M. Natarajan

Phase transitions and conductivity anomalies in solid solutions of VO2 with TiO2, NbO2, and MoO2

The solid solutions V1−xTixO2 (0·02<-x<-0·4) which are monoclinic at room temperature (with the monoclinicity decreasing with increasing x) transform to rutile structures at Tt. The Tt, H as well as the conductivity jump (at Tt) decrease with increase in x. Incorporation of 2 at.% of Nb (or Mo) lowers the Tt of VO2 considerably, while 10% Nb stabilizes the high-temperature rutile structure. The high temperature rutile phases of all the solid solutions show semiconducting behavior indicating that the conductivity anomalies correspond to semiconductor-semiconductor transitions.

Particle size effects and thermal hysteresis in crystal structure transformations

Particle size affects the temperature and enthalpy of phase transformations as well as the magnitude of thermal hysteresis ΔT, which is shown to be approximately equal to (1/Cpβ)ΔV2/V. The variation of ΔT with particle size (0–150 µm) is interpreted in terms of Turnbulls theory of heterogeneous nucleation. The dependence of the enthalpy of transformation on particle size seems to result from the variation of surface area and surface energy with particle size.

Thermal and particle size effects in magnesium oxide

X-ray line widths, surface areas and heats of solution of MgO samples prepared by vacuum dehydration of Mg(OH)2 at different temperatures in the range 350–1 200°C have been studied. While the surface energy of particles shows a steady decrease with the temperature of preparation, the heats of solution shows an abrupt decrease over a narrow temperature range. The results have been interpreted in terms of pseudocrystallization of the highly disordered oxide followed by sintering; the pseudocrystallization of colloidal oxides appear to be similar to glass transitions.

Antiferroelectric transition in CsPbCl3

Abstract The transformation of CsPbCl 3 at 47°C is associated with a change from the antiferroelectric phase to the paraelectric phase.

Pm3m-Fm3m transformations of alkali halides. Solid solutions of CsCl with KCl, CsBr, SrCl2

Pm3m-Fm3m transformations of solid solutions of CsCl with KCl and CsBr exhibit different behaviours. With increasing percentages of KCl, the NaCl structure gets stabilized in the CsCl+KCl system. In the CsCl+CsBr system, the transformation temperature increases with % CsBr and ΔH essentially remains constant. Both these behaviours can be satisfactorily explained in terms of the Born treatment of ionic solids. The Pm3m-Fm3m transformation retains its first-order characteristics in the CsCl+KCl system, but higher-order components seem to be present in the CsCl+CsBr system. Incorporation of vacancies do not affect the transformation of CsCl markedly.

Phase transitions in silver halides: silver iodide and its solid solutions with silver bromide

There is no doubt of the existence of the B3 polytype of Agl. Both the B3 and B4 phases of Agl transform into the B23 phase around 145 °C; there is no evidence for the B3 →B4 transformation. The phase transitions of Agl, AgBr, and AgCl can be satisfactorily explained on the basis of the Born treatment of ionic solids. About 10 moles % AgBr enters into solid solution with the B3, B4, or B23 phase of Agl; about 25% Agl enters into solid solution with the B1 form of AgBr. We have reported the phase transitions of the Agl–AgBr solid solutions and a partial phase diagram of the system. The Born model explains the stability of the B1 form of the solid solution above ca. 10% AgBr.

Defect energies in CsCl and its solid solutions with KCl, RbCl, and CsBr

The formation energies of Schottky defects in the CsCl (Pm3m) and the NaCl (Fm3m) structures of CsCl are found to be 1·4 and 2·0 eV respectively. The ground-state interaction energies between Sr2+Cs+ and the V–Cs+ in the Pm3m and Fm3m phases are ca. 0·45 and 0·35 eV respectively. The cation migration energies in CsCl are affected by the incorporation of K+, Rb+ or Br–. The formation energy of Schottky defects in the solid solutions of CsCl with KCl or RbCl is low (1·4 eV) in the Pm3m phase, but attains a much higher value (2 eV) at compositions where the Fm3m structure gets stabilized.