Rock-salt compounds ordered

In order to construct an MeX-type compound (X Me = 1) using only octahedral elements, the octahedrons must be linked via their faces i.e. by sharing of three angles. This arrangement of the octahedral polyhedrons yields a rock-salt type structure (NaCl type structure). 

In all cases, broad diffuse reflections are observed in the high interface distance range of X-ray powder diffraction patterns. The presence of such diffuse reflection is related to a high-order distortion in the crystal structure. The intensity of the diffuse reflections drops, the closer the valencies of the cations contained in the compound are. Such compounds characterizing by similar type of crystal structure also have approximately the same type of IR absorption spectra [261]. Compounds with rock-salt-type structures with disordered ion distributions display a practically continuous absorption in the range of 900-400 cm 1 (see Fig. 44, curves 1 - 4). However, the transition into a tetragonal phase or cubic modification, characterized by the entry of the ions into certain positions in the compound, generates discrete bands in the IR absorption spectra (see Fig. 44, curves 5 - 8). 

The magnetic stractures of the cubic fluoroperovskites AMeFs (Me + = Mn, Fe, Co, Ni) also belong to the G-type 273). Connected with the magnetic ordering below the Ne l-points, small distortions occur which have been studied in the case of potassium compounds by Okazaki and Suemune 237) and by Beckman and Knox 26, 27). A theoretical interpretation of such distortions, which were observed in antiferromagnetic oxides of the rock-salt type also, is given by Kanamori 179, 180). 

The structures within the homologous series Gdj + 2 2n + 3 C have been described in sect. 2 as a ccp arrangement of X and C atoms with Gd atoms in of the octahedral holes, i.e. as ordered defect derivatives of rock salt. That closed-shell situations do not always maximize stability is excellently demonstrated for the w = 1 and n = 2 members of the series because removal of one Gd atom per formula unit would produce closed-shell compounds in each case, Gd3X5C and Gd5X7C2 . Both metal-metal bonding interactions introduced by the three conduction electrons as well as the requirements of the highly charged interstitial atom to have a spherical shell of... 

---