Harmonic and anharmonic thermal vibrations in cubic ZnSe

The parameters of the interatomic one-particle potentials of cubic ZnSe wer e determined experimentally using X-ray diffraction methods. The harmonic force constants alpha of zinc and selenium were obtained from the temperature dependence of mean square displacements evaluated from the integrated intensities of selected reflections between 50 K and 300 K at a wavelength of lambda = 0.806 Angstrom. Least squares analysis of the experi mental data yields the room temperature mean square displacements (u(Zn)(2) )=0.0129(10)Angstrom (2) and (u(Se)(2))=0.0115(10)Angstrom (2) i.e. harmoni c force constants alpha (Zn) = 3.21(1) . 10(-19) J Angstrom (-2) and alpha (Se) = 3.60(1) . 10(-19) Angstrom (-2), respectively. The third order anhar monicity constant beta was determined exploiting the energy dependence of t he Bijvoet ratio between the reflections 666 and (6) over bar(6) over bar(6 ) over bar near the K-absorption edge of Se. It amounts to beta (Zn) = 1.48 (15) . 10(-19) J Angstrom (-3) assuming beta (Zn) = -beta (Se). The coefficients of the one-particle potentials of the series Ge, GaAs and ZnSe alpha and beta decreases with rising ionicity.