The refractive indices of structurally well-characterized crystals of the new optically nonlinear organic crystal (S)-3-methyl-5-nitro-N-(1-phenylethyl)-2-pyridinamine have been measured from 480 to 660 nm and fitted to four-parameter Sellmeier equations with an accuracy of ±3 × 10-3 to ±5 × 10-3. The value of n y was accurately verified by the measurement of the external angle of incidence required, at the (101) plane, to observe type I phase matching with a fundamental wavelength of 1064 nm. The phase matching locus has been established and is of Hobden class eight. The values of the n x , n y , and n z refractive indices at 532 nm were found from the Sellmeier equations to be 1.690, 1.901, and 1.725, respectively, and their relative magnitudes are correlated with the molecular arrangement in the crystal lattice. This material is a member of a series of pyridinamines that can exhibit the phenomenon of frequency-dependent orientation of their dielectric axes. Since this compound crystallizes in an orthorhombic structure, rotation of the dielectric axes is precluded for symmetry reasons.

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The heat capacity of crystalline zinc tris (thiourea) sulphate, has been measured in the range from 220 to 500 K by differential scanning calorimetry, and was found to obey the relationship Cp(T)=2.76×10−3 T+0.366 J g−1 K−1. Thermal expansion data have been measured in the range from 150 to 473 K. From these data, the principal thermal expansion coefficients were found to be α1=6.41×10−5 K−1, α2=4.52×10−5 K−1, and α3=−4.32×10−6 K−1. The thermal conductivity tensor of this orthorhombic crystal was calculated from values of the thermal diffusivity in the directions normal to the (100), (010), and (001) crystal planes by the laser flash method. The thermal conductivity coefficients at 295 K are k1=0.27 W m−1 K−1, k2=0.34 W m−1 K−1, and k3=0.54 W m−1 K−1.

The heat capacity of crystalline N-methylurea has been measured in the range from 223–303 K by differential scanning calorimetry, and was found to obey the relationship Cp(T)=(5.44±0.04) ×10−3 T−(0.02821±0.0016) J g−1 K−1. Thermal expansion data have also been measured in the range from 100–353 K, allowing the temperature dependent density to be calculated. From these data, the principal thermal expansion coefficients were found to vary linearly, e.g., over the range from 100–250 K. In this range α1=−(5.84±0.42)×10−5 K−1 and α2 and α3 were found to be (1.31±0.14)×10−4 K−1 and (1.04±0.10)×10−4 K−1, respectively. The thermal conductivity tensor of this orthorhombic crystal was then calculated from values of the thermal diffusivity in the directions normal to the (100), (010), and (001) planes determined by the laser flash method. The thermal conductivity coefficients at 295 K were found to be k1=0.47 W m−1 K−1, k2=0.26 W m−1 K−1, and k3=0.30 W m−1 K−1.

In addition to possessing strong optical nonlinearity, the (-)-2-(a-methylbenzylamino)-5-nitropyridine (MBANP) crystal has been shown to have unusual linear optical properties. Two methyl derivatives of MBANP have now been synthesized and grown as large single crystals; their refractive indices have been measured as a function of frequency and second-harmonic generation, induding the determination of the phase matching loci has been investigated. Data is also available on the racemic form of crystalline MBANP. The properties of this family of crystals have been reviewed and an attempt is made to relate the experimental responses of the crystals to their molecular structure and the calculated properties of the molecules.