Formation and crystal structure of a new double carbonate phase between Na and Cd

We report on the formation of a new phase Na₂Cd(CO₃)₂ (1) and its crystal structure. 1 belongs to the rare examples of double carbonates composed of d-block element metal elements. It is formed in a narrow window of temperature from 230 °C to slightly higher than 350 °C, because of which synthesis of 1 as a pure phase is practically impossible. We attempted to elucidate the crystal structure of 1 based on a powder X-ray diffraction pattern on a mixture between 1 and CdO as the major phase. Comparing with known structures of double carbonates of alkaline earth metal elements and introducing structural distortions to them, we identified the mother structure of 1 to be an orthorhombic variant of the Fairchildite structure reported for K₂Ca(CO₃)_2. Rietveld refinements with a model in which the two symmetry-independent carbonate ions are disordered resulted in a good agreement with the observed data, which leads to the composition of 1 to be Na₂Cd(CO₃)₂.