Detection of Negative Thermal Expansion in a Single Crystal of Sr0.75Ba0.25Nb6O2Relaxor Ferroelectric by X-Ray Diffraction Analysis

The atomic structure of a single crystal of Sr(0.75)Ba(0.25)Nb(6)O(2)representing a relaxor ferroelectric is determined from X-ray diffraction data. At nine temperatures within a range of 93-480 K, 12 experiments are performed. The experimental facilities (area detector, goniometer, and sample cooling system) are calibrated. The region of negative thermal expansion with respect to the unit-cell parametercis revealed at 272-324 K. In the same region, an electrical resistance anomaly is observed. It is found that the structural parameters have a singularity at 296 K, in particular, an unusual thermal motion of atoms is observed. This correlates with a kink in the resistance-temperature curve at 297.5 K. The obtained results and analysis of published data indicate that, near room temperature, several physical properties of Sr(0.75)Ba(0.25)Nb(6)O(2)relaxor at once have singularities, which can be explained by the revealed lattice instability.