Crystal structure of Ca2Na2(CO3)3 (shortite)

Abstract

Ca2Na2(CO3)3 crystallizes in the orthorhombic unit cell a = 4.947(1) ?, b = 11.032(2) ?, and c = 7.108(1) ? at 25 ?C with two formula weights in space-group Amm2. The structure has been redetermined, corrected, and refined to Rw = 0.025, R = 0.020 using 684 'observed' x-ray reflections from a single crystal. Corrections were made for absorption and isotropic extinction. In the extinction refinements, r refined to 0.00017(1) cm. The structure consists of Ca2NaCO3 layers interleaved with Na(CO3)2 layers. The Ca ion is coordinated strongly to nine oxygens, including three CO3 edges, with Ca. . . O distances varying from 2.401(2) ? to 2.576(2) ?. One Na ion is coordinated strongly to eight oxygens, including two CO3 edges, with Na. . . O distances from 2.429(2) ? to 2.605(1) ?. The other Na ion is coordinated strongly to six oxygens, including one CO3 edge, at 2.296(1) A to 2.414 A, and weakly to a seventh at 3.050(3) ?. One CO3 group is coordinated to seven cations, the other is coordinated to eight. The CO3 groups have seemingly maximized their edge sharing with Ca ions rather than Na ions.