Complex dynamics in low electron density lithium ammonia solutions. Interacting modes revealed by comparing neutron and x-ray inelastic scattering

We present an experimental study of the collective dynamics of lithium-ammonia solutions in order to investigate the role played by the metallic character of the liquid on the atomic dynamics and the relevance of almost free ammonia. To understand the intrinsically complex dynamical behavior due to the presence of clusters composed of at least four ammonia molecules coordinated by a Li atom, we measured the neutron dynamic structure factor in an extended region of wave-vector transfer Q, i.e., from a value as low as 0.2 Å−1 up to 2.4 Å−1, going beyond the limit of twice the Fermi wave vector 2kF 0.98 Å−1. This allowed us to probe the system with wavelengths between ≈3 Å (comparable with the ammonia size) and ≈30 Å, a range ensuring a good sensitivity to the collective atomic motions, where the metallic character is known to play a major role. Three different Li concentrations (0.10, 0.17, and 0.20 molar fraction) have been analyzed at 220 K and, in the case of the intermediate concentration, the behavior at 155 K (still in the liquid phase) has also been explored. The effect of the metallic nature, dominated by the electron response when Q < 2kF , is confirmed down to 0.1 molar fraction but, thanks to the availability of better data and the synergy enabled by the simultaneous analysis of neutron and previous x-ray data, we were able to detect the presence of a second excitation affecting the global dynamics of the system. This second mode shows, with respect to the acoustic collective mode, an increasing energy-integrated intensity as Q grows, i.e., in a range where the data obviously better reveal the local (short length scale) atomic behavior. Finally, by comparing the results of the mentioned analysis with those obtained recently by techniques specially focused on the intrinsic metallic properties, like photoemission spectroscopy, it has been possible to correlate the dynamics with the presence of metallic clusters, at almost constant electron density, embedded into free fluid ammonia. The present results also allow for a coherent interpretation of the elastic behavior of lithium ammonia solutions, as determined by ultrasound velocity data in a rather wide concentration range, and the identification of the different concentration regions where agreement with the interacting electron gas paramagnetic response and velocity of collective mode is obtained. Whether the dispersion curve anomaly at ≈2kF (observed down to 0.1 molar fraction also for the second mode) is simply related to the dielectric response or some other mechanism may also contribute to this phenomenon, remains an open question.