Anomalies of elastic properties of layered materials

Acoustic investigations of layered crystals KY(MoO₄)₂ and glassy alloys Si₂₀Te₈₀ (with inclusions of nanocrystallites) are performed with the purpose of elucidating the character of binding forces in layered materials. The absorption and velocity of sound, as well as the spatial evolution of the spectrum of acoustic fluxes in various directions in wide ranges of temperatures (90–300 K), frequencies (14–1800 MHz), and intensities (0.04–100 W/cm²) of sound, are measured. Acoustooptical and pulse-echo methods were used for the measurements. A theoretical analysis of the data obtained has revealed anomalously large values of the nonlinear elastic coefficients and anharmonicity constants of longitudinal phonon modes that are determined by the anharmonicity of binding forces across the layers and at boundaries with nanocrystallites. It is shown that the anisotropy of the mechanical strength of layered crystals is to a large extent determined by the anharmonicity of binding forces.