X-ray study of crystal structure and diffuse scattering spectra of ferroelectric SbSBr having pseudo-Jahn-Teller phase transition

The crystal structure of ferroelectric SbSBr was determined at 294, 150, 60 , 30 and 11 K and a phase transition was observed to occur at T-c = 22.8 K, where the crystal structure changed from orthorhombic Pnam with z = 4 to o rthorhombic Pna2(1) with z = 4. The unit cell volume decreased from 0.316 n m(3) (294 K) to 0.310 nm(3) (T-c) and increased to 0.312 nm(3) (11 K) below T-c and there was a drastic decrease of the volume near T-c. The diffuse X -ray scattering spectra of the (0 0 4) Bragg spot indicated that SbSBr had a quasi-one dimensional structure and when the temperature decreased to T-c , the interaction between the chains became strong in the a*-direction in a ddition to in the b*-direction. The temperature dependence of the unit cell volume was regarded to be due to two B-1u modes, which contributed as anha rmonic vibrations to stabilizing the paraelectric phase in the pseudo-Jahn- Teller mechanism. It was concluded that the decrease of the anharmonicity o f the low frequency B-1u mode induced a drastic decrease of the unit cell v olume below 30 K, and produced long range interaction causing the phase tra nsition to occur at 22.8 K. (C) 1999 Elsevier Science Ltd. All rights reser ved.