Monotropic Transition Mechanism of <i>m</i>‑Hydroxybenzoic Acid Investigated by Temperature-Resolved Second Harmonic Generation

Temperature-resolved second harmonic generation (TR-SHG) and SHG microscopy were used to study under normal pressure the solid–solid transition mechanism occurring between the two monotropically related polymorphic forms (metastable <i>Pna</i>2₁ and stable <i>P</i>2₁/<i>n</i>) of 3-hydroxybenzoic acid (MHBA). The activation energy <i>E</i>_a (as a measure of the barrier energy) of the irreversible transition was determined via isothermal TR-SHG (137–144 kJ·mol^–1). It fits well with that determined from differential scanning calorimetry (139 kJ·mol^–1). Regarding the two crystal structures, optical microscopy observations, and kinetics parameters from TR-SHG, a destructive/reconstructive mechanism is proposed for the solid–solid transition. The present study clearly demonstrates that TR-SHG is a relevant and accurate technique for monitoring solid–solid phase transitions

Hydrothermal Synthesis and Dehydration of CaTeO₃(H₂O): An Original Route to Generate New CaTeO₃ Polymorphs

CaTeO₃(H₂O) was obtained from microwave-assisted hydrothermal synthesis as a polycrystalline sample material. The dehydration reaction was followed by thermal analysis (thermogravimetric/differential scanning calorimetry) and temperature-dependent powder X-ray diffraction and leads to a new δ-CaTeO₃ polymorph. The crystal structures of CaTeO₃(H₂O) and δ-CaTeO₃ were solved ab initio from PXRD data. CaTeO₃(H₂O) is non-centrosymmetric: <i>P</i>2₁<i>cn</i>; <i>Z</i> = 8; <i>a</i> = 14.785 49(4) Å; <i>b</i> = 6.791 94(3) Å; <i>c</i> = 8.062 62(3) Å. This layered structure is related to the ones of MTeO₃(H₂O) (M = Sr, Ba) with layers built of edge-sharing [CaO₆(H₂O)] polyhedra and are capped of each side by [Te^IVO₃E] units. Adjacent layers are stacked along the <i>a</i>-axis and are held together by H-bonds via the water molecules. The dehydration reaction starts above 120 °C. The transformation of CaTeO₃(H₂O) into δ-CaTeO₃ (<i>P</i>2₁<i>ca</i>; <i>Z</i> = 8; <i>a</i> = 13.3647(6) Å; <i>b</i> = 6.5330(3) Å; <i>c</i> = 8.1896(3) Å) results from topotactic process with layer condensation along the <i>a</i>-axis and the 1/2<i>b⃗</i> translation of intermediate layers. Thus, δ-CaTeO₃ stays non-centrosymmetric. The characteristic layers of CaTeO₃(H₂O) are also maintained in δ-CaTeO₃ but held together via van der Waals bonds instead of H-bonds through water molecules. Electron localization function and dipole moment calculations were also performed. For both structures and over each unit cell, the dipole moments are aligned antiparallel with net dipole moments of 3.94 and 0.47 D for CaTeO₃(H₂O) and δ-CaTeO₃, respectively. The temperature-resolved second harmonic generation (TR-SHG) measurements, between 30 and 400 °C, show the decreasing of the SHG intensity response from 0.39 to 0.06 × quartz for CaTeO₃(H₂O) and δ-CaTeO₃, respectively

Enhanced Second Harmonic Generation from an Organic Self-Assembled Eutectic Binary Mixture: A Case Study with 3‑Nitrobenzoic and 3,5-Dinitrobenzoic Acids

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

In Situ Observation of Polymorphic Transition during Crystallization of Organic Compounds Showing Preferential Enrichment By Means Of Temperature-Controlled Video-Microscopy and Time-Resolved X‑ray Powder Diffraction

A solvent-assisted solid-to-solid polymorphic transition, which was assumed to be one of the key processes for the occurrence of preferential enrichment (PE), has been proven to occur by means of temperature-controlled video-microscopy (TCVM) and time-resolved in situ X-ray powder diffraction (XRPD) measurement during crystallization of two typical first-generation chiral racemic compounds (<b>1</b> and <b>2</b>) showing an excellent PE phenomenon