Observations of Borrmann–Lehmann interference patterns with synchrotron radiation

Abstract

The intrabranch interference effects expected under Laue-Bragg diffraction conditions [Saka, Katagawa & Kato (1972). Acta Cryst. A28, 102-113, 113-120] and first studied experimentally under high-absorption conditions by Borrmann & Lehmann [Crystallography and Crystal Perfection (1963), edited by G. N. Ramachandran, pp. 101-108. London and New York: Academic Press] have been investigated in the moderate (μ0t = 1.6) and low (μ0t = 0.47) absorption cases with Cu Kα1 and with synchrotron radiation of wavelengths 1.5 and 1.0 Å. Borrmann-Lehmann fringe patterns have been recorded with σ-mode and π-mode polarizations alone, utilizing the linearly polarized nature of synchrotron radiation. Specimens of natural diamonds possessing faces polished parallel to {100} and {110} were studied. The 220 reflection was recorded with the Laue entrance surface of {100} type and the Bragg-diffracting planes and internally reflecting Bragg surface oriented orthogonal to the entrance surface. Fringes of high visibility were obtained, even from specimens showing significant X-ray topographic evidence of lattice imperfections, but the fringe spacings observed were extremely sensitive to lattice distortions. Measurements of F220 from Borrmann-Lehmann fringe spacings gave values much in error, worse with the shorter wavelength. Borrmann-Lehmann fringe spacing measurement is advocated as a sensitive test of lattice perfection.