First hydrothermally synthesized polar lithium niobium 1M mica germanate–silicate KNb (Li0.9Nb0.02□0.08)2[(Ge0.4Si0.6)4O10]O2 with disorder and isomorphism, and topology symmetry analysis of the polytype family

Abstract

New niobium lithium 1M mica germanate–silicate KNb(Li_0.9Nb_0.02□_0.08)₂[(Ge_0.4Si_0.6)₄O₁₀]O₂ was synthesized hydrothermally at temperature 280–290 °C and a pressure 80–100 atm. It crystallises in the polar C2 space group. The simultaneous inclusion of lithium and niobium was achieved for the first time in the structure, and the presence of octahedral lithium was confirmed by Raman spectroscopy. The trioctahedral central layer possesses one octahedron fully occupied by Nb and two octahedra statistically occupied predominantly by Li with a small Nb impurity and vacancy. Ge–Si isomorphic substitution was revealed in tetrahedra, which made it possible to implement the inclusion of large Nb in the structure compared to conventional micas. The presence of disordered octahedral positions and isomorphism indicates structural defects overall. Between 1M, 2M, 2O, and 3T polytypes in the mica family, topology-symmetry analysis allowed us to find the origin of existence of the 1M or 2M (2O) polytypes based on the symmetry of heteropolyhedral octahedra and tetrahedra packets and symmetry operation of their multiplication in the structures. Under Nd:YAG laser illumination, powders of polar Nb–mica demonstrate second harmonic generation (SHG) with output as high as 20 compared to α-quartz powder but only 0.1 to LiNbO₃. Despite lower Nb concentration in lithium niobium 1M mica germanate–silicate, the structure–property relation here is similar to LiNbO₃, being determined by shortened bonds in the distorted Nb–O octahedron directed along the polar axis.