Fast Relaxation in a Fragile Liquid under Pressure

Abstract

The incoherent dynamic structure factor of ortho-terphenyl has been measured by neutron timeof-flight and backscattering technique in the pressure range from 0.1 MPa to 240 MPa for temperatures between 301 K and 335 K. Tagged-particle correlations in the compressed liquid decay in two steps. The α-relaxation lineshape is independent of pressure, and the relaxation time proportional to viscosity. A kink in the amplitude fQ(P ) reveals the onset of β relaxation. The β-relaxation regime can be described by the mode-coupling scaling function; amplitudes and time scales allow a consistent determination of the critical pressure Pc(T ). α and β relaxation depend in the same way on the thermodynamic state; close to the mode-coupling cross-over, this dependence can be parametrised by an effective coupling Γ ∝ nT −1/4 . 64.70. Pf, 62.50.+p, 61.25.Em, The glass transition can be induced by decreasing the temperature or by increasing the pressure. For practical reasons, most experimental investigations concentrate on temperature effects at ambient pressure P 0 = 0.1 MPa, although there is a clear interest in studying supercooled liquid dynamics in the full P, T parameter space. Variable pressure measurements have been decisive in demonstrating that density is not the only driving force behind the glass transition [1][2][3]. In the microscopic approach of mode-coupling theory [4], both pressure and temperature control the dynamics through variations of the static structure factor. Close to the dynamic cross-over, the effect of these variations can be expressed through a single separation parameter which however is not given by density alone, as was confirmed by depolarised light scattering in the fragile liquid cumene [3].Here we report on incoherent neutron scattering in the fragile van der Waals liquid ortho-terphenyl (OTP, T g (P 0 ) = 243 K) at variable pressure. As function of temperature, the microscopic dynamics of OTP has been studied extensively by neutron scattering [5][6][7] [5,10] and from direct observation of fast β relaxation [6-9] a cross-over was located at T c (P 0 ) = 290 ± 5K [16].OTP (Aldrich) was purified by repeated crystallisation from liquid methanol and vacuum destillation. An Al7049.T6 high pressure cell of the Institut LaueLangevin (ILL) with a ratio of outer to inner diameter of 2 allowed us to attain up to 250 MPa at temperatures up to 335 K; helium was used to transmit the pressure.The experiments were performed using backscattering (IN16) and time-of-flight (IN6) spectrometers of the ILL. Incident wavelengths 6.27Å −1 and 5.12Å −1 gave resolutions (fwhm) of about 1 µeV and 80 µeV, respectively.Vanadium was used to calibrate the detectors and to yield the resolution functions. The measured transmission of the empty pressure cell at P 0 was about 84%, that of the sample about 88%. Self shielding and multiple scattering turned out be negligible at large scattering angles. This was demonstrated by comparison of spectra taken with and without the pressure cell at 290 K: the normalised intermediate ...

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