STRUCTURAL CHARACTERIZATIONS OF 2 SYNTHETIC NI-LUDWIGITES, AND SOME SEMIEMPIRICAL EHTB CALCULATIONS ON THE LUDWIGITE STRUCTURE TYPE

The structures of two novel synthetic nickel-containing borates, Ni2Cr O2BO3 and Ni2VO2BO3, have been investigated by single crystal X-ray di ffraction techniques and shown to have the ludwigite structure. The sp ace group symmetry of both borates is Pbam with Z = 4. The unit cell d imensions are a = 9.209(1), b = 12.121(1), and c = 2.9877(3) angstrom for the Ni,Cr-borate, and a = 9.199(2), b = 12.211(2), and c = 2.988(1 ) angstrom for the Ni,V-borate. The structural model of the Ni,Cr-ludw igite has been refined on the basis of the 575 most significant (I > 3 sigma(I)) reflections with sin(theta)/lambda less-than-or-equal-to 0.9 angstrom-1 to an R-value of 0.029. For the Ni,V-ludwigite refinement, the 617 most significant (I > 3sigma(I)) reflections with sin(theta)/ lambda less-than-or-equal-to 0.8 angstrom-1 yielded an R-value of 0.02 3. Although the scattering powers of the cation pairs Ni/Cr and Ni/V a re very similar, the refined cation distributions at the mixed metal s ites yield approximately electroneutral compounds. The observed struct ural arrangement with the higher charged cations coplanar to the borat e anion planes in the crystal structure is apparently a salient featur e for all members of the pinakiolite structure family. A general model for the ludwigite structure, with alternating metal charges along the walls of octahedra, is supported by some semiempirical extended Hucke l electronic band calculations performed on the homometallic Fe- and C o-ludwigites. (C) 1994 Academic Press, Inc.