Low-temperature phase diagram of Fe${}_{1+y}$Te studied using x-ray diffraction

We used low-temperature synchrotron x-ray diffraction to investigate the structural phase transitions of Fe${}_{1+y}$Te in the vicinity of a tricitical point in the phase diagram. A detailed analysis of the powder diffraction patterns and temperature dependence of the peak widths in Fe${}_{1+y}$Te showed that two-step structural and magnetic phase transitions occur within the compositional range $0.11\le y\le 0.13$. The phase transitions are sluggish, indicating a strong competition between the orthorhombic and the monoclinic phases. We combine high-resolution diffraction experiments with specific heat, resistivity, and magnetization measurements and present a revised temperature-composition phase diagram for Fe${}_{1+y}$Te.

Figure 1

(a) X-ray diffraction diagram of samples with nominal composition Fe${}_{1+y}$Te for $y=0.04–0.17$, tetragonal Fe${}_{1+y}$Te as the main phase at room temperature. (Impurity phases: FeTe${}_2$ marked by filled circle and elemental Fe by $⧫$.) (b) Lattice parameters at room temperature in dependence of the nominal composition Fe${}_{1+y}$Te. The error bars here are smaller than the size of the symbols.

Figure 2

Determined compositions of Fe${}_{1+y}$Te with wavelength dispersive x-ray (WDX) analysis and chemical analysis by an inductively coupled plasma method (ICP). In calculating the standard deviation of the nominal compositions, mass loss after reaction was accounted for assuming that all loss is caused by tellurium evaporation. However, this error bar is smaller than the symbol size. The inset shows a representative backscattered electron (BSE) image of Fe${}_{1.13}$Te. The WDX analysis was performed on ten different spots, indicated by red dots on the image. The black spots on the image are pores in the sample.

Figure 3

Specific heat of Fe${}_{1+y}$Te for $y=0.11–0.15$. The $C_p\left(T\right)$ data for $y=0.115–0.15$ are shifted by the amounts given for each curve for clarity. Arrows show the disappearing first-order phase transition upon increasing Fe composition.

Figure 4

Magnetic susceptibility of Fe${}_{1+y}$Te for $y=0.11–0.15$ (a)–(e) and normalized resistance ($R/R_{300\mathrm{K}}$) during the heating cycle (f). The magnetic susceptibility was measured in a field of 0.1 T. The $R/R_{300\mathrm{K}}$ data for $y=0.14$ and 0.15 are multiplied by a factor of 1.25 and 1.5, respectively, for better visibility.

Figure 5

Resistivity as a function of temperature for Fe${}_{1+y}$Te measured in the heating and cooling cycles. (a) $y=0.11$, (b) $y=0.12$, (c) $y=0.13$, and (d) $y=0.14$.

Figure 6

Representative powder XRD patterns of Fe${}_{1.11}$Te in the temperature regime 38–58 K for the (112) and (200) Bragg reflections. The green and red curves indicate an onset of orthorhombic and monoclinic distortions, respectively.

Figure 7

Refined synchrotron powder x-ray diffraction patterns of Fe${}_{1.11}$Te at temperatures above (293 K) and below (10 K) the phase transition.

Figure 8

Representative powder XRD patterns of Fe${}_{1.12}$Te in the temperature regime 18–60 K. (a) The region of the (200) reflection between 46 and 60 K. (b) The combined region of (112) and (200) reflections between 18 and 48 K. The green and red curves indicate the onset of orthorhombic and monoclinic distortions, respectively.

Figure 9

Refined synchrotron powder x-ray diffraction patterns of Fe${}_{1.12}$Te at temperatures above (70 K) and below (10 K) the phase transition. At 10 K, the upper and lower Bragg reflections represent monoclinic and orthorhombic structures, respectively.

Figure 10

Representative powder XRD patterns of Fe${}_{1.14}$Te for the (112) and (200) Bragg reflections in the temperature regime 38–62 K. Broadening of the (200) peak sets in at 54 K (green line).

Figure 11

Refined synchrotron powder x-ray diffraction patterns of Fe${}_{1.15}$Te at temperatures above (293 K) and below (10 K) the phase transition.

Figure 12

Temperature dependence of powder x-ray diffraction peaks of Fe${}_{1+y}$Te, $y=0.11–0.14$. (a)–(c) The broadening of the reflections (112) and (200) for $0.11\le y\le 0.13$ demonstrates a monoclinic distortion at low temperatures, whereas in (d) constant values for (112) indicate an orthorhombic low-temperature phase for $y=0.14$. Dashed lines in (b) were drawn to mark the temperatures at which phase transitions occur in the thermodynamic measurements.

Figure 13

Representative powder XRD patterns of Fe${}_{1+y}$Te, $y=0.11–0.15$ at (a) 10 K and (b) 30 K. For $y=0.11$, both (200) and (112) peaks are clearly split at low temperatures, confirming the monoclinic structure. In comparison, the (112) peak of Fe${}_{1.12}$Te is broadened but not as nicely split (specifically at 30 K). This, in combination with the obvious separation of (200) and (020) peaks, is consistent with a mixture of orthorhombic and monoclinic phases at 10 K. For $y\ge 0.14$, the narrow (112) peak confirms a pure orthorhombic phase at 30 and 10 K.

Figure 14

Temperature dependence of lattice parameters $a$, $b$, and $c$ at various compositions. (a)–(d) Transition from tetragonal to monoclinic symmetry (with an intermediate orthorhombic phase, represented by half solid symbols) for $y=0.11$ and 0.12. (e)–(h) Orthorhombic phase transition for $y=0.14$ and 0.15. Below 46 K, Fe${}_{1.12}$Te consists of a mixture of orthorhombic and monoclinic phases. In (c) and (d), the lattice parameters below 46 K were evaluated assuming a monoclinic structure exclusively.

Figure 15

Revised temperature-composition phase diagram of Fe${}_{1+y}$Te. AFM and IC AFM stand for antiferromagnetic and incommensurate antiferromagnetic phase, respectively. The data points represent the transition temperatures determined from the specific heat measurements.