Centrosymmetric Rb2Mg3(SO4)4 and non-centrosymmetric Cs2Mg3(SO4)4 with a phase-matching nonlinear optical response

Non-centrosymmetric structure is the prerequisite of a nonlinear optical (NLO) material. However, most inorganic compounds tend to crystallize in centrosymmetric structures. Here, two new sulfates, Rb2Mg3(SO4)4 (1) and Cs2Mg3(SO4)4 (2), were successfully prepared by high-temperature solution method. Due to cationic size difference, 1 and 2 crystallize in centrosymmetric P21/c and chiral P212121, respectively. As a result, only 2 is NLO-active and exhibits a phase-matching second-harmonic generation response. Furthermore, diffuse reflectance spectra show that both 1 and 2 have short cutoff edges below 200 nm. In addition, they are congruently melting compounds with melting points higher than 780 °C. Calculations of dipole moment are carried out to explain the origin of NLO response. Cation-tuned structural strategies will shed a useful light on the development of new NLO materials.