First-order structural transition in the magnetically ordered phase of Fe${}_{1.13}$Te

Specific heat, resistivity, magnetic susceptibility, linear thermal expansion (LTE), and high-resolution synchrotron x-ray powder diffraction investigations of single crystals Fe${}_{1+y}$Te (0.06 $\le y\le$ 0.15) reveal a splitting of a single, first-order transition for $y\le$ 0.11 into two transitions for $y\ge$ 0.13. Most strikingly, all measurements on identical samples Fe${}_{1.13}$Te consistently indicate that, upon cooling, the magnetic transition at $T_N$ precedes the first-order structural transition at a lower temperature $T_s$. The structural transition in turn coincides with a change in the character of the magnetic structure. The LTE measurements along the crystallographic $c$ axis display a small distortion close to $T_N$ due to a lattice striction as a consequence of magnetic ordering, and a much larger change at $T_s$. The lattice symmetry changes, however, only below $T_s$ as indicated by powder x-ray diffraction. This behavior is in stark contrast to the sequence in which the phase transitions occur in Fe pnictides.

Figure 1

Specific heat $C_p\left(T\right)$ of Fe${}_{1+y}$Te for (a) $y=0.06$, 0.08, and 0.11 and (b) $y=0.13$ and 0.15.

Figure 2

Specific heat, dc magnetic susceptibility, and resistivity (from top to bottom) of Fe${}_{1.11}$Te (left) and Fe${}_{1.13}$Te (right) single crystals. The magnetic susceptibility was measured in a field of 0.1 T applied along the $ab$ plane.

Figure 3

(a) High-temperature-resolution specific-heat data for Fe${}_{1.13}$Te showing a clear $\lambda$-like transition at 57 K. (b) The temperature-time relaxation curve depicting the arrests (shaded region) due to latent heat at the first-order phase transition at $T\sim$ 46 K in Fe${}_{1.13}$Te.

Figure 4

Synchrotron x-ray diffraction of the powdered Fe${}_{1.13}$Te single crystal at temperatures above $T_N=57$ K, between $T_N$ and $T_s$, and below $T_s=46$ K, respectively. (a) Zoomed range $10.{75}^{\circ }\le 2\theta \le 12.{25}^{\circ }$. The single (112) peak and the splitting of the (200) tetragonal peak into two peaks (200) and (020) at $T=40\text{K}<T_s$ indicate an orthorhombic low-temperature structure. (b) Overview spectra $11.5^{\circ }\le 2\theta \le {23}^{\circ }$. The black lines represent fitted curves.

Figure 5

(a) The linear thermal expansion (LTE) of the Fe${}_{1.13}$Te single crystal measured along the crystallographic $c$ axis in both cooling and heating cycles. (b) The corresponding coefficients of the LTE, $\alpha \left(T\right)=(1/l_0)(dl/dT)$.

Figure 6

A tentative schematic temperature-composition phase diagram of Fe${}_{1+y}$Te based on our specific heat, LTE, and x-ray diffraction results (circles) as well as on results of Refs.16 and19. The double lines depict a first-order phase transition, the single line represents a continuous phase transition. The origin of the low-temperature first-order line is unclear, therefore it is marked by “?.” IC SDW denotes an incommensurate spin-density wave.