Crystal structure of incommensurate ηʺ-Cu_1.235Sn intermetallic

Abstract

The crystallographic parameters of the incommensurately ordered phase ηʺ of the composition Cu1.235Sn are reported. This phase belongs to the group of ordered Ni2In/NiAs-type phases, with a NiAs-type arrangement Cu(1)Sn and additional Cu(2) atoms partially occupying trigonal-bipyramidal interstices in an ordered fashion, leading to the formula Cu(1)Cu(2)0.235Sn = Cu1.235Sn. The structure model, afterward refined on the basis of powder X-ray diffraction data, has been derived on the basis of the slightly Cu-poorer commensurately ordered η′-Cu6Sn5 (=Cu1.2Sn) phase but also on previously reported commensurate structure models η8-Cu1.25Sn and η4+1-Cu1.243Sn derived from selected area electron diffraction data. In line with a recent work (Leineweber, Wieser & Hügel, Scr. Mater. 2020, 183, 66–70), the incommensurate ηʺ phase is regarded as a metastable phase formed upon partitionless ordering of the η high-temperature phase with absent long-range ordering of the Cu(2) atoms. The previously described η8 and η4+1 superstructure are actually of the same phase, and the corresponding superstructure models can be regarded as approximant structures of the ηʺ phase.The refined structure model is described in 3+1 dimensional superspace group symmetry C2/c(q10-q3)00 with a unit cell of the average structure with lattice parameters of aav = 4.21866(3) Å, bav = 7.31425(5) Å, cav = 5.11137(3) Å and bav = 90.2205(5)° and a unit cell volume V = 157.717(2) Å3. The modulation vector is with q1 = 0.76390(4), q3 = 1.51135(5), and governs the spatial modulation of the occupancy of the Cu(2) atoms described by a Crenel function. The occupational ordering is accompanied by displacive modulations of the atoms constituting the crystal structure, ensuring reasonable interatomic distances on a local level. In particular, the spatial requirements of pairs of edge-sharing Cu(2)Sn5 trigonal bipyramids (Cu(2)2Sn8) lead to a measurable splitting of some fundamental reflections in the powder diffraction data. This splitting is considerable smaller in η′-Cu1.20Sn, which lacks such pairs due to the lower Cu content.

Stable and Metastable Phase Equilibria Involving the Cu6Sn5 Intermetallic

“…This η′′ formation was associated with a monoclinic distortion of the pseudo-orthorhombic average structure evaluated in Ref. 13 (see Table I). Upon referring the reflections to a hexagonal unit cell, one expects a microstrain distribution due to the emerging lattice distortion associated with η′′ ordering with a large ⟨ 2…”

“…In the case of the presence of a wellestablished η′′ phase exhibiting narrow satellite reflections, a disordered structure having the monoclinic average unit cell was used for fitting, thus ignoring the relatively weak satellite reflections in the course of the refinements. Full fits considering the satellite reflections were presented in earlier works 11,13 . In the present paper, only a part of the PXRD scans is shown to illustrate the most prominent changes in the profiles.…”

“…This can usually be assumed to be valid in the case of small strains accompanying the transformation. The largest (deviatoric) component of the strain tensor is only 0.00213 , while the distortion of the also monoclinic 12 is even smaller.…”

“…Non-equilibrium η′ with δ deviating from 0.2 likely could, however, be obtained by low-temperature annealing of alloys which were previously equilibrated at higher temperatures 11 . Some remaining issues concerning the Cu 6 Sn 5 field/intermetallic in the Cu-Sn system were resolved in a series of recent works [10][11][12][13] : (i) Following the structural determination of the ordered η′ phase 9 , further superstructures different from η′ had been reported: η 8 and η 6 14 as well as η 4+1 15 .…”

“…This composition dependence implies that for higher Sn content, the ordering starts at lower temperature during cooling such that the applied quenching rate is sufficient to avoid emergence of appreciable ordering. Moreover, the η phase enters the region of η′ ordering with increasing Sn content, which shows a much lesser degree of unit cell distortion than η′′ 13 , hence leading to a lesser degree of line broadening even if ordering occurs. Note that the ordering kinetics towards η′′ (referred to as η 4+1 ) and η′ have been studied [29][30][31] without revealing the actual course of the effects as a function of composition of the intermetallic such that the particular composition dependence of the ordering/transus temperature was not revealed.…”