New low temperature phases of Sr 2 Ta 2 O 7 and Sr 2 Nb 2 O 7 were found by electron microscopy. Sr 2 Ta 2 O 7 undergoes a structural phase transition at about 170°C to a superlattice structure belonging to the monoclinic system with space group P2 1 / m , accompanying the appearance of characteristic twin and antiphase domains. Sr 2 Nb 2 O 7 has an incommensurate phase below about 220°C with an irrational lattice modulation in the [100] direction and an intricate fine domain-like texture.

New Phases of Sr 2 Ta 2 O 7 and Sr 2 Nb 2 O 7 Found by Electron Microscopy and Diffraction

Dielectric and light scattering spectroscopy are the most sensitive methods for the direct observation of low-frequency excitations in incommensurate phases. In several cases they have been successfully used for the observation of amplitudon and phason modqs. Whereas the amplitudon below the normal-incommensurate transition temperature behaves in a similar way to soft modes at common structural transitions, the phason represents a qualitatively new feature of incommensurate phases. All the corresponding experimental data published up to the middle of 1980 are reviewed and consistently interpreted taking into account the specific selection rules recenily derived. The necessary theoretical background is discussed qualitatively.

Dielectric and light scattering spectroscopy of incommensurate phases in crystals

The relation between the phase transition in the vicinity of 420 K discovered earlier and the formation of an incommensurate phase in the temperature range 410–460 K is established based on the X-ray, dielectric, and dilatometric studies of sodium niobate (NaNbO3) single crystals and ceramics. It is also established that this phase is characterized by temperature and temporal instabilities. Anomalies in some physical characteristics in the vicinity of 350 K are revealed. It is also shown that the thermodynamic history of the samples is important for the coexistence of the regions of the ferroelectric Q and antiferroelectric P phases in NaNbO3.

Structural instabilities, incommensurate modulations and P and Q phases in sodium niobate in the temperature range 300 500 K

Crystal chemical aspects of ferroelectrics AnMnO3n+2 with layered perovskite-like structure (the idealized and real structures, nature of phase transitions, the Curie temperatures, peculiar properties of Sr2Ta2O7, stability of the layered structure, formation of solid solutions) are discussed.

Crystal chemical aspects of the layered perovskite-like oxide ferroelectrics of the AnMnO3n 2 type

Optical mode softening in the incommensurate phase of Sr2Nb2O7

A new phase transition in Sr 2 Nb 2 O 7 was found at 215°C in addition to the already known phase transitions. The elastic compliance coefficients, the electromechanical coupling factor and the dielectric constant of an Sr 2 Nb 2 O 7 single crystal show anomaly around 215°C.

Haruhiko Ishida

Papers

Phase Transition in Sr 2 Nb 2 O 7 at 215°C