We report on pressure-induced structural changes in crystalline
oligo(para-phenylenes) containing two to six phenyl rings. The results are
discussed with particular emphasis put on the implications these changes in
intermolecular distances and molecular arrangement have on important
bulk properties of this class of materials, such as optical response and
charge transport. We performed energy dispersive x-ray diffraction in a
systematic study on polycrystalline powders of biphenyl, para-terphenyl,
p-quaterphenyl,
p-quinquephenyl
and p-sexiphenyl
under hydrostatic pressure up to 60 kbar. Revisiting the crystal structures at
ambient conditions reveals details in the packing principle. A linear relationship
between the density at ambient conditions and the number of phenyl rings is
found. High pressure data not only yields pressure-dependent lattice parameters
and hints towards pressure-induced changes in the molecular arrangement but
also allows for an analysis of the equations of state of these substances as a
function of oligomer length. We report the previously unknown bulk modulus of
p-quaterphenyl,
p-quinquephenyl
and p-sexiphenyl
(B0 = 83,
93 and 100 kbar, respectively) and its pressure derivative (B0 ′ = 6.4,
7.5 and 5.6). A linear dependence of the bulk modulus on the inverse number of
phenyl rings in the molecules and on their ambient conditions density is found.
The crystal structure of the low-symmetry phase Sm{V) was studied by synchrotron x-ray di8'raction under isothermal compression at room temperature up to 77 GPa {volume fraction=0. 45). Measured lattice spacings and difFraction intensities for Sm{U) in the pressure range beyond the phase mixing show that this phase can be identi5ed as a prototype structure with 3 atoms in the hexagonal unit cell. The occurrence of this low-symmetry structure gives strong evidence for a rapid increase of 4f-electron bonding in Sm in the pressure range above 37 GPa.
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