Effect of composition and pressure on phase transitions in Fe_xO at low temperature

We report the results of high-resolution neutron powder diffraction studies of Fe_xO (x = 0.925, 0.94) in the vicinity of the low-temperature antiferromagnetic transition and at pressures up to 8 GPa. Our analysis shows that the P-T phase diagram of Fe_xO is strongly affected by the composition and defect structure of the material. We observe the divergence of critical temperatures of magnetic and structural transitions. In contrast to the stoichiometric antiferromagnet MnO, we find no correlation between the magnitude of the magnetic moment of iron and the degree of the rhombohedral distortion in Fe_xO. We suggest that the defect structure of antiferromagnetic Fe_xO significantly influences the temperature of the structural transition.