A novel lithium zinc niobium oxide LiZnNb4O11.5 (LZNO) has been found in the Nb-rich part of Li2O–ZnO–Nb2O5 system. LZNO, with an original a-PbO2 related structure, has been synthesized by the routine ceramic technique and characterized by X-ray diffraction and transmission electron microscopy (TEM). Reflections belonging to the LZNO phase, observed in X-ray powder diffraction (XRPD) and electron diffraction, have been indexed as monoclinic with unit cell parameters a=17.8358(9) A˚ , b=15.2924(7) A˚ , c=5.0363(3)A˚ and g=96.607(5)1 or as a-PbO2-like with lattice constants a=4.72420(3) A˚ , b=5.72780(3) A˚ , c=5.03320(3) A˚ , g=90.048(16)1 and modulation vector q=0.3a*+1.1b* indicating a commensurately modulated a-PbO2 related structure. The monoclinic cell is a supercell related to the latter. Using synchrotron powder diffraction data, the structure has been solved and refined as a commensurate modulation (superspace group P1121/n(ab0)00) as well as a supercell (space group P21/b). The superspace description allows us to consider the LZNO structure as a member of the proposed a-PbO2-Z (3+1)D structure type, which unifies both incommensurately and commensurately modulated structures. HRTEM reveals several types of defects in LZNO and structural models for these defects are proposed. Two new phases in Li2O–ZnO–Nb2O5 system are predicted on the basis of this detailed HRTEM analysis.