Anharmonic Forces in the GaP Crystal

The infrared properties and the thermal expansion of gallium phosphide are treated on the basis of a simple model of the anharmonic forces. In this model the anharmonic forces are nearest neighbor central forces characterized by a single parameter. The value of this parameter for GaP is obtained from the integrated absorption recently measured for certain infrared combination bands. It is shown that the observed width of the fundamental resonance (reststrahl) and the observed shift with temperature of the combination bands are consistent with the absorption. This is presented as evidence that the anharmonic mechanism is predominantly responsible for the infrared absorption in the combination bands. It is also shown that the observed thermal expansion is consistent with the same anharmonic model.