Composition Dependences of Property and Conductivity in (Sr,Ca)3Nb2O7-δ
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 Sr1-xMxNbO3-δ (M = Nd, Ce), Sr2-xMxNbO4-δ (M = Ce, La), Sr3-xMxNb2-yM'yO7-δ (M = La, Ce; M’ = V), CaNbO3-δ, Ca2NbO4-δ and Ca3Nb2O7-δ, but these oxides did not exhit superconductivity.
From such background, we paid attention to Sr3-xCaxNb2O7-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.
We prepared NbO2, which was a starting material for Sr3-xCaxNb2O7-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 Sr3-xCaxNb2O7-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 Sr3-xCaxNb2O7-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 Sr3-xCaxNb2O7-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 Sr3-xCaxNb2O7 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 Sr3-xCaxNb2O7-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 Sr3-xCaxNb2O7-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.