Oxide Polymorphs
Many oxides can exist in more than one crystal structure (polymorph) with the same composition. One polymorph is usually the ground state under ordinary conditions, while others are metastable and can be reached by high pressure or temperature, by epitaxial stabilization in thin films, by nanoscale size effects, or by alloying. Because different polymorphs can share similar local coordination while differing in bond lengths and angles, they offer a way to change chemical properties without changing composition. Density functional theory can screen hypothetical polymorphs for stability and properties before synthesis is attempted. See also Epitaxial stabilization, Strain and ligand effects and Oxygen vacancies in oxides.
Relative stabilities of BO2 polymorphs (2014)
Mehta, Salvador and Kitchin computed equations of state and relative energies of anatase, brookite, rutile, columbite, pyrite and fluorite TiO2, VO2, RuO2, IrO2 and SnO2 with LDA, PBE, PBEsol and AM05. Rutile was the ground state for all but TiO2, whose anatase, brookite, rutile and columbite forms lay within 8 kJ/mol of each other. Volume generally increased from fluorite to pyrite, columbite, rutile, brookite and anatase, with bulk modulus decreasing roughly linearly with volume. Polymorphs within about 10 to 20 kJ/mol of rutile were flagged as epitaxial targets, notably columbite and brookite VO2 and pyrite and columbite RuO2; columbite SnO2 sat only about 3 to 5 kJ/mol above rutile. Because pyrite lies below columbite for RuO2 and IrO2 with most functionals, rutile converts directly to pyrite under pressure, leaving columbite RuO2 accessible only through epitaxy (Mehta et al. 2014). Summary: mehta2014.
Reactivity of hypothetical polymorphs of rutile dioxides (2015)
Xu and Kitchin computed O adsorption and bulk vacancy formation energies for anatase, brookite, columbite and pyrite polymorphs of RuO2, RhO2, IrO2 and PtO2, all of which are rutile-like in practice. Despite similar MO6 octahedral coordination in every structure, the polymorphs did not follow the trends obtained by isotropically straining rutile: changing structure often moved the adsorption energy more than strain, while polymorphs kept relatively endothermic vacancy formation energies, so they broke the adsorption/vacancy correlation of strained rutile. O, OH and OOH energies on polymorphs obeyed the rutile scaling relations, and anatase and columbite IrO2 and brookite RhO2 and PtO2 were predicted to be better oxygen evolution catalysts than their rutile forms. For IrO2, the about 0.15 eV change in O adsorption from rutile to columbite was traced to distortions of the Ir-O bond lengths and angles that redistribute the t2g states, something uniform strain does not do (Xu 2015). Summary: xu2015a.
Anatase and rutile TiO2 films on (Sr,Ba)TiO3 (2017)
Xu, Salvador and Kitchin computed which TiO2 polymorph should nucleate on low-index SrTiO3 and BaTiO3 surfaces. Anatase and the perovskites both have cubic-stacked close-packed oxygen layers, so all three anatase orientations could form coherent interfaces, while hexagonally stacked rutile was coherent only on the (111) facet. Because GGA functionals place anatase about 8 kJ/mol below rutile (the reverse of experiment) and strain penalties reached 5 to 21 kJ/mol, bulk energetics alone gave inconsistent predictions; including computed interface energies reproduced the experimental preference for anatase on (001) and rutile on (111) (Xu et al. 2017). Summary: xu2017.
Growing scrutinyite SnO2 (2017)
Rutile and scrutinyite (α-PbO2) dioxides share a nearly identical hexagonally stacked oxygen sublattice and differ in how cations fill half of the octahedral sites, so their competition is decided by cation arrangement rather than anion packing. Wittkamper et al. used SnO2, whose scrutinyite form is only about 6 kJ/mol less stable than rutile, as a model for harder targets such as scrutinyite IrO2 and RuO2, which have never been made. On columbite CoNb2O6 the scrutinyite polymorph covered slightly more than half of substrate orientation space, and the two polymorphs could be distinguished grain by grain by EBSD, confirmed by dictionary indexing against simulated patterns and by TEM (Wittkamper et al. 2017). Summary: wittkamper2017.