Change of structure of ferrous oxide at low temperature

The structure of ferrous oxide has been studied between room temperature and liquid-air temperature (90 ° K.) by X-ray powder photography.The results support the conclusion of Jette & Foote that at room temperature the structure is of the NaCl-type with a small and variable number of the Fe ~+ sites vacant, so that the stoichiometric composition :FeO is never attained.At low temperature (below 200 ° K.) the structure is rhombohedral, the new structure cell being derived from the originally cubic cell by an extension along one of the triad-axis directions.This is a change of the second order and appears to be directly connected with the transition from the paramagnetie to the antiferromagnetic state on cooling through 198 ° K.The magnitude of the change, as measured at liquid-air temperature, is very sensitive to the concentration of iron in the oxide.Thus, for the oxide of minimum iron content the rhombohedral angle a at 90 ° K. is within one minute of the value (60 °) which corresponds to cubic symmetry, whereas a is 59 ° 23' for the least iron-deficient oxide.It is shown that the powder photograph taken at liquid-air temperature provides a much more sensitive indication of the degree of homogeneity of the sample than that taken at room temperature.