The compounds Ta2NiS5 and Ta2NiSe5 have a novel layered structure: within the layers are rows of tantalum atoms separated by rows of nickel atoms. The lower dimensional structure of these compounds as well as the expected d1 non-bonding electron configuration of the tantalum 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 Ta2NiS5 a first-order transition occurs at 28 K and in Ta2NiSe5 a second-order transition occurs at 328 K. The latter transition is shown by electron microscopy to be martensitic-like.
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