Crystal and magnetic structure of the oxypnictide superconductor ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$: A neutron-diffraction study

High-resolution and high-flux neutron as well as x-ray powder-diffraction experiments were performed on the oxypnictide series ${\text{LaO}}_{1-x}{\text{F}}_x\text{FeAs}$ with $0\le x\le 0.15$ in order to study the crystal and magnetic structure. The magnetic symmetry of the undoped compound corresponds to those reported for $RE\text{OFeAs}$ (with $RE$ a rare earth) and for $A{\text{Fe}}_2{\text{As}}_2\left(A=\text{Ba},\text{Sr}\right)$ materials. We find an ordered magnetic moment of $0.63\left(1\right){\mu }_{\text{B}}$ at 2 K in LaOFeAs, which is significantly larger than the values previously reported for this compound. A sizable ordered magnetic moment is observed up to a F doping of 4.5% whereas there is no magnetic order for a sample with a F concentration of $x=0.06$. In the undoped sample, several interatomic distances and ${\text{FeAs}}_4$ tetrahedra angles exhibit pronounced anomalies connected with the broad structural transition and with the onset of magnetism supporting the idea of strong magnetoelastic coupling in this material.

Figure 1

(Color online) (a) Visualization of the tetragonal crystal structure of ${\text{LaO}}_{1-x}{\text{F}}_x\text{FeAs}$ and the definition of the As-Fe-As bond angles. (b) The magnetic structure only showing the Fe ions in the orthorhombic unit cell (straight line). The dashed line depicts the tetragonal cell. (c) Alternation of the moment direction along the $c$ axis due to the propagation vector $\mathbf{k}=\left(10\frac{1}{2}\right)$

Figure 2

(Color online) Diffraction patterns recorded with the $x=0.02$ sample at a temperature of $T=2\text{K}$ (a) at the high-resolution powder diffractometer D2B $\left(\lambda =1.594\text{Å}\right)$ and (b) at the high-flux powder diffractometer D20 $\left(\lambda =2.41\text{Å}\right)$. Dots represent the observed patterns while the respective upper (lower) lines show the calculated patterns (difference plots). The (green) markers indicate the positions of nuclear and magnetic [second row of markers in (b)] Bragg reflections.

Figure 3

(Color online) Extract of the observed pattern (dots), calculated patterns (upper lines), and difference plot (lower line) of two magnetic models for LaOFeAs. The calculated patterns result from magnetic structure models with the Fe magnetic moments along the $a$ axis (straight line) and along the $b$ axis (dashed line).

Figure 4

(Color online) Extract of the observed (dots) and calculated D20 pattern (solid line). The inset magnifies the magnetic $\left(10\frac{3}{2}\right)$ reflection. The respective integrated intensities are given in order to scale the magnetic scattering with respect to the nuclear one.

Figure 5

(Color online) Magnetic moment of the Fe ions in dependence of temperature and F doping (inset, 2 K). Power-law functions have been fitted to the data as guide to the eyes (solid lines).

Figure 6

(Color online) Data points show the magnetic $\left(10\frac{3}{2}\right)$ reflection of the $x=0$, 0.02, and 0.045 samples. The FWHM have been extracted from pseudo-Voigt fits and reveal a broadening with increasing doping level.

Figure 7

(Color online) The structural phase transition is revealed by the temperature- (left panel) and doping-dependent (right panel) splitting of the tetragonal (220) reflection into the orthorhombic (400) and (040) reflections.

Figure 8

(Color online) The tetragonal to orthorhombic phase transition expressed by the orthorhombicity $\left(a-b\right)/\left(a+b\right)$ (filled symbols, neutron diffraction) and the FWHM of the splitting ${\left(220\right)}_T$ reflection (open symbols, x-ray diffraction); the vertical dashed line indicates the structural transition temperature indicated by the resistivity and thermal-expansion anomalies in LaOFeAs.

Figure 9

(Color online) Structural parameters of the ${\text{LaO}}_{1-x}{\text{F}}_x\text{FeAs}$ compounds as a function of F doping [(a) $T=2\text{K}$ and (b) $T=\text{RT}$] and temperature [(c) $x=0$ and (d) $x=0.15$]. (Black) squares and (red) dots correspond to the left abscissa while (green) triangles belong to the right one. The gray shaded area in (a) denotes the doping-dependent structural phase transition regime. In (c) the dashed line marks the onset of static magnetism while the dotted line indicates the structural phase transition as taken from Ref. 17. Solid lines are guide to the eyes.