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Anisotropic x-ray absorption in layered compounds

1977; American Physical Society; Volume: 16; Issue: 12 Linguagem: Inglês

10.1103/physrevb.16.5549

0556-2805

Steve M. Heald, Edward A. Stern,

X-ray Diffraction in Crystallography

A study is presented of anisotropy effects in x-ray absorption in the layered compounds of $2H\ensuremath{-}\mathrm{W}{\mathrm{Se}}_{2}$ and $1T\ensuremath{-}\mathrm{T}\mathrm{a}{\mathrm{S}}_{2}$. In the measurements it was essential to separate the thickness effect from the true anisotropy effect which is dependent on the angle between the x-ray polarization and the crystal axes. The Se $K$ edge and the W and Ta $L$ edges were measured. Anisotropy in the white line of Se was found but no anisotropy was discerned in the W and Ta white lines. It is pointed out that x-ray absorption in general, and the extended x-ray absorption fine structure (EXAFS) in particular, have the anisotropy dependence of a second-order tensor and the theoretical expression for the EXAFS anisotropy at the ${L}_{2,3}$ edges is explicitly displayed. The anisotropy of the EXAFS in the Se and W absorption was measured and a good agreement with theory is found. The anisotropy of EXAFS at the ${L}_{2,3}$ edges has the new feature of a cross term between the final $s$ and $d$ states, which permits a determination from the measurements that the average contributions of the final $s$ state is 0.02 of that of the final $d$ states to the total absorption of the W ${L}_{3}$ edge. Finally, only qualitative agreement is obtained between band calculations and the near edge x-ray absorption structure, as expected theoretically.