Structural, Electronic, And Optical Properties Of Crystal Haloform Under High Pressure: First-principles Studies

Haloforms belong to the simplest halogen substituted methanes, which are made up of molecular units of the type CHX₃, where X = F, C1, Br or I. The molecules of Haloforms are C_3V symmetric, pyramidal, and polar (dipole moments in the gas phase: 3.37×10^-30 and 3.30×10^-30 C·m, respectively). As early as in 1981, David Y. Curtin and Ain C. Paul pointed out that polar molecules of organic matter are of particular interest in chemistry, physics, and materials science. The presence of polar molecules in crystals is an essential requirement for materials to have technological and industrial applications for nonlinear optics, optoelectronic transducers, ferro-, piezo-, and pyroelectric materials, actuators, and other applications. Iodorm is polar symmetry naturally, although the bromoform and chloroform is not polar symmetry under ambient pressure, but they changed to polar symmetry structure under high pressure.Haloforms also are ideal system for studying the nature of hydrogen bond and halogen bond. Hydrogen bond and halogen bond are non-bond week interaction, but they have important role in the crystal engineering, medical design, supermolecular chemisty, and attract more and more researching interest. In the last decade, more attention has been shifted to investigating weak intermolecular interactions such as hydrogen bonds (A-H…B) and halogen bond (halogen…halogen). It is demonstrated that these interactions play an important role in the formation of crystal structures, crystal packing, protein folding, and biological materials. Several studies have reported the significant effect of pressure on enhancing molecular aggregates and controlling the strength of C-H…X (X=F, Cl, Br, and I) and halogen…halogen interactions.At ambient pressure, the studies on the temperature dependence of bromoform (CHBr3) by neutron powder diffraction revealed three different crystalline phases, denoted asα,β, andγphase.Theαphase exists in the span from 281 to 268 K, which is dynamically disordered with their principal axes randomly parallel or anti-parallel to each other. The average symmetry is represented by the space group P63/m. Theβphase was observed between 268 K and 14 K. The structure ofβphase is centrosymmetric triclinic with space group P1|- , and contains two anti-parallel molecules in the unit cell. Theγphase exists in range from 200 K to 14 K. The structure is now described by centrosymmetric trigonal Bravais lattice with space group P 3|-, and contains two anti-parallel molecules in the unit cell. On the other hand, theγphase transforms irreversibly into theβphase by annealing above 200 K. In 1984, high pressure phase transitions at room temperature have been investigated for bromoform by Raman scatter experiment by H. Shimizu and K. Matsumoto. Theαphase transforms to theβphase at 1.0 GPa, and further transition into theγphase at 4.2 GPa. It is also noticed that theβphase andγphase coexist over an extended pressure range from 3.3 to 5.5 GPa. In 1986, Y. Zhao et al also carried out a investigation for bromoform by Raman scatter experiment at room temperature. They observed that theαphase transforms to theβphase at 0.8 GPa, and then transforms into theγphase at 5.15 GPa. Recently, a new high pressure phaseδ-CHBr₃ with polar symmetry (space group P6₃) is observed between 0.20 and 0.35 GPa by single-crystal X-ray diffraction at 295 K. This phase transition is attributed to intensifying of C-H…halogen, halogen…halogen interactions under high pressure.We observed a new structure of bromoform by performing a series of Forcite molecular dynamic calculation and density function theory (DFT) geometry optimization. This new structure is monoclinic Bravais lattices with CC (we call itεphase), which contains four molecular in its unit cell and characterizes polar symmetry. We calculated the enthalpies difference of all phases with pressure, phonon dispersion and elastic constants ofεphase at 100, 200 and 300 GPa, which demonstrate that the εphase is stable in the pressure range from 90 GPa to 300 GPa. It has been demonstrated that the electrostatic contribution to Br···Br and C–H···Br interactions in theεphase are favorable for the polar aggregation and these effects intensify with increasing pressure. The equation of state (EOS) for theεphase is determined by fitting the pressure as a function of volume to Murnaghan equation of state where V0 is the equilibrium volume at zero pressure, B0 is the bulk modulus at zero pressure, and B0 ' is the derivative of B0 with respect to pressure. On the basis of these results, V0, B0 and B0 'were calculated to be 218.56±8.58 A³, 29.63±4.48 GPa, and 3.08±0.04, respectively. On the other hands, our DFT calculations correctly describe the order from theβtoγphase transition observed experimentally, but with a different transition pressure. It is reported that theδphase naturally polar symmetry is observed at room temperature and 0.2-0.35 GPa.The calculated phonon dispersion curves and elastic constants ofδphase at 5 GPa indicate that this structure is dynamically and mechanically stable. However, its enthalpy is not the lowest one among all structures observed currently. Thus, theδ-CHBr₃ is a metastable phase at low temperature.The calculated energy band structures and partial density of state of bromoform indicate that the bromoform is an insulator at ambient pressure. The bands are flat, indicating that the solid bromoform is a typical molecular. It is also characterized by the coexistence of strong intramolecular and weak intermolecular bondings, implying that the electrons strongly locate at the intramolecular space. The van der Waals force here is mainly the dispersion forces originated by the instant molecular dipole interactions. From the calculated energy band structures, theγ-CHBr₃ exists as an insulator in the pressure range from 1 to 90 GPa, and theε-CHBr₃ begins to metalize at 130 GPa, which was caused by increasing of hybridization among C s, p and Br s, p states. There is no evidence of metallization in theγ-CHBr₃, implying the strong localization of electrons as well as the weaker attractive Br…Br interactions due to the hybridization way of carbon atoms is sp3.Iodoform (CHI₃) forms hexagonal ordered crystal (space group P63) at ambient pressure and room temperature according to the X-ray and neutron diffraction studies. This crystal has two molecules per unit cell and can be considered as built of layers of molecules perpendicular to [001] as shown in Fig. 1(a). Earlier studies of iodoform including Raman, infrared, and electrical measurements at high pressure and low temperature indicate no phase transition up to 10 GPa.Iodoform is a good polar molecules material with permanent dipole moments and rigid molecular arrangements. It is a nonlinear optical material and has been reported as a self-processing system for holographic recording with near-infrared sensitivity. The steady-state photoconductivity iodoform single crystal exhibits high photogeneration and yields interesting photoconductive characteristics. The color change with pressure was observed in the iodoform, which is attributed to the shifts of band edge in the visible spectrum. In addition, it has large thermal expansion coefficients and high compressibility. The study about the high-pressure behavior of the polar molecular crystal iodoform not only is important for physics science, but also provides information on hydrogenous structure.At the ambient pressure,α-CHI₃ phase is an insulator with indirect band gap of 2.5 eV. Energy band structures are flat, indicating the solidα-CHI₃ is a typical molecular crystal. It is also characterized by the coexistence of strong intramolecular and weak intermolecular bondings implying that the electrons are strongly located at the intramolecular space. When the pressure is increased to 32 GPa, the bands ofα-CHI₃ start to overlap and begin to metalize. The metallization pressure shows an attractive decrease with increasment of halogen radius. This trend is well correlated with both the electronegativity of halogen and band gap of halogen at ambient pressure. The electronic negativity of I atom is weaker than Br atom, and according to the earlier studies on the variation of the nature of homohalogen interactions, the I…I is the stronger interhalogen bonding than Br…Br within the same distance of shorter than sum of Van de Waals radii. The band gap of CHI₃ is 2.5 eV, which is lower than that of 3.55 eV for CHBr₃ at ambient pressure. We observed a new structure of iodoform by performing a series of Forcite molecular dynamic calculation and density function theory (DFT) geometry optimizations. This new structure is monoclinic Bravais lattices with Cc (we call itβ-CHI₃ phase), which contains four molecular in its unit cell and also characterizes polar symmetry. We calculated the enthalpies difference ofα-CHI₃ andβ-CHI₃ with pressure, phonon dispersion and elastic constants ofεphase at 41 and 150 GPa, and we find that theβ-CHI₃ phase is stable in the pressure range from 40.1 GPa to 150 GPa, which was the largest pressure we considered. It has been demonstrated that the formation ofβ-CHI₃ was attributed to the intensifying of I…I and C–H…I interactions with increasing pressure. The results of optical properties calculations for iodoform are in agreement with the experimental data to some extent. Further studies show that the absorption spectra of iodoform become broad with pressure.