Oxidation states, transport properties, and Te center dot center dot center dot Te short contacts in the ternary transition metal tellurides Ta3Pd3Te14 and Ta4Pd3Te16

The electronic structures of the layered ternary tellurides Ta3Pd3Te14 and Ta4Pd3Te16 are examined. It is found that the presence of bicapped trigonal prismatic chains with extensive Te ... Te interactions results in very wide tellurium-based bands along the direction of the chains. The local octahedral coordination of the Pd and one type of Ta atoms, as well as additional short Te ... Te contacts, allows for the participation of the remaining chains of the layer in such a delocalization. This leads to large tellurium to metal electron transfers and difficulties in proposing meaningful oxidation states for the Pd and Ta atoms. The interlayer Te ... Te interactions are found to exert a strong control of the density of states at the Fermi level and, hence, of the conductivity of these phases. The results are also compared with those for Ta3SiTe6, a structurally different layered ternary telluride with short Te ... Te contacts. For this phase, meaningful oxidation states for the Ta and Si atoms can be proposed. The different, but important, role of interlayer Te ... Te interactions in Ta3SiTe6 is also discussed.