The heat capacity of crystalline 4,4"-difluoro-p-terphenyl
was measured from 8 K to 303 K by adiabatic calorimetry. The anomaly
with a peak at (127.05 +- 0.05) K due to a structural phase transition
was detected between 100 and 160 K. The enthalpy and the entropy of
transition were determined as (180 +- 20) J mol-1 and
(1.41 +- 0.15) J K-1mol-1, respectively. The
properties of the transition are compared with those of p-terphenyl.
The possibility is pointed out that a transition in the present compound
lies in the "transition" region from order-disorder to a displacive
type of structural phase transition.
(Solid State Commun., 87, 903-906(1993))
The heat capacity of crystalline 4,4'''-difluoro-p-quaterphenyl
was measured from 8 K to 303 K by adiabatic calorimetry. A broad anomaly
was detected between 140 and 250 K and attributed to a structural phase
transition associated with molecular twisting. The temperature, the
enthalpy and the entropy of transition were detemined as (200.5 +- 0.5) K,
(322 +- 20) J mol-1 and (1.65 +- 0.10) J
K-1mol-1, respectively. The properties of
the transition are compared with those of unsubstituted and
fluorine-substituted p-polyphenyls (of two to four benzene rings
in a molecule) in the light of a unified model of displacive and
order-disorder types of structural phase transition.
(Solid State Commun., 92, 495-499(1994))
See also Twist transition in p-Poly(phenylene) Polymer