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 25mm/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 5mm×2mm×1mm 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 22K and evidence for long range antiferromagnetic order at approximately 14K, obtained by Fisher heat capacity analysis as well as heat capacity measurements. The susceptibility data for T>25K are well fit by a square lattice S=1 model, consistent with the Ni sublattice topology.