On the melting behaviour of calcium monoxide under different atmospheres: A laser heating study

The melting behaviour of calcium monoxide has been revisited by quasi-containerless laser heating under controlled atmosphere. The current results suggest that the large discrepancy in the literature data for the melting temperature of CaO is probably linked to the influence of the environmental oxidising/reducing conditions. The CaO solidification point measured in this work is (3222 ± 25) K in an oxidising environment, in agreement with previous research performed under similar conditions. In a reducing atmosphere, the liquid/solid transition occurs at a slightly, but systematically lower, less accurately reproducible temperature, (3192 ± 40) K. These phase transition temperatures are not depending on the external pressure, up to 0.3 MPa. In the latter case, radiance and optical spectroscopy measurements revealed a more abundant formation of colour centres, i.e. a higher concentration of oxygen defects in the material, which can affect the thermodynamic stability of the solid at very high temperatures. A similar behaviour may be expected to occur in multi-component material systems including CaO.