A single-crystal X-ray diffraction analysis has been performed on KDyP4O12 synthesized by a flux method. The new compound crystallizes at room temperature in the monoclinic space group C2/c with unit cell parameters: a=7.812(2) , b=12.318(3) , c=10.441(2) , β=111.09(2)°, V=937.42(4) 3 and Dcal=3.66 g cm−3 for Z=4. A full-matrix least square refinement gave R1=0.022, wR2=0.04 for 2421 independent reflections (I>2σ(I)) refined with 84 parameters. The structure is built up from P4O124− cyclotetraphosphate anions linked by DyO8 polyhedra to form a three-dimensional framework, which delimits intersecting oxygen tunnels in which the K+ ions are located. The atomic arrangement can be described as a succession of layers extending along the [010] direction. The P4O124− ring anion is centrosymmetrical is connected by irregularly shaped KO10 polyhedra to form a layer structure parallel to (001). Dysprosium and potassium are surrounded by eight and ten oxygen atoms respectively. Samples have been examined by impedance and infrared spectroscopy techniques. The reported IR absorption investigation, recorded at room temperature in the frequency range 200–4000 cm−1, shows some bands characteristic of cyclotetraphosphates. The electrical conductivity of KDyP4O12 has subsequently been measured as a function of temperature, it represents a significant ionic conductivity and activation energy (σ=2.15×10−4 Ω−1cm−1 at 453 K and Ea=0.387 eV) corresponding to the mobility of the K+ cations located within tunnels.< Réduire