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Electrical conductivity in the antiferromagnetic compounds FeVO4, FeVMoO7 and Fe4V2Mo3O20
Journal of Magnetism North-Holland
and Magnetic
Materials
M m+
101 (1991) 148-150
M
? s
A
Electrical conductivity in the antiferromagnetic FeVMoO, and Fe,V, Mo,O,, T. Groii a, J. Krok-Kowalski a, M. Kurzawa b and J. Walczak 0 Silesian University, Institute of Physics, al. Uniwersytecka 4, 40007 Katowice, Poland h Technical Uniuersity, Institute
of Fundamental
Chemistry,
compounds FeVO,,
b
al. Piastbw 42, 71065 Szczecin,
Poland
The compounds under study are n-type antiferromagnetic semiconductors. Their electrical properties the metallic component concentration in the compound. The big changes of the electrical conductivity connected with the processes strongly thermally activated. 1. Introduction
A literature survey has shown that both the oxides: V,O,, MOO,, Fe,,O, and their compounds occurring in the two-component systems: V,O,-MOO,, V,Os-FezO, and Fe,O,-Moo, have been an object of comprehensive studies for years, primarily due to their interesting catalytic properties. Vanadium (v> oxide is used as a catalyst in the process of oxidation of SO, to SO, and as a catalyst in the processes of sulfonation and oxidation of aromatic hydrocarbons [l]. Iron (III) oxide is a basic component of oxide contacts used in dehydrogenation of aliphatic and cyclic hydrocarbons [1,2]. On the other hand V,O, is also, beside MOO,, a basic component of catalysts applied in oxidation of benzene to maleic anhydride and methanol to formaldehyde [3]. Recently, the expensive silver catalyst, used in oxidation of methanol to formaldehyde and propene to acrolein, has been replaced, with a great success, by oxide catalysts which contain MOO, and Fe,(MoO,), as the active and selective components [4,5], the latter occurring in the Fe,O,-Moo, system [6]. The silver catalyst can also be replaced with Fe,O, and FeVO, [7] - one of the two compounds existing in the V,O,-Fe,O, system [8]. Iron orthovanadate is also one of the few substances showing a practical catalytic effect in a selective alkylation of phenol and methylophenol by methanol [9]. The structure of FeVO, is triclinic [lo] and for the remaining compounds the structure is not yet known. Miissbauer measurements suggest that the Fe ions exist in the valence state of 3 + in all the samples under study [lO,ll]. The investigations of the electrical and magnetic properties are limited to the FeVO, compound. It was found that FeVO, is an antiferromagnetic [12] p-type semiconductor [13]. However, the positive sign of the Seebeck coefficient was obtained for the samples which have been annealed earlier at 800 K for 24 h in a tightly closed platinum crucible to avoid oxidation at higher temperature as was done by Gupta et al. in ref. [13]. In the non-annealed samples of the system 0312-8853/91/$03.50
0 1991 - Elsevier Science Publishers
depend strongly vs. temperature
on are
(Fe,O,),(V,O,),_, (where 0 I x 5 0.25) [14] and Fe,V,_,Mo,O,, type phases (where 0.0 I x I 0.6) [15] the n-type sign of the Seebeck coefficient was observed. The aim of this paper is the presentation of the experiment concerning the magnetic susceptibility, the Seebeck effect and the electrical resistivity of the FeVO,, FeVMoO, and Fe,V,Mo,O,, compounds. 2. Experimental The magnetic susceptibility was measured with the use of a magnetic balance between 77 and 300 K using the Faraday method. The electrical resistivity measurements were made with the aid of the 2-point dc method in the temperature range of 170-500 K, using a semiautomatic electronic resistance bridge [16]. The accuracy of the measurements was *0.6%. The samples of the form of a parallelepiped were pressed (10 MPa). The contacts were made of silver paste. The Seebeck coefficient (Y was determined with the use of an electrical automatic measuring device in the temperature range of 300-400 K. This device generated a temperature difference AT = T, - Tl and keeping a constant average temperature T = (T, + T,)/2 between the two platinum blocks, where a sample was placed. In these blocks two thermocouples Chromel-Alumel were fixed in order to measure the temperature Tl and T2. In the range of the temperature difference 0 I AT I 7 K the dependence of the thermoelectric force on AT was linear. From the slope of this straight line the Seebeck-coefficient was determined with the accuracy of -t_lOpV/K. The measurement technique is in detail described in ref. [17]. The technology and the preparation of the samples under study are described in other papers [l&19]. 3. Results and discussion From the magnetic and electrical measurements (see table 1 and figs. 1 and 2) it follows that the compounds FeVO,, FeVMoO, and Fe,V,Mo,O,, are antiferromagnets with the paramagnetic Curie-Weiss temperature
B.V. All rights reserved
149
T. Grori et al. / Electrical conductivity in anfiferromagnets 15
100 1
XM
In7 (Rml
(kgT/Am*)
IO
50
0 -100
0
100
200
ci -’
300
i
o,, =
-64 K and are n-type semiconductors. It was also observed that: 1) with the temperature increase: a) the negative value of Seebeck coefficient (Y strongly decreases, particularly for FeVO,, b) the electrical resistivity p for all samples strongly decreases and changes 6 orders of magnitude in the temperature range of 170500 K; 2) with the increase of concentration of the metallic component in the compound: a) the Curie constant C, and the effective magnetic moment /Q remain almost constant, b) the electrical resistivity p, the negative Seebeck coefficient a, the negative coefficient of resistivity aTCR and the activation energy E, decrease. The values of peff p resented in table 1 suggest that for FeVO,, FeVMoO, and Fe,V,Mo,O,, the Fe3+ ions exist in a high-spin state (orbital singlet). The high values of aTCR and E, as well as the negative sign of a suggest that the compounds under study are characterized by the ionic conductivity and the charge transport occurs by means of the thermally activated hopping of 3d electrons of iron via oxygen vacancies (playing role of the double donor). Summing up, one has to conclude that: 1) in the case of antiferromagnetic ordering of the magnetic moments
I*
i
Fig. 1. Temperature dependence of the inverse magnetic susceptibility for (0) FeVO,, (x) FeVMoO, and (0) Fe,V,Mo,O,,.
d (I_ 2
I
I
I
3
I
5
6
103/T (K-l)
Fig. 2. The electrical resistivity (In p) vs. T-’ for (0) FeVO,, (x) FeVMoO, and (0) Fe,V,Mo,O,,. Inset: Temperature dependence of the Seebeck coefficient for the same samples.
usually one observes the low electrical conductivity (typical of dielectrics) [20,21] and the state corresponding to the compensated semiconductors [21], 2) in general the oxides exhibit n-type conductivity due to the non-stoichiometry (oxygen deficiencies) [14], 3) the use of V,O,, MOO,, Fe,O, and phases existing in two-component systems as catalysts in a number of processes of organic synthesis gives sufficient grounds to suppose that FeVMoO, and Fe,V,Mo,O, - the compounds firstly obtained by us [18,19,22] - may be the agents to possess catalytic properties of interest.
Table 1 The magnetic and electrical parameters of the compounds under study Compound
/J&f [PBI
cM
8
C+CR
& WI
[K/m4
[I(“;”
[lo-’ K-l]
T<3OOK
T>4OOK
P Pm1 (300 K)
a[mV/W (300 K)
0.43 0.30 0.24
1.72 1.13 0.86
4x106 lX105 5x103
- 14.4 - 2.2 - 0.4
(300 K) FeVO, FeVMoO, Fe,V#o30,
5.221 5.50 5.51
3.38 3.16 3.11
-70 -49 -71
- 31.5 - 25.5 - 22.5
150
T. Groin et al. / Elecirical conductiuity in antiferromagnets
References [l] Ya.A. Dorfman,
[2] [3] [4] [5] [6] [7] [8] [9]
[lo]
Katalizatory i mekhanizmy gidrirovaniya i okisleniya (Izd. Nauka, Alma Ata, 1984) pp. 240, 287, 326. G.M. Panczenkow and W.P. Lebiediew, Kinetyka Chemiczna i Katalityczna (WNT, Warsaw, 1964) p. 349. A. Inglot, React. Kinet. Catal. Lett. 39 (1989) 323. C.J. Machiels, U. Chowdhry, W.T.A. Harrison and A.W. Sleight, ACS Symp. Ser. Solid State Catal. 279 (1985) 103. N. van Truong, P. Tittarelli and P.L. Villa, Proc. III Intern. Conf. on Chem. Uses Molybdenum (1979) 161. W. Jager, A. Rahmed and K. Becker, Arch. Eisenhllttenwesen 30 (1959) 435. R. Malihski, J. Gallus-Olender and T. Kubicka, React. Kinet. Catal. Lett. 10 (1979) 219. J. Walczak, J. Zitikowski, M. Kurzawa, J. Osten-Sacken and M. Lysio, Polish J. Chem. 59 (1985) 255. V.I. Korenskii, I.S. Ignateva, I.P. Kolenko, A.A. Fotiev and L.L. Surat, Sb. Statei Osobennosti elektronnogo stroenniya i svoistva tverdofaznykh soedinenii titana i vanadiya (UNTS An SSSR, Sverdlovsk, 1982) p. 82. B. Robertson and K. Kostiner, J. Solid State Chem. 4 (1972) 29.
[ll] T. Panek, private communication. [12] L.M. Levinson and B.M. Wanklyn, J. Solid State Chem. 3 (1971) 131 [13] S. Gupta, Y.P. Yadava and R.A. Singh, J. Mater. Sci. Lett. 5 (1986) 736. [14] O.G. Palanna, A.L. Shashi Mohau and A.B. Biswas, Indian Acad. Sci. 87A (1978) 259. [15] T. Groh, J. Krok, M. Kurzawa and J. Walczak, J. Magn. Magn. Mater. 54-57 (1986) 1301. [16] I. Okonska-Kozlowska, H.D. Lutz, T. Groh, J. Krok and T. Mydlarz, Mater. Res. Bull. 19 (1984) 1. [17] M. Gogdowicz, T. Groh, Z. Ujma and J. Warczewski, Phase Transitions 5 (1985) 233. [18] J. Walczak, J. Ziblkowski, M. Kurzawa and L. Trzesniowska, Polish J. Chem. 69 (1985) 713. [19] J. Walczak, M. Kurzawa and E. Filipek, J. Therm. Anal. 31 (1986) 271. [20] T. Satoh and T. Tsushima, Mater, Res. Bull. 9 (1974) 1297. [21] T. Groh, H. Duda and J. Warczewski, Phys. Rev. B 41 (1990) 12424. [22] M. Kurzawa, J. Mater. Sci. Lett., in press.
and Magnetic
Materials
M m+
101 (1991) 148-150
M
? s
A
Electrical conductivity in the antiferromagnetic FeVMoO, and Fe,V, Mo,O,, T. Groii a, J. Krok-Kowalski a, M. Kurzawa b and J. Walczak 0 Silesian University, Institute of Physics, al. Uniwersytecka 4, 40007 Katowice, Poland h Technical Uniuersity, Institute
of Fundamental
Chemistry,
compounds FeVO,,
b
al. Piastbw 42, 71065 Szczecin,
Poland
The compounds under study are n-type antiferromagnetic semiconductors. Their electrical properties the metallic component concentration in the compound. The big changes of the electrical conductivity connected with the processes strongly thermally activated. 1. Introduction
A literature survey has shown that both the oxides: V,O,, MOO,, Fe,,O, and their compounds occurring in the two-component systems: V,O,-MOO,, V,Os-FezO, and Fe,O,-Moo, have been an object of comprehensive studies for years, primarily due to their interesting catalytic properties. Vanadium (v> oxide is used as a catalyst in the process of oxidation of SO, to SO, and as a catalyst in the processes of sulfonation and oxidation of aromatic hydrocarbons [l]. Iron (III) oxide is a basic component of oxide contacts used in dehydrogenation of aliphatic and cyclic hydrocarbons [1,2]. On the other hand V,O, is also, beside MOO,, a basic component of catalysts applied in oxidation of benzene to maleic anhydride and methanol to formaldehyde [3]. Recently, the expensive silver catalyst, used in oxidation of methanol to formaldehyde and propene to acrolein, has been replaced, with a great success, by oxide catalysts which contain MOO, and Fe,(MoO,), as the active and selective components [4,5], the latter occurring in the Fe,O,-Moo, system [6]. The silver catalyst can also be replaced with Fe,O, and FeVO, [7] - one of the two compounds existing in the V,O,-Fe,O, system [8]. Iron orthovanadate is also one of the few substances showing a practical catalytic effect in a selective alkylation of phenol and methylophenol by methanol [9]. The structure of FeVO, is triclinic [lo] and for the remaining compounds the structure is not yet known. Miissbauer measurements suggest that the Fe ions exist in the valence state of 3 + in all the samples under study [lO,ll]. The investigations of the electrical and magnetic properties are limited to the FeVO, compound. It was found that FeVO, is an antiferromagnetic [12] p-type semiconductor [13]. However, the positive sign of the Seebeck coefficient was obtained for the samples which have been annealed earlier at 800 K for 24 h in a tightly closed platinum crucible to avoid oxidation at higher temperature as was done by Gupta et al. in ref. [13]. In the non-annealed samples of the system 0312-8853/91/$03.50
0 1991 - Elsevier Science Publishers
depend strongly vs. temperature
on are
(Fe,O,),(V,O,),_, (where 0 I x 5 0.25) [14] and Fe,V,_,Mo,O,, type phases (where 0.0 I x I 0.6) [15] the n-type sign of the Seebeck coefficient was observed. The aim of this paper is the presentation of the experiment concerning the magnetic susceptibility, the Seebeck effect and the electrical resistivity of the FeVO,, FeVMoO, and Fe,V,Mo,O,, compounds. 2. Experimental The magnetic susceptibility was measured with the use of a magnetic balance between 77 and 300 K using the Faraday method. The electrical resistivity measurements were made with the aid of the 2-point dc method in the temperature range of 170-500 K, using a semiautomatic electronic resistance bridge [16]. The accuracy of the measurements was *0.6%. The samples of the form of a parallelepiped were pressed (10 MPa). The contacts were made of silver paste. The Seebeck coefficient (Y was determined with the use of an electrical automatic measuring device in the temperature range of 300-400 K. This device generated a temperature difference AT = T, - Tl and keeping a constant average temperature T = (T, + T,)/2 between the two platinum blocks, where a sample was placed. In these blocks two thermocouples Chromel-Alumel were fixed in order to measure the temperature Tl and T2. In the range of the temperature difference 0 I AT I 7 K the dependence of the thermoelectric force on AT was linear. From the slope of this straight line the Seebeck-coefficient was determined with the accuracy of -t_lOpV/K. The measurement technique is in detail described in ref. [17]. The technology and the preparation of the samples under study are described in other papers [l&19]. 3. Results and discussion From the magnetic and electrical measurements (see table 1 and figs. 1 and 2) it follows that the compounds FeVO,, FeVMoO, and Fe,V,Mo,O,, are antiferromagnets with the paramagnetic Curie-Weiss temperature
B.V. All rights reserved
149
T. Grori et al. / Electrical conductivity in anfiferromagnets 15
100 1
XM
In7 (Rml
(kgT/Am*)
IO
50
0 -100
0
100
200
ci -’
300
i
o,, =
-64 K and are n-type semiconductors. It was also observed that: 1) with the temperature increase: a) the negative value of Seebeck coefficient (Y strongly decreases, particularly for FeVO,, b) the electrical resistivity p for all samples strongly decreases and changes 6 orders of magnitude in the temperature range of 170500 K; 2) with the increase of concentration of the metallic component in the compound: a) the Curie constant C, and the effective magnetic moment /Q remain almost constant, b) the electrical resistivity p, the negative Seebeck coefficient a, the negative coefficient of resistivity aTCR and the activation energy E, decrease. The values of peff p resented in table 1 suggest that for FeVO,, FeVMoO, and Fe,V,Mo,O,, the Fe3+ ions exist in a high-spin state (orbital singlet). The high values of aTCR and E, as well as the negative sign of a suggest that the compounds under study are characterized by the ionic conductivity and the charge transport occurs by means of the thermally activated hopping of 3d electrons of iron via oxygen vacancies (playing role of the double donor). Summing up, one has to conclude that: 1) in the case of antiferromagnetic ordering of the magnetic moments
I*
i
Fig. 1. Temperature dependence of the inverse magnetic susceptibility for (0) FeVO,, (x) FeVMoO, and (0) Fe,V,Mo,O,,.
d (I_ 2
I
I
I
3
I
5
6
103/T (K-l)
Fig. 2. The electrical resistivity (In p) vs. T-’ for (0) FeVO,, (x) FeVMoO, and (0) Fe,V,Mo,O,,. Inset: Temperature dependence of the Seebeck coefficient for the same samples.
usually one observes the low electrical conductivity (typical of dielectrics) [20,21] and the state corresponding to the compensated semiconductors [21], 2) in general the oxides exhibit n-type conductivity due to the non-stoichiometry (oxygen deficiencies) [14], 3) the use of V,O,, MOO,, Fe,O, and phases existing in two-component systems as catalysts in a number of processes of organic synthesis gives sufficient grounds to suppose that FeVMoO, and Fe,V,Mo,O, - the compounds firstly obtained by us [18,19,22] - may be the agents to possess catalytic properties of interest.
Table 1 The magnetic and electrical parameters of the compounds under study Compound
/J&f [PBI
cM
8
C+CR
& WI
[K/m4
[I(“;”
[lo-’ K-l]
T<3OOK
T>4OOK
P Pm1 (300 K)
a[mV/W (300 K)
0.43 0.30 0.24
1.72 1.13 0.86
4x106 lX105 5x103
- 14.4 - 2.2 - 0.4
(300 K) FeVO, FeVMoO, Fe,V#o30,
5.221 5.50 5.51
3.38 3.16 3.11
-70 -49 -71
- 31.5 - 25.5 - 22.5
150
T. Groin et al. / Elecirical conductiuity in antiferromagnets
References [l] Ya.A. Dorfman,
[2] [3] [4] [5] [6] [7] [8] [9]
[lo]
Katalizatory i mekhanizmy gidrirovaniya i okisleniya (Izd. Nauka, Alma Ata, 1984) pp. 240, 287, 326. G.M. Panczenkow and W.P. Lebiediew, Kinetyka Chemiczna i Katalityczna (WNT, Warsaw, 1964) p. 349. A. Inglot, React. Kinet. Catal. Lett. 39 (1989) 323. C.J. Machiels, U. Chowdhry, W.T.A. Harrison and A.W. Sleight, ACS Symp. Ser. Solid State Catal. 279 (1985) 103. N. van Truong, P. Tittarelli and P.L. Villa, Proc. III Intern. Conf. on Chem. Uses Molybdenum (1979) 161. W. Jager, A. Rahmed and K. Becker, Arch. Eisenhllttenwesen 30 (1959) 435. R. Malihski, J. Gallus-Olender and T. Kubicka, React. Kinet. Catal. Lett. 10 (1979) 219. J. Walczak, J. Zitikowski, M. Kurzawa, J. Osten-Sacken and M. Lysio, Polish J. Chem. 59 (1985) 255. V.I. Korenskii, I.S. Ignateva, I.P. Kolenko, A.A. Fotiev and L.L. Surat, Sb. Statei Osobennosti elektronnogo stroenniya i svoistva tverdofaznykh soedinenii titana i vanadiya (UNTS An SSSR, Sverdlovsk, 1982) p. 82. B. Robertson and K. Kostiner, J. Solid State Chem. 4 (1972) 29.
[ll] T. Panek, private communication. [12] L.M. Levinson and B.M. Wanklyn, J. Solid State Chem. 3 (1971) 131 [13] S. Gupta, Y.P. Yadava and R.A. Singh, J. Mater. Sci. Lett. 5 (1986) 736. [14] O.G. Palanna, A.L. Shashi Mohau and A.B. Biswas, Indian Acad. Sci. 87A (1978) 259. [15] T. Groh, J. Krok, M. Kurzawa and J. Walczak, J. Magn. Magn. Mater. 54-57 (1986) 1301. [16] I. Okonska-Kozlowska, H.D. Lutz, T. Groh, J. Krok and T. Mydlarz, Mater. Res. Bull. 19 (1984) 1. [17] M. Gogdowicz, T. Groh, Z. Ujma and J. Warczewski, Phase Transitions 5 (1985) 233. [18] J. Walczak, J. Ziblkowski, M. Kurzawa and L. Trzesniowska, Polish J. Chem. 69 (1985) 713. [19] J. Walczak, M. Kurzawa and E. Filipek, J. Therm. Anal. 31 (1986) 271. [20] T. Satoh and T. Tsushima, Mater, Res. Bull. 9 (1974) 1297. [21] T. Groh, H. Duda and J. Warczewski, Phys. Rev. B 41 (1990) 12424. [22] M. Kurzawa, J. Mater. Sci. Lett., in press.