Structure description and element distribution

The characteristic crystal chemical feature of the two isostructural compounds M0.067Sb0.667Te0.266 (M = Ge, Sn) is the long-range ordered 39R stacking sequence of hexagonal

atom layers that extend parallel (001). A simplified description classifies the layer sequence as a stacking of alternating Sb2Te3-type slabs which consist of two cation and three anion layers and

four corrugated honeycomb layers with gray arsenic type (one such layer comprises two shifted planar hexagonal atom layers) as shown in Figure 4. These layers are quite similar to those in elemental antimony, (intra-layer distance 2.908 Å, inter-layer distance 3.355 Å).[18] The layers that are further away from the Sb2Te3-type slab are more similar to those in the element, those

that interact with the slab are more strongly distorted.

Within the Sb2Te3-type slab, the cations exhibit a 3+3 coordination with short Sb-Te distances

towards the Te layer terminating the slab. This can be explained with the one-sided coordination of the Te atoms involved. The Sb-Te distances towards the central Te atom layer are longer, and the latter exhibits an almost ideal octahedral coordination. This situation is rather similar to that in Sb10Te3, neither the introduction of Sn nor that of Ge significantly affects the distance set.[29]

With respect to the general formula (MTe)n(Sb2Te3)m(Sb2)k, M0.067Sb0.667Te0.266 = MSb10Te4

corresponds to n = m = 1 and k = 4, suggesting four corrugated honeycomb antimony layers between 7-layer rocksalt-like blocks as in GeSb2Te4 or SnSb2Te4.[22,26] However, the actual

structure does not match this prediction. Although, for instance, 9P-GeSb4Te4 and 51R-

Ge2-xSb2+xTe5·Sb8 prove that such structural combinations are possible,[24,25] the title compounds

are closer to Sb10Te3, which once more shows that simple rules for structure prediction often fail

for metastable compounds. This may be complicated by a certain range of homogeneity and minute deviations from the ideal composition as shown for Ge4Sb2Te7 whose layer sequence

corresponds to that expected for Ge3Sb2Te6.[20] Although the compounds (MTe)n(Sb2Te3)m (M =

Ge, Sn) exhibit mixed cation sites even in thermodynamically stable phases, the cation distribution is not completely random: Sb prefers the sites in the vicinity of the van der Waals gaps.

Figure 4. Atom distribution and selected interatomic distances (top, middle) in the refined structure models of 39R-M0.067Sb0.667Te0.266 (M = Ge, Sn) projected along [010] in comparison

with Sb10Te3 (left, all distances are given in Å, all e.s.d.s < 0.003 Å). The site occupation factors

for 39R-M0.067Sb0.667Te0.266 derived from the joint refinements on synchrotron data are given in

the center of the figure (the amount of Ge and Sn on position M5 is insignificant and not considered further). For comparison the occupancies in Sb10Te3 are also shown. The schematic

representation on the left depicts the element distribution, the slight difference between the Ge and Sn compounds is neglected; small gray spheres indicate that the corresponding element is absent on this position; enlarged spheres for M = Ge or Sn (white), Sb (gray), Te (black) represent the site occupancies, the size of the spheres is proportional to the occupancy. The brackets indicate the rocksalt-type slab and the gray arsenic type layers, which are formally present.

The single-crystal data (see above) clearly reveal the element distribution in M0.067Sb0.667Te0.266

(M = Ge, Sn). The occupancy factors correspond to concentration gradients as visualized in Figure 4. Interestingly, the arsenic-type layers do not exclusively contain Sb atoms. In part, the site occupancies may suggest that a certain proportion of corrugated honeycomb Sb layers is substituted by GeTe layers, which are simply a binary variant of the gray arsenic type layers and exhibit a very similar periodicity perpendicular to the stacking direction [001]: a = 4.308 Å for antimony and 4.164 Å for GeTe (compare Figure 1).[18,38] As both Sb and GeTe layers are charge neutral, one might expect that the formal replacement of Sb in the arsenic-type layers by Ge and/or Te is coupled. However, the oxidation state of Sb is variable; it may also be cationic near Te. Thus, charge neutrality does not necesssarily require to fully replace Sb by Ge or Sn on positions that neighbor those where Sb is replaced by Te. In fact, the element concentrations vary rather smoothly in both the Sb2Te3-type and the arsenic-type slabs, resembling a wave-like

concentration modulation. It is, therefore, just a simplification to distinguish the different types of slabs, the real structure is not well described as a stacking sequence of individual slabs. However, the variation which comprises both interatomic distances and occupancy factors is not smooth enough to be easily described as a commensurately modulated structure with a 3+1D superspace approach. This would be an unnecessary complication and cannot be used to simplify the refinement as very high satellite orders would be required unless observed reflections are omitted and the data/parameter ratio would not be reduced due to the complexity of the “modulation”. Cation-anion separation becomes less pronounced with increasing distance from the center of the formal Sb2Te3-type slabs. In fact, locally these might be extended by GeTe-type

layers on both sides to form slabs similar to those in Ge2Sb2Te5.[39] In the same way as the

distortion of the arsenic-type layers depends on the location within the stacking sequence, the site occupancies depend on the surrounding of the corresponding positions in the layer sequence. In the Sb2Te3-type slab, cation positions show a preferred occupancy for Ge and Sn which

decreases towards the center of the arsenic-type block. There, Sb is much more prevalent. No significant amount of Ge, Sn or Te in the arsenic-type layers that are not next to the Sb2Te3-type

slabs was found. The concentration gradient is slightly less pronounced for M = Sn as compared to M = Ge. In layered “superlattice” structures prepared by sputtering or similar techniques, interdiffusion between adjacent building blocks does not seem significant.[40,41] In contrast, both title compounds exhibit a rather continuous transition from predominantly ionic rocksalt-like Sb2Te3-type slabs to predominantly metallic antimony slabs. This bears similarity to an initial