# AsTe3: A novel crystalline semiconductor with ultralow thermal conductivity obtained by congruent crystallization from parent glass

J. B. Vaney, Cédric Morin, Julie Carreaud, Christophe Candolfi, Andrea

Piarristeguy, Mickael Bigot, Judith Monnier, Annie Pradel, V. Nassif, Gabriel Cuello, et al.

► To cite this version:

J. B. Vaney, Cédric Morin, Julie Carreaud, Christophe Candolfi, Andrea Piarristeguy, et al. AsTe3: A novel crystalline semiconductor with ultralow thermal conductivity obtained by congruent crystallization from parent glass. Journal of Alloys and Compounds, 2025, 1004, pp.175918. 10.1016/j.jallcom.2024.175918. hal-04671409

HAL Id: hal-04671409

https://hal.science/hal-04671409v1

Submitted on 14 Aug 2024

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# AsTe₃: A novel crystalline semiconductor with ultralow thermal conductivity obtained by congruent crystallization from parent glass

Jean-Baptiste Vaney<sup>a</sup>, Cédric Morin<sup>b</sup>, Julie Carreaud<sup>c</sup>, Christophe Candolfi<sup>a</sup>, Andrea Piarristeguy<sup>d</sup>, Mickael Bigot<sup>d</sup>, Judith Monnier<sup>b</sup>, Annie Pradel<sup>d</sup>, Vivian Nassif<sup>e</sup>, Gabriel Cuello<sup>f</sup>, Petr Levinsky<sup>g</sup>, Bertrand Lenoir<sup>a</sup>, Sylvian Cadars<sup>c</sup>, Assil Bouzid<sup>c</sup>, Olivier Masson<sup>c</sup>, Jean-Paul Laval<sup>c</sup>, Julie Cornette<sup>c</sup>, Maggy Colas<sup>c</sup>, Eric Alleno<sup>b</sup>, Gaëlle Delaizir<sup>c,∗</sup>

## ABSTRACT

The synthesis of novel narrow-band-gap semiconductors with ultralow thermal conductivity opens a pathway to the design of functional materials with high thermoelectric performance or with interesting optical sensitivity in the infrared range. Here, we report on the discovery of the novel crystalline binary AsTe₃ (c-AsTe₃) prepared by spark plasma sintering (SPS) from full and congruent crystallization of amorphous AsTe₃ (a-AsTe₃) previously prepared by twin roller quenching. X-ray diffraction suggests that the structure of c-AsTe₃ can be described as a superstructure of elementary Te with a specific distribution of As and Te atoms. More specifically, it appears as an intergrowth of a Te subunit (3 atoms) and As2Te5 subunit (6 As + 15 Te atoms) separated with interlayer spaces. The optical transmittance measured on both crystalline and amorphous AsTe₃ indicates a maximum transmittance of 22 % over the infrared range 10–25 µm. Transport properties measurements, performed between 5 and 375 K, reveal that AsTe₃ behaves as a lightly doped, *p*-type semiconductor. The complex crystal structure combined with a small-grain-size microstructure of the sample yields extremely low lattice thermal conductivity values of 0.35 W m⁻¹ K⁻¹ near 300 K. This poor ability to conduct heat is the main property that gives rise to an estimated dimensionless thermoelectric figure of merit *ZT* of ~0.3 at 375 K. These findings show that the recrystallization of amorphous phases by SPS provides an effective approach for stabilizing novel phases with interesting functional properties.

## 1. Introduction

Thermoelectric materials, which can convert heat into electricity and vice versa, constitute an appealing alternative technology for waste-heat harvesting. The thermoelectric performance of a material is evaluated by the dimensionless thermoelectric figure of merit $ZT = \frac{\alpha^2 T}{\rho \kappa} = PT/\kappa$ where $\alpha$ is the thermopower (or Seebeck coefficient), $P = \alpha^2/\rho$ is the power factor, $\rho$ is the electrical resistivity, $\kappa$ is the total thermal conductivity and $T$ is the absolute temperature [1,2].

A good thermoelectric material should simultaneously exhibit a large thermopower, a low total thermal conductivity and a low electrical resistivity. However, the mutual interdependence of these three transport properties via the carrier concentration makes the design of highly-

efficient thermoelectric materials a complex task. In most state-of-the-art thermoelectric compounds, high power factors are obtained when the carrier concentration is in the range $10^{18}-10^{21}$ cm⁻³ at 300 K i.e. in heavily-doped crystalline semiconductors or semimetals [1–4]. Another ingredient paramount to achieving high *ZT* values is the combination of a complex crystalline structure and a designed microstructure to limit the contribution of the heat-carrying acoustic modes in the thermal transport [5]. A different strategy consists in focusing on materials exhibiting intrinsically very low thermal conductivity values resulting from the presence of low-energy optical modes usually observed in materials with a crystal structure containing cages or tunnels where loosely-bounded cations can reside [6–12]. Recently, breakthroughs in thermoelectric performance have been achieved in representative

<footnote><sup>a</sup> Institut Jean Lamour, UMR 7198 CNRS – Université de Lorraine, Nancy, France</footnote>

<footnote><sup>b</sup> Université Paris Est Créteil, CNRS, Institut de Chimie et des Matériaux Paris Est (ICMPE), UMR 7182, Thiais, France</footnote>

<footnote><sup>c</sup> Institut de Recherche sur les Céramiques (IRCER), UMR CNRS 7315-Université de Limoges, France</footnote>

<footnote><sup>d</sup> ICGM, Univ Montpellier, CNRS, ENSCM, Montpellier, France</footnote>

<footnote><sup>e</sup> Institut Néel, Grenoble, France</footnote>

<footnote><sup>f</sup> Institut Laue Langevin, Grenoble Cedex 9, France</footnote>

<footnote><sup>g</sup> Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University, Prague, Czech Republic</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: gaelle.delaizir@unilim.fr (G. Delaizir).</footnote>
thermoelectric systems such as PbTe [4,13,14], PbSe [15–17], SnSe [18–21] and Bi₂Te₃ [22–24].

The monoclinic α-As₂Te₃ (space group C2/m) and its rhombohedral metastable polymorph β-As₂Te₃ (space group R3m) have been recently investigated for their interesting thermoelectric properties [25–32]. Both compounds are narrow-band gap semiconductors with lattice thermal conductivity values as low as 0.8 W m⁻¹ K⁻¹ above 300 K. This favorable combination of transport properties led to promising thermoelectric performance upon optimization of the carrier concentration with peak ZT values as high as 0.65 at 423 K and 0.85 at 523 K in Sn-substituted β-As₂Te₃ and α-As₂Te₃, respectively [25,26]. These high values show that binary compounds in the As-Te phase diagram may be worthy of further investigations.

In this system, monoclinic α-As₂Te₃ (space group C2/m, no. 12) and cubic AsTe (space group Fm$\overline{3}$m, no. 225) are the only stable crystalline phases that have been reported to date [33,34]. Here, we report on the characterizations of a new metastable crystalline binary phase with the chemical composition AsTe₃. This compound is located near the eutectic region of the phase diagram. This phase was first stabilized in a glass composition Cu₁₅As₂₀Te₆₅ after a thermal treatment near the crystallization temperature, Tc, but was not further characterized [35]. Some attempts to get this compound as single phase were undertaken and finally pure AsTe₃ was obtained by full and congruent crystallization from the parent glass of the same composition synthesized by twin-roller, ultrafast-quenching techniques. The structure of amorphous AsTe₃ has been already investigated and described on the basis of X-ray diffraction, first-principles molecular dynamics (FPMD), and machine learning interatomic potentials (ML-GAP) [36] but the structure of crystalline AsTe₃ remains unsolved.

Glass crystallization is a powerful, non-conventional elaboration process that allows to stabilize metastable crystalline phases [37]. The thermo-mechanical cycle applied during the spark plasma sintering (SPS) treatment enabled the consolidation of the amorphous powders into either dense amorphous or crystalline samples of AsTe₃ depending on the dwell time applied. Because chalcogenide glasses, and especially Te-based compositions, are very attractive optical materials in the infrared range for thermal cameras or optical sensors, the optical properties of both amorphous and crystalline AsTe₃ have also been investigated [38].

In the present paper, we report on a thorough experimental study regarding the physical properties of this new compound and their relationship with AsTe₃ crystal structure with a and particular focus on the transport properties.

## 2. Experimental section

### 2.1. Synthesis

AsTe₃ was synthesized from As (Goodfellow, 99.99 %) and Te ingots (5 N+, 99.99 %) that were inserted a silica ampoule pumped under secondary vacuum (10⁻⁵ mbar). The sealed tube was subsequently heated in a rocking furnace at 1123 K for 2 h and quenched in water. The resulting bulk was crushed into small pieces, melted again before being quenched using the twin roller technique. The pieces of material were then introduced in an Argon atmosphere glove box equipped with a set-up made of a quartz tube pierced at the end and a RF induction furnace. An Ar pressure gas jet was introduced in the tube forcing the melt to flow through the hole between the rotating twin rollers. Vitreous AsTe₃ quenched flakes were then finally obtained [39].

The consolidation of the AsTe₃ amorphous powders was achieved by spark plasma sintering (SPS) using a Dr.Sinter 825 Syntex machine. The powder was introduced in a graphite die and was consolidated under vacuum (5.10⁻² mbar) at 393 K under 50 MPa either for 2 min or 30 min to get amorphous or fully crystalline bulk samples, respectively. During the SPS process, the temperature was monitored by a

thermocouple positioned through the die in the vicinity of the sample.

### 2.2. Sample characterizations

Powder X-ray diffraction (PXRD) data and high-temperature X-ray diffraction (HT-XRD) were collected using a Bruker D8 Advance diffractometer with reflection mode (Cu Kα₁ radiation).

Raman spectra were recorded using an In Via Reflex Renishaw micro-Raman spectrometer at 785 nm. A grating of 1200 gr/mm has been used with an objective x50LWD and using a laser power of 0.02 mW to avoid any damage of the sample.

Neutron thermodiffraction experiments were performed using the D1B powder diffraction instrument at the Institut Laue-Langevin (Grenoble, France). The instrument is a two-axis diffractometer having a banana-like Position Sensitive Detector composed by ³He/CF4 1280 cells with a step of 0.1° in 2θ. The samples were placed in non-sealed cylindrical vanadium containers with an outer diameter of 8 mm and an inner diameter of 7.6 mm, under He exchange gas. A Radial Oscillating Collimator enables to remove diffraction peaks from the aluminum tail of the cryofurnace environment. The standard ILL cryofurnace and its control system allowed performing heating ramps at a heating rate of 0.1 K.min⁻¹ from room temperature up to 443 K, and then between 443 K and 473 K at 3 K.min⁻¹. The incident neutron wavelength (2.530219 Å) was selected by using a pyrolytic graphite monochromator (plane 002).

Scanning electron microscopy (SEM) was carried out using a ZEISS EVO HD15 to determine the chemical composition through energy dispersive spectroscopy (EDS-SEM) of the twin-roller-quenched flakes and to probe the microstructure of AsTe₃ bulk samples via observations of fracture on the cross section.

Transmission electron microscopy (TEM) was performed on a FEI Tecnai G2 F20 ST equipment cooled at 77 K to avoid possible sample damages. The powdered AsTe₃ was ultrasonically dispersed in ethanol and a minute fraction was deposited on a grid.

The thermal behavior of amorphous AsTe₃ was investigated by differential scanning calorimetry (DSC) using a Mettler DSC30 equipment. Around 10 mg of powder was placed inside sealed aluminum pans. Continuous heating experiments were performed from room temperature up to 800 K using different heating rates ranging from 5 up to 40 K. min⁻¹. The crystallization activation energy Eₐ was calculated using the Ozawa's method specifically designed for non-isothermal study [40]. Eₐ was deduced from the slope of ln v plotted as a function of 1000/Tₚ₁, where Tₚ₁ is the temperature at the top of the crystallization peak, following the relation:

$$ \frac{d(\ln v)}{d(\frac{1}{T_{p1}})} = - \frac{E_{c}}{R} \qquad (1) $$

where R is the gas constant.

The apparent density of the SPS bulk samples and the crystalline and amorphous AsTe₃ powders were determined through the Archimedean principle using a Kern balance and helium pycnometer (Micromeritics) respectively.

Optical measurements were carried out at 300 K on crystalline AsTe₃ powders of implementing diffuse reflectance spectroscopy with a Thermo Scientific Nicolet 6700 FTIR spectrophotometer equipped with integrated sphere. Reflectance data R were used to estimate the absorption coefficient using the Kubelka-Munk relation F(R) = (1-R)²/2 R. The data were plotted as (F(R)ħω)ⁿ versus ħω, where ħω is the incoming photon energy, to estimate the optical band gap Eₐ. The direct (n = 2) extrapolation of the absorption edge to zero was considered. The infrared transmittance of both amorphous and crystalline AsTe₃ was measured on bulk pieces using Thermo Scientific Nicolet 6700 FTIR.
## 2.3. Transport properties measurements

For transport properties measurements, the densified pellet of crystalline AsTe₃ was cut with a diamond-wire saw into samples of appropriate shape and dimensions.

Electrical resistivity, thermopower and thermal conductivity measurements were carried out at low temperatures (5 – 300 K) using the thermal transport option of a physical property measurement system (PPMS, Quantum Design). The sample, of dimensions 1.0×1.5×8 mm³, was cut perpendicularly to the pressing direction. Four copper bars were glued onto the sample using a small amount of silver epoxy. To establish a temperature gradient, a heat sink was attached to one end of the sample to maintain it at a constant temperature while applying heat at the other end. Calibrated cernox thermometers were screwed onto two copper bars glued between 2.5 and 3.5 mm apart. The temperature difference was monitored to amount to 2 % of the temperature at which the measurement was done. The voltage and temperature differences were measured at the same position along the sample. The collected data were corrected for thermal radiations by a model implemented in the TTO option software by Quantum Design that uses an accurate estimate of the bath temperature around the sample.

The electrical resistivity and thermopower were measured simultaneously from 300 up to 375 K using a ZEM-3 (ULVAC-RIKO) system on the same sample. The thermal conductivity was determined via thermal diffusivity measurements carried out between 300 and 375 K under argon atmosphere on a cylinder-shaped sample cut parallel to the pressing direction using a LFA 427 (Netzsch) equipment. Both properties are related by the formula $κ = dC_pα_V$ where d is the thermal diffusivity, $C_p$ is the specific heat and $α_V$ is the experimental density. Specific heat measurements were performed by the continuous scanning method under argon atmosphere using a DSC 403 F3 equipment (Netzsch). Density was determined from geometrical dimensions and weight and its temperature dependence has been neglected in the present case. The experimental uncertainties in $κ$, $α$ and $ρ$ are estimated to be 10, 5 and 5 %, respectively, giving rise to an overall uncertainty of 17 % in the ZT values [41].

# 3. Results and discussion

## 3.1. Thermal properties, crystal structure and microstructure of crystalline AsTe₃

The chemical composition of different amorphous flakes and zones after twin roller quenching and after SPS was checked by SEM-EDS in order to obtain information on the chemical homogeneity. As shown in Table 1, the results were quite reproducible regardless of the chosen area. The mean atomic percentages were 75.7 ± 1.0 at% Te, 24.3 ± 1.0 at% As and 75.5 ± 1.0 % Te, 24.5 ± 1.0 % As for the amorphous flakes and the SPS bulk sample, respectively (see Table 1). Both compositions are in good agreement with the initial stoichiometric composition AsTe₃ and with the fact that this compound, never before obtained in a pure form and yet-to-characterize, crystallizes in a full and congruent manner from the parent glass.

The density of amorphous and crystalline AsTe₃ powders was found to be 5.639 ± 0.002 and 6.204 ± 0.002 g.cm⁻³, respectively. The

<table><caption>**Table 1**

Chemical composition of amorphous flakes and AsTe₃ crystalline SPS sample (standard deviation ± 1 at%).</caption><thead><tr><th>Amorphous AsTe<sub>3</sub> flakes (twin-roller)</th><th>Zone 1</th><th>Zone 2</th><th>Zone 3</th></tr></thead><tbody><tr><td>% As (at)</td><td>24.3 ±1</td><td>24 ±1</td><td>24.6 ±1</td></tr><tr><td>% Te (at)</td><td>75.7 ±1</td><td>76 ±1</td><td>75.4 ±1</td></tr><tr><td>Crystalline AsTe<sub>3</sub> (SPS)</td><td>Zone 1</td><td>Zone 2</td><td>Zone 3</td></tr><tr><td>% As (at)</td><td>24.8 ±1</td><td>24.4 ±1</td><td>24.1 ±1</td></tr><tr><td>% Te (at)</td><td>75.2 ±1</td><td>75.6 ±1</td><td>75.9 ±1</td></tr></tbody></table>

former value is close to that reported by Cornet et al. who first reported amorphous AsTe₃ in 1973 [42]. The AsTe₃ amorphous powder was then sintered by SPS under vacuum at 393 K and 50 MPa for 30 min. The geometrical density of crystalline SPS pellet is 5.97 g.cm⁻³ leading to a compactness of 96 %.

Fig. 1a shows the DSC thermograms recorded for the AsTe₃ amorphous composition with varying the heating rate between 5 K min⁻¹ and 40 K min⁻¹. From the data collected with a heating rate of 5 K min⁻¹, a glass transition temperature $T_g$ and a temperature of the onset of crystallization $T_c$ of 365 K and 405 K, respectively, were inferred. The activation energy $E_c$ determined from the Ozawa's equation corresponding to the crystallization of AsTe₃ is about 180 kJ mol⁻¹ (Fig. 1b) which is consistent with values reported for telluride glasses. Activation energy ($E_c$) is one of the important parameters describing the transformation kinetics during crystallization. $E_c$ values are found to be larger at higher crystallization temperatures.

Fig. 2a shows the PXRD patterns measured on the flakes synthesized by the twin-roller ultrafast quenching and those measured on the SPS samples. As expected, the twin-rolled flakes are amorphous. On the contrary, the SPS sample shows diffraction peaks which correspond to the crystallization of the novel AsTe₃ phase.

The thermodiffractogram recorded on the initial amorphous composition AsTe₃ quenched from 923 K allows to determine the first steps of the crystallization process between room temperature and 443 K, by steps of 5/10 K at a heating rate of 10 K/min. In good agreement with DSC data, the crystallization begins at 403 K with the formation of AsTe₃ as predominant phase with concomitant presence of Te (JCPDS 36–1452) and β-As₂Te₃ (JCPDS 39–1043) phases (Fig. 2b). It is noteworthy that these two impurities were already present in the parent glass and could be explained by a lower quenching rate during the twin roller process. However, the intensity of these impurities does not evolve as the temperature increases and c-AsTe₃ remains the major phase.

Fig. 3 shows a 3D plot of the crystallization of amorphous AsTe₃ as observed by neutron diffraction between 375 K and 381 K with a heating rate of 0.1 K/min. From 300 up to 376 K (not shown here), no discernible changes were observed indicating that AsTe₃ remained amorphous. This temperature marks the crystallization onset with three peaks appearing at 2θ values of 48.1°, 63.4° and 75.2° and corresponding to the most intense reflections of the AsTe₃ PXRD pattern. This temperature of first crystallization differs from the one observed by DSC and this could be explained by both the temperature measurement localization and the heating rate that are different. At about 380 K, the background suddenly flattens due to the complete crystallization of the glass, with at least three new intense peaks discernable at 45.8°, 49.2° and 94.3° (see inset of Fig. 3). Beyond this temperature, no further evolution with increasing temperature up to 473 K occurs.

The PXRD pattern of sintered AsTe₃ (Fig. 2) shows 9 relatively intense main peaks in the 10–80° (2θ) range, in addition to many low-intensity peaks with varying widths. These 9 peaks could be easily indexed with a hexagonal unit cell of parameters a = b = 4.1270 Å and c = 6.1468 Å. Interestingly, these values were found to be relatively close to the unit cell parameters of elemental Te (a = b = 4.456 Å and c = 5.921 Å), which crystallizes in the trigonal P3̇121 space group with Te atoms in the 3a Wyckoff positions. In addition, the structure of elemental Te modified using the as-obtained cell parameters was able to reproduce well the 9 main peaks both in positions and intensities and led to an atomic density of $ρ = 0.0331$ at/Å³, which is very close to the experimental value obtained for c-AsTe₃ ($ρ = 0.0326$ at/Å³). These results suggested that the crystal structure of c-AsTe₃ could be described as a superstructure of elementary Te with a specific distribution of As and Te atoms in the crystallographic sites of the parent structure.

Unfortunately, it was not possible to go much further with conventional methods of structure determination. Full indexation of the PXRD diagram was not successful, nor were crystal growth experiments to obtain AsTe₃ single crystals and transmission electron microscopy
<figure><img src="image_2.png" /><figcaption>Fig. 1. (a) DSC thermograms of AsTe₃ amorphous powder (obtained after twin-roller quenching) at different heating rates (from 5 K/min to 40 K/min), (b) Plot of ln ν as a function of 1000/Tₚ allowing the calculation of the AsTe₃ crystallization energy E_c using Ozawa's method.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** (a) PXRD patterns of powder after twin-roller quenching (black curve) and powder sintered at 120°C for 30 min (50 MPa) by SPS (red curve), (b) HT-XRD of AsTe₃ amorphous powder (* unreported AsTe₃, 0 Te (JCPDS 36-1452) and + β-As₂Te₃, (JCPDS 39-1043)).</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 3.** 3D plots of the neutron thermodiffraction data (λ=2.530219 Å) collected between 33° and 85° as a function of temperature for the AsTe₃ amorphous sample. The inset shows a 2D mapping of the diffraction data between 0.8° and 128.6°.</figcaption></figure>

observations at 77 K. For this reason, we resorted to an unconventional approach combining PXRD, semi-automated atomic scale modeling and experimental atomic pair distribution function (PDF) analysis to propose a realistic structural model of c-AsTe₃ that highlights the structural complexity and local order of this phase. The details of this study is beyond the scope of the present manuscript and have been submitted elsewhere [43]. For discussion purposes, we describe below the proposed model in this investigation. It is important to keep in mind that, while providing a description in good overall agreement with experimental results, this model alone does not capture the entire complexity of the crystalline AsTe₃ phase.

The proposed model has monoclinic symmetry and is illustrated in Fig. 4a. It appears as an intergrowth of a Te subunit (3 Te atoms, highlighted in pale red in Fig. 4a) and an As₂Te₅ subunit (6 As + 15 Te

atoms) separated with interlayer spaces consistent with van der Waals interactions. The latter contains local structural features (highlighted in grey in Fig. 4a) similar to those found in α-As₂Te₃ (shown in Fig. 4b), albeit with substitutions of As atoms by Te atoms (highlighted in black in Fig. 4b and 4c) on both sides of the intergrown Te subunit. As a result of these As/Te substitutions, the unit cell of the AsTe₃ model structure is found to contain two domains of distinct compositions: a pure-Te composition domain almost 1 nm in thickness (9 Te atoms) with local structural arrangements visually similar to those found in elemental-Te (Fig. 4b) and a mixed-composition domain consisting of 6 As and 9 Te atoms (As₂Te₃). As illustrated in Fig. 4c, it translates at the atomic scale into a diversity of Te and As coordination states, albeit all in more or less distorted octahedral environments (considering a cutoff of 3.3 Å) with various combinations of Te and/or As bonded and/or non-bonded
<figure><img src="image_5.png" /><figcaption>**Fig. 4.** (a) The best candidate structure model proposed in [43] for c-AsTe₃ viewed along all three crystallographic axes. (b) Crystal structures of known α-As₂Te₃ and elemental-Te shown for comparison purposes. (c) Representative local environments identified with As-Te and Te-Te cutoff distances of 3.5 Å. Atomic proximities are shown as dashed black lines below a cutoff of 3.3 Å, which corresponds to a minimum in both As-Te and Te-Te partial radial distribution functions (not shown), and as dotted gray lines above this value.</figcaption></figure>
neighbors. These results are in good agreement with the recent work of An *et al.* who suggested that As-doped tellurium (as well as Bi or Sb dopants) samples feature a dual-phase structure consisting of a nearly undoped Te matrix as well as As₂Te₃ (respectively Bi₂Te₃ and Sb₂Te₃) precipitates that are responsible for improved thermoelectric properties [44].

The synthesis process (crystallization of the parent glass) and the far-
from-perfect crystallinity of the sample suggest that disorder of various
possible origins can be present in the structure, even though the sudden
plateau observed in variable-temperature neutron diffraction data
(Fig. 3) indicates a complete crystallization of the material.

To support the structure model proposed in [43] for c-AsTe₃, we
compare below the Raman spectra of c-AsTe₃ with those recorded on
elemental Te, β-As₂Te₃ and α-As₂Te₃ (Fig. 5a). Each spectrum was
decomposed, as shown in Fig. 5b-5e, in an attempt to reveal and locate
potentially-overlapping contributions. In As-Te crystalline compounds,
the modes with frequencies higher than 160 cm⁻¹ are dominated by
light As atoms, modes with frequencies between 100 and 150 cm⁻¹ are
defined by the vibration of heavy Te atoms and modes with frequencies
below 100 cm⁻¹ are collective or lattice modes of vibration where
groups of As and/or Te atoms move in or out of phase [45]. It must be
stressed that Raman spectra of α-As₂Te₃ (layered structure with mono-
clinic C2/m symmetry) and β-As₂Te₃ (rhombohedral structure with R3m
symmetry) resemble that of elemental Te, which crystallizes in a trigonal
space group since they both exhibit the three active Raman phonon
modes characteristics of Te but with various intensities and shifts in
frequencies. Further details about the Raman peak positions

corresponding to vibrational modes in α and β-As₂Te₃ phases, and their
corresponding attribution can be found in Refs. [45,46]. The most
intense Raman peak of elemental Te located at ~122 cm⁻¹ is related to
the A1 mode, corresponding to atoms moving in the basal plane (i.e.
perpendicular to the c-axis) (see Fig. 5e). Two degenerate E modes also
exist in Te, which split into predominately bond-bending and
bond-stretching types. The E1 mode at ~94 cm⁻¹ is caused by rotation
around the α-axis, and the E2 mode at 142 cm⁻¹ is assigned to asym-
metric stretching mainly along the c-axis [47,48]. It is noteworthy that
some Te-related modes (especially the A1 and E2 modes) can be indexed
as impurities in both α and β-As₂Te₃ phases (see black thick decomposed
curves in Fig. 5b and 5c). V. Cuenca-Gotor *et al.* and R. Vilaplana *et al.*
explained the presence of these unexpected crystalline Te modes by the
effect of laser irradiation during the Raman experiments that may
induce local heating even using a low power laser since no crystalline Te
can be indexed in both α and β-As₂Te₃ XRD patterns [45,46] (see Fig. 5b,
5c). Surprisingly the E1 mode related to crystalline Te is not found in our
α and β-As₂Te₃ Raman spectra suggesting that further investigations for
a better understanding of the presence of these modes may be
undertaken.

The decomposition of c-AsTe₃ Raman spectrum reveals five bands. Three of them (at 94, 122 and 137 cm⁻¹) match perfectly the Te Raman spectrum (in contrast to the peaks of unknown origin previously attributed to impurities in α- and β-As₂Te₃) and may correspond to the intergrown pure-Te sub-units depicted in the AsTe₃ crystalline structural model obtained from first-principles modeling. The two others modes observed at 169 cm⁻¹ and 191 cm⁻¹ may be assigned to the sub-unit

<figure><img src="image_6.png" /><figcaption>**Fig. 5.** (a) Raman spectra of crystalline (c-AsTe₃) AsTe₃, elemental Te, α-As₂Te₃ and β-As₂Te₃, Decomposition of Raman spectra (b) β-As₂Te₃, (c) α-As₂Te₃, (d) c-AsTe₃ and (e) Te spectra and its corresponding atom vibration directions. (Dashed lines serve as eye guide and thick black decomposed curves in (b) and (c) correspond to crystalline Te as impurity according to [45] and [46]).</figcaption></figure>
As₂Te₅ domains linked to the mode A_g in the *ac* plane also present in α-As₂Te₃ [46]. The Raman spectrum of c-AsTe₃ is thus intermediate between those of Te and α-As₂Te₃, in agreement with the local structural similarities identified from the modeling. It must also be emphasized that the Raman spectrum of c-AsTe₃ is significantly broader than those of crystalline As₂Te₃ and Te. This indicates the presence of a positional disorder in the structure, affecting both bond lengths and angles, the cause of which is not yet well established, but which could be related to an underlying As/Te chemical disorder.

Finally, the microstructure observed on a SPS fresh fractured crystalline sample reveals some residual porosity as shown in Fig. 6a and 6b. SEM observations show some nano-grains with size less than 50 nm. The microstructure of AsTe₃ was further probed at the atomic scale by TEM observations performed on a thin section of an AsTe₃ SPS sample. The size of the different monocrystalline domains does not exceed few tens of nm (Fig. 6c and 6d).

## 3.2. Transport properties

In light of the interesting thermoelectric properties already demonstrated in α and β-As₂Te₃, the transport properties of c-AsTe₃ were studied. Fig. 7a shows the temperature dependence of the electrical resistivity ρ. Upon heating, ρ rapidly decreases by five orders of magnitude, from around 10⁷ μΩ.m at 5 K to 150 μΩ.m at 375 K, indicating that AsTe₃ behaves as a semiconductor. The room-temperature ρ value is close to that of lightly-doped elemental Te from which the crystal structure of AsTe₃ derives [5]. The ρ(T) data were further analyzed by an activation model according to the relation lnρ = (E_g/(2k_B T)) + A where k_B is the Boltzmann constant and A is a constant. A linear fit to the lnρ versus 1/T data in the 300 – 375 K temperature range yields an estimated band gap E_g of 0.32 eV, in reasonable agreement with the optical band gap of 0.43 eV determined by absorption spectroscopy (see below). This small difference may be attributed to the narrow temperature range over which the ρ(T) data were fitted, with the activation model not being applicable at lower temperatures. Below 300 K, the data were rather analyzed using the variable-range-hopping (VRH)

model ρ(T) = ρ₀e^((T/T₀)ⁿ), characteristic of an impurity hopping conduction typically observed in semiconductors in which a large number of impurity states form a narrow band. In this equation, ρ₀ and T₀ are constants and the exponent n can be written as n = 1/(d+1) where d is the dimension of the conduction mechanism. This equation describes well the experimental data between 300 K down to about 30 K with n = ¼ suggesting that the electrical conduction is mostly three-dimensional (d = 3) (see inset in Fig. 7a). The lack of further decrease in ρ at high temperatures indicates the absence of minority carrier effects due to the moderate temperature range over which AsTe₃ is stable as shown by HT-XRD (Fig. 2b).

The temperature dependence of the thermopower α is shown in Fig. 7b. As in α- and β-As₂Te₃, the α values are positive in the whole temperature range indicating that holes are the dominant charge carriers in AsTe₃. The α(T) dependence confirms the semiconducting nature of crystalline AsTe₃, with a non-linear variation with temperature below 50 K usually observed in lightly-doped semiconductors. Above this temperature, α increases monotonically with increasing temperature to reach 185 μV K⁻¹ at 375 K.

The temperature dependence of the specific heat C_p is shown in Fig. 8a. Near 300 K, the C_p values approach the Dulong-Petit limit 3NR where N is the number of atoms per formula unit and R is the ideal gas constant. The low-temperature C_p/T data obeys the conventional free-electron formula C_p/T = γ + βT² where γ is the Sommerfeld coefficient reflecting the electronic contribution and β is a parameter related to the lattice contribution.

The best fit to the data below 3.5 K yields a γ value indistinguishable from zero to within experimental uncertainty corresponding to a null density of states of the Fermi level, consistent with the semiconducting nature of AsTe₃. The parameter β corresponds to a Debye temperature θ_D = (12π² NR/5β)¹/³ of 154 K. Plotting the phonon contribution to the specific heat C_pph/T³ as a function of T (Fig. 8b) shows a pronounced maximum centered near 10 K that reflects an excess of vibrational density of states over the Debye term. Insights into the phonon spectrum can be usually obtained by fitting these data using a combination of

<figure><img src="image_7.png" /><figcaption>**Fig. 6.** SEM images of a fresh fracture of crystalline AsTe₃ (SPS sample sintered at 393 K for 30 min under 50 MPa), (a) low magnification, (b) high magnification. TEM observations of AsTe₃ crystalline SPS sample: (c) bright field, (d) dark field.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 7. (a) Electrical resistivity and (b) Seebeck coefficient of polycrystalline AsTe$_{3}$ as a function of temperature. The inset of figure (a) shows the lnρ versus T$^{-1/4}$ according to the variable-range-hopping (VRH) model.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 8. (a) Specific heat C$_{p}$ as a function of temperature of AsTe$_{3}$. The horizontal solid black line stands for the Dulong-Petit limit of 3NR. (b) C$_{ph}$/T$^{3}$plotted as a function of T (C$_{ph}$ is the phonon contribution to the specific heat C$_{p}$). The solid black curve corresponds to the best fit to the data according to Eq.(1).</figcaption></figure>

Einstein and Debye terms. This approach has been widely used in cage-like compounds such as skutterudites or clathrates for which the large thermal vibrations of the encaged ions are associated with low-energy optical phonon modes, which can be adequately described by Einstein-like contributions. In this context, the low-temperature C$_{p}$ (T) data are then modelled by a combination of Debye and Einstein terms through the relation

$$C_p = C_D + C_{Ei} = 9N_D R \left(\frac{T}{\theta_D}\right)^3 \int_0^{\theta_D/T} \frac{x^4 e^x}{(e^x - 1)^2} dx + \sum_i p_i N_{Ei} R \left(\frac{\theta_{Ei}}{T}\right)^2 \frac{e^{\theta_{Ei}/T}}{(e^{\theta_{Ei}/T} - 1)^2} \quad (1)$$

where C$_{D}$ is the lattice contribution described within the Debye model, C$_{Ei}$ is the Einstein contribution of the i$^{th}$ vibrational mode of the guest atoms, x = $\hbar\omega/k_{B}T$ with $\hbar$ the reduced Planck constant and k$_{B}$ the Boltzmann constant, N$_{D}$ is the number of Debye oscillators per formula unit and p$_{i}$, N$_{Ei}$ and θ$_{Ei}$ are the degrees of freedom, the number of Einstein oscillators, and the Einstein temperature related to the i$^{th}$ vibrational mode, respectively. The best fit to the data (Fig. 8b), with the two Einstein temperatures and the spectral weights allowed to vary, shows that this model captures reasonably well the temperature dependence of the C$_{p}$ data, yielding an effective Debye temperature θ$_{D}$ of 134 K, and Einstein temperatures θ$_{E1}$ = 44.1 K and θ$_{E2}$ = 172.8 K. The θ$_{E1}$ value

corresponds to a low-energy optical mode at 3.8 meV (0.93 THz). This value is similar to those determined in α-As$_{2}$Te$_{3}$ (around 0.6 THz, that is, 2.5 meV) and β-As$_{2}$Te$_{3}$ (around 0.8 THz, that is, 3.3 meV) by lattice dynamics calculations and Raman spectroscopy. [7,8]. The presence of these low-energy optical modes strongly limits the energy window over which the acoustic branches disperse, thereby lowering the lattice thermal conductivity.

In agreement with these considerations, AsTe$_{3}$ exhibits extremely low thermal conductivity values κ of around 0.35 W m$^{-1}$ K$^{-1}$ in the whole temperature range (Fig. 9), similar to those reported for α- and β-As$_{2}$Te$_{3}$. Due to the rather high ρ values, the measured κ values nearly entirely reflect the lattice contribution κ$_{L}$ according to the Wiedemann-Franz law κ$_{e}$ = LT/ρ that relates the electronic contribution κ$_{e}$ to ρ via the Lorenz number L. The very low κ$_{L}$ values and the absence of an Umklapp peak indicate that the thermal transport in AsTe$_{3}$ mirrors that observed in amorphous compounds. The κ$_{L}$ values approach the glassy limit, according to the model of Cahill and Pohl, estimated to be equal to 0.33 W m$^{-1}$ K$^{-1}$ from the longitudinal (ν$_{L}$ = 2967 m s$^{-1}$) and transverse (ν$_{T}$ = 1422 m s$^{-1}$) sound velocities measured at 300 K by a pulse-echo method. The significant consequences on phonon-phonon scattering processes implied by the presence of low-energy optical modes explains both the very low κ$_{L}$ values of AsTe$_{3}$ and their temperature dependence. Indeed, these modes open an additional channel of three-phonon Umklapp scattering processes, making them still active at low
<figure><img src="image_10.png" /><figcaption>**Fig. 9.** Thermal conductivity of polycrystalline AsTe$_{3}$ as a function of temperature.</figcaption></figure>

temperatures and explaining the absence of a pronounced crystalline peak. The key role played by these modes in shaping the temperature dependence of $\kappa_L$ has been demonstrated in several low-$\kappa_L$ compounds such as tetrahedrites, InTe or type-I clathrates [49–52]. Furthermore, the presence of nano-crystallites with an average grain size of 50 nm observed by SEM and TEM in these polycrystalline AsTe$_{3}$ specimens also contributes to lower the $\kappa_L$ values through enhanced grain boundary and point-defect scattering that predominantly scatter low-energy and high-energy phonons, respectively. Both mechanisms are thus expected to concomitantly reduce the $\kappa_L$ values and weakens their temperature dependence [53].

Fig. 10 shows the temperature dependence of the dimensionless thermoelectric figure of merit ZT. A peak ZT value of 0.28 is achieved in AsTe$_{3}$ at 375 K, a value higher than those measured in pristine α- and β-As$_{2}$Te$_{3}$ at the same temperature. We note that these ZT values should be taken with some caution due to the anisotropic crystal structure of AsTe$_{3}$ that may lead to some degree of anisotropy in the transport properties, as observed in both α- and β-As$_{2}$Te$_{3}$. Thus, the combination of transport properties measured parallel and perpendicular to the

<figure><img src="image_11.png" /><figcaption>**Fig. 10.** Dimensionless thermoelectric figure of merit ZT of polycrystalline AsTe$_{3}$ as a function of temperature. The error bars represent the uncertainties (17 %).</figcaption></figure>

pressing direction likely results in overestimated ZT values. Nevertheless, the comparison between the room-temperature $\kappa$ values of the low and high-temperature data sets are close, suggesting that the possible anisotropy between the parallel and perpendicular directions should be small. Thus, although possibly overestimated, the present ZT values are expected to provide a reasonable estimation of the actual ZT values of AsTe$_{3}$.

### 3.3. Optical measurements

Fig. 11 shows the optical absorption spectra collected on both amorphous and crystalline AsTe$_{3}$ compounds assuming direct transitions. The spectrum of crystalline AsTe$_{3}$ shows an increase up to around 0.4 eV where a steep rise is observed and that likely corresponds to the optical band gap $E_g$ with an estimated value of 0.43 eV. This value is close to those determined from similar optical data for α-As$_{2}$Te$_{3}$ (0.43 eV) and β-As$_{2}$Te$_{3}$ (0.30 eV) [25,26]. The spectrum of amorphous AsTe$_{3}$ exhibits a different behavior with a band gap of around 0.70 eV which is in good agreement with the band gap observed in transmittance experiments at ~ 2 µm (Fig. 8b).

The measurements of the optical transmittance of the AsTe$_{3}$ SPS glass and corresponding crystalline bulk samples (Fig. 8b) show that glass AsTe$_{3}$ exhibits a transmittance of up to 22 % in the range 10–25 µm. The absorption band around 12 µm is attributed to As-O bonds of impurities. While the corresponding crystalline AsTe$_{3}$ compound exhibits a nano-structure with grains less than 50 nm, its transmittance is zero in the range 2–25 µm. This result could be explained by the numerous micro-pores observed on SEM images (see Fig. 6a) that induces light scattering, especially in the visible range. Moreover, the difference in densities between the AsTe$_{3}$ glass and its crystalline counterpart is high (almost 9 %). Thus, the crystallization induces mechanical strain resulting in micro-cracks that can act as scattering centers.

## 4. Conclusion

We reported on a detailed investigation of the synthesis, crystal structure, thermal, optical and transport properties of the novel binary AsTe$_{3}$ compound. Bulk crystalline samples of AsTe$_{3}$ have been successfully synthesized from the full and congruent crystallization of the parent glass by spark plasma sintering. X-ray diffraction and X-ray total scattering coupled with atomic scale modeling methods suggest that the crystal structure of AsTe$_{3}$ derives from the structure of elemental Te with suspected intergrowth of pure Te sheets and α-As$_{2}$Te$_{3}$-like domains. Optical properties of AsTe$_{3}$ glass are promising as a transmittance of 22 % up to 25 µm was recorded with no prior element purification and optimization of the SPS thermomechanical cycle. The small grain size (~60 nm) evidenced in the crystalline AsTe$_{3}$ compound should also favor good optical properties and garner some interest in this field. Transport properties measurements combined with absorption spectroscopy revealed that AsTe$_{3}$ is a narrow-band-gap semiconductor and a poor thermal conductor, as its cousin compounds α- and β-As$_{2}$Te$_{3}$. Despite being undoped, the beneficial combination of relatively low electrical resistivity and thermal conductivity and high thermopower results in a promising peak ZT value of 0.3 at 375 K. Further investigations would be worthwhile to determine whether the synthesis method used herein allows for a further optimization of the thermoelectric properties by extrinsic doping. On a more general aspect, these results show that using the thermomechanical SPS process to crystallize amorphous phases can lead to the discovery of novel metastable phases with interesting functional properties.

### CRediT authorship contribution statement

Vivian Nassif: Investigation. Cédric Morin: Formal analysis, Data curation, Conceptualization. Sylvian Cadars: Methodology, Formal analysis. Jean-Baptiste Vaney: Methodology, Investigation, Data
<figure><img src="image_12.png" /><figcaption>**Fig. 11.** (a) Optical absorption spectrum for amorphous (●) and crystalline (■) AsTe₃ plotted as $[F(R)×E]²$ vs (E = hω) (that is for direct transition). The extrapolation of the solid black line to zero provides an estimate of the optical band gap Eg. (b) Transmittance in infrared range of the AsTe₃ glass and fully crystallized samples (thickness: 0.5 mm).</figcaption></figure>

curation. Bertrand Lenoir: Supervision, Project administration. Gaëlle Delaizir: Writing – original draft, Project administration, Funding acquisition. Petr Levinsky: Investigation. Gabriel Cuello: Data curation. Mickael Bigot: Methodology, Investigation. Maggy Colas: Formal analysis, Data curation. Andrea Piarristeguy: Resources, Methodology. Jean-Paul Laval: Methodology, Data curation. Christophe Candolfi: Resources, Methodology, Investigation. Olivier Masson: Formal analysis, Data curation, Conceptualization. Julie Carreaud: Supervision, Resources, Methodology. Assil Bouzid: Software, Data curation, Conceptualization. Annie Pradel: Supervision. Eric Alleno: Supervision, Methodology, Investigation, Conceptualization. Judith Monnier: Investigation, Formal analysis. Julie Cornette: Formal analysis, Data curation.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

Data will be made available on request.

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