Modulations in B8-type Intermetallics

Abstract: The B8-type regions of the Mn-Sn and Cu-In systems have been investigated by means of electron and X-ray diffraction. The importance of the combination of these techniques in the study of B8-type phases is pointed out. In addition to the B8-type phases an alloy with a quasicrystal approximant structure has been studied. In the Mn-Sn system at least three B8-type phases exist. An X-ray powder pattern of the low-temperature phase Mn3Sn2 has been refined with the Rietveld method. The structure of one of the high-temperature phases, Mn8Sn5, has been refined by using the JANA94 program for incommensurate structures, and for the other one, Mn12Sn7, a structural model has been proposed. All three structures are orthorhombic with different length of the a axis - 2ahex, 5ahex and 7ahex for Mn3Sn2, Mn8Sn5 and Mn12Sn7, respectively, in a commensurate approximation of the phases. ahex refers to the a axis of the base NiAs/Ni2In type cell. In the Cu-In system at least four B8-type phases, A, B, C and Cu7In3, exist. A fifth phase, Cu11In9, is probably also based on the B8-type structure. The system is very complicated. The A, B and C phases seem to be related through gradual phase transitions. From the monoclinic high-temperature phase A the intermediate phase B is formed by a change in the modulation vector of the A-phase. There are different appearances of the B-phase because of the gradual change in its modulation vector. However, they all resemble each other, and all are closely related to the A-phase. At the end of this transition the C-phase appears as an orthorhombic superstructure with a=9ahex, b=sqrt(3)bhex, c=4chex. The structure of the quasicrystal approximant (Mg,Na,Al)2(Al,Zn)3 has been refined from single-crystal data. The structure description is made in terms of successive shells of atoms with increasing size. The quasisymmetry of the phase arises from three shells with icosahedral (five-fold) symmetry. The fourth and outermost shell breaks this symmetry and reduces the symmetry to cubic.