Structural relationships between Bi2O2CO3 and β-Bi2O3

The structure of Bi2O2CO3 has been investigated using powder neutron diffraction. Not all observed peaks could be indexed on the previously reported tetragonal unit cell, and a larger orthorhombic cell was adopted (Pna21, a=5.468(1), b=27.32(2), c=5.468(1) Å). The structure refinement was not totally satisfactory due to apparent orientational disorder of the CO32− groups. Bi2O2CO3 of small crystallite size decomposes at about 400°C to β-Bi2O3, which is thermodynamically unstable relative to α-Bi2O3. Structural relationships between Bi2O2CO3 and β-Bi2O3 are suggested as a means of rationalising this phenomenon.