Phase equilibrium and oscillation of crystal-structure parameters of ammonium perchlorate in the polymorphic transition region

Abstract

Polymorphism of crystalline materials is a part of their structural-phase changes at any type (thermal, baric, etc) of impacts. There are at least two known types of polymorphic transitions, i.e. enantiotropic and monotropic. As is well known, ammonium perchlorate (AP) is revealed to have the polymorphous transition of the first kind within 511–517 K and the data on the isotropic change of thermal expansion coefficients in the pressed AP are analyzed. In this paper, qualitative results on the changes in the diffraction pattern of x-rays reflected from crystals are obtained to perform detailed analysis of the polymorphous transition in AP under atmospheric pressure. The structure of AP crystals at final temperatures was thoroughly investigated with the help of the x-ray diffractometer within 150 and 550 K. Changes recorded in the diffraction pattern indicate changes in the crystalline structure, phase composition, and oscillation of the structural state in AP. Crystallographic models of two modifications of AP, i.e. orthorhombic and cubic, are developed to perform precise, standard-free phase and structural analyses. The full-profile analysis (quantum simulation combined with the differential Fourier-transforms) used to process x-ray patterns. So, the polymorphous transition is proved to be incomplete in AP under atmospheric pressure until its complete decomposition. A simple exponential model is proposed for description of the phase equilibrium process in AP.