A structural theory for non-stoicheiometry. Part 4. Defect fluorite-type structures: vacancy superstructures in ordered calcium oxide-hafnium dioxide ternary oxides

Abstract

The structures of each of the known ternary phases of the CaO–HfO₂ system are interpreted in terms of the coordinated defect (c.d.) model. In all the three phases the integrity of the c.d. is retained with an essential aspect being the cation centring of c.d.s along 〈111〉_F directions. The Φ₁-Ca₂Hf₇O₁₆ phase is based on a close-packed arrangement of isolated [111]_F related metal-centred pairs of c.d.s which occupy only one third of the (111)_F anion layers. For the Φ₂-Ca₆Hf₁₉O₄₄ phase each [111]_F-related metal-centred pair of c.d.s is connected to two further pairs along other 〈111〉_F directions, forming helices of c.d.s throughout the structure. The c.d.s are confined to prisms of sesquioxide composition M₂O₃[CaHfO₃□] which are separated from each other by intersecting {10}_F planes of composition HfO₂. The Φ-CaHf₄O₉ phase has been shown to be comprised of a complex system of intergrowths involving three previously unrecognised distinct phases, one of which also has the overall parent composition CaHf₄O₉ while the other two have the new stoicheiometries CaHf₆O₁₃ and CaHf₂O₅. Even for the regions of the most reduced phase, CaHf₂O₅, the c.d. remains intact by the subtle overlaying of (100)_F strips of CaHf₂O₅ with domains of the more oxidised phases. The analysis has suggested that, as well as the unique topological requirements of the c.d., both cation ordering and the retention of a three-fold axis of symmetry from the parent fluorite structure are important structure-determining features of Φ-Ca₂Hf₇O₁₆ and other defect fluorite-related oxide phases.