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Electronic state of Sr3Fe2O7−y studied by specific heat and Mössbauer spectroscopy
ELSEVIER
Physica B 237-238 (1997) 105-107
Electronic state of Sr3Fe207_y studied by specific heat and M6ssbauer spectroscopy H. K o b a y a s h i a'*, M . K i r a a, H. O n o d e r a b, T.
Suzuki a, T. K a m i m u r a a
a Department of Physics, Tohoku University, Sendai 980-77, Japan b lnstitute for Materials Research, Tohoku University, Sendai 980-77, Japan
Abstract
The low-temperature specific heat and M6ssbauer spectra have been measured for Sr3Fe207_y.The iron sites are subdivided into two electronically nonequivalent sites, that is, one is a tetravalent site and the other a trivalent one. The concentration and the isomer shift of tetravalent site depend on the oxygen vacancies. The electronic specific heat of Sr3Fe206.92is smaller by one order than those of Sr3Fe207-y above y = 0.2. It is concluded that the electronic state of Sr3Fe2OT-ychanges around y ,-, 0.15.
Keywords: Sr3Fe2OT-y;Specific heat; M6ssbauer effect
In the series of compounds Srn+lFenO3n+l (n = 1, 2 and ~ ) with formally tetravalent iron, SrFeO3 (n=~) has a cubic perovskite structure which consists of a three-dimensional network of cornersharing FeO6 octahedra, whereas Sr2FeO4 (n = 1) is a K2NiFn-type structure with a two-dimensional network of the octahedra. The structure of Sr3Fe207 (n = 2) can be described as a stacking of the sheets of the double octahedra and the Sr layers alternately along the c-axis, which is called as the RuddelesdenPopper phase and then is an intermediate structure between those of n = 1 and c~ compounds. The electrical resistivity of Sr2FeO4_y shows a semiconducting tempera~re-dependence, whereas that of S r F e O 3 _ y is temperature independent [I]. It was found by the measurement of optical reflectivity for Sr2FeO4 under pressure [2] that the lowest optical transition shifts to lower energy with pressure, which is interpreted in terms of a gap narrowing. Hence, the variation of electric properties in the series of these compounds seems to be responsible for the degree of condensation of the FeO6 octahedra and strength of the d-p hybridiza* Corresponding author. 0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 0 0 6 5 - 3
tion. As observed in high-To superconductors, the electronic properties in these compounds are strongly affected by the oxygen deficiency. In this study, we investigate the electronic properties of Sr3Fe2OT_y for three different oxygen-deficient samples using the specific heat and Mfssbauer effect measurements. The polycrystalline samples of Sr3Fe207_y were synthesized by a direct reaction. SrCO3 (purity: 99.994%) and Fe203 (99.999%) were mixed together in the nominal molar proportion and heated at 1100°C for 25 h in air; the product was ground and heated at 1100°C for further 25 h . This sample is referred to as ~ 1. Moreover, we obtained the oxygen-doped samples by following two different annealing processes. The sample ~2 was produced by annealing at 900°C for 50h and at 450°C for 170h in flowing 02 gas. The sample g3 was yielded by annealing at 500°C for 50h under 1000 atm of 02 and Ar mixed gas in the molar ratio of 1 : 4. The Mrssbauer spectra were measured with a conventional transmission spectrometer. The specific heat measurements were carried out from 1.8 to 100K by an adiabatic method. The three samples were confirmed to be in a single phase by the powder X-ray diffraction using a
106
H. Kobayashi et at/Physica B 237 238 (1997) 105-107
80
[Sr3Fe2Oe.(#1) ss
*
Sr3Fe206.68 ( #i )
o
Sr3Fe206.80 ( #2 ) # Sr3Fe206.92 (
60 "~
¢'
~ ~
~40E ~ tO
.on E
~ 2o
I(i,)
0
e0
i 0
i
i
t
I
i
i
102
T
i
~
I
J
L
20
[ K I
Fig. 2. Low-temperature specific heats of Sr3ge2OT-y in a form of C/T versus T 2. The solid lines indicate fitting curves.
n,"
,
-3
I -2
=
I -1
,
I 0
~
I 1
,
I
2
3
Velocity (mm/s) Fig. 1. M6ssbauer spectra of Sr3Fe207_y at 295K. The closed circles indicate the observed spectrum. The fitted full spectrum as well as the calculated subspectra are shown by the solid and broken lines, respectively.
CuKct radiation. The lattice parameters, a and c, of these samples were obtained from the powder X-ray diffraction patterns and then the values of the oxygen deficiency, y, for the samples ~1, ~2 and g3 were estimated to be 0.32, 0.20 and 0.08, respectively, using the linear relation between a and the oxygen content [3]. As shown in Fig. 1, M6ssbauer spectra of these samples at 295 K consist of two doublets with a small quadrupole interaction. Thus, all iron magnetic moments are paramagnetic at 295 K. The isomer shifts, 6, of two subspectra reveals that one main component is the tetravalent iron site and the other the trivalent iron one. The Fe4+ concentration is evaluated to be about 81% of iron in Sr3Fe206.92, 64% in Sr3Fe206.80 and 68% in Sr3Fe206.68 from the intensity ratio between two subspectra, in which we assume that all iron nuclei have a same recoil-free fraction. It was found
by the powder neutron diffraction experiment on the oxygen-deficient compounds [3] that the vacancies are located only on the O(2a) site and all other O sites are fully occupied. Charge compensation requires the formation of two Fe 3+ ions for each oxygen deficiency. The Fe4+ concentration in two-less vacancy samples is in good correspondence with these results. The 6 value of Fe4+ in Sr3Fe206.92 is (0.00-t0.01)mm/s relative to o~-Fe at room temperature while those in Sr3Fe206.80 and Sr3Fe206.68 are -0.04 and -0.03 (i0.01) mm/s, respectively. On the other hand, the 6 value of Fe 3+ is hardly affected by the oxygen content. This result indicates that the total s-electron density at the iron nucleus in the octahedra of Sr3Fe206.92 is smaller than those of Sr3Fe206.80 and Sr3Fe206.68, which is probably caused by a change in the degree of covalence for the chemical Fe-O bond. Fig. 2 shows the low-temperature specific heat, C, of three samples plotted in the form of C/T versus T 2. As shown in this figure, there is an almost linear relation between C/T and T 2 in all the three samples below 5.5 K. Although it has been found by our preliminary measurements that the electrical resistivities of these samples increase rapidly with decreasing temperature, these electronic specific heat coefficients, 7, have finite values. The estimated y value, 4.0mJ/K2mol, of Sr3Fe206.92 is smaller by one order than those of Sr3Fe206.80 and Sr3Fe206.68. This result indicates that the density of states at the Fermi energy decreases drastically around y ,-~0.15.
H. Kobayashi et al. / Physica B 237-238 (1997) 105-107
From the oxygen-content dependence o f y and 6 for Fe 4+, we conclude that the electronic state o f Sr3Fe206.92 differs greatly from those o f Sr3Fe206.80 and Sr3Fe206.68. We would thank Mr. H. Tokuno for helping in the preparation o f the highest oxygen-doped sample.
References [1] P. Adler et al., Hperfine Interact. 95 (1995) 71. [2] P. Adler et al., Phys. Rev. B 50 (1994) 11396. [3] S.E. Damn et al., J. Solid State Chem. 97 (1992) 179.
107
Physica B 237-238 (1997) 105-107
Electronic state of Sr3Fe207_y studied by specific heat and M6ssbauer spectroscopy H. K o b a y a s h i a'*, M . K i r a a, H. O n o d e r a b, T.
Suzuki a, T. K a m i m u r a a
a Department of Physics, Tohoku University, Sendai 980-77, Japan b lnstitute for Materials Research, Tohoku University, Sendai 980-77, Japan
Abstract
The low-temperature specific heat and M6ssbauer spectra have been measured for Sr3Fe207_y.The iron sites are subdivided into two electronically nonequivalent sites, that is, one is a tetravalent site and the other a trivalent one. The concentration and the isomer shift of tetravalent site depend on the oxygen vacancies. The electronic specific heat of Sr3Fe206.92is smaller by one order than those of Sr3Fe207-y above y = 0.2. It is concluded that the electronic state of Sr3Fe2OT-ychanges around y ,-, 0.15.
Keywords: Sr3Fe2OT-y;Specific heat; M6ssbauer effect
In the series of compounds Srn+lFenO3n+l (n = 1, 2 and ~ ) with formally tetravalent iron, SrFeO3 (n=~) has a cubic perovskite structure which consists of a three-dimensional network of cornersharing FeO6 octahedra, whereas Sr2FeO4 (n = 1) is a K2NiFn-type structure with a two-dimensional network of the octahedra. The structure of Sr3Fe207 (n = 2) can be described as a stacking of the sheets of the double octahedra and the Sr layers alternately along the c-axis, which is called as the RuddelesdenPopper phase and then is an intermediate structure between those of n = 1 and c~ compounds. The electrical resistivity of Sr2FeO4_y shows a semiconducting tempera~re-dependence, whereas that of S r F e O 3 _ y is temperature independent [I]. It was found by the measurement of optical reflectivity for Sr2FeO4 under pressure [2] that the lowest optical transition shifts to lower energy with pressure, which is interpreted in terms of a gap narrowing. Hence, the variation of electric properties in the series of these compounds seems to be responsible for the degree of condensation of the FeO6 octahedra and strength of the d-p hybridiza* Corresponding author. 0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 0 0 6 5 - 3
tion. As observed in high-To superconductors, the electronic properties in these compounds are strongly affected by the oxygen deficiency. In this study, we investigate the electronic properties of Sr3Fe2OT_y for three different oxygen-deficient samples using the specific heat and Mfssbauer effect measurements. The polycrystalline samples of Sr3Fe207_y were synthesized by a direct reaction. SrCO3 (purity: 99.994%) and Fe203 (99.999%) were mixed together in the nominal molar proportion and heated at 1100°C for 25 h in air; the product was ground and heated at 1100°C for further 25 h . This sample is referred to as ~ 1. Moreover, we obtained the oxygen-doped samples by following two different annealing processes. The sample ~2 was produced by annealing at 900°C for 50h and at 450°C for 170h in flowing 02 gas. The sample g3 was yielded by annealing at 500°C for 50h under 1000 atm of 02 and Ar mixed gas in the molar ratio of 1 : 4. The Mrssbauer spectra were measured with a conventional transmission spectrometer. The specific heat measurements were carried out from 1.8 to 100K by an adiabatic method. The three samples were confirmed to be in a single phase by the powder X-ray diffraction using a
106
H. Kobayashi et at/Physica B 237 238 (1997) 105-107
80
[Sr3Fe2Oe.(#1) ss
*
Sr3Fe206.68 ( #i )
o
Sr3Fe206.80 ( #2 ) # Sr3Fe206.92 (
60 "~
¢'
~ ~
~40E ~ tO
.on E
~ 2o
I(i,)
0
e0
i 0
i
i
t
I
i
i
102
T
i
~
I
J
L
20
[ K I
Fig. 2. Low-temperature specific heats of Sr3ge2OT-y in a form of C/T versus T 2. The solid lines indicate fitting curves.
n,"
,
-3
I -2
=
I -1
,
I 0
~
I 1
,
I
2
3
Velocity (mm/s) Fig. 1. M6ssbauer spectra of Sr3Fe207_y at 295K. The closed circles indicate the observed spectrum. The fitted full spectrum as well as the calculated subspectra are shown by the solid and broken lines, respectively.
CuKct radiation. The lattice parameters, a and c, of these samples were obtained from the powder X-ray diffraction patterns and then the values of the oxygen deficiency, y, for the samples ~1, ~2 and g3 were estimated to be 0.32, 0.20 and 0.08, respectively, using the linear relation between a and the oxygen content [3]. As shown in Fig. 1, M6ssbauer spectra of these samples at 295 K consist of two doublets with a small quadrupole interaction. Thus, all iron magnetic moments are paramagnetic at 295 K. The isomer shifts, 6, of two subspectra reveals that one main component is the tetravalent iron site and the other the trivalent iron one. The Fe4+ concentration is evaluated to be about 81% of iron in Sr3Fe206.92, 64% in Sr3Fe206.80 and 68% in Sr3Fe206.68 from the intensity ratio between two subspectra, in which we assume that all iron nuclei have a same recoil-free fraction. It was found
by the powder neutron diffraction experiment on the oxygen-deficient compounds [3] that the vacancies are located only on the O(2a) site and all other O sites are fully occupied. Charge compensation requires the formation of two Fe 3+ ions for each oxygen deficiency. The Fe4+ concentration in two-less vacancy samples is in good correspondence with these results. The 6 value of Fe4+ in Sr3Fe206.92 is (0.00-t0.01)mm/s relative to o~-Fe at room temperature while those in Sr3Fe206.80 and Sr3Fe206.68 are -0.04 and -0.03 (i0.01) mm/s, respectively. On the other hand, the 6 value of Fe 3+ is hardly affected by the oxygen content. This result indicates that the total s-electron density at the iron nucleus in the octahedra of Sr3Fe206.92 is smaller than those of Sr3Fe206.80 and Sr3Fe206.68, which is probably caused by a change in the degree of covalence for the chemical Fe-O bond. Fig. 2 shows the low-temperature specific heat, C, of three samples plotted in the form of C/T versus T 2. As shown in this figure, there is an almost linear relation between C/T and T 2 in all the three samples below 5.5 K. Although it has been found by our preliminary measurements that the electrical resistivities of these samples increase rapidly with decreasing temperature, these electronic specific heat coefficients, 7, have finite values. The estimated y value, 4.0mJ/K2mol, of Sr3Fe206.92 is smaller by one order than those of Sr3Fe206.80 and Sr3Fe206.68. This result indicates that the density of states at the Fermi energy decreases drastically around y ,-~0.15.
H. Kobayashi et al. / Physica B 237-238 (1997) 105-107
From the oxygen-content dependence o f y and 6 for Fe 4+, we conclude that the electronic state o f Sr3Fe206.92 differs greatly from those o f Sr3Fe206.80 and Sr3Fe206.68. We would thank Mr. H. Tokuno for helping in the preparation o f the highest oxygen-doped sample.
References [1] P. Adler et al., Hperfine Interact. 95 (1995) 71. [2] P. Adler et al., Phys. Rev. B 50 (1994) 11396. [3] S.E. Damn et al., J. Solid State Chem. 97 (1992) 179.
107