Attenuation of acoustic waves and Grüneisen tensor in lithium fluoride crystals

Abstract

The anisotropy of acoustic attenuation in lithium fluoride crystals in the range of 0.4–1.6 GHz was studied by the method of Bragg light diffraction on acoustic waves. The obtained values of the velocity and attenuation coefficient of acoustic waves were used to determine all independent real and imaginary elastic constants. Based on the Akhiezer attenuation mechanism, the anisotropy of acoustic attenuation was determined for the propagation of longitudinal and transverse waves in the (001) and (110) planes. All directions along which pure longitudinal and transverse acoustic waves propagate are found. To describe the anisotropy of acoustic attenuation, the Grüneisen acoustic tensor is used for the first time, the components of which have been determined from the values of the attenuation coefficient of acoustic waves along the crystallographic axes [100], [110], and [111]. It is shown that the orientational dependence of the effective Grüneisen constant, defined in terms of the components of this tensor, describes well the anisotropy of the attenuation of acoustic waves in the studied frequency range. The proposed approach can be used to all cubic crystals in which the main attenuation mechanism is the phonon–phonon Akhiezer mechanism.