High-pressure structural properties of naphthalene up to 6 GPa

The elastic and structure behavior of solid naphthalene (C10H8) was studied by in situ synchrotron powder X-ray diffraction up to 5.6 GPa in a diamond anvil cell. Rietveld refinements were applied to obtain atomic coordinates and lattice parameters as a function of pressure. Within the studied pressure range, the intramolecular C-C and C-H distances decrease by 3.5%, whereas intermolecular distances within and between the herringbone layers decrease by 18 and 10%, respectively. Noindication of a symmetry change (space group P2(1)/a) was observed. A markedly anisotropic compression is preserved up to 5.6 GPa, with linear compressibilities beta(a): beta(b): beta(c): beta([200]) similar or equal to 7:5:3:10. Fitting of the P-V experimental data to the Vinet equation of state yields K-0 = 7.9 (3) GPa and K' = 7.5 (3) at V-0 = 361 angstrom(3). At the highest pressure of 5.6 GPa, the unit-cell volume is decreased by 23%. The overall regularity of compression confirms the absence of major structural transformations in naphthalene up to 5.6 GPa. However, a distinct bend of the pressure dependence of the interlayer C center dot center dot center dot C distances observed at 2 GPa suggests a minor structural irregularity, which is apparently associated with spectroscopic anomalies observed previously.