The Mineral Stützite: a Zintl-Phase or Polar Intermetallic? A Case Study Using Experimental and Quantum-Chemical Techniques

Abstract

Differing reports regarding the structural features of the mineral stützite, AgTe (-0.25 ≤ x ≤ 1.44), and the quest for tellurides with low-dimensional fragments stimulated our impetus to review this system by employing experimental as well as quantum-chemical methods. Determination of the crystal structures for three samples with compositions AgTe (I), AgTe (II), and AgTe (III) revealed considerable positional disorders for the Ag and Te sites and previously unknown structure models for I and II, which differ from that of III through the stacking sequences of honeycomb-fashioned Te layers. The crystal structures comprise [Te@Ag]@Te units in the forms of bicapped hexagonal Te antiprisms that enclose Te-centered tricapped trigonal Ag prisms, while each Te atom is encapsulated by Ag atoms assembling diverse types of coordination polyhedra. The vibrational and electronic properties were determined for three models approximating the actual crystal structure of stützite by means of techniques based on first principles. From analyses of the electronic structures and projected crystal orbital Hamilton populations (pCOHP), it is clear that the amounts and distributions of the Ag atoms within the Te network should be influenced by the subtle interplay between the attempts to achieve an electronically favorable situation with a gap at E and minimize the occupations of antibonding states.

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

“…Under consideration of previous research [37,38] on the electronic structures of alkaline-metal-containing polar intermetallics, this outcome suggests that the Rb-Te interactions should be considered as ionic interactions, while the Pr-Te and Ag-Te interactions should be regarded as rather strong, polar, mixed-metal bonds. This outcome also indicates that a full valence-electron transfer as proposed by an application of the Zintl-Klemm concept should be regarded with suspicion-a consequence which has also been identified by recent explorations of the electronic structures of rare-earth transition-metal tellurides [22] as well as silver tellurides [21]. The homoatomic Ag-Ag interactions change from bonding to considerable antibonding states at −4.41 eV, which may be regarded as attributes of the d 10 -d 10 closed-shell [39,40] interactions; however, the Ag-Ag -ICOHP values are indicative of a net bonding character for these interactions.…”

“…To provide an insight into the bonding situation in RbPr2Ag3Te5, we followed up with an examination of the crystal orbital Hamilton population curves (COHP; Figure 2) and their respective integrated values (Table 3). In this context, the cumulative -ICOHP/cell values, i.e., the sums of the negative ICOHP/bond values of all nearest neighboring contacts within one unit cell, were projected as percentages of the net bonding capabilities for each sort of interaction to identify roles of the diverse interactions in overall bonding-a procedure that has been largely employed elsewhere [21,22,[33][34][35][36]. (Figure 2) reveals that the occupied states close to the Fermi level, E F , originate to a large extent from the Ag-d and Te-p atomic orbitals beside minor contributions from the Pr-d states.…”

“…The distributions of the valence-electrons in the tellurides have been typically described by applying the Zintl-Klemm concept [19,20]; however, the application of the latter to certain tellurides containing low-dimensional tellurium fragments resulted in the assignments of non-electron-precise charges to the tellurium atoms [20]. In that connection, recent research [21,22] on the electronic structures of certain tellurides, i.e., LaCu x Te 2 (0.28 ≤ x ≤ 0.50), Gd 3 Cu 2 Te 7 , and Ag 5−x Te 3 (−0.25 ≤ x ≤ 1.44), indicated that the valence-electron transfers as suggested by Zintl-Klemm treatments should be taken with a grain of salt. In fact, analyzing the nature of bonding for the aforementioned tellurides revealed the presence of strong rare-earth-tellurium as well as transition-metal-tellurium bonding interactions being in stark contrast to the (complete) valence-electron transfers as assumed by Zintl-Klemm treatments.…”

Development of a robust tool to extract Mulliken and Löwdin charges from plane waves and its application to solid-state materials

“…Both orbital-based Mulliken and Löwdin population analyses as implemented in LOBSTER automatically and accurately derive atomic charges in solid-state materials, and we will demonstrate their performance as regards ionic compounds, polar intermetallics, including commonly known archetypal Zintl phases, as well as another polar intermetallic compound, i.e., stützite. 54 The latter mineral was chosen for the prototypical application of the tool, as more recent research 54 revealed that applying the Zintl-Klemm concept to it should be taken with a pinch of salt. For comparison, density-based Bader charges were also calculated for the present compounds.…”

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