Structural, electronic, magnetic, and thermal properties of single-crystalline UNi${}_{0.5}$Sb${}_2$

We studied the properties of the antiferromagnetic (AFM) UNi${}_{0.5}$Sb${}_2$ ($T_N$ ≈ 161 K) compound in Sb flux-grown single crystals by means of measurements of neutron diffraction, magnetic susceptibility (χ), specific heat ($C_p$), thermopower ($S$), thermal conductivity (κ), linear thermal expansion (Δ$L$/$L$), and electrical resistivity (ρ) under hydrostatic pressures ($P$) up to 22 kbar. The neutron diffraction measurements revealed that the compound crystallizes in the tetragonal $P$4${}_2$/nmc structure, and the value of the U-moments yielded by the data at 25 K is ≈1.85 ± 0.12 μ${}_B$/U-ion. In addition to the features in the bulk properties observed at $T_N$, two other hysteretic features centered near 40 and 85 K were observed in the measurements of χ, $S$, ρ, and Δ$L$/$L$. Hydrostatic pressure was found to raise $T_N$ at the rate of ≈0.76 K/kbar, while suppressing the two low temperature features. These features are discussed in the context of Fermi surface and hybridization effects.

Figure 1

A portion of the ($h$0$l$) plane for UNi${}_{0.5}$Sb${}_2$ at ambient temperature, indexed according to the HfCuSi${}_2$-type structure. The arrows indicate four of the weaker reflections with $l$ $=$ (2$n$ $+$ 1)/2 indices. The $l$ indices are integers when indexed according to the $P$4${}_2$/nmc structure. The dark band across the pane is due to the space between the two detectors.

Figure 2

Schematic diagram of the crystal and magnetic structures for UNi${}_{0.5}$Sb${}_2$. The box represents the unit cell, while the few additional atoms outside of the unit cell are drawn for clarity. The faded green spheres represent the unoccupied 2a Ni positions in space group $P$4${}_2$/nmc (see Table ). The arrows indicate the unit-cell configuration of the U moments at 25 K.

Figure 3

(a) χ($T$) and (b) ${\chi }^{-1}$($T$) for UNi${}_{0.5}$Sb${}_2$ for $H$ $=$ 1 T parallel and perpendicular to the $c$-axis, respectively. These data yield $T_N$ ≈ 161 K. Fits of χ($T$) to a Curie-Weiss law yield Θ ≈ 100 K and ${\mu }_{\mathrm{eff}}$ ≈ 3.15 μ${}_B$ for $H$//$c$, and Θ ≈ −240 K and ${\mu }_{\mathrm{eff}}$ ≈ 3.25 μ${}_B$ for $H$⊥$c$.

Figure 4

$C_p$($T$) data for UNi${}_{0.5}$Sb${}_2$ between 1.8 and 300 K. The large peak centered at 161 K is due to the onset of AFM order. The lower inset shows how the magnetic entropy was extracted, yielding Δ$S_{\mathrm{mag}}$ ≈ 3.0 J/mole K, which corresponds to 52$\%$ of $R$ ln2. The upper inset shows a plot of $C$/$T$ vs $T^2$ at low temperatures, yielding γ ≈ 9.5 mJ/mole K${}^2$.

Figure 5

Thermal conductivity for UNi${}_{0.5}$Sb${}_2$ between 1.8 and 300 K for Δ$T$ across the $ab$-plane.

Figure 6

Thermopower vs temperature for UNi${}_{0.5}$Sb${}_2$ between 1.8 and 300 K. The dashed line is a fit to the one-band model.

Figure 7

Thermal expansion parallel and perpendicular to the $c$-axis in UNi${}_{0.5}$Sb${}_2$. The onset of AFM at 161 K and the two lower temperature features can be clearly identified. The Δ$L$/$L$ data for the $c$-axis is offset by 0.5 × 10${}^{-3}$ (main panel) and 0.2 × 10${}^{-3}$ (inset) for clarity. The lower inset details the hysteretic nature of the two features near 40 and 85 K upon cooling and warming. The upper inset shows the temperature dependence of the linear coefficients of thermal expansion ${\alpha }_{ab}$ and ${\alpha }_c$.

Figure 8

Normalized electrical resistivity $\rho /{\rho }_{300\mathrm{K}}$ vs temperature for UNi${}_{0.5}$Sb${}_2$ in pressures up to 22 kbar for (a) $I$//$ab$-plane and (b) $I$//$c$. The curves for $P$ > 0 are offset for clarity. The discontinuity in ${\rho }_{ab}$ and ${\rho }_c$ near 161 K is due to the onset of AFM order, and it is shifted upward with $P$, while the 2 hysteretic features centered near 40 and 85 K are suppressed. The inset of the upper pane shows the $P$ dependence of $T_N$. The pressures indicated in both panes are the values for $T$ < 90 K. The $P$ values between ambient temperature and $T$ > 90 K were estimated by linear interpolation (see text).