$T_d$ to $1T^{\prime }$ structural phase transition in the ${\mathrm{WTe}}_2$ Weyl semimetal

Elastic neutron scattering on a single crystal combined with powder x-ray diffraction measurements were carried out to investigate how the crystal structure evolves as a function of temperature in the Weyl semimetal $\mathrm{W}{\mathrm{Te}}_2$. A sharp transition from the low-temperature orthorhombic phase ($T_d$) to the high-temperature monoclinic phase ($1T^{\prime }$) was observed at ambient pressure in the single crystal near $\sim 565$ K. Unlike in ${\mathrm{MoTe}}_2$, the solid-solid transition from $T_d$ to $1T^{\prime }$ occurs without the cell doubling of the intermediate $T_d^{*}$ phase with AABB (or ABBA) layer stacking. In powders, however, the thermal transition from the $T_d$ to the $1T^{\prime }$ phase was broadened and a two-phase coexistence was observed until 700 K, well above the structural transition.

Figure 1

(a) The crystal structure of $1T^{\prime }-{\mathrm{Mo}}_{1-x}{\mathrm{W}}_x{\mathrm{Te}}_2$ projected in the $a-c$ plane. (b) Stacking sequences for the $T_d$ and $1T^{\prime }$ phases of $\mathrm{W}{\mathrm{Te}}_2$. (c) Temperature and field dependence of resistivity in $\mathrm{W}{\mathrm{Te}}_2$, for current along the $b$ direction and $H\parallel c$. The relative error of each data point is $\sim 0.001$. (d), (e) Scans of neutron scattering intensity along $(2,0,L)$ collected on a single crystal of $\mathrm{W}{\mathrm{Te}}_2$ on cooling and warming. The Bragg peaks labeled $D1$ and $D2$ refer to the two $1T^{\prime }$ twins. (f) Intensity as a function of temperature of ${\left(203\right)}_{T_d}$ and ${\left(20\overline{3}\right)}_{1T^{\prime }}$, obtained from fits of scans along $(2,0,L)$. [inset of (f)] The temperature dependence of the interlayer spacing, obtained from fits to longitudinal scans along $\left(004\right)$.

Figure 2

(a), (b) A plot of the x-ray diffraction pattern compared to the refined model for the average symmetry of powder $\mathrm{W}{\mathrm{Te}}_2$, collected at 300 and 700 K on warming. Pure Te Bragg peaks are observed at 700 K. (c)–(h) Diffraction data plotted in a narrow range (blue dashed lines) for 300 K (c)–(e) and 700 K (f)–(h) for the $(0,0,2)$ peak (c), (f) and two other peaks. The red curves correspond to the calculated intensity for the $T_d$ phase or a $T_d-1T^{\prime }$ phase coexistence, respectively. (i) The volume fractions of the $T_d$ and $1T^{\prime }$ phases as a function of temperature. (j) Simulated XRD intensities for $T_d$ ($p=0$), $1T^{\prime }$ ($p=1$), and disordered stacking ($0<p<1$), intermediate between $T_d$ and $1T^{\prime }$, for the same regions as (d) and (g). $p$ is the probability of a random swap of “A” with “B”-type stacking for every other interlayer boundary in the $T_d$ AAAA... stacking.

Figure 3

(a) The temperature dependence of the $\delta$ parameter of $\mathrm{W}{\mathrm{Te}}_2$ from powder XRD (black) and single crystal neutron diffraction (SCND) (red) measured on HB1A. (b)–(d) The temperature dependence of the lattice constants $a, b$, and $c$. The error bars for the points in (a)–(d) are smaller than the symbols except for the XRD $\delta$ points in (a).