Lattice Distortion and Magnetic Quantum Phase Transition in ${\mathrm{CeFeAs}}_{1-x}{\mathbf{P}}_x\mathbf{O}$

We use neutron diffraction to study the structural and magnetic phase diagram of ${\mathrm{CeFeAs}}_{1-x}{\mathrm{P}}_x\mathrm{O}$. We find that replacing the larger arsenic with smaller phosphorus in ${\mathrm{CeFeAs}}_{1-x}{\mathrm{P}}_x\mathrm{O}$ simultaneously suppresses the AFM order and orthorhombic distortion near $x=0.4$, thus suggesting the presence of a magnetic quantum critical point. Our detailed structural analysis reveals that the pnictogen height is an important controlling parameter for their electronic and magnetic properties, and may play an important role in electron pairing and superconductivity of these materials.

Figure 1

(a) The three dimensional crystal lattice structure of CeFeAsO. (b) The magnetic unit cell of Fe in CeFeAsO. The Fe moments lie in the $a\mathrm{\text{−}}b$ plane and form an antiferromagnetic collinear structure similar to that of LaFeAsO [4], while nearest-neighbor spins along the $c$ axis are parallel and so there is no need to double the magnetic cell along the $c$ axis [13]. (c) Magnetic scattering at the Fe $(1,0,2{)}_M$ peak, normalized to the nuclear Bragg peak $(1,1,1{)}_O$. The vanishing magnetic peak intensity is clearly seen near $x\sim 0.4$. (d) Experimentally obtained structural and magnetic phase transitions temperatures as a function of P substitution of As. The inset in (d) shows the P dependence of the Ce AFM ordering temperatures. (e) The ordered AFM moment of Fe is proportional to the orthorhombicity (defined as $a_o/b_o$) of the system and becomes zero at $a_o/b_o-1$ (f) P-doping dependence of the orthorhombicity and Fe ordered moments. The Fe ordering temperatures are noted in the inset.

Figure 2

(a) Temperature dependence of the Ce $(0,0,1{)}_M$ magnetic Bragg peak for various $x$. The inset shows the $(0,0,1{)}_M$ peak profile at 1.6 K as a function of $x$. (b) Doping dependence of the Fe $(1,0,2{)}_M$ magnetic Bragg peak normalized to the nuclear $(1,1,1{)}_O$ peak intensity. The AFM peaks appear at the commensurate ordering wave vectors $Q=1.851(2)$, 1.850(2), 1.857(2), 1.851(2), 1.855(3), 1.854(4), $1.848(4){\AA }^{-1}$ for $x=0.05$, 0.10, 0.20, 0.25, 0.30, 0.35, 0.37, respectively. The corresponding spin-spin correlation lengths are $\xi =205(24)$, 179(17), 175(17), 157(23), 174(28), 140(33), 130(29) Å. The peak positions are essentially doping independent, while the spin-spin correlation length decreases with increasing P concentration. (c) Temperature dependence of the order parameter at the Fe magnetic Bragg peak position $(1,0,2{)}_M$ as a function of P concentration. The Fe ordered moments were estimated from comparing the magnetic scattering intensity with nuclear Bragg intensity at 40 K for $x\le 0.3$, at 20 K for $x=0.35$, and at 8 K for $x=0.37$, 0.40, 0.43. The solid lines are mean field fits to the data.

Figure 3

(a)–(f) Temperature dependence of the $(2,2,0{)}_T$ ($T$ denotes tetragonal) nuclear reflection at temperatures above and below the structural phase transition for various $x$. In most cases ($x\le 0.35$), the low-temperature width is broader than that at high temperatures. (g) Temperature dependence of the $(2,2,0{)}_T$ Bragg peak intensity for different samples, which shows clear kinks indicating tetragonal to orthorhombic phase transitions.

Figure 4

(a) schematic diagrams defining the Fe-As-Fe and CeO blocks and illustrating various bond distances and bond angles. (b) $a$, $b$, $c$ lattice constants of the orthorhombic unit cell as a function of $x$. The inset shows that $a/c$ ratio saturates when the Fe moment vanishes. (c) Fe-Fe and Fe-As distances as a function of P concentration. The Fe-As distance falls on the critical value of 2.36 Å where the Fe ordered moment is suppressed. (d) $\mathrm{Ce}\mathrm{\text{−}}\mathrm{O}/\mathrm{F}$ and Ce-As distances are essentially P concentration independent. The O-Ce-O block size is also P-substitution independent. The effect of P substitution of As is to reduce the Fe-As bond distance and As-Fe-As block size. This is purely due to the size (lattice) differences between P and As. (e) Fe-As-Fe bond angles as defined in (a) versus P doping. The most dramatic effect of P doping is to increase the Fe-As-Fe angle to 114.6° and decrease the corresponding As-Fe-As angle to 107.3°. Thus, P doping makes the FeAs tetrahedra deviate further from the ideal As-Fe-As angle of 109.5°.