Abstract
1. Introduction
Samarium monosulfide (SmS) is one of the most studied rare-earth semiconductors, due to its unique properties, such as: record low pressure of the isostructural NaCl–NaCl semiconductor–metal phase transition (6.5 kbar at 300 K) associated with the transition of SmS to a state with an intermediate valence of samarium ion (Sm2+ → Sm2,7+)[
2. Experiment
Polycrystalline samarium sulfide was prepared by synthesis from simple Sm and S. The technology for obtaining samples includes two stages. First, the rare-earth metal Sm and chalcogen S are heated; upon reaching temperatures of 770–870 K sulfur reacts with samarium to form the Sm, SmS, Sm3S4, Sm2S3, SmS2 phases. Heat treatment of the mixture 920–970 K leads to the thermal decomposition of SmS2 into Sm2S3 and S, which, interacting with Sm, increases the yield of SmS. The obtained substance according to X-ray phase analysis is a mixture of phases, which, however, does not contain free metal and chalcogen. The second stage is high temperature annealing (1773–2273 K) obtained by briquetting bulk samples. Only after high temperature annealing do the samples become dense and well-formed. A sample with dimensions of 19.15 × 3.38 × 2.95 mm3 had the following parameters: lattice constant
An acoustic technique is used as a research method. We study the Young's modulus and attenuation decrement in a wide temperature range using the three-component piezoelectric ultrasonic composite oscillator technique (PUCOT)[
3. Results and discussion
The results of measurements of the Young's modulus under different annealing temperatures are presented in Fig. 1. Cooling of the sample leads to the increase of the elastic modulus. However, as the annealing temperature grows, the Young's modulus decreases maintaining the general trend of spectra, Fig. 1. The drop of the Young's modulus is about several percent and reaches 6.5% at an annealing temperature of 1073 K comparing with data without annealing. At the same time the disappearance of the “step” of the Young's modulus at about 135 K with increase of the annealing temperature is noticeable, indicating stress relaxation in the structure[
Figure 1.Young's modulus temperature dependencies in polycrystalline SmS at various annealing temperatures: 1: without annealing, 2: 673 K, 3: 913 K, 4: 1073 K. The “step” observed at approximately 135 K in spectra 1 and 2 disappears with an increase in the annealing temperature (see spectrum 4).
During annealing of metals and semiconductors, the changes of the elastic modulus occur due to the structural relaxation in the bulk of the material, which reduces the level of internal stresses, changes the size of grains and subgrains, transforms nonequilibrium boundaries into equilibrium, and leads to the gradual elimination of various defects in the crystal lattice, in particular, dislocations[
Figure 2.X-ray diffraction patterns of the sample: 1: initial state (sample surface), 2: after heat treatment (sample cleavage). The reflection peak corresponding to plane (111) disappears after annealing; the reflection intensity peak (200) decreases twice after annealing.
According to Ref. [15] the estimation of the Young's modulus during annealing allows one to determine the transition from an isotropic to anisotropic crystal. If we assume that SmS polycrystal has a large amount of grains and their orientations are equally probable, then it is possible to consider the material as an isotropic medium. The elastic properties of such medium can be characterized using effective elastic coefficients that relate averaged stress and strain characteristics over the entire volume[
Table 1 shows the theoretically and experimentally obtained Young's moduli for the SmS polycrystal. Young's modulus values at room temperature without annealing and after annealing at temperatures of 613 and 913 K are within the upper limits (Voigt criterion
In the PUCOT method the damping (attenuation decrement) is automatically measured with the frequency. The dependencies of the attenuation decrement at different annealing temperatures in the temperature range 80–260 K were obtained (Fig. 3). As is well known, various thermally activated dislocation relaxation processes are manifested at low temperatures, such as the generation of pairs of kinks on the dislocations (overcoming the first order Peierls barrier, Bordoni peak), diffusion of single (geometric) kinks in the potential second order Peierls barrier (Niblett-Wilks peak), the interaction of dislocations with impurity pinning centers, “dragging” or “sweeping” of impurity atmospheres during the motion of dislocations and etc. Such relaxation processes are very clearly visible in the attenuation decrement spectra. There are two low temperature peaks in Fig. 3. The first peak located about 125 K is the well-studied Bordoni peak (P2) observed in all fcc metals, and in our case in fcc semiconductor[
Figure 3.Temperature dependencies of the attenuation decrement at different annealing temperatures: 1: without annealing, 2: 673 K, 3: 913 K, 4: 1073 K. Niblett-Wilks and Bordoni peaks are shown as
4. Conclusion
We found that the Young’s modulus of SmS polycrystal is more influenced by the texturing of the material than the contribution of the dislocation processes. Thermally-activated relaxation peaks of the dislocation nature have been first detected in the SmS semiconductor. Strong decrease of relaxation peaks after heat treatment indicates the release of stresses due to a drop of the dislocation density, which in turn should lead to an increase in Young's modulus. However, due to the specific orientation of the grains during the annealing, the Young’s modulus drops to values at which the material can no longer be considered isotropic. Thus, our results show that the Young's modulus changes abnormally after the annealing process. Varying the annealing temperature significantly affects the Young's modulus and allows one to change it to the necessary values required in the technological processes.
Acknowledgements
This research was supported by Russian Science Foundation under Grant 19–72–30004.
References
[1] K Takenaka, D Asai, R Kaizu et al. Giant isotropic negative thermal expansion in Y-doped samarium monosulfides by intra-atomic charge transfer. Sci Rep, 9, 122(2019).
[2] V V Kaminsky, S M Soloviev, G D Khavrov et al. Mechanism of the semiconductor –metal phase transition in Sm 1 –
[3] N V Sharenkova, V V Kaminskii, A V Golubkov et al. The structure of a metallic-phase film produced by mechanical polishing of polycrystalline SmS. Phys Solid State, 47, 622(2005).
[4] V V Kaminskii, S Lanyi. Semiconductor–metal phase transition under a strain induced by a spherical indenter. Tech Phys, 43, 314(1998).
[5] A Sousanis, P F Smet, D Poelman. Samarium monosulfide (SmS): reviewing properties and applications. Materials, 10, 953(2017).
[6] V V Kaminskiĭ, V A Didik, M M Kazanin et al. Thermovoltaic effect in polycrystalline samarium sulfide. Tech Phys Lett, 35, 981(2009).
[7] V V Kaminskii, S M Solov’ev, G D Khavrov et al. Structural features of Sm1 –
[8] I A Pronin, I A Averin, A S Bozhinov et al. The thermovoltaic effect in zinc oxide inhomogeneously doped with mixed-valence impurities. Tech Phys Lett, 41, 930(2015).
[9] V V Kaminsky, A A Molodykh, N N Stepanov et al. The application peculiarities of semi-conducting resistive-strain sensors and baroresistors on the basis of samarium sulphide. Nauchnoe Priborostroenie (Scientific Instrumentation), 2, 53(2011).
[10] S Kustov, S Golyabdin, A Ichino et al. A new design of automated piezoelectric composite oscillator technique. Mater Sci Eng A, 442, 532(2006).
[11] W H Robinson, A Edgar. The piezoelectric method of determining mechanical damping at frequencies of 30 to 200 kHz. IEEE Trans Sonics Ultrason, 21, 98(1974).
[12] P P Pal’-Val’, L N Pal’-Val’. Low-temperature internal friction and nanostructured metal stability. Met Sci Heat Treat, 54, 234(2012).
[13] D Lin, L Xu, H Jing et al. Effects of annealing on the structure and mechanical properties of FeCoCrNi high-entropy alloy fabricated via selective laser melting. Addit Manufact, 32, 101058(2020).
[14] Z Wang, I Baker. Effects of annealing and thermo-mechanical treatment on the microstructures and mechanical properties of a carbon-doped FeNiMnAl multi-component alloy. Mater Sci Eng A, 693, 101(2017).
[15] P P Pal-Val, N Loginov Yu, S L Demakov et al. Unusual Young׳s modulus behavior in ultrafine-grained and microcrystalline copper wires caused by texture changes during processing and annealing. Mater Sci Eng A, 618, 9(2014).
[16] U Scharer, P Wachter. Brillouin spectroscopy on doped SmS. Physica B, 721, 230(1997).
[17]
[18]
[19]
[20] V V Kaminskii, Y V Lyubimova, A E Romanov. Probing of polycrystalline magnesium at ultrasonic frequencies by mechanical spectroscopy. Mater Phys Mechan, 44, 19(2020).
Set citation alerts for the article
Please enter your email address