Atsushi Okazaki

The Crystal Structures of KMnF3, KFeF3, KCoF3, KNiF3 and KCuF3 above and below their Néel Temperatures

The crystal structures of the antiferromagnets KMnF 3 , KFeF 3 KCoF 3 , KCNiF 3 and KCuF 3 have been determined above and below their Neel temperatures ( T N ) by X-ray diffraction using single crystals. At room temperature (above T N ) the structures of these compounds are of the ideal perovskite type (cubic) except for that of KCuF 3 Which crystallizes as a tetragonal modification ( a > c ) of the perovskite type. At 78°K (below T N ) the lattice symmetries of KMnF 3 , KFeF 3 and KCoF 3 are monoclinic, rhombohedral (α c ), respectively, while KNiF 3 and KCuF 3 retain their own symmetries at room temperature. The lattice constants have been determined between these temperatures and their changes near the transition temperature were closely followed. This temperature region was extended up to 670°K for KCuF 3 .

The Crystal Structure of Germanium Selenide GeSe

The crystal structure of germanium selenide has been analysed by X-ray method. The structure is orthorhombic with the space group D 2 h 16 - Pcmn . The dimensions of the unit cell containing four chemical units, GeSe, are a =4.38, b =3.82 and c =10.79 A. It follows from the positions of all atoms determined by means of the electron-density projection that the structure is of a distorted NaCl type isomorphous with SnS and SnSe, The distances Ge–Se are 2.54, 2.58, 3.30 and 3.39 A.

The Superstructures of Iron Selenide Fe7Se8

Iron vacancies in Fe 7 Se 8 are confined to the alternate iron layers parallel to the c plane of the fundamental lattice with the NiAs structure. The origin of the ferrimagnetisni of Fe 7 Se 8 ( T c =187°C) may be attributed to this ordered arrangement similarly to the case of Fe 7 Se 8 . There are two types of the superstructure: low- and high-temperature ones. The unit cell dimensions and the lattice symmetries of these structures are as follows:

Structural Study of Iron Selenides FeSex. I Ordered Arrangement of Defects of Fe Atoms

The ordered arrangements of the defects of iron atoms in FeSe x have been studied for several compositions by X-ray analysis. These defects tend to assume an ordered arrangement as in the case of pyrrhotite FeS 1.14 . New phases are found in the region ( x =8/7∼4/3) which have been considered as the single phase region. If we choose the orthohexagonal and pseudo-orthohexagonal unit cells for x =8/7 and 4/3, respectively, the unit cell dimensions of the superstructure resulting from this ordering are twice as large as that of the fundamental structure along the a - and b -axes, and three times along the c -axis for x =8/7, and on the other hand, twice only along the c -axis for x =4/3.

Neutron Diffraction Study of Fe7Se8

Ferrimagnetic orderings of the moments of iron atoms in Fe 7 Se 8 are confirmed by neutron diffraction method; the moment is parallel to that of each atom on the same atomic layer parallel to the (001), while it is antriparallel to that on the next layer. When the temperature of the crystal is lowered from the Neel temperature (455°K), the direction of the moment, which is at first on the (001), changes towards the [001]. The behaviour of this “stand up” process depends on the type of the crystallographic superstructure of the specimen: gradually in the 4 c type crystal and abruptly in the 3 c type one. The ionic models of the magnetic structure, used in the case of pyrrhotite, for example, are not consistent with the result of the present experiment. A model with a more metallic character seems to be acceptable, which is supported by the Mossbauer effect experiment.