Neutron diffraction study of phase transitions and thermal expansion of SrFeAsF

The magnetic ordering and crystal structure of iron pnictide SrFeAsF was investigated by using neutron powder-diffraction method. With decreasing temperature, the tetragonal to orthorhombic phase transition is found at 180(2) K while the paramagnetic to antiferromagnetic phase-transition set in at 133(3) K. The big difference between structural and magnetic phase transitions in SrFeAsF indicates the weak magnetic exchange coupling between Fe layers. Similar to the parent compound of other iron-pnictide system, the striped Fe magnetism is confirmed in antiferromagnetic phase and the Fe moment of 0.58(6)μ_B aligned along long a axis. Given the fact that long-range antiferromagnetic order can be described by a power law with β=0.124(16), SrFeAsF can be treated as a quasi-two-dimensional Ising system. Thermal expansion of orthorhombic phase of SrFeAsF is also investigated. Based on the Grüneisen approximation and Debye approximation for internal energy, the volume of SrFeAsF can be well fitted with Debye temperature of 347(5) K. The experimental atomic-displacement parameters for different crystallographic sites in SrFeAsF are analyzed with Debye model. Thermal expansion analysis for SrFeAsF suggested that the expansion of FeAs layers plays an important role in determining the thermal expansion coefficient.