Series of the mixed fluorides with the general composition M1-xNdxF2+x (0.00 less than or equal to x less than or equal to 1.00; M = Sr2+ and Ca2+) were prepared by a vacuum heat treatment of the appropriate mixtures of MF2 (M = Ca and Sr) and NdF3. The products obtained were analyzed by powder X-ray diffraction, which revealed the phase relations in these systems. At the MF2-rich compositions a fluorite-type solid solution was observed in both systems. The typical solid solution limits of NdF3 in the SrF2 and CaF2 lattice are about 40 and 45 mol%, respectively. The incorporation of NdF3 in the fluorite lattice of CaF2 causes an expansion of the unit cell volume. The unit cell volume of the fluorite lattice in Sr1-xNdxF2+x system surprisingly remains constant throughout the solid solution range. The NdF3 (tysonite) type phase separates out beyond the solid solution limit, in both systems. Also, it was observed that about 15 and <10 mol% of CaF2 and SrF2, respectively, could be retained in the NdF3, maintaining the tysonite lattice. No fluorite or tysonite-related ordered phases were observed in these systems. (C) 2003 Elsevier Ltd. All rights reserved.