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 (<b>I</b>) and Ca₂La₂CuTi₃O₁₂ (<b>II</b>), by solid-state metathesis reaction between K₂La₂Ti₃O₁₀ and A₂CuO₂Cl₂ (A = Sr, Ca). <b>I</b> is formed at 920 °C/12 h, and <b>II</b>, at 750 °C/24 h. Both the oxides crystallize in a tetragonal (<i>P</i>4/<i>mmm</i>) quadruple perovskite structure (<i>a</i> = 3.9098(2) and <i>c</i> = 15.794(1) Å for <b>I</b>; <i>a</i> = 3.8729(5) and <i>c</i> = 15.689(2) Å for <b>II</b>). We have determined the structures of <b>I </b>and <b>II</b> 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 <i>c</i>-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 <b>I</b> and 0.0033 μ_B for <b>II</b>) suggesting that the Cu(II) spins are in an antiferromagnetically ordered state. Both <b>I</b> and <b>II </b>transform at high temperatures to 3D perovskites where La/Sr and Cu/Ti are disordered, suggesting that <b>I</b> and <b>II</b> 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.