A(II)GeTeO₆ (A = Mn, Cd, Pb): Non-Centrosymmetric Layered Tellurates with PbSb₂O₆-Related Structure

A(II)GeTeO₆ (A = Mn, Cd, Pb), new non-centrosymmetric (NCS) honeycomb-layered tellurates, were synthesized and characterized. A(II)GeTeO₆ (A = Mn, Cd, Pb) crystallize in trigonal space group P312 (No. 149) of edge-sharing Ge⁴⁺O₆ and Te⁶⁺O₆ octahedra, which form honeycomb-like-layers in the ab-plane with A(II) (A = Mn, Cd, Pb) cations located between the layers. Their crystal structures are PbSb₂O₆-related, and the ordering of Ge⁴⁺ and Te⁶⁺ in octahedral environment breaks the inversion symmetry of the parent PbSb₂O₆ structure. The size of A(II) cation in six coordination is an important factor to stabilize PbSb₂O₆-based structure. Temperature-dependent optical second harmonic generation measurements on A(II)GeTeO₆ confirmed non-centrosymmetric character in the entire scanned temperature range (0 to 600 °C). The materials exhibit a powder SHG efficiency of ∼0.37 and ∼0.21 times of KH₂PO₄ for PbGeTeO₆ and CdGeTeO₆, respectively. Magnetic measurements of MnGeTeO₆ indicate anti-ferromagnetic order at T_N ≈ 9.4 K with Weiss temperature of -22.47 K.