Crystal structure of In-based cuprates: (In,Cu)S 2YCu 2O 6+ δ (1212), (In,Cu)(Sr,Ho) 2(Ho,Ce 4+) 2Cu 2O 8+ δ (1222)

Abstract

The indium-based cuprates with perovskite-like structure (In,Cu)Sr 2YCu 2O 6+ δ and (In,Cu)(Sr,Ho) 2(Ho,Ce) 2Cu 2O 8+ δ , belonging to the homologous series (In,Cu)(Sr,Ln) 2(Ln,Sr) k−1 Cu k O 2 k+2+ δ with k=1 (In,Cu-1212) and (In,Cu)(Sr,Ln) 2(Ln,Ce) k+1 Cu 2O 2 k+6+ δ with k=1 (In,Cu-1222) have been obtained. Their structure has been refined at room temperature using X-ray powder diffraction data. The samples with the nominal compositions (In 0.5Cu 0.5)Sr 2YCu 2O x and (In 2/3Cu 1/3)(Ho 1/3Ce 2/3) 3Sr 2Cu 2O x appeared to contain phases with lattice parameters a=3.8163(1), c=11.9785(6) Å (In,Cu-1212, sp. gr. P4/mmm) and a=3.8272(0), c=29.0774(3) Å (In,Cu-1222, sp. gr. I4/mmm), respectively. Rietveld refinements were carried out. The real composition of these phases may be written as (In 0.89Cu 0.11(3))Sr 2YCu 2O 6.98(4) (In,Cu-1212) and (In 0.98Cu 0.02(1))(Sr 1.25Ho 0.75(2))(Ho 1.89Ce 0.11(5) 4+)Cu 2.00O 9.00 (In,Cu-1222). The refined composition of these phases after oxygen treatment was formal to be (In 0.82Cu 0.18(2))Sr 2YCu 2O 7.00 (In, Cu ∗ -1212) and (In 0.87Cu 0.13(1))(Sr 1.52Ho 0.48(2))(Ho 1.95Ce 0.05(3) 4+)Cu 2O 9.00 (In,Cu ∗ -1222). The Cu formal charge (FC Cu) in the (In,Cu)-1212, calculated from electroneutrality condition of the refined phase composition, was almost the same as before (FC Cu∼2.04) and after (FC Cu∼2.09) the oxygen treatment (underdoped phase). On the contrary, the FC Cu for the (In,Cu)-1222 increased after oxygen annealing (from −2.08 to −2.30) (overdoped phase). The non-optimal values of the Cu formal charge could be a possible reason for the absence of superconductivity in these samples.