Composition Dependences of Property and Conductivity in (Sr,Ca)3Nb2O7-δ

INTRODUCTION

Superconductors which shows complete conductivity at low temperature is expected to be applied for many electronic technologies, such as power transmission line and an electricity accumulation system like the flywheel, and operation tests for these technologies have been in progress nowadays. Most of superconductors with a high superconduction transition temperature which can be worthy of practical use  include copper as a constituent element. However, the high temperature superconduction mechanism has not been understood completely yet. In order to get new knowledge for the high temperature superconduction mechanism, we have tried to discover new superconduction oxide withough copper. In our previous works, we performed systematic investigation on electrical conductivities of alkaline-earth niobium oxides, especially Sr_1-xM_xNbO_3-δ (M = Nd, Ce), Sr_2-xM_xNbO_4-δ (M = Ce, La), Sr_3-xM_xNb_2-yM'_yO_7-δ (M = La, Ce; M’ = V), CaNbO_3-δ, Ca₂NbO_4-δ and Ca₃Nb₂O_7-δ, but these oxides did not exhit superconductivity.

From such background, we paid attention to Sr_3-xCa_xNb₂O_7-δnewly in this study, and prepared the samples with various Ca-substitution amounts. For these samples, we examined properties and composition dependence of the conductivity.

EXPERIMENTAL

We prepared NbO₂, which was a starting material for Sr_3-xCa_xNb₂O_7-δ synthesis, by firing mixed powder of Nb and Nb₂O₅ with a molar ratio of 1.1:2 at 1080 ^oC for 36 h under 10^-4 Pa using a small metal vacuum furnace. In addition, we heat-treated SrCO₃ and CaCO₃  at 1000 ^oC, for 24 h in air and then made SrO and CaO. We mixed NbO₂, SrO and CaO in Ar atmosphere with an excess amount of Nb compoud in order to controll Nb valence to around 4. After pellet molding, we sintered the pellet at 1080 ^oC for 36 h under 10^-4 Pa, and then obtained Sr_3-xCa_xNb₂O_7-δ(0≤x≤1.5). We put a Nb small piece as a getter at the sintering.

For these samples, we performed phase identification by powder X-ray diffraction (XRD), metal composition analysis by the ICP emission spectrometry analysis, and calculation of the  mean valence of the Nb by TG-DTA. Their electrical conductivities were also estimated by the resistivity measurement by 4 probe direct current method at the temperature range from room temperature to 4.2 K. In order to clarify the crystal structures,  the Rietveld analysis (Rietan-FP) using synchrotron X-ray diffraction data (BL02B2, SPring-8) was carried out.

RESULTS AND DISCUSSION

XRD measurements confirmed that a main phase of Nb oxide used as a starting material for Sr_3-xCa_xNb₂O_7-δ synthesis was NbO₂, and the mean Nb valence was evaluated as 4.11 from a weight increase due to Nb oxidation measured by TG-DTA. We used this NbO₂ as a raw material and prepared Sr_3-xCa_xNb₂O_7-δ. In this process, 7.5 % excess Nb compound was added. In order to identify the phase of the obtained sample, the XRD pattern was collected. Because a crystal structure of Sr₃Nb₂O₇ is unknown, we assigned the diffraction peaks to the same crystal structure as Sr₃Mo₂O₇¹⁾ where Mo was tetravalent as well as Nb in Sr₃Nb₂O₇ and Mo⁴⁺ has a close ionic radius to Nb⁴⁺. As a result, we confirmed that Sr_3-xCa_xNb₂O₇ could be obtained as a main phase of the products regardless of the Ca content. Because the peak intensity ratio was dependent on the Ca-substituted quantity, the crystal structure was supposed to be changed by the partial substitution of Ca for Sr. In addition, from the analytical metal composition estimated by ICP, it was confirmed that compositions were controlled successfully. TG-DTA measurement demonstrated that the mean Nb valence was controlled to near 4 in Sr_3-xCa_xNb₂O_7-δ (0≤x≤1.5).

As for these samples, we measured resistivity from room temperature to 4.2 K (Fig. 1). From this figure, it was found that the resistivity decreased with increasing the Ca-substituted quantity up to x=0.5. However, the sudden drop of the resistance which is one of the characteristics of a superconductor was not observed, and thus the superconductivity could not be achieved in Sr_3-xCa_xNb₂O_7-δ. In addition, the resistivity behaviors against temperature were changed below 150 K. This result suggests a change of the crystal structure with decreasing temperature.

REFERENCES

1) Shinji Kouno, Naoki Shirakawa, Physica B, 403, 1029 (2008).