Control of phase formation and film orientation by molar volume stress during MgO-GeO2 thin-film solid-solid reactions
Abstract Experimental investigations were performed on the role molar volume stresses play during thin-film solid-solid reactions and phase transformations. Reactions and transformations in the thin-film system MgO-GeO 2 were studied under two different conditions, viz. a planar thin-film geometry on a rigid substrate involving stress, and a microbeam geometry ensuring the immediate relaxation of any stresses via microbeam bending. Induced by reactions or transformations, respectively, the formation of the different phases (MgGeO 3 , Mg 2 GeO 4 , Mg 28 Ge 10 O 48 ) and their polymorphs (clino- and orthopyroxene, spinel and olivine) was investigated by RBS, XRD, TEM/SAED and EDX as well as by in situ-TEM-observations applying a double-tilt heating holder. The results show that phase formation under thin-film conditions involving stress indeed differs from that under stress-free conditions. Stability ranges and transformation temperatures were distinctly shifted under conditions involving stress. Moreover, the crystallographic orientation of the olivine phase was stress-dependent. The observations are discussed in terms of different phase transformation mechanisms and a contribution of the stress enthalpy to the Gibbs free energy.