Crystal growth and magnetic characterization of a tetragonal polymorph of NiNb2O6

A previously unidentified polymorph of nickel niobate, NiNb2O6, was grown and stabilized in single crystalline form using an optical floating zone furnace. Key parameters of the growth procedure involved use of a slight excess of NiO (1.2% by mol), an O2 atmosphere and a growth rate of 25 mm/h. The resulting boule consisted of a polycrystalline exterior shell of the columbite structure – columbite is the thermodynamically stable form of NiNb2O6 under ambient conditions – and a core region consisting of transparent yellow-green single crystals up to 5 mm×2 mm×1 mm in dimension of the previously unidentified phase. The crystal structure, solved from single crystal x-ray diffraction data, is described in the P42/n space group. Interestingly, this is not a subgroup of P42/mnm, the rutile space group. The Ni2+ ions form layers which are displaced such that interlayer magnetic frustration is anticipated. Magnetic susceptibility data shows a broad maximum at approximately 22 K and evidence for long range antiferromagnetic order at approximately 14 K, obtained by Fisher heat capacity analysis as well as heat capacity measurements. The susceptibility data for T > 25 K are well fit by a square lattice S = 1 model, consistent with the Ni sublattice topology.