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, A 6 M 2 M‘ 8 O 30 (A = Sr 2+, Ba 2+, or Pb 2+; M = Ti 4+, Zr 4+, or Hf 4+; M‘ = Nb 5+ or Ta 5+ ), have been synthesized and characterized. Although structurally very similar, the nine materials are not isostructural. The materials crystallize in noncentrosymmetric space groups, P 4 bm (no. 100) for Ba 6 Ti 2 Nb 8 O 30, Ba 6 Ti 2 Ta 8 O 30, Ba 6 Zr 2 Nb 8 O 30, Ba 6 Zr 2 Ta 8 O 30, Ba 6 Hf 2 Nb 8 O 30, and Ba 6 Hf 2 Ta 8 O 30; Pba 2 (no. 52) for Sr 6 Ti 2 Nb 8 O 30 and Sr 6 Ti 2 Ta 8 O 30; and Cm 2 m (no. 38) for Pb 6 Ti 2 Nb 8 O 30 . 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 × SiO 2 . 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/cm 2 . The magnitude of the SHG efficiency and ferroelectric polarization are strongly dependent on the A, M, and M‘ cations.