KEu(MoO₄)₂: Polymorphism, Structures, and Luminescent Properties

In this paper, with the example of two different polymorphs of KEu(MoO 4 ) 2, the influence of the ordering of the A -cations on the luminescent properties in scheelite related compounds ( A ′, A ″) n [( B ′, B ″)O 4 ] m is investigated. The polymorphs were synthesized using a solid state method. The study confirmed the existence of only two polymorphic forms at annealing temperature range 923–1203 K and ambient pressure: a low temperature anorthic α-phase and a monoclinic high temperature β-phase with an incommensurately modulated structure. The structures of both polymorphs were solved using transmission electron microscopy and refined from synchrotron powder X-ray diffraction data. The monoclinic β-KEu(MoO 4 ) 2 has a (3+1)-dimensional incommensurately modulated structure (superspace group I 2 /b (αβ0)00, a = 5.52645(4) Å, b = 5.28277(4) Å, c = 11.73797(8) Å, γ = 91.2189(4) o, q = 0.56821(2) a *–0.12388(3) b *), whereas the anorthic α-phase is (3+1)-dimensional commensurately modulated (superspace group I 1̅(αβγ)0, a = 5.58727(22) Å, b = 5.29188(18)Å, c = 11.7120(4) Å, α = 90.485(3) o, β = 88.074(3) o, γ = 91.0270(23) o, q = 1/2 a * + 1/2 c *). In both cases the modulation arises due to Eu/K cation ordering at the A site: the formation of a 2-dimensional Eu 3+ network is characteristic for the α-phase, while a 3-dimensional Eu 3+ -framework is observed for the β- phase structure. The luminescent properties of KEu(MoO 4 ) 2 samples prepared under different annealing conditions were measured, and the relation between their optical properties and their structures is discussed.