A computer program is presented for finding the unit-cell constants for a crystal phase from X-ray powder data containing multi-phase diffraction lines. Input data are observed diffraction angles, 2θ_obs (or d-values), allowable errors in diffraction angles, Δ2θ, approximate unit-cell constants and space group (or lattice type). The procedure of the program is as follows:
(1) Five consecutive low-angle lines are selected by using the input cell constants and the space group (or lattice type).
(2) Trial unit-cell constants are obtained from the first 35 lines by using the algorithms given by Takaki et al. [Yogyo-Kyokai-Shi, 95, 610 (1987), Seramikkusu Ronbunshi, 96, 13 (1988)].
(3) For each of the trial cell constants, diffraction angles are calculated and compared with observed ones. Then, number of indexed lines N and the reliability index R for the trial unit cell are calculated.
(4) After the calculation, the 10 sets of the unit-cell constants having largest values of N×R are selected. The user can obtain the most suitable unit-cell constants comparing the values of N, R, and NÅR.
The reliability index R is given by R=100Å(Number of observed reflections)/(Number of all calculated reflections), where the observed reflections are those belonging to the observed lines within window widths, ±Δ2θ. The program is written in N₈₈-BASIC.