Na3AMg7(PO4)6 (A = K, Rb and Cs): Structures, properties and theoretical studies of alkali metal magnesium orthophosphates

Abstract

• Na 3 KMg 7 (PO 4 ) 6 and Na 3 CsMg 7 (PO 4 ) 6 were determined for the first time. • Theoretical calculations were performed to understand the electronic structures. • The title compounds melt congruently and exhibit a wide transparent range with the ultraviolet cut-off edge below 210 nm. Abstract Two new alkali metal magnesium orthophosphates, Na 3 KMg 7 (PO 4 ) 6 and Na 3 CsMg 7 (PO 4 ) 6 , have been successfully synthesized using the high-temperature solid state method. The crystal structure has been determined by single crystal X-ray diffraction. Besides, Na 3 RbMg 7 (PO 4 ) 6 , a known compound, has also synthesized and characterized for comparison. The title compounds crystallize in the same space group, C 2 /c (No. 15). They are isostructural and feature a three-dimensional network composed of isolated PO 4 tetrahedra, AO x ( A = K, Rb and Cs, x = 8 and 12), and MgO x ( x = 5 and 6) distorted polyhedra. The thermal properties, infrared spectroscopies, and UV–Vis–NIR diffuse reflectance spectrum have been performed for the first time. Meanwhile, first-principle theoretical studies were carried out to understand the relationship between the electronic structures and optical properties. The title compounds melt congruently and exhibit a wide transparent range with the ultraviolet (UV) cut-off edge below 210 nm, which makes Na 3 AMg 7 (PO 4 ) 6 ( A = K, Rb, Cs) as potential UV optical materials. Na 3 AMg 7 (PO 4 ) 6 ( A = K, Rb, Cs) are isostructural and the structures have 3D framework composed of the isolated PO 4 and MgOx ( x = 5 and 6), then the K, Rb, Cs and Na cations locate in the tunnels to form the final structure, respectively.