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Semiconductor-metal transition in v-doped Ti3O5
~'olume 61A, number 4
PHYSICS LETTERS
SEMICONDUCTOR-METAL
16 May 1977
TRANSITION
IN V-DOPED Ti 3 0 5
C.N.R. RAO ~ and G. RAMA RAO Department of Chemistry, Indian Institute of Technology, Kanpur 208016, bMia Received 1 April 1977 The semiconductor-metal transition in Ti305 is significantly affected by vanadium doping. The unit cell volume, AH, Ap as well as A×M decrease markedly up to 0.1% V305 and gradually thereafter until at ~ 10% V305, the transition disappears. Ti305 is a semiconductor with monoclinic structure (a = 9.80A, b = 3.79A, c = 9.45 A,/3 = 91.75 °) at ordinary temperatures; it transforms to a metallic phase around 450 K accompanied by a first order transition with an enthalpy change, ~ H , o f 1.5 -+ 0.4 kcal mol 1 [1,2]. At the transition temperature, there is a sharp increase in magnetic susceptibility [31, but the electrical resistivity does not show an abrupt change through the semiconductor-metal transition [4]. V305 is also a semiconductor with monoclinic structure (a = 9.98A, b = 5.03A, c = 9 . 8 4 A and 13 = 138.8°), but it does not undergo any phase transition upto 1270 K [2, 5, 6]. We were interested to investigate the effect o f vanadium doping on the semiconductormetal transition in Ti305. Such studies have been of' great value in understanding mechanisms of semiconductor-metal transitions in analogous oxide systems typical of them being, V and Sc doped Ti203 [7,8] and Ti, Cr and AI doped V203 [9 11]. Results of our studies on the T i 3 0 5 - V 3 0 5 system are summarized in fig. 1. Incorporation o f V in Ti305 affects the semiconductor-metal transition significantly. Thus, around 10% V 3 0 5 , no transition is observed and the material remains a semiconductor even upto high temperatures. The transition temperature, Tt, shows a slight increase with initial V-doping, but decreases thereafter linearly until it sharply drops off (presumably) to 0 K at ~ 10% V 3 0 5 . The ZXH of the transition, however, decreases markedly at 0.1% V305 ; above this composition, the decrease is linear until A t t = 0 around 10% V 3 0 5 . The unit cell volume also To whom all correspondence should be addressed at: Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India.
A×M
ZX9
CGSunits
AH
k cai/mol
o h m cm
o
Tt,K
V , ~3
,_._.-o
q
-
<
(
il
i
i i
6 Fig. 1. Variation of unit cell volume, V, transition temperature, Tt, enthalpy change, AH, resistivity discontinuity at the transition, Ap and magnetic susceptibility discontinuity at the transition, aXM, with % V305 in the Ti305-V305 system. Values of Tt from differential thermal analysis (circles), XM measurements (triangles) and resistivity measurements (squares) axe shown. decreases significantly at 0.1% V305 and gradually thereafter. The jump in magnetic susceptibility at the transition, AXM, follows the same trend as the AH. The discontinuity in resistivity at the transition, Ap, also follows the trend in 2xH. It appears that A p is generally a good measure of 2xH in semiconductor-metal transitions. The activation energy for conduction, Ea, in the semiconducting phase (prior to the transition) 247
Volume 61A, number 4
PHYSICS LETTERS
decreases at 0.1% V305 and shows an increasing trend thereafter until at 10% V305 it again decreases to a small value. Variations in the different physical characteristics accompanying the semiconductor-metal transition in the T i 3 0 5 - V 3 0 5 system are reminiscent of behaviours found in the transitions of other mixed oxide systems. Thus, the variation in T t with % V305 is somewhat similar to that found in A1 doped V 2 0 3 [10]. The sharp decrease in unit cell volume and the accompanying decrease in E a with the initial incorporation of the dopant are found in Sc doped T i 2 0 3 [8]. The correspondence between the variations of A H and At) has been noticed in Ti doped V 2 0 3 [11]. The present results along with the analogies mentioned above seem to indicate that the semiconductor-metal transition in the T i 3 0 5 - V 3 0 5 system is largely latticedominated. Bartholomew and Frankl [4] had indeed suggested that the transition in Ti305 essentially conforms to the crystal distortion model of Adler and Brooks [12] which seems to be applicable to the transition in VO 2 as well. In many ways, the transition in Ti305 is comparable to that in VO 2. Thus, both transitions are thermodynamically first order and are associated with large AXM ; Ap is, however, considerably large and more abrupt in the case of VO 2 [2]. In the VO 2 - T i O 2 system, variation of AH, Ap, AXM and Tt with % TiO 2 has been found to be governed by the magnitude of crystal distortion of the low-temperature semiconducting phase [ 13].
248
16 May 1977
-References [1] C.N.R. Rao, S. Ramdas, R.E. Loehman and J.M. Ilonig, J. Solid State Chem. 3 (197l) 83. [2] C.N.R. Rao and G.V. Subba Rao, Transitional metal oxides, NSRD-NBS Monograph 49 (National Bureau of Standards, Washington, D.C. 1974). [3] L.N. Mulay and W.J. Danley, J. Appl. Phys. 41 (1970) 877. [4] R.E. Bartholomew and D.R. Frankl, Phys. Rev. 187 (1969) 828. [5] S. Asbrink, S. Friberg, A. Magneli and G. Andersson, Acta Chem. Stand. 13 (1959) 603. [6] S. Kachi, K. Kosuge and H. Okinaka, J. Solid State Chem. 6 (1973) 258. [7] G.V. Chandrashekhar, Q.W. Choi, J. Moyo and J.M. Honig, Mat. Res. Bull. 5 (1970) 999. [8] G.V. Chandrashekhar, L.L. Van Zandt, J.M. ltonig and A. Jayaraman, Phys. Rev. B10 (1974) 5063. [9] A.P.B. Sinha, G.V. Chandrashekhar and J.M. Honig, J. Solid State Chem. 12 (1975) 402, and the references cited therein. [10] H. Kuwamoto, D.L. Dickerson, H.V. Keer and J.M. ftonig Mat. Res. Bull. 11 (1976) 1301. [11] ft. Kuwamoto, H.V. Keer, J.E. Keem, S.A. Shivashankar, L.L. Van Zandt and J.M. Honig, J. Physique, Paris, to be published. [12] D. Adler and H. Brooks, Phys. Rev. 145 (1967) 826. [13] C.N.R. Rao, G.V. Subba Rao, M. Natarajan and R.E. Loehman, J. Phys. Chem. Solids 32 (1971) 1147.
PHYSICS LETTERS
SEMICONDUCTOR-METAL
16 May 1977
TRANSITION
IN V-DOPED Ti 3 0 5
C.N.R. RAO ~ and G. RAMA RAO Department of Chemistry, Indian Institute of Technology, Kanpur 208016, bMia Received 1 April 1977 The semiconductor-metal transition in Ti305 is significantly affected by vanadium doping. The unit cell volume, AH, Ap as well as A×M decrease markedly up to 0.1% V305 and gradually thereafter until at ~ 10% V305, the transition disappears. Ti305 is a semiconductor with monoclinic structure (a = 9.80A, b = 3.79A, c = 9.45 A,/3 = 91.75 °) at ordinary temperatures; it transforms to a metallic phase around 450 K accompanied by a first order transition with an enthalpy change, ~ H , o f 1.5 -+ 0.4 kcal mol 1 [1,2]. At the transition temperature, there is a sharp increase in magnetic susceptibility [31, but the electrical resistivity does not show an abrupt change through the semiconductor-metal transition [4]. V305 is also a semiconductor with monoclinic structure (a = 9.98A, b = 5.03A, c = 9 . 8 4 A and 13 = 138.8°), but it does not undergo any phase transition upto 1270 K [2, 5, 6]. We were interested to investigate the effect o f vanadium doping on the semiconductormetal transition in Ti305. Such studies have been of' great value in understanding mechanisms of semiconductor-metal transitions in analogous oxide systems typical of them being, V and Sc doped Ti203 [7,8] and Ti, Cr and AI doped V203 [9 11]. Results of our studies on the T i 3 0 5 - V 3 0 5 system are summarized in fig. 1. Incorporation o f V in Ti305 affects the semiconductor-metal transition significantly. Thus, around 10% V 3 0 5 , no transition is observed and the material remains a semiconductor even upto high temperatures. The transition temperature, Tt, shows a slight increase with initial V-doping, but decreases thereafter linearly until it sharply drops off (presumably) to 0 K at ~ 10% V 3 0 5 . The ZXH of the transition, however, decreases markedly at 0.1% V305 ; above this composition, the decrease is linear until A t t = 0 around 10% V 3 0 5 . The unit cell volume also To whom all correspondence should be addressed at: Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India.
A×M
ZX9
CGSunits
AH
k cai/mol
o h m cm
o
Tt,K
V , ~3
,_._.-o
q
-
<
(
il
i
i i
6 Fig. 1. Variation of unit cell volume, V, transition temperature, Tt, enthalpy change, AH, resistivity discontinuity at the transition, Ap and magnetic susceptibility discontinuity at the transition, aXM, with % V305 in the Ti305-V305 system. Values of Tt from differential thermal analysis (circles), XM measurements (triangles) and resistivity measurements (squares) axe shown. decreases significantly at 0.1% V305 and gradually thereafter. The jump in magnetic susceptibility at the transition, AXM, follows the same trend as the AH. The discontinuity in resistivity at the transition, Ap, also follows the trend in 2xH. It appears that A p is generally a good measure of 2xH in semiconductor-metal transitions. The activation energy for conduction, Ea, in the semiconducting phase (prior to the transition) 247
Volume 61A, number 4
PHYSICS LETTERS
decreases at 0.1% V305 and shows an increasing trend thereafter until at 10% V305 it again decreases to a small value. Variations in the different physical characteristics accompanying the semiconductor-metal transition in the T i 3 0 5 - V 3 0 5 system are reminiscent of behaviours found in the transitions of other mixed oxide systems. Thus, the variation in T t with % V305 is somewhat similar to that found in A1 doped V 2 0 3 [10]. The sharp decrease in unit cell volume and the accompanying decrease in E a with the initial incorporation of the dopant are found in Sc doped T i 2 0 3 [8]. The correspondence between the variations of A H and At) has been noticed in Ti doped V 2 0 3 [11]. The present results along with the analogies mentioned above seem to indicate that the semiconductor-metal transition in the T i 3 0 5 - V 3 0 5 system is largely latticedominated. Bartholomew and Frankl [4] had indeed suggested that the transition in Ti305 essentially conforms to the crystal distortion model of Adler and Brooks [12] which seems to be applicable to the transition in VO 2 as well. In many ways, the transition in Ti305 is comparable to that in VO 2. Thus, both transitions are thermodynamically first order and are associated with large AXM ; Ap is, however, considerably large and more abrupt in the case of VO 2 [2]. In the VO 2 - T i O 2 system, variation of AH, Ap, AXM and Tt with % TiO 2 has been found to be governed by the magnitude of crystal distortion of the low-temperature semiconducting phase [ 13].
248
16 May 1977
-References [1] C.N.R. Rao, S. Ramdas, R.E. Loehman and J.M. Ilonig, J. Solid State Chem. 3 (197l) 83. [2] C.N.R. Rao and G.V. Subba Rao, Transitional metal oxides, NSRD-NBS Monograph 49 (National Bureau of Standards, Washington, D.C. 1974). [3] L.N. Mulay and W.J. Danley, J. Appl. Phys. 41 (1970) 877. [4] R.E. Bartholomew and D.R. Frankl, Phys. Rev. 187 (1969) 828. [5] S. Asbrink, S. Friberg, A. Magneli and G. Andersson, Acta Chem. Stand. 13 (1959) 603. [6] S. Kachi, K. Kosuge and H. Okinaka, J. Solid State Chem. 6 (1973) 258. [7] G.V. Chandrashekhar, Q.W. Choi, J. Moyo and J.M. Honig, Mat. Res. Bull. 5 (1970) 999. [8] G.V. Chandrashekhar, L.L. Van Zandt, J.M. ltonig and A. Jayaraman, Phys. Rev. B10 (1974) 5063. [9] A.P.B. Sinha, G.V. Chandrashekhar and J.M. Honig, J. Solid State Chem. 12 (1975) 402, and the references cited therein. [10] H. Kuwamoto, D.L. Dickerson, H.V. Keer and J.M. ftonig Mat. Res. Bull. 11 (1976) 1301. [11] ft. Kuwamoto, H.V. Keer, J.E. Keem, S.A. Shivashankar, L.L. Van Zandt and J.M. Honig, J. Physique, Paris, to be published. [12] D. Adler and H. Brooks, Phys. Rev. 145 (1967) 826. [13] C.N.R. Rao, G.V. Subba Rao, M. Natarajan and R.E. Loehman, J. Phys. Chem. Solids 32 (1971) 1147.