Syntheses, Structures, Second-Harmonic Generating, and Ferroelectric Properties of Tungsten Bronzes:  A₆M₂M‘₈O₃₀ (A = Sr²⁺, Ba²⁺, or Pb²⁺; M = Ti⁴⁺, Zr⁴⁺, or Hf⁴⁺; M‘ = Nb⁵⁺ or Ta⁵⁺)

Nine tungsten bronze oxides, A₆M₂M‘₈O₃₀ (A = Sr²⁺, Ba²⁺, or Pb²⁺; M = Ti⁴⁺, Zr⁴⁺, or Hf⁴⁺; M‘ = Nb⁵⁺ or Ta⁵⁺), have been synthesized and characterized. Although structurally very similar, the nine materials are not isostructural. The materials crystallize in noncentrosymmetric space groups, P4bm (no. 100) for Ba₆Ti₂Nb₈O₃₀, Ba₆Ti₂Ta₈O₃₀, Ba₆Zr₂Nb₈O₃₀, Ba₆Zr₂Ta₈O₃₀, Ba₆Hf₂Nb₈O₃₀, and Ba₆Hf₂Ta₈O₃₀; Pba2 (no. 52) for Sr₆Ti₂Nb₈O₃₀ and Sr₆Ti₂Ta₈O₃₀; and Cm2m (no. 38) for Pb₆Ti₂Nb₈O₃₀. All of the compounds exhibit second-order nonlinear optical behavior, i.e., second-harmonic generation (SHG), by frequency doubling 1064 nm radiation to 532 nm. The SHG efficiencies range from 10 to 600 × SiO₂. The reported materials are also ferroelectric, as demonstrated by hysteresis loops (polarization vs electric field). Spontaneous polarization values (P_s) range from 0.79 to 12.1 μC/cm². The magnitude of the SHG efficiency and ferroelectric polarization are strongly dependent on the A, M, and M‘ cations.