A WARNING IN INDEXING TRICLINIC POWDER DIFFRACTION DATA

Because of experimental errors, in the indexing of a triclinic powder pattern, the discrimination between two cells which roughly differ onl y for having supplementary angles (type I and type II cells) might mee t some difficulties in the following cases: (i) reflections with at le ast one index null are dominant in the large d(hkl) region or/and (ii) one interaxial angle is close to 90 degrees. In case (i), e.g. for l = 0, d(hko) = (h(2)a(*2) + k(2)b(*2) + 2hka*b* cos gamma*)(1/2) can e xactly match the relevant d(obs)'s both with all positive indexes and gamma > 90 degrees and with one negative index and gamma* < 90 degree s. In case (ii), e.g. if alpha approximate to 90 degrees, d*(hkl) app roximate to (h(2)a(2) + k(2)b(*2) + l(2)c(*2) + 2hla*c* cos beta* + 2 hkab* cos gamma*)(1/2) can match, within experimental errors, the rel evant d(obs)'s both for positive h with beta and gamma* > 90 degrees, and negative h, with (beta)' = 180 degrees - beta* and (gamma*)' = 1 80 degrees - gamma.