STRUCTURE AND MAGNETIC-SUSCEPTIBILITY OF MN11TA4O21 AND REFINEMENT OFTHE MN4TA2O9 STRUCTURE

The structure of Mn11Ta4O21 has been determined and refined using the Rietveld method with combined CuK alpha(1) X-ray and time-of-flight ne utron powder data in space group P (3) over bar c1 to R-F2 = 1.3% (neu tron data) and R-F2 = 7.8% (X-ray data). The unit cell is a = 5.3776(2 ) Angstrom, c = 34.040(2)Angstrom, V = 852.5 Angstrom(3), and Z = 2, T he structure consists of a 14-layer hexagonal sequence of close packed oxygen atoms and an ordered distribution of metal atoms in octahedral coordination. It can be described as built up of corundum-type Mn4Ta2 O9 blocks, with six layers of octahedra, alternating with single MnO l ayers of octahedra, The structure is compared with that of corundum-re lated Mn4Ta2O9, with unit cell a = 5.3306(2) Angstrom, c = 14.336(1) A ngstrom, V = 352.8 Angstrom(3), and Z = 2, which has been refined in s pace group P (3) over bar c1 to R-F2 = 1.9%, using constant-wavelength (lambda = 1.47 Angstrom) neutron powder diffraction data, The magneti c susceptibility of Mn11Ta4O21 exhibits a maximum at 23 K and a Curie- Weiss behavior at higher temperatures with Theta(a) = -240 K and mu(ef f) = 5.7 mu(B) per Mn atom. (C) 1998 Academic Press.