The combination of Pb(2+) cations with lone-pair electrons and F(-) anions with the largest electronegativity into the carbonate generates a new nonlinear optical material, CsPbCO3F, with the largest powder second-harmonic generation (SHG) response among carbonates of about 13.4 times that of KDP (KH2PO4), and transparency over the near-UV to middle-IR region. The optical characterization of the compound indicates that it is phase matchable. Its crystal structure exhibits the stacking of [CsF]∞ and [Pb(CO3)]∞ layers, and the coplanar alignment of [CO3] triangles which are oriented in the same direction. Yet the Pb(2+) cation has an inert or nonstereoactive lone-pair, as indicated by its more spherical shape. Theoretical calculations confirm that the extremely large SHG efficiency indeed originates from enhancement via p-π interaction between Pb(2+) and [CO3](2-) within the [Pb(CO3)] layers.