SYNTHESES AND STRUCTURES OF LAGE2-X AND LAALGE - INTERRELATIONSHIPS AMONG THE ALPHA-THSI2, ALPHA-GDSI2, AND LAPTSI STRUCTURE TYPES

Alpha-LeGe2-x has been synthesized by powder sintering and shown to have the composition range 0.33 less-than-or-equal-to x less-than-or-equal-to 0.40 and the orthorhombic alpha-GdSi2-type structure (Imma, Z = 4) with deviations from near-tetragonal dimensions that increase with decreasing x. The structure of LaGe1.60(1) at 20-degrees-C was refined from single-crystal data (a = 4.2680 (7), b = 4.2735 (6), c = 14.404 (1) angstrom; R, R(w) = 2.5%, 2.3%) with the germanium deficiency predominantly on one of two independent non-metal sites. The allowed second-order phase transition of alpha-LaGe2-x to the beta-form in the tetragonal alpha-ThSi2-type structure (I4(1)/amd) has been observed by powder diffractometry over the range of 425-635-degrees-C with decreasing x. The structural nature of the transition is described. The TbGe1.67 superstructure reported in space group Fdd2 is noted to also be consistent with a second-order transition from a high-temperature alpha-ThSi2-type structure. The fully stoichiometric Zintl phase LaAlGe was synthesized and shown to exist in the ordered LaPtSi version of the ThSi2 structure (I4(1)md; Z = 4; a = 4.336 (1), c = 14.828 (5) angstrom; R, R(w) = 3.2%, 2.1%). The stability of tetragonal, fully stoichiometric LaAlxGe2-x extends well into the ternary system to x less-than-or-equal-to 0.2. Oxygen has a similar effect and stabilizes tetragonal LaGe2-x(O) (with small x) at room temperature.