View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Institutionelles Repositorium der Leibniz Universität Hannover Dalton Transactions PAPER View Article Online View Journal | View Issue Characterization of powellite-based solid solutions Cite this: Dalton Trans., 2013, 42, 8387 by site-selective time resolved laser fluorescence spectroscopy Moritz Schmidt,*a Stephanie Heck,a Dirk Bosbach,b Steffen Ganschow,c Clemens Waltherd and Thorsten Stumpfa We present a comprehensive study of the solid solution system Ca2(MoO4)2–NaGd(MoO4)2 on the mole- cular scale, by means of site-selective time resolved laser fluorescence spectroscopy (TRLFS). Eu3+ is used as a trace fluorescent probe, homogeneously substituting for Gd3+ in the solid solution crystal structure. Site-selective TRLFS of a series of polycrystalline samples covering the whole composition range of the solid solution series from 10% substitution of Ca2+ to the NaGd end-member reveals it to be homo- geneous throughout the whole range. The trivalent ions are incorporated into the powellite structure in Received 15th January 2013, – Creative Commons Attribution 3.0 Unported Licence. only one coordination environment, which exhibits a very strong ligand metal interaction. Polarization- Accepted 4th April 2013 dependent measurements of a single crystal of NaGd(Eu)(MoO4)2 identify the coordination geometry to DOI: 10.1039/c3dt50146a be of C2v point symmetry. The S4 symmetry of the Ca site within the powellite lattice can be transformed www.rsc.org/dalton into C2v assuming minor motion in the first coordination sphere. 2+ Introduction substituted for the divalent Ca ion charge compensation has been found to proceed via coupled substitution with mono- 10,13,14 Powellite (CaMoO4) is a naturally occurring mineral that forms as valent cations, e.g. alkali metal ions. This mechanism 15 This article is licensed under a an alteration product of molybdenite MoS2 under oxidizing con- has been identified for other solid solutions as well. In other ditions. Powellite has also been shown to form as a second- ary mineral upon corrosion of Mo-bearing high-level nuclear 1 Open Access Article. Published on 04 April 2013. Downloaded 22/10/2015 17:12:27. waste (HLW) borosilicate glasses. During this process radio- nuclides may form thermodynamically stable solid solutions with powellite, and thus remain effectively immobilized after the – initial failure of the technological barrier.1 6 Powellite solid solu- tions have also recently attracted interest from materials scientists – as materials with noteworthy dielectric and optical properties.7 10 Powellite crystallizes in the scheelite structure with the 2+ space group I41/a in which the central Ca ion is coordinated by eight singly-bound molybdate groups (Fig. 1). The point symmetry of the Ca site in this structure has been reported as 11 12 D2d as well as S4 in synthetic samples. The structure is ideally suited for the formation of solid solutions, as it exhibits significant compositional flexibility.3 When trivalent ions are aInstitute for Nuclear Waste Disposal, Karlsruhe Institute of Technology, Karlsruhe, Germany. E-mail: [email protected], [email protected], [email protected] Fig. 1 Top: Coordination of Ca2+ in the Ca site of the powellite lattice, blue: Ca, bInstitut für Energie- und Klimaforschung (IEK-6), Forschungszentrum Jülich, Jülich, red: O, pink: Mo (left), and unit cell of the powellite structure, blue: Ca, red: O, Germany. E-mail: [email protected] white: Mo (right). Ca, Na, and Ln are coordinated by eight O belonging to eight c 2− Leibniz-Institut für Kristallzüchtung, Berlin, Germany. unique MoO4 -tetrahedra. Bottom: SEM micrographs of the crystal mor- E-mail: steff[email protected] phology of the series members with 10 and 100% substitution, GdPow10G and dInstitut für Radioökologie und Strahlenschutz, Universität Hannover, Hannover, GdPow100G, respectively, are shown in the bottom of the figure. The crystallites Germany. E-mail: [email protected] are approx. 10 μm in size. This journal is © The Royal Society of Chemistry 2013 Dalton Trans., 2013, 42, 8387–8393 | 8387 View Article Online Paper Dalton Transactions Ln3+-molybdates defect structures have been identified where In aqueous systems, the fluorescence of Eu3+ can only be two trivalent cations substitute for three divalent calcium ions.16 effectively quenched by energy transfer to O–H-vibrational For either application, as a dielectric material or for the modes, due to the magnitude of the energy gap. Under this con- immobilization of nuclear waste, the efficacy of the material dition, the number of water molecules coordinating Eu3+ can be will depend on its stability. Material stability is again related determined through Horrocks’ equation.32,33 In the presence of to material properties such as crystallinity, phase purity, and transitionmetalsadifferent path for radiationless deexcitation distribution of the luminescent probe,9 or radionuclide, becomes available, and fluorescence lifetimes are no longer a respectively. The stability of a solid solution is to a large simple function of the number of water molecules in the first degree dependent on the stress induced in the lattice by the coordination sphere of Eu3+.Thiseffect has been well documen- substitution.17 Fully understanding the structural effects of a ted for systems containing Fe, as well as other transition – substitution mechanism will require analytical techniques that metals34 36 and can be described theoretically as direct metal- are capable of probing local structural features in great detail. to-metal energy transfer.37,38 Similar effects were observed for The overall structure of the solid solution is commonly accessi- Eu3+ fluorescence emission lifetimes in the presence of Mo in ble by X-ray diffraction (XRD) techniques; however, XRD will this study (see the Results and discussion section). not easily reveal local structural effects around the guest ion. There are some previous studies on the luminescence of The local coordination environment of guest ions in a solid rare earths incorporated into powellite; however, most studies solution can be characterized by time-resolved laser fluo- focus on members of the series with low concentrations of the – rescence spectroscopy (TRLFS).15,18 22 For this study we guest ion, and a comprehensive study of a complete solid solu- applied site-selective Eu3+ TRLFS at low temperatures (T < tion series has to the best of our knowledge not been pub- 20 K), which is particularly efficient for the speciation and lished thus far. There is also no detailed investigation of the – characterization of multi-species systems.18,19,23 26 The tech- structural effects of the substitution on the powellite lattice. nique allows determination of the different species in a system Thomas et al. studied solid solutions with the general compo- 7 5 3+ – Creative Commons Attribution 3.0 Unported Licence. by excitation from the F0 ground state to the D0 state. In this sition CaGd1−xNbMoO8: xEu for x =530%. They find emis- 5 7 transition both levels are non-degenerate and thus only one sion spectra with strongly enhanced ( D0 → F2)-transitions 5 7 singlet signal is observed for each species. Determination of compared to relatively weak ( D0 → F1)-bands upon UV exci- 5 → 7 the number of non-equivalent species is possible by simple tation. The ( D0 F2)-transitions are clearly enhanced by the counting of observed transitions as long as resolution and line hypersensitive effect, indicating strong coordination. The widths allow an unambiguous separation. In addition, the spectra for series members with 5% to 30% trivalent metal signal position of the F0 band gives a first indication of the content are very similar; however, a detailed structural analysis local environment of the respective Eu3+ species: stronger was not performed as spectra lack the necessary resolution.9 coordination generally results in lower energy transition, i.e. a Mendoza et al. also studied the early members of the solid This article is licensed under a – stronger bathochromic shift of the signal.26 28 Consecutive solution series with up to 7% Ln3+ substitution before and selective excitations of the respective species yield “single after irradiation by Ar and Pb ion beams. Once again the species”-emission spectra and -lifetimes. The emission spectra spectra show a strong hypersensitive effect. The spectra show a Open Access Article. Published on 04 April 2013. Downloaded 22/10/2015 17:12:27. 5 → 7 allow identification of the ions coordination environment in very complex behavior of the ( D0 F0) band, which was fit this site, using the splitting pattern and the relative intensity using nine Gaussian profiles, pointing to a manifold of 5 7 5 7 of the ( D0 → F1)- and ( D0 → F2)-transitions, the so-called different, but similar, coordination environments for the guest hypersensitive effect.29 ion. The spectra after irradiation are simpler, exhibiting fewer 5 7 We can improve the level of detail of the structural charac- species and a red shift of the ( D0 → F0) transition. This terization by making use of the polarization dependence of the suggests a higher degree of order around the trivalent guest 3+ − 4 selection rules for transitions of the Eu fluorescence emis- ion and a stronger coordination by the MoO4 -ligands. sion.30 In single crystals, where orientations are not averaged Here, we present a comprehensive investigation of the solid isotropically, emission