Single phase samples of the monoclinic and orthorhombic polymorphs of ${\mathrm{Nb}}_{12}{\mathrm{O}}_{29}$ have been identified and isolated. These polymorphs have different arrangements of nanometer dimension structural columns of $\mathrm{Re}{\mathrm{O}}_3$-type niobium oxygen arrays: They differ in structure after a $3\text{−}\mathrm{nm}$ translation in one of three dimensions while being the same in the other two. Magnetic susceptibility measurements show that one polymorph displays an antiferromagnetic transition at $12\mathrm{K}$, along with short-range order fluctuations up to $25\mathrm{K}$, while the other shows strict Curie-Weiss behavior down to $2\mathrm{K}$.

• Received 1 April 2005

DOI:https://doi.org/10.1103/PhysRevB.72.033413