A photograph method of determining the size, shape and orientation of the unit cell of an unknown spherical crystal

Abstract

The method is based on oscillation photographs of, say, 5° range. Several such photographs, having consecutive ranges of oscillation, are taken with unfiltered radiation and the exposures are made long enough to ensure that a visible white radiation streak passes through a sufficient number of the characteristic spots. The θ value of each characteristic spot is read off by a chart as are the ρ, values of the left-hand end of the white radiation streak passing through the characteristic spot. The coordinates of the reflecting reciprocal lattice point at the setting corresponding to the left-hand end of the white radiation streak are thus obtained. A plot of a suitable number of reciprocal lattice points near to the origin is made and the three shortest non-coplanar vectors between neighbouring lattice points are chosen as a*, b*, c*. The three components of each reciprocal axis can thus be obtained approximately. These values are refined progressively by finding the indices of reciprocal points further from the origin and then using suitable sets of three reflections to obtain more accurate values of the components of the reciprocal axes. A practical illustration of the method is given.