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Homogeneity range of the 1T phase in the systems IrSe2–V (Ta)Se2
Journal of Alloys and Compounds 442 (2007) 117–118
Homogeneity range of the 1T phase in the systems IrSe2–V (Ta)Se2 K. Hayashi ∗ , Y. Tanino, K. Kawachi, Y. Nakata, K. Inoue, N. Maeda Department of Chemistry, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan Received 20 June 2006; received in revised form 5 September 2006; accepted 5 September 2006 Available online 30 January 2007
Abstract The phase relations in the IrSe2 –MSe2 (M = V, Ta) systems are investigated to confirm the 1T structure stabilization mechanism. The homogeneity range of the 1T phase extends up to the compositions of 48%IrSe2 in the IrSe2 –VSe2 system and 42%IrSe2 in IrSe2 –TaSe2 system at 1000 ◦ C. The 1T-sample of each system is semi-metallic. The 1T-V1−x Irx Se2 is paramagnetic and the 1T-Ta1−x Irx Se2 , a diamagnetic. The present experimental results are consistent with the explanation of the 1T structure stabilization mechanism. © 2007 Elsevier B.V. All rights reserved. Keywords: Transition metal alloys and compounds; Solid state reaction; Electronic properties; Magnetic measurement
1. Introduction In the previous paper [1], we reported the 1T-phase was stabilized in the IrSe2 –NbSe2 system in which both end members did not crystallize into the 1T structure. We considered that this 1T structure stabilization must be induced by electron-transfer from a high energy Nb-t2g band to a low energy Ir-t2g band. Consequently, this stabilization mechanism functions as long as the electrons remain in the Nb-t2g band. In the present investigation, we study phase relations in the Vgroup diselenides and iridium diselenide systems to confirm this mechanism. The IrSe2 crystallizes into a 3D double Marcasite type structure [2]. This structure suggests that the chemical formula of the IrSe2 can be written as Ir2 Se2 (Se2 ) and the oxidation numbers of Ir, Se, and (Se2 ) are +3, −2, and −2, respectively. The Ir-atom is surrounded by six Se-atoms forming an octahedron. Hence, an Ir-t2g band is completely filled with six electrons and the Se p-band has a hole to form the (Se2 ) molecule from one-half of all the Se atoms. The VSe2 crystallizes into a 2DCdI2 type structure called “1T” [3]. The TaSe2 also crystallizes into layered-structures [4–7]. The 1T-polytype is stable above 700 ◦ C and the 2H-polytype is stable below 600 ◦ C [8]. The octahedral coordination is stabilized at high temperature and the trigonal prismatic coordination is stabilized at a low temperature. At a temperature from 600 to 700 ◦ C, the mixture of these ∗
Corresponding author. Fax: +81 86 256 97 57. E-mail address: [email protected] (K. Hayashi).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.09.148
two coordination-types is stabilized in TaSe2 . Therefore, in the present experimental temperature range from 800 to 1000 ◦ C VSe2 and TaSe2 are the 1T-phases. In the present investigation, the phase relations were studied on the basis of the components, IrSe2 , 1T-VSe2 , and 1T-TaSe2 . The electrical and magnetic properties of the layered transition metal dichalcogenides are also an interesting subject in terms of a charge density wave and a superconducting phenomena [9–10]. The magnetic susceptibility and the electrical resistivity of the 1T-pahse are reported also in the present investigation. 2. Experimental All the samples were prepared by so-called “sealed ampoule method” [1]. The starting charge was powdered metals, V (3N), Ta (3N), and selenium shots (5N). A stoichiometric mixture of the metal powder and the selenium shots was evacuated in a silica glass ampoule and sealed. The mixture in the ampoule was heated gradually up to the equilibrium temperature of 800–1000 ◦ C, annealed for 3 days and quenched to the room temperature with cold water. This process was repeated twice to achieve the phase equilibrium. The phases of the products were identified by the powder X-ray diffraction method. The chemical compositions of the products were determined by the EPMA method. The magnetic properties were measured between 4 K and the room temperature with the MPMS of Quantum Design Co. and the electron transport properties were measured between 10 K and the room temperature.
3. Results Homogeneity range of the 1T phases in the IrSe2 –VSe2 , and IrSe2 –TaSe2 systems between 800 and 1000 ◦ C are shown in
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of the electrical resistivity and the decrement of the weak paramagnetism of conduction electrons. The other phenomenon is the increment in the average electron-hopping distance, since the V–V connections are interrupted by the substituted Ir atoms. This second phenomenon induces a large increment of the electrical resistivity with an increment of the Ir atom substitution. The observed electrical resistivity values of the 1TV1−x Irx Se2 and the 1T-Ta1−x Irx Se2 are as low as that of a semimetal. The observed electrical resistivity tendency of the 1T-V1−x Irx Se2 and the 1T-Ta1−x Irx Se2 is well explained by the 1T-structure stabilization mechanism. 5. Conclusions Fig. 1. Homogeneity range of the 1T phase in the systems IrSe2 –VSe2 (1) and IrSe2 –TaSe2 (2) within the temperature range from 800 to 1000 ◦ C.
Fig. 1. Both the vanadium diselenide and the tantalum diselenide have the CdI2 -type (1T) structure in this temperature range. Hence, the niobium diselenide has the 4Ha-type structure. Therefore, the phase relations in the VSe2 –IrSe2 and TaSe2 –IrSe2 systems are similar to each other: only two phases, the 1T and the IrSe2 , are observed. The 1T-phase has a wide homogeneity range between 0 and 48%IrSe2 at 1000 ◦ C in the VSe2 –IrSe2 system and similarly between 0 and 42%IrSe2 in the TaSe2 –IrSe2 system. The phase relations in the NbSe2 –IrSe2 system is different from that of these two systems. The phase boundaries between the 1T and the IrSe2 phase in these two systems are similar to each other. The phase boundary of the 1T phase is just below 50%IrSe2 and the IrSe2 phase is a line phase. Both the 1T-V1−x Irx Se2 and the 1T-Ta1−x Irx Se2 show a semimetallic electrical resistivity value depending on the Ir-atom substitution. The Ir-atom substitution makes the electrical resistivity value large. The magnetic susceptibilities of both 1T-phases are very small, an order of diamagnetism or Pauli paramagnetism. 4. Discussion The present phase relations in the IrSe2 –VSe2 and IrSe2 – TaSe2 systems suggest that the 1T-single phase is stabilized up to a near 50%IrSe2 as suggested in the IrSe2 –NbSe2 system. These results verify that the 1T structure stabilization mechanism caused by electron-transfer from a high energy Nb-t2g band to a low energy Ir-t2g band works in all the V-group dichalcogenide-IrSe2 system. In VSe2 and TaSe2 , a conduction band is the M-t2g band. The Ir atom substitution causes two phenomena. The number of conduction electrons decreases as the Ir atom substitution increases, since the electron transfers from the M-t2g conduction band to the Ir-t2g discrete band. This phenomenon induces the increment
The phase relations in the IrSe2 –V1−x Se2 and IrSe2 –Ta1−x Se2 systems in the temperature range between 800 and 1000 ◦ C were established. The homogeneity range of the 1T phase expanded up to the composition of 48%IrSe2 in the former system, and 42%IrSe2 in the latter system at 1000 ◦ C. The 1T-compound of each system is essentially semimetallic. The electrical resistivity value of the 1T-Ta1−x Irx Se2 is 100 times larger than that of the 1T-V1−x Irx Se2 . The Ir-atom substitution enhances the electrical resistivity value of both 1T compounds. The 1T structure stabilization mechanism induced by the electron-transfer from a high energy M-t2g band to a low energy Ir-t2g band works in all the V-group diselenides and the iridium diselenide systems. We will apply this study of the 1T-structure stabilization mechanism to the other group dichalcogenide systems, in the near future. References [1] M. Shimakawa, K. Kawachi, S. Nishikawa, K. Hayashi, J. Solid State Chem. 129 (1997) 242–249. [2] S. Jobic, P. Deniard, R. Brec, J. Rouxel, M.G.B. Drew, W.I.F. David, J. Solid State Chem. 89 (1990) 315–327. [3] J. Rigoult, C. Guidi-Morosini, A. Tomas, P. Molinie, Acta Crystallogr. B 38 (1982) 1557–1559. [4] G.A. Wiegers, J.L. de Boer, A. Meetsma, S. van Smaalen, Z. Kristallogr. 216 (2001) 45–50. [5] J. Luedecke, S. van Smaalen, A. Spijkerman, J.L. de Boer, G.A. Wiegers, Phys. Rev. B 59 (9) (1999) 6063–6071. [6] W.Y. Zhou, A. Meetsma, J.L. de Boer, G.A. Wiegers, Mater. Res. Bull. 27 (1992) 563–572. [7] E. Bjerkelund, A. Kjekshus, Acta Chem. Scand. 21 (1967) 513–526. [8] A. Kawamura, Thesis of Okayama University of Science (1987) 38–43. [9] V. Vescoli, L. Degiorgi, H. Berger, L. Ferro, Phys. Rev. Lett. 81 (2) (1998) 453–456. [10] T. Endo, S. Nakao, W. Yamaguchi, T. Hasegawa, K. Kitazawa, Solid State Commun. 116 (1) (2000) 47–50.
Homogeneity range of the 1T phase in the systems IrSe2–V (Ta)Se2 K. Hayashi ∗ , Y. Tanino, K. Kawachi, Y. Nakata, K. Inoue, N. Maeda Department of Chemistry, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan Received 20 June 2006; received in revised form 5 September 2006; accepted 5 September 2006 Available online 30 January 2007
Abstract The phase relations in the IrSe2 –MSe2 (M = V, Ta) systems are investigated to confirm the 1T structure stabilization mechanism. The homogeneity range of the 1T phase extends up to the compositions of 48%IrSe2 in the IrSe2 –VSe2 system and 42%IrSe2 in IrSe2 –TaSe2 system at 1000 ◦ C. The 1T-sample of each system is semi-metallic. The 1T-V1−x Irx Se2 is paramagnetic and the 1T-Ta1−x Irx Se2 , a diamagnetic. The present experimental results are consistent with the explanation of the 1T structure stabilization mechanism. © 2007 Elsevier B.V. All rights reserved. Keywords: Transition metal alloys and compounds; Solid state reaction; Electronic properties; Magnetic measurement
1. Introduction In the previous paper [1], we reported the 1T-phase was stabilized in the IrSe2 –NbSe2 system in which both end members did not crystallize into the 1T structure. We considered that this 1T structure stabilization must be induced by electron-transfer from a high energy Nb-t2g band to a low energy Ir-t2g band. Consequently, this stabilization mechanism functions as long as the electrons remain in the Nb-t2g band. In the present investigation, we study phase relations in the Vgroup diselenides and iridium diselenide systems to confirm this mechanism. The IrSe2 crystallizes into a 3D double Marcasite type structure [2]. This structure suggests that the chemical formula of the IrSe2 can be written as Ir2 Se2 (Se2 ) and the oxidation numbers of Ir, Se, and (Se2 ) are +3, −2, and −2, respectively. The Ir-atom is surrounded by six Se-atoms forming an octahedron. Hence, an Ir-t2g band is completely filled with six electrons and the Se p-band has a hole to form the (Se2 ) molecule from one-half of all the Se atoms. The VSe2 crystallizes into a 2DCdI2 type structure called “1T” [3]. The TaSe2 also crystallizes into layered-structures [4–7]. The 1T-polytype is stable above 700 ◦ C and the 2H-polytype is stable below 600 ◦ C [8]. The octahedral coordination is stabilized at high temperature and the trigonal prismatic coordination is stabilized at a low temperature. At a temperature from 600 to 700 ◦ C, the mixture of these ∗
Corresponding author. Fax: +81 86 256 97 57. E-mail address: [email protected] (K. Hayashi).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.09.148
two coordination-types is stabilized in TaSe2 . Therefore, in the present experimental temperature range from 800 to 1000 ◦ C VSe2 and TaSe2 are the 1T-phases. In the present investigation, the phase relations were studied on the basis of the components, IrSe2 , 1T-VSe2 , and 1T-TaSe2 . The electrical and magnetic properties of the layered transition metal dichalcogenides are also an interesting subject in terms of a charge density wave and a superconducting phenomena [9–10]. The magnetic susceptibility and the electrical resistivity of the 1T-pahse are reported also in the present investigation. 2. Experimental All the samples were prepared by so-called “sealed ampoule method” [1]. The starting charge was powdered metals, V (3N), Ta (3N), and selenium shots (5N). A stoichiometric mixture of the metal powder and the selenium shots was evacuated in a silica glass ampoule and sealed. The mixture in the ampoule was heated gradually up to the equilibrium temperature of 800–1000 ◦ C, annealed for 3 days and quenched to the room temperature with cold water. This process was repeated twice to achieve the phase equilibrium. The phases of the products were identified by the powder X-ray diffraction method. The chemical compositions of the products were determined by the EPMA method. The magnetic properties were measured between 4 K and the room temperature with the MPMS of Quantum Design Co. and the electron transport properties were measured between 10 K and the room temperature.
3. Results Homogeneity range of the 1T phases in the IrSe2 –VSe2 , and IrSe2 –TaSe2 systems between 800 and 1000 ◦ C are shown in
118
K. Hayashi et al. / Journal of Alloys and Compounds 442 (2007) 117–118
of the electrical resistivity and the decrement of the weak paramagnetism of conduction electrons. The other phenomenon is the increment in the average electron-hopping distance, since the V–V connections are interrupted by the substituted Ir atoms. This second phenomenon induces a large increment of the electrical resistivity with an increment of the Ir atom substitution. The observed electrical resistivity values of the 1TV1−x Irx Se2 and the 1T-Ta1−x Irx Se2 are as low as that of a semimetal. The observed electrical resistivity tendency of the 1T-V1−x Irx Se2 and the 1T-Ta1−x Irx Se2 is well explained by the 1T-structure stabilization mechanism. 5. Conclusions Fig. 1. Homogeneity range of the 1T phase in the systems IrSe2 –VSe2 (1) and IrSe2 –TaSe2 (2) within the temperature range from 800 to 1000 ◦ C.
Fig. 1. Both the vanadium diselenide and the tantalum diselenide have the CdI2 -type (1T) structure in this temperature range. Hence, the niobium diselenide has the 4Ha-type structure. Therefore, the phase relations in the VSe2 –IrSe2 and TaSe2 –IrSe2 systems are similar to each other: only two phases, the 1T and the IrSe2 , are observed. The 1T-phase has a wide homogeneity range between 0 and 48%IrSe2 at 1000 ◦ C in the VSe2 –IrSe2 system and similarly between 0 and 42%IrSe2 in the TaSe2 –IrSe2 system. The phase relations in the NbSe2 –IrSe2 system is different from that of these two systems. The phase boundaries between the 1T and the IrSe2 phase in these two systems are similar to each other. The phase boundary of the 1T phase is just below 50%IrSe2 and the IrSe2 phase is a line phase. Both the 1T-V1−x Irx Se2 and the 1T-Ta1−x Irx Se2 show a semimetallic electrical resistivity value depending on the Ir-atom substitution. The Ir-atom substitution makes the electrical resistivity value large. The magnetic susceptibilities of both 1T-phases are very small, an order of diamagnetism or Pauli paramagnetism. 4. Discussion The present phase relations in the IrSe2 –VSe2 and IrSe2 – TaSe2 systems suggest that the 1T-single phase is stabilized up to a near 50%IrSe2 as suggested in the IrSe2 –NbSe2 system. These results verify that the 1T structure stabilization mechanism caused by electron-transfer from a high energy Nb-t2g band to a low energy Ir-t2g band works in all the V-group dichalcogenide-IrSe2 system. In VSe2 and TaSe2 , a conduction band is the M-t2g band. The Ir atom substitution causes two phenomena. The number of conduction electrons decreases as the Ir atom substitution increases, since the electron transfers from the M-t2g conduction band to the Ir-t2g discrete band. This phenomenon induces the increment
The phase relations in the IrSe2 –V1−x Se2 and IrSe2 –Ta1−x Se2 systems in the temperature range between 800 and 1000 ◦ C were established. The homogeneity range of the 1T phase expanded up to the composition of 48%IrSe2 in the former system, and 42%IrSe2 in the latter system at 1000 ◦ C. The 1T-compound of each system is essentially semimetallic. The electrical resistivity value of the 1T-Ta1−x Irx Se2 is 100 times larger than that of the 1T-V1−x Irx Se2 . The Ir-atom substitution enhances the electrical resistivity value of both 1T compounds. The 1T structure stabilization mechanism induced by the electron-transfer from a high energy M-t2g band to a low energy Ir-t2g band works in all the V-group diselenides and the iridium diselenide systems. We will apply this study of the 1T-structure stabilization mechanism to the other group dichalcogenide systems, in the near future. References [1] M. Shimakawa, K. Kawachi, S. Nishikawa, K. Hayashi, J. Solid State Chem. 129 (1997) 242–249. [2] S. Jobic, P. Deniard, R. Brec, J. Rouxel, M.G.B. Drew, W.I.F. David, J. Solid State Chem. 89 (1990) 315–327. [3] J. Rigoult, C. Guidi-Morosini, A. Tomas, P. Molinie, Acta Crystallogr. B 38 (1982) 1557–1559. [4] G.A. Wiegers, J.L. de Boer, A. Meetsma, S. van Smaalen, Z. Kristallogr. 216 (2001) 45–50. [5] J. Luedecke, S. van Smaalen, A. Spijkerman, J.L. de Boer, G.A. Wiegers, Phys. Rev. B 59 (9) (1999) 6063–6071. [6] W.Y. Zhou, A. Meetsma, J.L. de Boer, G.A. Wiegers, Mater. Res. Bull. 27 (1992) 563–572. [7] E. Bjerkelund, A. Kjekshus, Acta Chem. Scand. 21 (1967) 513–526. [8] A. Kawamura, Thesis of Okayama University of Science (1987) 38–43. [9] V. Vescoli, L. Degiorgi, H. Berger, L. Ferro, Phys. Rev. Lett. 81 (2) (1998) 453–456. [10] T. Endo, S. Nakao, W. Yamaguchi, T. Hasegawa, K. Kitazawa, Solid State Commun. 116 (1) (2000) 47–50.