Polymorphism of Eu8In3 and the Solid Solution (CaxEu1–x)8In3

Abstract

AbstractIn the binary intermetallic system Eu/In, a new compound with the composition Eu8In3 was observed and the crystal structures of two polymorphs determined by single‐crystal X‐ray diffraction. The low‐temperature β‐modification crystallizes in the trigonal crystal system with the Yb8In3 structure type (R$\bar3$c (No. 167), a = 957.7(1) pm, c = 5674(1) pm, Z = 12, R(F) = 0.020, wR(F2) = 0.038), the high‐temperature α‐phase triclinic (P$\bar1$ (No. 2), a = 992.67(6) pm, b = 998.08(4) pm, c = 1007.89(5) pm, α = 99.140(4)°, β = 101.172(5)°, γ = 119.452(6)°, Z = 2, R(F) = 0.035, wR(F2) = 0.080) in the Ca8In3 structure type. The densities (ρtriclinic = 6.36 g·cm–1; ρtrigonal = 6.90 g·cm–1) as well as the synthesis conditions are compatible with the assignment as high‐ and low‐temperature phase. The two polymorphs can be rationalized as polytypes of close packings of In with occupation of all octahedral holes and 5/6 of the tetrahedral voids. In this description, indium atoms realize the motif of a double‐hexagonal stacking for the low‐temperature β‐modification and a cubic stacking for the high‐temperature α‐phase. Alternatively, both structures can be described by the occurring coordination polyhedra (tetrahedra for europium, cubes, icosahedra and cubicosahedra surrounding indium). α‐Eu8In3 is versatile to europium substitution by calcium, with little preference for either europium or calcium on the sites exhibiting the highest degree of distortion from tetrahedral shape (Ca4Eu4In3: P$\bar1$, a = 975.2(2) pm, b = 987.1(2) pm, c = 992.8(2) pm, α = 99.12(3)°, β = 101.06(3)°, γ = 119.59(3)°, Z = 2, R(F) = 0.023, wR(F2) = 0.044).