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 <i>P</i>312 (No. 149) of edge-sharing Ge⁴⁺O₆ and Te⁶⁺O₆ octahedra, which form honeycomb-like-layers in the <i>ab</i>-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 <i>T</i>_N ≈ 9.4 K with Weiss temperature of −22.47 K