Compton-scattering study of trigonal, monoclinic, and amorphous phases of Se

Abstract

The Compton profiles (CP) and autocorrelation functions (AF) for crystalline trigonal Se ($t$-Se), polycrystalline trigonal Se (pct-Se), polycrystalline monoclinic Se (pcm-Se), and black amorphous Se (ba-Se), have been determined experimentally from inelastic $\ensuremath{\gamma}$-ray scattering. Two different $\ensuremath{\gamma}$-ray spectrometers have been used, namely one using the 412-keV line of $^{198}\mathrm{Au}$ and the other using the 59.5-keV line of $^{241}\mathrm{Am}$. The momentum resolutions were 0.41 and 0.56 a.u., respectively. The present results for $t$-Se, pct-Se, and ba-Se differ significantly from earlier results. The data are analyzed using theoretical results obtained from atomic restricted Hartree-Fock models, free-electron (FE) models, one-dimensional potential array studies, and from self-consistent orthogonalized-plane-wave (SCOPW) calculations based on a local density formalism. Good agreement for $t$-Se and pct-Se has been obtained between both sets of experimental data and the SCOPW results. The FE model, fulfilling the correct translation symmetry and occupation of the higher Brillouin zones, is shown to explain long-range oscillations of the AF qualitatively but is inadequate for describing the short-range bonding effects. No significant difference within the present resolution was detected between the CP and the AF of pct-Se and ba-Se, contrary to earlier findings. Reasons for this are discussed using the above models. A significant difference between pct-Se and pcm-Se has not been obtained with the Am spectrometer.