High-temperature crystal structure and transport properties of the layered cuprates Ln2CuO4, Ln=Pr, Nd and Sm

High-temperature crystal structure of the layered cuprates Ln2CuO4, Ln=Pr, Nd and Sm with tetragonal T′-structure was refined using X-ray powder diffraction data. Substantial anisotropy of the thermal expansion behavior was observed in their crystal structures with thermal expansion coefficients (TEC) along a- and c-axis changing from TEC(a)/TEC(c)≈1.37 (Pr) to 0.89 (Nd) and 0.72 (Sm). Temperature dependence of the interatomic distances in Ln2CuO4 shows significantly lower expansion rate of the chemical bond between Pr and oxygen atoms (O1) belonging to CuO2-planes (TEC(Pr–O1)=11.7 ppm K−1) in comparison with other cuprates: TEC (Nd–O1)=15.2 ppm K−1 and TEC (Sm–O1)=15.1 ppm K−1. High-temperature electrical conductivity of Pr2CuO4 is the highest one in the whole studied temperature range (298–1173 K): 0.1–108 S/cm for Pr2CuO4, 0.07–23 S/cm for Nd2CuO4 and 2×10−4–9 S/cm for Sm2CuO4. The trace diffusion coefficient (DT) of oxygen for Pr2CuO4 determined by isotopic exchange depth profile (IEDP) technique using secondary ion mass spectrometry (SIMS) varies in the range 7.2×10−13 cm2/s (973 K) and 3.8×10−10 cm2/s (1173 K) which are in between those observed for the manganese and cobalt-based perovskites.