Syntheses, Structures, Second-Harmonic Generating, and Ferroelectric Properties of Tungsten Bronzes:  A<sub>6</sub>M<sub>2</sub>M‘<sub>8</sub>O<sub>30</sub> (A = Sr<sup>2+</sup>, Ba<sup>2+</sup>, or Pb<sup>2+</sup>; M = Ti<sup>4+</sup>, Zr<sup>4+</sup>, or Hf<sup>4+</sup>; M‘ = Nb<sup>5+</sup> or Ta<sup>5+</sup>) ChiEun Ok GandiniAlberto OkKang Min ZhangLei HalasyamaniP. Shiv 2004 Nine tungsten bronze oxides, A<sub>6</sub>M<sub>2</sub>M‘<sub>8</sub>O<sub>30</sub> (A = Sr<sup>2+</sup>, Ba<sup>2+</sup>, or Pb<sup>2+</sup>; M = Ti<sup>4+</sup>, Zr<sup>4+</sup>, or Hf<sup>4+</sup>; M‘ = Nb<sup>5+</sup> or Ta<sup>5+</sup>), have been synthesized and characterized. Although structurally very similar, the nine materials are not isostructural. The materials crystallize in noncentrosymmetric space groups, <i>P</i>4<i>bm</i> (no. 100) for Ba<sub>6</sub>Ti<sub>2</sub>Nb<sub>8</sub>O<sub>30</sub>, Ba<sub>6</sub>Ti<sub>2</sub>Ta<sub>8</sub>O<sub>30</sub>, Ba<sub>6</sub>Zr<sub>2</sub>Nb<sub>8</sub>O<sub>30</sub>, Ba<sub>6</sub>Zr<sub>2</sub>Ta<sub>8</sub>O<sub>30</sub>, Ba<sub>6</sub>Hf<sub>2</sub>Nb<sub>8</sub>O<sub>30</sub>, and Ba<sub>6</sub>Hf<sub>2</sub>Ta<sub>8</sub>O<sub>30</sub>; <i>Pba</i>2 (no. 52) for Sr<sub>6</sub>Ti<sub>2</sub>Nb<sub>8</sub>O<sub>30</sub> and Sr<sub>6</sub>Ti<sub>2</sub>Ta<sub>8</sub>O<sub>30</sub>; and <i>Cm</i>2<i>m</i> (no. 38) for Pb<sub>6</sub>Ti<sub>2</sub>Nb<sub>8</sub>O<sub>30</sub>. 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<sub>2</sub>. The reported materials are also ferroelectric, as demonstrated by hysteresis loops (polarization vs electric field). Spontaneous polarization values (<i>P</i><sub>s</sub>) range from 0.79 to 12.1 μC/cm<sup>2</sup>. The magnitude of the SHG efficiency and ferroelectric polarization are strongly dependent on the A, M, and M‘ cations.