- Email: [email protected]
On the synthesis and the electric and magnetic properties of superconducting barium–niobium–oxide compounds
Physica C 291 Ž1997. 207–212
On the synthesis and the electric and magnetic properties of superconducting barium–niobium–oxide compounds G.K. Strukova a,) , V.V. Kedrov b, V.N. Zverev a , S.S. Khasanov a , I.M. Ovchinnikov a , I.E. Batov a , V.A. Gasparov a a b
Institute of Solid State Physics RAS, 142432 ChernogoloÕka, Russian Federation Institute of Chemical Physics RAS, 142432 ChernogoloÕka, Russian Federation Received 9 July 1997; accepted 6 August 1997
Abstract In the system Ba–Nb–O–N a superconducting ceramic material with Tc s 22 K was synthesized. The temperature dependencies of magnetic susceptibility and resistivity of these ceramics with different oxygen concentrations was studied. A large increase of Tc was observed, from 12 K for samples prepared in O 2 up to 23 K prepared in air due to the variation of oxygen composition and Y doping. The XRD structure and Auger electron spectroscopy composition investigations indicate ˚ which is apparently the formation of oxinitrides BaNbO x N y with a cubic structure having a lattice parameter a s 4.32 A, responsible for the highest Tc observed. The incorporation of nitrogen into the crystal lattice results in an increase of Tc and stabilization of the superconducting structure. q 1997 Elsevier Science B.V. PACS: 61.10.-i; 72.15.Fy; 74.25.Ha; 74.62.-c; 74.70-b; 74.80.Bj
1. Introduction This work is the continuation of our researches, directed on finding new conducting compounds on the basis of refractory metals with variable valency, in particular, on the basis of niobium. Recently we have shown that in the system Ba x –Nb–O y with x s 0.25, 0.33 and 0.5 there exists superconductivity at a temperature as high as 18.6 K w1x. We believe that this compound represents a third class of high-Tc superconducting oxides after layered copper and three-dimensional perovskite bismuth oxide structures. The material synthesized was never obtained
)
Corresponding author. E-mail: [email protected]
as a monophase, but contained three phases: metallic niobium, a cubic niobium mononitride Ž d-NbN. like phase and an oxide phase. The main lines of the X-ray spectrum of this oxide phase coincided with the lines of the Ba 4 Nb 2 O 9 compound. Taking into account that the maximum value of Tc for d-NbN is 17.3 K w2x, and the Ba 4 Nb 2 O 9 compound is dielectric, it was assumed that the superconducting transition observed at 18.6 K is caused by an unknown oxide phase of the barium–niobium compound having a large oxygen deficiency. The superconducting properties of niobium nitride can be imposed on this transition. Superconductivity in ŽSr1y x Ln x .Nb 2 O6yy compounds Žwhere Ln s La, Nd, Pr, Ce, Gd. at 12–17 K was also reported w3–5x. The nonlinear voltage–cur-
0921-4534r97r$17.00 q 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 4 5 3 4 Ž 9 7 . 0 1 7 1 2 - 7
208
G.K. StrukoÕa et al.r Physica C 291 (1997) 207–212
rent dependencies observed in Ln–Sr–Nb–O compounds lead to a more intrigue conclusion about the localization of superconductivity with Tc as high as ¨ 160 K w6x. Ogushi et al. observed superconductivity in Nb oxides at 100–150 K Žsee Refs. w7,8x and references therein., which however was never confirmed. Electric and magnetic investigations of LaTiO x , CaNbO x , and SrNbO x Ž x s 3.35–3.5. systems have revealed the minimum in the magnetic susceptibility along the perovskite layers with an onset point at 100 K while the resistivity exhibits a thermally activated behavior at low temperatures with very low activation energies in the meV range w9x. This observation indicates that Nb oxide compounds could be very sensitive to oxygen stoichiometry. In this paper we report on the synthesis, identification of superconducting phases in the Ba–Nb–O–N system, and on the study of the electric and magnetic properties of these compounds with an equimolar BarNb ratio in a wide range of synthesis conditions. We describe the sample preparation using the solidstate quick burning reaction technique, which we believe is creating a largely reduced oxygen stoichiometry in BaNbO x superconducting compounds.
2. Experimental The preparation of ceramic BaNbO x samples is similar to that described in Ref. w1x, based on the quick burning solid-state reaction with some additional improvements. The starting materials were pure metallic Nb Ž99.99% purity., oxide, BaO, and peroxides, BaO 2 Žboth 99.9% purity. powders. The latter were added in different ratios with the aim to get different oxygen stoichiometries at a constant Ba percentage. After mixing appropriate amounts of powders in an agate mortar, the respective powders were pressed into pellets about 1 mm thick and 10 mm in diameter. The synthesis was performed in a two-zone vertical quartz reactor, which allowed us to change both the temperature of the synthesis and the composition of the gas phase. This setup consists of two parts with a common volume. The bottom part is a thermostated quartz cylindrical can. The upper part was kept at room temperature and equipped with a holder that allows to drop a pellet of initial reagents to the
bottom hot zone, so that samples were heated very quickly. The device was evacuated to 0.01 Torr, filled with an appropriate gas pressure, and then the bottom part was heated up to 9008C. Pellets of Nb and barium oxide ŽBaO. or peroxide ŽBaO 2 . were placed in the setup, and evacuated. Then the pellets were dropped in the hot zone and exposed to 9008C. The pellets were sintered during the quick burning due to a solid-state reaction and were additionally annealed at 9008C for a short time ranging between 0.5 and 70 min, with a gas adsorption control provided by the pressure change inside the device. The samples were then rapidly cooled to room temperature. The effect of the gas phase was studied, performing the synthesis in oxygen, hydrogen, nitrogen, or in air. The initial hydrogen pressures ranged from 270 to 550 Torr, the oxygen, nitrogen, and air pressures from 0.2 to 2000 Torr, respectively. The real part of the ac susceptibility ŽRe X . was measured between room temperature and 4.2 K for various shapes of the samples obtained after fracturing the sintered pellets. The temperature was measured by a Cu–CuFe thermocouple which was additionally calibrated by the superconducting transition of a V3 Si single crystal ŽT s 15.1 K.. Resistivity measurements between room temperature and 4.2 K were performed by a standard four probe ac Ž9 Hz. method on bars of about 1 mm in diameter and 5 mm in length, obtained by fracturing the sintered pellets and lapping these samples on abrasive tapes. Electrical contacts were made with thin silver wires and silver paint. Bulk structural analysis was done by X-ray powder diffraction ŽSiemens Diffractometer D500. with Cu K a 1 radiation, and 2 u scans were made in 0.028 steps, typically from 258 to 758. An 09IOS-3 CMA spectrometer was used for the Auger electron spectra ŽAES. investigation of these pellets, which is a very useful tool for the examination of light elements like carbon, nitrogen, and oxygen in high-Tc superconductors w10,11x. After insertion into the spectrometer the sample surface was Arq ion etched with an energy of 2 keV, and then annealed in vacuum at 8008C for 1 h, in order to eliminate the contaminated surface layer. AES spectra were recorded at 1 = 10y7 Pa before and after in situ annealing of the ion etched samples Žsee details in Ref. w11x.. Since we had no standards, we were
G.K. StrukoÕa et al.r Physica C 291 (1997) 207–212
not able to determine the exact sample composition and oxygen stoichiometry from AES. However, our estimations are in qualitative agreement with the BaNbO 3y x composition, with x F 1.5 indicating very high oxygen deficiency.
3. Results and discussion Whenever the synthesis was made in an atmosphere of air, hydrogen or oxygen compact grayishblack samples with metallic conductivity were obtained. Electron microprobe analysis of these samples gave a BarNb ratio of 1 " 0.1, in good agreement with the expected value. After long annealing times of these samples in oxygen at 9008C as well as sintering in nitrogen, one obtains grey nonconducting samples. XRD data were collected for all synthesized BaNbO 3y x compounds Žsee Fig. 1., and the cubic perovskite related structure was assumed. XRD phase analysis shows the presence of a new phase along with the cubic niobium mononitride in the samples at molar ratios of BarNbs 1r1, which becomes very clear in the samples sintered in oxygen ŽFig. 1.. Crystallographic data of the refined unit cell parameter a are given in Table 1 for different synthesis environments, together with published data for BaNbO 3 , BaNbO 2 N and d-NbN compounds. The a values for our Ba–Nb–O samples compared to published data for oxides and oxinitrides are higher, they
Fig. 1. XRD spectra of samples synthesized in different environments with equimolar ratios of BarNbs1 : 1 and BaOrBaO2 s 1 : 1; Ža. the sample prepared in air, Žb. the same sample after etching in HCl, Žc. the sample prepared in O 2 .
209
Table 1 Comparative data on the lattice parameter, a, in the Ba–Nb–O system Composition
Environment
˚. a ŽA
air O2 H2
4.04 4.13 4.373–4.379 4.32 4.35 4.24
BaNbO 3 w15x BaNbO 2 N w12x d-NbN w2x BaO 2 q4BaOq5Nb
are however very close to those of d-NbN. Nevertheless, Fig. 1 clearly shows the presence of a new cubic phase along with a cubic d-NBn-like phase for samples Žwith a molar ratio of BarNbs 1. synthesized in air, which becomes even clearer for samples synthesized in oxygen. Since the samples were not single phase, we were not able to collect a full set of X-ray intensity data for the Rietveld refinement to determine the exact crystallographic structure. However, the main measured intensities are almost consistent with calculated intensities based on the primitive cubic structure. The temperature dependencies of the magnetic susceptibility, X ŽT ., and the resistivity, RŽT ., are shown in Fig. 2Ža.,Žb., for three samples, synthesized in air from an equimolar mixture of Ba and Nb and with an amount of BaO varying from 20 to 80 mol.% of barium peroxide and oxide mixture. It is clear, that these samples exhibit two-step superconducting transitions with onset points Tc1 and Tc2 in the region from 13 to 22 K, largely varying at different contents of oxygen. The highest Tc1 transition is likely caused by the presence of the d-NbN-like phase in the samples, and the lower Tc2 by the presence of the BaNbO 3y x phase. Tc2 was strongly increased as the content of active oxygen in the system decreased. However, we can rule out this conclusion because it is too high for d-NbN Tc1 Žonset point is equal to 22 K.. We rather believe in replacement of oxygen vacancies by nitrogen for samples sintered in air and thus the formation of oxide–nitride–barium–niobium compounds. The assumption of the formation of the complex barium–niobium–oxide–nitrides is confirmed by the published data. In fact, several works on the synthesis of AMO 2q x N1yx compounds, where A is Ca, Sr
210
G.K. StrukoÕa et al.r Physica C 291 (1997) 207–212
or Ba and M represents group IV–VI metals, which, however, are not superconducting, appeared recently w12–14x. The cubic structure with a lattice parameter ˚ was determined for BaNbO2 N w12x. It of a s 4.13 A is important to note that the cubic lattice type of the BaNbO 3 composition w15x was conserved when oxygen was partly substituted by nitrogen. However, we have to stress that the BaNbO 2 N and BaNbO 3 compounds are dielectric w12,15x and therefore, we can only speculate that superconductivity is caused by the large nonstoichiometry of oxygen and nitrogen in our samples. Indeed, AES studies of the samples synthesized in air show the presence of a small amount of nitrogen, which was absent in samples synthesized in hydroFig. 3. Auger electron spectra of samples synthesized in air Ža. and in hydrogen Žb. after etching with Arq ions at an energy of 2 keV and annealing in vacuum at 8008C for 1 h.
Fig. 2. Temperature dependencies of: Ža. the real part of ac magnetic susceptibility, X ŽT ., and Žb. resistivity, RŽT ., for samples synthesized in air with a constant ratio of BarNbs1 : 1 and with a varying ratio of BaOrBaO2 .
gen and oxygen Žsee Fig. 3.. These studies allowed us to propose the empirical formula of the new oxide–nitride phase BaNbO 3y x N y . The difference in the lattice parameters of these samples compared to BaNbO 2 N ŽTable 1. can be related to: Ži. the change in the ratio of oxygen and nitrogen and Žii. the change of their occupation sites in the lattice. It is likely that these deviations of the oxide–nitride compounds from stoichiometry are the reason for the appearance of the superconducting transition in the BaNbO 3y x N y system. In order to clarify the quantitative contribution of the new phase to the superconductivity of the double-phase system, the ceramic sample synthesized in air was grind and etched with 18% hydrochloric acid at room temperature for two days until the BaNbO 3y x N y-like phase completely disappeared in the XRD spectra. From the weight loss of the sample we concluded an initial phase, 90 wt.% of the oxide–nitride phase. XRD spectra revealed the presence of cubic d-NbN and some metallic Nb in the remaining product. The temperature dependencies of the magnetic susceptibility of two equal weighted samples Ž0.2 g. of the former double-phase sample and the remaining product after acid treatment are presented in Fig. 4. It can be concluded that the large transition at Tc1 s 20 K occurs in the former sample
G.K. StrukoÕa et al.r Physica C 291 (1997) 207–212
and 10 times weaker at 16 and 12 K in the etched one. We concluded that the 20 K transition is due to the BaNbO 3y x N y phase and the 16 K transition is due to the d-NbN phase Žpresented in the XRD spectra, see Fig. 1.. We also succeeded in the preparation of superconducting samples containing no nitrogen according to AES ŽFig. 3. with Tc ranging from 15 to 17 K, during synthesis in an atmosphere of pure hydrogen and oxygen ŽFig. 5.. In these experiments, the BarNb ratios were taken also equal to 1 : 1, and the amount of BaO was varied from 20 to 80 mol.% of the mixture of oxide and peroxide. Upon argon annealing the samples prepared in O 2 exhibited an increase of Tc from 15 up to 18.5 K. In previous works w3,4x, similar oxygen-containing systems based on niobium exhibited superconductivity between 5 and 17 K. However, these samples were unstable, and the percentage of the superconducting phase was small. In our method of synthesis, with low oxygen pressure and quenching of the samples at high temperatures, a nonstoichiometric metastable compound of the BaNbO 3y x type was formed, which also looses superconductivity upon storage in air for a few days at room temperature. Apparently, the nitrogen in the composition of the samples synthesized in air stabilizes the superconducting structure, because these
Fig. 4. Temperature dependencies of the real part of ac magnetic susceptibility, X ŽT ., of the sample synthesized in air and the same sample after etching with HCl. The X ŽT . curve for the last one is multiplied by a factor of 10, indicating a very small amount of d-NbN phase in the former sample.
211
Fig. 5. Temperature dependencies of the real part of ac magnetic susceptibility, X ŽT ., for samples synthesized in oxygen, in hydrogen, in oxygen with subsequent annealing in argon, and in air ŽBarNbs1 : 1.. Also shown are the data for Y0.2 Ba 0.8 NbO 3yx , prepared in air.
samples are much more stable than those prepared in hydrogen and oxygen. The large amount of oxygen vacancies makes this compound suitable for intercalation. Doping of these systems with different chemical elements and using a low oxygen pressure can result into a stabilization of similar compounds and an increase in Tc . In fact, when some part of BaO was replaced by Y2 O 3 , an increase in Tc was observed up to 23 K ŽFig. 5.. Thus, the results of the present studies make it possible to assert that the variation of the medium in which niobium reacts with barium oxides allows to obtain stable superconducting phases from complex barium–niobium–oxide–nitrides to pure Ba–Nb oxides with Tc above 20 K. Actually, cubic niobium mononitride with Tc s 16 K appears in some samples due to the formation of barium–niobium–oxide– nitride followed by its disproportionation. In order to clarify the Ba–Nb–O phase which is responsible for the high-Tc transition and its crystallographic structure, further investigations on the single crystals and thin films are necessary. This work is in progress now and thin epitaxial films on SrTiO 3 , NdGaO 3 , A l 2O 3 and YSZ have been deposited. Very unusual electron transport properties of these films have been observed which will be published elsewhere. Note also, that we synthesized SrNbO x
212
G.K. StrukoÕa et al.r Physica C 291 (1997) 207–212
superconducting compounds with Tc s 19.7 K by means of the same technique. In conclusion, we have synthesized a new ceramic material, whose superconducting properties and stability are crucially dependent on the composition of the gas medium in which the synthesis was performed. We believe that Tc s 22 K of this ceramic material is due to the presence of a new superconducting phase with a perovskite-like cubic lattice of the general formula BaNbO 3y x N y . Magnetic and resistivity measurements have proven the superconductivity transition to be as high as 23 K for Y-doped compounds having a large oxygen deficiency. The incorporation of nitrogen into the crystal lattice results into an increase of Tc and a stabilization of the superconducting structure. We propose, that a novel class of superconducting compounds can be formed by means of the developed synthesis method, using oxides of alkali-earth ŽCa, Sr. and rare-earth ŽY, Nd. elements as dopants.
Acknowledgements The authors wish to acknowledge the great interest and stimulating discussions of this work by I.F. Shchegolev, and the authors will keep a deep memory of Igor Fomich. We wish to thank V.F. Gantmakher, R. Huguenin, D. Khomskii, D. van der Marel, D. Pavuna, W. Richter, L. Rinderer, G.A. Sawatzky and P. Seidel for helpful discussions, S.F. Kosterev and V.D. Gurova for magnetic susceptibil-
ity measurements, and M. Riehl-Chudoba for his help in editing this paper. This work was supported by the International Science Foundation Žgrant NKS300., US Civilian Research and Development Foundation Žgrant RE1-356. and Russian Council on High-Tem perature Superconductivity Ž grant N93071.. References w1x V.A. Gasparov, G.K. Strukova, S.S. Khasanov, IETP Lett. 60 Ž1994. 440. w2x G. Oya, Y. Onodera, J. Appl. Phys. 45 Ž1974. 1389. w3x J. Akimitsu, J. Amano, H. Sawa et al., Jpn. J. Appl. Phys. 30 Ž1991. L1151. w4x A. Nakamura, Jpn. J. Appl. Phys. Part 2, N 4B Ž1994. 583. w5x S.F. Solodovnikov, L.I. Yudanova, V.V. Gurov et al., Zh. Neorg. Khim. 25 Ž1995. 179–183. w6x A.V. Mitin, G.M. Kusjmicheva et al., Zh. Eksp. Teor. Fiz. 107 Ž1995. 1943. ¨ w7x T. Ogushi, R. Obara, T. Anauama, Jpn. J. Appl. Phys. 22 Ž1983. L523. ¨ w8x T. Ogushi, S. Higo, N.G. Suresha et al., J. Low Temp. Phys. 73 Ž1988. 305. w9x F. Lichtenberg, T. Williams, A. Reller et al., Z. Physik B 84 Ž1991. 369. w10x V.A. Gasparov, A.F. Dite, V.V. Bondarev et al., Vacuum 41 Ž1990. 989. w11x I.M. Ovchinnikov, N.M. Sorokin, G.A. Emel’chenko et al., Superconductivity 7 Ž1994. 340, in Russian. w12x F. Pors, R. Marchand, Y. Laurent et al., Mater. Res. Bull. 23 Ž1988. 1447. w13x R. Marchand, F. Pors, Y. Laurent, Ann. Chim. 16 Ž1991. 553. w14x J. Grins, G. Svensson, Mater. Res. Bull. 29 Ž1994. 801. w15x R. Kreiser, R. Ward, J. Solid State Chem. 1 Ž1970. 368.
On the synthesis and the electric and magnetic properties of superconducting barium–niobium–oxide compounds G.K. Strukova a,) , V.V. Kedrov b, V.N. Zverev a , S.S. Khasanov a , I.M. Ovchinnikov a , I.E. Batov a , V.A. Gasparov a a b
Institute of Solid State Physics RAS, 142432 ChernogoloÕka, Russian Federation Institute of Chemical Physics RAS, 142432 ChernogoloÕka, Russian Federation Received 9 July 1997; accepted 6 August 1997
Abstract In the system Ba–Nb–O–N a superconducting ceramic material with Tc s 22 K was synthesized. The temperature dependencies of magnetic susceptibility and resistivity of these ceramics with different oxygen concentrations was studied. A large increase of Tc was observed, from 12 K for samples prepared in O 2 up to 23 K prepared in air due to the variation of oxygen composition and Y doping. The XRD structure and Auger electron spectroscopy composition investigations indicate ˚ which is apparently the formation of oxinitrides BaNbO x N y with a cubic structure having a lattice parameter a s 4.32 A, responsible for the highest Tc observed. The incorporation of nitrogen into the crystal lattice results in an increase of Tc and stabilization of the superconducting structure. q 1997 Elsevier Science B.V. PACS: 61.10.-i; 72.15.Fy; 74.25.Ha; 74.62.-c; 74.70-b; 74.80.Bj
1. Introduction This work is the continuation of our researches, directed on finding new conducting compounds on the basis of refractory metals with variable valency, in particular, on the basis of niobium. Recently we have shown that in the system Ba x –Nb–O y with x s 0.25, 0.33 and 0.5 there exists superconductivity at a temperature as high as 18.6 K w1x. We believe that this compound represents a third class of high-Tc superconducting oxides after layered copper and three-dimensional perovskite bismuth oxide structures. The material synthesized was never obtained
)
Corresponding author. E-mail: [email protected]
as a monophase, but contained three phases: metallic niobium, a cubic niobium mononitride Ž d-NbN. like phase and an oxide phase. The main lines of the X-ray spectrum of this oxide phase coincided with the lines of the Ba 4 Nb 2 O 9 compound. Taking into account that the maximum value of Tc for d-NbN is 17.3 K w2x, and the Ba 4 Nb 2 O 9 compound is dielectric, it was assumed that the superconducting transition observed at 18.6 K is caused by an unknown oxide phase of the barium–niobium compound having a large oxygen deficiency. The superconducting properties of niobium nitride can be imposed on this transition. Superconductivity in ŽSr1y x Ln x .Nb 2 O6yy compounds Žwhere Ln s La, Nd, Pr, Ce, Gd. at 12–17 K was also reported w3–5x. The nonlinear voltage–cur-
0921-4534r97r$17.00 q 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 4 5 3 4 Ž 9 7 . 0 1 7 1 2 - 7
208
G.K. StrukoÕa et al.r Physica C 291 (1997) 207–212
rent dependencies observed in Ln–Sr–Nb–O compounds lead to a more intrigue conclusion about the localization of superconductivity with Tc as high as ¨ 160 K w6x. Ogushi et al. observed superconductivity in Nb oxides at 100–150 K Žsee Refs. w7,8x and references therein., which however was never confirmed. Electric and magnetic investigations of LaTiO x , CaNbO x , and SrNbO x Ž x s 3.35–3.5. systems have revealed the minimum in the magnetic susceptibility along the perovskite layers with an onset point at 100 K while the resistivity exhibits a thermally activated behavior at low temperatures with very low activation energies in the meV range w9x. This observation indicates that Nb oxide compounds could be very sensitive to oxygen stoichiometry. In this paper we report on the synthesis, identification of superconducting phases in the Ba–Nb–O–N system, and on the study of the electric and magnetic properties of these compounds with an equimolar BarNb ratio in a wide range of synthesis conditions. We describe the sample preparation using the solidstate quick burning reaction technique, which we believe is creating a largely reduced oxygen stoichiometry in BaNbO x superconducting compounds.
2. Experimental The preparation of ceramic BaNbO x samples is similar to that described in Ref. w1x, based on the quick burning solid-state reaction with some additional improvements. The starting materials were pure metallic Nb Ž99.99% purity., oxide, BaO, and peroxides, BaO 2 Žboth 99.9% purity. powders. The latter were added in different ratios with the aim to get different oxygen stoichiometries at a constant Ba percentage. After mixing appropriate amounts of powders in an agate mortar, the respective powders were pressed into pellets about 1 mm thick and 10 mm in diameter. The synthesis was performed in a two-zone vertical quartz reactor, which allowed us to change both the temperature of the synthesis and the composition of the gas phase. This setup consists of two parts with a common volume. The bottom part is a thermostated quartz cylindrical can. The upper part was kept at room temperature and equipped with a holder that allows to drop a pellet of initial reagents to the
bottom hot zone, so that samples were heated very quickly. The device was evacuated to 0.01 Torr, filled with an appropriate gas pressure, and then the bottom part was heated up to 9008C. Pellets of Nb and barium oxide ŽBaO. or peroxide ŽBaO 2 . were placed in the setup, and evacuated. Then the pellets were dropped in the hot zone and exposed to 9008C. The pellets were sintered during the quick burning due to a solid-state reaction and were additionally annealed at 9008C for a short time ranging between 0.5 and 70 min, with a gas adsorption control provided by the pressure change inside the device. The samples were then rapidly cooled to room temperature. The effect of the gas phase was studied, performing the synthesis in oxygen, hydrogen, nitrogen, or in air. The initial hydrogen pressures ranged from 270 to 550 Torr, the oxygen, nitrogen, and air pressures from 0.2 to 2000 Torr, respectively. The real part of the ac susceptibility ŽRe X . was measured between room temperature and 4.2 K for various shapes of the samples obtained after fracturing the sintered pellets. The temperature was measured by a Cu–CuFe thermocouple which was additionally calibrated by the superconducting transition of a V3 Si single crystal ŽT s 15.1 K.. Resistivity measurements between room temperature and 4.2 K were performed by a standard four probe ac Ž9 Hz. method on bars of about 1 mm in diameter and 5 mm in length, obtained by fracturing the sintered pellets and lapping these samples on abrasive tapes. Electrical contacts were made with thin silver wires and silver paint. Bulk structural analysis was done by X-ray powder diffraction ŽSiemens Diffractometer D500. with Cu K a 1 radiation, and 2 u scans were made in 0.028 steps, typically from 258 to 758. An 09IOS-3 CMA spectrometer was used for the Auger electron spectra ŽAES. investigation of these pellets, which is a very useful tool for the examination of light elements like carbon, nitrogen, and oxygen in high-Tc superconductors w10,11x. After insertion into the spectrometer the sample surface was Arq ion etched with an energy of 2 keV, and then annealed in vacuum at 8008C for 1 h, in order to eliminate the contaminated surface layer. AES spectra were recorded at 1 = 10y7 Pa before and after in situ annealing of the ion etched samples Žsee details in Ref. w11x.. Since we had no standards, we were
G.K. StrukoÕa et al.r Physica C 291 (1997) 207–212
not able to determine the exact sample composition and oxygen stoichiometry from AES. However, our estimations are in qualitative agreement with the BaNbO 3y x composition, with x F 1.5 indicating very high oxygen deficiency.
3. Results and discussion Whenever the synthesis was made in an atmosphere of air, hydrogen or oxygen compact grayishblack samples with metallic conductivity were obtained. Electron microprobe analysis of these samples gave a BarNb ratio of 1 " 0.1, in good agreement with the expected value. After long annealing times of these samples in oxygen at 9008C as well as sintering in nitrogen, one obtains grey nonconducting samples. XRD data were collected for all synthesized BaNbO 3y x compounds Žsee Fig. 1., and the cubic perovskite related structure was assumed. XRD phase analysis shows the presence of a new phase along with the cubic niobium mononitride in the samples at molar ratios of BarNbs 1r1, which becomes very clear in the samples sintered in oxygen ŽFig. 1.. Crystallographic data of the refined unit cell parameter a are given in Table 1 for different synthesis environments, together with published data for BaNbO 3 , BaNbO 2 N and d-NbN compounds. The a values for our Ba–Nb–O samples compared to published data for oxides and oxinitrides are higher, they
Fig. 1. XRD spectra of samples synthesized in different environments with equimolar ratios of BarNbs1 : 1 and BaOrBaO2 s 1 : 1; Ža. the sample prepared in air, Žb. the same sample after etching in HCl, Žc. the sample prepared in O 2 .
209
Table 1 Comparative data on the lattice parameter, a, in the Ba–Nb–O system Composition
Environment
˚. a ŽA
air O2 H2
4.04 4.13 4.373–4.379 4.32 4.35 4.24
BaNbO 3 w15x BaNbO 2 N w12x d-NbN w2x BaO 2 q4BaOq5Nb
are however very close to those of d-NbN. Nevertheless, Fig. 1 clearly shows the presence of a new cubic phase along with a cubic d-NBn-like phase for samples Žwith a molar ratio of BarNbs 1. synthesized in air, which becomes even clearer for samples synthesized in oxygen. Since the samples were not single phase, we were not able to collect a full set of X-ray intensity data for the Rietveld refinement to determine the exact crystallographic structure. However, the main measured intensities are almost consistent with calculated intensities based on the primitive cubic structure. The temperature dependencies of the magnetic susceptibility, X ŽT ., and the resistivity, RŽT ., are shown in Fig. 2Ža.,Žb., for three samples, synthesized in air from an equimolar mixture of Ba and Nb and with an amount of BaO varying from 20 to 80 mol.% of barium peroxide and oxide mixture. It is clear, that these samples exhibit two-step superconducting transitions with onset points Tc1 and Tc2 in the region from 13 to 22 K, largely varying at different contents of oxygen. The highest Tc1 transition is likely caused by the presence of the d-NbN-like phase in the samples, and the lower Tc2 by the presence of the BaNbO 3y x phase. Tc2 was strongly increased as the content of active oxygen in the system decreased. However, we can rule out this conclusion because it is too high for d-NbN Tc1 Žonset point is equal to 22 K.. We rather believe in replacement of oxygen vacancies by nitrogen for samples sintered in air and thus the formation of oxide–nitride–barium–niobium compounds. The assumption of the formation of the complex barium–niobium–oxide–nitrides is confirmed by the published data. In fact, several works on the synthesis of AMO 2q x N1yx compounds, where A is Ca, Sr
210
G.K. StrukoÕa et al.r Physica C 291 (1997) 207–212
or Ba and M represents group IV–VI metals, which, however, are not superconducting, appeared recently w12–14x. The cubic structure with a lattice parameter ˚ was determined for BaNbO2 N w12x. It of a s 4.13 A is important to note that the cubic lattice type of the BaNbO 3 composition w15x was conserved when oxygen was partly substituted by nitrogen. However, we have to stress that the BaNbO 2 N and BaNbO 3 compounds are dielectric w12,15x and therefore, we can only speculate that superconductivity is caused by the large nonstoichiometry of oxygen and nitrogen in our samples. Indeed, AES studies of the samples synthesized in air show the presence of a small amount of nitrogen, which was absent in samples synthesized in hydroFig. 3. Auger electron spectra of samples synthesized in air Ža. and in hydrogen Žb. after etching with Arq ions at an energy of 2 keV and annealing in vacuum at 8008C for 1 h.
Fig. 2. Temperature dependencies of: Ža. the real part of ac magnetic susceptibility, X ŽT ., and Žb. resistivity, RŽT ., for samples synthesized in air with a constant ratio of BarNbs1 : 1 and with a varying ratio of BaOrBaO2 .
gen and oxygen Žsee Fig. 3.. These studies allowed us to propose the empirical formula of the new oxide–nitride phase BaNbO 3y x N y . The difference in the lattice parameters of these samples compared to BaNbO 2 N ŽTable 1. can be related to: Ži. the change in the ratio of oxygen and nitrogen and Žii. the change of their occupation sites in the lattice. It is likely that these deviations of the oxide–nitride compounds from stoichiometry are the reason for the appearance of the superconducting transition in the BaNbO 3y x N y system. In order to clarify the quantitative contribution of the new phase to the superconductivity of the double-phase system, the ceramic sample synthesized in air was grind and etched with 18% hydrochloric acid at room temperature for two days until the BaNbO 3y x N y-like phase completely disappeared in the XRD spectra. From the weight loss of the sample we concluded an initial phase, 90 wt.% of the oxide–nitride phase. XRD spectra revealed the presence of cubic d-NbN and some metallic Nb in the remaining product. The temperature dependencies of the magnetic susceptibility of two equal weighted samples Ž0.2 g. of the former double-phase sample and the remaining product after acid treatment are presented in Fig. 4. It can be concluded that the large transition at Tc1 s 20 K occurs in the former sample
G.K. StrukoÕa et al.r Physica C 291 (1997) 207–212
and 10 times weaker at 16 and 12 K in the etched one. We concluded that the 20 K transition is due to the BaNbO 3y x N y phase and the 16 K transition is due to the d-NbN phase Žpresented in the XRD spectra, see Fig. 1.. We also succeeded in the preparation of superconducting samples containing no nitrogen according to AES ŽFig. 3. with Tc ranging from 15 to 17 K, during synthesis in an atmosphere of pure hydrogen and oxygen ŽFig. 5.. In these experiments, the BarNb ratios were taken also equal to 1 : 1, and the amount of BaO was varied from 20 to 80 mol.% of the mixture of oxide and peroxide. Upon argon annealing the samples prepared in O 2 exhibited an increase of Tc from 15 up to 18.5 K. In previous works w3,4x, similar oxygen-containing systems based on niobium exhibited superconductivity between 5 and 17 K. However, these samples were unstable, and the percentage of the superconducting phase was small. In our method of synthesis, with low oxygen pressure and quenching of the samples at high temperatures, a nonstoichiometric metastable compound of the BaNbO 3y x type was formed, which also looses superconductivity upon storage in air for a few days at room temperature. Apparently, the nitrogen in the composition of the samples synthesized in air stabilizes the superconducting structure, because these
Fig. 4. Temperature dependencies of the real part of ac magnetic susceptibility, X ŽT ., of the sample synthesized in air and the same sample after etching with HCl. The X ŽT . curve for the last one is multiplied by a factor of 10, indicating a very small amount of d-NbN phase in the former sample.
211
Fig. 5. Temperature dependencies of the real part of ac magnetic susceptibility, X ŽT ., for samples synthesized in oxygen, in hydrogen, in oxygen with subsequent annealing in argon, and in air ŽBarNbs1 : 1.. Also shown are the data for Y0.2 Ba 0.8 NbO 3yx , prepared in air.
samples are much more stable than those prepared in hydrogen and oxygen. The large amount of oxygen vacancies makes this compound suitable for intercalation. Doping of these systems with different chemical elements and using a low oxygen pressure can result into a stabilization of similar compounds and an increase in Tc . In fact, when some part of BaO was replaced by Y2 O 3 , an increase in Tc was observed up to 23 K ŽFig. 5.. Thus, the results of the present studies make it possible to assert that the variation of the medium in which niobium reacts with barium oxides allows to obtain stable superconducting phases from complex barium–niobium–oxide–nitrides to pure Ba–Nb oxides with Tc above 20 K. Actually, cubic niobium mononitride with Tc s 16 K appears in some samples due to the formation of barium–niobium–oxide– nitride followed by its disproportionation. In order to clarify the Ba–Nb–O phase which is responsible for the high-Tc transition and its crystallographic structure, further investigations on the single crystals and thin films are necessary. This work is in progress now and thin epitaxial films on SrTiO 3 , NdGaO 3 , A l 2O 3 and YSZ have been deposited. Very unusual electron transport properties of these films have been observed which will be published elsewhere. Note also, that we synthesized SrNbO x
212
G.K. StrukoÕa et al.r Physica C 291 (1997) 207–212
superconducting compounds with Tc s 19.7 K by means of the same technique. In conclusion, we have synthesized a new ceramic material, whose superconducting properties and stability are crucially dependent on the composition of the gas medium in which the synthesis was performed. We believe that Tc s 22 K of this ceramic material is due to the presence of a new superconducting phase with a perovskite-like cubic lattice of the general formula BaNbO 3y x N y . Magnetic and resistivity measurements have proven the superconductivity transition to be as high as 23 K for Y-doped compounds having a large oxygen deficiency. The incorporation of nitrogen into the crystal lattice results into an increase of Tc and a stabilization of the superconducting structure. We propose, that a novel class of superconducting compounds can be formed by means of the developed synthesis method, using oxides of alkali-earth ŽCa, Sr. and rare-earth ŽY, Nd. elements as dopants.
Acknowledgements The authors wish to acknowledge the great interest and stimulating discussions of this work by I.F. Shchegolev, and the authors will keep a deep memory of Igor Fomich. We wish to thank V.F. Gantmakher, R. Huguenin, D. Khomskii, D. van der Marel, D. Pavuna, W. Richter, L. Rinderer, G.A. Sawatzky and P. Seidel for helpful discussions, S.F. Kosterev and V.D. Gurova for magnetic susceptibil-
ity measurements, and M. Riehl-Chudoba for his help in editing this paper. This work was supported by the International Science Foundation Žgrant NKS300., US Civilian Research and Development Foundation Žgrant RE1-356. and Russian Council on High-Tem perature Superconductivity Ž grant N93071.. References w1x V.A. Gasparov, G.K. Strukova, S.S. Khasanov, IETP Lett. 60 Ž1994. 440. w2x G. Oya, Y. Onodera, J. Appl. Phys. 45 Ž1974. 1389. w3x J. Akimitsu, J. Amano, H. Sawa et al., Jpn. J. Appl. Phys. 30 Ž1991. L1151. w4x A. Nakamura, Jpn. J. Appl. Phys. Part 2, N 4B Ž1994. 583. w5x S.F. Solodovnikov, L.I. Yudanova, V.V. Gurov et al., Zh. Neorg. Khim. 25 Ž1995. 179–183. w6x A.V. Mitin, G.M. Kusjmicheva et al., Zh. Eksp. Teor. Fiz. 107 Ž1995. 1943. ¨ w7x T. Ogushi, R. Obara, T. Anauama, Jpn. J. Appl. Phys. 22 Ž1983. L523. ¨ w8x T. Ogushi, S. Higo, N.G. Suresha et al., J. Low Temp. Phys. 73 Ž1988. 305. w9x F. Lichtenberg, T. Williams, A. Reller et al., Z. Physik B 84 Ž1991. 369. w10x V.A. Gasparov, A.F. Dite, V.V. Bondarev et al., Vacuum 41 Ž1990. 989. w11x I.M. Ovchinnikov, N.M. Sorokin, G.A. Emel’chenko et al., Superconductivity 7 Ž1994. 340, in Russian. w12x F. Pors, R. Marchand, Y. Laurent et al., Mater. Res. Bull. 23 Ž1988. 1447. w13x R. Marchand, F. Pors, Y. Laurent, Ann. Chim. 16 Ž1991. 553. w14x J. Grins, G. Svensson, Mater. Res. Bull. 29 Ž1994. 801. w15x R. Kreiser, R. Ward, J. Solid State Chem. 1 Ž1970. 368.