Polymorphism phase transition and the relative stability of various phases in the LiIO3 crystal.

Abstract

The polymorphism phase transition and relative stability of various phases in ${\mathrm{LiIO}}_{3}$ crystal at normal pressure have been further studied by means of \ensuremath{\mu}DTA, constant-temperature heat treatment, specific heat, and room- and high-temperature powder x-ray diffractions. There are three phases, \ensuremath{\beta}, \ensuremath{\eta}, and \ensuremath{\delta}, existing relatively stably at high temperature, and they can directly melt at 432, 421, and 416 \ifmmode^\circ\else\textdegree\fi{}C, respectively. The melting points, thermal processes, and the existing temperature ranges all show that the order of their thermodynamical stability is: \ensuremath{\beta}>\ensuremath{\eta}>\ensuremath{\delta}. At room temperature the \ensuremath{\alpha}, \ensuremath{\beta}, and \ensuremath{\zeta} can coexist in dry air. Their stabilities were determined by Gibbs free energy curves versus temperature. The \ensuremath{\alpha} phase is stable at room temperature, while the \ensuremath{\beta} phase is stable above 300 \ifmmode^\circ\else\textdegree\fi{}C. In the range 210--290 \ifmmode^\circ\else\textdegree\fi{}C, the stable phase is the \ensuremath{\zeta} phase. During heating both \ensuremath{\alpha} and \ensuremath{\zeta} phases change into the \ensuremath{\beta} phase and cannot be transformed into each other. The existence of the \ensuremath{\beta} phase in the \ensuremath{\alpha} or \ensuremath{\zeta} phase has a promotive effect on the phase transition of \ensuremath{\alpha} or \ensuremath{\zeta} into \ensuremath{\beta}. The existence of the \ensuremath{\zeta} phase in the \ensuremath{\alpha} phase also has a promotive effect on the \ensuremath{\alpha} to \ensuremath{\beta} transformation. The \ensuremath{\gamma} phase is an intermediate metastable phase in the phase transition \ensuremath{\alpha}\ensuremath{\rightarrow}\ensuremath{\beta}. The \ensuremath{\theta} phase, like the \ensuremath{\gamma} phase, is also an intermediate metastable phase through which the \ensuremath{\delta} phase changes into the \ensuremath{\alpha} phase on cooling or changes into the \ensuremath{\eta} phase on heating.