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Synthesis, Characterization and Optical Properties of NH 4Dy(PO3)4. -Polycrystalline NH 4Dy(PO3)4 is grown from a mixture of Dy2O3 and excess (NH4)2HPO4 acting as reactive flux (350 C, 2 d) and further transformed to Dy(PO 3)3 by calcination (800 C, 2 h). NH4Dy(PO3)4 crystallizes in the monoclinic space group P21/n and is isotopic with NH 4Ce(PO3)4 and RbHo(PO3)4. The emission spectra show the characteristic emission bands of Dy 3+ in both compounds, before and after calcination. The integrated emission intensity ratios of the yellow to blue transitions and the chromaticity properties are determined from emission spectra. The decay curves are found to be double-exponential. The non-exponential behavior of the decay rates is related to the resonant energy transfer as well as cross-relaxation between the donor and acceptor Dy 3+ ions. The results reveal that NH 4Dy(PO3)4 is promising for white light generation and Dy(PO 3)3 is a potential candidate for field emission display (FED) and plasma display panel (PDP) devices. -(CHEMINGUI, S.; FERHI*, M.; HORCHANI-NAIFER, K.; FERID, M.; J. Solid State Chem. 217 (2014) 99-104, http://dx.