Phonons with wave vectors along [100], [110], and [111] directions and energies $\lesssim 36$ meV were measured in a large (1.2 ${\mathrm{cm}}^3$) single crystal using triple-axis spectroscopy. Scattering intensities were used to make symmetry assignments for the $\overrightarrow{\mathrm{q}}=0\mathrm{}$ modes. A total of 15 of the expected 24 $\overrightarrow{\mathrm{q}}=0\mathrm{}$ modes are identified and show much accidental near-degeneracy. The ${\Gamma }_{25}^{\prime }$ mode frequency is in agreement with Raman results. A short-ranged central-force Born—von Karman model is constructed and used to discuss the results.

• Received 21 December 1981

DOI:https://doi.org/10.1103/PhysRevB.25.5856