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

Crystalline 2,5-di(4-nitrophenyl)-1,3,4-oxadiazole (DNO) has been investiga ted at pressures up to 5 GPa using Raman and optical spectroscopy as well a s energy dispersive X-ray techniques. At ambient pressure DNO shows an orth orhombic 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 spectroscopi c investigations three phase transitions have been detected at 0.88, 1.28, and 2.2 GPa, respectively. These transitions have also been confirmed by ab sorption spectroscopy and X-ray measurements. Molecular modeling simulation s have considerably contributed to the interpretation of the X-ray diffract ograms. In general, the nearly hat structure of the oxadiazole molecule is preserved during the transitions. All subsequent structures are characteriz ed by a stack-like arrangement of the DNO molecules. Only the mutual positi on of these molecular stacks changes due to the transformations so that thi s process may be described as a topotactical reaction. Phases II and III sh ow a monoclinic symmetry with space group P2(1)/c with cell parameters a = 1.990 nm, b = 0.500 nm, c = 1.240 nm, beta = 91.7 degrees, 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, beta = 89.0 degrees, density 1.733 g cm(-3) (phase III, determi ned at 2.0 GPa), respectively. The high-pressure phase IV stable at least u p to 5 GPa shows again an orthorhombic structure with space group Peen 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 sh ift 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 ons et of absorption shifts to increased wavelengths due to larger intermolecul ar interactions and enhanced excitation delocalization with decreasing inte rmolecular spacing. (C) 1999 Elsevier Science Ltd. All rights reserved.