Modified two-beam description of X-ray fields and intensities near a three-beam diffraction point. Second-order solution

The previously developed first-order modification of two-beam diffraction near a third reciprocal-lattice point [Juretschke (1984). Acta Cryst. A40, 379-389] is extended to second order. To this order a modified two-beam description is still retainable, and the normal modes persist in their original polarization, but now with respect to two-beam asymptotes rotated and with a Bragg angle altered relative to the original two-beam case. Integrated intensities are evaluated for modified strong and weak primary two-beam symmetric Bragg reflections, in the Bragg-Bragg and Bragg-Laue configurations, and some implications of the results are discussed.