A simplified criterion for the reliability of a powder pattern indexing

A figure of merit is defined for an indexing of a given pattern as M20 = Q20/2 20. Here N20 is the number of different calculated Q values up to Q20, which is the Q value for the 20th observed and indexed line; is the average discrepancy in Q for these 20 lines. From a number of indexings which have been disproved by single-crystal analysis, the conclusion is drawn that M20 < 6 must give rise to considerable doubt about the result. A number of confirmed indexings shows values of 20–60 for good routine work on pure, well crystallized samples, and down to 6 for retrievable correct indexings of less accurate data. If the number of unindexed lines below Q20 is not more than two, a value M20 > 10 guarantees that the indexing is substantially correct.