Pressure-dependent modifications in the $\mathrm{LaAu}{\mathrm{Sb}}_2$ charge density wave system

Hydrostatic pressure response of $\mathrm{LaAu}{\mathrm{Sb}}_2$ charge density wave (CDW) system is investigated using electrical transport, x-ray diffraction (XRD), and density-functional theory (DFT). Resistivity data at ambient pressure evidence a clear CDW transition at 83 K. X-ray diffraction at ambient pressure reveals that in plane and out of plane axes show opposite behavior with decreasing temperature, in particular, out of plane $c$ axis develops a distinct change at ∼250 K, much above the observed CDW transition at 83 K. The CDW transition shifts to low temperature with increasing pressure. Our resistivity data indicate a complete suppression of the CDW transition at ∼3.6 GPa. High-pressure XRD revealed a change from the linear trend for the out of plane ($c$) and the in plane ($a$) lattice parameters for pressure above 3.8 GPa. With compression, DFT indicated an anomaly in the c/a ratio around 8 GPa. The calculated electronic structure also indicated minor changes in the band structure in this pressure range. In addition, high-pressure DFT structural investigations reveal the $\mathrm{LaAu}{\mathrm{Sb}}_2$ system to be stable up to pressures as high as 150 GPa.

Figure 1

SEM image reconstructed from the backscattered electrons (main frame) together with the EDX elemental maps. The scale bar in all cases indicates 10 μm. The atomic percentage obtained from the EDX analysis are Sb: 50.17, Au: 24.13, and La: 25.7.

Figure 2

Measured x-ray-diffraction pattern of $\mathrm{LaAu}{\mathrm{Sb}}_2$ sample at ambient pressure and 400 K (symbols) together with the Rietveld refinement results (solid red lines). The 0.7-Å wavelength was used. Green vertical bars below the diffractogram indicate the Bragg peak positions and * indicates the Sb impurity peaks. Inset shows the tetragonal crystal structure of $\mathrm{LaAu}{\mathrm{Sb}}_2$ where antimony occupies two distinct positions Sb1 (0.75, 0.25, 0.5) in between La layers and Sb2 (0.25, 0.25, 0.1706) close to Au.

Figure 3

Calculated phonon spectra (a), site-projected phonon density of states (b), electronic band structure (c), and total electronic DOS (d) for $\mathrm{LaAu}{\mathrm{Sb}}_2$ at ambient pressure. General gradient approximation (GGA) and Perdew-Wang (PW91) functionals are used for the computation.

Figure 4

Temperature dependence of the out of plane to in plane lattice parameters (a). Temperature dependence of the unit-cell volume (b) and c/a ratio (c). Green arrows in (b) and (c) indicate temperature at which a deviation from the expected linear trend (solid line) occurs.

Figure 5

(a) Temperature dependence of electrical resistivity measurements under various hydrostatic pressures. (b) First derivative of the temperature dependence of electrical resistivity demonstrating the CDW transition temperature and (c) $T_{\mathrm{CDW}}$ vs pressure.

Figure 6

Measured x-ray-diffraction pattern of $\mathrm{LaAu}{\mathrm{Sb}}_2$ at several pressures up to 9 GPa using 4:1 methanol-ethanol as pressure-transmitting medium. Patterns are shifted vertically for clarity in presentation.

Figure 7

Pressure dependence of lattice parameters. Solid and dotted lines in the panels correspond to a linear fit of the data in the pressure below and above 3.8 GPa regimes. Corresponding slopes are indicated in the graph.

Figure 8

Pressure dependence of unit-cell volume. Open circles are data from the high-pressure run. Filled circle at 0 GPa is from the temperature-dependent study. Solid red line is the result of a second-order Birch-Murnaghan equation of state fit to the data. Inset (a) shows the out of plane to in plane lattice parameter ratio from experiments (symbol diamond) and DFT (symbol star).

Figure 9

Calculated electronic structure (a) and total density of states close to Fermi level (b) of $\mathrm{LaAu}{\mathrm{Sb}}_2$ with GGA and PW91 functional as a function of energy under various pressures up to 8 GPa.

Figure 10

Calculated $c/a$ ratio as a function of pressure up to 150 GPa obtained from optimized unit cell.