The diffuse x-ray scattering and small-angle neutron scattering of a Pt-47 at. % Rh single crystal aged at 923 K were measured to determine the local atomic arrangement. The separated short-range order scattering including the small-angle scattering range showed weak intensity modulations, with the maximum at 1 1 2 0 positions, thus indicating the presence of local order. In contrast to short-range order scattering, size-effect scattering is already well visible in the raw data, in spite of the small difference of 3% in the atomic sizes. Size-effect scattering is mainly due to Rh-Rh displacements.

We report an fcc structure for the epitaxial Gd 2 O 3 films grown on GaAs͑100͒. This fluorite-derived structure appears to be stabilized by epitaxy with the substrate and has a great similarity to the GaAs structure. A model calculation supports this finding. Our secondary-electron imaging studies of these nanometer-thick films reveal that the films are cubic as deposited, and this structure can be derived from the stable ␣-phase of the Gd 2 O 3 by a mild Ne ϩ-ion bombardment and a subsequent anneal. These epitaxial Gd 2 O 3 films have great importance because of their excellent surface passivation properties. RAPID COMMUNICATIONS R10 616 PRB 62 C. STEINER et al.

A fluorite-related phase of Gd 2 O 3 , with a tetragonal unit cell of aϭ5.65 Å and cϭ5.37 Å, was attained in this study. The new phase was found either in a thin Gd 2 O 3 film ͑ϳ18 Å͒, which was epitaxially grown on GaAs͑100͒, or in a disordered ͑by mild Ne ϩ -ion sputtering͒ and recrystallized ͑by UHV annealing͒ thin cubic ␣-Gd 2 O 3 film. The structural characteristics of the new oxide films were studied using in situ reflection high-energy electron diffraction, secondary-electron imaging, and single-crystal x-ray diffraction.

Diffuse x-ray scattering under grazing incidence was measured of Pt-47 at. % Rh͑110͒ and ͑111͒ surfaces at 1000 K. The elastic scattering contributions of short-range order scattering and displacement scattering were separated from the inelastic scattering contributions that served for data calibration. The modulations in shortrange order scattering are small and indicate differences in the near-surface microstructure of the two surfaces: a global maximum at 1

Gallium arsenide crystals exhibit specific electrical and optical properties important for both electronic and optical device applications. However its material characteristics are still of significant interest to the electric industry. Beryllium is commonly used as a p-type dopant in GaAs grown by molecular beam epitaxy to increase thermal stability and to shorten the lifetimes of charge carriers. The aim of the presented paper is to find how the beryllium is incorporated into gallium arsenide lattice and study the microscopic structures around the beryllium atoms. High resolution X-ray study of GaAs:Be was already reported in our paper [3] indicating that Be position in the crystal lattice may play important role. Some reflections like (002) and (006), called quasi-forbidden reflections, have very small intensity due to the small difference in scattering factors between gallium and arsenic. The integral intensity of these reflections is very sensitive to composition and hardly sensitive to crystal lattice defects. The X-ray quasi-forbidden reflection method is based on comparison quasi-forbidden reflection intensities for the investigated crystal and for the ideal reference crystal [4]. References [1] G.