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

Abstract

The elastic properties of para-terphenyl C₁₈H₁₄ in the monoclinic and triclinic phases, as well as its positional isomer ortho-terphenyl in the crystalline and glassy states, were studied with the use of ultrasonic techniques over a wide P–T 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 para-terphenyl both under isothermal compression at 0.34 GPa (77 K) and under isobaric heating (182 K, 0.05 GPa). For ortho-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 ortho-terphenyl have a similar bulk modulus, but the shear modulus of the glass is almost two times lower than that of crystalline ortho-terphenyl.