Structural studies by electron and X-ray diffraction of the disordered phases II': (Sn1−x Tb x )Tb₄Rh₆Sn₁₈ and (Sn1−x Dy x )Dy₄Os₆Sn₁₈

This is the authors' abstract. We don't add key points for this paper.

(Snl_xTbx)Tb4Rh6Sn18 and (Snl_xDyx)Dy4OsrSnls belong to a large stannide series where there exist two chemical formulae: SnM3M~Sn~2 (phase I and I') and SnM4M~Sn~8 (phase II and II'), and four structures: I, I', II and II'. That of phase II' appeared to be cubic (a = 13.7/~ and space group Fm3m) while the structure of phase II is tetragonal (a = 13-7, c ~27.4A and space group I4~/acd). Phase II' of the M = Tb, M'= Rh stannide was found to be a disordered microtwinned phase II. The disorder concerned the Tb(2) and Sn(2) sublattices of formula: [Sn(1) ~_xTbx]Tb(2)4Rh6Sn(2)4Sn(3)12Sn(4)2. This qualitative model is based on the observation of weak intermittent diffuse streaks and on refinements of the average cubic structure, carried out by taking into account only Bragg reflections. The results proved that phases II and II' had related structures and the same chemical formulae. The complete reciprocal lattice (Bragg+diffuse scattering) was obtained by electron diffraction for the M = Dy, M'=Os stannide. The diffuse streaks are lines parallel to the {100} cubic (a~i,x an,× an,) directions and going through the F-centering-forbidden nodes. The three sets of lines are related by the cubic threefold [lll]~r axis. It is observed that phase II structure contains eightatom [4Sn(2), 4M(2)] blocks which can have two orientations (here called A and B). It can be surmised that the same blocks exist in phase II', but in a partially disordered arrangement. Two equivalent descriptions of this arrangement are given: (1) ordered (100) planes of A and B blocks are stacked at random and the threefold-axis-twinned individuals give rise to three sets of streaks; (2) the A and B blocks form an f.c.c, lattice which is comprised of tetrahedra formed of nearest neighbours; it is shown that when each cluster contains two A and two B blocks, diffuse scattering condenses into the observed