Verbindungen A 3 M 5 (A = Erdalkalimetall, M = Triel/Tetrel): Eine Fallstudie zur strukturellen und elektronischen Stabilität polarer intermetallischer Phasen

Compounds A 3 M 5 (A = alkaline earth, M = triel/tetrel): A Case Study on Structural and Electronic Factors Stabilizing Polar Intermetallics Starting from the non electron precise binary compounds Ca 3 Ga 5 /Sr 3 In 5 (Hf 3 Ni 2 Si 3 type) and Ba 3 Al 5 at one hand and Ba 3 Pb 5 (Pu 3 Pd 5 type) at the other hand, a series of new ternary intermetallics of the general formula A 3 M 5 (A: alkaline earth, M: triel/tetrel) has been synthesized, structurally characterized and studied by band structure calculations. The chemical substitution of M in A 3 M 5 allows, via the continous variation of the radius ratio (r A :r M ) and the valence electron number (VE/M) the detection of the geometrically and electronically determined stability ranges of the three structure types formed by the binary compounds. At values of r A :r M between 1.30 and 1.52 in the triel rich region of A 3 M′ x M″ 5−x the Hf 3 Ni 2 Si 3 type (orthorhombic, space group Cmcm ) is formed: In Ca 3 Ga 5 up to 1.8 Ga can be substituted by Al, in Sr 3 In 5 similar amount of In can be replaced by either Al or Ga. The mixed trielide Sr 3 Al 2.6 Ga 2.4 ( a = 468.4(1), b = 1132.5(1), c = 1570.0(2) pm, R 1 = 0.0261) can be obtained, although both corresponding binary phases are not known. At larger values of the ratio r A /r M as in Ba 3 Al 3 Ga 2 (Ba 3 Al 5 type, hexagonal, space group P 6 3 / mmc , a = 598.9(1), c = 1456.0(3) pm, R 1 = 0.0353) layers of condensed M 5 building blocks with Al‐Al partial bonds are formed. Substituting one In position in Sr 3 In 5 against Pb results in the isotypic, but electron precise Zintl compound Sr 3 In 4 Pb ( a = 506.1(1), b = 1191.8(3), c = 1650.2(4) pm, R 1 = 0.0286), where the Fermi level in shifted into a distinct minimum of the density of states. Conversely, at the tetrele rich end of the series A 3 In x Pb 5−x , characterized by compounds of the Pu 3 Pd 5 type (orthorhombic, space group Cmcm ) with almost isolated nido clusters M 5 , a minimum of the DOS can be reached, if Pb is partially substituted by In (A 3 In x Pb 5−x with A = Sr/Ba: x = 0.7/0.6; a = 1084.6(2)/1118.6(2), b = 867.1(2)/904.4(1), c = 1104.8(2)/1133.9(2) pm, R 1 = 0.0394/0.0434).