Abstract:
The three known varieties of FeS are described and some peculiar features of iron sulfide chemistry are discussed. One introduces the new tool of "similarity operators" for the study of the displacive transitions of FeS (Ta transition) and of MnAs (MnP NiAs type). In pnictides the tran— sitions magnetic -'- non magnetic (MnP type) are first order in conformity with observed changes of volumes and of transition temperatures versus pres— sure and/or concentration.
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2.3. Application to the high temperature transition NiAs MnP
2.3.1. Determination of the similarity operator..
The geometrical relations between the NiAs (, , ) and MnP (, , ) structure types have been extensively.described by Kjekshus and his coworkers (46) so that we shall only restate the main results in terms of the similarity operators. One has the vector relations .
The geometrical relations between the NiAs (, , ) and MnP (, , ) structure types have been extensively.described by Kjekshus and his coworkers (46) so that we shall only restate the main results in terms of the similarity operators. One has the vector relations .
a=C;b=A;c=A+2B
........
2.3.4. Search of the subgroup g.
The P subgroup of G must admit an orthorhombic lattice. As in the case of FeS, we proceed by steps of maximal subgroups. G = P63/mmc admits a maximal subgroup Cmcm of index 3 which has the orthohexagonal (centred) translation lattice. The full notation of cmcm is C. There are eight maximal subgroups of class mumi obtained by the loss of the translation L £ 0. One of them is Pmcn which in the standard notation of the International Tables becomes g = Pnma. The (orthorhombic) translation — 0 coincides with the translation B. The index of g in G is 6. Remark. It is quite interesting to state that the image of = Pnma in G (see table 2) would c&tncide with Gk = Pmcm if in the glide plane operation (mzII I ) one could suppress the glide component 1 1 0. We shall investigate this point in another study (53).
2.3.4. Search of the subgroup g.
The P subgroup of G must admit an orthorhombic lattice. As in the case of FeS, we proceed by steps of maximal subgroups. G = P63/mmc admits a maximal subgroup Cmcm of index 3 which has the orthohexagonal (centred) translation lattice. The full notation of cmcm is C. There are eight maximal subgroups of class mumi obtained by the loss of the translation L £ 0. One of them is Pmcn which in the standard notation of the International Tables becomes g = Pnma. The (orthorhombic) translation — 0 coincides with the translation B. The index of g in G is 6. Remark. It is quite interesting to state that the image of = Pnma in G (see table 2) would c&tncide with Gk = Pmcm if in the glide plane operation (mzII I ) one could suppress the glide component 1 1 0. We shall investigate this point in another study (53).
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