Role of vacancies of a strongly strained crystal in the melting process

It has been shown that, apart from classical vacancies formed as a result of the thermal fluctuations, a crystal can contain so-called nonclassical vacancies of nonfluctuation nature. The latter vacancies appear when the temperature exceeds a critical value T _C . The factor responsible for their formation is a mechanical instability of an ideal crystal. The temperature T _C is a second-order phase transition point. The vacancies formed as a result of this phase transition are joined together into small clusters with sizes of the order of several atoms. The above transition makes a substantial contribution to the premelting process observed experimentally.     

Influence of polytypism on structural phase transformations in TlGaSe2 crystals

The polytypism is found to have a significant influence on the structural transformation of the crystal lattice of the TlGaSe₂ ferroelectric with variations in temperature. In the 2C-polytype, unlike the C-polytype, a first-order structural phase transition, which leads to a change in the translational symmetry along the C axis, is not observed in the temperature range T = 90–300 K and a second-order phase transition due to the formation of an incommensurate phase occurs at a higher temperature.     

Magnetostructural phase transitions in manganese arsenide single crystals

The effect of an external magnetic field with a strength up to 140 kOe on the phase transitions in manganese arsenide single crystals has been investigated. The existence of unstable magnetic and crystal structures at temperatures above the Curie temperature T _C = 308 K has been established. The displacements of manganese and arsenic atoms during the magnetostructural phase transition and the shift in the temperature of the first-order magnetostructural phase transition in a magnetic field have been determined. It has been shown that the magnetocaloric effect in a magnetic field of 140 kOe near the Curie temperature T _C is equal to ??T ?? 13 K. A model of the superparamagnetic state in MnAs above the temperature T _C has been proposed using the data on the magnetic properties and structural transformation in the region of the first-order magnetostructural phase transition. It has been demonstrated that, at temperatures close to T _C, apart from the contribution to the change in the entropy from the change in the magnetization there is a significant contribution from the transformation of the crystal lattice due to the magnetostructural phase transition.     

Effect of electron and gamma irradiation on elastic characteristics of KTiOPO4 crystals

The elastic moduli C ₁₁ and C ₃₃ of KTiOPO₄ crystals unirradiated and irradiated by electrons and gamma quanta in the temperature range 100–330 K have been measured by the echo-pulse technique. It has been shown that C ₁₁ < C ₃₃ and, with increasing temperature, their values smoothly decrease; moreover, in the temperature range of the second-order phase transition at T ～ 281 K, the curves C ₁₁ = f(T) and C ₃₃ = f(T) exhibit anomalies in the form of a kink. It has been established that, under electron irradiation, the elastic moduli decrease and the phase transition temperature increases. Irradiation of KTiOPO₄ crystals by gamma quanta with a dose of 10⁷ R has no substantial effect on the dynamic characteristics of this crystal.     

Low-temperature structural phase transition in a monoclinic KDy(WO4)2 crystal

The low-temperature thermal and magnetic-resonance properties of a monoclinic KDy(WO₄)₂ single crystal are investigated. It is established that a structural phase transition takes place at T _c=6.38 K. The field dependence of the critical temperature is determined for a magnetic field oriented along the crystallographic a and c axes. The initial part of the H-T phase diagram is plotted for H∥a. The prominent features of the structural phase transition are typical of a second-order Jahn-Teller transition, which is not accompanied by any change in the symmetry of the crystal lattice in the low-temperature phase. The behavior of C(T) in a magnetic field shows that the transition goes to an antiferrodistortion phase. An anomalous increase in the relaxation time (by almost an order of magnitude) following a thermal pulse is observed at T>T _c(H), owing to the structural instability of the lattice. A theoretical model is proposed for the structural phase transition in a magnetic field, and the magnetic-field dependence of T _c is investigated for various directions of the field. Fiz. Tverd. Tela (St. Petersburg) 40, 750–758 (April 1998)     

A transverse electric current in triglycine sulphate ferroelectric crystal

The application of a prolonged transverse electric field at a temperature T_A < T_C leads to unexpected qualitative changes in dielectric and thermal properties of the uniaxial ferroelectric triglycine sulphate (TGS) crystal, where T_C is the critical temperature of the paraelectric-ferroelectric phase transition. The new properties can be still observed even after the transverse field has ceased to be applied as long as the temperature of the sample does not exceed T_A. However, annealing the sample above T_C leads to the restoration of the original state of the crystal. An electric current flowing along the direction perpendicular to the polar axis of the uniaxial TGS ferroelectric crystal was measured below the temperature T_A at which the prolonged transverse electric field had been formerly applied to the crystal for a few hours. The experimental data resemble the classic pyroelectric current flowing along the polar axis.     

Origin of the metallic to insulating transition of an epitaxial Bi(111) film grown on Si(111)

Transport characteristics of single crystal bismuth films on Si(111)-7×7 are found to be metallic or insulating at temperatures below or above T_C, respectively. The transition temperature T_C decreases as the film thickness increases. By combining thickness dependence of the films resistivity, we find the insulating behaviour results from the states inside film, while the metallic behaviour originates from the interface states. We show that quantum size effect in a Bi film, such as the semimetal-to-semiconductor transition, is only observable at a temperature higher than T_C.     

Anomalies in the Young modulus at structural phase transitions in rare-earth cobaltites RBaCo<Subscript>4</Subscript>O<Subscript>7</Subscript> (R = Y,Tm-Lu)

The elastic properties of rare-earth cobaltites RBaCo₄O₇ (R = Y, Tm-Lu) have been experimentally studied in the temperature range of 80–300 K. The strong softening of the Young modulus ΔE(T)/E ₀ ≈ −(0.1–0.2) of cobaltites with Lu and Yb ions has been revealed, which is due to the instability of the crystal structure upon cooling and is accompanied by an inverse jump at the second-order structural phase transition. The softening of the Young modulus and the jump at the phase transition decrease by an order of magnitude and the transition temperature T _s and hysteresis ΔT _s increase from a compound with Lu to that with Tm. A large softening of the Young modulus at the structural transition in Lu- and Yb cobaltites indicates that the corresponding elastic constant goes to zero, whereas this constant in Tm cobaltite is not a “soft” mode of the phase transition. It has been found that the structural phase transition in Lu- and Yb cobaltites is accompanied by a large absorption maximum at the phase transition point and an additional maximum in the low-temperature phase and absorption anomalies in Tm cobaltite is an order of magnitude smaller.     

NMR study of guanidinium cation dynamics in C(NH2)3SbCl6

Proton NMR second moment and spin—lattice relaxation times T ₁ and T _1p have been studied for polycrystalline guanidinium hexachloroantimonate C(NH₂)₃SbCl₆ in a wide temperature range. A dynamic inequivalence of two cations has been detected in spite of their crystal-lographical equivalence. Activation parameters for C₃ reorientation and self-diffusion of the more mobile cation have been determined. It was shown that the para–ferroelastic phase transition at 351 K is connected with abrupt changes in the dynamics of the two cations. The weaker, second-order transition at 265 K is thought to be related to a change in the dynamics of one of the cations.     

Effect of γ irradiation on the heat capacity of (CH3)2NH2Al(SO4)2 · 6H2O crystals near the ferroelectric phase transition

The heat capacity of dimethyl ammonium-aluminum sulfate crystals (DMAAS), both nonirradiated and γ-irradiated to fluences of 10⁷, 5×10⁷, and 10⁸ R, has been measured by the adiabatic method near the ferroelectric phase transition (PT) within the 80–300 K temperature range. The C _p =f(T) curve exhibits a λ-shaped anomaly near the phase-transition point T _C =152 K. The PT temperature and the magnitude of the anomaly are shown to decrease with increasing γ-irradiation fluence. It has been established that the ferroelectric PT at T _C =152 K, which lies close to the tricritical point, shifts progressively more under γ irradiation toward the second-order PT, and that the behavior of the anomalous part of the heat capacity in the ferroelectric phase is described by the thermodynamic theory of Landau. The experimental heat-capacity data have been used to calculate the variation of the thermodynamic functions of the DMAAS crystal.     

On the composition-dependent isotope effect of HTSC in the two-band model

The oxygen isotope effect in high-T_C superconductors has been investigated on the basis of a two-band model where superconducting phase transition is induced by interband (mainly Coulombic) interaction. The isotope shift of T_C appears due to the dependence of averaged interband electronphonon coupling constant on oxygen mass. This coupling has a repulsive nature and gives a relatively small contribution to the total interaction inducing superconductivity. The calculated isotope effect exponent depending on the carrier concentration has been compared with the experimental one as a function of x for La_2-xSr_xCuO₄.     

Ferroelectric phase transitions of the 0.32PIN-0.345PMN-0.335PT single crystals studied by temperature-dependent Raman spectroscopy,dielectric and ferroelectric performance

The ferroelectric phase transition characteristics of the 0.32Pb(In_1/2Nb_1/2)O₃-0.345Pb(Mg_1/3Nb_2/3)O₃-0.335PbTiO₃ (0.32PIN-0.345PMN-0.335PT) single crystals were studied by the temperature-dependent Raman spectroscopy and some electrical properties. Ferroelectric monoclinic phase was confirmed at room temperature by the numbers of the Raman modes. Successive ferroelectric phase transitions, i.e. ferroelectric monoclinic phase to ferroelectric tetragonal phase transition (FE_M-FE_T) and ferroelectric tetragonal phase to paraelectric cubic phase transition (FE_T-P_C), are evidenced by the anomalies of Raman modes line width, peaks intensity and their ratios around T_M-T and T_C/T_m temperatures. The temperature dependent permittivity derivative ξ = d?/dT not only provides further evidence of the successive ferroelectric phase transitions, but also demonstrates the second-order transition characteristic of the FE_M-FE_T phase transition and the first-order transition feature of the FE_T-P_C phase transition. The FE_M-FE_T phase transition is also confirmed by the abnormal narrowing of the P-E loops, decrease of the P_r and E_c values, and extremums of the pyroelectric performance.     

Fermi surface reconstruction in strongly correlated Fermi systems as a first-order phase transition

A quantum phase transition in strongly correlated Fermi systems beyond the topological quantum critical point has been studied using the Fermi liquid approach. The transition takes place between topologically equivalent states with three Fermi surface sheets, but one of them is characterized by a quasiparticle halo in the quasiparticle momentum distribution n(p), and the other one is characterized by a hole pocket. It has been found that the transition between these states is a first-order phase transition for the interaction constant g and temperature T. The phase diagram in the vicinity of this transition has been constructed.     

Structural and magnetic phase transitions in Pr<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at high pressures

The crystal structure and Raman spectra of Pr_0.7Ca_0.3MnO₃ manganite at high pressures of up to 30 GPa and the magnetic structure at pressures of up to 1 GPa have been studied. A structural phase transition from the orthorhombic phase of the Pnma symmetry to the high-pressure orthorhombic phase of the Imma symmetry has been observed at P ∼ 15 GPa and room temperature. Anomalies of the pressure dependences of the bending and stretching vibrational modes have been observed in the region of the phase transition. A magnetic phase transition from the initial ferromagnetic ground state (T _C = 120 K) to the A-type antiferromagnetic state (T _N = 140 K) takes place at a relatively low pressure of P = 1 GPa in the low-temperature region. The structural mechanisms of the change of the character of the magnetic ordering have been discussed.     

Depression of the ferroelastic phase transition in LiCsSO<Subscript>4</Subscript> by uniaxial stress

In the lithium-caesium sulphate crystal in the temperature range of ferroelastic phase transition, the uniaxial stress σ _X induced changes in velocity and attenuation of the longitudinal ultrasonic wave propagating in the direction [010] are studied. The phase transition is close to the three-critical point and the critical exponent is κ = 0.27 ± 0.02. The stress applied drastically decreases the stepwise change in the wave velocity at T _C up to its disappearance at 2 MPa. In the temperature range between T _C and T _C − 6 K, the stress leads to an increase in the wave velocity and a decrease in its attenuation. This range was interpreted as that of co-existence of ferroelastic and incommensurate ordering, in which the stress influences the density of solitons leading to stiffening of the crystal lattice.     

Specific heat of KTiOPO4 within the 80–300 K range

The specific heat of the KTiOPO₄ crystal has been measured with a vacuum adiabatic calorimeter within the 80–300 K range. A peak-shaped anomaly in the specific heat indicating a phase transition has been revealed in the C _p (T) curve at T≅279 K. Numerical integration of smoothened experimental C _p (T) curves yielded the thermodynamic functions of KTiOPO₄, namely, the entropy, enthalpy, and reduced Gibbs energy. The entropy and enthalpy of the observed transition have been determined. Fiz. Tverd. Tela (St. Petersburg) 41, 497–498 (March 1999)     

Specific features of the formation of the ferroelectric phase in polytypes of TlGaSe2 crystals

The polytypism of layered crystals of thallium gallium diselenide TlGaSe₂ has been found to substantially affect the temperature of phase transformations and the mechanism of formation of the polar state in these ferroelectrics. In particular, it is shown that the phase transition observed in the C-TlGaSe₂ polytype is an improper ferroelectric phase transition occurring at a temperature T _c ≈ 108 K, whereas the phase transition observed in the 2C-TlGaSe₂ polytype is a proper ferroelectric phase transition occurring at a higher temperature T _c ≈ 111 K. It is concluded that the elucidation of the polytype of a particular sample is a necessary stage of investigation of the TlGaSe₂ crystals.     

Phase transition and ferroelastic property studied by using the Cs nuclear magnetic resonance in a CsHSO4 single crystal

The ¹³³Cs spin-lattice relaxation time in a CsHSO₄ single crystal was measured in the temperature range from 300 to 450 K. The changes in the ¹³³Cs spin-lattice relaxation rate near T_c1 (=333 K) and T_c2 (=415 K) correspond to phase transitions in the crystal. The small change in the spin-lattice relaxation time across the phase transition from II to III is due to the fact that during the phase transition, the crystal lattice does not change very much; thus, this transition is a second-order phase transition. The abrupt change of T₁ around T_c2 (II-I phase transition) is due to a structural phase transition from the monoclinic to the tetragonal phase; this transition is a first-order transition. The temperature dependences of the relaxation rates in phases I, II, and III are indicative of a single-phonon process and can be represented by T₁⁻¹=A+BT. In addition, from the stress-strain hysteresis loop and the ¹³³Cs nuclear magnetic resonance, we know that the CsHSO₄ crystal has ferroelastic characteristics in phases II and III.     

DSC study of superionic phase transition in (NH4)4H2(SeO4)3 single crystals

DSC and complex impedance studies of the protonic conductor (NH₄)₄H₂(SeO₄)₃, which undergoes a superionic phase transition of first order at T_s = 378 K show that the activation energy of ionic conductivity d(lg σ)/dt and the ordering enthalpy ΔC_p of the crystal are proportional: d(lg σ)/dT = XΔC_p/RT_s + const, as found for MAg₄I₅ crystals undergoing a second-order superionic phase transition. Thus the short-range order environment of the species involved in fast-ion transport plays the main role in the superionic phase transition. This is also supported by the value of the entropy change at T_s, ΔS = 43 J/mole·K. A new metastable phase was found to be induced on heating the (NH₄)₄H₂(SeO₄)₃ crystal above T_s.     

Magnetocaloric effect in Gd6Co1.67Si3 compound with a second-order phase transition

The magnetic properties and the magnetic entropy change AS have been investigated for Gd6Co1.67Si3 compounds with a second-order phase transition. The saturation moment at 5 K and the Curie temperature TC are 38.1μB and 298 K, respectively. The AS originates from a reversible second-order magnetic transition around TC and its value reaches 5.2 J/kg.K for a magnetic field change from 0 to 5T. The refrigerant capacity （RC） of Gd6Co1.67Si3 are calculated by using the methods given in Refs.[12] and [21], respectively, for a field change of 0 5T and its values are 310 and 440 J/kg, which is larger than those of some magnetocaloric materials with a first-order phase transition.