Crystallographic Phase Transformation Research Articles

Single-crystal growth of MF 2 : UF 4 : CeF 3 (M = Ca, Sr, Ba) solid solutions

The melt growth of solid-solution single crystals of MF 2 : UF 4 : CeF 3 (M = Ca, Sr, Ba) is described. Comments on relevant parts of the binary MF 2-UF 4 systems applicable to the crystal-growth problem and some new information are also presented. The question of the existence of a crystallographic phase transformation in pure UF 4 was investigated.

On the Crystallographic Phase Transformation in Mn0.9Fe0.1As.

On the Crystallographic Phase Transformation in Mn0.9Fe0.1As.

Structural, Magnetic, and Magneto-Optic Properties of Ti-Substituted MnBi Films

Structural, magnetic, and magneto-optic properties of Mn1−xTixBi films (0≤x≤0.2) prepared by a vacuum-evaporation technique are reported. The crystallographic structure and the order of phase transformations of unsubstituted MnBi films were not altered by Ti. Both low-temperature (LT) phase and quenched high-temperature (HT) phase films were oriented with the hexagonal c axis and the magnetization normal to the film plane. The Curie temperature of HT films was decreased by Ti. There is a strong effect of Ti on the kinetics of the crystallographic phase transformations in the films. Particularly, the speed of the HT-phase-to-LT-phase transformation is reduced by up to three orders of magnitude, depending on the Ti concentration and the temperature. At room temperature the specific Faraday rotation of LT (HT) phase films decreased with increasing Ti concentration from 9.8 × 105 deg/cm (4.3 × 105 deg/cm) for x = 0 to 3.8 × 105 deg/cm (1.6 × 105 deg/cm) for x = 0.18, measured with light of 632.8-nm wavelength. The optical absorption coefficient, typically 5.3 × 105 cm−1 at 25°C and 632.8 nm, was not changed appreciably by Ti.

Two switching devices utilizing VO2

Vanadium dioxide undergoes a crystallographic phase transformation at 68°C which is accompanied by a large and abrupt change in resistivity. Two devices, a two-terminal threshold switch and a four-terminal thermal relay, based on this effect are investigated theoretically and experimentally with regard to switching applications. The formation of thermal filaments is found to have a strong influence on both the dc characteristics and on the switching properties. One consequence for both devices is that the power required to sustain the "on" condition is significantly less than that required for switching. It is shown that both devices should be capable of submicrosecond switching times at milliwatt power levels.

Thermodynamic theory of nonhydrostatically stressed solid involving finite strain

A thermodynamic theory under nonhydrostatic stresses is developed to discuss the role of thermodynamic functions in chemical processes, based on the treatment of elasticity involving finite deformation. This treatment makes it possible to examine the attempts made by many investigators to define the chemical potential as a function of the state alone. It is shown that these treatments were not quite rigorous because relative stability depends not only on the state, but also on the process. The chemical potential introduced by the ‘local equilibrium’ theory developed by J. W. Gibbs, W. B. Kamb, and A. G. McLellan is shown to be valid under certain restrictions. An expression of the chemical potential applicable to the case under a small nonhydrostatic stress superimposed on a finite hydrostatic pressure is derived. The states are classified as ‘simple’ or ‘complex’ state, when the bulk stability is treated on the basis of this chemical potential. Application of the theory to crystallographic phase transformation explains the spreading of transition pressure resulting from nonhydrostatic components of stress. Preferred orientations of cubic, hexagonal, and trigonal crystals are discussed, and the result expected from the theory seems to be consistent with the experiments or observations for LaAlO3, α quartz, and ice.

Electron Spin Resonance of Mn2+ in α-Sr2P2O7

Single crystals of Mn2+-doped α-Sr2P2O7 have been studied by electron spin resonance at 35 Gc/sec. The space-group ambiguity has been resolved because the Mn2+ ions lie on a mirror plane, and therefore the unit cell must be centric. The spin-Hamilitonian parameters have been determined to be gx = 2.0021(10), gy = 2.0015(4), gz = 2.0012(4), b20 = − 177.1(10) G, b22 = − 118.5(46) G, Ax = − 91.9(10) G, Ay = − 92.8(13) G, and Az = − 90.8(7) G. The z principal axis lies in the ab plane 17° from the b crystallographic axis. Only one EPR active site has been found rather than the two anticipated on crystallographic grounds. No evidence of a crystallographic phase transformation was found in the temperature range 100°–300°K.

On reaction in the solid state

Using the concept of phonon-lattice interaction, an expression is derived for the minimum temperature, T R , of a solid at which it may enter into solid-state reaction. This expression leads to a more precise idea of single-component reaction (sintering) temperatures than those given by Hüttig. Reaction between two different solids has been re-examined in the light of their physical and crystallographic properties. The rate law for such an additive reaction has been deduced from the quantum rate theory. It is concluded that crystallographic phase transformations and the formation of transitional superstructures constitute the phase-boundary processes and that the kinetics are governed by the dynamics of the diffusion process.