ИСТИНА

Introduction Recently layered cuprates have attracted attention as materials for high-temperature electrochemical de-vices like cathode materials in solid oxide fuel cells (SOFC) due to high total electrical and oxide-ion con-ductivities and thermal expansion coefficients (TECs) matching with GDC electrolyte [1-3]. In the present work we report on the study of high-temperature crystal structure, thermal expansion behavior and electrical conductivity properties of Pr-contained layered cuprates Pr2-xSrxCuO4 having T’ (x=0.0), T* (x=0.4) and T (x=1.0) structures (see Fig. 1). Experiments Samples of Pr2-xSrxCuO4-δ, x=0.0, 0.4 and 1.0 were prepared by annealing stoichiometric amounts CuO, SrCO3 and Pr6O11 at 1000-1100°C for 20h in air. High-temperature X-ray powder diffraction (HT XRPD) data were collected in air using Bruker D8-Advance diffrac-tometer. Netzsch 402C dilatometer operated in air (25–900oC, 10oC/min) was used for the thermal expansion coefficient measurements. High-temperature electrical conductivity of the ceramic samples was measured at 25–900oC and p(O2)=0.21-1.6•10-4 atm. The 18O pene-tration profiles for dense ceramic samples of Pr2-xSrxCuO4-δ, x=0.0 and 0.4 were determined on a TOF-SIMS 5 instrument operated in depth profile mode. Results and Discussion In comparison with Pr2CuO4 (TEC=11.8 ppm K-1), Pr2-xSrxCuO4-δ, x=0.4 and 1.0 expand non-linearly due to loss of oxygen with TEC 14.9 (150-500°C); 17.3 (500-1000°C) and 14.1 (150-500°C); 13.7 (600-1000°C), respectively. HT XRPD study revealed substantial ani-sotropy of thermal expansion along a- and c-axis of their crystal structure. However, in comparison with x=0.4 and 1.0 (TEC(a)/TEC(c)≈0.65 at 25-500оC), Pr2CuO4 exhibits larger expansion along a-axis (TEC(a)/TEC(c)≈1.37) due to strong interactions be-tween Pr and O atoms from the CuO2 layer. Decrease of TEC for x=1.0 in comparison with x=0.4 observed at t>600oC is discussed in connection with their high-temperature crystal structures. Electrical conductivity measurements revealed that Pr2CuO4 exhibits highest conductivity among studied compounds of ~100 S/cm in air at 900оC. Dependences of the electrical conductivities of x=0.4 and 1.0 on oxy-gen partial pressure are discussed. Calculated from the analysis of TOF-SIMS data values of activation energies for oxygen diffusion (D*) and surface exchange coefficients (k) for Pr2CuO4 are much larger in comparison with Pr1.6Sr0.4CuO4: 290±28 kJ/mol (D*), 197±28 kJ/mol (k) and 175±15 kJ/mol (D*), 85±3 kJ/mol (k), respectively. D* values for Pr1.6Sr0.4CuO4 are between those observed for La2CuO4 and Pr2CuO4. This result is discussed in connection with substantial compressed fluorite block responsible for oxide-ion conductivity in T’-structure in comparison with rock-salt blocks present in T- and T*-structures. Conclusions Pr2CuO4, although showing low diffusion coeffi-cient, can be considered as a perspective cathode mate-rial for IT-SOFC due to high electrical conductivity and TEC matching with GDC electrolyte. Current study shows that the presence of rock-salt block in the crystal structure of layered cuprates is required for their high oxide-ion conductivity. Acknowledgments This work was partially supported by State con-tracts 14.740.12.1358 and 14.740.11.0033, RFBR (No. 11-08-01159a and 11-03-01225) and MSU-development Program up to 2020. References 1. M.S. Kaluzhskikh et al. J. Solid State Chem. 184 (2011) 698–704 2. N.V. Lyskov et al., Int. J.Hydrogen Energy 37 (2012) 18357-18364 3. E. Boehm et al. Solid State Sci. 5 (2003) 973–981