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Phase transformation in CuAgSe: a DSC and electron diffraction examination
Journal of Alloys and Compounds 385 (2004) 169–172
Phase transformation in CuAgSe: a DSC and electron diffraction examination K. Chrissafis, N. Vouroutzis, K.M. Paraskevopoulos, N. Frangis, C. Manolikas∗ Physics Department, Aristotle University of Thessaloniki, GR-54124, Thessaloniki, Greece Received 10 April 2004; received in revised form 28 April 2004; accepted 28 April 2004
Abstract The phase transformation of CuAgSe is examined by DSC and electron diffraction methods. It is found that the transformation occurs at the onset temperature of about 201.8 ± 0.2 ◦ C and it is followed by a remarkable hysteresis between heating and cooling. The electron diffraction patterns indicate an orthorhombic superstructure at room temperature. By heating above the transition temperature fcc structure appears. A remarkable short range order, however, persists in this temperature range. © 2004 Elsevier B.V. All rights reserved. Keywords: Semiconductors; Crystal growth; Crystal structure and symmetry; Transmission electron microscopy; Thermal analysis
1. Introduction Silver and copper dichalcogenides are considered as promising in electronic technology due to their interesting physicochemical properties [1–3] and references therein. The Cu2 Se and Ag2 Se superionic conductors form quasi-binary alloys with mixed cationic conductivity [4,5]. To these belongs CuAgSe, the rare mineral eucairite. It is well-known that CuAgSe undergoes a phase transformation from an ordered low-temperature phase (-CuAgSe) to a disordered high temperature one (␣-CuAgSe) [6]. The latter is characterized by high mobility of cations, i.e. it is a superionic conductor. The transition temperature has been measured by different methods and has been determined in the temperature range 195–231 ◦ C [6–8]. Concerning the structure of the two phases, the ␣-phase has been reported as cubic with the Se anions forming a rigid fcc lattice and the mobile Ag and Cu cations randomly distributed over tetrahedral and trigonal sites [7], i.e. in a quite large number of interstices, equivalent from the point of view of energy and responsible for the high mobility of cations and their weak binding to the rigid framework [9]. The structure of the -phase has been initially reported by ∗ Corresponding author. Tel.: +30-2310-99-8081; fax: +30-2310-99-8019. E-mail address: [email protected] (C. Manolikas).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.04.119
Earley [10], as tetragonal, however later, Frueh et al. [11] in a more rigorous study by X-rays, found that the real structure is a superstructure of a pseudotetragonal one—orthorhombic in actual case—and the supercell consists of five such pseudotetragonal unit subcells stacked along one at -axis. What has not cleared up as yet is the transformation process between the two phases, which would elucidate also the relationship between the structures of the two phases and this is the purpose of our work, by using DSC and electron diffraction methods.
2. Experimental The material was prepared by mixing the appropriate quantities of Ag, Cu and Se in a quartz tube, sealed under vacuum. After finishing the reaction of the elements, by heating at 500 ◦ C, the material was melted by heating at 800 ◦ C, rested at that temperature for 24 h and then slowly cooled through the melting point (780 ◦ C) and faster down to room temperature. The as above obtained crystalline material was sliced and the composition and homogeneity of the slices was checked by SEM–EDS analysis. The thermal behavior of CuAgSe was studied using a Setaram DSC 141 calorimeter. The temperature and energy calibrations of the instrument were performed using the well-known melting temperatures and melting enthalpies
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of high purity standards, i.e. In, Sn and Zn. The CuAgSe bulk samples, about 8 mg in weight, were thermally treated using aluminum crucibles under a constant nitrogen flow. All the DSC measurements were non-isothermal and consisted of a complete thermal cycle (heating and cooling) with the same heating/cooling rate (β). The rates varied from 1 to 20 ◦ C/min for different measurements. The same sample was used for all the heating–cooling cycles. However, in order to testify the reproducibility of the results, the whole set of measurements was carried out on different samples. Specimens suitable for electron microscopic observations were prepared by cutting discs of 3 mm diameter and thinning them by ion bombardment. They were examined in a Jeol 100CX electron microscope equipped with a goniometer hot-stage. Fig. 2. Thermo-analytical curves recorded at different cooling rates. 1:1 and 2:5 ◦ C/min.
3. Results 3.1. DSC measurements In Fig. 1 are shown the DSC curves (endothermic peaks) of a typical CuAgSe sample obtained at different heating rates (β = 1, 5, 10, 15 and 20 ◦ C/min). For different values of β the peak maximum temperature (Tp ) shifts slightly to higher temperatures as the rate increases. The transformation enthalpy was evaluated from the area below the endothermic peak for each heating rate and its mean value is 36.6 ± 0.4 J/g. In Fig. 2 are shown the thermograms (exothermic peaks) of the CuAgSe samples taken during cooling at different rates (β = 1 and 5 ◦ C/min) that correspond to the endothermic curves in Fig. 1. It can be seen that there is a significant shift in the position of the peak maximum (Tp ) towards lower temperatures with the increase of the cooling rate. The mean value of the enthalpy that was estimated from the
Fig. 1. The influence of the heating rate β on the thermo-analytical curves of CuAgSe 1:1, 2:5, 3:10, 4:15 and 5:20 ◦ C/min.
exothermic peaks is 38.3 ± 0.8 J/g, slightly larger than the one obtained during heating. A typical complete thermal cycle (heating–cooling) of the phase transformation obtained at a rate β = 5 ◦ C/min is given in Fig. 3, where the existence of a significant hysteresis of T = 13.5 ◦ C in Tp is easily understood. This hysteresis is present in all the complete cycles taken at different rates and increases with the increase of the rate β. The hysteresis increase is due to the observed shift in the peak temperature (Tp ) of the phase transformation that takes place during cooling, and not due to the reverse transformation (heating), which happens approximately at the same temperature. During the transformation, the lattice distortion caused by cooling due to the formation of the β-phase, creates stresses, which needs a certain period of time to be absorbed. As the rate increases, the crystal has to undergo a larger change in temperature within the same period. As also reported [12], this thermal hysteresis is something expectable and is related to the difference V in cell volumes before and after the transformation.
Fig. 3. Thermo-analytical curves of CuAgSe on heating and cooling with the rate β = 5 ◦ C/min. 1, heating and 2, cooling.
K. Chrissafis et al. / Journal of Alloys and Compounds 385 (2004) 169–172
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Fig. 4. Electron diffraction patterns of -CuAgSe. (a) (0 0 1)∗t section, (b) (100)∗t section, (c) (0 1 0)∗t section, (d) a section slightly inclined with respect to the (0 0 1)∗t one. Indices are referred to the tetragonal subcell. Note that in (d) the successive rows of spots are shifted one with respect to the other over half period, revealing the doubling of the supercell period. From the inclination also of this section with respect to the (0 0 1)∗t one, doubling of the c-parameter is deduced.
3.2. Electron diffraction observations Initially, diffraction patterns across various zones at room temperature were taken. Some of them are reproduced in Fig. 4. It is deduced from them that the basic structure is indeed a tetragonal (or pseudotetragonal) one and a superstructure exists along one at -axis. However, a careful observation of inclined sections (Fig. 4d) leads to a 10at period of the superstructure instead of 5at reported in [6]. Also taking
into account the inclination of the latter section with respect to the (0 0 1)∗ section, a 2ct period for the superstructure is extracted. Subsequently, the specimens were heated, in situ, above ∼200 ◦ C. We observed at that temperature a gradual disappearance of superstructure spots and appearance of diffuse spots connected by lines of diffuse intensity around the remaining basic spots (Fig. 5). The geometry of the basic spots in the diffraction pattern is in agreement with the fcc
Fig. 5. Electron diffraction patterns of ␣-CuAgSe. (a) (1 0 0)∗fcc section, (b) the same section after slight inclination. Note in (b) the presence of diffuse lines terminated in rather diffuse spots.
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Fig. 6. Crystallographic relation between the fcc unit cell of ␣-CuAgSe and the basic tetragonal one of -CuAgSe.
structure suggested for ␣-CuAgSe. However, the existence of diffuse lines and spots indicates that a kind of short range order of cations also exists, at least in the transition state, i.e. in the narrow temperature range just above the transition point. An important observation also is the coexistence of the ␣ and -phases at that temperature range. This is shown by diffraction patterns where both the superstructure spots, as well as the diffuse lines are presented. Furthermore, the electron diffraction patterns from corresponding zones of the two phases, allow the determination of the relation between the tetragonal subcell of the -phase and the fcc cell of the ␣-phase. A construction which ascribes this relation is shown in Fig. 6. Obviously the ct -axis is parallel with one edge of the fcc unit cell; whereas the at -axis coincides with the diagonal of the perpendicular cubic side.
4. Discussion and conclusion From the DSC experimental results it is followed that during heating the onset of the endothermic peak remains almost constant for all the different heating rates. Consequently, this leads to the conclusion that this phase transformation can be considered as an athermal one. It is apparent from the DSC measurements that the phase transformation occurs at an onset temperature of about 201.8 ± 0.2 ◦ C followed by a remarkable hysteresis between heating and cooling, which increases with the increase of the rate β of cooling. Also, there is a change in the transformation temperature as the rate changes during cooling. According to Ubbelohde [13] and Rao and Rao [14] the thermal hysteresis may be attributed to the formation of a hybrid phase in a narrow temperature range around the transition temperature. It consists of a mixture of the two phases, i.e. it coincides with the well-known transition state [15], already seen in CuAgSe by the electron diffraction
patterns. Because of the volume change of the tetragonal unit cell with respect to the cubic one, the coexistence of the two phases causes the formation of considerably strong strains inside the material. The corresponding energy which is stored in the hybrid phase is the main factor controlling the hysteresis. The increasing of the hysteresis on the other hand, with increasing the cooling rate may be attributed to kinetic barriers which the nucleation barriers must overcome in order for the nucleation of the -phase to start [14]. The transformation itself is an order–disorder one. Taking into account the structural models for the two phases and also the relation between the tetragonal and cubic unit cells illustrated in Fig. 6, it is deduced that in the -phases the Ag and Cu cations in a first step occupy octahedral and tetrahedral sites, respectively, and then considerable displacements of atoms along c-axis occurs. There are strong evidences, even from the diffraction patterns, that the superstructure of the -phase is due to further slight displacements of cations from their ideal position in the basic pseudotetragonal unit cell. However, this needs further study by high resolution electron microscopy, a work which is in progress and the results will be published elsewhere.
References [1] A. Husmann, J.B. Betts, G.S. Boebinger, A. Migliori, T.F. Rosenbaum, M.L. Saboungi, Nature 417 (2002) 421. [2] Yi Xie, Xiuwen Zheng, Xuchuan Jiang, Jun Lu, Liying Zhu, Inorg. Chem. 41 (2002) 387. [3] Yang Jiang, Bo Xie, Ji Wu, Shengwen Yuan, Yue Wu, Hao Huang, Yitai Qian, J. Solid State Chem. 167 (2002) 28. [4] S. Miyatani, J. Phys. Soc. Jpn. 34 (1973) 423. [5] S. Miyatani, Yu. Miura, H. Ando, J. Phys. Soc. Jpn. 46 (1979) 1825. [6] Sh.K. Kiazimov, K.M. Jafarov, Yu.G. Asadov, Phase Trans. 21 (1990) 11. [7] T. Shimoyama, M. Arai, T. Sakuma, Crystal structure of superionic phase of CuAgSe, in: 1st International Meeting on Superionic Conductor Physics (IDMSICP), Kyoto, 2003. [8] V.L. Barone, I.L. Botto, I.B. Schalamuk, Latin Am. Appl. Res. 33 (2003) 1. [9] R.F. Kadrgulov, A.I. Livshits, R.F. Yorkshibaev, Sov. Phys. Solid State 34 (1992) 1146. [10] J.W. Earley, Am. Mineral. 35 (1950) 337. [11] A.J. Frueh, G.K. Czamanske, Ch. Knight, Zeitsch. Kristall. 108 (1957) 389. [12] C.N. Rao, K.J. Rao, Phase Transitions in Solids, McGraw-Hill, New York, 1978. [13] A.R. Ubbelohde, in: J.H. de Boer (Ed.), Reactivity of Solids, 1961. [14] K.J. Rao, N.R. Rao, J. Mater. Sci. 1 (1966) 238. [15] R. de Ridder, G. van Tendeloo, S. Amelinckx, Acta Cryst. 32 (1976) 216.
Phase transformation in CuAgSe: a DSC and electron diffraction examination K. Chrissafis, N. Vouroutzis, K.M. Paraskevopoulos, N. Frangis, C. Manolikas∗ Physics Department, Aristotle University of Thessaloniki, GR-54124, Thessaloniki, Greece Received 10 April 2004; received in revised form 28 April 2004; accepted 28 April 2004
Abstract The phase transformation of CuAgSe is examined by DSC and electron diffraction methods. It is found that the transformation occurs at the onset temperature of about 201.8 ± 0.2 ◦ C and it is followed by a remarkable hysteresis between heating and cooling. The electron diffraction patterns indicate an orthorhombic superstructure at room temperature. By heating above the transition temperature fcc structure appears. A remarkable short range order, however, persists in this temperature range. © 2004 Elsevier B.V. All rights reserved. Keywords: Semiconductors; Crystal growth; Crystal structure and symmetry; Transmission electron microscopy; Thermal analysis
1. Introduction Silver and copper dichalcogenides are considered as promising in electronic technology due to their interesting physicochemical properties [1–3] and references therein. The Cu2 Se and Ag2 Se superionic conductors form quasi-binary alloys with mixed cationic conductivity [4,5]. To these belongs CuAgSe, the rare mineral eucairite. It is well-known that CuAgSe undergoes a phase transformation from an ordered low-temperature phase (-CuAgSe) to a disordered high temperature one (␣-CuAgSe) [6]. The latter is characterized by high mobility of cations, i.e. it is a superionic conductor. The transition temperature has been measured by different methods and has been determined in the temperature range 195–231 ◦ C [6–8]. Concerning the structure of the two phases, the ␣-phase has been reported as cubic with the Se anions forming a rigid fcc lattice and the mobile Ag and Cu cations randomly distributed over tetrahedral and trigonal sites [7], i.e. in a quite large number of interstices, equivalent from the point of view of energy and responsible for the high mobility of cations and their weak binding to the rigid framework [9]. The structure of the -phase has been initially reported by ∗ Corresponding author. Tel.: +30-2310-99-8081; fax: +30-2310-99-8019. E-mail address: [email protected] (C. Manolikas).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.04.119
Earley [10], as tetragonal, however later, Frueh et al. [11] in a more rigorous study by X-rays, found that the real structure is a superstructure of a pseudotetragonal one—orthorhombic in actual case—and the supercell consists of five such pseudotetragonal unit subcells stacked along one at -axis. What has not cleared up as yet is the transformation process between the two phases, which would elucidate also the relationship between the structures of the two phases and this is the purpose of our work, by using DSC and electron diffraction methods.
2. Experimental The material was prepared by mixing the appropriate quantities of Ag, Cu and Se in a quartz tube, sealed under vacuum. After finishing the reaction of the elements, by heating at 500 ◦ C, the material was melted by heating at 800 ◦ C, rested at that temperature for 24 h and then slowly cooled through the melting point (780 ◦ C) and faster down to room temperature. The as above obtained crystalline material was sliced and the composition and homogeneity of the slices was checked by SEM–EDS analysis. The thermal behavior of CuAgSe was studied using a Setaram DSC 141 calorimeter. The temperature and energy calibrations of the instrument were performed using the well-known melting temperatures and melting enthalpies
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of high purity standards, i.e. In, Sn and Zn. The CuAgSe bulk samples, about 8 mg in weight, were thermally treated using aluminum crucibles under a constant nitrogen flow. All the DSC measurements were non-isothermal and consisted of a complete thermal cycle (heating and cooling) with the same heating/cooling rate (β). The rates varied from 1 to 20 ◦ C/min for different measurements. The same sample was used for all the heating–cooling cycles. However, in order to testify the reproducibility of the results, the whole set of measurements was carried out on different samples. Specimens suitable for electron microscopic observations were prepared by cutting discs of 3 mm diameter and thinning them by ion bombardment. They were examined in a Jeol 100CX electron microscope equipped with a goniometer hot-stage. Fig. 2. Thermo-analytical curves recorded at different cooling rates. 1:1 and 2:5 ◦ C/min.
3. Results 3.1. DSC measurements In Fig. 1 are shown the DSC curves (endothermic peaks) of a typical CuAgSe sample obtained at different heating rates (β = 1, 5, 10, 15 and 20 ◦ C/min). For different values of β the peak maximum temperature (Tp ) shifts slightly to higher temperatures as the rate increases. The transformation enthalpy was evaluated from the area below the endothermic peak for each heating rate and its mean value is 36.6 ± 0.4 J/g. In Fig. 2 are shown the thermograms (exothermic peaks) of the CuAgSe samples taken during cooling at different rates (β = 1 and 5 ◦ C/min) that correspond to the endothermic curves in Fig. 1. It can be seen that there is a significant shift in the position of the peak maximum (Tp ) towards lower temperatures with the increase of the cooling rate. The mean value of the enthalpy that was estimated from the
Fig. 1. The influence of the heating rate β on the thermo-analytical curves of CuAgSe 1:1, 2:5, 3:10, 4:15 and 5:20 ◦ C/min.
exothermic peaks is 38.3 ± 0.8 J/g, slightly larger than the one obtained during heating. A typical complete thermal cycle (heating–cooling) of the phase transformation obtained at a rate β = 5 ◦ C/min is given in Fig. 3, where the existence of a significant hysteresis of T = 13.5 ◦ C in Tp is easily understood. This hysteresis is present in all the complete cycles taken at different rates and increases with the increase of the rate β. The hysteresis increase is due to the observed shift in the peak temperature (Tp ) of the phase transformation that takes place during cooling, and not due to the reverse transformation (heating), which happens approximately at the same temperature. During the transformation, the lattice distortion caused by cooling due to the formation of the β-phase, creates stresses, which needs a certain period of time to be absorbed. As the rate increases, the crystal has to undergo a larger change in temperature within the same period. As also reported [12], this thermal hysteresis is something expectable and is related to the difference V in cell volumes before and after the transformation.
Fig. 3. Thermo-analytical curves of CuAgSe on heating and cooling with the rate β = 5 ◦ C/min. 1, heating and 2, cooling.
K. Chrissafis et al. / Journal of Alloys and Compounds 385 (2004) 169–172
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Fig. 4. Electron diffraction patterns of -CuAgSe. (a) (0 0 1)∗t section, (b) (100)∗t section, (c) (0 1 0)∗t section, (d) a section slightly inclined with respect to the (0 0 1)∗t one. Indices are referred to the tetragonal subcell. Note that in (d) the successive rows of spots are shifted one with respect to the other over half period, revealing the doubling of the supercell period. From the inclination also of this section with respect to the (0 0 1)∗t one, doubling of the c-parameter is deduced.
3.2. Electron diffraction observations Initially, diffraction patterns across various zones at room temperature were taken. Some of them are reproduced in Fig. 4. It is deduced from them that the basic structure is indeed a tetragonal (or pseudotetragonal) one and a superstructure exists along one at -axis. However, a careful observation of inclined sections (Fig. 4d) leads to a 10at period of the superstructure instead of 5at reported in [6]. Also taking
into account the inclination of the latter section with respect to the (0 0 1)∗ section, a 2ct period for the superstructure is extracted. Subsequently, the specimens were heated, in situ, above ∼200 ◦ C. We observed at that temperature a gradual disappearance of superstructure spots and appearance of diffuse spots connected by lines of diffuse intensity around the remaining basic spots (Fig. 5). The geometry of the basic spots in the diffraction pattern is in agreement with the fcc
Fig. 5. Electron diffraction patterns of ␣-CuAgSe. (a) (1 0 0)∗fcc section, (b) the same section after slight inclination. Note in (b) the presence of diffuse lines terminated in rather diffuse spots.
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Fig. 6. Crystallographic relation between the fcc unit cell of ␣-CuAgSe and the basic tetragonal one of -CuAgSe.
structure suggested for ␣-CuAgSe. However, the existence of diffuse lines and spots indicates that a kind of short range order of cations also exists, at least in the transition state, i.e. in the narrow temperature range just above the transition point. An important observation also is the coexistence of the ␣ and -phases at that temperature range. This is shown by diffraction patterns where both the superstructure spots, as well as the diffuse lines are presented. Furthermore, the electron diffraction patterns from corresponding zones of the two phases, allow the determination of the relation between the tetragonal subcell of the -phase and the fcc cell of the ␣-phase. A construction which ascribes this relation is shown in Fig. 6. Obviously the ct -axis is parallel with one edge of the fcc unit cell; whereas the at -axis coincides with the diagonal of the perpendicular cubic side.
4. Discussion and conclusion From the DSC experimental results it is followed that during heating the onset of the endothermic peak remains almost constant for all the different heating rates. Consequently, this leads to the conclusion that this phase transformation can be considered as an athermal one. It is apparent from the DSC measurements that the phase transformation occurs at an onset temperature of about 201.8 ± 0.2 ◦ C followed by a remarkable hysteresis between heating and cooling, which increases with the increase of the rate β of cooling. Also, there is a change in the transformation temperature as the rate changes during cooling. According to Ubbelohde [13] and Rao and Rao [14] the thermal hysteresis may be attributed to the formation of a hybrid phase in a narrow temperature range around the transition temperature. It consists of a mixture of the two phases, i.e. it coincides with the well-known transition state [15], already seen in CuAgSe by the electron diffraction
patterns. Because of the volume change of the tetragonal unit cell with respect to the cubic one, the coexistence of the two phases causes the formation of considerably strong strains inside the material. The corresponding energy which is stored in the hybrid phase is the main factor controlling the hysteresis. The increasing of the hysteresis on the other hand, with increasing the cooling rate may be attributed to kinetic barriers which the nucleation barriers must overcome in order for the nucleation of the -phase to start [14]. The transformation itself is an order–disorder one. Taking into account the structural models for the two phases and also the relation between the tetragonal and cubic unit cells illustrated in Fig. 6, it is deduced that in the -phases the Ag and Cu cations in a first step occupy octahedral and tetrahedral sites, respectively, and then considerable displacements of atoms along c-axis occurs. There are strong evidences, even from the diffraction patterns, that the superstructure of the -phase is due to further slight displacements of cations from their ideal position in the basic pseudotetragonal unit cell. However, this needs further study by high resolution electron microscopy, a work which is in progress and the results will be published elsewhere.
References [1] A. Husmann, J.B. Betts, G.S. Boebinger, A. Migliori, T.F. Rosenbaum, M.L. Saboungi, Nature 417 (2002) 421. [2] Yi Xie, Xiuwen Zheng, Xuchuan Jiang, Jun Lu, Liying Zhu, Inorg. Chem. 41 (2002) 387. [3] Yang Jiang, Bo Xie, Ji Wu, Shengwen Yuan, Yue Wu, Hao Huang, Yitai Qian, J. Solid State Chem. 167 (2002) 28. [4] S. Miyatani, J. Phys. Soc. Jpn. 34 (1973) 423. [5] S. Miyatani, Yu. Miura, H. Ando, J. Phys. Soc. Jpn. 46 (1979) 1825. [6] Sh.K. Kiazimov, K.M. Jafarov, Yu.G. Asadov, Phase Trans. 21 (1990) 11. [7] T. Shimoyama, M. Arai, T. Sakuma, Crystal structure of superionic phase of CuAgSe, in: 1st International Meeting on Superionic Conductor Physics (IDMSICP), Kyoto, 2003. [8] V.L. Barone, I.L. Botto, I.B. Schalamuk, Latin Am. Appl. Res. 33 (2003) 1. [9] R.F. Kadrgulov, A.I. Livshits, R.F. Yorkshibaev, Sov. Phys. Solid State 34 (1992) 1146. [10] J.W. Earley, Am. Mineral. 35 (1950) 337. [11] A.J. Frueh, G.K. Czamanske, Ch. Knight, Zeitsch. Kristall. 108 (1957) 389. [12] C.N. Rao, K.J. Rao, Phase Transitions in Solids, McGraw-Hill, New York, 1978. [13] A.R. Ubbelohde, in: J.H. de Boer (Ed.), Reactivity of Solids, 1961. [14] K.J. Rao, N.R. Rao, J. Mater. Sci. 1 (1966) 238. [15] R. de Ridder, G. van Tendeloo, S. Amelinckx, Acta Cryst. 32 (1976) 216.