Large Anharmonicity of Amorphous and Crystalline Phases of a Pd-Si Alloy

Abstract

The uniaxial-stress dependence of the Young's modulus $E$ in ${\mathrm{Pd}}_{0.775}$ ${\mathrm{Si}}_{0.165}$ ${\mathrm{Cu}}_{0.06}$ is found to have the unusually large values $\frac{\mathrm{dE}}{d\sigma }=-12\mathrm{} \mathrm{and} -16\mathrm{}$ at low stresses in the amorphous and crystalline phases, respectively. In the amorphous phase a sharp reversible decrease in $\frac{\mathrm{dE}}{d\sigma }$ also occurs for uniaxial stresses greater than 1.2 × ${10}^9$ dyn/${\mathrm{cm}}^2$. No explanation is given to account for the abrupt change in behavior for small tetragonal strains. A discussion of the anharmonicity is given including a comparison of the calculated and observed high-temperature specific-heat contribution (linear in temperature) from the anharmonicity. The ultrasonic attenuation calculated for the Akhieser effect is also presented and compared with the experimental value. Reasonable agreement is obtained in both cases. Finally, some qualitative inferences of the anharmonicity to the structure are given.