Syntheses and structures of LaGe sub 2 minus x and LaAlGe: Interrelationships among the. alpha. -ThSi sub 2 ,. alpha. -GdSi sub 2 , and LaPtSi structure types

{alpha}-LaGe{sub 2{minus}x} has been synthesized by powder sintering and shown to have the composition range 0.33 {le} x {le} 0.40 and the orthorhombic {alpha}-GdSi{sub 2}-type structure (Imma, Z = 4) with deviations from near-tetragonal dimensions that increase with decreasing x. The structure of LaFe{sub 1.60(1)} at 20C was refined from single-crystal data (a = 4.2680 (7), b = 4.2735 (6), c = 14.404 (1) {angstrom}; R, R{sub 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}-LaGe{sub 2{minus}x} to the {beta}-form in the tetragonal {alpha}-ThSi{sub 2}-type structure (I4{sub 1}/amd) has been observed by powder diffractometry over the range of 425-635C with decreasing x. The structural nature of the transition is described. The TbGe{sub 1.67} superstructure reported in space group Fdd2 is noted to also be consistent with a second-order transition from a high-temperature {alpha}-ThSi{sub 2}-type structure. The fully stoichiometric Zintl phase LaAlGe was synthesized and shown to exist in the ordered LaPtSi version of the ThSi{sub 2} structure (I4{sub 1}md; Z = 4; a = 4.336 (1), c = 14.828 (5) {angstrom}; R, R{sub w} = 3.2%, 2.1%). The stability of tetragonal fully stoichiometric LaAl{sub x}Ge{sub 2{minus}x} extends well into the ternary system to x {le} 0.2. Oxygen has a similar effect and stabilizes tetragonal LaGe{sub 2{minus}x}(O) (with small x) at room temperature.