Absence of long-range magnetic order in ${\mathrm{Fe}}_{1-\delta }{\mathrm{Te}}_2$ ($\delta \approx$ 0.1) crystals

Transition metal dichalcogenides attract considerable attention due to a variety of interesting properties, including long-range magnetism in nanocrystals. Here we investigate the magnetic, thermal, and electrical properties of an ${\mathrm{FeTe}}_2$ single crystal with iron vacancy defects. Magnetic measurements show a paramagnetic state and the absence of magnetic order with low anisotropy in the magnetic susceptibility. Fe $3d$ orbitals are well hybridized, contributing to the bad metal electrical resistivity. Observed thermal conductivity values below room temperature are rather low and comparable to those of high-performance thermoelectric materials. Our results indicate that ${\mathrm{FeTe}}_2$ can form in a highly defective marcasite crystal structure which can be exploited in future materials design.

Figure 1

(a) The Rietveld refinement of the background subtracted iron ditelluride synchrotron powder x-ray diffraction up to $Q\sim 6.5 {\text{Å}}^{-1}$. Plots show the observed (dots) and calculated (red solid line) powder patterns with a difference curve. The black vertical tick marks represent Bragg reflections in the Pnnm space group, whereas the red tick marks represent Bragg reflections of the small amount (0.6 t %) of residual Te on the crystal surface during pulverization of the single-crystal specimen. The inset shows the crystal structure of ${\mathrm{FeTe}}_2$ in the Pnnm space group; the unit cell consists of $2\times {\mathrm{Fe}}_2$. (b) PDF analysis of ${\mathrm{Fe}}_{0.88\left(4\right)}{\mathrm{Te}}_2$. The PDF model (red solid line) is applied to observed data (open circles) at 300 K up to $r$ = 30 Å. The difference curve is shown displaced below (green solid line).

Figure 2

(a) Temperature dependence of the heat capacity. The inset shows the low-temperature fit to $C/T=\gamma +{\beta }_3{T}^2+{\beta }_5{T}^4$. (b) Thermopower $S\left(T\right)$, (c) thermal conductivity $\kappa \left(T\right)$, and (d) resistivity $\rho \left(T\right)$ for ${\mathrm{Fe}}_{1-\delta }{\mathrm{Te}}_2$ single crystals. (e)–(g) Magnetic susceptibility $\chi$ taken in $H=1$ kOe magnetic field. (h) and (i) XPS Fe $2p$ and Te $3d$ peaks.

Figure 3

(a) Reflectance for two different polarizations and unpolarized light (inset). (b). Optical conductivity for polarized (main panel) and unpolarized (left inset) radiation obtained from Kramers-Kronig transformations. The right inset shows a sketch of the energy levels involved in optical transitions (in eV; see the text). Dashed and dotted black lines represent Lorentz-Drude fits for both the unpolarized (inset) and polarized (main panel) conductivities.

Figure 4

(a) Total and atom-projected electronic densities of states for ${\mathrm{FeTe}}_2$. (b) Projected density of states of Fe $3d$ orbital states in the vicinity of the Fermi level. (c) Calculated GGA band structure for ${\mathrm{FeTe}}_2$. (d) Mössbauer spectrum recorded within the low-velocity range at room temperature. The observed data are presented by the gray solid circles, the fit is given by the red solid line, and the difference ($I_{\mathrm{calc}}-I_{\mathrm{obs}}$) is shown by the dark gray line. The QSDs obtained with the VBF method are depicted by the blue and navy blue lines. The vertical arrow denotes the relative position of the lowest experimental point with respect to the background (relative absorption of 1.89%). The absolute difference is less than 0.054%. The inset on the left side shows the spectrum recorded within the high-velocity range as evidence that there is no magnetic phase in the sample at room temperature. The orange line is just a guide for the eye. The inset on the right side shows the QSD calculated using the histogram method.

Figure 5

Total DOS around the Fermi level in (a) pure ${\mathrm{FeTe}}_{1-\delta }{\mathrm{Te}}_2$, (b) the $2\times 2\times 2$ supercell with one Fe vacancy, and (c) the $2\times 2\times 2$ supercell with two Fe vacancies.