Structural distortions leading to potential polar materials in n \(=\) 3 and 4 Dion–Jacobson-layered perovskite series

This study describes the synthesis and comprehensive characterization of solid solutions in the n \(=\) 3 and 4 members of the Dion–Jacobson (DJ) family of layered perovskites, \(\text{A}^\prime{\text{Ca}}_{2-x}\)Cd_xNb₃O₁₀ and A′Ca_2-xCd_xNaNb₄O₁₃ (A′ \(=\) Rb, Cs) (0 ≤ x ≤ 2). In the n \(=\) 3 series, both Rb and Cs oxides exhibit a transition from centrosymmetric (CS) towards non-centrosymmetric (NCS) polar structures (S.G. Ima2, a_O ~ 29.7 Å, b_O ~ 5.39 Å, c_O ~ 5.43 Å) from x \(=\) 1 resembling x \(=\) 2 phases, A′Cd₂Nb₃O₁₀ (A′ \(=\) Rb, Cs). In the n \(=\) 4 series, the transitions to acentric polar oxides occur for the Rb analogs (x ≥ 1.75). The polar RbCd₂NaNb₄O₁₃ (n \(=\) 4) was confirmed through positive second harmonic generation and Le Bail refinements of the PXRD data. Structures of the Cs analogs and the rest of solid solution members including that for Rb adopt a CS tetragonal symmetry (S.G. P4/mmm, a_T ~ 3.9 Å; c_T ~ 18.9 Å), as demonstrated by the Rietveld refinements. Raman spectroscopy corroborates the local distortions of NbO₆ units. Estimation of the optical band gaps and valence and conduction bands energy levels illustrate their utilization for potential photocatalytic applications.

Graphical abstract

The colour wheel reveals the transition from centrosymmetric (CS) to non-centrosymmetric (NCS) structures based on the Cd content in the Dion-Jacobson (n \(=\) 3, 4) layered perovskite oxides, \(\text{A}^\prime{\text{Ca}}_{2-x}\)CdxNan-3NbnO3n+1 (A′ \(=\) Rb, Cs; 0 ≤ x ≤ 2.0), forming a significant group of potential functional materials.