Electronic band structures and physical properties of ALnTe 4 and ALn3Te 8 compounds ( A=alkali metal; Ln=lanthanoid)

Abstract

We report about the LMTO-ASA band structure, ELF and COHP calculations for a number of alkali metal rare earth tellurides of the formulas ALnTe 4 ( A=K, Rb, Cs and Ln=Pr, Nd, Gd) and K Ln 3Te 8 ( Ln=Pr, Nd) to point out structure-properties relations. The ALnTe 4 compounds crystallize in the KCeSe 4 structure type with Te ions arranged in the form of 4.3 2.4.3 nets, in which interatomic homonuclear distances indicate an arrangement of isolated dumbbells. This could be verified by the COHP and ELF calculations, both of which revealed isolated [Te 2] units. But in contrast to the ionic formulation as A + Ln 3+ ([Te 2] 2−) 2, which can be deduced from this observation, the band structure calculations for KPrTe 4, KNdTe 4, RbNdTe 4 and CsNdTe 4 reveal metallic conductivity. This behavior was verified for KNdTe 4 by resistivity measurements performed by a standard four-probe technique. We explain these results by an incomplete carryover of electrons from the rare earth cation onto tellurium due to covalent bonding leaving parts of the Te–Te ppπ * antibonding states unoccupied. On the other hand the calculations suggest insulating behavior for KGdTe 4 resulting from a complete filling of the Te–Te ppπ * antibonding states due to the increased stability of the half filled 4 f shell. The ALn 3Te 8 compounds crystallize in the KNd 3Te 8 structure type, a distorted addition-defect variant of the NdTe 3 type with 4 4 Te nets. As polyanionic fragments L-shaped [Te 3] 2− and infinite zig-zag chains ∞ 1[Te 4] 4− are observed (with interatomic homonuclear distances in the range 2.82–3.00 Å), which are separated from each other by distances in the range 3.27–3.49 Å. Again COHP calculations made evident that these latter interactions are secondary. Within the infinite zig-zag chains ∞ 1[Te 4] 4− the Te ions at the corners of the chain have a higher negative charge than the linear coordinated ones in the middle. KPr 3Te 8 and KNd 3Te 8 are semiconductors, verified for the latter by resistivity measurements.