Temperature dependence of integrated thermal diffuse scattering

The temperature dependence of the integrated thermal diffuse scattering (T.D.S.) surrounding Bragg reflexions is discussed, with particular reference to cryogenic applications. Detailed calculations of the T.D.S. and its ratio to the Bragg intensity are presented for NaCI over the range 0 to 800°K. The T.D.S. includes not only the one-phonon contribution but also multiphonon processes. The temperature dependence of the T.D.S.-to-Bragg ratio is shown to fall off more quickly at low temperatures than the Debye-Waller exponent values. Some insight is gained into the importance of multiphonon processes, and it is concluded that there is much to be gained both in the reduction of T.D.S. and in the increase in intensity of higher-order reflexions by cooling to liquid-nitrogen temperatures.