Influence of Element Substitution on the Cluster Arrangement in the Novel Structures Ca₃Tl₅, Sr₃Tl₅, and Sr₃Sn_5–xTl_x (x = 1.8 and 2.2) 

Abstract

.78 (I), Sr3Sn2.84Tl2.16 (II), Ca3Tl5 (III), and Sr3Tl5 (IV) are prepared from the elements (Nb ampules, 750-940°C, 6 h, followed by annealing at 500-600°C for 5-7 d) and characterized by single crystal XRD and TB-LMTO-ASA calculations. (I) and (II) crystallize with the Tm3Ga5-type structure in the space group Pnma with Z = 4, and (III) and (IV) crystallize with the Pu 3 Pd 5 -type structure in the space group Cmcm with Z = 4. Both structure types consist of zigzag chains of distorted square pyramidal {M5} (M: Tl, Sn/Tl) clusters. In (I) and (II) the clusters are reoriented with respect to each other and have only two exo-contacts each, whereas in (III) and (IV) the clusters are aligned parallel and show a considerable higher degree of polymerization.

Novel barium triel/tetrelides with the structure type

“…numbers. For the even more electron-depleted, but formally electron precise phase Sr 3 Tl ∼2 Sn ∼3 [9], similar tendencies can be observed despite its different structure type, as it contains the same type of clusters. As the partial substitution by thallium in itself leads to an increase of the bond lengths by ∼ 9 pm, the distances a and c are effectively decreased relative to Sr 3 Sn 5 , while b is subjected to the expected elongation from 300 to 310 pm.…”

“…although it should be theoretically possible and practically all element combinations have been studied. Instead, the only known compound intermediate between the two classes given above, the mixed strontium triel/tetrelide Sr 3 Tl ∼2 Sn ∼3 (24 ± ı v.e./M 5 , also with only a very small phase width) forms the closely related Tm 3 Ga 5 type with the very same nido clusters [9]. Mixed lanthanum indium plumbides between La 3 In 3.7 Pb 1.3 and La 3 In 0.8 Pb 4.2 , which might be expected to resemble the alkaline earth compounds, instead show a completely different structural chemistry as they crystallise with the La 3 In 4 Ge structure type [17].…”

“…2. The isotypic Sr tetrelides (Sr 3 Sn 5 [12][13][14] and its In [16] and Tl [9] derivatives) as well as the binary Ca/Sr thallides (Ca/Sr) 3 Tl 5 [9] are not included in this scheme, but they are mentioned where appropriate in the following discussion. To provide a convenient overview of the element distribution over the three M sites (i.e.…”

“…(5.2 v.e./M) already suggests an excess of two electrons per cluster. Accordingly, EH, LMTO [9,13] and FP-LAPW [16,17] band structure calculations for these compounds all show, that the Fermi level is situated above a distinct minimum of the total density of states. In the Sr/Ba plumbides [16] this v.e.…”

“…At one hand, the electron poor trielides (Ca/Sr) 3 Tl 5 [9] (21 v.e./f.u.) and the lanthanum indide La 3 In 5 [10] form this structure type, while the analogous Ln gallides crystallize with the closely related Tm 3 Ga 5 type [11].…”