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

Abstract

A(II)GeTeO 6 (A = Mn, Cd, Pb), new non-centrosymmetric (NCS) honeycomb-layered tellurates, were synthesized and characterized. A(II)GeTeO 6 (A = Mn, Cd, Pb) crystallize in trigonal space group P312 (No. 149) of edge-sharing Ge 4+ O 6 and Te 6+ O 6 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 PbSb 2 O 6 -related, and the ordering of Ge 4+ and Te 6+ in octahedral environment breaks the inversion symmetry of the parent PbSb 2 O 6 structure. The size of A(II) cation in six coordination is an important factor to stabilize PbSb 2 O 6 -based structure. Temperaturedependent optical second harmonic generation measurements on A(II)GeTeO 6 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 KH 2 PO 4 for PbGeTeO 6 and CdGeTeO 6 , respectively. Magnetic measurements of MnGeTeO 6 indicate anti-ferromagnetic order at T N ≈ 9.4 K with Weiss temperature of −22.47 K.

PbMnTeO6: a chiral quasi 2D magnet with all cations in octahedral coordination and the space group problem of trigonal layered A2+M4+TeO6

“…Its phase purity is illustrated by the XRD pattern (Fig. S1 of the ESI), its refined lattice parameters are in excellent agreement with the initial report 6 and differ significantly from the most recent single-crystal data 7 (Table 1). Uniaxial hot pressing yielded 95% dense ceramic discs with preferred orientation of the three-fold axis normal to the disc (Fig.…”

“…7 It is, however, unnatural that lower preparation temperature in the presence of flux results in cation disorder 7 whereas the high-temperature product is cationordered. 6 The polar displacement of Pb 2+ , in contrast to Sr 2+ , might be due to the stereochemically active lone electron pair of Pb 2+ but the same displacements of Mn 4+ and Te 6+ seem strange. Moreover, such small static displacements cannot be proved definitely by diffraction methods alone due to strong correlation with thermal displacement parameters.…”

“…The large difference in lattice parameters might be due to the experimental errors but might also reflect the real difference in the structure and/ or composition because the solidstate preparation of non-polar PbGeTeO6 samples was made at 700 °C (ref. 6) or 730 °C (in this work, see the ESI) whereas the P31m crystals were reportedly grown using the TeO2 flux at 600 °C and below. 7 It is, however, unnatural that lower preparation temperature in the presence of flux results in cation disorder 7 whereas the high-temperature product is cationordered.…”

“…1,2 Besides these geometrical constraints, a very high crystal field stabilization energy for d 3 cations in an octahedral environment is another argument against the prismatic coordination for Mn 4+ . The adopted structural model for AMnTeO6 (A = Pb and Sr) is especially strange compared with AGeTeO6 (A = Sr 5 and Pb 6 ), having essentially the same unit cell sizes (Table 1) but refined in the space group P312 with all cations in octahedral coordination.…”

Crystal Structure and Properties of a UV‐Transparent High‐Pressure Polymorph of Mg3TeO6 with Second Harmonic Generation Response

“…Furthermore, alkali sulfate tellurates: (Na 2 (SO 4 ))(Te(OH) 6 ), CsK(SO 4 )Te(OH) 6 , (Rb 1.12 (NH 4 ) 0.88 (SO 4 ))(Te(OH) 6 ), the alkali selenate tellurate: K 2 (SeO 4 )(Te(OH) 6 ), and the tellurate K 3 Na 2 LiTeO 6 were reported to crystallize acentric. Also in the case of transition metal tellurates: Ni 3 TeO 6 , Ni 2 CoTeO 6 , Ni 2.5 M 0.5 TeO 6 ( M = Mg, Mn, Cu), and a huge number of mixed tellurates: Li 2 M TeO 6 ( M = Zr 4+ , Hf 4+ or Ge 4+ ), Li 2– x M O 6 ( M = Zr 4+ , Nb 5+ or Te 6+ ; x ≤ 0.5) 2 ,[20a] A GeTeO 6 ( A = Mn 2+ , Cd 2+ , Pb 2+ , Ba 2+ or Sr 2+ ),[20a], LiMn 2 TeO 6 often with corundum related non‐centrosymmetric structure types are known. Only a few of these compounds like KIO 3 · Te(OH) 6, Ni 3 TeO 6 , Ni 2 CoTeO 6 , Ni 2.5 M 0.5 TeO 6 ( M = Mg, Mn, Cu) and A GeTeO 6 ( A = Mn 2+ , Cd 2+ or Pb 2+ )[20a], were examined for their NLO properties, whereby the maximum SHG signal of 1.2 × KH 2 PO 4 (KDP) was measured for KIO 3 · Te(OH) 6 and Ni 2.5 Mg 0.5 TeO 6 …”