Reversible Reconstructive Phase Transition of Ba₂SnSe₅: A New High Temperature Modification with Completely Different Structural Motifs

Abstract

A new modification of Ba(2)SnSe(5) was prepared by high temperature synthesis. In contrast to its low temperature modification that adopts the orthorhombic space group P2(1)2(1)2(1), the new beta-Ba(2)SnSe(5) crystallizes in the monoclinic system, space group P2(1)/c, with the lattice parameters a = 9.3949(6) A, b = 8.8656(6) A, c = 12.5745(7) A, beta = 113.299(4) degrees, V = 961.9(1) A(3), Z = 4. alpha-Ba(2)SnSe(5) is comprised of Sn(3)Se(10)(8-) units, SnSe(4)(4-) tetrahedra, and isolated Se(3)(2-) units, while beta-Ba(2)SnSe(5) contains only SnSe(5)(4-) units, wherein Sn is tetrahedrally coordinated by four Se atoms. The fifth Se atom is connected to one Se atom of the SnSe(4)(4-) tetrahedron, thereby forming a Se(2)(2-) dumbbell. Different band gaps are a result of the different structure motifs, which are reflected in different colors of the two Ba(2)SnSe(5) modifications, the alpha-form being dark brown and the beta-form being red.

Distant-Atom Mutation for Better Earth-Abundant Light Absorbers: A Case Study of Cu2BaSnSe4

“…Moreover, by forming the Se i , the coordination number of the two adjacent Ba atoms increases from 8 to 9. The 9-coordinated Ba with Se has been observed in Se-rich compounds such as Ba 2 SnSe 5 (Figure e), indicating that under Se-rich conditions, the Ba atom also prefers the 9-coordination, because of its large ionic radius. These results qualitatively explain the low formation energy of Se i .…”

A Polyselenide with a Novel Se78– Unit: the Structure of Sr19–xPbxGe11Se44 with x = 5.0 and 6.4

“…Among the binaries, SnSe, was also recently shown to exhibit outstanding thermoelectric properties, in addition to the well‐established variants of PbTe . Among the ternaries, several complex materials were uncovered in the last 15 years, including SrSn 2 Se 4 with mixed valent Sn, the polyselenides Sr 4 Sn 2 Se 9 and Sr 4 Sn 2 Se 10 with interesting redox chemistry, Ba 2 SnSe 5 with its three different modifications with either V‐shaped Se 3 2– or Se 2 2– units, and the dimorphic polytelluride Ba 2 SnTe 5 exhibiting either a unique Te 5 4– unit or the more common Te 2 2– dumbbells . As these examples demonstrate, the polychalcogenides may comprise a plethora of oligomeric Q (S, Se or Te) units; in addition to the common Q 2 2– dumbbells and Q n 2– zigzag chains, one can also find hypervalent units with elongated bond lengths such as Se 3 4– , Se 4 4– , Se 5 4– , and Te 5 4– …”

New Quaternary Rare-Earth Chalcogenides BaLnSn2Q6 (Ln = Ce, Pr, Nd, Q = S; Ln = Ce, Q = Se): Synthesis, Structure, and Magnetic Properties

“…(Although the M–M bonding is possible for some p-block elements, which might affect the assignment of the formal oxidation states, it will not change the electronic configuration and coordination geometry.) In comparison, Ge and Sn can be stabilized in both the +2 oxidation state with electron lone pair and the +4 oxidation state typically in a tetrahedral environment, as shown by a number of compounds ranging from binary to quaternary ones such as MQ (M = Ge, Sn; Q = S, Se), − MQ 2 (M = Ge, Sn; Q = S, Se), − Ba 7 Sn 5 S 15 , Ba 6 Sn 6 Se 13 , Ba 2 SnSe 5 , − Sr 4 Sn 2 Se 9 , La 2 Ga 2 GeS 8 , K 2 FeGe 3 Se 8 , and A 2 Hg 3 M 2 S 8 (A = K, Rb; M = Ge, Sn) . Such mixed valence property of Ge and Sn will increase the diversity in the stoichiometries and structures and may also lead to compounds with interesting properties.…”