Lanthan-Triel/Tetrel-ide La(Al,Ga)x(Si,Ge)1-x. Experimentelle und theoretische Studien zur Stabilität intermetallischer 1 : 1-Phasen / Lanthanum Triel/Tetrel-ides La(Al,Ga)x(Si,Ge)1−x. Experimental and Theoretical Studies on the Stability of Intermetallic 1:1 Phases

Abstract

Systematic studies of the phase formation at the binary sections LaSi - LaGa and LaGe - LaAl have been carried out by means of synthetic, crystallographic and bond theoretical methods. The hightemperature forms of the two binary monotetrelides LaSi and LaGe crystallize with the FeB structure type, whereas LaGa forms the related CrB type and LaAl the significantly different CeAl type. Starting from LaSi/LaGe, the FeB type (orthorhombic, space group Pnma, Z = 4, a = 839.2(1)/842.7(1), b = 399.9(1)/412.3(1), c = 606.2(2)/612.2(1) pm, R1 = 0.0356/0.0298) remains stable only down to a valence electron number per M atom (M = Si, Ge, Al, Ga) of 6.9 (LaGa0.10Si0.90: a = 840.14(7), b = 404.12(12), c = 608.5(2) pm, R1 = 0.0513; LaAl0.15Ge0.85: a = 845.40(7), b = 414.08(13), c = 614.08(14) pm, R1 = 0.0213). In the system LaGaxSi1−x, the stability range of the CrB type (orthorhombic, space group Cmcm, Z = 4) starts at a gallium proportion of 25% (LaGa0.25Si0.75: a = 450.03(8), b = 1140.5(2), c = 406.05(6) pm, R1 = 0.0163) and extends to the border compound LaGa (v. e./M = 6). The CrB type also occurs in the system La-Al-Ge, but is in this case only formed around the 1 : 1 composition in between LaAl0.42Ge0.58 (a = 455.90(12), b = 1161.1(3), c = 418.05(9) pm, R1 = 0.0474) and LaAl0.61Ge0.39 (a = 454.89(10), b = 1168.8(2), c = 420.41(11) pm, R1 = 0.0447). These stability ranges, the variations of several key geometric parameters such as the M-M distances or the heights of the trigonal prisms, and the main aspects of the chemical bonding in these lanthanum monometallides are analyzed using FP-LAPWband structure methods. The structure of the new compound La2Al2Ge (V2B3 structure type, orthorhombic, space group Cmcm, a = 416.69(4), b = 2719.7(4), c = 450.46(5) pm, Z = 2, R1 = 0.0458) combines the structural elements of the CrB/FeB structure family (two-bonded M atoms) with the trigonally planar bonded M atoms of the ThSi2 type in a fully ordered Al and Ge atom distribution and thus without phase width