Structural Transformations in the Decomposition of Mg(OH) ₂ and MgCO ₃

A transmission electron microscopy study has been carried out to understand the structural transformation mechanisms of decomposition of Mg(OH) 2 and MgCO 3 under vacuum. Both Mg(OH) 2 and MgCO 3 decompose topotactically to yield porous pseudomorphic MgO of normal structure with definite orientation relationships. The decomposition of Mg(OH) 2 yields MgO with a single orientation relationship, but that of MgCO 3 leads to one major and two minor orientation relationships with two, six, and three variants, respectively. The major orientation relationships in the formation of 2‐ to 3‐nm cubic MgO particles are discussed in terms of a correlation between oxygen octahedra in the reactant solids. The small cubic MgO particles may aggregate spontaneously to reduce the excess surface energy and result in minute cracks.