Ovchinnikov Alexander, Darone Gregory M, Saparov Bayrammurad, Bobev Svilen
Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716, USA.
Charter School of Wilmington, Wilmington, DE 19807, USA.
Materials (Basel). 2018 Oct 31;11(11):2146. doi: 10.3390/ma11112146.
Investigation of the quaternary system, Ca⁻Eu⁻Cd⁻Sb, led to a discovery of the new solid solutions, CaEuCd₂Sb₂, with the CaAl₂Si₂ structure type ( ≈ 0.3⁻0.9, 5, 3 ¯ 1, = 4.6632(5)⁻4.6934(3) Å, = 7.630(1)⁻7.7062(7) Å), CaEuCdSb₂ with the Yb₂CdSb₂ type ( ≈ 0.6, 20, 2₁, = 4.646(2) Å, = 17.733(7) Å, = 7.283(3) Å), and EuCaCd₆Sb with the SrCd₆Sb type ( ≈ 1, 58, 2/, = 32.407(4) Å, = 4.7248(5) Å, = 12.377(1) Å, = 109.96(1)°). Systematic crystallographic studies of the CaEuCd₂Sb₂ series indicated expansion of the unit cell upon an increase in the Eu content, in accordance with a larger ionic radius of Eu vs. Ca. The CaEuCdSb₂ composition with ≈ 0.6 adopts the non-centrosymmetric space group, 2₁, although the parent ternary phase, Ca₂CdSb₂, crystallizes in the centrosymmetric space group, . Two non-equivalent Ca sites in the layered crystal structure of CaEuCdSb₂ get unevenly occupied by Eu, with a preference for the interlayer position, which offers a larger available volume. Similar size-driven preferred occupation is observed in the EuCaCd₆Sb solid solution with ≈ 1.
对四元体系Ca⁻Eu⁻Cd⁻Sb的研究发现了新的固溶体CaEuCd₂Sb₂，其具有CaAl₂Si₂结构类型（≈0.3⁻0.9，5，3 ¯ 1，= 4.6632(5)⁻4.6934(3) Å，= 7.630(1)⁻7.7062(7) Å）、具有Yb₂CdSb₂结构类型的CaEuCdSb₂（≈0.6，20，2₁，= 4.646(2) Å，= 17.733(7) Å，= 7.283(3) Å）以及具有SrCd₆Sb结构类型的EuCaCd₆Sb（≈1，58，2/，= 32.407(4) Å，= 4.7248(5) Å，= 12.377(1) Å，= 109.96(1)°）。对CaEuCd₂Sb₂系列进行的系统晶体学研究表明，随着Eu含量的增加，晶胞会膨胀，这与Eu相对于Ca更大的离子半径一致。CaEuCdSb₂组成（≈0.6）采用非中心对称空间群2₁，尽管其母体三元相Ca₂CdSb₂结晶于中心对称空间群。在CaEuCdSb₂的层状晶体结构中，两个不等价的Ca位点被Eu不均匀占据，Eu更倾向于占据层间位置，因为该位置有更大的可用体积。在EuCaCd₆Sb固溶体（≈1）中也观察到了类似的由尺寸驱动的优先占据情况。
