This work illustrates the use of
powder second harmonic generation
(powder SHG), temperature-resolved second harmonic generation (TR-SHG),
and second harmonic generation microscopy (SHGM) in monophasic and
multiphasic sample studies. The commercial powder of 3,5-dinitrobenzoic
acid was found to exhibit a significant second harmonic generation
signal, whereas only two centrosymmetric polymorphic forms have been
reported for this compound. Second harmonic generation techniques
were used in combination with chromatography, differential scanning
calorimetry, and powder X-ray diffraction to show that the SHG activity
of 3,5-dinitrobenzoic acid powder originates from a chemical impurity
(3-nitrobenzoic acid) present in the commercial powder under the form
of a new metastable noncentrosymmetric polymorph. The metastable equilibria
between 3,5-dinitrobenzoic acid and 3-nitrobenzoic acid were studied,
and SHG analyses performed on crystallized binary mixtures showed
significant enhancements of the SHG signal compared to that of the
pure noncentrosymmetric phase. This is due to the formation of a suitable
eutectic microstructure that enables quasi phase matching (QPM). In
particular, powders from near-eutectic compositions exhibit SHG signals
up to 20 times higher than that of the powder containing pure 3-nitrobenzoic
acid noncentrosymmetric phase. This observation could provide the
basis for a new route to achieve new, efficient materials for second-order
frequency conversion.