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The signs of the second-harmonic-generation (SHG) coefficients have been determined for SiC(d₃₁+,d₃₃-), AgGaS₂(d₃₆+), ZnGeP₂(d₃₆+), PbTiO₃(d₃₁-,d₃₃+), Ba0.5Sr0.5Nb₂O₆(d₃₁-,d₃₃-), and PbNb₄O₁₁(d₃₂-,d₃₃-). Where comparisons between theory and experiment can be made, the agreement is good except for PbTiO₃. In addition, the phases of the SHG coefficients for CdTe, CdSe, and ZnTe have been redetermined in a spectral region where the crystals are transparent and the phases were found to agree with expectations-all d's positive except d₃₁ for CdSe.
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Miller et al. (1972) studied this question.
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