Brillouin scattering on rubidium iodide under high pressure

Brillouin scattering measurements were made on single crystals of rubidium iodide (RbI) in the 100> and 110> directions at high pressures up to 4 kbar. RbI shows a pressure-induced first-order phase transition from the NaCl ($B1$) to CsCl ($B2$) structure at about 3.5 kbar. The elastic constants C₁₁ and C₄₄ determined from the Brillouin shifts (at frequencies at about 10¹⁰ Hz) show a linear pressure dependence in agreement with ultrasonic measurements (at sound frequencies at about 10⁶ Hz). No significant deviations from the linear pressure dependence were found in the vicinity of the pressure-induced phase transition. The results from the present Brillouin measurements, probing the long-wavelength limit of the phonon dispersion of the transverse-acoustic (TA) 110> branches, contrast with earlier neutron scattering data, which exhibit small anomalies in frequency and intensity in the pressure dependence of the TA 100> and 110> phonons with small values of the reduced wave vector ($q0.5$) at the onset of the phase transition.