Knowledge Bank

Phase transitions have been detected in biphenyl at about $42\circ K$ and $17\circ K$ by noting changes in the transmitted light intensity when the crystal is placed, at extinction, between crossed polarizers and the crystal temperature slowly varied. The phase transition at $42\circ K$ is gradual (i.e. it occurs over 3 range of temperature) while the $17\circ K$ transition is abrupt. The effect is observed only for light propagating normal to an $a\ast b$ section which implies that the atomic displacements at the phase transitions are largely restricted to this plane. An analogous behavior is observed for bipheny $1-d10$ except that the onset of the gradual transition on cooling is $38\circ K$ and the abrupt transition is at $24\circ K$. Further evidence for the $17\circ K$ phase transition has been obtained through a study of the temperature variation of the two-photon absorption origin linewidth.