JOURNAL OF SOLID STATE CHEMISTRY 137, 332 — 345 (1998) ARTICLE NO. SC977746 Modeling the Thermal Decomposition of Solids on the Basis of Lattice Energy Changes Part 1: Alkaline–Earth Carbonates Annemarie de La Croix, Robin B. English,- and Michael E. Brown Chemistry Department, Rhodes University, Grahamstown, 6140 South Africa E-mail: chmb@warthog.ru.ac.za and Leslie Glasser Centre for Molecular Design, Chemistry Department, University of the Witwatersrand, P O Wits, 2050 South Africa E-mail: glasser@aurum.chem.wits.ac.za Received July 22, 1997; in revised form December 21, 1997; accepted December 30, 1997 Decompositions of many solids are of the form: A (s)PB (s)  gases. As real examples of this reaction pattern, the decompositions of some alkaline-earth metal (Ca, Sr, Ba) carbonates MCO 3 (s)PMO(s)  CO 2 (g) were selected for modeling. The crystal structures the reactants and the solid product oxides have been reported in the literature. Symmetry-controlled routes for transforming the reactant into the solid product oxide were devised as possible decomposition pathways. Lattice energies of the reactants, the conjectured transient intermediate structures, and the final products were then estimated by crystal modeling procedures, and profiles of energy changes during the proposed decomposition routes were constructed. Barriers in these energy profiles are compared with experimental values reported for the activation energies of the thermal decompositions.  1998 Academic Press INTRODUCTION The processes which occur during solid decompositions are complex, leading to experimental observations which can be very different under even slightly changed conditions. These problems arise from the great variety of possibly - Deceased.  To whom correspondence should be addressed. uncontrolled system variables, such as the nature of the solid reactant (single crystal, powder), its pretreatment (grinding, annealing, etc., which influence the defect content and its nature), sample mass or size (which affect mass and energy transfer), and experimental conditions (temperature, rate of temperature rise, pressure, nature of the surrounding atmosphere, removal or otherwise of evolved gases, and so forth) (1). Consequently, decomposition processes occur along different microscopic (molecular and atomic) paths, depend- ing on the sample history and local conditions, such as chemical composition and crystal structure. The energetics of the processes will also differ as one compares conditions within the bulk of the crystal, on a planar surface, at a crystal corner or protuberance, along a grain boundary, adjacent to a dislocation, and within fine cracks. In experimental terms, decompositions are studied by low-resolution bulk measurements such as changes in mass of a sample, or accumulated pressure of gas evolved from the sample. Such measurements average out the detailed behavior which occurs at the microscopic level. The micro- scopic behavior can only be revealed, to some extent, by repeated measurements with altered sample histories. A very few experimental measurements have been per- formed on complex decompositions in such a way as to attempt to isolate all but a few of the experimental variables. An example is work by Powell and Searcy (2) who examined the evolution of gases in vacuum from selected crystal faces of carbonates. Given this exceedingly complex relation between obser- vation and interpretation, it becomes worthwhile to attempt 332 0022-4596/98 $25.00 Copyright  1998 by Academic Press All rights of reproduction in any form reserved.