2D−3D Transformation of Layered Perovskites through Metathesis:  Synthesis of New Quadruple Perovskites A₂La₂CuTi₃O₁₂ (A = Sr, Ca)

We describe the synthesis of two new quadruple perovskites, Sr₂La₂CuTi₃O₁₂ (I) and Ca₂La₂CuTi₃O₁₂ (II), by solid-state metathesis reaction between K₂La₂Ti₃O₁₀ and A₂CuO₂Cl₂ (A = Sr, Ca). I is formed at 920 °C/12 h, and II, at 750 °C/24 h. Both the oxides crystallize in a tetragonal (P4/mmm) quadruple perovskite structure (a = 3.9098(2) and c = 15.794(1) Å for I; a = 3.8729(5) and c = 15.689(2) Å for II). We have determined the structures of I and II by Rietveld refinement of powder XRD data. The structure consists of perovskite-like octahedral CuO_4/2O_2/2 sheets alternating with triple octahedral Ti₃O_18/2 sheets along the c-direction. The refinement shows La/A disorder but no Cu/Ti disorder in the structure. The new cuprates show low magnetization (0.0065 μ_B for I and 0.0033 μ_B for II) suggesting that the Cu(II) spins are in an antiferromagnetically ordered state. Both I and II transform at high temperatures to 3D perovskites where La/Sr and Cu/Ti are disordered, suggesting that I and II are metastable phases having been formed in the low-temperature metathesis reaction. Interestingly, the reaction between K₂La₂Ti₃O₁₀ and Ca₂CuO₂Cl₂ follows a different route at 650 °C, K₂La₂Ti₃O₁₀ + Ca₂CuO₂Cl₂ → CaLa₂Ti₃O₁₀ + CaCuO₂ + 2KCl, revealing multiple reaction pathways for metathesis reactions.