. The lattice relaxation mechanism was connected with the appearance of mesoscopic modulations of the ferroelectric domain structure, and with anomalies in the dielectric behaviour near the transition.

A Brillouin scattering study of the hexagonal modification of barium titanate ( $h$-BaTi${\mathrm{O}}_{3}$) was done to elucidate the soft mode behavior in the ferroelectric phase transition at 74 K. Measurements were made in the temperature region $20<T<300$ K. A clear softening of the ${c}_{66}$ shear acoustic mode was found at the phase transition. The squared frequency of the ${c}_{66}$ mode exhibits a linear dependence on the temperature difference from ${T}_{c}$, both above and below ${T}_{c}$. We conclude from this that the phase transition at ${T}_{c}$ is the proper ferroelastic transition with order parameter the elastic strain ${s}_{6}$.

Lead-free (Ba1−xCax)(Zr0.1Ti0.9)O3(BCZT) ceramics were synthesized by conventional solid-state sintering process. The Ca ion substitutions on phase transition behavior, microstructure, ferroelectric and piezoelectric properties of BCZT ceramics were systemically investigated. The diffuse phase transition (DPT) behavior was suppressed, while the orthorhombic → tetragonal (O→T) and rhombohedral → orthorhombic (R→O) phase transitions were observed when the Ca content is at x = 0.05. Further increasing the Ca content, the DPT behavior gradually enhanced, and both the O→T and R→O phase transitions gradually evolved into R→T phase transition when the Ca content increases up to 0.15 at 25 °C. This behavior was related with phase structure and morphology, which significantly impacted the ferroelectric and piezoelectric properties near the R→T phase boundary. As a result, the sample with Ca contents of 0.15 shows the outstanding piezoelectric properties (with d33 = 572 pC/N, kp = 57%, Qm = 125, εr = 4821, and tan δ = 0.015) while negligible change for the ferroelectric properties. The abnormal ferroelectric behavior and potential factor contributing to large piezoelectric response also were discussed.

Structural phase transition and thermochromic behavior of synthesized W-substituted CuMoO4

Tailored phase transition temperature and negative thermal expansion of Sc-substituted Fe2Mo3O12 synthesized by the co-precipitation method

It was reported that electrical conductivity increased with increasing Ni content in LaNi1-x Fe x O3-δ for 0.4≤x≤1.0 and that LaNi0.6Fe0.4O3-δ showed superior property as cathode material for solid oxide fuel cells (SOFCs). However, deterioration of stability at high temperature and sintering property with increasing Ni content was also reported as problem for practical application. As a method to produce high electrical conducting material with high stability, partial substitution of divalent alkaline earth ion for trivalent La in LaNi1-x Fe x O3-δ with low Ni content can be proposed. Although investigation of La1-x Sr x Ni0.6Fe0.4O3-δ as cathode material for SOFCs was reported, report on effect of partial substitution of alkaline earth ion for La site in LaNi1-x Fe x O3-δ with low Ni content has been limited. In this study, preparation of La2-x Ae x Ni0.2Fe0.8O3-δ (Ae: Ca, Sr and Ba) was examined. The crystal structure, structural phase transition behavior, sintering property and electrical property of the prepared specimens were evaluated and effect of kinds and composition of Ae on material property was investigated.La1-x Ae x Ni0.2Fe0.8O3-δ was prepared by Pechini method with starting materials of La2O3, AeCO3, Ni(NO3)2·6H2O and Fe(NO3)3·9H2O. Before dissolution, AeCO3 was dried at 150 ºC and La2O3 was heated at 1000 °C in air. Purity of Ni(NO3)2·6H2O and Fe(NO3)3·9H2O was verified by ignition loss method. The solutions were mixed together with nominal ratio of the metal ions. After addition of citric acid and ethylene glycol, the mixed solution was heated at 450 °C until the residual materials was fired, resulting in precursor. The molar ratio of metal ions, citric acid and ethylene glycol was approximately 1:2:8. The obtained precursor was crushed into powder and calcined at 1000 °C for 10-12 h in air or O2, followed by heating at 1200 °C for 10 h in air or O2. For the specimens in which impurity was observed, additional heating at 900 °C for 12 h in O2 was carried out.X-ray diffraction measurement (CuKα: 50 kV, 250 mA, RINT-2500, Rigaku Co., Ltd.) was performed at room temperature to distinguish whether single phase was obtained or not. Crystal system, lattice constants and molar volumes were also evaluated from X-ray diffraction patterns. The structural phase transition behavior of La1-x Ae x Ni0.2Fe0.8O3-δ was investigated with high temperature X-ray diffraction measurements in air. Measurement temperature range was 100-1000 °C. The structural phase transition behavior was evaluated also with dilatometer (TMA8310, Rigaku Co., Ltd.). The single phase powder was pressed into pellet, followed by sintering at 1000-1200 °C for 10 h in air. The sintered pellet was subjected to dilatometry. The electrical conductivity of cylindrical La1-x Ae x Ni0.8Fe0.2O3-δ sintered specimens was measured with DC four probe method with Pt electrodes and wires. The measurement temperature range was 300-800 °C in air. The cylindrical specimens were prepared by pressing the single phase powder followed by sintering at 1000-1200 °C for 10 h in air.So far, no second phase is observed in X-ray diffraction patterns of La1-x Ca x Ni0.2Fe0.8O3-δ and La1-x Sr x Ni0.2Fe0.8O3-δ with x=0.2 or less. For La1-x Ba x Ni0.2Fe0.8O3-δ , single phase is obtained for the specimens with x=0.3 or less. The diffraction patterns of La1-x Ca x Ni0.2Fe0.8O3-δ (x=0.1-0.2) could be indexed as orthorhombic distorted perovskite with space group of Pnma (No. 62). Also, the diffraction patterns of La0.9Sr0.1Ni0.2Fe0.8O3-δ and La0.9Ba0.1Ni0.2Fe0.8O3-δ could be indexed as orthorhombic distorted perovskite; however, crystal system varied with increasing Sr or Ba content. The pattern of La0.8Sr0.2Ni0.2Fe0.8O3-δ could be indexed as mixture of orthorhombic Pnma (No. 62) and rhombohedral R3barc (No. 167). The patterns of La1-x Ba x Ni0.2Fe0.8O3-δ with x=0.2 and 0.3 could be indexed as rhombohedral distorted perovskite with space group of R3barc(No. 167). This structural variation shows fair correspondence with tolerance factor approaching 1 with increasing Ba and Sr content. Figure attached shows temperature dependence of electrical conductivity of La0.9Ae0.1Ni0.2Fe0.8O3-δ in air. In the figure, the electrical conductivity of LaNi0.2Fe0.8O3-δ is presented for comparison. With partial substitution of Ae, electrical conductivity increased by more than one order regardless of kinds of Ae. The increase can be attributed to increase of hole concentration by substitution of divalent Ae for trivalent La. Among La0.9Ae0.1Ni0.2Fe0.8O3-δ , La0.9Ba0.1Ni0.2Fe0.8O3-δ showed the highest electrical conductivity. Dependence of logarithm of product of electrical conductivity, σ, and temperature, T, on reciprocal of temperature showed almost linear relationship, which indicated hopping conduction model could be applicable. From the slope of the relationship between logσT and 103/T, activation energy was estimated. The activation energy of La0.9Sr0.1Ni0.2Fe0.8O3-δ and La0.9Ca0.1Ni0.2Fe0.8O3-δ was 0.22 eV, whereas that of La0.9Ba0.1Ni0.2Fe0.8O3-δ was 0.19 eV, which was lower than those of La0.9Sr0.1Ni0.2Fe0.8O3-δ and La0.9Ca0.1Ni0.2Fe0.8O3-δ . Figure 1