Protons in oxides B y C. R. A. C atlow 1, P. S. B aram 2, S. C. P arker 2, J. P urton 2 and K. V. W right 3 1Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21Albemarle Street, London W1X U.K. 2 Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K. 3 MaterialsScience Centre, University of Manchester and UMIST, Grosvenor Street, Manchester Ml 7HS, U.K. We review the nature of the defects and defect reactions involved in the incor- poration of water and hydrogen in oxides and silicates. We describe the role of computational techniques in investigating the structures and energies of hydrogen containing defects in these materials. 1. Introduction ’ The presence of water and of hydrogen containing defects is known often to have a controlling influence on both the chemical and physical properties of oxide and silicate materials; examples range from the role of Brpnsted acidic hydroxyl groups in catalytic aluminosilicates (Thomas 1993) through solid state proton conduction (Iwahara 1992) to the crucial yet poorly understood mechanisms of the reaction of water with quartz (Hobbs 1984; Cordier Sz Doukhan 1991) and other minerals including olivine, [(Mg/Fe)2 Si04] where the nature and mech- anisms of water dissolution are of considerable geochemical significance (Miller et al. 1987; Bell & Rossman 1992; Bai & Kohlstedt 1992). There is nevertheless relatively little knowledge of the atomistic structures and reaction mechanisms involved in water and hydrogen dissolution in these materials. In this paper we describe the types of defect and defect reactions which may occur when hydrogen is incorporated in oxide and silicate systems and then show how computational techniques - both simulations, based in interatomic potentials and ab initio quan- tum mechanical techniques - may provide valuable quantitative information as to both the structure and stabilities of the hydrogen containing defects and to the energetics of the corresponding defect reactions. The emphasis of this paper is on bulk defect species; although the importance of hydroxyl groups at surfaces is well known as illustrated by, for example, recent studies of the surface structure of the industrial mineral sodalite (Carr et al. 1994). This study will consider binary oxides (specifically MgO), perovskite structured oxides and a range of minerals, notably quartz, garnets, olivine and microporous catalytic aluminosilicates. We will show how these different classes of oxidic com- pound exhibit different facets of hydrogen containing defect chemistry. Phil. Trans. R. Soc. Lond. A (1995) 350, 265—276 Printed in Great Britain 265 (c) 1995 The Royal Society TpX Paper Downloaded from https://royalsocietypublishing.org/ on 20 February 2022