Crystals (May 2018)
Mixed Sr and Ba Tri-Stannides/Plumbides AII(Sn1−xPbx)3
Abstract
The continuous substitution of tin by lead (M IV ) allows for the exploration geometric criteria for the stability of the different stacking variants of alkaline-earth tri-tetrelides A II M 3 IV . A series of ternary Sr and Ba mixed tri-stannides/plumbides A II (Sn 1 − x Pb x ) 3 (A II = Sr, Ba) was synthesized from stoichiometric mixtures of the elements. Their structures were determined by means of single crystal X-ray data. All structures exhibit close packed ordered A M 3 layers containing M kagomé nets. Depending on the stacking sequence, the resulting M polyanion resembles the oxygen substructure of the hexagonal (face-sharing octahedra, h stacking, Ni 3 Sn-type, border compound BaSn 3 ) or the cubic (corner-sharing octahedra, c stacking, Cu 3 Au-type, border compound SrPb 3 ) perovskite. In the binary compound BaSn 3 (Ni 3 Sn-type) up to 28% of Sn can be substituted against Pb (hP8, P 6 3 / mmc, x = 0.28(4): a = 726.12(6), c = 556.51(6) pm, R1 = 0.0264). A further increased lead content of 47 to 66% causes the formation of the BaSn 2.57 Bi 0.43 -type structure with a ( hhhc ) 2 stacking [hP32, P 6 3 / mmc, x = 0.47(3): a = 726.80(3), c = 2235.78(14) pm, R1 = 0.0437]. The stability range of the BaPb 3 -type sequence ( hhc ) 3 starts at a lead proportion of 78% (hR36, R 3 ¯ m, a = 728.77(3), c = 2540.59(15) pm, R1= 0.0660) and reaches up to the pure plumbide BaPb 3 . A second new polymorph of BaPb 3 forms the Mg 3 In-type structure with a further increased amount of cubic sequences [ ( hhcc ) 3 ; hR48, a = 728.7(2), c = 3420.3(10) pm, R1 = 0.0669] and is thus isotypic with the border phase SrSn 3 of the respective strontium series. For the latter, a Pb content of 32% causes a small existence region of the PuAl 3 -type structure [hP24, P 6 3 / mmc, a = 696.97(6), c = 1675.5(2) pm, R1 = 0.1182] with a ( hcc ) 2 stacking. The series is terminated by the pure c stacking of SrPb 3 , the stability range of this structure type starts at 75% Pb (cP4, Pm 3 ¯ m; a = 495.46(9) pm, R1 = 0.0498). The stacking of the close packed layers is evidently determined by the ratio of the atomic radii of the contributing elements. The Sn/Pb distribution inside the polyanion (’coloring’) is likewise determined by size criteria. The electronic stability ranges, which are discussed on the basis of the results of FP-LAPW band structure calculations are compared with the Zintl concept and Wade’s/mno electron counting rules. Still, due to the presence of only partially occupied steep M-p bands the compounds are metals exhibiting pseudo band gaps close to the Fermi level. Thus, this structure family represents an instructive case for the transition from polar ionic/covalent towards (inter)metallic chemistry.
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