Crystals of KDyP4O12 have been grown by the flux technique and characterized by single-crystal X-ray diffraction. KDyP4O12 crystallizes in the monoclinic C2/c space group with lattice parameters: a = 7.8158(3), b = 12.3401(5), c = 10.4382(3) Angstrom, beta = 111.053degrees(2), V = 939.6(4) Angstrom(3), Z = 4. The crystal structure has been refined yielding a final R(F-2) = 0.034 and R-w(F-2) = 0.082 for 902 independent reflections (F-o(2) greater than or equal to 2sigma(F-o(2))). The structure of KDYP4O12 consists of DyO8 polyhedra and cyclotetraphosphate P4O12 groups sharing oxygen atoms to form a three-dimensional framework, delimiting intersecting tunnels in which the potassium ion is located. Each K+ ion is bonded to 10 oxygen atoms. The energies of the vibrational modes of the crystal were obtained from measurements of the infrared and Raman spectra. (C) 2002 Elsevier Science Ltd. All rights reserved.