Revealing the Nature of Chemical Bonding in an ALn2Ag3Te5-Type Alkaline-Metal (A) Lanthanide (Ln) Silver Telluride

Abstract

Although the electronic structures of several tellurides have been recognized by applying the Zintl-Klemm concept, there are also tellurides whose electronic structures cannot be understood by applications of the aforementioned idea. To probe the appropriateness of the valence-electron transfers as implied by Zintl-Klemm treatments of ALn2Ag3Te5-type tellurides (A = alkaline-metal; Ln = lanthanide), the electronic structure and, furthermore, the bonding situation was prototypically explored for RbPr2Ag3Te5. The crystal structure of that type of telluride is discussed for the examples of RbLn2Ag3Te5 (Ln = Pr, Nd), and it is composed of tunnels which are assembled by the tellurium atoms and enclose the rubidium, lanthanide, and silver atoms, respectively. Even though a Zintl-Klemm treatment of RbPr2Ag3Te5 results in an (electron-precise) valence-electron distribution of (Rb+)(Pr3+)2(Ag+)3(Te2−)5, the bonding analysis based on quantum-chemical means indicates that a full electron transfer as suggested by the Zintl-Klemm approach should be considered with concern.

Revisiting the Zintl‒Klemm Concept for ALn2Ag3Te5-Type Alkaline-Metal (A) Lanthanide (Ln) Silver Tellurides

“…An inspection of the unit-cell volumes for the herein and previously reported ALn2Ag3Te5-type tellurides (A = Rb, Ln = Pr, Nd, Sm, Gd−Er; A = Cs, Ln = La−Nd, Sm, Gd−Er) [18,28] shows that the volumes decrease, while the atomic number of the rare-earth elements increases. For instance, a comparison of the lattice parameters of CsLn2Ag3Te5 (Ln = La, Ce) to those of the isostructural CsLn2Ag3Te5 (Ln = Pr, Nd, Sm, Gd−Er) reveals that the molar volumes of the lanthanum-and ceriumcontaining tellurides are slightly larger than those of the remaining tellurides (see Figure 2).…”

“…A phase analysis was accomplished based on the powder X-ray diffraction patterns collected for the samples, and it revealed that the tellurides were obtained in considerable yields; yet, all tellurides were accompanied by by-products that were binary lanthanide tellurides, Ag 2 Te, CsAg 5 Te 3 , and an unknown phase (see Figure 1). The lattice parameters of the holmium-containing telluride have been recently reported elsewhere [18], while the crystal structures of the previously unknown RbEr 2 Ag 3 Te 5 and CsLn 2 Ag 3 Te 5 (Ln = La, Ce) have been determined for the first time (see Tables 1 and 2). Experimental and theoretical powder X-ray diffraction patterns of (a) CsCe2Ag3Te5, (b) CsLa2Ag3Te5, (c) RbHo2Ag3Te5, and (d) RbEr2Ag3Te5; reflections arising from side-products [23][24][25][26][27] are marked by the diverse circles.…”

“…The alkaline-metal atoms occupy the centers of bicapped trigonal tellurium prisms, whose triangular bases are condensed to 1 ∞ All silver atoms occupy the centers of the tellurium tetrahedra, which are condensed to linear 1 ∞ [Ag@Te 4 chains, but there are two different types of 1 ∞ [Ag@Te 4 chains in the crystal structure of the ALn 2 Ag 3 Te 5 -type tellurides (see Figure 3 5 . Indeed, such a Zintl-Klemm picture is oversimplistic because more recent research [18] on the nature of bonding for this particular type of telluride revealed strong mixed-metal interactions for the Ln−Te and Ag−Te contacts. As the A−Te interactions were considered to be rather ionic, one may therefore question the valence-electron distribution in these tellurides.…”

“…In the light of such relevance for research and technical applications, one could also expect that the nature of bonding in tellurides is typically examined employing first-principles-based approaches; yet, the electronic structures, particularly the nature of bonding, in many tellurides are frequently (and simplistically) interpreted by applying the Zintl-Klemm concept rather than first-principles-based bonding analyses [15]. More recent research [16][17][18] on the electronic structures of certain tellurides, however, demonstrated that such Zintl-Klemm pictures should be taken with a pinch of salt. In fact, examining the electronic structures of (active-metal) transition-metal tellurides (active-metal = group I and III elements) revealed that the bonding nature in these tellurides is dominated by strong transition-metal−tellurium interactions beside minor transition-metal−transition-metal or tellurium−tellurium interactions.…”

“…Further research on the nature of bonding in a quaternary ALn 2 Ag 3 Te 5 -type telluride [18] (A = alkaline-metal; Ln = lanthanide), which also comprised strong mixed-metal transition-metal−tellurium interactions, showed that the alkaline-metal−tellurium interactions appear as ionic bonds. Because the alkaline-metal-tellurium contacts should be viewed at as ionic contacts while the Zintl-Klemm approach should be considered with concern, how can the charge of the alkaline-metal be determined?…”