Doping evolution of antiferromagnetic order and structural distortion in ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$

We use neutron scattering to study the structural distortion and antiferromagnetic (AFM) order in ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$ as the system is doped with fluorine (F) to induce superconductivity. In the undoped state, LaFeAsO exhibits a structural distortion, changing the symmetry from tetragonal (space group $P4/nmm$) to orthorhombic (space group $Cmma$) at 155 K, and then followed by an AFM order at 137 K. Doping the system with F gradually decreases the structural distortion temperature, but suppresses the long range AFM order before the emergence of superconductivity. Therefore, while superconductivity in these Fe oxypnictides can survive in either the tetragonal or the orthorhombic crystal structure, it competes directly with static AFM order.

Figure 1

(Color online) (a) The Fe spin ordering in the LaFeAsO chemical unit cell. (b) The Fe magnetic unit cell of LaFeAsO in the Fe-As plane. The Fe moments lie in the $a\text{−}b$ plane along the $a$ axis and form an antiferromagnetic collinear spin structure similar to ${\text{BaFe}}_2{\text{As}}_2$, ${\text{SrFe}}_2{\text{As}}_2$, and ${\text{CaFe}}_2{\text{As}}_2$ (Refs. 7, 8, 9, 10) (c) The structural and magnetic phase diagram determined from our neutron measurements on ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$ with $x=0,0.03,0.05,0.08$. The red circles indicate the onset temperature of the $P4/nmm$ to $Cmma$ phase transition. The black squares designate the Néel temperatures of Fe as determined from neutron measurements in Fig. 3. The superconducting transition temperatures $T_c$ for $x=0.05,0.08$ are determined from susceptibility measurements. The AFM to superconducting phase transition happens between $x=0.03$ and 0.05. The inset in (d) shows the F doping dependence of the Fe moment as determined from the intensity of the ${\left(1,0,3\right)}_M$ magnetic peak at 4 K.

Figure 2

(Color online) Dependence of the low-temperature crystal structure of ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$ as a function of F-doping $x$. (a) $2\theta$ scans, showing the reduction of the orthorhombic lattice distortion with increasing F doping. The $\left(2,2,0\right)$ peak for $x=0.05$ is clearly broader than the resolution. [(b)–(d)] Temperature dependence of the ${\left(2,2,0\right)}_{\text{T}}$ (T denotes tetragonal) nuclear reflection indicative of a structural phase transition for various $x$ (Ref. 7). The temperature of the tetragonal to orthorhombic lattice distortion reduces with increasing $x$. The insets show the ${\left(2,2,0\right)}_{\text{T}}$ reflection above and below the transition temperatures.

Figure 3

(Color online) (a) Wave vector dependence of the AFM ordering peak (1,0,3) for $x=0,0.03,0.05$ at 2 K. The intensity of scattering is normalized to the nuclear Bragg peaks and can be compared directly. (b) and (c) Temperature dependence of the magnetic scattering for $x=0,0.03$, respectively.

Figure 4

(Color online) Low temperature structural evolution of ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$ as a function of F doping obtained from analysis of the BT-1 data. There is no sudden structural transition as the AFM order is replaced by the superconducting phase. The atomic positions of ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$ and their temperature dependence are shown in Tables . (a) schematic defining the As-Fe-As block and illustrating the process of electron doping. (b) $a$, $b$, $c$ lattice constants of the orthorhombic unit cell and the two Fe-Fe nearest-neighbor distances as a function of F doping. Similar to ${\text{CeFeAsO}}_{1-x}{\text{F}}_x$, F doping only suppresses the long axis of the orthorhombic structure. (c) La-O/F and La-As distances as a function of F doping. The slight increase in the La-O/F block size is compensated by a much larger reduction in the La-As distance, resulting in an overall $c$-axis lattice contraction as shown in (b). (d) Fe-As-Fe bond angles as defined in the inset versus F doping. While angle 1 hardly changes with doping, angles 2 and 3 decrease substantially with increasing F doping. (e) The Fe-As bond distance and As-Fe-As block size versus F doping. The Fe-As distance is independent of F doping.