Crystal structures of carbonates Cs₂Sr₂(CO₃)₃ and Rb₂Sr₂(CO₃)₃ from powder data

Abstract

Two carbonates Cs 2 Sr 2 (CO 3 ) 3 and Rb 2 Sr 2 (CO 3 ) 3 were synthesized by solid-state reaction at high temperatures, and their crystal structures were determined from X-ray powder diffraction data. Because of the presence of heavy atoms and heavy peak overlapping, proper restrains on light-atom groups were found essential to obtain satisfactory results. The title compounds are isostructural to an early reported compound Cs 2 Ba 2 (CO 3 ) 3 and crystallize in cubic space group I 2 1 3, with unit-cell dimensions a =10.072 64(2) Å for the cesium phase and a =9.855 56(7) Å for the rubidium phase. Unlike most carbonates, the title compounds feature in mutually perpendicular CO 3 atomic groups. Our results also indicate that the global instability index is a more sensitive parameter in evaluating the structure rationality over the agreement factors. Moreover, differential thermal analysis and thermogravimetric analysis measurements reveal that Cs 2 Sr 2 (CO 3 ) 3 and Rb 2 Sr 2 (CO 3 ) 3 are stable in air up to 796 and 880 °C, respectively.