2D-3D transformation of layered perovskites through metathesis: synthesis of new quadruple perovskites A<SUB>2</SUB>La<SUB>2</SUB>CuTi<SUB>3</SUB>O<SUB>12</SUB> (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&#176;C/12 h, and II, at 750 &#176;C/24 h. Both the oxides crystallize in a tetragonal (P4/mmm) quadruple perovskite structure (a = 3.9098(2) and c = 15.794(1) &#197; for I; a = 3.8729(5) and c = 15.689(2) &#197; 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 CuO4/2O2/2 sheets alternating with triple octahedral Ti3O18/2 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 &#956;B for I and 0.0033 &#956;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&#176;C, K2La2Ti3O10 + Ca2CuO2Cl2 &#8594; CaLa2Ti3O10 + CaCuO2 + 2KCl, revealing multiple reaction pathways for metathesis reactions