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.
Highlights
In view of the complex bonding situation between ionic, covalent and metallic, the synthetic, crystallographic and bond theoretical studies on geometric and electronic parameters determiningCrystals 2018, 8, 204; doi:10.3390/cryst8050204 www.mdpi.com/journal/crystalsCrystals 2018, 8, 204 the structure chemistry of polar intermetallics of the alkali and alkaline-earth compounds of the p-block elements are still a fascinating field of research
Similar to the variation of the alkaline-earth elements (A) in the pure tri-stannides [10] and tri-plumbides [11], a continuous substitution of tin by lead allows for the exploration of geometric and electronic (Sn/Pb distribution) criteria for the stability of different stacking variants of the hexagonal close packed layers
The tri-plumbides APb3 of calcium and strontium, which both form the cubic Cu3 Au-type structure (|:ABC:| or c stacking after Jagodzinski), have been structurally characterized using X-ray powder data in the past and some physical properties as well as the electronic band structure were reported in the 1960s and 1970s by Havinga et al [6,12,13]
Summary
In view of the complex bonding situation between ionic, covalent and metallic, the synthetic, crystallographic and bond theoretical studies on geometric and electronic parameters determining. Van Vucht published an elaborate work [7], in which he, based on powder diffraction data, already suggested the existence of further stacking variants for the mixed alkaline-earth plumbides A x Ba1 − x Pb3 (A = Ca, Sr) These two series have been fully explored by our group 10 years ago [11]: Both the Ca and the Sr series start with the pure c stacking of (Ca/Sr)Pb3. The tin series terminates with the pure h stacking of BaSn3 ; the stability range of this Ni3 Sn-type structure starts at the composition Sr0.22 Ba0.78 Sn3 Beyond these pure geometric/size effects, van Vucht and Havinga [8,16] demonstrated the electronic influence on the h/c stacking of the layers in several Tl-substituted plumbides A(Tl/Pb). The substitution of tin by lead does change the geometric relations, but in addition the occupation of the crystallographically different M positions of the more complex stacked sequences with tin and/or lead, i.e., the ’coloring’ of the polyanion [19] can be studied
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