Stacking polytypes of mixed alkali gallides/indides A _1–2(Ga/In)₃ (A=K, Rb, Cs): synthesis, crystal chemistry and chemical bonding

Abstract

The Ga/In phase width (y) and the distribution of the two triels within the polyanions of known binary (x = 1, 2) and new ternary ‘intermediate’ (x = 1–2) alkali trielides Ax(Ga1−yIny)3 (A = K, Rb, Cs) was investigated in a synthetic (slow cooling of the melts of the three elements), crystallographic (X-ray single crystal) and bond theoretical (FP-LAPW DFT bandstructure calculation) study. The Cs2In3-type structure (x = 2, series A, tetragonal, I 4/mmm) exhibits layers of four-connected closo octahedra [M6]4−. Ternary K compounds of this type were yielded within a large range (y = 0–0.87), whereas isotypic Rb/Cs trielides exist only at higher In contents (>52/69%). Geometric criteria determine not only the Ga/In stability ranges but also the occurrence of a commensurate superstructure at approx. 33% In (K2Ga2.17In0.83: P 42/ncm, a = 879.83(4), c = 1557.66(10) pm, R 1 = 0.0887), in which the octahedra are slightly tilted against the layers. Cesium compounds of the RbGa3-type structure (x = 1, C, tetragonal, I 4̅m2), which exhibits a 3D network of all-exo bonded closo dodecahedra [M8]2− and four-bonded M− anions, are stable throughout the whole substitutional range CsGa3–CsIn3. The maximum possible In content increases with increasing size of A+ (Cs: 100%, Rb: 30%, K: 8% In). The similarities between these two tetragonal structures are consistent with the occurrence of two new structure types of ‘intermediate’ compounds A7M15 (x = 1.4, 1B/2B), which differ in the stacking sequences of double layers of novel six-fold exo-bonded pentagonal bipyramidal closo clusters [M7]3− connected via four-bonded M− (e.g. 1B: Cs7Ga8.4In6.6, P 4̅m2, a = 656.23(3), c = 1616.0(1) pm, R 1 = 0.0742; 2B: Rb7Ga8.1In6.9, P 42/nmc, a = 665.64(2), c = 3140.9(2) pm, R 1 = 0.0720). The Rb/Cs compounds of these types are only stable in a limited Ga/In region and with a distinct Ga/In distribution within the [M7] clusters. According to the close relation between the structures A, B and C, the structure family is characterised by the occurrence of stacking faults and diffuse scattering, indicating the existence of further members of this series. The new compound Cs5Ga3.1In5.9 (x = 1.667, P 4̅m2, a = 654.62(2), c = 3281.5(2) pm, R 1 = 0.1005) is a reasonably periodically ordered stacking variant containing layers A and double layers B in parallel.

A Coloring Study of the Ga Richest Alkali Gallides: New In- and Hg-Containing Gallides with the RbGa7- and the K3Ga13-Type Structure

“…The present work on the "coloring" [21] of the polyanions in the two Ga-rich gallides K 3 Ga 13 and (Rb/Cs)Ga 7 with the larger In atoms extends our recent studies on mixed Ga/In trielides of the RbGa 3 , Cs 2 In 3 [22], BaAl 4 [9], and Cs 8 Ga 11 [12] structure types. Due to the closed-shell character of the two gallides, the partial substitution of Ga by Hg, which differs not only in size and electronegativity, but in v.e.…”

“…The calculated valence electron densities (ρ) and the results of their Bader analyses fit those of the trielides AM 3 and A 2 M 3 discussed in [22], as well as those of the complex cluster structure of K 3 Ga 9.6 In 1.4 [25]: all endoand exohedral Ga-Ga bonds carry bond critical points (cf. the individual values in Table 9); ring and cage critical points are again located at the triangular faces and the cluster centers, respectively.…”

“…Likewise, several In-substituted derivatives of this unknown gallide and of RbGa 7 could be obtained, again from triel-rich samples. In accordance with the incongruent melting of both types of compounds, samples with an overall composition AM 7 (i.e., 12.5% Rb; Rb1 and Rb2) yielded crystals of ternary KGa 3 -type derivatives only [22]. At a reduced Rb content of 6% (A 3 M 47 ) and for In poorer samples [Rb 3 Ga 32 In 15 (Rb3) still contained KGa 3 -type compounds besides elemental In], i.e., from the samples Rb4 (Rb 3 Ga 37 In 10 ) and Rb5a-c (Rb 3 Ga 42 In 5 ), ternary In-derivatives of RbGa 7 and the K 3 Ga 13 -type structure could be obtained.…”

“…[28] In the ternary system K-Ga-In, indium richer samples at the pseudo-binary section A 3 M 13 (e.g., samples K1 and K2; Table 1) yielded members of the recently newly obtained orthorhombic A 3 M 11 phases K 3 Ga 11-x In x (x = 1.16 − 1.36, [25]) if the maximum temperature was kept at 500 • C. At a reduced In content, i.e., below a Ga:In ratio of 11:2 (samples K3 and K4), variable amounts (depending on the temperature program) of the A 3 M 11 trielides (for T max = 500 • C) and ternary In-substituted derivative of the K 3 Ga 13 -type (T max = 700 • C) are formed. As the binary gallide K 3 Ga 13 [20] is known to melt incongruently and the peritectic line at 510 • C meets the liquidus curve at approximately 6% K [18,29], well-developed needle-shaped crystals of the most In-rich K 3 Ga 13 -type phase were obtained from the triel-rich sample K 3 Ga 42 In 5 (K5), again at a maximum temperature of 700 • C. By-products were In-substituted KGa 3 -type phases [5,22] and elemental gallium. Attempts to crystallize the incongruently melting Ga-rich gallides from liquid mercury in samples of overall composition KGa 3 Hg 3 (K6) and KGaHg 3 (K7) yielded-together with KHg 6 [26] and K 2 Hg 7 [27], respectively-nice needle-shaped crystals of the ternary Hg-containing derivative K 3 Ga 12.65 Hg 0.35 of the K 3 Ga 13 -type.…”

Spotlight on Alkali Metals: The Structural Chemistry of Alkali Metal Thallides

“…A small number of ternary compounds involving sodium and a heavier congener were reported subsequently. There are also known more complex systems like ternary compounds involving lighter group 13 elements (Ga, In) [25][26][27][28], elements from the d-block [29,30] or thallide oxo compounds [31][32][33], but in this review the focus is set on the most simple combination of alkali metal and thallium, in order to point out structural effects of different alkali metals.…”