Anomalous polariton dispersion in LiTaO3 near Tc

Anomalous leveling of the A₁(Z) polaritons of ferroelectric LiTaO₃ was observed below certain wavevectors on approaching the Curie temperature. The saturation is attributed to the destruction of the polaritons propagation length due to the crystal domain variations, which is interpreted as a new phase transition model. The almost linear dependence of the propagation length does indicate the Curie temperature to be 873 K.     

铌酸钡钠和钽酸锂铁电相变的实地观察

本文报道了用透射电子显微镜实地观察铌酸钡钠和钽酸锂铁电相变的实验结果。在室温电畴的尺寸为0.2—1μm。当温度增加到接近居里点时,形成准规则排列的微畴结构,其尺寸为几百埃,实验结果用稍微修正了的模型进行了解释。在钽酸锂中当接近铁电相变温度时,发现了强烈的辐照损伤,这可能跟相变的软模有关。 关键词：     

Relationship between phase structure and electrical properties of (K0.5Na0.5)NbO3-LiTaO3 lead-free ceramics

Lead-free piezoelectric ceramics of (1−x)K_0.5Na_0.5NbO₃-xLiTaO₃ (KNN-LT) system have been investigated in this work. X-ray diffraction, Raman spectra measurements, DSC (Differential Scanning Calorimetric), and dielectric constant versus temperature provide direct evidence that the phase transition temperature between tetragonal and orthorhombic shift to lower temperature with the increasing of LT content. The KNN-0.05LT ceramics exhibit the highest high-field d₃₃ up to 220 pm/V. At the same time, we also investigated the relationship between phase structure and electric properties, showing that the orthorhombic phase presents better piezoelectric temperature stabilities than the tetragonal phase. The result may provide a new way for KNN-based lead-free ceramics.     

Raman scattering from soft phonons in NdP5O14

Raman spectra of NdP₅O₁₄ were recorded between 4.2 and 600 K and analyzed. The monoclinic-orthorhombic phase transition temperature was determined to be 420 K by measuring the energy of an A_g-B_2g and a B_g-B_3g soft optic phonon mode as a function of temperature (symmetries refer to the monoclinic and orthorhombic phase, respectively). Coupling of these optic phonons to the soft acoustical phonon which is associated with the phase transition is discussed.     

Raman spectrum of α and β-Pb3(PO4)2

The Raman spectrum of Pb₃(PO₄)₂ exhibits a progressive modification from the α to the β phase spectrum. No discontinuities are observed at the transition point.One soft mode of A_g type due to a translation mode parallel to the binary axis appears in the α phase.A strong broadening of the Rayleigh line is also observed above and below the transition point.     

High temperature Raman scattering and phase transition in EuAlO3

Polarized Raman spectra of EuAlO₃ have been obtained at various temperatures from 77–1500 K. Three phonon modes are observed to soften in frequency with increasing temperature. Discontinuity in the frequency temperature dependence appears at 1420 K. This result is consistent with a first order phase transition. Analysis of the Raman data shows evidence of a coupling between the soft mode of Bg(XY) symmetry and one mode of same symmetry.     

Raman scattering in K2Pt(CN)4Br0.3 · 3H2O

Raman scattering experiments on K₂Pt(CN)₄Br_0.3 · 3H₂O are reported between 5 and 300 K as a function of temperature. A line of A₁ symmetry detected at 44 cm^?1 shows interesting temperature dependent properties. It is concluded from a comparison of the frequency, symmetry, and scattering intensity of this line with theoretical predictions that the excitation concerned represents the amplitude mode of the charge density wave (the line observed in infrared absorption being the phase mode). No Peierls transition is observed, but the results are consistent with a Peierls distortion present at all temperatures. The findings are correlated with inelastic neutron scattering and infrared studies. Finally, the CN stretching modes at 2189 and 2173 cm^?1 and the water mode at 3490 cm^?1 are studied as a function of temperature.     

钽酸锂铁电相变的喇曼光谱研究

本文测定了钽酸锂晶体从室温到居里温度以上(20—800℃)的喇曼光谱。没有观察到软模现象。我们发现在居里点附近,铁电相和顺电相的喇曼光谱十分一致,经过居里点喇曼光谱不发生变化。根据这个新的实验结果和文献上关于用中子衍射测定的晶体结构资料,我们做出了钽酸锂的铁电相变为有序无序型的推论。 关键词：     

On the order parameter of RbH3(SeO3)2

Rubidium trihydrogen selenite RbH₃(SeO₃)₂ undergoes an improper ferro-electric phase transition due to the condensation of a doubly degenerate soft mode at the zone boundary. Superlattice reflections have been measured using neutron diffraction, and after taking domains into account, it is shown that the measured reflections scale onto the same curve as a function of temperature. The consequences of this scaling are explained in terms of the temperature dependence of the order parameter of the transition.     

CDW evidence in one-dimensional K0.3MoO3 by means of Raman scattering

Raman scattering measurements on the one-dimensional (1-D) blue bronze K_0.3MoO₃ are reported as function of temperature. At about 50 cm^-1 a Raman line of A_g(A) symmetry was found with an anomalous temperature dependence. The line symmetry, the softening of the eigenfrequency and the apparent disappearing of the coupling constant at the critical temperature, allow us to assign this line to the amplitude mode of the CDW, predicted by the theory of the Peierls transition. A Peierls precursor at room temperature was also observed and is discussed.     

Symmetry and lattice-dynamic aspects of structural phase transitions in (CH3NH3)2MnCL4 and related compounds

A group theoretical analysis of the second-order structural phase transition in (CH₃NH₃)₂MnCL₄ at 394°K and of similar transitions in ethyl and propyl compounds (D¹⁷_4h→D¹⁸_2h) was performed. The soft mode transforms according to the τ^x₅-irreducible representation at the X-point of the Brillouin-zone boundary and its eigenvector is discussed. The transition is of the order-disorder type and is caused by a slowing down of the hindered rotation of NH₃-groups. Knowing the symmetry of the order parameter, a thermodynamic potential expansion was constructed and expected anomalies in material constants around the transition temperature are briefly discussed. The high temperature phase transitions in analogous copper compounds are explained as a sequence D¹⁷_4h→D¹⁸_2h→D¹⁵_2h. The second of these phase transitions is driven by a soft mode transforming as the τ^Y₇-representation at the D¹⁸_2h Brillouin zone boundary.     

Epitaxial strain effects on the metal-insulator transition in V2O3 thin films

Effects of epitaxial stress on the metal-insulator transition of V₂O₃ have been studied for in the form of epitaxial thin films grown on α-Al₂O₃ (0001) and LiTaO₃ (0001) substrates. A metallic phase is stabilized down to 2 K in the V₂O₃ thin film on α-Al₂O₃ (0001), where the a-axis is compressed by 4% owing to large epitaxial stress. On the other hand, the transition temperature T_MI is raised by 20 K from the value of 170 K in bulk samples in the film on LiTaO₃ (0001), where the a-axis is expanded. These results suggest an intimate relationship between the a-axis length and T_MI in V₂O₃. The conductivity of the metallic ultrathin films shows logarithmic temperature dependence below 20 K, probably due to the Anderson localization in two-dimensional systems.     

Theoretical explanation of g factors for Ti ions in LiNbO3 and LiTaO3 crystals

The EPR g factors g_// and g_⊥ for Ti³⁺ ions at the trigonal octahedral Li⁺ sites of LiNbO₃ and LiTaO₃ crystals are calculated from the third-order perturbation formulas of g factors for 3d¹ ion in trigonal symmetry. In the calculations, the crystal-field parameters are obtained from the structural data by using the superposition model. The calculated values are in reasonable agreement with the observed values. The results are discussed.     

A comment on the phase transition in RbCaF3

The structural phase transition in RbCaF₃ has been examined by observing super lattice points by means of neutron diffraction. There is one structural phase transition with the R₂₅^x soft mode phonon condensation up to 4.2 K. The rotation angle of CaF₆ octahedra, Ø, is calculated as 8.64 degrees at 60 K. The temperature dependence of Ø is also given.     

Observation of soft-modes in a single crystal of Rb2ZnBr4 by Raman scattering

The Raman spectra of an oriented single crystal of Rb₂ZnBr₄ shows three soft modes. One of them is interpreted as the amplitude mode of the incommensurate phase. The second one, seen well below the lock-in transition temperature appears as the phase mode. Moreover, an additional soft mode is observed between T=120 K and T=20 K, in the b(c, c)a scattering geometry. By extrapolation it is found that its frequency would vanish around T₃=140 K ± 10 K. This soft mode is likely to be related to a new phase transition.     

Electron spin resonance investigation of the structural phase transitions in Alurea6(ClO4)3 and Ga urea6(ClO4)3

Second order structural phase transitions in Alur₆(ClO₄)₃ and Gaur₆(ClO₄)₃ with T_c ～ 300 K are studied by means of ESR on single crystals doped with the analogous Cr(III) compound. The transitions are antiferrodistortive and of the displacive type, the displacements resulting from the condensation of a X₂ mode $\text{(k = (0}\text{1}\text{2}\text{1}\text{2}\text{))}$ of the ClO₄ ions. The ESR parameters have the same temperature dependence as the order parameters and can be described by D and E～φ～. The space group describing the structure changes from S₆² to S₂¹, and the number of domains is multiplied by three. Above 300 K the crystals already consist of two domains, resulting from a ferrodistortive phase transition D_3d⁶ → S₆². The actual transition temperature of the latter phase transition lies at some temperature above the decomposition temperature of the crystals.     

Enhancement of the superconducting transition temperature near a phase instability in NaxWO3

The superconducting transition temperature of tetragonal I sodium tungsten bronze (Na_xWO₃) has been found to increase rapidly as the x-value is decreased to the metal-semiconductor phase transition. It is suggested that a soft mode instability is responsible for the increased electron-phonon interaction near the phase transition.     

Magnetic phase diagram of Ni(NO3)2 6NH3

The magnetic phase diagram of Ni(NO₃)₂6NH₃ was determined from the field and temperature dependence of the magnetic susceptibility. The zero temperature exchange and anisotropy fields were determined to be H_E(0) ≈ 26 kOe and H_A(0) ≈ 0.7 kOe respectively.     

Temperature Raman scattering study of CaAl0.5Ta0.5O3 perovskite crystal

Temperature Raman scattering studies were performed for CaAl_0.5Ta_0.5O₃ crystal. This material features NaCl-type ordering of Al³⁺ and Ta⁵⁺ ions at the B-site superimposed onto b⁻b⁻c⁺ octahedral tilting. Because of the existing twin-related domains in crystal structure, the micro-Raman measurements were carried out at room temperature. Some differences in Raman spectra obtained using macro- and micro-Raman system were revealed. Most of the Raman modes show monotonous red-shift with the increase in temperature. Only the cubic-like fully symmetric A_1g mode slightly increases its frequency with an increase in temperature. There are no uncontinuous changes of mode frequency in the temperature range studied. It indicates the stable character of static distortions of crystal structure relying on changes of octahedra tilt angle.     

Raman study of NH4NO3 and ND4NO3—250–420K

Raman spectra of NH₄NO₃, and ND₄NO₃, were studied from 250 to 420K. The results show that there are four phases separated by first order transitions. No evidence of the previously reported phase II' was observed.The present results combined with the results of other experiments present the following picture of the state of order of the molecules.In phase I, the highest temperature phase, the NH₄⁺ groups are in a free rotation and the nitrate groups are likely in random reorientation among 12-equivalent positions. In phase II, the NH₄⁺ groups are likely in rapid random reorientation under the local force field of S₄ symmetry. The nitrate groups are in hindered rotation but are disordered with one of the O-N bonds directed in one sense or the other along the c-axis. In phase III, the absence of the librational mode indicates that the NH₄⁺ groups are in nearly free rotation but the rotational motion is restricted by the local force field of C₃ symmetry. The nitrate groups are probably ordered as suggested by the well polarized character of the modes associated with the nitrate groups. In phase IV, the nitrate groups are ordered with their molecular planes perpendicular to the b-axis. The NH₄⁺ groups are in orientational disorder but may undergo bindered rotations. An optical mode was observed to couple to an anomalous mode which is believed to be a zone edge acoustical mode.