Etude structurale des composés Fe 0,25TiSe 2 et Co 0,25TiSe 2 à cristaux maclés. Surstructures et degré d'ordre des lacunes

Abstract

Fe 0.25TiSe 2 and Co 0.25TiSe 2 crystals have been studied. Meriedric twins are detected by means of X-ray diffraction, from extinction rules observed on the apparent cells that are multiples of the C6 hexagonal unit cell ( a′, c′). In Fe 0.25TiSe 2 [quasi-orthohexagonal monoclinic I true unit cell: a (≈a′ 3 1 2 ) = 3.566(1) × 3 1 2 ; b (≈a′) = 3.566(1); c (≈2c′) = 5.964(1) × 2 Å; β ≈ 90°] existence of a M 3□ X 4 ordering of vacancies, previously observed on powders, is confirmed by this study. In Co 0.25TiSe 2, the true unit cell is monoclinic F, (a ≈ 2a′ 3 1 2 , b ≈ 2a′, c ≈ 2c′) . The basic C6 lattice is apparently undistorted and the structure may be described through a hexagonal unit cell ( a 1 = b 1 = 2 a′; c 1 = 2 c′); on the studied crystal we have found a 1 = 7.1014(9); b 1 = 7.0993(8); c 1 = 11.800(2) Å; γ 1 = 119.97(3)°. The structure refinement ( R = 0.036 from 120 “ C6” reflections; R = 0.056 from 70 superstructure reflections) is carried out with the M 5□ 3 X 8 model, 2 c′ variety, space group C2 m . Assuming Co atoms to be in vacancy-containing layers, occupation rates of 0.55 on “metal” sites and 0.092 on “vacant” sites are found.