Physical and structural properties of the new layered compounds Ta(2)NiS(5) and Ta(2)NiSe(5).

The compounds Ta(2)NiS(5) and Ta(2)NiSe(5) have a novel layered structure. Within the layers are rows of Ta atoms separated by rows of Ni atoms. The lower dimensional structure of these compounds as well as the expected d(l) nonbonding electron configuration of the Ta atom leads to the expectation that classical charge density wave behavior would be observed in their properties. An examination of the electrical, magnetic, and structural properties shows that this is not the case, rather, the properties are more consistent with the materials being small band gap semiconductors. Further, both compounds show structural phase transitions: in Ta(2)NiS(5) a first-order transition occurs at 28K and in Ta(2)NiSe(5) a second-order transition occurs at 328K. The latter transition is shown by electron microscopy to be martensitic-like.