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 (D_T) 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.