Synthesis and characterisation of novel bismuthates
The synthesis of phases in the Na-Bi-Cu-O system was investigated using the technique of hydroxide flux growth. Highly oxidised bismuthates (NaBiO3.2H2O, Na3BiO4) and cuprates (NaCuO2) were prepared, but these compounds were poorly crystalline and multiphase. The synthesis of a copper doped form of Na3BiO4 (Na2.8(2)BiCu0.1(1)O4) was also achieved, but characterisation of this phase was limited, due to the poorly crystalline nature of the compound and the presence of impurity phases. The synthesis of a new phase, NaBiO2.5, was accomplished by the thermal decomposition of NaBiO3.2H2O in either oxygen or argon about 400°C. As a result of this method of synthesis, NaBiO2.5 had a very small crystal size (<100nm), which hampered the structure determination of this phase. The more crystalline and isostructural compound, Pb2Bi6O11, was used as a model for the structure of NaBiO2.5. Pb2Bi6O11 was found to have a monoclinic unit cell with a=3.9110(2)A, b=5.5499(2)A, c=3.7671(2)A, and β=102.513(4)°. Analysis of X-ray and electron diffraction data showed that Pb2Bi6O11 had an incommensurate supercell which could be described by the vector q*=0.3141a* - 0.1937c*. The accuracy of the structure model obtained from analysis of neutron diffraction data was severely limited by the presence of this supercell. The NaBiO2.5 phase had a slightly smaller unit cell (a=3.8381(4)A, b=5.5468(5)A, c=3.7488(4)A, and β=104.407(9)° and a similar type of supercell, q*=0.3121a* - 0.1934c*. Conductivity measurements revealed that Pb2Bi6O11 was a poor electronic conductor in the temperature range of 300-500°C. NaBiO2.5 was shown to be an n-type semiconductor in the temperature region of 230-380°C. The new phase, β-Na3BiO4, was synthesised by heating together Na2O2 and Bi2O3 at 380°C in oxygen overnight. β-Na3BiO4 had a face centred cubic unit cell of length 4.7398A. This phase can be described as a rocksalt-type structure with the sodium and bismuth atoms randomly disordered on the cation site.