SOLID SOLUBILITY IN THE (Ln,Ln')O(F,Cl,Br) PHOSPHOR MATERIALS

J. Hölsä1, K. Koski1, R.-J. Lamminmäki1, M. Lastusaari1, H. Rahiala2, E. Säilynoja3, J. Valkonen4, and J. Viljanen1

1 University of Turku, Department of Chemistry, FIN-20014 Turku, Finland,
2 Abo Akademi University, Department of Physical Chemistry, FIN-20500 Turku, Finland,
3 Turku Centre for Biomaterials, Tykistökatu 4, FIN-20520 Turku, Finland,
4 University of Jyväskylä, Department of Chemistry, P.O. Box 35, FIN-40351 Jyväskylä, Finland.

The lanthanide oxyhalides, LnOX (X=F, Cl, Br, and I) have proven to be stable and efficient phosphors in several industrial applications [1]. The extraordinary stability of these luminescent materials is based on the structurally rigid unit, the (LnO)+ complex cation which, in addition to yielding stability to the phosphor, largely determines the luminescent properties of the material [2]. The halide - as the anions in general in the lanthanide oxycompounds - plays only a minor role in luminescence properties.

The structure of the lanthanide oxyhalides is composed of alternating layers - most often perpendicular to the c-axis - of two dimensional networks of the complex cations and the anions [3]. There are, however, two individual types of the (LnO)+ complex cation: the trigonal or tetragonal systems resulting from either a three or two dimensional network of the complex cations, respectively. In the lanthanide oxyhalide series the compounds with a small cation or anion size tend to form the hexagonal (trigonal) structures, since only the oxyfluorides and heavier (smaller) oxychlorides (ErOCl-LuOCl) have a stable hexagonal form with R-3m (Z=6) as the space group [4]. All the other lanthanide oxyhalides crystallize in the tetragonal system with space group P4/nmm (Z=2) [5].

For practical applications the host lattice of the luminescent material is modified by the inclusion of the luminescent centre - activator - which should have a random and uniform distribution in the host lattice to ensure the efficient luminescence. The luminescent properties of the phosphor as the absorption efficiency can also be modified to a certain extent by the inclusion of other ions in the lattice. The solid solubility of the species embedded into the host lattice is of outmost importance since defects and non-stoichiometry result usually in a decrease in the luminescence efficiency.

In this work both the anion and cation solubility in the lanthanide oxyhalide series has been studied by X-ray powder diffraction analysis followed by the Rietveld refinement of the diffraction data. The bond valence calculations [6] were used to derive additional information on the stability of the compounds and phases obtained. The results show that the cation solid solubility in the oxychloride series exists between lanthanum and gadolinium in all proportions although the stability of the phases in the middle of the series decreases according to the bond valence calculations [7]. The formation of a species of lower symmetry in the (La,Gd)OCl series has been subsequently confirmed by UV-excited photoluminescence measurements [8]. In spite of only a slight increase in the difference in the size of the host cation, the solid solubility in the (La,Y)OCl system breaks down to form separate phases with either a lanthanum or yttrium rich composition.

The solid solubility in the anion substituted systems has been studied more extensively [9, 10]. The results show that there exists practically no solid solubility between the fluoride and chloride in good accordance with the Vegard's law [11] which states in a phenomenological way that the difference in the ionic radii between the ions constituting the solid solution should not exceed ca. 15 per cent. This limit is exceeded in the LaO(F,Cl) series and no solid solubility exists. On the other hand, the results were quite surprising since no stabilization of the tetragonal, non-stoichiometric form, LaO1-xF1+2x, in the oxyfluoride system by the chloride anion could be observed although the LaO1-xF1+2x structures are quite similar to the tetragonal LnOCl structure, i.e. they have the same space group.

In contrast to the LaO(F,Cl) system, the solid solubility in the LaO(Cl,Br) series extends throughout the whole composition range. The structural parameters evolve rather smoothly across the series indicating complete solid solubility. The Global Instability Index (GII) representing the stability of the compounds has - as in the (La,Gd)OCl series - a local minimum in the middle of the series but indicates also the inherent instability of the oxychloride phase vis-a-vis the oxybromide one. The results for the LaO(Cl,Br) series agree well with the prediction by the Vegard's rule since the complete solid solubility would be expected due to the small difference in the ionic radii between the chloride and bromide anions.

The Rietveld profile refinements based on the X-ray powder diffraction data on the LnOX system showed that the solid solubility is quite well restricted only to the systems where the Vegard's rule is valid, i.e. the difference in the ionic radii of the component ions does not exceed ca. 15 per cent. There exist, however, according to the bond valence calculations local instability regions in the solid solutions.

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