Ultrasonic study of terphenyls under high pressure: 2nd order phase transition in <i>para</i> -terphenyl and glass transition in <i>ortho</i> -terphenyl

The elastic properties of <i>para</i>-terphenyl C₁₈H₁₄ in the monoclinic and triclinic phases, as well as its positional isomer <i>ortho</i>-terphenyl in the crystalline and glassy states, were studied with the use of ultrasonic techniques over a wide <i>P</i>-<i>T</i> region (temperatures 77-300 K, pressures up to 1 GPa). We observed experimentally a second-order phase transition between the monoclinic and triclinic structures of <i>para</i>-terphenyl both under isothermal compression at 0.34 GPa (77 K) and under isobaric heating (182 K, 0.05 GPa). For <i>ortho</i>-terphenyl, we compared the elastic properties of the crystalline and glassy states at low temperatures (77 K) up to 1 GPa and also observed a glass-supercooled liquid-crystal transition with increasing temperature in the range of 77-300 K. Comparison of the elastic properties of these modifications showed that glassy and crystalline <i>ortho</i>-terphenyl have a similar bulk modulus, but the shear modulus of the glass is almost two times lower than that of crystalline <i>ortho</i>-terphenyl.