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, $Sr_2La_2CuTi_3O_{12} (I)$ and $Ca_2La_2CuTi_3O_{12} (II),$ by solidstate metathesis reaction between $K_2La_2Ti_3O_{10}$ and $A_2CuO_2Cl_2 (A = Sr, Ca). I$ 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) A for I; a ) 3.8729(5) and c ) 15.689(2) A 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 $CuO_{4/2}O_{2/2}$ sheets alternating with triple octahedral $Ti_3O_{18/2}$ 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 $(0.0065_{\mu{B}}$ for I and $0.0033_{\mu{B}}$ 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 $K_2- La2_Ti_3O_{10}$ and $Ca_2CuO_2Cl_2$ follows a different route at 650 °C, $K_2La_2Ti_3O_{10} + Ca_2CuO_2Cl_2 \rightarrow CaLa_2Ti_3O_{10} + CaCuO_2 + 2KCl,$ revealing multiple reaction pathways for metathesis reactions.