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대한화학회 제124회 학술발표회 및 총회 Chemical Driving Force for Phase-Transition in the Ca2-xRExCdSb2 (RE = Yb, Eu; 0.11(1) ≤ x ≤1.36(2)) System

2019년 8월 13일 17시 06분 54초
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목 09시 : 20분
Inorganic Chemistry - Oral Presentation of Young Inorganic Chemistry
저자 및
Ki Won Kim, Tae-Soo You*
Department of Chemistry, Chungbuk Natioanl University, Korea

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, both of which were very similar but slightly different homotypic structure types, depending upon the Ca2+/Yb2+ mixed-ratio. On the other hand, the three compounds in the Ca2-xEuxCdSb2 (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-xYbxCdSb2 system can be attributed to the two kinds of stacking sequence of the octahedral [(Ca/Yb)Sb6] building block and the resultant interatomic interactions between two neighboring Ca2+/Yb2+ mixed-sites in two distinctive 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 was conducted, and the resultant DOS, COHP, and ELF diagrams were thoroughly interrogated. In particular, the COHP analysis successfully revealed 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-xYbxMCdSb2 system can be explained by the electronic-factor criterion based on the Q value of each site, whereas that in the Ca2-xEuxCdSb2 system should be elucidated by the size-factor criterion based on 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.