On the relation between molybdenum trioxide and rhenium trioxide type crystal structures

The structural relatiion between MoO₃-type and ReO₃-type is clarified by considering layers of anions parallel to (13)_MoO3 and (111)_ReO3. MoO₃ is then seen to be approximately cubic close-packed anions while ReO₃ is cubic close-packed anions but with one-quarter of the anion sites unoccupied. The two structures may then be interconverted by a rotation operation resembling that which links ReO₃- and PdF₃-type structures. Photographs of ball models are used to illustrate the relations. No compounds are known which exhibit direct transition from ReO₃-type to either MoO₃- or PdF₃-type. This is probably because of the large change in volume per anion. The interface between ReO₃ and the close packed structures may be described as a dilation fault.