Further evidence for phase transitions in biphenyl near 40 K and 16 K

Phase transitions have been detected in biphenyl at about 42 and 17 K by noting changes in the transmitted light intensity when the crystal is placed at extinction between crossed polaroids and the crystal temperature slowly varied. The phase transition at 42 K is gradual (i.e. it occurs over a range of temperature) while the 17 K transition is abrupt. The effect is observed only for light propagating normal to an a*b section which implies that the atomic displacements at the phase changes are largely restricted to this plane. An analogous behaviour is observed for biphenyl-d₁₀ except that the onset of the gradual transition on cooling is at 38 K and the abrupt transition is at 24 K.