Pressure-induced electronic and structural transformations in bulk GeSe2 glass

The pressure dependence of the electrical resistivity of bulk GeSe₂ glass shows a semiconductor-to-metal transition at 7 GPa pressure. The high pressure phase is examined using the x-ray diffractometer and is found to be crystalline, with a face-centred cubic structure havinga = 4·06 A. The electrical conductivity has also been studied as a function of temperature at various pressures.     

Pressure-induced transformations in crystalline and vitreous PbGeO3

Structural transitions in crystalline and vitreous PbGeO₃ were studied at pressures up to 20 GPa. Crystalline PbGeO₃ was observed to undergo a pressure-induced amorphization between 12–18 GPa. Vitreous PbGeO₃ was found to exhibit an amorphous-to-amorphous transition in a similar pressure range. The structural and thermal properties of the pressure-cycled PbGeO₃ materials were further studied with high-energy x-ray diffraction and differential scanning calorimetry. The properties were then compared to those of thermally quenched glass and ball-milled PbGeO₃ samples. The structure of pressure-amorphized PbGeO₃ was found to closely resemble that of ball-milled PbGeO₃. However, the thermal properties probed by differential scanning calorimetry exhibited significant differences to those of thermally quenched PbGeO₃ glass.     

Mean-field and anomalous behavior on a small-world network

We consider various equilibrium statistical mechanics models with combined short- and long-range interactions and identify the crossover to mean-field behavior, finding anomalous scaling in the width of the mean-field region, as well as in the mean-field amplitudes. We then show that this model enables us, in many cases, to determine the universal critical properties of systems on a small-world network. Finally, we consider nonequilibrium processes.     

Pressure-induced anomalous insulating behavior in frustrated iridate La_3Ir_3O_(11)

The geometrically frustrated iridate La_3Ir_3O_(11) with strong spin–orbit coupling and fractional valence was recently predicted to be a quantum spin liquid candidate at ambient conditions. Here, we systematically investigate the evolution of structural and electronic properties of La_3Ir_3O_(11) under high pressure. Electrical transport measurements reveal an abnormal insulating behavior rather than metallization above a critical pressure P_c ～ 38.7 GPa. Synchrotron x-ray diffraction(XRD)experiments indicate the stability of the pristine cubic KSbO_3-type structure up to 73.1 GPa. Nevertheless, when the pressure gradually increases across P_c, the bulk modulus gets enhanced and the pressure dependence of bond length d_(Ir-Ir) undergoes a slope change. Consistent with the XRD data, detailed analyses of Raman spectra reveal an abnormal redshift of Raman mode and a change of Raman intensity around P_c. Our results demonstrate that the pressure-induced insulating behavior in La_3Ir_3O_(11) can be assigned to the structural modification, such as the distortion of IrO_6 octahedra. These findings will shed light on the emergent abnormal insulating behavior in other 5 d iridates reported recently.     

Renormalization group study of anomalous acoustic behavior in critical percolation network

We study the acoustic behavior of critical percolation network within a real-space renormalization group framework recently proposed by Ohtsuki and Keyes. Using large cell Monte Carlo renormalization group calculations, we obtain the exponent for anomalous sound dispersion K ^1+x/v . Our estimate 2x/v0.80 is in agreement with the exponent for anomalous diffusion in percolation clusters =(–)/v.     

PbTiO3在压力下的异常相变和极强压电效应

运用第一性原理计算预言了在一纯化合物中可由压力诱导出顺序为叫方晶-单斜体-菱形体-立方体的新的相变，而且存在有变形相界面。在相变区有可与在复杂的单晶固溶体压电材料，如人们期待在机电应用方面引起革命性变化的Pb（Mg1/3Nb2/3）O3-PbTiO3可比的，极大的介电和压电耦合常数。我们的结果表明变形相界面和巨压电效应并不需要内禀的无序，并打开了在简单系统中研究这一效应的可能性，     

Pressure-induced anomalous phase transitions and colossal enhancement of piezoelectricity in PbTiO3

We find an unexpected tetragonal-to-monoclinic-to-rhombohedral-to-cubic phase transition sequence induced by pressure, and a morphotropic phase boundary in a pure compound using first-principles calculations. Huge dielectric and piezoelectric coupling constants occur in the transition regions, comparable to those observed in the new complex single-crystal solid-solution piezoelectrics such as Pb(Mg(1/3)Nb(2/3))O3-PbTiO3, which are expected to revolutionize electromechanical applications. Our results show that morphotropic phase boundaries and giant piezoelectric effects do not require intrinsic disorder, and open the possibility of studying this effect in simple systems.     

Pressure-induced phase transition and structural behavior of MgO at high temperatures: a model study

A realistic interaction potential model approach by including temperature effects is developed to study phase transition, elastic properties and thermo-physical properties at very high pressures and temperatures. This approach is effectively able to explain the inter-atomic interaction involved at high temperature and high pressure as it includes the three-body interactions. Earlier works overlooked the three-body interactions at high temperature and pressures. Moreover, the phase-transition pressures of MgO crystal at high temperatures including the three-body interaction are computed for the first time. Elastic behavior, anisotropic factor and Debye temperature of MgO at high pressures and temperatures are also reported.     

PbTiO3在压力下的异常相变和极强压电效应

运用第一性原理计算预言了在一纯化合物中可由压力诱导出顺序为四方晶－单斜体－菱形体－立方体的新的相变,而且存在有变形相界面.在相变区有可与在复杂的单晶固溶体压电材料,如人们期待在机电应用方面引起革命性变化的Pb(Mg1/3Nb2/3)O3-PbTiO3可比的,极大的介电和压电耦合常数.我们的结果表明变形相界面和巨压电效应并不需要内禀的无序,并打开了在简单系统中研究这一效应的可能性.     

Shear viscosity of glass-forming melts in the liquid-glass transition region

A new approach to interpreting the hole-activation model of a viscous flow of glass-forming liquids is proposed. This model underlies the development of the concept on the exponential temperature dependence of the free energy of activation of a flow within the range of the liquid-glass transition in complete agreement with available experimental data. The “formation of a fluctuation hole” in high-heat glass-forming melts is considered as a small-scale low-activation local deformation of a structural network, i.e., the quasi-lattice necessary for the switching of the valence bond, which is the main elementary event of viscous flow of glasses and their melts. In this sense, the hole formation is a conditioned process. A drastic increase in the activation free energy of viscous flow in the liquid-glass transition region is explained by a structural transformation that is reduced to a limiting local elastic deformation of the structural network, which, in turn, originates from the excitation (critical displacement) of a bridging atom like the oxygen atom in the Si-O-Si bridge. At elevated temperatures, as a rule, a necessary amount of excited bridging atoms (locally deformed regions of the structural network) always exists, and the activation free energy of viscous flow is almost independent of temperature. The hole-activation model is closely connected with a number of well-known models describing the viscous flow of glass-forming liquids (the Avramov-Milchev, Nemilov, Ojovan, and other models).     

Viscosity of oxide melts in the Doremus model

It is shown that a model recently proposed by R.H. Doremus for the viscosity of oxide melts correctly reproduces a two-exponential expression for viscosity, which is observed experimentally. For both low and high temperatures, the temperature dependence of viscosity is of the Arrhenius type. The viscosity activation energy at low temperatures is high and equal to the sum of enthalpies of defect formation and their motion. The temperature of transition from high to low viscosity-activation energies is determined by the enthalpy and entropy of defect formation and is not associated with configurational changes.     

Pressure-induced phase transition and structural properties of CrO2

The structural properties and pressure-induced phase transitions of CrO₂ have been investigated using the pseudopotential plane-wave method based on the density functional theory (DFT). The rutile-type (P4₂/mnm), CaCl₂-type (Pnnm), pyrite-type (Pā3), and CaF₂-type (Fm-3m) phases of CrO₂ have been considered. The structural properties such as lattice parameters, bulk moduli and its pressure derivative are consistent with the available experimental data. The second-order phase-transition pressure of CrO₂ from the rutile phase to CaCl₂ phase is 10.9?GPa, which is in good agreement with the experimental result. The sequence of these phases is rutile-type?→?CaCl₂-type?→?pyrite-type?→?CaF₂-type with the phase-transition pressures 10.9, 23.9, and 144.5?GPa, respectively. The equation of state of different phases has also been presented. It is more difficult to compress with the increase of pressure for different phases of CrO₂.     

Behaviour and phase transformations of nickel sulphide inclusions in glass melts

Nickel sulphide inclusions with the NiS composition can cause spontaneous failure of thermally toughened glass. During annealing a phase transformation from high to low-temperature phase takes place, which has a higher volume. This phase transformation, which is very slow at room temperature, generates internal stress in the glass, and after some months up to several years spontaneous fracture can occur. To obtain more information about the nickel sulphide behaviour, NiS and Ni₃S₂ were prepared and investigated in melts of different glass types at various temperatures and melting times. Analysis of the nickel sulphides was done before and after treatment in the melts. Knowing that additives modify the thermodynamics and kinetics of dissolution behaviour, experiments were carried out with 5 mol% Fe or 5 mol% Sn added to Ni₃S₂. The oxidation of the intermediate phases (Ni,Fe)₉S₈ and Ni₃Sn₂S₂ was preferred over the oxidation of the pure nickel sulphides. Additionally, the possible forming process of nickel sulphide in glass was tested. The normally formed phase was Ni₃S₂, but some additional complex metallic phases were also found, depending on the glass type.