Preparation and complete struitural characterization by (\mathrm{X})-ray diffraction, IR absorption. DTA, and GTA are given for a new lithium cyclohexaphosphate hydrate: (\mathrm{Li}{6} \mathrm{P}{6} \mathrm{O}{18} \cdot 9 \mathrm{H}{2} \mathrm{O}). This compound is monoclinic, (\mathrm{C} 2 / c), with (a=15.429(8), b=11.794(5)), (c=14.369(8) \AA, \beta=115.95(5), V=2351(5) \AA^{3}, Z=4, R=) (0.049, R_{\mathrm{w}}=0.052) for 1959 independent reflections with (I>3 \sigma(I)). Layers of (\mathrm{P}{6} \mathrm{O}{18}) rings, with the (\overline{1}) internal symmetry, are stacked along the (\mathrm{c}) axis. Three Li coordination polyhedra (tetrahedra, pseudosquare or pseudosquare pyramid) share common edges as to form (\mathrm{Li}{3} \mathrm{O}{8}) or (\mathrm{Li}{3} \mathrm{O}{9}) groups. These cationic groups interconnect the rings in a three-dimensional way. Among the nine water molecules, five participate in the different lithium environments. The other ones are nonbonded water molecules and are disordered over several sites lying between the superimposed rings of two successive layers. O 1994 Academic Press, Inc.