The combination of Pb2+ 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](infinity) and [Pb(CO3)](infinity), layers, and the coplanar alignment of [CO3] triangles which are oriented in the same direction. Yet the Pb2+ 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-pi interaction between Pb2+. and [CO3](2-) within the [Pb(CO3)] layers.