Synthesis, Structure, and Characterization of New Li+ – d0 –Lone-Pair – Oxides: Noncentrosymmetric Polar Li6(Mo2O5)3(SeO3)6 and Centrosymmetric Li2(MO3)(TeO3) (M = Mo6+ or W6+)

New quaternary lithium – d0 cation – lone-pair oxides, Li6(Mo2O5)3(SeO3)6 (Pmn21) and Li2(MO3)­(TeO3) (P21/n) (M = Mo6+ or W6+), have been synthesized and characterized. The former is noncentrosymmetric and polar, whereas the latter is centrosymmetric. Their crystal structures exhibit zigzag anionic layers composed of distorted MO6 and asymmetric AO3 (A = Se4+ or Te4+) polyhedra. The anionic layers stack along a 2-fold screw axis and are separated by Li+ cations. Powder SHG measurements on Li6(Mo2O5)3(SeO3)6 using 1064 nm radiation reveal a SHG efficiency of approximately 170 × α-SiO2. Particle size vs SHG efficiency measurements indicate Li6(Mo2O5)3(SeO3)6 is type 1 nonphase-matchable. Converse piezoelectric measurements result in a d33 value of ∼28 pm/V and pyroelectric measurements reveal a pyroelectric coefficient of −0.43 μC/m2K at 50 °C for Li6(Mo2O5)3(SeO3)6. Frequency-dependent polarization measurements confirm that Li6(Mo2O5)3(SeO3)6 is nonferroelectric, i.e., the macroscopic polarization is not reversible, or ‘switchable’. Infrared, UV–vis, thermogravimetric, and differential thermal analysis measurements and electron localization function calculations were also done for all materials.