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

Abstract

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 (23) o , q = 1/2a* + 1/2c*). 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.

New red phosphor Na3EuB8O15 with an open-window tubular structure

“…In most cases, a low doping of Eu 3+ activator leads to a weak luminescence, whereas heavy doping will lead to concentration quenching. For some fully concentrated Eu 3+ compounds, no concentration quenching occurs at all, for example, Eu 2 WO 6 (Qin et al, 2009), KEu(MoO 4 ) 2 (Morozov et al, 2015), K 5 Eu(P 2 O 7 ) 2 (Zhao et al, 2019a) and EuZnB 5 O 10 (Hao et al, 2014). These materials are very likely to suffer from high photon flux during their application.…”

Trigonal double molybdates and tungstates: Ferroelastic phase transitions

“…The succession of structural types with change in the ionic radii of R 3+ and M + , met in double molybdates and tungstates, has been described in a review paper of Klevtsov and Klevtsova [5] .The compounds can be listed in 22 structural types distributed into four basic classes taken as parent types: scheelite CaWO4, wolframite (Fe,Mn)WO4, γ-RbPr(MoO4)2 and glaserite-type (trigonal) KAl(MoO4)2. Recently the incommensurately modulated scheelite-like structure have been found in KNd(MoO4)2 [6] , KSm(MoO4)2 [7] and KEu(MoO4)2 [8] .…”

Single Crystals of KRE(MoO4)2, (RE‐ Ce, Pr) Obtained from Fluorides: Scheelite‐Related Structure and Luminescence

“…Some of them represent a tetragonal structure, namely NaGd(MoO 4 ) 2 , with the space group I4 1 /a . When metals distribution is more complicated the symmetry of the framework lowers toward a monoclinic one or became a modulated one, like KEu(MoO 4 ) 2 and NaEu(MoO 4 ) 2 …”

“…A previous X‐ray single diffraction study has shown that the symmetry of the ARE(MoO 4 ) 2 compounds depends on the A/RE content, temperature and synthetic procedure . The A/RE substitution process is expected to be quite complex leading to incommensurately modulated structures .…”

“…A previous X‐ray single diffraction study has shown that the symmetry of the ARE(MoO 4 ) 2 compounds depends on the A/RE content, temperature and synthetic procedure . The A/RE substitution process is expected to be quite complex leading to incommensurately modulated structures . Because these modifications occur for some potassium‐containing binary molybdates, the X‐ray diffraction technique, including powder diffraction should be done precisely enough to locate first‐, second‐, and third‐order satellites, observed in the X‐ray experiment, that are the first significant characteristic of the incommensurately modulated scheelite‐like structure .…”