Isothermal kinetics and relaxation recovery of high-frequency shear modulus in the course of structural relaxation of Pd<Subscript>40</Subscript>Cu<Subscript>30</Subscript>Ni<Subscript>10</Subscript>P<Subscript>20</Subscript> bulk glass

Isothermal kinetics of relaxation of the high-frequency (1.4 MHz) shear modulus during structural relaxation of Pd₄₀Cu₃₀Ni₁₀P₂₀ bulk metallic glass below the glass transition temperature is studied by an in situ method of contactless electromagnetic acoustic transformation. The kinetic law of relaxation is established. It is shown that quenching of aged samples from the supercooled liquid state leads to a decrease in the absolute value of shear modulus to below the initial value; the degree of subsequent isothermal relaxation of the modulus may be several times higher than the initial value. Possible reasons for relaxation and recovery of the shear modulus are considered.     

Features of the pressure-induced phase transitions in Eu<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals

The structural changes induced by a 9-GPa pressure in Eu₂(MoO₄)₃ single crystals at room temperature have been studied using x-ray diffraction. It is established that a structural phase transition from the initial tetragonal phase to the new high-pressure tetragonal phase occurs rather than solid-phase amorphization that was observed previously in polycrystalline samples. The samples in the observed transition remain single-crystalline despite a significant difference (ΔV ～ 18%) between the specific volumes of the initial and final phases. It is shown that the transition from the initial state to the high-pressure phase occurs via the formation of broad transition zones featuring a continuous and smooth change of the crystal lattice parameters.     

On the nature of the KH<Subscript>2</Subscript>PO<Subscript>4</Subscript> high-temperature transformation

For over two decades, the high-temperature phase transition (HTPT) at around T _p = 180 °C on KH₂PO₄ (KDP), which involves an ionic conductivity increase, constitutes a controversial subject; while most authors ratify a physical transformation (tetragonal → monoclinic phase transition), others defend the chemical transformation. A careful high-temperature phase behavior examination of this acid salt by means of modulated and conventional differential scanning calorimetry, thermogravimetric analysis, simultaneous thermogravimetric and differential scanning calorimetry, impedance spectroscopy, and temperature evolution of X-ray diffraction was performed to provide a possible solution to this long-standing issue. We found that the structural phase transition does not take place. Instead, a chemical transformation occurs at T _p. When KDP is heated through this temperature, the sample initially corresponding to a single phase (tetragonal) transforms to a sample composed of two solid phases: tetragonal KDP, located at its bulk, and monoclinic potassium metaphosphate (KPO₃), located at its surface. Most of the water produced evaporates, but a small portion of liquid water bonds to KPO₃. Because this is of polymeric nature, it takes the role of a host matrix that contains liquid water regions. Consequently, given that part of the water dissolves a portion of surface salt (providing protons), the surface sample system behaves in a similar manner to a polymer electrolyte membrane where the proton transport mechanism includes the vehicle type, using hydronium (H₃O⁺) as a charge carrier. On further heating, the bulk tetragonal KDP phase reduced to its total decomposition. The metastability of the high-temperature phase below T _p is also explained.     

Heterophase states in 0.10PbTiO<Subscript>3</Subscript>-0.90Pb(Zn<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript> crystals

A new model of the elastic matching of phases is proposed, and heterophase structures near the morphotropic phase boundary in 0.10PbTiO₃-0.90Pb(Zn_1/3Nb_2/3)O₃ crystals are studied. Unique behavior of the unit cell parameters is found to favor the elastic matching of the ferroelectric tetragonal and orthorhombic phases under the conditions of complete or partial relaxation of internal mechanical stresses at a volume concentration ratio of these phases of about 20/80% and temperatures of T=20–300 K. Interrelations between the volume concentrations of different domain (twin) types and of the coexisting phases are analyzed.     

Microstructure of Cu<Subscript>60</Subscript>Zr<Subscript>20</Subscript>Ti<Subscript>20</Subscript> bulk metallic glass rolled at different strain rates

The structural evolution of Cu₆₀Zr₂₀Ti₂₀ bulk metallic glass during rolling at different strain rates and cryogenic temperature was investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and high-resolution transmission electron microscopy (HRTEM). It is revealed that the deformation-induced transformation is strongly dependent on the strain rate. At the lowest experimental strain rate of 1.0×10⁻⁴ s⁻¹, no phase transformation occurs until the highest deformation degree reaches 95%. In a strain rate range of 5.0×10⁻⁴−5.0×10⁻² s⁻¹, phase separation occurs in a high deformation degree. As the strain rate reaches 5.0×10⁻¹ s⁻¹, phase separation and nanocrystallization concur. The critical deformation degree for occurrence of phase transformation decreases with the strain rate increasing. Supported by the National Natural Science Foundation of China (Grant No. 50471016)     

Shear viscosity of the Pd<Subscript>40</Subscript>Cu<Subscript>40</Subscript>P<Subscript>20</Subscript> metallic glass under conditions of isochronous heating below the glass transition temperature

The shear viscosity is measured under conditions of isochronous (linear) heating below the glass transition temperature of the Pd₄₀Cu₄₀P₂₀ metallic glass, which is characterized by the polymorphic crystallization into the Pd₂Cu₂P tetragonal phase with a lower density than the initial glass. It is shown that the rate dependence of the shear viscosity can be interpreted as a result of the irreversible structural relaxation by analogy with the case of the previously studied metallic glasses despite the unusual ratio of the densities of the material in noncrystalline and crystalline states.     

Glass-forming ability and thermal stability of Fe<Subscript>62</Subscript>Nb<Subscript>8−x</Subscript>Zr<Subscript>x</Subscript>B<Subscript>30</Subscript> and Fe<Subscript>72</Subscript>Zr<Subscript>8</Subscript>B<Subscript>20</Subscript> amorphous alloys?

Glass-forming ability (GFA) and thermal stability of Fe₆₂Nb₈B₃₀, Fe₆₂Nb₆Zr₂B₃₀ and Fe₇₂Zr₈B₂₀ at % amorphous alloys were investigated by calorimetric (DSC and DTA) measurements. The crystallization kinetics was studied by DSC in the mode of continuous versus linear heating and it was found that both the glass transition temperature, T _g , and the crystallization peak temperature, T _p , display strong dependence on the heating rate. The partial replacement of Nb by Zr leads to lower T _g and T _x temperatures and causes a decrease of the supercooled liquid region. JMA analysis of isothermal transformation data measured between T _g and T _x suggests that the crystallization of the Fe₆₂Nb₈B₃₀ and Fe₆₂Nb₆Zr₂B₃₀ amorphous alloys take place by three-dimensional growth with constant nucleation rate. Nb enhances the precipitation of the metastable Fe₂₃B₆ phase and stabilizes it up to the third crystallization stage. Zr addition increases the lattice constant of Fe₂₃B₆ and, at the same time, decreases the grain size.     

Enhanced electrochemical performances of Li<Subscript>2</Subscript>MnO<Subscript>3</Subscript> cathode materials by Al doping

Al-doped Li₂MnO₃ (Li₂Mn_0.9Al_0.1O₃) lithium-rich layered oxide is prepared and investigated as cathode material for lithium-ion batteries (LIBs). X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive spectrometer (SEM-EDS) analyses reveal that the Al element is distributed in the sample homogenously. The Al-LMO sample exhibits a great improvement on the rate capability and cycling stability compared to the LMO sample. The differential capacity versus voltage (dQ/dV) results reveal that Al doping would be to prevent the first charge phase transformation from a layered phase to a cubic spinel-like phase and also slowdown the rate of transformation upon cycling. Electrochemical impedance spectroscopy (EIS) results confirm that Al doping decreases the charge-transfer resistance and improves the electrochemical reaction kinetics.     

Thermal reaction characterization of micron-sized aluminum powders in CO<Subscript>2</Subscript>

The thermal reaction characterization of micron-sized aluminium powder in carbon dioxide were investigated by simultaneous thermal analysis technology (TG/DSC), using a series of heating rates (5, 10, 15, 20°C/min). The results showed that the reaction process of micron-sized aluminium powder in carbon dioxide was divided into three stages: the initial slow oxidation stage, the sharp oxidation stage and the last oxidation stage. The thermal performance was increased with the increase in the heating rates. Evolution of the samples was determined by collecting the products at the initial, sharp, and last oxidation stages of the process. The reaction products morphology was examined using scanning electron microscopy (SEM). The corresponding chemical changes were analysed by X-ray diffraction spectrometry (XRD). The effects of heating rate on the thermal reaction characteristics were discussed. A new reaction mechanism of micron-sized Al particle in CO₂ with gradually increased temperature was proposed.     

Manifestation of a ferroelectric-antiferroelectric phase transition in Li<Subscript>0.12</Subscript>Na<Subscript>0.88</Subscript>Ta<Subscript>0.4</Subscript>Nb<Subscript>0.6</Subscript>O<Subscript>3</Subscript> in its Raman scattering spectra

The ferroelectric-antiferroelectric phase transition in the Li_0.12Na_0.88Ta_0.4Nb_0.6O₃ ceramic solid solution has been studied by the Raman scattering technique. As the temperature approached the transition point from below, we observed an appreciable broadening of the lines associated with the vibrations of the cations occupying octahedral and cubooctahedral cavities of the structure and with the oxygen network vibrations (which implies a substantial increase in disorder on the cation sublattices), as well as a decrease to zero intensity of the 875-cm^?1 line corresponding to stretch vibrations of the bridging oxygen in the BO₆ octahedral anion in the vicinity of the transition. The temperature dependence of the 875-cm_?1 line intensity near the transition was used to study the behavior of the phase transition order parameter η. The behavior of η was found to disagree markedly with the Landau theory of second-order phase transitions. It is shown that discrepancies originate from the increase in disorder in the niobium and tantalum sublattices in the Li_0.12Na_0.88Ta _y Nb_1-y O₃ solid solution system with increasing y. The order of the transition is lowered.     

The influence of chromium impurity centers on the critical properties of a weakly polar ferroelectric Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript>

The Cr³⁺ EPR spectra of Li₂Ge₇O₁₅ (LGO) crystals are analyzed in the temperature range of the ferroelectric phase transition. The temperature dependence of the local order parameter is determined from the measured splittings of the EPR lines in the polar phase. The experimental critical exponent of the order parameter β=0.31 in the range from the phase transition temperature T _C to (T _C -T) ～ 40 K corresponds to the critical exponent of the three-dimensional Ising model. Analysis of the available data demonstrates that, away from the phase transition temperature T _C , the macroscopic and local properties of LGO crystals are characterized by a crossover from the fluctuation behavior to the classical behavior described in terms of the mean-field theory. The temperature dependence of the local order parameter for LGO: Cr crystals does not exhibit a crossover from the Ising behavior (β=0.31) to the classical behavior (β=0.5). This is explained by the defect nature of Cr³⁺ impurity centers, which weaken the spatial correlations in the LGO host crystal. The specific features of the critical properties of LGO: Cr³⁺ crystals are discussed within a microscopic model of structural phase transitions.     

Electrochemical properties of Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>/C cathode materials synthesized via ethylene glycol-assisted solvothermal method

The Li₃V₂(PO₄)₃/C (LVP/C) cathode materials for lithium-ion batteries were synthesized via ethylene glycol-assisted solvothermal method. The phase composition, phase transition temperature, morphology, and fined microstructure were studied using X-ray diffraction (XRD), differential thermal analyzer (DTA), scanning electron microscope (SEM), and transmission electron microscope (TEM), respectively. The electrochemical properties, impedance, and electrical conductivity of LVP/C cathode materials were tested by channel battery analyzer, the electrochemical workstation, and the Hall test system, respectively. The results shown that the appropriate amount of water added to ethylene glycol solvent contributes to the synthesis of pure phase LVP. The LVP₁₀/C cathode material can exhibit discharge capacities of 128, 126, 126, 123, 124, and 114 mAh g^?1 at 0.1, 0.5, 2, 5, 10, and 20 C in the voltage range of 3.0–4.3 V, respectively. Meanwhile, it shows also a stable cycling performance with the capacity retention of 89.6% after 180 cycles at 20 C.     

Phase behaviour and superionic phase transition in K<Subscript>3</Subscript>H(SeO<Subscript>4</Subscript>)<Subscript>2</Subscript>

The phase behaviour of K₃H(SeO₄)₂ (TKHSe) above room temperature has been studied by differential scanning calorimetric (DSC), thermogravimetric analysis (TGA), simultaneous thermogravimetric and mass spectroscopy analysis (TG-MS), impedance spectroscopy (IS) and X-ray powder diffraction (XRD). Our results show that the previously claimed superionic phase transition in TKHSe at around 388 K (114.85 °C) is also the onset temperature of a slow thermal dehydration that occurs at reaction sites distributed over the surface of the crystal. That is, we propose that the TKHSe undergoes simultaneously a superionic phase transition and a decomposition process with a very slow reaction rate that is evident when the sample is pulverized to fine powder, both starting at the same temperature. As a matter of fact, we observe a decrease of the magnitude of the dc-conductivity on successive thermal runs in powdered sample attributed to sample decomposition that starts at the surface of the TKHSe grains, but the jump in conductivity is only a consequence of the order–disorder transition in the TKHSe phase that remains inside the grains.     

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.     

Optical studies of delayed relaxation in Pb<Subscript>0.94</Subscript>Ba<Subscript>0.06</Subscript>Sc<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> (PBSN-6) solid solutions

The temperature dependences of the permittivity, birefringence, optical transmittance, and small-angle light scattering and their variations with time are studied for single crystals of the Pb_0.94Ba_0.06Sc_0.5Nb_0.5O₃ relaxor (PBSN-6) in the heterophase region of coexistence of different phases. It is shown that an electric field induces a phase transition to the ferroelectric state, which manifests itself within some time (delay time τ) after application of the electric field to the crystal. The observed dependence of the temperature of this transition on the heating rate of the sample and the changes in the birefringence and small-angle light scattering intensity with time confirm the kinetic character of the induced transition. Temperature dependences of the delay time τ for different electric fields are constructed. It is revealed that, at low temperatures, the delay time τ decreases with increasing temperature. This agrees with the behavior of τ in classical relaxors. At the Vogel-Fulcher temperature, however, one observes that dτ/dT reverses sign and τ increasing as the temperature continues to increase. This anomalous behavior of τ in the heterophase region is accounted for by the coexistence of the cubic relaxor and rhombohedral macrodomain phases.     

New magnetic states in copper metaborate CuB<Subscript>2</Subscript>O<Subscript>4</Subscript>

The static and resonance properties of copper metaborate CuB₂O₄ were experimentally studied in a magnetic field applied in the crystal tetragonal plane. The field-induced second-order phase transition to a weakly ferromagnetic state was observed in the temperature range 10–20 K. The low-field state is characterized by the absence of spontaneous moment, and it represents, presumably, a long-period helicoid. At temperatures below 2 K, two sequential first-order phase transitions were observed. They were accompanied by jumps in resonance absorption with a hysteresis upon changing field-scan direction. These transitions can be caused by the transformation of the incommensurate spin structure into the helicoidal states with periods commensurate with the lattice translation period.     

On the possible coexistence of spiral and collinear structures in antiferromagnetic KFe(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript>

The static and resonance properties of a quasi-two-dimensional antiferromagnet phase on a distorted triangular lattice of KFe(MoO₄)₂ have been experimentally studied. Magnetization curves exhibit features corresponding to the spin-flop transition in a collinear biaxial antiferromagnet and simultaneously show a magnetization plateau characteristic of a triangular spin structure. The magnetic resonance spectra also display absorption lines corresponding to the spin structures of both types. The experimental data are described in terms of a model comprising alternating weakly bound magnetic layers, in which the main two exchange integrals have different values. Below the Néel temperature (T _N =2.5 K), some of these layers possess a collinear antiferromagnetic structure, while the other layers have a triangular or spiral structure.     

Kinetics of structural relaxation of bulk and ribbon Pd<Subscript>40</Subscript>Cu<Subscript>30</Subscript>Ni<Subscript>10</Subscript>P<Subscript>20</Subscript> glasses determined from electrical resistance measurements

The electrical resistances of ribbon and bulk Pd₄₀Cu₃₀Ni₁₀P₂₀ metallic glasses, whose quenching rates differ by four orders of magnitude, were precisely measured during cyclic heating. Three stages of electrical resistance relaxation are detected as the maximum heating temperature increases. The first and third stages decrease the electrical resistance, and the second stage increases it. The first stage is shown to be caused by the relaxation of deformation-induced internal stresses and not to be related to the excess free volume concentration, which differs by a factor of about 2 in the ribbon and bulk samples. The second stage reflects structural relaxation in the glass and is only partly related to its free volume. The third relaxation stage is assumed to be caused by fine precrystallization phenomena like phase separation. The effect of deformation by rolling or quenching from the temperature range of a supercooled melt on the resistance relaxation kinetics was also studied.     

Pressure-induced transformations in two-dimensional polymeric phases of C<Subscript>60</Subscript>

The structural stability of the tetragonal and rhombohedral two-dimensional (2D) polymeric phases of C₆₀ was studied under pressures up to 27 GPa at room temperature by means of in situ Raman scattering spectroscopy. The results show that the tetragonal 2D phase undergoes an irreversible transformation in the region of 20 GPa while no pressure-induced transitions were observed for the rhombohedral 2D phase. The obtained data are discussed within the framework of recent numerical calculations, which predict the pressure-induced transformation of the 2D polymeric phases of C₆₀ into three-dimensional (3D) polymers in the pressure range 14–20 GPa.     

Structural disorder in the paraelectric phase of the Fe<Subscript>3</Subscript>B<Subscript>7</Subscript>O<Subscript>13</Subscript>Br boracite

A structural model of the cubic paraelectric phase of a Fe₃B₇O₁₃Br crystal belonging to the boracite family has been developed using the data obtained by single-crystal X-ray diffraction with due regard for the results of extended X-ray absorption fine structure (EXAFS) spectroscopy. It has been shown that the best agreement between the data obtained by these two methods is achieved within a model assuming a disorder in the arrangement of both the Fe and Br atoms and a high degree of correlation of their displacements. It has been found that, during the phase transition from the rhombohedral ferroelectric phase to the cubic paraelectric phase, no significant transformation of the structure is observed on a local level. In this case, a change in the macroscopic symmetry occurs predominantly as a result of the variation in the set of possible spatial orientations of stable structural fragments, which is characteristic of order-disorder phase transitions.