Band Jahn-Teller structural phase transition in ${\mathrm{Y}}_2\mathrm{In}$

The number of paramagnetic materials that undergo a structural phase transition is rather small, which can perhaps explain the limited understanding of the band Jahn-Teller mechanism responsible for this effect. Here we present a structural phase transition observed in paramagnetic ${\mathrm{Y}}_2\mathrm{In}$ at temperature $T_0=250\pm 5$ K. Below $T_0$, the high-temperature hexagonal $P{6}_3/mmc$ phase transforms into the low-temperature orthorhombic $Pnma$ phase. This transition is accompanied by an unambiguous thermal hysteresis of about 10 K, observed in both magnetic susceptibility $M/H\left(T\right)$ and resistivity $\rho \left(T\right)$, indicating a first-order transition. Band structure calculations suggest a band Jahn-Teller mechanism, during which the degeneracy of electron bands close to the Fermi energy is broken. We establish that this structural phase transition does not have a magnetic component; however, the possibility of a charge density wave formation has not been eliminated.

Figure 1

(a) The zero-field-cooled (black line) and field-cooled (gray line) temperature-dependent magnetic susceptibility $M\left(T\right)/H$ data of ${\mathrm{Y}}_2\mathrm{In}$, measured in $H=0.04$ T. Inset: the thermal hysteresis around the $T_0=250\pm 5$ K transition. [Note: 1 emu/(${\mathrm{mol}}_{\mathrm{F}.\mathrm{U}.}$ Oe) = $4\pi {10}^{-6}{\mathrm{m}}^3$/mol.] (b) Magnetization isotherms $M\left(H\right)$, measured at $T=200$ K (circle) and 300 K (triangle).

Figure 2

The temperature-dependent resistivity $\rho \left(T\right)$ of ${\mathrm{Y}}_2\mathrm{In}$ in $H=0$. Inset: the thermal hysteresis in $H=0$ (line) and $H=5$ T (symbols).

Figure 3

The transition temperature $T_0=250\pm 5$ K, determined from the derivatives of susceptibility $\mathit{dM}/\mathit{dT}$ ($H=0.04$ T, gray line, left axis) and resistivity $d\rho$/$dT$ ($H=0$, black line, right axis).

Figure 4

Neutron-diffraction patterns of ${\mathrm{Y}}_2\mathrm{In}$ (symbols) for (a) $T=300$ K and (b) $T=200$ K. The data were fit (black line) with (a) $P{6}_3/mmc$ and (b) $Pnma$ space groups, with calculated peak positions represented by the vertical symbols. The difference between the measured and fit data is shown as a gray line. (c) Evolution of the x-ray diffraction pattern (solid lines) between 200 and 300 K compared with the neutron data (symbols). The high- and low-temperature crystal structures are shown in (d) $P{6}_3/mmc$ and (e) $Pnma$, with the unit cell outlined in black.

Figure 5

(a) The total (solid line) and $d$-electron (dashed line) nonmagnetic density of states for the $Pnma$ (blue line) and the $P{6}_3/mmc$ (red line) phases of ${\mathrm{Y}}_2\mathrm{In}$. Fermi surface plots for the (b) $P{6}_3/mmc$ and (c) $Pnma$ phases of ${\mathrm{Y}}_2\mathrm{In}$.