Gergana Alexieva, Anna Amova

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ACOUSTIC AND OPTICAL PROPERTIES OF Ag4SSe.2PbTe

Gergana Alexieva1, Anna Amova2

ABSTRACT

The acoustic and optical properties of Ag4SSe.2PbTe solid solution are experimentally studied. A low longitudinal acoustic velocity of 2994 m/s is measured while the transverse acoustic wave is found to exhibit a very high attenu-ation due to viscous losses related to the polycrystalline nature of the material. A high refractive index of about 4 is measured in the IR range of the lower optical loss between 1400 nm and 2500 nm. Combined with the low acoustic velocity, it provides a very high acoustooptic figure of merit in the studied spectral range.

Keywords: acoustic properties, optical properties, IR range, solid solution, Ag4SSe.2PbTe.

Received 16 October 2018Accepted 04 February 2019

Journal of Chemical Technology and Metallurgy, 54, 5, 2019, 1035-1039

1 Sofia University, Faculty of Physics Department of Solid State Physics and Microelectronics 5 J. Bourchier Blvd., 1164 Sofia, Bulgaria 2 University of Architecture, Civil Engineering and Geodesy Hydrotechnical Faculty, Department of Physics 1 Hristo Smirnenski blvd., 1046 Sofia, Bulgaria E-mail: gerry@phys.uni-sofia.bg

INTRODUCTION

The solid solutions have been an object of consider-able interest because of their potential for valuable tech-nological properties [1, 2]. In principle, they represent mixtures of two crystalline solids which coexist as a new crystalline solid. The mixing is usually accomplished by combining the two melts at high temperatures and sub-sequent cooling to form the new solid. Like the liquids, the solids have different degrees of mutual solubility depending on their chemical properties and crystalline structure. This determines their atoms disposition in the mixed crystal lattice. The substances may be solu-ble over a range of concentrations producing a crystal whose properties vary continuously over the range. This provides a way to tailor the properties of the solid solu-tion aiming specific applications. In addition, the newly obtained properties may differ considerably from those of the initial compounds.

The compound Ag4SSe.2PbTe has been studied [3] for the first time as a part of a comprehensive project

involving an investigation of the three-component chal-cogenide system As2Se3-Ag4SSe-PbTe [4]. Specifically, Ag4SSe.2PbTe is found to be an intermediate phase of Ag4SSe-PbTe system. Our further interest in this mate-rial is determined by the fact that Ag4SSe and PbTe are narrow-band semiconductors of similar properties. Ac-cording to Petruk et al. [5] the low-temperature (below about 100oC) polymorphous modification a-Ag4SSe has an orthorhombic structure with unit cell parameters: a = 0.433 nm, b = 0.709 nm and c = 0.776 nm. PbTe has a face-centered cubic lattice of NaCl type of a unit cell parameter a = 0.6452 nm [6]. In addition, as a result of this structure closeness, the optical band gap of Ag4SSe is 0.28 eV at 300 K [7], while this of PbTe is 0.32 eV [8]. The intermediate crystalline Ag4SSe.2PbTe phase is found to be monoclinic with unit cell parameters: a = 0.3510 nm, b = 0.3053 nm, c = 0.2951 nm, α = 99.07o, β = 94.37o, γ = 89.31o [3]. As a whole, the binary system Ag4SSe-PbTe does not provide glassy phases [4].

As mentioned above, the similarity of the two constituents suggests the possibility of obtaining novel

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properties of the solid solution realized on their basis. A verification of this assumption has already been demonstrated in a recent paper [9] on the optical conduct of Ag4SSe-PbTe system in the mid-infrared spectral range. A spectral window of reduced absorbance (7000 cm-1 - 4000 cm-1) is unexpectedly found deep within the otherwise intrinsic absorption range of the two semi-conductors (approximately above 2500 cm-1) (Fig. 1). This window is practically composition independent and thus refers also to the crystalline intermediate phase Ag4SSe(33)-PbTe(67) hereby discussed. On the other hand, as commented in ref. [10], the index of refraction of these multi-component systems depends strongly on the presence of heavy elements. So, it is to be expected that the combination of Ag and Pb in the presently discussed system would have a pronounced positive effect on its refraction index. It is also interesting to see how these elements affect the sound velocity as it has generally been known that a possible downshift can be observed in such cases (see for example ref. [11]).

The above considerations reveal the need to study the discussed properties provoked by both the fundamental interest and the potential of future applications.

It is important to note that due to the method by which the solid solution has been obtained (a direct monotemperature synthesis in quartz ampoules evacu-ated and sealed at a residual pressure of ≈ 0.1 Pa with subsequent vibration stirring and homogenization of the melt at 500oC for 2 h [3]), the compound resides essentially in a polycrystalline state.

EXPERIMENTALAcoustic measurements

A cylindrical sample of a length of 9.82 mm and a diameter of 9.32 mm was specifically fabricated aiming this analysis. Piezoelectric ceramic ultrasound transduc-ers were attached with epoxy glue on its two polished sides. Two types of transducers were used. Thickness-polarized plates of a fundamental resonant frequency of 7.7 MHz were applied for exciting and detecting a longitudinal acoustic wave in the sample. In the second experiment, in-plane polarized plates of a frequency of 4.4 MHz were used to produce transverse waves. The plate surfaces were connected to copper electrodes through which RF electric field from HMF 2550 function signal generator was supplied. The output signal was fed to Tektronix TDS 2022B digital oscilloscope. The RF pulse was of a resonant frequency and a length less than 500 ns. The delay of the output echo with respect to the electromagnetic feed-through was measured. The acoustic velocity was readily derived from it. Because of the considerable loss observed, the input signal was passed through a 40 dB 603L RF linear power amplifier.

Optical studyThe refractive index n and the extinction coefficient

k of the material were initially studied in the visible range. The two characteristic values were determined on the ground of the multiangle ellipsometric data col-lected with a null type ellipsometer (LEM 2) working at a single wavelength of 632.8 nm.

Then the focus was on the IR range of seeming transparency (7000 cm-1 - 4000 cm-1; 1400 nm - 2500 nm) where the absorption dropped well below the high intrinsic values expected (Fig. 1). The reflectance spec-trum was recorded following the procedure described in ref. [10].

Fig. 1. IR spectra of samples of a varying composition from the binary system Ag4SSe-PbTe [9].

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RESULTS AND DISCUSSION

Fig. 2 is an oscillogram obtained in the longitudinal wave case. The delay of the first echo over the length of the sample is 3280 ns measured with an accuracy of 20 ns. The corresponding value of the longitudinal acoustic velocity refers to 2994 m/s (an accuracy of 0.8 %, mainly limited by the time error). The overall loss of the signal amounts to 39 dB and, although a considerable part of it comes from the input and the output piezoelectric conversion as well as the impedance mismatching, it still indicates a high acoustic loss within the material itself.

The transverse wave experiment does not reveal a well distinguishable echo. The cylindrical sample is replaced by a thin (1 mm) plate with polished sides aiming to reduce the travelled acoustic path. The input pulse length is decreased to one or two oscillations only to prevent the input and the output signals from overlap-ping along this short path. The result obtained is shown in Fig. 3a,b. A weak echo is observed at about 560 ns suggesting a transverse velocity of approximately 1780 m/s (Fig. 3a). The fact that the echo disappears out of resonance (Fig. 3(b)) is a proof of its acoustic nature. However, its amplitude is so weak even after massive amplification that neither a precise result can be ob-tained, nor any possible application can be expected. This result obviously reveals a highly viscous behavior of the grainy matrix of the material studied.

The longitudinal sound velocity obtained is consid-erably lower than that reported for pure PbTe (3590 m/s at a room temperature [12]) and lower than that of many technological materials. Taking into account the simple

method of preparation of this compound, it seems attrac-tive for non-dispersive piezoelectric delay lines, but the considerable attenuation requires signal amplification. This attenuation obviously results from the friction be-tween the constitutive crystalline grains of the material. It is much more pronounced in case of the transverse wave leading to its practical lack of propagation.

The result from the ellipsometric measurements is: n = 3.4 +/- 0.1; k = 1.52 +/- 0.05. The reflectance spec-trum obtained from the plate sample discussed above is shown in Fig. 4. It is attempted to get transmittance data from the plate sample but a detectable output is not obtained despite the moderately small (1 mm) thickness. The further decrease of the thickness turns out to be impossible because the grainy structure of the material provokes its breakage. The lack of transmittance data

Fig. 3. A transverse wave response at a resonance of 4.4 MHz (a) and 10 MHz (b). The upper channel refers to the input pulse.

Fig. 2. An oscillogram indicating the delay of the first longitudinal echo. A frequency of 7.7 MHz.

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does not provide the simultaneous derivation of the refractive index and the extinction coefficient in the studied range following the method described in ref. [10]. However, side data on the extinction in the visible range indicates that it drops by at least one order of magnitude from 633 nm to 1000 nm. Its contribution to the reflection amounts to 6 % at 633 nm in accordance with ellipsometric data obtained.

It should be much weaker in the IR range. Thus, the extinction is neglected and the refractive index is directly obtained from the reflection coefficient. Its dispersion is shown in Fig. 5. The results are quantitatively sound as they correlate to the exact experimental value in the visible range under the anomalous dispersion exhibited.

The material demonstrates a high, weakly dispersive refractive index (about 4) in the IR range of reduced light absorption (1400 nm - 2500 nm). Meanwhile, the reasons for this lower absorption are not definitely clear. They appear somehow related to the lower absorption measured in the visible range – the k-value of 1.42 obtained at 633 nm yields an absorption coefficient α = 2.8 x 105 cm-1. It is about 3 times lower than the one reported for pure PbTe (7.5 x 105 cm-1 [13]). It seems possible that the solution matrix introduces prohibition rules for some of the electron band-to-band transitions, which are characteristic for the studied spectral ranges in these narrow-band materials. This result might also be due to a quantum confinement effect in the polycrystal-line material. It is worth noting that a blue shift of the absorption onset of 1.26 eV is noticed in PbTe thin films compared to that of the bulk PbTe material (0.32 eV)

due to the small particles size [14]. Among other possible applications, the high value of

the refractive index combined with a low acoustic veloc-ity appears very attractive for use in IR acoustooptics. It has been known that the figure of merit of the acoustoop-tic interaction varies as the 6th power of the index and the inverse 3rd power of acoustic velocity [15]. Thus, a severe advantage over fused silica used as a reference material is obtained. Fused silica has an index of 1.46 and a velocity of 5960 m/s. This makes a superiority coefficient of over 3300. It is much higher than those of all other known acoustooptic materials in the infrared range. However this advantage is partly compromised by the lack of sufficient transparency. As evident from the previously described experiments, despite the con-siderable decrease, the extinction coefficient and hence the optical absorption of the compound studied are still efficient enough to stop the IR transmittance in the range of interest over travelled paths of the order of 1 mm. Thus, lower optical paths (for example using acoustic wave geometries referring to thin film surfaces) should be created or intensive light sources have to be applied to ensure a profit from the attractive material properties in the optic or acoustooptic fields.

CONCLUSIONS

A low longitudinal acoustic velocity is found in the solid solution of Ag4SSe.2PbTe most probably due to the presence of heavy elements. Meanwhile, the transverse acoustic wave exhibits a very high attenuation caused

Fig. 5. Dispersion of the index of refraction obtained from the reflection observed in the IR range.

Fig. 4. A reflectance spectrum of the plate sample.

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by the viscosity of the polycrystalline material. A high refractive index is found in the IR range of the previously established reduced optical absorption. It is attributed to the composition of the material. Combined with the low acoustic velocity, it results in a very high figure of merit of acoustooptic interaction in the studied spectral range. The obtained results indicate possible applications of the material for time delay or acoustooptic purposes.

AcknowledgementsThe authors thank Prof. T. Babeva and Dr. G. Tsut-

sumanova for their assistance in optical experiments conductance.

REFERENCES

1. W. Callister, Jr., D. Rethwish, Material Science and Engineering: an Introduction, Wiley, Eighth Edition, 2009.

2. https://www.britannica.com/science/solid-solution.3. A. Amova, T. Hristova-Vasileva, V. Vassilev,

Phase equilibria in the Ag4SSe-PbTe system, Thermochimica Acta, 531, 2012, 42-45.

4. A. Amova, T. Hristova - Vasileva, L. Aljihmani, I. Bineva, V. Vassilev, Region of glass formation and main physicochemical properties of glasses from the As2Se3 - Ag4SSe - PbTe system, J. Alloys Comp., 573, 2013, 32-36.

5. W. Petruk, D.R. Owens, J.M. Stewart, E.J. Murray, Observations on Acanthite, Aguilarite and Naumannite, Can. Mineral., 12, 6, 1974, 365-369.

6. N.H. Abrikosov, L.E. Shelimova, Poluprovodnikovie materialii na osnove AIVBVI, Nauka, Moskva, 1975, (in Russian).

7. Sh. M. Alekperova, I.A. Akhmedov, G.S. Gadzhieva, Kh.D. Dzhalilova, Giant magnetoresistance and ki-netic phenomena in n-Ag4SSe in the vicinity of a

phase transition, Physics of the Solid State, 49, 3, 2007, 512-515.

8. M.G. Kanatzidis, Nanostructured Thermoelectrics: The New Paradigm?, Chem. Mater., 22, 3, 2010, 648-659.

9. G. Alexieva, P. Petkova , I. Kolev, I. Ismailov, A. Amova, V. Vassilev, V. Strashilov, Mid-Infrared Optical Spectra of Chalcogenide Glasses from the System As2Se3-Ag4SSe-PbTe, C. R. Acad. Bul. Sci.,70, 11, 2017, 1501-1508.

10. T. Babeva, V. Vassilev, P. Gushterova, A. Amova, G. Alexieva, V. Strashilov and P. Petkova, Optical Properties of Chalcogenide Glasses from the System As2Se3-Ag4SSe-PbTe, J. Optoelectron. Adv. M., 19, 3-4, 2017, 204-210.

11. W. Li, S. Lin, B. Ge, J. Yang, W. Zhang and Y. Pei, Low Sound Velocity Contributing to the High Thermoelectric Performance of Ag8SnSe6, Adv. Sci., 3, 2016, doi: 10.1002/advs.201600196.

12. O. Madelung, U. Rössler, M. Schulz (Eds.), Lead telluride (PbTe) sound velocities, elastic moduli, In: Non-Tetrahedrally Bonded Elements and Binary Compounds I. Landolt-Börnstein - Group III Condensed Matter (Numerical Data and Functional Relationships in Science and Technology), v. 41C, Springer, Berlin, Heidelberg, 1998.

13. Ch.E. Ekuma, D.J. Singh, J. Moreno, M. Jarrell, Optical properties of PbTe and PbSe, Phys. Rev., B 85, 2012; doi: https://doi.org/10.1103/PhysRevB.85.085205.

14. L. Kungumadevi, R. Sathyamoorthy, Structural, Electrical, and Optical Properties of PbTe Thin Films Prepared by Simple Flash Evaporation Method, Advances in Condensed Matter Physics, v. 2012, Article ID 763209, 5 pages, doi:10.1155/2012/763209.

15. I.C. Chang, Acoustoptic Devices and Applications, in Handbook of Optics (M. Bass Editor in chief), v. II, Ch. 12, McGraw-Hill, 1995.