Phase transitions in 1,3,4-oxadiazole crystals under high pressure

• Published in: The Journal of physics and chemistry of solids 1999-12, Vol.60 (12), p.1949-1965
• Main Authors: Orgzall, I., Lorenz, B., Mikat, J., Reck, G., Knochenhauer, G., Schulz, B.
• Format: Article
• Language: English
• Subjects: A. Organic compounds; C. High-pressure; C. Raman spectroscopy; C. X-ray diffraction; Condensed matter: structure, mechanical and thermal properties; Crystalline state (including molecular motions in solids); Crystallographic aspects of phase transformations; pressure effects; D. Phase transitions; Equations of state, phase equilibria, and phase transitions; Exact sciences and technology; Physics; Solid-solid transitions; Specific phase transitions; Structure of solids and liquids; crystallography
• ISSN: 0022-3697; 1879-2553
• DOI: 10.1016/S0022-3697(99)00204-8

Crystalline 2,5-di(4-nitrophenyl)-1,3,4-oxadiazole (DNO) has been investigated at pressures up to 5 GPa using Raman and optical spectroscopy as well as energy dispersive X-ray techniques. At ambient pressure DNO shows an orthorhombic unit cell ( a=0.5448 nm, b=1.2758 nm, c=1.9720 nm, density 1.513 g cm −3) with an appropriate space group Pbcn. From Raman spectroscopic investigations three phase transitions have been detected at 0.88, 1.28, and 2.2 GPa, respectively. These transitions have also been confirmed by absorption spectroscopy and X-ray measurements. Molecular modeling simulations have considerably contributed to the interpretation of the X-ray diffractograms. In general, the nearly flat structure of the oxadiazole molecule is preserved during the transitions. All subsequent structures are characterized by a stack-like arrangement of the DNO molecules. Only the mutual position of these molecular stacks changes due to the transformations so that this process may be described as a topotactical reaction. Phases II and III show a monoclinic symmetry with space group P2 1/c with cell parameters a=1.990 nm, b=0.500 nm, c=1.240 nm, β=91.7°, density 1.681 g cm −3 (phase II, determined at 1.1 GPa) and a=1.890 nm, b=0.510 nm, c=1.242 nm, β=89.0°, density 1.733 g cm −3 (phase III, determined at 2.0 GPa), respectively. The high-pressure phase IV stable at least up to 5 GPa shows again an orthorhombic structure with space group Pccn with corresponding cell parameters at 2.9 GPa: a=0.465 nm, b=1.920 nm, c=1.230 nm and density 1.857 g cm −3. For the first phase a blue pressure shift of the onset of absorption by about 0.032 eV GPa −1 has been observed that may be explained by pressure influences on the electronic conjugation of the molecule. In the intermediate and high-pressure phases II–IV the onset of absorption shifts to increased wavelengths due to larger intermolecular interactions and enhanced excitation delocalization with decreasing intermolecular spacing.