Effect of the isoelectronic substitution of Sb for As on the magnetic and structural properties of LaFe(As${}_{1-x}$Sb${}_x$)O

The antiferromagnetic (AFM) order and structural distortion in the LaFe(As${}_{1-\mathrm{x}}$Sb${}_{\mathrm{x}}$)O system have been investigated by neutron powder diffraction and physical properties measurements. Polycrystalline samples of LaFe(As${}_{1-\mathrm{x}}$Sb${}_{\mathrm{x}}$)O ($x$ < 0.5) were prepared using solid state synthesis at ambient and high pressure. We find that the isoelectronic substitution of Sb for As decreases the structural and magnetic transition temperatures, but, contrary to the effects of phosphorus substitution, superconductivity is not induced. Instead a slight increase in the Fe-magnetic moment is observed.

Figure 1

Observed (red cross), calculated (green line) and difference (pink line, below) patterns from the Rietveld analysis of (a) x-ray powder diffraction data collected at 300 K and (b) neutron powder diffraction data collected at 2 K of LaFeAs${}_{0.6}$Sb${}_{0.4}$O. The major peaks of the La${}_2$O${}_3$/La(OH)${}_3$ (5%) and FeSb (6%) impurities are marked with “${}^{*}$.”

Figure 2

Lattice parameters versus Sb content at 300 K for LaFeAs${}_{1-\mathrm{x}}$Sb${}_{\mathrm{x}}$O samples made with ambient (blue squares) and high (red circles) pressure synthesis. The stars represent the values predicted by DFT calculations.[24, 30] Dotted lines are a guide for the eye.

Figure 3

Powder neutron diffraction data for LaFeAs${}_{1-\mathrm{x}}$Sb${}_{\mathrm{x}}$O at variable temperatures. Selected 2 θ range showing the change in diffraction intensities of the tetragonal (220) reflection with temperature for (a) LaFeAs${}_{0.8}$Sb${}_{0.2}$O and (b) LaFeAs${}_{0.6}$Sb${}_{0.4}$O. (c) Temperature dependence of the (1,0,3/2) and (1,2,1/2) magnetic Bragg peaks of LaFeAs${}_{0.6}$Sb${}_{0.4}$O.

Figure 4

Temperature dependence of (a) the unit-cell lengths for LaFeAs${}_{0.8}$Sb${}_{0.2}$O and LaFeAs${}_{0.6}$Sb${}_{0.4}$O based on neutron diffraction data (the error bars are smaller than the markers). (b) The Fe-magnetic moment of LaFeAs${}_{0.8}$Sb${}_{0.2}$O and LaFeAs${}_{0.6}$Sb${}_{0.4}$O. Critical law curves are shown as a guide to the eye. The inset of (b) shows the structural phase transition expressed by the orthorhombicity parameter ($a$ − $b$)/(a $+$ b). Data for $x$ $=$ 0 are taken from Ref. 32. In (a) and the inset of (b) the dashed lines are there as a guide to the eye.

Figure 5

Behavior of the physical properties of LaFeAs${}_{1-\mathrm{x}}$Sb${}_{\mathrm{x}}$O $x$ $=$ 0, 0.2, and 0.4 near the structural and magnetic phase transitions. (a) The anomalous contributions Δ$c_p$/$T$ to the specific heat as described in the text. The inset shows the raw data $C_p$/$T$. (b) Magnetization M/H versus temperature measured at an applied field of 1 T. A Curie-Weiss type background due to the FeSb impurity has been removed from the LaFeAs${}_{0.6}$Sb${}_{0.4}$O data to give a clearer picture of the transition. Note that the negative values only indicate a negative deviation from the background, not a negative magnetization. The inset shows the derivative, $d$($M$/$H$)/$\mathit{dT}$. (c) Temperature dependence of zero-field resistivity ($\rho /{\rho }_0$). The inset shows the derivative dρ/$\mathit{dT}$. The Δ$c_p$/$T$, $C_p$/$T$, $d$(M/H)/$d$T and dρ/$\mathit{dT}$ data sets have all been normalised by its maximum value, whereas M/H and ρ were normalised by the value at 280 K.

Figure 6

(a) The variation of the structural ($T_S$) and magnetic ($T_N$) transition temperatures with Sb content estimated from resistivity (ρ), magnetization ($M$/$H$), specific heat ($C_P$) measurements (following the approach of Klauss et al.[35] and Kondrat et al.[36]) and neutron diffraction data (NPD). $T_S$ is represented by open symbols, while $T_N$ is indicated by full ones. (b) Evolution of the structural parameters of LaFeAs${}_{1-\mathrm{x}}$Sb${}_{\mathrm{x}}$O as a function of Sb doping (positive $x$) obtained from the Rietveld fits of the 300 K neutron diffraction data. Negative values of $x$ indicate $P$ doping, and the corresponding structural parameters were taken from Wang et al.[9] For comparison the parameters for $x$ $=$ 1.0 from Lébegue et al.[30] are also plotted. Dotted lines are added as a guide to the eye. The data was normalised by the values for $x$ $=$ 0.