CRYSTALLOGRAPHIC DISCUSSION OF AgTa_xNb_1-xO₃ (ATN) PEROVSKITE SYSTEM

Zdenek Zikmund

Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 180 40 Praha 8, Czech Republic, zikmund@fzu.cz

Keywords: perovskite, silver niobate, silver tantalate, phase transitions, symmetry reduction, distortions, (octahedral) tilt

Structural discussion and symmetry analysis of AgTa_xNb_1-xO₃ (ATN) system is given. Using a close analogy of the system to NaNbO₃ crystals and supposed tilt character of ATN phases symmetry space groups of the phases are derived including their embedding in a prototypic symmetry group and crystal structure. From point of view of structure distortion and symmetry reduction ATN phase transitions can be distinguished into two types: potentially continuous and (almost) necessarily discrete. Consequently, set of all ATN phases can be partishioned into few sequences of the phases with increasing distortion and decreasing symmetry separated by potentially continuous transitions and realizing, as a rule, with decreasing temperature. Nevertheless, a reversal has been found between two phases, M₃ and O, in such a sequence with decreasing temperature (having no analogy in NaNbO₃ crystals). It should be stressed that very high permittivity and low dielectric losses make these materials highly perspective for electronic (microwave) applications.