An X-ray diffraction single crystal study of the monoclinic phase of RbLaP4O12, which occurs at room temperature, has been performed with the aim of determining the atomic structure. This compound crystallizes in the space group C2/c with unit cell dimensions a= 7.946(6)Å, b= 12.835(6)Å, c= 10.769(10)Å, â= 110.624(7)°, Z= 4 and V= 1027.9(13)Å3. The refinement converged to R= 0.019 and wR= 0.019 using 6412 independent reflections (I > 2(I)). The P4O12 anion has a twofold symmetry, the La atom is eightfold coordinated. The main characteristic of this atomic arrangement is a stacking of three dimensionally framework delimiting intersecting tunnels in which the Rb+ cations are inserted, linked LaO8 layers parallel to (001) planes connected together by P4O12 groups lying in (010) planes. Infrared and Raman spectrum was investigated at room temperature in the frequencies range, 4001600 cm1, showing some characteristic vibration bands of infinite chain structure of PO4 tetrahedra linked by bridging oxygen.