A new deep-ultraviolet nonlinear optical material, RbMgCO3F, has been synthesized and characterized. The achiral nonpolar acentric material is second harmonic generation (SHG) active at both 1064 and 532 nm, with efficiencies of 160 x alpha-SiO2 and 0.6 x beta-BaB2O4, respectively, and exhibits a short UV cutoff, below 190 nm. RbMgCO3F possesses a three-dimensional structure of corner-shared Mg(CO3)(2)F-2 polyhedra. Unlike other acentric carbonate fluorides, in this example, the inclusion of Mg2+ creates pentagonal channels where the Rb+ resides. Our electronic structure calculations reveal that the denticity of the carbonate linkage, monodentate or bidendate, to the divalent cation is a useful parameter for tuning the transparency window and achieving the sizable SHG response.