Phase transition effects in the incommensurate modulated single crystal of Sr0.7Ba0.3Nb2O6

On mixed crystals of tetragonal tungsten bronze-type structure, Sr0.7Ba0.3Nb2O6, the incommensurate (IC) modulated structure and phase transition (PT) phenomena are investigated at room temperature (RT) and at four steps up to 773 K, using the neutron time-of-flight method and at RT the triple-axis technique, too. At RT the influence of an electric poling field on the structure is measured. The satellites of first, second, and third rank of the modulation vector are found. The satellite positions are not sensitive on electric field influence and on temperature. The satellite intensity decreases strongly with temperature and at 773 K the IC structure is nearly disappeared. At cooling below TC a change of structure is indicated at first only by streaks of diffuse elastic scattering, widely extended perpendicularly to the c-axis around (00l) reflections. The PT into the polar phase, which is influenced by aging and favoured by an electric poling field, becomes visible due to a change of the lattice parameters and the appearance of a (h/2 h/2 l/2) superstructure. The existence of another PT is indicated by a strong change of intensity of distinct Bragg reflections between 500 and 773 K. (author)