Structure block stacking in intermetallic compounds. I. The rhombohedral-hexagonal M _n+1 X _5n−1 and the monoclinic-hexagonal-trigonal-orthorhombic M _n+1 X _5n+2 structure series

Abstract

In binary systems containing rare-earth elements (M) and transition elements (X) several intermetallic compounds with composition between MX2 and MX5 are formed which, according to their mechanical properties, fall into two groups. The brittle alloys are hexagonal or rhombohedral and belong to the M,+IXs,-~ structure series. This series may be considered to be formed by stacking M2X4 blocks (characteristic of Laves phases) with a varying number of MXs blocks (characteristic of the Haucke phase CaCus). The soft alloys with pronounced cleavage have monoclinic symmetry. They belong to a new M, + 1X5, + 2 structure series which contains a new structure block, M2X7, found in the ZrzNi7 structure. A consideration of the intraplanar translation vectors and of the possible rotations of the M2X7 blocks shows that this structure series has three other subseries apart from the monoclinic Zr2Ni7 subseries, these being hexagonal, trigonal and orthorhombic. Point positions for these hypothetical structures are given together with a ready method of recognizing their presence from precession photographs.

Crystal structure, layer defects, and the origin of plastic deformability of Nb2Co7

“…The structural data of Zr 2 Ni 7 [18,20] confirm the results from the assumptions leading to Eq. (1aec), since within the aeb plane the experimentally observed displacements from the ideal coordinates are smaller than 0.05 Å.…”

“…Zr 2 Ni 7 has initially been reported with space group C2/m and Pearson symbol mC36 (this structure was also reported later for Hf 2 Co 7 [19]). On the basis of the initially refined structure parameters [18], other authors [20] proposed that a smaller unit cell should be sufficient to describe the crystal structure of Zr 2 Ni 7 (C2/m, mC18, see Fig. 3).…”

“…The latter maxima nevertheless turn out to originate from the Nb 2 Co 7 phase, too, revealing important information about its defect structure (see section 3.3.1). Extracted reflection positions yield monoclinic lattice parameters a, b, c and b after refinement À referring as all data to the small unit cell [20] 1 À as listed in Table 1.…”

“…3a and 4d) can be constructed, which are of particular use for the following discussion (it should be mentioned that these blocks are, however, not identical with those discussed in Ref. [20]). The C-centred unit blocks have orthorhombic metrics and are spanned by a ¼ a hex þ b hex , b ¼ a hex À b hex andc.…”

“…Consequently, within the 2q range of the diffraction pattern only h ¼ 0 reflections emanate from the phase with reflection positions given by the values of b* ¼ b À1 and c* ¼ (ccosb) À1 (the * indicates a reciprocal lattice parameter). (ii) Initial values for fractional coordinates were taken from the data for the small unit-cell version of Zr 2 Ni 7 [20] and the x coordinates were fixed, since they have no impact on the intensities of the h ¼ 0 reflections. (iii) Certain 2q ranges (excluded regions) containing h s 0 reflections were defined such that they are not considered for refinement and some peaks with refinable intensity, width [4].…”