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Low-spin-high-spin transition in the Co3O4 spinel
Journal of Magnetism and Magnetic Materials 104-107 (1992) 405-406 North-Holland
Low-spin-high-spin transition in the C0304 spinel V.A.M. Brabers and A.D.D. Broemme Department of Physics, Eindhoven University of Technology, P.O. Pox 513, 5600 MB Eindhot'en, Netherlands The anomalous increase of the thermal expansion of Co304 above 600 K is explained by a second-order LS-HS transition of the octahedral Co 3+ ions with a transition energy of about 0.3 eV. The analysis of thermoelectric measurements on pure and doped C030 a does not support other electronic charge transitions as the origin of the thermal-expansion anomaly. The compound Co304 crystallizes in the.cubic normal spinel structure with lattice parameter a = 0.8086 nm and an oxygen parameter ~ = 0.3887 _+ 0.0001 [1,2]. The normal cation distribution has been found from N M R [3,4], magnetic susceptibility [5,6] and M6ssbauer spectra measurements [7]. Below 40 K the magnetic structure is antiferromagnetic with high-spin Co 2+ ions on tetrahedral sites, whereas the Co 3+ ions on octahedral sites are in the non-magnetic low-spin state [5,6]. Touzelin [8] reported an anomalous increase of the lattice parameter in the range of 600-1200 K, which was related [9] to a possible electron-spin unpairing transition of the octrahedral Co 3+ ions: Co 3+ (LS)-~ Co 3+ (HS), reaction A. The anomalous increase of the lattice parameter is due to the higher ionic radius of the HS-Co 3+ ion compared with the LS-Co 3+ ion in octahedral sites (0.061 and 0.053 nm, respectively [10]). A direct indication for the L S - H S transition might be formed in the plot of the inverse susceptibility versus temperature [5]. The negative deviation from the linear behaviour can be explained by the increasing number of HS-Co 3+ ions. However, the anomalous thermal expansion might also be caused by three other reactions: (B) the disproportionation of octahedral Co 3+ ions into Co 2+ and Co 4+, (C) an electron transfer from tetrahedral to octahedral Co 3+ ions, ( D ) t h e formation of a defect spinel structure due to the chemical reaction with the gas atmosphere. Because the reactions B, C or D will influence the electrical properties a closer analysis of the electrical properties of C 0 3 0 4 can be useful in proving whether the anomalous thermal dilatation is caused by the L S - H S Co 3+ transition or not. Thermal expansion was measured with a dilatometer, described in ref. [11]. The polycrystalline samples were prepared by the usual ceramic techniques [12]. X-ray diffraction and optical microscopy proved that all the specimens were single-phase spinels. The electrical conductivity and the thermoelectric power were measured using the experimental set-up given in refs. [12,13]. In fig. 1 the linear thermal expansion is plotted against temperature for C 0 3 0 4. Measurements were performed in oxygen pressures between 1 and 10 3 atmospheres. U p till 1100 K no influence of the oxygen pressure, nor any hysteresis effect was found between
ColO 4
measured dilat
0.008
0.006
0.004 0.002
~ . ~ . ~ /
400
600
~esd~mated normal lattice dilatation
800 T (K)
1000
1200
Fig. 1. Linear thermal expansion of C0304. the rising and decreasing temperature runs, which excludes the reaction D. The excess thermal expansion above 600 K, which is determined as the difference between the experimental and the theoretical expansions with a constant c~ = 6 × 10 -6 above the Debye temperature of 525 K [6], has now been analysed separately for the three reactions A, B and C. Using the radii of the different ionic cobalt species [10], thc excess dilation for the three reactions can be calculated supposing a linear dependence of the lattice parameter upon the ionic concentration. The maximum values obtained for the completed reactions are 4.2, 5.0 and 1.6%, respectively. Using the temperature dependence of the excess expansion, the progress of the reactions can be calculated as functions of temperature and from these values the activation energies for the three reactions are calculated as functions of temperature and plotted in fig. 2. For the L S - H S transition an energy of the order of 0.3 eV is found. For the reactions B and C energies in the range of 0.5-0.8 eV are calculated, which are of the same order of magnitude as the activation energies of the electrical conductivity [i2,14-16]. These similar activation energies could be taken as evidence that either reactions B or C is the cause of the expansion anomaly. However, just in the temperature range above 800 K the energy for the electrical conductivity is 0.8 eV (see fig. 3), whereas the calculated E B and E c values are lowered with increas-
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
4(16
V.A.M. Brabers and A.D.D. Broemme / Low-spin-high-spin transition 8O0
0.8
:
EB
.""
600 •
40O 0.6
~
EC
•
\'
• •
~200
o000o00
&~
ov ~1 ° ~ 00 a t, -.~)a ~ a A o
o Oo~' o O0 o "0
o eo
o o
0.4
0
Ti - doped C%O 4
o -200
o
0.2 -400 400
600
800
1000
•
x=0
0
x : 0.001 x = 0.0l x=O.1
a o
o o
1200 0
T (K)
400
600
800
1000
1200
Fig. 2. Activation energy calculated from the excess thermal expansion for the reactions A, B and C.
T (K) Fig. 4. Thermoelectric power of C 0 3 0 4 and Ti-doped C0304 as a function of temperature.
ing temperature. Moreover, the thermopower data on pure and substituted C0304 presented in fig. 4 show that a relatively large concentration of 0.1 Ti per formula unit is needed to change the conduction from p to n type. Since the impurity contents in our pure C0304+ ~ specimens are well below 10 - 3 and the oxygen excess y is lower than 0.005, the large Ti concentration required to change the sign of the thermopower indicates that in pure Co304 the electrical conduction is intrinsic with high-mobility holes and low-mobility electrons• This means that the energy gap for the electrical conduction, which is just the reaction energy of either reactions B or C, must be about twice the activation energy of the electrical conduction. A quantitative analysis of the thermoelectric properties revealed that the energy gap is strongly temperature dependent, which was related to the anomalous thermal expansion and amounts at 300 and 900 K, to 1.65 and 1.05 eV, respectively [12]. The room temperature
value of 1.65 eV is in good agreement with the absorption band near 1.7 eV in the optical spectra [17] and recent XPS and BIS experiments revealed a value of 1.6 eV for the gap energy [18]. Consequently, the discrepancies between the reaction energies calculated for the reactions B and C and the electrical gap energy are in favour of the second order L S - H S transition of the octahedral Co 3+ ions with a transition energy of about 0.3 eV.
% B~
Co304
Oao ~o
own data
~Oo
~-, 0
a aoe • • ,-8,
eoe
a~
~-2
*~
[KOU 81]
a
[MAS 881
o
t~ -4
A
-8
-10
8
12
L 16
104/T
,
~ 20
24
28
(K d )
Fig. 3. Electrical conductivity of Co304 against reciprocal temperature.
References
[1] G. Will, N. Mascionani, W. Parrish and M. Hart, J. Appl. Crystallogr 20 (1987) 394. [2] C.E. Infante-Barros, Ph.D. Thesis, Oxford (19751. [3] K. Miyatami, K. Kohn, H. Kamimura and S. Iida, J. Phys. Soc. Japan 21 (1966) 464. [4] H. Kamimura, J. Phys. Soc. Japan 21 (1966) 484. [5] P. Cossee, J. Inorg. Nucl. Chem. 8 (1958) 483; Ph.D. Thesis, Leiden (1956). [6] W.L. Roth, J. Phys. Chem. Solids 25 (19641 1. [7] W. Kiindig, M. Kobelt, H. Appel, G. Constabaris and R.H. Lindquist, J. Phys. Chem. Solids 30 (1969) 819. [8] B. Touzelin, Rev. Int. Htes Temp. Refract. 15 (19781 33. [9] H.St. O'Neill, Phys. Chem. Minerals 12 (19841 149. [10] R.D. Shannon and C.T. Prewitt, Acta Crystallogr. B 25 (19691 925, B 26 (1970) 1046, A 32 (1976) 751. [11] V.A.M. Brabers, Ph.D. Thesis, Eindhoven (1970). [12] A.D.D. Broemme, Ph.D. Thesis, Eindhoven (19901. [13] A.J.M. Kuipers, Ph.D. Thesis, Eindhoven (19781. [14] J.A.K. Tareen, A. Malecki, J.P. Doumerc, J.C. Launay, P. Dordor, M. Pouchard and P. Hagenmuller, Mater. Res. Bull. 19 (1984) 989. [15] K. Koumoto and H. Yanagida, J. Am. Ceram. Soc. 64 (1981) C-156. [16] T.O. Mason, Physica B 150 (1988) 37. [17] J.G. Cook and M.P. van der Meer, Thin Solid Films 144 (1986) 165. [18] J. van Elp, J.L. Wieland, H. Eskes, P. Kuiper, G.A. Sawatzky, F.M.F. de Groot and T.S. Turner, Phys. Rev. B 44 (1991) 6090.
Low-spin-high-spin transition in the C0304 spinel V.A.M. Brabers and A.D.D. Broemme Department of Physics, Eindhoven University of Technology, P.O. Pox 513, 5600 MB Eindhot'en, Netherlands The anomalous increase of the thermal expansion of Co304 above 600 K is explained by a second-order LS-HS transition of the octahedral Co 3+ ions with a transition energy of about 0.3 eV. The analysis of thermoelectric measurements on pure and doped C030 a does not support other electronic charge transitions as the origin of the thermal-expansion anomaly. The compound Co304 crystallizes in the.cubic normal spinel structure with lattice parameter a = 0.8086 nm and an oxygen parameter ~ = 0.3887 _+ 0.0001 [1,2]. The normal cation distribution has been found from N M R [3,4], magnetic susceptibility [5,6] and M6ssbauer spectra measurements [7]. Below 40 K the magnetic structure is antiferromagnetic with high-spin Co 2+ ions on tetrahedral sites, whereas the Co 3+ ions on octahedral sites are in the non-magnetic low-spin state [5,6]. Touzelin [8] reported an anomalous increase of the lattice parameter in the range of 600-1200 K, which was related [9] to a possible electron-spin unpairing transition of the octrahedral Co 3+ ions: Co 3+ (LS)-~ Co 3+ (HS), reaction A. The anomalous increase of the lattice parameter is due to the higher ionic radius of the HS-Co 3+ ion compared with the LS-Co 3+ ion in octahedral sites (0.061 and 0.053 nm, respectively [10]). A direct indication for the L S - H S transition might be formed in the plot of the inverse susceptibility versus temperature [5]. The negative deviation from the linear behaviour can be explained by the increasing number of HS-Co 3+ ions. However, the anomalous thermal expansion might also be caused by three other reactions: (B) the disproportionation of octahedral Co 3+ ions into Co 2+ and Co 4+, (C) an electron transfer from tetrahedral to octahedral Co 3+ ions, ( D ) t h e formation of a defect spinel structure due to the chemical reaction with the gas atmosphere. Because the reactions B, C or D will influence the electrical properties a closer analysis of the electrical properties of C 0 3 0 4 can be useful in proving whether the anomalous thermal dilatation is caused by the L S - H S Co 3+ transition or not. Thermal expansion was measured with a dilatometer, described in ref. [11]. The polycrystalline samples were prepared by the usual ceramic techniques [12]. X-ray diffraction and optical microscopy proved that all the specimens were single-phase spinels. The electrical conductivity and the thermoelectric power were measured using the experimental set-up given in refs. [12,13]. In fig. 1 the linear thermal expansion is plotted against temperature for C 0 3 0 4. Measurements were performed in oxygen pressures between 1 and 10 3 atmospheres. U p till 1100 K no influence of the oxygen pressure, nor any hysteresis effect was found between
ColO 4
measured dilat
0.008
0.006
0.004 0.002
~ . ~ . ~ /
400
600
~esd~mated normal lattice dilatation
800 T (K)
1000
1200
Fig. 1. Linear thermal expansion of C0304. the rising and decreasing temperature runs, which excludes the reaction D. The excess thermal expansion above 600 K, which is determined as the difference between the experimental and the theoretical expansions with a constant c~ = 6 × 10 -6 above the Debye temperature of 525 K [6], has now been analysed separately for the three reactions A, B and C. Using the radii of the different ionic cobalt species [10], thc excess dilation for the three reactions can be calculated supposing a linear dependence of the lattice parameter upon the ionic concentration. The maximum values obtained for the completed reactions are 4.2, 5.0 and 1.6%, respectively. Using the temperature dependence of the excess expansion, the progress of the reactions can be calculated as functions of temperature and from these values the activation energies for the three reactions are calculated as functions of temperature and plotted in fig. 2. For the L S - H S transition an energy of the order of 0.3 eV is found. For the reactions B and C energies in the range of 0.5-0.8 eV are calculated, which are of the same order of magnitude as the activation energies of the electrical conductivity [i2,14-16]. These similar activation energies could be taken as evidence that either reactions B or C is the cause of the expansion anomaly. However, just in the temperature range above 800 K the energy for the electrical conductivity is 0.8 eV (see fig. 3), whereas the calculated E B and E c values are lowered with increas-
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
4(16
V.A.M. Brabers and A.D.D. Broemme / Low-spin-high-spin transition 8O0
0.8
:
EB
.""
600 •
40O 0.6
~
EC
•
\'
• •
~200
o000o00
&~
ov ~1 ° ~ 00 a t, -.~)a ~ a A o
o Oo~' o O0 o "0
o eo
o o
0.4
0
Ti - doped C%O 4
o -200
o
0.2 -400 400
600
800
1000
•
x=0
0
x : 0.001 x = 0.0l x=O.1
a o
o o
1200 0
T (K)
400
600
800
1000
1200
Fig. 2. Activation energy calculated from the excess thermal expansion for the reactions A, B and C.
T (K) Fig. 4. Thermoelectric power of C 0 3 0 4 and Ti-doped C0304 as a function of temperature.
ing temperature. Moreover, the thermopower data on pure and substituted C0304 presented in fig. 4 show that a relatively large concentration of 0.1 Ti per formula unit is needed to change the conduction from p to n type. Since the impurity contents in our pure C0304+ ~ specimens are well below 10 - 3 and the oxygen excess y is lower than 0.005, the large Ti concentration required to change the sign of the thermopower indicates that in pure Co304 the electrical conduction is intrinsic with high-mobility holes and low-mobility electrons• This means that the energy gap for the electrical conduction, which is just the reaction energy of either reactions B or C, must be about twice the activation energy of the electrical conduction. A quantitative analysis of the thermoelectric properties revealed that the energy gap is strongly temperature dependent, which was related to the anomalous thermal expansion and amounts at 300 and 900 K, to 1.65 and 1.05 eV, respectively [12]. The room temperature
value of 1.65 eV is in good agreement with the absorption band near 1.7 eV in the optical spectra [17] and recent XPS and BIS experiments revealed a value of 1.6 eV for the gap energy [18]. Consequently, the discrepancies between the reaction energies calculated for the reactions B and C and the electrical gap energy are in favour of the second order L S - H S transition of the octahedral Co 3+ ions with a transition energy of about 0.3 eV.
% B~
Co304
Oao ~o
own data
~Oo
~-, 0
a aoe • • ,-8,
eoe
a~
~-2
*~
[KOU 81]
a
[MAS 881
o
t~ -4
A
-8
-10
8
12
L 16
104/T
,
~ 20
24
28
(K d )
Fig. 3. Electrical conductivity of Co304 against reciprocal temperature.
References
[1] G. Will, N. Mascionani, W. Parrish and M. Hart, J. Appl. Crystallogr 20 (1987) 394. [2] C.E. Infante-Barros, Ph.D. Thesis, Oxford (19751. [3] K. Miyatami, K. Kohn, H. Kamimura and S. Iida, J. Phys. Soc. Japan 21 (1966) 464. [4] H. Kamimura, J. Phys. Soc. Japan 21 (1966) 484. [5] P. Cossee, J. Inorg. Nucl. Chem. 8 (1958) 483; Ph.D. Thesis, Leiden (1956). [6] W.L. Roth, J. Phys. Chem. Solids 25 (19641 1. [7] W. Kiindig, M. Kobelt, H. Appel, G. Constabaris and R.H. Lindquist, J. Phys. Chem. Solids 30 (1969) 819. [8] B. Touzelin, Rev. Int. Htes Temp. Refract. 15 (19781 33. [9] H.St. O'Neill, Phys. Chem. Minerals 12 (19841 149. [10] R.D. Shannon and C.T. Prewitt, Acta Crystallogr. B 25 (19691 925, B 26 (1970) 1046, A 32 (1976) 751. [11] V.A.M. Brabers, Ph.D. Thesis, Eindhoven (1970). [12] A.D.D. Broemme, Ph.D. Thesis, Eindhoven (19901. [13] A.J.M. Kuipers, Ph.D. Thesis, Eindhoven (19781. [14] J.A.K. Tareen, A. Malecki, J.P. Doumerc, J.C. Launay, P. Dordor, M. Pouchard and P. Hagenmuller, Mater. Res. Bull. 19 (1984) 989. [15] K. Koumoto and H. Yanagida, J. Am. Ceram. Soc. 64 (1981) C-156. [16] T.O. Mason, Physica B 150 (1988) 37. [17] J.G. Cook and M.P. van der Meer, Thin Solid Films 144 (1986) 165. [18] J. van Elp, J.L. Wieland, H. Eskes, P. Kuiper, G.A. Sawatzky, F.M.F. de Groot and T.S. Turner, Phys. Rev. B 44 (1991) 6090.