Crystallographic phase transition within the magnetically ordered state of ${\mathrm{Ce}}_2{\mathrm{Fe}}_{17}$

X-ray diffraction experiments were performed on polycrystalline and single-crystal specimens of ${\mathrm{Ce}}_2{\mathrm{Fe}}_{17}$ at temperatures between 10 and $300\mathrm{K}$. Below $T_t=118\pm 2\mathrm{K}$, additional weak superstructure reflections were observed in the antiferromagnetically ordered state. The superstructure can be described by a doubling of the chemical unit cell along the $\mathbf{c}$ direction in hexagonal notation with the same space group $R\overline{3}m$ as the room-temperature structure. The additional antiferromagnetic satellite reflections observed in earlier neutron diffraction experiments can be conclusively related to the appearance of this superstructure.

Figure 1

(Color online) Diffraction patterns of the ${\mathrm{Ce}}_2{\mathrm{Fe}}_{17}$ single crystal measured using $\mathrm{Cu}K\alpha$ x rays at $T=110$ and $130\mathrm{K}$, well below and above the crystallographic phase transition, respectively. Parts of the patterns around the $(0015∕2)$ position are enlarged in the inset.

Figure 2

(Color online) High-energy x-ray diffraction patterns of ${\mathrm{Ce}}_2{\mathrm{Fe}}_{17}$ powder measured with $\lambda =0.141\mathrm{\AA }$. The upper panel shows diffraction patterns at $T=10$ and at $130\mathrm{K}$. The difference between the intensities at both temperatures is shown in the lower panel. Parts of the patterns around the $(003∕2)$ position are enlarged in the inset.

Figure 3

(Color online) Special rocking technique to measure entire reciprocal planes of a ${\mathrm{Ce}}_2{\mathrm{Fe}}_{17}$ single crystal using high-energy x rays. Here, it is illustrated for the recording of the $\left(H0L\right)$ plane. Dimensions and angles are not to scale and, partly, strongly exaggerated for a better illustration. (a) Instrumental geometry with the image-plate detector mounted centered and perpendicular to the incident x-ray beam behind the sample. The direct x-ray beam is blocked by a small beam catcher not shown here. Tilting by the angle $\eta$ is visualized by the resulting fan of the ${\mathbf{a}}^{*}$ direction. (b) Ewald illustration of the scattering geometry in reciprocal space. The tilting of the reciprocal lattice caused by the sample tilting is visualized by the fan of the ${\mathbf{a}}^{*}$ direction. (c) View onto the reciprocal $\left(H0L\right)$ plane in beam direction. The circle and the annuli represent the intersection between the Ewald sphere and the $\left(H0L\right)$ plane for the different tilting angles $\eta$ shown in (a) and (b).

Figure 4

High-energy x-ray diffraction patterns of the ${\mathrm{Ce}}_2{\mathrm{Fe}}_{17}$ single crystal. The intensity is encoded by the gray tone in the contour map of the (a) $\left(HHL\right)$, (b) $\left(HK0\right)$, and (c) $\left(H0L\right)$ planes. The left and right sides show measurements at $T=25$ and at $130\mathrm{K}$, respectively, which is above the crystallographic phase transition. The arrows point to superstructure reflections appearing in the low-temperature phase.

Figure 5

(Color online) Temperature $\left(T\right)$ dependence of the integrated intensity $\left(I\right)$ of superstructure reflections measured with high-energy x rays on the ${\mathrm{Ce}}_2{\mathrm{Fe}}_{17}$ single crystal. The lines represent power-law curves describing the combined data for the Friedel-pair reflections $(00\pm 3∕2)$ and $(00\pm 9∕2)$, respectively.