Third-order elastic constants and acoustic-mode vibrational anharmonicity of cobalt

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Abstract
The third-order elastic constants of cobalt have been determined from measurements of the hydrostatic and uniaxial pressure dependences of the velocities of ultrasonic waves propagated through single crystals. The hydrostatic pressure dependences of the second-order elastic constants have also been obtained. Comparison between these experimental results and predicted values show that, not unexpectedly, a simple Lennard-Jones model employed by previous workers for the lattice potential does not give a realistic assessment of the elastic behaviour of cobalt under pressure. The volume dependence of cobalt upon pressure, i.e. the compression, is estimated by extrapolation up to high pressure of the ultrasonic wave velocity data. The lack of acoustic-mode softening establishes that the h.c.p. to f.c.c. transition in cobalt is not associated with ferroelastic behaviour. The anharmonicity of the long-wavelength acoustic modes is discussed in terms of mode Grüneisen parameters obtained in the elastic continuum approximation.

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