Homoatomic Stella Quadrangula [Tl₈]⁶⁻in Cs₁₈Tl₈O₆, Interplay of Spin−Orbit Coupling, and Jahn−Teller Distortion

Abstract

Cs(18)Tl(8)O(6) was synthesized reacting the binary compounds CsTl and Cs(2)O. According to single crystal X-ray analysis, the title compound crystallizes as a novel structure type in the cubic space group I23 and is diamagnetic. The electronic structure of the extended solid and of excised Cs(6)Tl(8) clusters has been examined by relativistic density functional calculations including spin-orbit coupling. Cs(18)Tl(8)O(6) comprises a clusteranion [Tl(8)](6-) in the shape of a tetrahedral star. An isoelectronic cluster was found previously in Cs(8)Tl(8)O, however, with the shape of a parallelepiped. Both clusteranions can be derived from a homocubane unit by displacive distortions. It has been shown by quantum mechanical analyses that the closed-shell electronic structure of the parallelepiped is the result of a Jahn-Teller distortion, while in contrast the tetrahedral star in Cs(18)Tl(8)O(6) would still exhibit an open-shell degenerate HOMO within a scalar relativistic approximation. Only if spin-orbit coupling is considered, a closed-shell electronic system is obtained in accordance with the diamagnetic behavior of Cs(18)Tl(8)O(6).

Regular [Tl6]6– Cluster in Cs4Tl2O Exhibiting Closed‐Shell Configuration and Energetic Stabilization due to Relativistic Spin–Orbit Coupling

“…In contrast, the nearly ideal octahedral shape of [Tl 6 ] 6– in the title compound and the opening of a band gap can only be reproduced computationally if spin–orbit coupling is considered. A similarly crucial effect of spin–orbit coupling for the correct description of the electronic structure was reported recently for Cs 18 Tl 8 O 6 , containing [Tl 8 ] 6– clusters 11. By comparison of the latter cluster with an isoelectronic, but topologically different one found in Cs 8 Tl 8 O,7 it has been revealed that spin–orbit coupling is a mechanism alternative to Jahn–Teller distortion in order to obtain a closed shell electronic system.…”

“…Cs 4 Tl 2 O was prepared by reaction of the binary compounds CsTl and Cs 2 O. The detailed reaction techniques for preparation of both binary starting compounds have been described previously 11. The mixture of CsTl and Cs 2 O in the molar ratio 1:0.95 (total amount 0.5–1 g) was placed into a tantalum tube, which was sealed under argon with an arc‐welder.…”

“…The reaction mixture was heated with a rate of 50 K · h –1 up to 573 K, annealed at this temperature for one week, and then cooled to room temperature with a rate of 5 K · h –1 . The as‐synthesized products were pure, sometimes containing a small amount of Cs 18 Tl 8 O 6 (1–3 at‐%),11 according to X‐ray powder diffraction. The atom ratio of heavy elements in the product was additionally confirmed by using a scanning electron microscope (XL 30 TMP, Philips, Netherlands, tungsten electrode, 25 kV), equipped with an integrated EDAX–EDX system (S–UTW–Si(Li)–detector).…”

Neutron and X-ray diffraction studies on the high temperature phase of Mn3(VO4)2, the new isostructural compound NaMn4(VO4)3 and their mixed crystals Na Mn4.5−/2(VO4)3 (0≤x≤1)

“…16 Fig. 1), which is known as Hoard dodecahedron [16] and also as stella quadrangula [17,18]. From a structural analysis by means of XRD data through a Rietveld analysis, the refined Mn-O distances for the 4b site appear to be too large compared to what would be expected for Mn 2 þ according to the Shannon radii [36].…”

“…In a recent publication we presented a high temperature phase of this compound (h.t.-Mn 3 (VO 4 ) 2 , space group I4 2d) which was shown to exist at temperatures above 946 1C [8]. This phase shows an unusual coordination for the Mn-atoms at the 4b site, which is known as Hoard dodecahedron [16] or as stella quadrangula [17,18] and this site is only half occupied. Co 3 (AsO 4 ) 2 [16] and Mg 3 (AsO 4 ) 2 [19] were previously reported to crystallize in that structure.…”