In Search of Inorganic Nonlinear Optical Materials for Second Harmonic Generation

In a search for inorganic oxide materials showing second-order nonlinear optical (NLO) susceptibility, we investigated several borates, silicates, and a phosphate containing trans-connected MO6 octahedral chains or MO5 square pyramids, where M=d0: Ti(IV), Nb(V), or Ta(V). Our investigations identified two new NLO structures: batisite, Na2Ba(TiO)2Si4O12, containing trans-connected TiO6 octahedral chains, and fresnoite, Ba2TiOSi2O7, containing square-pyramidal TiO5. Investigation of two other materials containing square-pyramidal TiO5, viz., Cs2TiOP2O7 and Na4Ti2Si8O22·4H2O, revealed that isolated TiO5 square pyramids alone do not cause a second harmonic generation (SHG) response; rather, the orientation of TiO5 units to produce –Ti–O–Ti–O– chains with alternating long and short Ti–O distances in the fresnoite structure is most likely the origin of a strong SHG response in fresnoite.