Cation Nonstoichiometry of La2CoO4
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In this thesis, the cation nonstoichiometry and composition range of the La2CoO4 phase in the La-Co-O system were investigated and compared to earlier studies. There is a controversy over the cation nonstoichiometry of La2COO4, and the La/Co ratio has been reported to be both 1.83 and 2 in different papers. The objective of this work is to clarify the cation nonstoichiometry of La2CoO4.Two samples were prepared with different La:Co ratio, one La-rich and one Co-rich sample. The samples were synthesised using the nitrate freeze-drying method before being heat treated at the temperatures 900°C and 1100°C. In addition, low-temperature oxidation was performed. The La/Co ratios were investigated using XRD, EPMA-WDS and TEM-EDS.Heat treatment of the La-rich and Co-rich samples at 1100°C resulted in samples containing the La2CoO4 phase with the average La/Co ratio of 2.0550±0.0158 and 1.997±0.0056, respectively. This indicated that the cation stoichiometry lies close to the stoichiometric value, but showed evidence that the compound might have a small composition range. The results of low-temperature oxidation of these samples did not show any evidence of any changes which indicated that a La-deﬁcient La2CoO4 phase was formed. Changes in lattice parameters were observed, which was consistent with earlier studies indicating that the oxidation leads to oxygen excess in the compound, giving the formula La2CoO4+δ.Heat treatment of the samples at 900°C resulted in the same three-phase composition containingLa2CoO4. The slow reaction rate led to the samples not being fully equilibrated which made it difﬁcult to draw any conclusions on cation nonstoichiometry of La2CoO4 from the data collected.