Anomalies of elastic properties of layered materials

Acoustic investigations of layered crystals KY(MoO4)(2) and glassy alloys S i20Te80 (with inclusions of nanocrystallites) are performed with the purpos e of elucidating the character of binding forces in layered materials. The absorption and velocity of sound, as well as the spatial evolution of the s pectrum of acoustic fluxes in various directions in wide ranges of temperat ures (90-300 K), frequencies (14-1800 MHz), and intensities (0.04-100 W/cm( 2)) of sound, are measured. Acoustooptical and pulse-echo methods were used for the measurements. A theoretical analysis of the data obtained has reve aled anomalously large values of the nonlinear elastic coefficients and anh armonicity constants of longitudinal phonon modes that are determined by th e anharmonicity of binding forces across the layers and at boundaries with nanocrystallites. It is shown that the anisotropy of the mechanical strengt h of layered crystals is to a large extent determined by the anharmonicity of binding forces. (C) 2001 MAIK "Nauka/Interperiodica".