Thermoelectric (TE) materials can be used to transfer the wasted heat from many different heat sources into electricity via the Seebeck effect. Among various candidate materials, Zintl phase compounds are one of the most attractive choices for TE application due to their intrinsically complex crystal structures and semiconducting characteristics. Two title Zintl compounds Ca₉Zn_4.47(1)Sb₉ and Ca_8.15(2)Yb_0.85Zn_4.46(1)Sb₉ have been synthesized by using the high temperature reaction and characterized by PXRD and SXRD. Both compounds crystallized in the orthorhombic Ca₉Mn₄Bi₉-type structure (space group Pbam, Pearson code oP48). According to our SXRD refinement results, the unit cells of two isotypic title compounds include total 13 crystallographically independent atomic sites including five cationic sites, five Sb sites, and three Zn sites. In particular, one of the Zn site (Wyckoff 4g), which acts as a bridge connecting the chains of dimerized tetrahedral ∞[Zn₄Sb₉] moieties, shows some vacancies. Since it is known that the partial occupation of this bridging Zn significantly affects the zT value, we attempted to fine tune the Zn amount to adjust the hole carrier concentration and introduce the mixed cations of Ca²⁺/Yb²⁺ to increase disordering in this system. A series of theoretical calculations using TB-LMTO method were also performed using several different structural models with different chemical compositions to understand the correlation between the given composition and the electronic structure. Detailed electronic energy of each model is further investigated, and DOS, COHP and ELF were also interrogated.