Organic-inorganic compounds with strong nonlinear optical properties based on 2,4,6-trimethylpyridinium and tetrahedral BF4−networks

Abstract

A different organic-inorganic crystal---[2,4,6-trimethylpyridinium][BF${}_{4}$]---of nonlinear optical properties at room temperature was synthesized and characterized. The compound is built up of the organic [2,4,6-trimethylpyridinium] cations incorporated into inorganic, tetrahedral BF${}_{4}$ anions. It crystallizes at room temperature in the polar space group $\mathit{Pmn}{2}_{1}$, and undergoes three first-order phase transitions at [cooling (heating)] 241 (245) K, 297 (328) K, and 389 (406) K. The lowest temperature ferroic phase transition (ferroelastic; $\mathit{mm}2\ensuremath{\rightarrow}m$ type) is related to the significant pyroelectric effect. The compound was studied by single-crystal x-ray diffraction at several temperatures, using thermal (differential scanning calorimetry and thermogravimetric analysis) methods and dielectric spectroscopy. The piezoelectric, pyroelectric, and second-harmonic generation (SHG) properties were determined. Density-functional theory calculations in two stable phases are given. The [2,4,6-trimethylpyridinium][BF${}_{4}$] crystal exhibits a SHG efficiency of 1.7 times that of KDP. The mechanism of structural phase transitions in the title compound is discussed.