Structural Characterizations of Two Synthetic Ni-Ludwigites, and Some Semiempirical EHTB Calculations on the Ludwigite Structure Type

Abstract The structures of two novel synthetic nickel-containing borates, Ni 2 CrO 2 BO 3 and Ni 2 VO 2 BO 3 , have been investigated by single crystal X-ray diffraction techniques and shown to have the ludwigite structure. The space group symmetry of both borates is Pbam with Z = 4. The unit cell dimensions are a = 9.209(1), b = 12.121(1), and c = 2.9877(3) A, for the Ni,Cr-borate, and a = 9.199(2), b = 12.211(2), and c = 2.988(1) A for the Ni,V-borate. The structural model of the Ni,Cr-ludwigite has been refined on the basis of the 575 most significant ( I > 3σ( I )) reflections with sin(θ)/λ ≤ 0.9 A, -1 to an R -value of 0.029. For the Ni,V-ludwigite refinement, the 617 most significant ( I > 3σ( I )) reflections with sin(θ)/λ ≤ 0.8 A -1 yielded an R -value of 0.023. Although the scattering powers of the cation pairs Ni/Cr and Ni/V are very similar, the refined cation distributions at the mixed metal sites yield approximately electroneutral compounds. The observed structural arrangement with the higher charged cations coplanar to the borate anion planes in the crystal structure is apparently a salient feature 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 semi-empirical extended Huckel electronic band calculations performed on the homometallic Fe- and Co-ludwigites.