Lattice and magnetic structures of PrFeAsO, ${\text{PrFeAsO}}_{0.85}{\text{F}}_{0.15}$, and ${\text{PrFeAsO}}_{0.85}$

We use powder neutron diffraction to study the spin and lattice structures of polycrystalline samples of nonsuperconducting PrFeAsO and superconducting ${\text{PrFeAsO}}_{0.85}{\text{F}}_{0.15}$ and ${\text{PrFeAsO}}_{0.85}$. We find that PrFeAsO exhibits abrupt structural phase transitions at 153 K followed by static long-range antiferromagnetic order at 127 K. Both the structural distortion and magnetic order are similar to other rare-earth oxypnictides. Electron doping the system with either fluorine or oxygen deficiency suppresses the structural distortion and static long-range antiferromagnetic order, therefore placing these materials into the same class of FeAs-based superconductors.

Figure 1

(Color online) Lattice and magnetic structures in undoped PrFeAsO. (a) The three-dimensional antiferromagnetic structures of Fe as determined from the refinements of our neutron-diffraction data. (b) The magnetic structure of Fe in the FeAs plane. (c) Observed (crosses) and calculated (solid line) neutron-diffraction intensities of PrFeAsO at 175 K in the tetragonal structure with space group $P4/nmm$. The inset shows the detailed data for $18°<2\theta <38°$, where most of the observable magnetic peaks are located. $2\theta$ is the diffraction angle, and the short vertical lines show the Bragg-peak positions. No magnetic peaks are observed at 175 K. The (purple) trace indicates the intensity difference between the observed and calculated structures. (d) Diffraction data at 5 K fitted with the orthorhombic structure of space group $Cmma$. The inset plots the detailed data in $18°<2\theta <38°$ showing three indexed magnetic peaks at 5 K along with the observed splitting of the structural peak. The magnetic peaks are accounted for by the combined contributions of Fe and Pr. The (1,0,0) peak vanishes completely above the Pr Néel temperature of 14 K, while the (1,0,1) and (1,0,2) peaks persist above 14 K and vanish at 127 K as shown in Fig. 2b.

Figure 2

(Color online) Temperature dependence of ${\left(2,2,0\right)}_T$ nuclear Bragg peak and magnetic ${\left(1,0,0\right)}_M$ and ${\left(1,0,1\right)}_M$ peaks. The data in (a) and (b) are collected on BT-1 and HB-3, respectively. (a) Temperature dependence of the ${\left(2,2,0\right)}_T$ ($T$ denotes tetragonal) nuclear Bragg peak showing that the onset of the structure phase transition is about 153 K. The inset shows the ${\left(2,2,0\right)}_T$ reflection above and below the transition temperature. (b) Temperature dependence of the order parameter for the ${\left(1,0,1\right)}_M$ ($M$ denotes magnetic) magnetic Bragg peak. The large increase in the intensity below 14 K is due to Pr ordering as confirmed by the temperature dependence of the ${\left(1,0,0\right)}_M$ magnetic Bragg peak, which has only an intensity contribution from Pr. The intensity of the ${\left(1,0,1\right)}_M$ peak vanishes at the Néel temperature of 127 K for the iron-spin ordering.

Figure 3

(Color online) Structural diffraction data for the ${\text{PrFeAsO}}_{0.85}{\text{F}}_{0.15}$ and ${\text{PrFeAsO}}_{0.85}$ superconducting samples. The data were collected on the BT-1 diffractometer. (a) Observed (crosses) and calculated (solid line) neutron-diffraction intensities of ${\text{PrFeAsO}}_{0.85}{\text{F}}_{0.15}$ at 5 K for the tetragonal structure (space group $P4/nmm$). The short vertical lines show the Bragg-peak positions. The (purple/gray) trace indicates the intensity difference between the observed and calculated structures. (b) Observed (crosses) and calculated (solid line) neutron-diffraction intensities of ${\text{PrFeAsO}}_{0.85}$ at 5 K refined with the tetragonal space group $P4/nmm$.