BRILLOUIN-SCATTERING STUDY OF THE ELASTIC ANOMALIES AT THE STRUCTURAL PHASE-TRANSITION IN ANTHRACENE-TETRACYANOBENZENE

Brillouin-scattering experiments performed at room temperature indicate unambiguous relations between the crystalline structure and the optical and elastic properties of this charge-transfer crystal. The largest refractive index and the large longitudinal sound velocity are found along the molecular long axis direction, which is [201»]. All the elastic constants are determined, and the anisotropy of the derived linear compressibility is found in reasonably good agreement with the thermal-expansion anisotropy. Two kinds of elastic anomalies have been recorded at low temperatures and analyzed as being mainly due to an interaction that occurs along the [201»] direction. Elastic anomalies of relaxational character as large as 22% have been recorded, and a strong temperature dispersion allows the determination, in the low-temperature phase, of the order-parameter relaxation time 5×10-11 (Tc-T)-1 s K-1. © 1990 The American Physical Society.