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 (Ps) 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.