Structural and magnetic characterization of the elusive Jahn-Teller active ${\mathrm{NaCrF}}_3$

We report on the structural and magnetic properties of the elusive Jahn-Teller active compound ${\mathrm{NaCrF}}_3$, for first time synthesized in large quantities allowing detailed characterization. The crystal structure of ${\mathrm{NaCrF}}_3$ is initially described from a DFT model, which helped serve as the basis for indexing and structure determination confirmed by high-resolution synchrotron x-ray diffraction experiments. ${\mathrm{NaCrF}}_3$ adopts the triclinic space group $P\overline{1}$ (isostructural with ${\mathrm{NaCuF}}_3$). Magnetometry studies at low temperature show that ${\mathrm{NaCrF}}_3$ is a weak antiferromagnet, Weiss temperature $\theta =-4$ K. The Néel temperature is $T_N=21.3$ K and the paramagnetic moment $\mu =4.47{\mu }_{{}_B}$ is in accordance with the theoretical $S=2$. Field-dependent measurements between 2 and 12 K unveil the onset of metamagnetic behavior. Our experiments revealed a weakly canted $A$-type magnetic structure observed by neutron powder diffraction, with a magnetic propagation vector $(1/2,1/2,0)$ and a magnetic moment of $3.51{\mu }_B$ at 1.5 K. Our results shed further light on the Jahn-Teller effects and strong correlations as a function of $A$-ion size in the family $A{\mathrm{CrF}}_3$.

Figure 1

(a) Final observed (black dots), calculated (red line) and difference (blue line) synchrotron x-ray powder diffraction profiles ($\lambda =0.4501$ Å) for ${\mathrm{NaCrF}}_3$ at 298 K $[a=5.51515(2)$ Å, $b=5.68817\left(3\right)$ Å, $c=8.18349\left(3\right)$ Å, $\alpha =90.5039{\left(3\right)}^{\circ }, \beta =92.2554{\left(3\right)}^{\circ }, \gamma =86.0599{\left(2\right)}^{\circ }]$. $R_{wp}=11.52\%$; $R_{\exp }=5.53\%$. Inset: Close up of the high angle region of the refined pattern. (b) Structure of ${\mathrm{NaCrF}}_3$ viewed along the [110] direction.

Figure 2

(a) $ls$-bond length motif of the four crystallographic chromium sites of ${\mathrm{NaCrF}}_3$. Cr1, Cr2, Cr3, and Cr4 are represented here with blue, cyan, purple, and green spheres, respectively. The Cr-F-Cr bond angles are labeled as follows: ${\xi }_1^{\circ }, \mathrm{Cr}1-\mathrm{F}1-\mathrm{Cr}2$; ${\xi }_6^{\circ }, \mathrm{Cr}2-\mathrm{F}6-\mathrm{Cr}4$; ${\xi }_3^{\circ }, \mathrm{Cr}4-\mathrm{F}3-\mathrm{Cr}3$; ${\xi }_5^{\circ }, \mathrm{Cr}3-\mathrm{F}5-\mathrm{Cr}1$. (b) Packed crystal structure of ${\mathrm{NaCrF}}_3$ with red and blue planes marking layers of ${\mathrm{Cr}}^{2+}$ in which the ls-bond length motif (a) is rotated by ${90}^{\circ }$ relative to the adjacent layers. The midplane (green) cuts through the connecting $m$-bond distances, and represents the stacking directions of the canted antiferrodistortive orbital ordering (AOO). The unit cell is shown in pale gray.

Figure 3

(a) ZFC-FC temperature dependency of the magnetic susceptibility measured $\chi \left(T\right)$ at $H=1$ T (left axis), and their inverse ${\chi }^{-1}$ (right) with the linear regression at $\theta =-4$ K. (b) Isothermal half-loop magnetization curves magnetic field $\left[M\right(H\left)\right]$ applied from 0–14 T and then back to 0 T at 2, 4, 12, and 23 K. The inset is the full $M\left(H\right)$ hysteresis loop at 2 K to show the symmetry at the negative quadrant.

Figure 4

First derivative $dM/dH$ of the isothermal half-loops at 2, 4, 12, and 23 K. The upswing is represented with connected line guides. A metamagnetic transition occurs at 8 T in the half-loops at 2 and 4 K. Up and downswing data are emphasized by arrows for the 4 T data.

Figure 5

(a) Rietveld refinements of NPD data set of ${\mathrm{NaCrF}}_3$ at 1.5 K from detector bank 2 (lowest resolution bank used in refinements) showing the peak at 7.63 Å with the inlet showing small peaks of the magnetic phase. The purple and green tics correspond to the crystal and magnetic phase, respectively. (b) Magnetic structure of ${\mathrm{NaCrF}}_3$ in the [1, $-1$, 0] direction. The antiparallel alignment of the spins is represented by the blue-red sequence. Blue, cyan, purple, and green atoms correspond to Cr1, Cr2, Cr3, and Cr4 respectively.

Figure 6

(a) Magnetic moment of the chromium cations in ${\mathrm{NaCrF}}_3$ determined by neutron diffraction as a function of temperature. Temperature evolution of the (b) polar and (c) azimuthal angles. Constraints are described in Table 2.

Figure 7

(a) Temperature dependence of unit cells dimensions ($a,b,\gamma ,$ V) and ${\xi }^{\circ }$ angles. (b) Temperature dependence of the unit cell lattice parameters (c) Perovskite angles composing the canted $ls$ motif ${\mathrm{NaCrF}}_3$ as function of temperature: ${\xi }_1^{\circ }=\mathrm{Cr}1-\mathrm{F}1-\mathrm{Cr}2, {\xi }_6^{\circ }=\mathrm{Cr}2-\mathrm{F}6-\mathrm{Cr}4, {\xi }_3^{\circ }=\mathrm{Cr}4-\mathrm{F}3-\mathrm{Cr}3, {\xi }_5^{\circ }=\mathrm{Cr}3-\mathrm{F}5-\mathrm{Cr}1$. Vertical dashed line at $T_N$ to emphasize the place where the magnetic long-range order sets in.