Syntheses, characterization, and theoretical calculation of Rb2Mg3(P2O7)2 polymorphs with deep-ultraviolet cutoff edges

By the combination of the isolated P2O7 dimers and MgO4 tetrahedra, alpha- and beta-Rb2Mg3(P2O7)(2) polymorphs were synthesized by a high-temperature solution method. alpha-Rb2Mg3(P2O7)(2) crystallizes in non-centrosymmetric space group P2(1)2(1)2(1), while beta-Rb2Mg3(P2O7)(2) crystallizes in centrosymmetric P2(1)/c. Both structures contain a three dimensional [Mg3P4O14](2-) anionic framework, while Rb+ cations are in the space. Structure analyses show that the isolated P2O7 dimers can easily adjust their variable configurations and orientations to fit the different coordination environments of the cations, which is conducive to the formation of polymorphs. The phase transformation process from alpha- to beta-Rb2Mg3(P2O7)(2) was further investigated by powder X-ray diffraction and thermal gravimetric/differential scanning calorimetry measurements. In addition, UV-vis-NIR diffusion spectra indicate both materials have deep-ultraviolet cut-off edges (below 190 nm). alpha-Rb2Mg3(P2O7)(2) is second-harmonic generation (SHG)-active and the origin of SHG response was investigated by the SHG density calculations. The first-principle calculations were also carried out to illuminate their structure-property relationships.