Hydrostatic pressure effect on Tc of new BiS2‐based Bi4O4S3 and NdO0.5F0.5BiS2 layered superconductors

We investigate the external hydrostatic pressure effect on the superconducting transition temperature (T_c) of new layered superconductors Bi₄O₄S₃ and NdO_0.5F_0.5BiS₂. Though the T_c is found to have a moderate decrease from 4.8 K to 4.3 K (dT_c^onset/dP = –0.28 K/GPa) for Bi₄O₄S₃ superconductor, the same increases from 4.6 K to 5 K (dT_c^onset/dP = 0.44 K/GPa) up to 1.31 GPa followed by a sudden decrease from 5 K to 4.7 K up to 1.75 GPa for NdO_0.5F_0.5BiS₂ superconductor. The variation of T_c in these systems may be correlated to an increase or decrease of the charge carriers in the density of states under externally applied pressure. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural studies of the P-T phase diagram of sodium niobate

The crystal structure of sodium niobate (NaNbO₃) has been investigated by energy-dispersive X-ray diffraction at high pressures (up to 4.3 GPa) in the temperature range 300–1050 K. At normal conditions, NaNbO₃ has an orthorhombic structure with Pbcm symmetry (antiferroelectric P phase). Upon heating, sodium niobate undergoes a series of consecutive transitions between structural modulated phases P-R-S-T(1)-T(2)-U; these transitions manifest themselves as anomalies in the temperature dependences of the positions and widths of diffraction peaks. Application of high pressure leads to a decrease in the temperatures of the structural transitions to the R, S, T(1), T(2), and U phases with different baric coefficients. A phase diagram for sodium niobate has been build in the pressure range 0–4.3 GPa and the temperature range 300–1050 K. The dependences of the unit-cell parameters and volume on pressure and temperature have been obtained. The bulk modulus and the volume coefficients of thermal expansion have been calculated for different structural modulated phases of sodium niobate. A phase transition (presumably, from the antiferroelectric orthorhombic P phase to the ferroelectric rhombohedral N phase) has been observed at high pressure (P = 1.6 GPa) and room temperature.     

Effect of high pressure on the crystal and magnetic structures of La<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>CoO<Subscript>3</Subscript> cobaltite

The atomic and magnetic structures of La_0.5Ca_0.5CoO₃ cobaltite have been studied by the neutron diffraction technique at high pressures of up to 4 GPa in the 10- to 300-K temperature range. The pressure dependences of the structural parameters have been obtained. The Curie temperature increases with the pressure with the coefficient dT _C/dP = 1 K/GPa, demonstrating the stability of the ground ferromagnetic (FM) state. The pressure dependence of the ground FM state in La_0.5Ca_0.5CoO₃ is in drastic contrast with that for La_{1 − x} Ca _x CoO₃ at a lower calcium content (x < 0.3). For the latter compound, the pressure suppressed the ground FM state and a large negative pressure coefficient of the Curie temperature (dT _C/dP ∼ −10 K/GPa) was observed. The nature of such a phenomenon is analyzed in the framework of the double exchange model also taking into account the changes in the electron configuration of Co³⁺ ions.     

High-pressure effect on the ferroelectric-paraelectric transition in PbTiO<Subscript>3</Subscript>

The crystal structure of lead titanate PbTiO₃ was investigated by energy dispersive X-ray diffraction at high pressures up to 4 GPa in a temperature range of 300–950 K. At the ambient conditions, the PbTiO₃ structure is tetragonal with the space group P4mm (ferroelectric phase). A structural phase transition into the cubic phase with a space group Pm[`3]mPm\bar 3m is observed at T = 747 K. It was found that the phase transition temperature decreases upon applying the high pressure with the coefficient dT _C /dP = -65 K/GPa. Dependences of parameters and volume of the unit cell on the pressure and temperature was found, and the bulk modulus and thermal expansion coefficients for the tetragonal and cubic phases of lead titanate have been calculated.     

Structural transformations in a single‐crystal Rb2NaYF6: Raman scattering study

This paper reports Raman spectroscopy investigation of phase transitions in Rb₂NaYF₆ crystal. The experimental spectra were compared with the calculated one. The spectra were obtained in temperature range from 8 to 300 K. The Raman spectra shows anomalous temperature‐dependent behavior at T₁ = 154 and T₂ = 122 K. Soft mode restoration has been found, which allows us to attribute first transition at 154 K to displacive type. Detailed analysis temperature dependencies of the line positions and widths have been performed. We found no effects of possible lattice disorder anywhere, except narrow (about 20 K) range above the T₁ temperature. The Raman spectra of Rb₂NaYF₆ crystal have been obtained and analyzed under hydrostatic pressure up to 4.33 GPa (at T = 295 K). The high pressure experiment up to 4.33 GPa did not disclose any effects associated with phase transitions. The lattice vibration spectra were calculated up to 10 GPa. The calculation has been demonstrated that the Rb₂NaYF₆ does not undergo high pressure phase transition. Copyright © 2013 John Wiley & Sons, Ltd.     

Quantum dots of Cd0.5Mn0.5Te semimagnetic semiconductor formed by the cold isostatic pressure method

Cd_0.5Mn_0.5Te is a semimagnetic semiconductor, which crystallizes in the zinc-blende structure (ZB) and exhibits a magnetic spin glass like transition at 21 K. Under pressure it shows a first-order phase transition around 2.6 GPa to the NaCl like structure. In this work, the pressure cycled method using a Paris–Edinburgh cell up to 8 GPa has been applied to Cd_0.5Mn_0.5Te samples in order to obtain recovered nanocrystals. The nanoparticles have been characterized by EDX and electron microscopy. The X-ray and electron diffraction results confirmed the existence of nanocrystals in the ZB phase with an average size of 7 nm. Magnetization measurements made in the range of 2–300 K at low field show that the temperature of the magnetic transition decreases when the crystallites’ size is reduced.     

Infrared absorption spectra of a Nd0.5Ho0.5Fe3(BO3)4 crystal

Infrared absorption spectra of a Nd_0.5Ho_0.5Fe₃(BO₃)₄ crystal in the spectral range of 30–1700 cm^–1 have been measured at temperatures from 6 to 300 K. The experimental spectra have been analyzed based on the semiempirical calculation of the lattice dynamics and the analysis of correlation diagrams of borate complexes. No changes associated with structural phase transitions have been detected in the temperature range of measurements; the effect of magnetic ordering on the infrared absorption spectra has not been observed.     

X-Ray powder diffraction study of structural phase transitions in (Ba0.5Sr0.5)PbO3 perovskite

Study of structural properties of (Ba_0.5Sr_0.5)PbO₃ has been carried out in a wide temperature range from 10 to 1223 K using the X-ray powder diffraction technique. Two temperature-induced structural phase transitions at 948 and 1198 K have been found. For polycrystallines the XPS and electrical resistivity measurements have been performed.     

Electronic and vibrational Raman spectroscopy of Nd0.5Sr0.5MnO3 through the phase transitions

Raman scattering experiments have been carried out on single crystals of Nd_0.5Sr_0.5MnO₃ as a function of temperature in the range of 320–50 K, covering the paramagnetic insulator-ferromagnetic metal transition at 250 K and the charge-ordering antiferromagnetic transition at 150 K. The diffusive electronic Raman scattering response is seen in the paramagnetic phase which continue to exist even in the ferromagnetic phase, eventually disappearing below 150 K. We understand the existence of diffusive response in the ferromagnetic phase to the coexistence of the different electronic phases. The frequency and linewidth of the phonons across the transitions show significant changes, which cannot be accounted for only by anharmonic interactions.     

Surface spin-glass-like behavior of monodispersed superparamagnetic Mn0.5Zn0.5Fe2O4 magnetic fluid

Zero field cooled dc-magnetization measurements of monodispersed Mn_0.5Zn_0.5Fe₂O₄ nanoparticles dispersed in kerosene exhibit two transitions at low temperatures. These transitions correspond to (i) the superparamagnetic to blocked superparamagnetic and (ii) the blocked superparamagnetic to surface spin-glass like/quantum superparamagnetic state upon lowering the temperature. The existence of a disorder surface is confirmed by recording small-angle neutron scattering data below and above the Curie temperature. Magnetic relaxation analysis shows a plateau at low temperature (below 5 K) with a slight minimum at 3 K, which is a characteristic of the surface spin-glass-like state. This is analyzed considering the energy distribution n(E)∼1/E. The existence of surface disorder dominates at low temperature and mimics the transition from superparamagnetic to quantum superparamagnetic states.     

Electrical properties of the high-pressure phases of gallium and indium tellurides

A study has been made of the resistance ρ, the thermopower S, and magnetoresistance MR of Ga₂Te₃ and α-In₂Te₃ single crystals at pressures P up to 25 GPa. It is found that the resistance ρ and |S| sharply decrease at ∼0–5 and 1.5–3 GPa, respectively. The semiconductor-metal phase transitions in the temperature range from 77 to 300 K are established from the sign reversal of the temperature coefficient of ρ to occur at P>4.4 and >1.9 GPa. The values S ≈+(10–20)μ V/K for the metallic phases with a Bi₂Te₃-type structure agree with those for liquid In₂Te₃ and Ga₂Te₃. Negative MR is revealed in In₂Te₃ at P≈1.9 GPa. No MR is observed in Ga₂Te₃ up to 25 GPa. The variation of the electronic structure of In₂Te₃ and Ga₂Te₃ under pressure is discussed. __________ Translated from Fizika Tverdogo Tela, Vol. 42, No. 6, 2000, pp. 1004–1008. Original Russian Text Copyright ? 2000 by Shchennikov, Savchenko, Popova.     

Low-temperature phase transitions in LiKSO4: A neutron study

A single crystal neutron diffraction study of the low temperature structural phase transitions in LiKSO₄ has been carried out. Detailed temperature dependence of the Bragg peak profiles and integrated intensities for a group of about 20 reflections has been investigated upon cooling and heating in the temperature range 300 K to 96 K. The crystal undergoes three clear phase transitions at 205 K (from sp. gr. P6₃ to sp. gr. P31c), at 189 K and at about 135 K on cooling. The corresponding temperatures are 250 K, 190 K and 138 K on the heating cycle. These transitions are extremely sluggish, the kinetics depending on the thermal treatment of the crystal.     

New crystallographic dielectric phase boundary in K0.5Na0.5NbO3‐based lead‐free ceramics

0.979K_0.5Na_0.5Nb_1‐xSb_x O₃‐0.021Bi_0.5Na_0.5TiO₃ (KNNSx ‐BNT) lead‐free piezoelectric ceramics were fabricated by conventional solid state reaction technique, and their phase transition and electrical properties were studied. With the increase of x, the rhombohedral‐orthorhombic phase transition temperature of the ceramics increases. Finally, both the rhombohedral‐orthorhombic and orthorhombic‐tetragonal phase transitions of the ceramics were modified to be around room tempera‐ ture when about 6% Sb were substituted for the Nb site, resulting in the formation of a new phase boundary separating rhombohedral and tetragonal phases. The formation of the new phase boundary results in excellent properties for the ceramics, that is, the KNNS0.05‐BNT ceramic shows an enhancement in piezoelectric properties: d₃₃ = 380 pC/N and k_P = 0.438. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of crystal and magnetic structures of manganite Pr<Subscript>0.7</Subscript>Ba<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at high pressure

The crystal and magnetic structures of manganite Pr_0.7Ba_0.3MnO₃ have been studied at high pressures of up to 5.1 GPa and temperatures from 10 to 300 K by means of the neutron diffraction. At normal pressure and a temperature T _C = 200 K, a ferromagnetic state forms in Pr_0.7Ba_0.3MnO₃. At high pressures P ≥ 1.9 GPa and T < T _N ≈ 153 K, a new antiferromagnetic state of A-type have been observed. Under high pressure, the Curie temperature T _C increases with the characteristic quantity dT _C/dP ≈ 2.4 K/GPa. A possible reason for the appearance of an A-type antiferromagnetic phase in Pr_0.7Ba_0.3MnO₃ at high pressures may be anisotropic uniaxial compression of oxygen octahedra along the b axis of the orthorhombic structure.     

Dielectric and structural relaxation phenomena in Na0.5Bi0.5TiO3 single crystal

Abstract

Dielectric permittivity studies of Na_0.5Bi_0.5TiO₃ single crystals in a broad range of frequency up to 10 MHz and temperature 300—823 K are reported. In this temperature range dielectric dispersion below 1 MHz has been found. The obtained data were fitted to the Cole-Cole relation. The mean relaxation time τ is strongly temperature dependent (0.04 ? 2.6 × 10^?5 s). A remarkable hysteresis effect in the values of τ on cooling and heating took place. The Δε(T) dependence (the maximal value of Δε ～ 400) is similar to the global ε′(T) response at low frequency. An isothermal structural transformation in Na_0.5Bi_0.5TiO₃ was observed by X-ray measurements. The order of the time in which the transformation takes place (～300 minutes) corresponds to the time in which the strongest time evolution of electric permittivity and time changes of dielectric dispersion were detected.     

Calorimetric and x-ray diffraction studies of the (NH4)3WO3F3 and (NH4)3TiOF5 perovskite-like oxyfluorides

The heat capacity and unit cell parameters of the (NH₄)₃WO₃F₃ and (NH₄)₃TiOF₅ perovskite-like oxyfluorides were measured in the temperature interval from 80 to 300 K; the existence of two and one phase transitions in these compounds, respectively, was demonstrated, and their thermodynamic parameters were determined. The effect of a hydrostatic pressure of up to 0.5 GPa on the phase transition temperatures was studied. Triple points and high-pressure phases were found in the T vs. p diagrams. An analysis of entropy changes suggests that all the structural transformations revealed are associated with the ordering of structural blocks. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 5, 2004, pp. 888–894. Original Russian Text Copyright ? 2004 by Flerov, Gorev, Fokina, Bovina, Laptash.     

Temperature-dependent Brillouin scattering studies of surface acoustic modes in Nd0.5Sr0.5MnO3

Brillouin scattering experiments are carried out to study the surface acoustic waves in Nd_0.5Sr_0.5MnO₃ as a function of temperature in the range of 40-300 K covering the metal-insulator and charge-ordering phase transitions. The surface modes include surface Rayleigh wave, pseudo-surface acoustic wave (PSAW) and high velocity PSAW. The observed softening of the sound velocities for the surface modes below paramagnetic to ferromagnetic transition, T_c is related to the softening of the C₄₄ elastic constant. The subsequent hardening of the sound velocity below the charge ordering transition temperature T_co is attributed to the coupling of the acoustic phonon to the charge ordered state via long range ordering of the strong Jahn-Teller (JT) distortion.     

Structural phase transition in Bi2Te3 under high pressure

Structural change in Bi₂Te₃ under high pressure up to 16.6 GPa has been studied by powder x-ray diffraction. We observed two times of phase transitions at room temperature at the pressures of 8 and 14 GPa, respectively. According to our preliminary result on electrical resistance, it is reasonable to suppose that superconducting transition with T _c =2.8 K at the pressures of 10.2 GPa is observed in phase II. On the other hand, we found anomalies of the pressure dependences of lattice parameters and volume at around 2 GPa, which probably means the change in electrical structure on the Fermi surface.     

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.     

Structural phase transitions in KCdF3 and K0.5Rb0.5CdF3

Successive structural transitions of KCdF₃ and K_0.5Rb_0.5CdF₃ have been studied by means of thermal diffusivity, polarized light scattering and X-ray diffraction. It is found that transitions occur at about 460 K and 475 K in KCdF₃, and at 222 K, 232 K, 250 K, 258 K and 268 K in K_0.5Rb_0.5CdF₃. The partial exchange between K⁺ and Rb⁺ ions gives rise to simultaneous condensation of the M^z ₃ and R^x ₂₅ soft phonons.