In Situ Synthesis and Single Crystal Synchrotron X-ray Diffraction Study of ht-Sn₃Sb₂: An Example of How Complex Modulated Structures Are Becoming Generally Accessible

Abstract

Recent developments in X-ray sources and detectors and the parallel development of software for nonstandard crystallography has made analysis of very complex structural problems accessible to nonexperts. Here, we report the successful solution of the structure of ht-SnSb, an analysis that presents several challenges but that is still manageable in a relatively straightforward way. This compound exists only in a narrow temperature regime and undergoes an unquenchable phase transformation on cooling to room temperature; it contains two elements with close to identical scattering factors, and the structure is incommensurately modulated with four symmetry dependent modulation wave vectors. In this study, an attempt was first made to synthesize the title compound by in-house crystal growth in the stability region of ht-SnSb, followed by cooling to room temperature. This is known to produce mutiply twinned stistaite and elemental tin, and this sample, freshly prepared, was then reheated in situ at the single crystal materials beamline Crystal at the synchrotron Soleil. This method was unsuccessful as reheating the sample led to loss of Sn from stistaite as revealed by a change in the measured modulation wave vector. The compound was instead successfully synthesized in situ at the beamline by the topochemical reaction of single crystalline stistaite and liquid tin. A well-formed crystal of stistaite was enclosed in a quartz capillary together with a large excess of tin and heated above the melting point of tin but below the melting point of ht-SnSb. The structure was probed by sychrotron X-ray diffraction using a wavelength close to the absorption edge of Sn to maximize elemental contrast. In the diffraction patterns, first order satellites were observed, making the structure of ht-SnSb incommensurately modulated. Further analysis exposes four q-vectors running along the body diagonals of the cubic unit cell (q' = α α α, q' = -α α -α, q' = -α -α α, q' = α -α -α). To facilitate the analysis, the q vectors were instead treated as axial (q = α 0 0, q = 0 α 0, q = 0 0 α) and an F-type extinction condition for satellites was introduced so that only reflections with hklmnp, mnp all odd or all even, were considered. Further, the modulation functions F(q) were set to zero, and only modulation functions of the type F(q') were refined. The final model uses the four modulation functions, F(q'), F(q'), F(q'), and F(q'), to model occupancy Sn/Sb and positional modulation. The model shows a structure that comprises small NaCl type clusters, typically 7 × 7 × 7 atoms in extension, interspersed between single layers of elemental tin. The terminating layers of tin are slightly puckered, emulating an incipient deformation toward the structure of the layers perpendicular to the [001] direction in elemental tin. It is notable that this model is complementary to that of stistaite. In stistaite, two-dimensionally infinite slabs of rock salt are interspersed between layers of antimony along the trigonal [001] direction, so that the terminatin...

New Insights into Phase Equilibria of the Sb-Sn System

“…Due to the complexity of the Sb 3 Sn 4 and SbSn crystal structures, as described by different authors, [33][34][35][36] and the limited resolution of XRD patterns, only a few samples were investigated by XRD, and the investigation was discontinued because the diffractometer available in our laboratory does not resolve the reflexes of the two Sb-Sn intermediate phases.…”

“…It is worth considering that the two solid phases involved in the invariant reaction have close chemical composition and related crystal structures. [33][34][35][36] Therefore, the heat associated to this reaction should be small and slow, whereas the observed DSC effects around 321°C are strong and sharp, as for the peritectic effect. Furthermore, the observation of thermal effects at * 321°C for samples from 44.0 (#11) to 48.5 at.% Sb (#18), even if weak, is in conflict with a possible catatectic reaction and in line with a peritectic one.…”

“…On the other hand, an alternative interpretation may suggest a structural transformation of Sb 3 Sn 4 to (SbSn), as suggested by Schmetterer et al [33] In agreement with this hypothesis, the sharp invariant effect observed at 321 ± 1°C should belong to the peritectic decomposition of Sb 3 Sn 4 as proposed by all other authors. The in situ HT-XRD investigations by Schmetterer et al [33] and in situ synthesis and XRD investigation by synchrotron radiation by Lidin and Folkers [36] have shown that Sb 3 Sn 4 /ht-Sb 3 Sn 2 forms by local stacking within (SbSn). Perhaps small differences in stacking sequence may lead to the sluggish signals recorded around 332°C in the present research, using the highly sensitive Calvet-type calorimeter.…”

“…[22,25,34,35] Lidin et al investigated the SbSn phase by single crystal x-ray Diffraction (XRD) and EDS analysis; according to their findings, [34,35] the SbSn has a rhombohedral distorted and incommensurately modulated NaCl-type crystal structure. Lidin and Folkers [36] carried out a single crystal synchrotron XRD investigation in order to solve the structure of the ''ht-Sb 2 Sn 3 '' phase. At the same time, Schmetterer et al [33] found that the Sb 3 Sn 4 phase crystallizes in a rhombohedral structure with pronounced structural similarities to the (SbSn) phase.…”