Some crystal chemical aspects of ferroelectrics with structure of the hexagonal tungsten bronze type

Some crystal chemical aspects of ferroelectrics with formula AB(x)W(3-x)O(9) and structure of the hexagonal tungsten bronze (HTB) type are discussed.The compunds have spontaneous polarization either along the c axis (and are hexagonal) or perpendicular to it (and are orthorhombic). The probability of the orthorhombic symmetry increases with the size of the B ions. Compounds have diverse superstructures which seem to be due to different ordering of the B and W ions and octahedra tilting. There are also slow phase transitions in certain of these compounds, described earlier, which seem to be caused by some rebuilding of the ionic order. It strongly changes the compound's properties. The superstructure itself and the compound properties (particularly the electric conductivity) are strongly influenced by deviations from the stoichiometry. The paper deals also with other problems of the HTB-type ferroelectrics.