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Candidate compounds for superconductivity
PHYSICA
PhysicaC 190 (1991) 104-106 North-Holland
Candidate compounds for superconductivity Tetsuo Shimura, Masahiro Shikano, Mitsuru Itoh and Tetsur6 Nakamura Research Laboratory of Engineering Materials, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan
Non-cupreous compounds with perovskite-related structures containing V4+ (3d ~), C03+(3&), and Rh 3+ (4&) were synthesized and their properties were investigated. A metallic d ~compound Sr4V3Ojo_~ with the layered structure showed a semiconducting resistivity at low temperatures by an increase of the oxygen vacancies or by a substitution of V by Ti. Semiconducting perovskite-related compound LaRhO3 with the low spin configuration of Rh 3÷ (4d 6) could be hole-doped in the d~ orbitals by the substitution of La by Sr or Ca, and showed a metallic conductivity, while LaSrRhO4 with the K2NiF4 structure and the low spin configuration could not be hole-doped by decreasing the La/Sr ratio.
1. Introduction In the cupreous superconductors with perovskite-related layered structures, holes or electrons are doped in the d r orbitals of the Cu ions by the substitutions of A-site cations. As a strategy of searching for new non-cupreous superconductors we first perceived the electronic structure of the compounds and secondly considered the possibility of doping of holes or electrons in the compounds. If some d ° compounds containing the tetravalent IVa elements such as Ti 4+, Zr 4+, or H P + or pentavalent Va elements such as W +, Nb 5+, or Ta 5+ in their 6-coordinating positions are available, these compounds have the potentiality to be doped with electrons in the d, orbitals by the introduction of oxygen vacancies or the elemental substitution. Electron doping may be possible also in the d ~compounds containing V 4÷, Nb 4+, or Ta 4+ by the same method. On the other hand, a low spin configuration is possible in the d 6 compounds containing Co 3÷ (3d6), Rh 3+ (4d 6 ) m or Ir 3+ ( 5d 6) in their 6-coordinating positions. In these compounds, hole-doping in the d, orbitals or electron-doping in the d r orbitals keeping the low spin state may be possible by the elemental substitution. In this paper, the following experimental results are indicated: ( 1 ) the change of electrical resistivity of the d z layered compound SraV30~o_~ via changing the number of oxygen vacancies and substituting V by Ti. (2) Hole-doping for the d 6 compounds LaRhO3 and LaSrRhO4 with the low spin configuration.
2. Experimental Samples of the vanadium compounds were prepared by the method reported previously [ 1 ]. Samples of rhodium
compounds were prepared by the solid-state reaction technique. A stoichiometric mixture of powders of Rh203 (3N), La203(4N) and SrCO3 or CaCO3(3N) was thoroughly mixed, pressed into a pellet, and calcined in an oxygen flow at 1473 K for 12 h. The calcined sample was reground, pressed into a pellet and sintered at 1523 K for 36-60 h in an oxygen flow. Only for Lao.9oSro.~oRhO3 quenching to room temperature was needed to obtain a single-phased sample. The lattice symmetry of the sample was determined by the powder X-ray diffraction method using a Rigaku 0-0 diffractometer equipped with the curved graphite monochromator. The magnetic susceptibility of the sample was measured by a Shimadzu MB-2 magnetic balance above 77 K and a Quantum Design SQUID magnetometer. Resistivity of the sample was measured by DC four- or two-probe method.
3. Results and discussion
Figures 1 and 2 show the electrical resistivities of Sr4V30~o_~(0.20< t~< 0.56) and Sr4(V~_~Ti~)3Oto_~(0_
0921-4534/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.
T. Shimura et aL / Candidate compounds for superconductivity
105
0.03 0
s 0.02
Sr4V3Ol~
oo
oO0 0
0.01 6=
0.49
0.00
4
~
l
m
0
o:t
l
~
t
o
-50
0.51
~O
oO
° o° ~:)
0 /
0.0 i ,,
I
i
100
,
I
200
0.2
0.6
0.8
300
T/K Fig. I. Temperature dependence of electrical resistivity of the system Sr4V30~o_~.
Fig. 3• Dependences of the Curie constant C, characteristic temperature 0, and Xoon 6 o f the system Sr4V30~o_~. Open and closed circles represent the data obtained by magnetic balance and SQUID magnetometer, respectively.
3
25
La l xA x RhO3
Sr4(Vl.~i x) ~O10_a 20
~
0.4
5 in Sr,V~O~
i
,
o o •
x = 0.00 x=0.05"1 ~r x = 0.I0 J °
~. ~v~
° x = 0.05 n - x = 0.10 l ca
15
0 15
"10
~-0ps, ~040
10
~0~04;~02s
~o5=OO1~-o4s~oo5 ~o 0
100
200
_Q
300
T/K Fig. 2. Temperature dependence of electrical resistivity of the system Sr4(Vl_xTix)3Olo_ ~.
0
100
200
300
T/K
(t)
Fig. 4. Temperature dependence of electrical resistivity and magnetic susceptibility of the system (La~ _xAx)RhO3 (A=Ca, Sr).
When the oxygen vacancies are increased the Curie constant which expressed the localized magnetic moment of vanadium ions and Zo value which is related to the density of state at the Fermi surface are almost constant• By contrast, the Weiss temperature 0 changes to zero from a negative value. This indicates that the antiferromagnetic interaction be-
tween vanadium ions along the c-axis vanishes by the introduction of oxygen vacancies in the O (2) (4e) sites between V( 1 ) (2a) and V ( 2 ) (4e). The increase in the number of oxygen vacancies leads to the increase both in the numbers of electron carriers and point defects, while the substitution of V by Ti reduces the number of electron carriers. The roles
C
x= -~.-b +xo.
106
T. Shimura et al. / Candidate compounds for superconductivity
of the number of carriers, oxygen vacancies, and the Ti substitution in the complex behaviors of resistivity and magnetic susceptibility will be made clear by investigating the structure and properties of stoichiometric SraV3OIo which is difficult to obtain because of the difficulty in the control of in the hydrogen atmosphere. Figure 4 shows the electrical resistivities and magnetic susceptibilities of the (Lal _xAx) RhO3 system (A = Sr, Ca). The semiconducting LaRhO3 with the perovskite-type structure [2 ] changes to a metallic one by the substitution of La by Sr or Ca. By the measurement of the sign of the Zeebeck coefficient of the substituted samples, the carriers were found to be holes. The values of C, 0 and go for LaRhO3 were 8.83XI0 -4 emu K m o l - l , - 1 . 3 K and 4.18X10 -5 emumol-I, respectively. These results suggest that the six electrons of Rh 3+ ions in LaRhO take the low spin configuration. By the substitution of La by Sr or Ca up to x=0.20, the Curie constant C and go increased to a value of several times as large and 0 decreased in proportion to the amount of substitution. By contrast, these values for LaCoO3, which shows an antiferromagnetic-like and broad transition below 150 K, analyzed using the data in the temperature range from 150 to 300 K were C=1.90 emu K tool -t, 0 = - 2 1 8 K, and Xo= - 6 . 7 7 × 10 -4 emu tool -~. This fairly large Curie constant is due to an intermediate spin configuration caused by the thermal excitation of electrons from d, to d r orbital. Resistivities of substituted samples at 300 K are smaller than
LaRhO3 in one order of magnitude. The substitution of La by Sr was more effective in lowering of resistivity compared to the substitution by Ca. The sign of dp/dT around 300 K of all the substituted samples is positive, except for Lao.9sCao.osRhO3. However, the sign was negative for all samples below 50 K. Minimum resistivity temperature decreases as x increases. Semiconducting LaSrRhO4 with the K2NiF4 structure could be successfully obtained, and holedoping was attempted by decreasing the La/Sr ratio for this compound. However, no marked change was observed in the values of electrical resistivity and magnetic susceptibility.
Acknowledgement A part of this work was made possible by a Grant-in-Aid for Scientific Research "'Chemistry of New Superconductors" from the Ministry of Education, Science and Culture of Japan.
References [ 1] M. ltoh, M. Shikano, R. Liang, H. Kawaji and T. Nakamura, J. Solid State Chem. 88 (1990) 597. [ 2 ] A. Wold, B. Post and E. Banks, J. Am. Chem. Soc. 79 ( 1957 ) 6365.
PhysicaC 190 (1991) 104-106 North-Holland
Candidate compounds for superconductivity Tetsuo Shimura, Masahiro Shikano, Mitsuru Itoh and Tetsur6 Nakamura Research Laboratory of Engineering Materials, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan
Non-cupreous compounds with perovskite-related structures containing V4+ (3d ~), C03+(3&), and Rh 3+ (4&) were synthesized and their properties were investigated. A metallic d ~compound Sr4V3Ojo_~ with the layered structure showed a semiconducting resistivity at low temperatures by an increase of the oxygen vacancies or by a substitution of V by Ti. Semiconducting perovskite-related compound LaRhO3 with the low spin configuration of Rh 3÷ (4d 6) could be hole-doped in the d~ orbitals by the substitution of La by Sr or Ca, and showed a metallic conductivity, while LaSrRhO4 with the K2NiF4 structure and the low spin configuration could not be hole-doped by decreasing the La/Sr ratio.
1. Introduction In the cupreous superconductors with perovskite-related layered structures, holes or electrons are doped in the d r orbitals of the Cu ions by the substitutions of A-site cations. As a strategy of searching for new non-cupreous superconductors we first perceived the electronic structure of the compounds and secondly considered the possibility of doping of holes or electrons in the compounds. If some d ° compounds containing the tetravalent IVa elements such as Ti 4+, Zr 4+, or H P + or pentavalent Va elements such as W +, Nb 5+, or Ta 5+ in their 6-coordinating positions are available, these compounds have the potentiality to be doped with electrons in the d, orbitals by the introduction of oxygen vacancies or the elemental substitution. Electron doping may be possible also in the d ~compounds containing V 4÷, Nb 4+, or Ta 4+ by the same method. On the other hand, a low spin configuration is possible in the d 6 compounds containing Co 3÷ (3d6), Rh 3+ (4d 6 ) m or Ir 3+ ( 5d 6) in their 6-coordinating positions. In these compounds, hole-doping in the d, orbitals or electron-doping in the d r orbitals keeping the low spin state may be possible by the elemental substitution. In this paper, the following experimental results are indicated: ( 1 ) the change of electrical resistivity of the d z layered compound SraV30~o_~ via changing the number of oxygen vacancies and substituting V by Ti. (2) Hole-doping for the d 6 compounds LaRhO3 and LaSrRhO4 with the low spin configuration.
2. Experimental Samples of the vanadium compounds were prepared by the method reported previously [ 1 ]. Samples of rhodium
compounds were prepared by the solid-state reaction technique. A stoichiometric mixture of powders of Rh203 (3N), La203(4N) and SrCO3 or CaCO3(3N) was thoroughly mixed, pressed into a pellet, and calcined in an oxygen flow at 1473 K for 12 h. The calcined sample was reground, pressed into a pellet and sintered at 1523 K for 36-60 h in an oxygen flow. Only for Lao.9oSro.~oRhO3 quenching to room temperature was needed to obtain a single-phased sample. The lattice symmetry of the sample was determined by the powder X-ray diffraction method using a Rigaku 0-0 diffractometer equipped with the curved graphite monochromator. The magnetic susceptibility of the sample was measured by a Shimadzu MB-2 magnetic balance above 77 K and a Quantum Design SQUID magnetometer. Resistivity of the sample was measured by DC four- or two-probe method.
3. Results and discussion
Figures 1 and 2 show the electrical resistivities of Sr4V30~o_~(0.20< t~< 0.56) and Sr4(V~_~Ti~)3Oto_~(0_
0921-4534/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.
T. Shimura et aL / Candidate compounds for superconductivity
105
0.03 0
s 0.02
Sr4V3Ol~
oo
oO0 0
0.01 6=
0.49
0.00
4
~
l
m
0
o:t
l
~
t
o
-50
0.51
~O
oO
° o° ~:)
0 /
0.0 i ,,
I
i
100
,
I
200
0.2
0.6
0.8
300
T/K Fig. I. Temperature dependence of electrical resistivity of the system Sr4V30~o_~.
Fig. 3• Dependences of the Curie constant C, characteristic temperature 0, and Xoon 6 o f the system Sr4V30~o_~. Open and closed circles represent the data obtained by magnetic balance and SQUID magnetometer, respectively.
3
25
La l xA x RhO3
Sr4(Vl.~i x) ~O10_a 20
~
0.4
5 in Sr,V~O~
i
,
o o •
x = 0.00 x=0.05"1 ~r x = 0.I0 J °
~. ~v~
° x = 0.05 n - x = 0.10 l ca
15
0 15
"10
~-0ps, ~040
10
~0~04;~02s
~o5=OO1~-o4s~oo5 ~o 0
100
200
_Q
300
T/K Fig. 2. Temperature dependence of electrical resistivity of the system Sr4(Vl_xTix)3Olo_ ~.
0
100
200
300
T/K
(t)
Fig. 4. Temperature dependence of electrical resistivity and magnetic susceptibility of the system (La~ _xAx)RhO3 (A=Ca, Sr).
When the oxygen vacancies are increased the Curie constant which expressed the localized magnetic moment of vanadium ions and Zo value which is related to the density of state at the Fermi surface are almost constant• By contrast, the Weiss temperature 0 changes to zero from a negative value. This indicates that the antiferromagnetic interaction be-
tween vanadium ions along the c-axis vanishes by the introduction of oxygen vacancies in the O (2) (4e) sites between V( 1 ) (2a) and V ( 2 ) (4e). The increase in the number of oxygen vacancies leads to the increase both in the numbers of electron carriers and point defects, while the substitution of V by Ti reduces the number of electron carriers. The roles
C
x= -~.-b +xo.
106
T. Shimura et al. / Candidate compounds for superconductivity
of the number of carriers, oxygen vacancies, and the Ti substitution in the complex behaviors of resistivity and magnetic susceptibility will be made clear by investigating the structure and properties of stoichiometric SraV3OIo which is difficult to obtain because of the difficulty in the control of in the hydrogen atmosphere. Figure 4 shows the electrical resistivities and magnetic susceptibilities of the (Lal _xAx) RhO3 system (A = Sr, Ca). The semiconducting LaRhO3 with the perovskite-type structure [2 ] changes to a metallic one by the substitution of La by Sr or Ca. By the measurement of the sign of the Zeebeck coefficient of the substituted samples, the carriers were found to be holes. The values of C, 0 and go for LaRhO3 were 8.83XI0 -4 emu K m o l - l , - 1 . 3 K and 4.18X10 -5 emumol-I, respectively. These results suggest that the six electrons of Rh 3+ ions in LaRhO take the low spin configuration. By the substitution of La by Sr or Ca up to x=0.20, the Curie constant C and go increased to a value of several times as large and 0 decreased in proportion to the amount of substitution. By contrast, these values for LaCoO3, which shows an antiferromagnetic-like and broad transition below 150 K, analyzed using the data in the temperature range from 150 to 300 K were C=1.90 emu K tool -t, 0 = - 2 1 8 K, and Xo= - 6 . 7 7 × 10 -4 emu tool -~. This fairly large Curie constant is due to an intermediate spin configuration caused by the thermal excitation of electrons from d, to d r orbital. Resistivities of substituted samples at 300 K are smaller than
LaRhO3 in one order of magnitude. The substitution of La by Sr was more effective in lowering of resistivity compared to the substitution by Ca. The sign of dp/dT around 300 K of all the substituted samples is positive, except for Lao.9sCao.osRhO3. However, the sign was negative for all samples below 50 K. Minimum resistivity temperature decreases as x increases. Semiconducting LaSrRhO4 with the K2NiF4 structure could be successfully obtained, and holedoping was attempted by decreasing the La/Sr ratio for this compound. However, no marked change was observed in the values of electrical resistivity and magnetic susceptibility.
Acknowledgement A part of this work was made possible by a Grant-in-Aid for Scientific Research "'Chemistry of New Superconductors" from the Ministry of Education, Science and Culture of Japan.
References [ 1] M. ltoh, M. Shikano, R. Liang, H. Kawaji and T. Nakamura, J. Solid State Chem. 88 (1990) 597. [ 2 ] A. Wold, B. Post and E. Banks, J. Am. Chem. Soc. 79 ( 1957 ) 6365.