INFRARED-ABSORPTION IN DEFECT-INDUCED ANHARMONIC SOLIDS

The infrared absorption in isotopically disordered anharmonic crystals is theoretically investigated using double time thermal Green's funct ion technique. The Hamiltonian used in the study includes the cubic an d quartic anharmonicities alongwith the terms associated with the crys tal defects. The total i.r. absorption in impurity induced anharmonic crystals is obtained as the sum of diagonal and non-diagonal terms. Th e diagonal terms can be further separated into defect dependent, anhar monic and impurity-anharmonicity cross term contributions, the non-dia gonal terms chiefly depend on mass change parameters and vanishes in t he absence of impurities. The impurity-anharmonicity interactions invo lve defect and anharmonic parameters simultaneously and contribute sig nificantly to the absorption coefficient. A brief description of impur ity modes, phonon life times and strength of absorption is given with special reference to impurity-anharmonicity interactions to study the effects of temperature, frequency and impurity concentration on optica l absorption.