Effects of anion replacement on the physical properties of CaCd2X2 (X = P, As, Sb, Bi)

Abstract

The transport properties of the Cd-based Zintl phase have been reported previously, so we investigated the transport properties of CaCd2Sb2 and similar compounds with the same structure in the present study. We determined the opto-electronic properties using the full potential augmented plane wave method implemented in the WIEN2k code and calculated the thermoelectric properties using the interface BoltzTraP code. We relaxed the internal and external structural parameters for the compounds and obtained the ground state energies for all compounds using the Perdew–Burke–Ernzerhoff generalized gradient approximation functional. The lattice parameters increased with the replacement of anions from P to Bi, and the bulk modulus increased in reverse order with these replacements. The electronic band structures calculated using the Tran and Blaha modified Becke-Johnson approach indicated direct band gaps for CaCd2P2 and CaCd2As2 but an indirect band gap for CaCd2Sb2, and CaCd2Bi2 exhibited a metallic nature. The band gap value calculated for CaCd2Sb2 agreed well with previous experimental and theoretical data. We also found that anion replacement affected the optical properties, where all of the compounds exhibited high optical conduction and light absorption in the energy range from 2.5 to 5.0 eV. The behavior of the electronic/thermal conductivity was linear with temperature and all of the compounds were confirmed as n-type based on the Seebeck coefficient. For all of the compounds, the highest figure of merit occurred at 450 K, thereby indicating that these compounds are favorable for thermoelectric applications.