Structural and Optothermal Properties of Iron Ditelluride Layered Structures in the Framework of the Lattice Compatibility Theory

This study concerns structural and optothermal properties of iron ditelluride layered structures which were fabricated via a lowcost protocol.Themain precursors were FeCl 3 ⋅ 6H 2 O and Fe 2 O 3 . After a heat treatment within a tellurium-rich medium at various temperatures (470C, 500C, and 530C) during 24 h, classical analyses have been applied to the iron ditelluride layered structures. A good crystalline state with a preferential orientation of the crystallites along (111) direction has been recorded. Moreover, additional opto-thermal investigation and analyses within the framework of the Lattice Compatibility Theory gave plausible explanation for prompt temperature-dependent incorporation of tellurium element inside hematite elaborated matrices.


Introduction
Iron ditelluride is a good representative of crystalline transition metal ditelluride in the 3d series.It has been identified as a particularly conductive marcasite containing the narrow 3d band (about 1.1 eV).Nanocrystalline iron ditelluride has been elaborated by Zhang et al. [1] by a reaction of alkaline aqueous solutions dissolving elemental tellurium and iron (II) complex Na 2 [Fe(EDTA)] at 140 ∘ C under atmospheric pressure, while Liu et al. [2] used a hydrothermal coreduction method in order to produce room temperature magneticsensitive frohbergite iron ditelluride nanocrystallites [2] using N 2 H 4 ⋅H 2 O as reductant.
Consecutive attempts of chemical doping induced antiferromagnetic (AFM) [3,4] and orbital orders [5,6] in iron ditelluride compounds, but their applications faced several problems [2][3][4][5][6].The actual study reports structural and optothermal properties of iron ditelluride layered structures which were developed by a simple process consisting on anneal amorphous iron oxide layers, predeposited by spray pyrolysis of FeCl 3 ⋅ 6H 2 O based aqueous solution onto heated pyrex glass substrates, under tellurium atmosphere.This simple and low cost process has been used by Ouertani et al. to obtain FeS 2 and FeSe 2 and allowed us to obtain FeTe 2marcasite phase thin films [7,8].

Experiment
An aqueous solution of FeCl 3 -6H 2 O (0.015 M) was prepared and sprayed with a gas vector (N 2 ) on glass substrates, which were placed on a hotplate heated at 623 K. 50 mL of the solution was sprayed on the heated glass substrates, by means of a nozzle allowing a jet flow rate of 4 mL/min.The height distance between the nozzle and the substrate is about 30 cm and the N 2 flow is about 4 L/min.Layers obtained are amorphous and have a red rust color.These as-prepared amorphous iron oxide layers were placed with a small amount of tellurium in a vacuum-sealed Pyrex tube and subjected to a heat treatment under tellurium atmosphere in an electric tubular oven at different temperatures (470 ∘ C, 500 ∘ C, and 530 ∘ C) for 24 h.

Optical and Atomic Force Microscopy Analyses
The optical transmission and reflectance spectra of iron ditelluride films in the wavelength region of 300-1800 nm are shown in Figures 1 and 2, respectively.All films are a thickness of the order of 200 nm; it can also be seen that the interference fringe patterns are absent in all transmittance and reflectance spectra due to weak multiple reflections at the interface.The films show a high transparency within the visible range with an average transmittance lying between 78-88%.This may be due to the thickness of the films.Consecutively, band gap, refractive index, and extinction coefficient of films have been obtained by fitting from the transmittance spectra of the thin films in the spectral domain varying from 300 to 1800 nm.The model details have been calculated using optical experimental measurements and the method of Bathe and Patil [9] and Belgacem and R. Bennaceur [10].Spectra patterns confirm the records of Si et al. [11] and Nie et al. [12], who stated that for deposition temperatures beyond 500 ∘ C, the rhombohedral phase of the oxide Fe 2 O 3 appears and prevents the substitution of oxygen by tellurium.On the other hand, AFM analyses have been applied to investigate the topography of the FeTe 2 thin films.Root mean square surface roughness value was determined on an area of 3 m × 3 m for all samples.Figure 3 shows AFM surface morphologies of iron telluride films synthesized at different temperatures.The analysis results are presented in Table 1; indeed the aspects of the explored layers areas show that the optimized layer, elaborated at 500 ∘ C, has the highest values of rms and average height.
The Amlouk-Boubaker optothermal expansivity Ψ  is a thermophysical parameter defined in precedent studies [13,14], as a 3d expansion velocity of the transmitted heat inside the material.It is expressed in m 3 s −1 , and calculated by where  is the thermal diffusivity and α is the effective absorptivity.Values of the Amlouk-Boubaker optothermal expansivity Ψ  of the studied films iron telluride films (at 470 ∘ C, 500 ∘ C, and 530 ∘ C) are gathered in Figure 4.

Lattice Compatibility Theory LCT Analysis. The Lattice
Compatibility Theory, as mentioned in some recent studies [36][37][38], is based on the interaction of doping-element lattice behavior versus host edifice.Preludes to this theory have been established in the context of analysing Urbach tailing controversial behaviour in some nanocompounds.It was also confirmed by Boubaker [39] on the bases of investigation on some copper-doped compounds.An original formulation of the Lattice Compatibility Theory [38,39] has been established as follows: "The stability of doping agents inside host structures is favorized by geometrical compatibility, expressed in terms of matching patterns between doping agent intrinsic lattice and those of the host." In the actually discussed case (FeTe 2 lattice), the nature of the highest occupied bands and the location of holes in elemental iron have been demonstrated to be determinant.In this context, fundamental geometrical observations concerning the structure of Te intrinsic lattice (Figure 5) along with the host matrix were interpreted in terms of conventional lattice-linked parameters (i.e.,  and   in Figure 5).The similarities between the two structures, as evoked elsewhere [40,41], plea in favor of an easy substitution process between copper and silver ions within ternary structures.
The Lattice Compatibility Theory LCT gives hence an explanation to the incorporation of tellurium in the FeTe 2 orthorhombic phase matrix via the substitution of oxygen by tellurium [42,43].

Conclusion
We have studied the structure and the morphology of iron ditelluride layered structures obtained by a heat treatment within a Te-rich atmosphere of amorphous iron oxide sprayed thin films.Morphological observations show that all films have a relatively perturbed surface state.Additional and original investigations in terms of Amlouk-Boubaker optothermal expansivity Ψ  showed amorphous iron oxide oxygen substitution patterns.Temperature-dependent alterations have been outlined in the framework of the Lattice Compatibility Theory.

Figure 2 :
Figure 2: Reflectance spectra of the as-grown iron ditelluride layers.

Table 1 :
Layers mean surface roughness and average height versus substrate temperature.