Temperature evolution of crystallographic parameters in tetragonal tungsten bronzes

A large group of materials are known to crystallize in the tetragonal tungsten bronze (TTB) structure, having the general formula A12A24C4B12B28O30. Depending on the chemistry and structure a wide range of interesting magnetic and electronic properties are known to exist among these materials, including ferroelectricity. However, there is no general consensus on the origin of ferroelectricity in these materials, and our understanding of the composition-structure-properties relationship is further complicated by the complex modulations, octahedral tilting patterns and mixed occupancy that is also known to exist for this class of materials. Here we propose a temperature dependent high-resolution X-ray powder diffraction investigation of the structures, modulations and mixed occupancies of a wide group of TTBs, namely ferroelectric Ba4A12Nb10O30 (A1 = Li, Na, K, Rb) and non-ferroelectric A24Bi2Nb10O30 (A2 = Na, K, Rb, Ba).