ANGULAR VARIATIONS IN CORE-LEVEL XPS PEAK INTENSITY RATIOS FROM SINGLE-CRYSTAL SOLIDS

Oscillatory angular variations of up to ±30% in core-level XPS peak intensity ratios from single-crystal layered silicates, due to diffraction of the outgoing photoelectrons by the surrounding solid and characteristic of the crystallographic locations of the emitting atoms, are described. Determination of the relative atomic abundances of the major non-transitional elements in single-crystal surfaces from such intensity ratios is shown to be possible only after integration over a considerable angular range, ≥40° for the AEI ES 200A spectrometer. Implications for the quantitative estimation of surface coverage in single-crystal chemisorption experiments by XPS are discussed. Comparisons of XPS surface analyses with bulk chemical analyses reveal that the cleavage surfaces of trioctahedral micas are often rich in Al and deficient in Mg. The distribution of the Al between the tetrahedral and octahedral sites is deduced from the surface analyses and (independently and more directly) from the X-ray photoelectron diffraction patterns. Good agreement (12%) is found, confirming the validity of both approaches. Distortion caused by the high proportion of tetrahedral Al and an apparent shortage of octahedral cations is postulated to explain the exposure of these compositionally anomalous surfaces on cleavage. © 1979.