Complex Structure, Chemical Bonding, and Electrical Transport Properties of a La-Doped Zintl Phase

The La-doped ternary Zintl phase Ca10.43(3)La0.57Sb9.69(1) was successfully synthesized by arc melting, and the title compound adopted the Ho11Ge10-type structure with a tetragonal I4/mmm space group (Z = 4, Pearson code tI84). The complex crystal structure is composed of (1) the four different kinds of cationic Ca or Ca/La mixed sites surrounded by seven or nine Sb atoms and (2) the 3-dimensional cage-shaped anionic frameworks built by the other two types of Sb atoms. In particular, the La dopants preferred to occupy the Ca4 and Ca1 sites, and this specific cationic-site preference can be rationalized by both electronic and size-factor criteria. Moreover, the ca. 16% occupational deficiency observed at the Sb3 site was attributed to the energetically unfavorable antibonding character of the Sb3–Sb3 bond in the [Sb3]4 tetramers, according to a series of DFT calculations. A crystal Hamilton overlap population curve analysis also proved that the title compound Ca10.43(3)La0.57Sb9.69(1) tried to keep the valence electron count below 71.02 to remain energetically stable in the Ho11Ge10-type phase. Measurements of temperature-dependent electrical transport properties revealed that the La doping indeed enhanced the electrical conductivity of Ca10.43(3)La0.57Sb9.69(1) compared to the un-doped Ca11Sb10. However, unlike other rare earth metal (RE)-doped compounds in the Ca11−xRExSb10 (RE = Nd and Sm) system that display semiconducting behavior, the La-doped title compound showed poor metallic electrical properties. The positive values of Seebeck coefficients indicated the p-type character of the title compound despite the successful n-type La doping, and this should be attributed to Sb deficiency.