Chemical Driving Force for Phase-Transition in the Ca2-xRExCdSb2 (RE = Yb, Eu; 0.11(1) ≤ x ≤ 1.36(2)) System

A total of nine solid-solution Zintl phases in the Ca2-xRExCdSb2 (RE = Yb, Eu; 0.11(1) <= x <= 1.36(2)) system with two types of cationic mixtures have been synthesized by the Pb metal-flux method, and their crystal structures have been characterized by powder and single-crystal X-ray diffraction (PXRD and SXRD) measurements. In particular, a series of solid-solution Ca2-xYbxCdSb2 (0.43(2) <= x <= 1.36(2)) compounds showed a phase-transition from the Ca2CdSb2-type to the Yb2CdSb2-type structure depending upon the Ca2+/Yb2+ mixed-ratio. These two structure types contained nearly identical anionic structural moieties, but their spatial arrangements were distinctive. On the other hand, the three compounds in the Ca2-xYbxCdSb2 (0.11(1) <= x <= 1.04(2)) system crystallized only in the Yb2CdSb2 type phase regardless of the Eu amounts. The observed phase-transition in the Ca2-xRExCdSb2 system can be attributed to the two kinds of stacking sequence of the octahedral [(Ca/Yb)Sb-6] building block and the resultant interatomic interactions between two neighboring Ca2+/Yb2+ mixed-sites in two distinctive title structure types. Moreover, according to SXRD refinements, two types of mixed-cations of Ca2+/Yb2+ or Ca2+Eu2+ showed noticeable site-preferences between two available atomic sites. To understand the driving force for this phase-transition and the origin of the cationic site-preference, a series of DFT calculations using the TB-LMTO-ASA method were conducted, and the resultant DOS, COHP, and ELF diagrams were thoroughly interrogated. In particular, the COHP analysis successfully supported that the observed phase-transition was triggered by the energetically unfavorable shorter (Ca/Yb)1-(Ca/Yb)1 interaction in the Yb-rich Ca2CdSb2-type phase. Moreover, the cationic site-preference in the Ca2-xYbxCdSb2 system can be interpreted by the electronic-factor criterion based on the Q value of each site, whereas that in the Ca2-xEuxCdSb2 system can be justified by the size-factor criterion on the basis of the size of cationic elements. The chemical compositions and the appearance of selected single-crystals were analyzed by EDS and SEM, and the thermal stability of a sample was also checked by TGA analysis.