Single crystals of the trihydrated praseodymium cyclotriphosphate PrP3O9 center dot 3H(2)O were grown for the first time, using a classical method of aqueous chemistry and characterized by single crystal X-ray diffraction. PrP309.3HO is isostructural with LnP(3)O(9)center dot 3H(2)O (Ln La, Ce and Nd). It crystallizes in the hexagonal system with space group P-6(-) (C-3h(1)), with lattice parameters: a = 6.7677(4) angstrom, b = 6.7677(4) angstrom, c = 6.0501(4) angstrom, V = 239.98(3) angstrom(3), Z = 1 and D-x = 2.988 g cm(-3). The crystal structure is resolved for the first time, with final R(F-2) = 0.0175 and R-w(F-2) = 0.0417 for 396 independent reflections (F-0(2) >= 2 sigma(F-0(2))). The P3O93- cyclic anions have a plane configuration with C-3h symmetry. The nearest neighbours of the rare earth ion are six oxygen atoms belonging to the P3O93- anions. Pr3+ ions occupy sites with C-3h symmetry. The energies of the crystal vibrational modes are obtained from infrared (IR) and Raman spectra. Optical absorption measurements and emission spectra under selective excitation in the Pr3+ (4f) P-3(J) (J = 0, 1, 2) levels, at room and liquid-helium temperatures, are reported. The observed fluorescence originates mainly from P-3(0) with a decay time on the order of 10 ns, regardless of temperature. (C) 2006 Elsevier Ltd. All rights reserved.