JP2007320829A - Superconducting compound and method for producing the same - Google Patents

Abstract

In a perovskite-type copper oxide, a high-temperature superconductor exceeding 100 K has been found, but a room temperature superconductor has not yet been found.
A superconducting transition was found in a compound represented by the chemical formula LaFeOPh (Ph is at least one of P, As, and Sb) and having a crystal structure of the ZrCuSiAs type (space group P4 / nmm). LaFeOPh is a member of a layered structure compound group having a transition metal ion represented by the general chemical formula LnMOPn (M is a transition metal) in the skeleton structure. Where Ln
Y and rare earth metal elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy,
Ho, Er, Tm, Yb, Lu), and M is a transition metal element (Fe,
Ru, Os), and Pn is a pnictide element (N, P, As, Sb).
). This compound can control the transition temperature by changing the number of carriers by adding F ions or the like.
[Selection] Figure 2

Description

The present invention relates to a layered superconducting compound having Fe ions in a skeleton structure and a method for producing the same.

Since the superconducting phenomenon was found in mercury in 1911, superconductivity was found in many compounds and put into practical use as superconducting magnets and magnetic sensors (SQUID). In addition, since the discovery of high-temperature superconductors (perovskite-type copper oxides), research and development of materials aimed at room temperature superconductors has been actively conducted, and superconducting compounds having superconducting transition temperatures (Tc) exceeding 100K have been developed. It was found.

The understanding of the mechanism of superconductivity of perovskite-type copper oxide is also progressing (for example, Non-Patent Documents 1 and 2). Further, as a compound containing a transition metal ion other than copper, or a novel compound, Sr ₂ RuO ₄ (Tc = 0.93K) (Non-patent Document 3), magnesium diboride (
Tc = 39K) (Non-Patent Document 4, Patent Document 1), Na _0.3 CoO ₂ .1.3H ₂ O (T
c = 5K) (Non-Patent Document 5, Patent Documents 2 and 3) and the like were newly found.

A strongly correlated electron compound in which the interaction between conduction electrons is larger than the conduction band width may be a superconductor having a high superconducting transition temperature when the number of d electrons is a specific value. It is known to be expensive. The strongly correlated electron system is realized by a layered compound having a transition metal ion in a skeleton structure. Many of these layered compounds are Mott insulators and are antiferromagnetic at room temperature.

However, for example, in La ₂ CuO ₄ that is a perovskite-type copper oxide, in La _2−x Sr _x CuO ₄ in which Sr ²⁺ ions are added to La ³⁺ ion sites, the value of x is 0.
In the range from 05 to 0.28, a spin glass state showing metal conduction is obtained, a superconductor state is observed at a low temperature, and the highest Tc = 40K is obtained at x = 0.15 (Non-Patent Document 6). .

That is, in the strong electron correlation system, when the number of d electrons is a specific value, it becomes a spin glass state showing metal conduction, and when the temperature is lowered, the superconducting state is reached at a specific temperature (superconducting transition temperature) or lower. Metastasize. Furthermore, the transition temperature of this superconductor varies from 5K to 40K depending on the number of conductive carriers. Further, since the transition temperature changes due to oxygen isotope substitution, it is known that lattice vibration and interaction between electrons are involved in the superconducting generation mechanism in a complex manner.

Ikuo Tsuda, Shinichiro Nasu, Kei Fujimori, Kiichi Shiratori Revised edition "Electrically Conductive Oxides", pp. 350-452, Hanawabo, (1993) Maekawa, Y., Applied Physics, Vol. .75, No. 1, pp. 17-25, (2006) Y. Maeno, H. Hashimoto, K. Yoshida S. Nishizaki T. Fujita J. G. Bednorz F. Lichtenberg, Nature, 372, pp. 532-534 (1994) J. Nagamatsu, N. Nakagawa, T. Muranaka Y. Zenitani and J. Akimitsu, Nature, 410, pp. 63-64, (2001) K. Takada H. Sakurai E. Takayama-Muromachi F. Izumi R. A. Dilanian T. Sasaki, Nature, 422. 53-55, (2003) J.B.Torrance et al., Phys. Rev., B40, pp. 8872-8877, (1989) Japanese Patent Laid-Open No. 2002-219916 JP 2004-262675 A JP 2005-350331 A

In order to dramatically expand the application of superconductivity, discovery of room temperature superconductors is strongly desired. In the layered perovskite-type copper oxide, a high-temperature superconductor exceeding 100 K has been found, but a room temperature superconductor has not yet been found. One strategy for developing room temperature superconductors is to find a new group of layered compounds having transition metal ions in the skeleton structure instead of perovskite-type copper oxides.

One powerful guideline for searching for high-temperature superconductors is to find a group of layered compounds having transition metal ions in the skeleton structure that can greatly change the number of conductive carriers, and change the type of transition metal in the group of compounds. In other words, the number of conductive carriers is optimized so that the superconducting transition temperature is maximized by doping electrons or holes by adding impurities or the like while keeping the type of transition metal constant.

In addition, since the interaction between d electrons of transition metals plays a major role in the superconducting mechanism, it is also necessary to control the interaction. One strategy for this is to find an anion that replaces the oxygen ion. For example, in the phosphorus ion, the energy level of p electrons is close to that of d electrons, so that the pd orbital hybridization becomes large and the interaction between carriers and electrons becomes large. Further, since the phosphorus ion has a larger atomic weight than the O ion, the lattice vibration energy is reduced and acts to reduce the electron-lattice interaction.

As a result of intensive research on many layered structure compounds having transition metal ions in the skeleton structure based on such a material search policy, LaFeOPh (Ph is P, As) having a crystal structure of ZrCuSiAs type (space group P4 / nmm). And at least one of Sb) was found to be a superconductor having a superconducting transition temperature in the vicinity of 5K.

That is, the present invention is characterized in that (1) it is represented by the chemical formula LaFeOP (Ph is at least one of P, As and Sb) and has a crystal structure of ZrCuSiAs type (space group P4 / nmm). A superconducting compound.

Further, the present invention provides (2) fluorine-doped chemical formula LaFeOPh: F (Ph is P,
ZrCuSiAs type (space group P4 / n)
mm) crystal structure.

In addition, the present invention is (3) the superconducting compound according to (1) or (2) above, which comprises a sintered body.
The present invention also provides (4) the superconducting compound according to (1) or (2) above, which comprises a thin film.

Further, the present invention provides (5) La ₂ O ₃ , LaPh, FePh, and Fe ₂ Ph (Ph is
Each powder of at least one of P, As, and Sb) is mixed at a molar ratio of 1: 1.02: 1.02: 1 and heated to 1100 ° C. to 1250 ° C. in an inert gas atmosphere. A method for producing a superconducting compound according to the above (3), wherein a sintered body is produced.

Further, the present invention provides (6) La ₂ O ₃ , LaPh, FePh, and Fe ₂ Ph (Ph is
At least one of P, As, and Sb) was mixed at a molar ratio of 1: 1.02: 1.02: 1, and LaF ₃ was mixed with La ₂ O _{3 at} a molar ratio of 0. .01 over 0.
The method for producing a superconducting compound sintered body according to (3) above, wherein the sintered body is produced by adding less than 1 and heating and holding at 1100 ° C. to 1250 ° C. in an inert gas atmosphere. is there.

Further, the present invention uses (7) LaFeOPh or LaFeOPh: F sintered body (Ph is at least one of P, As and Sb) prepared by the method of (5) or (6) above as a target. (4) The method for producing a superconducting compound thin film according to (4) above, wherein the film is formed by a vapor deposition method.

In the group of layered structure compounds having the transition metal ion in the skeleton structure,
1) A part or all of a divalent iron ion (Fe ²⁺ ion) is converted into a divalent transition metal ion M ²⁺ (
M = Ti, V, Mn, Co, Ni, Cu)
2) Trivalent lanthanum ions (La ³⁺ ions) are partially or entirely converted to trivalent rare earth ions (
Ln = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, L
u) or a method of substitution with Y ³⁺ ions,
3) A method of substituting La ³⁺ ions with metal ions having different valences,
4) A method of replacing O ²⁺ ions and / or P ³⁺ ions with anions having different valences,
5) A method of shifting the chemical composition of O ²⁺ and / or P ^{3+ from} the stoichiometric composition,
Thus, the carrier concentration in LaFeOP can be changed significantly from 10 ²⁰ to 10 ²¹ .

As a result, the number of conductive carriers can be optimized so as to maximize the interaction between d-electrons that cause superconductivity, so that the superconducting transition temperature can be changed from 5K of LaFeOP. As an example, O and P which are anions in LaFeOP or LaFeOAs
Alternatively, in LaFeOP: F or LaFeOAs: F in which O and As are substituted with fluorine ions, the superconducting transition temperature can be increased to about 10K.

Unlike the known superconducting compound, the superconducting compound of the present invention is an oxypnictide compound containing iron, which is the most abundant transition metal, and can be produced at low cost.

The superconducting compound of the present invention is represented by the chemical formula LaFeOP (Ph is at least one of P, As and Sb). This compound is ZrCuSiAs type (space group P4 / nmm)
The crystal structure is Further, this compound has a chemical formula LaFeOPh: F doped with fluorine.
The compound represented by (Ph is at least one of P, As and Sb) is also ZrCuS.
It is a superconducting compound having a crystal structure of iAs type (space group P4 / nmm).

LaFeOPh is a member of a layered structure compound group having a transition metal ion represented by the general chemical formula LnMOPn (M is a transition metal) in the skeleton structure. Here, Ln is Y and rare earth metal elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,
Yb, Lu) is at least one of transition metal elements (Fe, Ru, Os), and Pn is at least one of pnictide elements (N, P, As, Sb).

LaFeOP does not change the crystal structure even if the Fe ions are substituted with divalent transition metal ions or the La ions are substituted with trivalent rare earth ions or Y ions. 4f of rare earth ions
Some of the electrons move to the 3d orbit. Therefore, by these substitutions, the number of d electrons in LaFeOP can be controlled without changing the crystal structure, and as a result, the superconducting transition temperature can be changed.

That is, some or all of the divalent iron ions (Fe ²⁺ ions) are replaced with divalent transition metal ions M ²⁺ (M = Ti, V, Mn, Co, Ni, Cu) and / or Some or all of the valent lanthanum ions (La ³⁺ ions) are converted to trivalent rare earth ions (Ln = Ce, Pr,
Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) or Y ³⁺ ions, and the number of conduction electrons is controlled by the type and / or amount of metal ions to be substituted Can do.

In LaFeOP, even when La ³⁺ ions are replaced with cations having different valences, the carrier concentration can be changed. In the case of the substitution, electrons are generated in the LaO layer.
The electrons immediately move to the FeP layer and become conductive carriers. Therefore, it is possible to control only the number of conduction carriers without changing the magnitude of interaction between d electrons.

That is, La ³⁺ ions are substituted with metal ions having different valences, and the number of conductive carriers can be controlled by the type and / or addition amount of metal ions to be substituted.

Further, some or all of the phosphorus ions are converted into pnictide ions Ph (Ph = N, As, Sb).
The superconducting transition temperature can be changed by controlling the crystal lattice constant and / or the lattice frequency depending on the type and / or addition amount of the pnictide ion to be substituted.

In addition, the number of conductive carriers can be obtained by substituting O ²⁺ ions and / or P ³⁺ ions with anions having different valences and / or shifting the chemical composition of O ²⁺ and / or P ^{3+ from} the stoichiometric composition. And the superconducting transition temperature can be changed.

In addition, the superconducting transition temperature can be changed by combining at least two of the methods for changing the superconducting transition temperature.

The method for producing the superconducting compound of the present invention will be described by taking LaFeOP as an example. La ₂ O ₃ , La
A mixture in which P, FeP, and Fe ₂ P powders were mixed at a molar ratio of 1: 1.02: 1.02: 1 was placed in a quartz tube, evacuated to vacuum, and then an inert gas was discharged at room temperature. Introduce. As the inert gas, Ar gas is suitable, but is not limited thereto. The pressure of the inert gas is
It may be less than 1 atm. By enclosing the inert gas, shrinkage burst of the quartz glass tube during sintering can be prevented. In addition, there is an effect that the evaporation of the phosphide having a high vapor pressure is suppressed and the composition deviation from the stoichiometric composition of the sintered body is prevented. When LaFeOP is doped with fluorine, LaF ₃ may be added to the mixture at a molar ratio of LaF ₃ to La ₂ O ₃ of more than 0.01 and less than about 0.1.

Sintering is performed by heating and maintaining at 1100 ° C. to 1250 ° C. in an inert gas atmosphere. Preferably, the quartz tube containing the mixture is heated to about 1200 to 1250 ° C. and held for about 40 hours. Considering that P of the phosphide tends to sublimate, the mixed powder has about 2 P.
A compound having a chemical equivalent composition can be obtained by adding the compound in excess of a mol%. Next, the quartz tube is cooled to about 1100 to 1050 ° C. and held for about 10 hours, and then cooled to room temperature at a rate of about 250 ° C./hour. The reason why the temperature is raised to about 1200 to 1250 ° C. is to increase the chemical reaction rate and obtain a single phase of LaFeOP, and thereafter the temperature is kept at about 1100 to 1050 ° C. for a long time to increase the degree of firing. It is. If kept at about 1200 to 1250 ° C. for a long time, phosphorus volatilizes and a heterogeneous phase is generated.

FIG. 1 shows an X-ray diffraction pattern of the sintered body obtained in Example 1. From this FIG.
It can be seen that the crystal structure of the sintered body is a ZrCuSiAs type (space group P4 / nmm) and is composed of a single phase.

Using the sintered body as a target, by pulse laser deposition, 900 ° C. to 550 ° C.,
More preferably, a LaFeOP thin film can be formed on an MgO substrate heated to 600 ° C. to 700 ° C. As the substrate, MgO is suitable, but a substrate such as silica glass, alumina, yttrium-stabilized zirconia that can withstand a temperature of 900 ° C. can be used. In addition, it is deposited on a substrate at room temperature and sealed together with an inert gas in silica glass.
You may anneal by heating from 0 degreeC to 1100 degreeC for less than 10 hours. As a method for depositing the LaFeOP film, a pulse laser deposition method is simple, but other gas phase methods such as a sputtering method and a vapor deposition method can also be used.

In FIG. 2, the temperature change of the electrical resistivity of the sintered compact obtained in Example 1 is shown. From FIG.
It can be seen that the obtained sintered body had an electric resistance of zero at about 5K and transitioned to the superconducting phase. In the superconducting phase, due to the Meissner effect, the magnetic susceptibility exhibits complete diamagnetism, so that it can be confirmed from the temperature change of the magnetization of this sintered body shown in FIG.
Hereinafter, the present invention will be described in more detail based on examples.

<Synthesis of LaFeOP>
La (Shin-Etsu Chemical 99.9%) and P (rare metallic purity 99.9999%) were mixed at a molar ratio of 1: 1, sealed in a quartz tube, heated to 400 ° C., held for 10 hours, and then 700 ° C. Raised to
After holding for 10 hours, it was cooled to room temperature at a rate of about 250 ° C./hour to synthesize LaP. Moreover, Fe (high purity chemical research institute purity 99.9%), P (rare metallic purity 99.99)
99%) in a 1: 1 molar ratio, sealed in a quartz tube, heated to 400 ° C. and held for 10 hours, then heated to 700 ° C. and held for 10 hours, then 250 ° C./hour Cooling to room temperature at a moderate rate, FeP was synthesized. Furthermore, Fe (high purity chemical laboratory purity 99.9%) and P (rare metallic purity 99.9999%) are mixed at a molar ratio of 2: 1, sealed in a quartz tube, heated to 400 ° C. and held for 10 hours. Then, the temperature was raised to 700 ° C. and held for 10 hours, and then cooled to room temperature at a rate of about 250 ° C./hour to synthesize Fe ₂ P.

Using LaP, FeP, and FeP synthesized under the above conditions, each powder of La ₂ O ₃ (High Purity Chemical Laboratory, purity 99.99%), LaP, FeP, and Fe ₂ P was added at 1: 1.02: 1. .0
The mixture was mixed at a molar ratio of 2: 1, sealed in a quartz tube together with argon gas, heated to 1250 ° C. and held for 40 hours. Considering that P tends to sublime, P was added to the mixed powder in excess of about 2 mol%. Further, the temperature was lowered to about 1100 ° C. and held for 10 hours. FIG. 1 shows an X-ray diffraction pattern of the obtained sintered body. From FIG. 1, it was found that it was LaFeOP having a crystal structure of ZrCuSiAs type (space group P4 / nmm).
<Measurement of electrical resistivity>

The electrical resistivity was measured in the range from room temperature (300K) to 2K by a four-terminal method using silver paste as the electrode. The result is shown in FIG. The electrical resistivity decreased with a decrease in temperature, decreased rapidly around 7K, and became zero near 4K. That is, it was shown that LaFeOP is a superconductor having a superconducting transition temperature in the vicinity of 5K.

FIG. 3 shows an enlarged view of the temperature change in electrical resistivity when a magnetic field is applied near the superconducting transition temperature. The superconducting transition temperature was found to be 5K. Here, a temperature indicating a resistivity value that is 1/2 of the electrical resistivity before the superconducting transition is defined as a superconducting transition temperature (Tc). Tc decreased with application of the magnetic field.
<Measurement of magnetization>

Using a sample vibration type magnetometer, the magnetization of LaFeOP having a superconducting transition temperature of 5K was measured. The sample was cooled to 2K without applying a magnetic field (zero field cooling: ZFC), and a magnetic field of 10 Oe was applied during measurement. FIG. 4 shows the temperature change of magnetization near Tc. The magnetization decreased from around 4K, and at a lower temperature, the magnetization became a negative value.

This is because the magnetic flux cannot enter the superconductor and thus exhibits complete diamagnetism (Meissner effect), and is evidence that LaFeOP has transitioned to the superconducting state. Note that the superconducting state volume estimated from the magnetization value is 10 K or less at around 10K.
It became 0%. In addition, when cooling is performed while applying a magnetic field of 10 Oe (field cooling: FC), no decrease in magnetization is observed below 4K. That is, it was found that the obtained LaFeOP sintered body is a type 2 superconductor in which the Meissner effect is apparently not observed because the magnetic flux is trapped by internal defects or the like.

<Synthesis of LaFeO _0.94 F _0.06 P>
Using LaP, FeP, and FeP synthesized under the conditions described in Example 1, La ₂ O ₃ (
High Purity Chemical Laboratory, purity 99.9%), La (Shin-Etsu Chemical purity 99.9%), LaF ₃ (
Morita Chemical Industries purity 99%), each powder of LaP, Fe ₂ P and FeP was 0.94: 0.0
Mixed at a molar ratio of 6: 0.06: 1: 1: 1, sealed in a quartz tube with argon gas,
The temperature was raised to 1200 ° C. and held for 40 hours. Thereafter, it was cooled to room temperature at a rate of about 250 ° C./hour.

<Measurement of electrical resistivity>
The electrical resistivity was measured in the range from room temperature (300K) to 2K by a four-terminal method using silver paste as the electrode. The result is shown in FIG. The electrical resistivity decreases with decreasing temperature, 10K
It decreased rapidly around zero and became zero near 5.5K. That is, L doped with fluorine
aFeO _0.94 F _0.06 P was shown to be a superconductor having a superconducting transition temperature in the vicinity of 7K.

<Synthesis of LaFeO _0.94 F _0.06 As>
La (Shin-Etsu Chemical purity 99.9%), As (High-Purity Chemical Laboratory purity 99.99%) 1: 1
And sealed in a quartz tube, heated to 400 ° C. and held for 10 hours, then 600
After raising the temperature to 0 ° C. and holding it for 10 hours, it is cooled to room temperature at a rate of about 250 ° C./hour, and La
As was synthesized. Also, Fe (High Purity Chemical Laboratory, purity 99.9%) and As (High Purity Chemical Laboratory, purity 99.99%) were mixed at a molar ratio of 1: 1, sealed in a quartz tube, and 400 ° C. Then, the temperature was raised to 600 ° C. and held for 10 hours, and then cooled to room temperature at a rate of about 250 ° C./hour to synthesize FeAs. Furthermore, Fe (High Purity Chemical Laboratory, purity 99
. 9%), As (high purity chemical laboratory purity 99.99%) in a 2: 1 molar ratio, sealed in a quartz tube, heated to 400 ° C. and held for 10 hours, then 600 ° C. Then, the temperature was maintained for 10 hours, and then cooled to room temperature at a rate of about 250 ° C./hour to synthesize Fe ₂ As.

Using LaAs, FeAs, and FeAs synthesized under the above conditions, La ₂ O ₃ (high purity chemical research laboratory purity 99.9%), La (Shin-Etsu Chemical purity 99.9%), LaF ₃ (Morita Chemical Industries) Purity 99%), each powder of LaAs, Fe ₂ As and FeAs was 0.94: 0.
The mixture was mixed at a molar ratio of 06: 0.06: 1: 1: 1, sealed with argon gas in a quartz tube, heated to 1230 ° C., and held for 40 hours. Then, at a rate of about 250 ° C./hour,
Cooled to room temperature.

<Measurement of electrical resistivity>
The electrical resistivity was measured in the range from room temperature (300K) to 2K by a four-terminal method using silver paste as the electrode. The result is shown in FIG. The electrical resistivity decreases with decreasing temperature, 10K
It decreased rapidly in the vicinity and became zero near 3K. That is, LaF doped with fluorine
eO _0.94 F _0.06 As was shown to be a superconductor having a superconducting transition temperature in the vicinity of 7K.

In the present invention, a newly discovered superconducting material is used to conduct a conduction quantum interference device (SQUID: Supercond
ucting Quantum Interference Device) and Josephson devices can be created.

2 is an X-ray diffraction pattern of a sintered body obtained in Example 1. FIG. 4 is a graph showing measurement results of electrical resistivity of the sintered body obtained in Example 1. It is an enlarged view which shows the temperature change of the electrical resistivity at the time of the magnetic field application in the superconducting transition temperature vicinity of the sintered compact obtained in Example 1. FIG. 4 is a graph showing a temperature change of magnetization in the vicinity of a superconducting transition temperature of a sintered body obtained in Example 1. It is a graph which shows the measurement result of electrical resistivity in the superconducting transition temperature vicinity of the sintered compact obtained in Example 2. FIG. 4 is a graph showing measurement results of electrical resistivity of a sintered body obtained in Example 3.

Claims (7)

It is represented by the chemical formula LaFeOPh (Ph is at least one of P, As and Sb),
A superconducting compound having a crystal structure of ZrCuSiAs type (space group P4 / nmm). Fluorine-doped chemical formula LaFeOPh: F (Ph is at least one of P, As, and Sb), and has a ZrCuSiAs type (space group P4 / nmm) crystal structure. . The superconducting compound according to claim 1 or 2, comprising a sintered body. The superconducting compound according to claim 1, wherein the superconducting compound comprises a thin film. Each powder of La ₂ O ₃ , LaPh, FePh, and Fe ₂ Ph (Ph is at least one of P, As, and Sb) are mixed at a molar ratio of 1: 1.02: 1.02: 1. The method for producing a superconducting compound according to claim 3, wherein the sintered body is produced by heating and maintaining at 1100 ° C to 1250 ° C in an inert gas atmosphere. Each powder of La ₂ O ₃ , LaPh, FePh, and Fe ₂ Ph (Ph is at least one of P, As, and Sb) were mixed at a molar ratio of 1: 1.02: 1.02: 1. LaF ₃ is added to the mixture at a molar ratio to La ₂ O ₃ of more than 0.01 and less than 0.1, and a sintered body is produced by heating and maintaining at 1100 ° C. to 1250 ° C. in an inert gas atmosphere. The method for producing a superconducting compound sintered body according to claim 3, wherein: A LaFeOPh sintered body (Ph is at least one of P, As, and Sb) prepared by the method according to claim 5 or 6 is used as a target, and a film is formed by a vapor deposition method. Item 5. A method for producing a superconducting compound thin film according to Item 4.

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