Organic-inorganic compounds with strong nonlinear optical properties based on 2,4,6-trimethylpyridinium and tetrahedral BF${}_4$${}^{-}$ networks

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; $\mathit{mm}2\to m$ 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.

Figure 1

Orientation of the sample of $[\mathrm{TMP}][{\mathrm{BF}}_4]$.

Figure 2

DSC curves for $[\mathrm{TMP}][{\mathrm{BF}}_4]$.

Figure 3

Simultaneous thermogravimetric analysis (TGA) and differential thermal analysis (DTA) scans for $[\mathrm{TMP}][{\mathrm{BF}}_4]$. The scans were performed in flowing nitrogen with a ramp rate of 5 K/min (selected single crystals, atmosphere: nitrogen; flow rate 1 dm${}^3/$h${}^{-1}$).

Figure 4

Thermal evolution of lattice parameters in $[\mathrm{TMP}][{\mathrm{BF}}_4]$on cooling. The lattice parameters are numbered using the cell choice of phase III.

Figure 5

Powder diagrams of $[\mathrm{TMP}][{\mathrm{BF}}_4]$ at high temperatures.

Figure 6

Structure transformation in PT at 241 K from orthorhombic phase III to low-temperature monoclinic structure, phase IV. The 2,4,6-trimethylpyridinium cation and [BF${}_4$]${}^{-}$ are elemental motifs building the crystal structure. Red arrows stand for the dipole moments.

Figure 7

Packing of the crystal structure in the low-temperature phase IV (monoclinic, $\mathit{Pn}$), $T=205$ K. Dashed lines represent hydrogen bonds.

Figure 8

Electromechanical coupling coefficient $k$ (left-hand side) and the reciprocal component $s$${}_{22}$ of elastic compliance tensor (right-hand side) of the longitudal normal vibration vs temperature in the vicinity of the III$\leftrightarrow$IV phase transition.

Figure 9

The electromechanical coupling coefficient $k$ (left-hand side) and the component $c_{66}^{\text{'}}$ of elastic stiffness tensor (right-hand side) of the face-shear vibration vs temperature in the vicinity of the III$\leftrightarrow$IV phase transition.

Figure 10

The pyroelectric current and the change of the $P_s$ measured along [1 0 $\overline{1}$] for cooling and heating scans in the vicinity of the III$\leftrightarrow$IV phase transition.

Figure 11

The pyroelectric current and the change of the $P_s$ measured along [1 0 $\overline{1}$] on heating in the high-temperature region.

Figure 12

Particle-size dependence of the SHG intensity for $[\mathrm{TMP}][{\mathrm{BF}}_4]$ crystal (${\lambda }_f=1064$ nm). The $y$ scale is normalized to the SHG intensity of KDP powder with 160–180 $\mu$m grain size.

Figure 13

Free energy for LT (phase IV) and HT (phase III) phases calculated at PBE-$D$* level.

Figure 14

Linear thermal expansion of the $[\mathrm{TMP}][{\mathrm{BF}}_4]$crystal along the [1 0 1] and [1 0 $\overline{1}$] directions upon cooling and heating.

Figure 15

The change of intensity of selected Bragg peaks in $[\mathrm{TMP}][{\mathrm{BF}}_4]$ in phase IV to III transition (on heating).

Figure 16

The real (a) and imaginary (b) part of the complex electric permittivity of $[\mathrm{TMP}][{\mathrm{BF}}_4]$ measured along the [1 0 $\overline{1}$] direction in the vicinity of the III$\leftrightarrow$IV PT; (c) the electric permittivity measured along the [0 1 0] direction; (d) Cole-Cole plots at selected temperatures; (e) the Arrhenius relation ($\ln \tau$ vs 1000/$T$).

Figure 17

The real (a) and imaginary (b) part of the complex electric permittivity of $[\mathrm{TMP}][{\mathrm{BF}}_4]$ measured along the [1 0 $\overline{1}$] direction in the high-temperature region on cooling; (c) Cole-Cole plots at selected temperatures; (d) the Arrhenius relation ($\ln \tau$ vs 1000/$T$).

Figure 18

The ferroelastic domain structure of $[\mathrm{TMP}][{\mathrm{BF}}_4]$ developed in phase IV along the [0 1 0] direction.