- Email: [email protected]
Infrared and transport properties of the layered perovskite related oxide Ba5Nb4O15 and its oxygen deficient phases
Physiea C 235-240 (1994) 755-756
PHYSICA
North-Holland
Infrared and Transport Properties of the Layered Perovskite Related Oxide BasNb40~s and its Oxygen Deficient Phases S. Pagola ", N.E. Massa b, G. Polla °, G. Leyva °, and R.E. Carbonio" " INFIQC, Dpto. de Fisicoquimica, Fac. Cs. Quimicas, U.N.C., CC 61, 5016, C6rdoba, Argentina. b LANAIS en Espectroscopia 6ptica and QUINOR - Dpto. de Quimica and Dpto. de Fisica, U.N.L.P., CC 962, 1900, La Plata, Argentina. ° CNEA, Dpto. de Fisica, Av. del Libertador 8250, 1429, Buenos Aires, Argentina. BasNb40~., oxides were studied by infrared, electrical resistivity and thermogravimmetric analysis (TGA). FIR reflectivity, measurementsreveal a strong ionic compound that has well defined features in groups that we assign to oxygen stretching, bending and lattice phonons splitted by the lower symmetry of this layered compound. For the sample with x = 0.56, oxygen vacancies do not affect phonon b~ld profiles, indicating that carriers are not free enough to interact with longitudinal modes. Electrical resistivity vs. temperature measurements show that the oxygen deficient compounds, for low values of x, are small band gap semiconductors. Since the discovery of high-temperature superconductivity, perovskite related oxides have 0een extensively studied in order to understand its mechanism and search for new materials. Among these, the non copper ones are growing in interest. Cations with a n d ~ (n=3, 4, 5) configuration such as Ti(III), Nb(IV) or Ta(IV) all having s=l/2 spin state, are of particular interest. Among the niobium oxides, Li,,NbO2 and (Sq.x P-,, )NbzO6 where R is a lantanide, were reported to be superconducting [1,2]. BasNb40~5 is a layered perovskite related oxide [3L that can be prepared with Nb(IV) as BasNb40~3. /dl these characteristics make this compound a good candidate to search for new superconductors. BasNb40~s was obtained by heat treatment of the stoichiomellic mixture of BaCO3 and Nb205 at 1000°C in air. The oxygen deficient phases were prepared by reduction of BasNb40~5 in a 95% Ar + 5% H: atmosphere by changing the temperature between 1300 and 1500°C and the time of reduction betwee md 54 hours. Ele~. ,a~ resistivity measuremenls have been done by he four probe method, between 80 and 300 K. The samples were characterized by TGA on a Mettler TG 50 equipment, x values were determined by the mass gain measured in flowing O~ at 600 °C. X ray powder diffraction measurements were done in a Philips PW 3710 Diffractometer, ,and temperature dependent reflective measurements have
been done with and interferometer Bruker 113V from 5,000 to 30 cm 4 , the samples were mounted on an Oxford DN 1754 cryostat. Figure 1 shows our preliminary reflectivity
0921-4534/94/$07.00 © 1994 - Elsevier Science B,V. All rights reserved. SSDI 0921-4534(94)00941-4
spectra of BasNb4Ol5 and BasN~4014.44 at 80 K.
X-ray diffraction powder patterns correspond to the Ba~Nb4Ot5 phase [3]. i00
..............................
" ~ 60 40
0 1500
/~
~ . . . . . 1000 500
0
Frequency (cm -1)
Figure 1. IR reflectivity spectra in the phonon region of Ba~Nb4Ots (solid line) and BasNb4Ot4.44(dashed line), at 80 K. The oxidized sample has a typical white ionic insulator color, while in contrast, a lead like shine is the characteristic for the one treated in the reducing atmosphere. Our spectra confirms earlier X-ray data showing that this layered compo,md has a perovskite distorted lattice. Unscreened pi~onon features from 800 to 30 cm ~ are found in g~oups at four, at the same frequency positions of the room temperature three infrared active reflection bands of
756
S. Pagola et al./Physica C 235-240 (1994) 755-756
SrTiO.,. Since on cooling we only observe a decrease in the damping of those twelve vibrational modes, we conclude that this compound does not undergo a structaral phase transition in the range from 300 to 80 K. Their proper assignment together with the now in progress Raman spectra, to confirm the centrosymmetry and the reported P.~ml space group, will be published elsewhere. The distinctive feature between the reflectivity spectra of both compounds is a weak band, shown in fig. 2, at about 3,300 cm"~, active only at low temperatures for BasNb4Ots.44 . The origin of this feature is a matter of current discussion [4]. In our case, in principle, we could assign its activity to the localized levels introduced by Nb(IV) in the energy gap, close to the conduction band.
a weakly localized system. Fig. 3 shows the resistivity (p) vs. "FTM plots for different x values. As can be seen the linearity is good. From the slope, B, the characteristic energy for the hopping state can be obtained [5]. These values are shown in table 1. It is clear that B decreases with increasing x, indicating that carriers are getting less localized as x increases.
'•100
S
KI
x=
~
x =0.56
Q. 10 l
l
i
0.24 0.26
l
I
0.21t 0.30 0.32 0.34 I/Tt/4(Kq/4)
Figure 3. Electrical resistivity vs. T"" for BasNb40~5.x.
4000
3500
3000
2500
Frecuency(cm'l) Figure 2. Defect induced reflectivity band at 80 K. BasNb4Ois (solid line), BasNb40 14.44(dashed line). From the logarithmic electrical resistivity vs. reciprocal temperature for samples with different numbers of oxygen deficiencies, not shown, a linear relation is obtained at high temperatures. From this relation the activation energy E~, can be obtained and the results are shown in Table 1. Table 1 Parameters obtained from the electrical resistivity data for BasNb40 ts-xcompounds.
It is expected that for larger x values a metal-non metal transition will be obtained. With the reduction method discussed in the present communication we could not obtain x values larger than 0.56. Other methods are being currently explored in order to increase the number of oxygen deficiencies and will be informed in a future publication. In conclusion, several common characteristics to the oxide superconductors, namely, layered structure, a transition metal in a 4d ~configuration, oxygen deficiencies and a probable metal-non metal transition with changiag number of oxygen deficiencies, make this series of compounds good candidates to look for new superconductors.
REFERENCES x
E, / eV
B / K'l/4
0.08 0.13 0.56
0.13 0.08 0.035
204 130 53
Activation energies in the order of 10.2 eV with decreasing values as x increases are obtained. These extremelv low values of ~, do nnt , - ' n r r , ~ , n . 4 ta a.q activation across the energy gap from the valence band, but to ,an activation from a donor level introduced by the oxygen deficiency or Nb(IV). On the lower temperature side, this plots deviate from linear relations. This implies that E~ is temperature dependent, which may be caused by the so called "variable range hopping" , characteristic of
1. M. J. Geselbracht, T. J. Richardson and A. M. Stacy, Nature 345, 324 (1990). 2. J. Akimitsu, J. Amano, H. Sawa, O. Nagase, K. Gyoda and M. Kogai, Jpn. J. Appl. Phys. 30, Ll155 (1991). 3. a) F. G. Galasso and L. Katz , Acta Cryst. 14, 647 (1961). k~
l
CHL. . . . . . . .
..1 1[
TP--~-.
A
~ -
U} J. OIIO.IUIUII i~UlU L~..['~dlL'.,,Z " ~ L d
~
.....
lr'~f
1,~l~'~t. D ~ O
1 ~
, ILP~
(1970). 4. P. Calvani, M. Capizzi, F. Donato, S. Lupi, P. Maselli and D. Peschiaroli , Phys. Rev. B 47, 8917 (1993). 5. N. F. Mort and E. A. Davis, "Electronic Processes in Non-Crystal.line Materials", Clarendon Press, Oxford (1979).
PHYSICA
North-Holland
Infrared and Transport Properties of the Layered Perovskite Related Oxide BasNb40~s and its Oxygen Deficient Phases S. Pagola ", N.E. Massa b, G. Polla °, G. Leyva °, and R.E. Carbonio" " INFIQC, Dpto. de Fisicoquimica, Fac. Cs. Quimicas, U.N.C., CC 61, 5016, C6rdoba, Argentina. b LANAIS en Espectroscopia 6ptica and QUINOR - Dpto. de Quimica and Dpto. de Fisica, U.N.L.P., CC 962, 1900, La Plata, Argentina. ° CNEA, Dpto. de Fisica, Av. del Libertador 8250, 1429, Buenos Aires, Argentina. BasNb40~., oxides were studied by infrared, electrical resistivity and thermogravimmetric analysis (TGA). FIR reflectivity, measurementsreveal a strong ionic compound that has well defined features in groups that we assign to oxygen stretching, bending and lattice phonons splitted by the lower symmetry of this layered compound. For the sample with x = 0.56, oxygen vacancies do not affect phonon b~ld profiles, indicating that carriers are not free enough to interact with longitudinal modes. Electrical resistivity vs. temperature measurements show that the oxygen deficient compounds, for low values of x, are small band gap semiconductors. Since the discovery of high-temperature superconductivity, perovskite related oxides have 0een extensively studied in order to understand its mechanism and search for new materials. Among these, the non copper ones are growing in interest. Cations with a n d ~ (n=3, 4, 5) configuration such as Ti(III), Nb(IV) or Ta(IV) all having s=l/2 spin state, are of particular interest. Among the niobium oxides, Li,,NbO2 and (Sq.x P-,, )NbzO6 where R is a lantanide, were reported to be superconducting [1,2]. BasNb40~5 is a layered perovskite related oxide [3L that can be prepared with Nb(IV) as BasNb40~3. /dl these characteristics make this compound a good candidate to search for new superconductors. BasNb40~s was obtained by heat treatment of the stoichiomellic mixture of BaCO3 and Nb205 at 1000°C in air. The oxygen deficient phases were prepared by reduction of BasNb40~5 in a 95% Ar + 5% H: atmosphere by changing the temperature between 1300 and 1500°C and the time of reduction betwee md 54 hours. Ele~. ,a~ resistivity measuremenls have been done by he four probe method, between 80 and 300 K. The samples were characterized by TGA on a Mettler TG 50 equipment, x values were determined by the mass gain measured in flowing O~ at 600 °C. X ray powder diffraction measurements were done in a Philips PW 3710 Diffractometer, ,and temperature dependent reflective measurements have
been done with and interferometer Bruker 113V from 5,000 to 30 cm 4 , the samples were mounted on an Oxford DN 1754 cryostat. Figure 1 shows our preliminary reflectivity
0921-4534/94/$07.00 © 1994 - Elsevier Science B,V. All rights reserved. SSDI 0921-4534(94)00941-4
spectra of BasNb4Ol5 and BasN~4014.44 at 80 K.
X-ray diffraction powder patterns correspond to the Ba~Nb4Ot5 phase [3]. i00
..............................
" ~ 60 40
0 1500
/~
~ . . . . . 1000 500
0
Frequency (cm -1)
Figure 1. IR reflectivity spectra in the phonon region of Ba~Nb4Ots (solid line) and BasNb4Ot4.44(dashed line), at 80 K. The oxidized sample has a typical white ionic insulator color, while in contrast, a lead like shine is the characteristic for the one treated in the reducing atmosphere. Our spectra confirms earlier X-ray data showing that this layered compo,md has a perovskite distorted lattice. Unscreened pi~onon features from 800 to 30 cm ~ are found in g~oups at four, at the same frequency positions of the room temperature three infrared active reflection bands of
756
S. Pagola et al./Physica C 235-240 (1994) 755-756
SrTiO.,. Since on cooling we only observe a decrease in the damping of those twelve vibrational modes, we conclude that this compound does not undergo a structaral phase transition in the range from 300 to 80 K. Their proper assignment together with the now in progress Raman spectra, to confirm the centrosymmetry and the reported P.~ml space group, will be published elsewhere. The distinctive feature between the reflectivity spectra of both compounds is a weak band, shown in fig. 2, at about 3,300 cm"~, active only at low temperatures for BasNb4Ots.44 . The origin of this feature is a matter of current discussion [4]. In our case, in principle, we could assign its activity to the localized levels introduced by Nb(IV) in the energy gap, close to the conduction band.
a weakly localized system. Fig. 3 shows the resistivity (p) vs. "FTM plots for different x values. As can be seen the linearity is good. From the slope, B, the characteristic energy for the hopping state can be obtained [5]. These values are shown in table 1. It is clear that B decreases with increasing x, indicating that carriers are getting less localized as x increases.
'•100
S
KI
x=
~
x =0.56
Q. 10 l
l
i
0.24 0.26
l
I
0.21t 0.30 0.32 0.34 I/Tt/4(Kq/4)
Figure 3. Electrical resistivity vs. T"" for BasNb40~5.x.
4000
3500
3000
2500
Frecuency(cm'l) Figure 2. Defect induced reflectivity band at 80 K. BasNb4Ois (solid line), BasNb40 14.44(dashed line). From the logarithmic electrical resistivity vs. reciprocal temperature for samples with different numbers of oxygen deficiencies, not shown, a linear relation is obtained at high temperatures. From this relation the activation energy E~, can be obtained and the results are shown in Table 1. Table 1 Parameters obtained from the electrical resistivity data for BasNb40 ts-xcompounds.
It is expected that for larger x values a metal-non metal transition will be obtained. With the reduction method discussed in the present communication we could not obtain x values larger than 0.56. Other methods are being currently explored in order to increase the number of oxygen deficiencies and will be informed in a future publication. In conclusion, several common characteristics to the oxide superconductors, namely, layered structure, a transition metal in a 4d ~configuration, oxygen deficiencies and a probable metal-non metal transition with changiag number of oxygen deficiencies, make this series of compounds good candidates to look for new superconductors.
REFERENCES x
E, / eV
B / K'l/4
0.08 0.13 0.56
0.13 0.08 0.035
204 130 53
Activation energies in the order of 10.2 eV with decreasing values as x increases are obtained. These extremelv low values of ~, do nnt , - ' n r r , ~ , n . 4 ta a.q activation across the energy gap from the valence band, but to ,an activation from a donor level introduced by the oxygen deficiency or Nb(IV). On the lower temperature side, this plots deviate from linear relations. This implies that E~ is temperature dependent, which may be caused by the so called "variable range hopping" , characteristic of
1. M. J. Geselbracht, T. J. Richardson and A. M. Stacy, Nature 345, 324 (1990). 2. J. Akimitsu, J. Amano, H. Sawa, O. Nagase, K. Gyoda and M. Kogai, Jpn. J. Appl. Phys. 30, Ll155 (1991). 3. a) F. G. Galasso and L. Katz , Acta Cryst. 14, 647 (1961). k~
l
CHL. . . . . . . .
..1 1[
TP--~-.
A
~ -
U} J. OIIO.IUIUII i~UlU L~..['~dlL'.,,Z " ~ L d
~
.....
lr'~f
1,~l~'~t. D ~ O
1 ~
, ILP~
(1970). 4. P. Calvani, M. Capizzi, F. Donato, S. Lupi, P. Maselli and D. Peschiaroli , Phys. Rev. B 47, 8917 (1993). 5. N. F. Mort and E. A. Davis, "Electronic Processes in Non-Crystal.line Materials", Clarendon Press, Oxford (1979).