High-Pressure Nmr And Compressibility Evidence For A Phase-Transition In The Protonic Conductor (NH4)4Fe(CN)6.1.5H2o

1H NMR at high hydrostatic pressures and compressibility studies show that the protonic conductor (NH4)4Fe(CN)6·1.5H2O undergoes a phase transition around 0.45 GPa. The transition is characterized by a large hysteresis. From the NMR studies, an activation volume of 6% is obtained below the phase transition, indicating the dominance of Frenkel defects.

The role of transition metal oxides in the decomposition and explosion of arylammonium perchlorates

The influence of MnO2, CuO, and NiO on the thermal decomposition and explosivity of arylammonium perchlorates has been studied by differential thermal analysis (DTA) and explosive sensitivity measurements. The metal oxides considerably sensitize both decomposition and explosion and the sensitizing effect is in the order NiO < CuO < MnO2. The accelerated decomposition or explosion seems to occur via the formation of an intermediate, metal perchlorate arylamine complex. The experimental evidence for the mechanism put forward has been included.

Mechanistic studies on the pyrolysis and combustion of polystyrene/ammonium perchlorate propellants in the presence of transition metal oxides

The effect of transition metal oxides (Fe2O3, MnO2, Ni2O3 and Co2O3) on polystyrene/ammonium perchlorate propellant systems has been examined. The mechanism of action of the oxides in increasing the burning rate was examined by studying the effect of the oxides on the thermal decomposition and combustion of the oxidizer and the propellant. It has been concluded that one of the mechanisms by which the oxides act is by promoting the charge-transfer process, which is indicated by the enhancement of the electron-transfer process in ammonium perchlorate and by the correlation between the redox potential of the metal ions and the corresponding burning rates of the propellant.

Metal Auger Intensity Ratios in Transition Metals and Their Compounds

Metal Auger intensity ratios of the type Z(CVV)/I(CC'V) and Z(CVV)/Z(CC'C"), where C, C' and C" denote core levels and V stands for a valence level, are shown to increase progressively with the number of valence electrons in the metal in the case of second-row transition metals and their oxides. Metal Auger intensity ratios in chalcogenides of transition metals can be correlated by taking the effective atomic charge on the metal into consideration. The possible use of metal Auger intensity ratios in the study of surface oxidation of second-row transition metals is illustrated in the case of zirconium.

Pressure sensitive flow and constraint factor in amorphous materials below glass transition

The constraint factor, C (given by the hardness-yield strength ratio H/Y in the fully lastic regime of indentation), in metallic glasses, is greater than three, a reflection of the sensitivity of their plastic flow to pressure. Furthermore, C increases with increasing temperature. In this work, we examine if this is true in amorphous polymers as well, through experiments on amorphous poly(methyl methacrylate) (PMMA). Uniaxial compression as well as spherical indentation tests were conducted in the 248-348 K range to construct H/Y versus indentation strain plots at each temperature and obtain the C-values. Results show that C increases with temperature in PMMA as well. Good correlation between the loss factors, measured using a dynamic mechanical analyzer, and C, suggest that the enhanced sensitivity to pressure is possibly due to beta-relaxation. We offer possible mechanistic reasons for the observed trends in amorphous materials in terms of relaxation processes.

Esr study of (SO4)2- dynamics in relation to the ferroelectric phase transition in ammonium sulfate

Ferroelectric phase transition in ammonium sulfate has been studied by ESR of CrO43- radical substituting for SO42- ion in (NH4)2SO4. In addition to discontinuous changes at Tc, certain continuous changes are observed in ESR parameters of this probe below Tc, which reflect the role of the sulfate ion in the phase transition. A microscopic mechanism of the phase transition is proposed and discussed in terms of the change of orientation of the sulfate tetrahedron through a finite angle. The degree of the change of orientation below Tc is thought to be the possible order parameter of the phase transition.

XPES studies of oxides of second- and third-row transition metals including rare earths

X-ray photoelectron spectroscopy has been employed to investigate oxides of second- and third-row transition metals, including those of rare earths. Systematics in the spin—orbit splittings and binding energies of core levels of the metals are described. In most of the cases studied, the dependence of the spin—orbit splittings on the atomic number Z is given by the relation ΔE = a(Z - Z0)4, where a is the quantum defect parameter and Z0 is the effective screening. Core-level binding energies are found to increase with the oxidation state of the metal. Most of the core-level binding energies are related to the atomic number Z by the expression E = x(Z - Z0)2, giving rise to linear plots of ln E versus ln Z. Specific features of individual oxides, with respect to satellites, multiplet structure, configuration mixing, and other properties are also discussed. The spectra of PrO2, Pr6O11, TbO2 and Tb4O7 are reported for the first time.

Predicting a new phase (T′′) of two-dimensional transition metal di-chalcogenides and strain-controlled topological phase transition

Single layered transition metal dichalcogenides have attracted tremendous research interest due to their structural phase diversities. By using a global optimization approach, we have discovered a new phase of transition metal dichalcogenides (labelled as T′′), which is confirmed to be energetically, dynamically and kinetically stable by our first-principles calculations. The new T′′ MoS2 phase exhibits an intrinsic quantum spin Hall (QSH) effect with a nontrivial gap as large as 0.42 eV, suggesting that a two-dimensional (2D) topological insulator can be achieved at room temperature. Most interestingly, there is a topological phase transition simply driven by a small tensile strain of up to 2%. Furthermore, all the known MX2 (M = Mo or W; X = S, Se or Te) monolayers in the new T′′ phase unambiguously display similar band topologies and strain controlled topological phase transitions. Our findings greatly enrich the 2D families of transition metal dichalcogenides and offer a feasible way to control the electronic states of 2D topological insulators for the fabrication of high-speed spintronics devices.

Pressure effect on the phase transition of trissarcosine calcium chloride

The dielectric measurement of ferroelectric trissarcosine calcium chloride (TSCC) was made under various pressures up to 6 kbar. A striking decrease in the peak value of the permittivity, epsilon r, at the transition temperature, Tc, was observed with increasing pressure. The value of Tc increases linearly with a pressure coefficient dTc/dp=11.1K kbar-1 at low pressures. This increase in Tc supports the suggestion that the ferroelectric transition is of the pure order-disorder type. It is suggested on the basis of the behaviour of epsilon r with pressure that the order of the ferroelectric transition changes from second to first order on application of pressure.

Universality class of the reversible-irreversible transition in sheared suspensions

Collections of non-Brownian particles suspended in a viscous fluid and subjected to oscillatory shear at very low Reynolds number have recently been shown to exhibit a remarkable dynamical phase transition separating reversible from irreversible behavior as the strain amplitude or volume fraction are increased. We present a simple model for this phenomenon, based on which we argue that this transition lies in the universality class of the conserved directed percolation models. This leads to predictions for the scaling behavior of a large number of experimental observables. Non-Brownian suspensions under oscillatory shear may thus constitute the first experimental realization of an inactive-active phase transition which is not in the universality class of conventional directed percolation.

On factors influencing the ductile-to-brittle transition in a bulk metallic glass

An experimental study to ascertain the ductile-to-brittle transition (DBT) in a bulk metallic glass (BMG) was conducted. Results of the impact toughness tests conducted at various temperatures on as-cast and structurally relaxed Zr-based BMG show a sharp DBT. The DBT temperature was found to be sensitive to the free-volume content in the alloy. Possible factors that result in the DBT were critically examined. It was found that the postulate of a critical free volume required for the amorphous alloy to exhibit good toughness cannot rationalize the experimental trends. Likewise, the Poisson's ratio-toughness correlations, which suggest a critical Poisson's ratio above which all glasses are tough, were found not to hold good. Viscoplasticity theories, developed using the concept of shear transformation zones and which describe the temperature and strain rate dependence of the crack-tip plasticity in BMGs, appear to be capable of capturing the essence of the experiments. Our results highlight the need for a more generalized theory to understand the origins of toughness in BMGs.

Doping Dependence of Semiconductor-Metal Transition in InP at High Pressures

The resistivity of selenium-doped n-InP single crystal layers grown by liquid-phase epitaxy with electron concentrations varying from 6.7 x 10$^18$ to 1.8 x 10$^20$ cm$^{-3}$ has been measured as a function of hydrostatic pressure up to 10 GPa. Semiconductor-metal transitions were observed in each case with a change in resistivity by two to three orders of magnitude. The transition pressure p$_c$ decreased monotonically from 7.24 to 5.90 GPa with increasing doping concentration n according to the relation $p_c = p_o [1 - k(n/n_m)^a]$, where n$_m$ is the concentration (per cubic centimetre) of phosphorus donor sites in InP atoms, p$_o$ is the transition pressure at low doping concentrations, k is a constant and $\alpha$ is an exponent found experimentally to be 0.637. The decrease in p$_c$ is considered to be due to increasing internal stress developed at high concentrations of ionized donors. The high-pressure metallic phase had a resistivity (2.02-6.47) x 10$^{-7}$ $\Omega$ cm, with a positive temperature coefficient dependent on doping.

Auger studies of transition metal oxides

Intra-atomic Auger transitions involving metal energy levels are found to be useful in studying the surface oxidation state as well as the oxidation of metals. Transition Metal oxides also exhibit interatomic Auger transitions, the intensities of which depend on the occupation of the metal d level. The probability of the interatomic transition is therefore highest in oxides where the metal has the d' configuration. The competition between intra-atomic and interatomic Auger transitions in oxides will be discussed as also the use of the interatomic transitions in the study of metal oxidation.

NMR studies of phase transition in ammonium hydrogen bischloroacetate, NH4H(CH2ClCOOO)2

Proton second moment (M2) and spin-lattice relaxation time (T1) of Ammonium Hydrogen Bischloroacetate (ABCA) have been measured in the range 77-350 K. A value of 6.5 G2 has been observed for the second moment at room temperature, which is typical of NH4+ reorientation and also a second moment transition in the range 170-145 K indicates the freezing of NH4+ motion. The NMR signal disappears dicontinuously at 128 K. Proton spin-lattice relaxation time (T1) Vs temperature, yielded only one sharp miniumum of 1.9 msec which is again typical of NH4+ reorientation. A slope change at 250 K is also observed, prbably due to CH2 motion. Further, the FID signal disappears at 128 K. Thus the Tc appears to be 128 K (of two reported values 120 K and 128 K). Activation energies have been calculated and the mechanism of the phase transition is discussed.