The Madelung constant and bonding angle as “sensors” of the high-pressure induced amorphization and first-order isostructural phase transition in Mg(OH)2 and Ca(OH)2

Abstract

With the use of high-pressure experimental results available in literature, this study theoretically investigates, for M(OH)2 (M= Mg, Ca) compounds, the pressure dependences of the structural parameters (lattice parameters, the Madelung constant, bonding angle.O-M-O, unit cell volume and its contributions from constituent sublattices) and bulk compressibility. These pressure dependences and their anomalies are analyzed in terms of the amorphization of the H-sublattice and isostructural transition in a compound. It is demonstrated that (i) for Mg(OH)2 and Ca(OH)2, the Madelung constant and bonding angle O-M-O decrease with increase of pressure, (ii) for Mg(OH)2 and Ca(OH)2, both the Madelung constant and bonding angle O-M-O achieve a minimum at pressure of the onset of a strong amorphization of the H-sublattice, and (iii) for Mg(OH)2, the isostructural phase transition and above critical amorphization occur practically at the same pressure. It is assumed that the critical amorphization is mainly responsible for isostructural phase transition.