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

Abstract

In this paper, with the example of two different polymorphs of KEu­(MoO₄)₂, the influence of the ordering of the <i>A</i>-cations on the luminescent properties in scheelite related compounds (<i>A</i>′,<i>A</i>″)_<i>n</i>[(<i>B</i>′,<i>B</i>″)­O₄]_<i>m</i> 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(MoO₄)₂ has a (3+1)-dimensional incommensurately modulated structure (superspace group <i>I</i>2<i>/b</i>(αβ0)­00, <i>a</i> = 5.52645(4) Å, <i>b</i> = 5.28277(4) Å, <i>c</i> = 11.73797(8) Å, γ = 91.2189(4)^o, <b>q</b> = 0.56821(2)<b>a</b>*–0.12388­(3)<b>b</b>*), whereas the anorthic α-phase is (3+1)-dimensional commensurately modulated (superspace group <i>I</i>1̅(αβγ)­0, <i>a</i> = 5.58727(22) Å, <i>b</i> = 5.29188(18)­Å, <i>c</i> = 11.7120(4) Å, α = 90.485(3)^o, β = 88.074(3)^o, γ = 91.0270(23)^o, <b>q</b> = 1/2<b>a</b>* + 1/2<b>c</b>*). In both cases the modulation arises due to Eu/K cation ordering at the <i>A</i> site: the formation of a 2-dimensional Eu³⁺ network is characteristic for the α-phase, while a 3-dimensional Eu³⁺-framework is observed for the <i>β-</i>phase structure. The luminescent properties of KEu­(MoO₄)₂ samples prepared under different annealing conditions were measured, and the relation between their optical properties and their structures is discussed