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The behavior of longitudinal (LA) and transverse (TA) acoustic phonons in a cubic relaxor PbMg 1/3 Ta 2/3 O 3 (PMT) ferroelectric has been investigated by Brillouin light scattering. Analysis of the temperature dependences of phonon velocity in the temperature range from 50 to 870 К has revealed anomalies in the vicinity of Burns temperature (Т В ≈ 570К) for LA phonons and a wide frequency-dependent minimum of the velocities of LA and TA phonons in the same temperature region as the dielectric response anomaly. Using experimental data, temperature dependences of the С 11 and С 44 elastic constants have been calculated. The elastic constants have been found to be frequency-independent in the gigahertz range. The results obtained are discussed in the framework of modern ideas on the crystalline lattice dynamics of relaxor ferroelectrics.

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