Inelastic-neutron-scattering studies on glassy and liquidCa0.4K0.6(NO3)1.4

Triple-axis inelastic-neutron-scattering measurements have been carried out on the ionic glass material Ca0.4 K0.6(NO3)1.4. Our measurements of the temperature dependence of the position of the main elastic diffraction peak in the liquid give a linear coefficient of thermal expansion of (2.3±0.2)× 10−4, which is markedly greater than the value of 1.2× 10−4 from density measurements. This shows that the expansion is accompanied by structural rearrangements. The Debye-Waller factor, as determined from the temperature dependence of the intensity of the main elastic diffraction peak, shows an anomaly at the calorimetric glass transition temperature, Tg=335 K, but the anomaly at the critical temperature Tc predicted by mode-coupling theory is not seen. In contrast, the change in the effective root-mean-square displacement, ⟨r2⟩effT−⟨r2⟩eff300 deduced from the Q dependence of the temperature variation of the quasielastic scattering over a wide range of wave vector Q from 2 to 4 Å−1, shows a break of slope at a temperature that can be identified as Tc, but there is no anomaly at Tg. Our results give Tc=368±5 K. The inelastic scattering for constant Q scans at frquencies ν=ω2π<~0.5 THz deviates from harmonic-phonon-like behavior at temperatures above Tg. This can be identified as a fast relaxation or β process, as predicted by mode-coupling theory. Such a deviation is not observed at higher frequencies. In the present work, we establish the Q and ν dependences of the β process.