Phonon density of states in Na beta'' alumina

Although lithium batteries now play a central role in our everyday lives, lithium is a scarce and expensive resource, and its future availability is becoming a bottleneck to more widespread adoption of e.g. electric vehicles. Sodium batteries have been proposed as a solution to this problem, but high-performance materials are required to compensate for the higher mass of sodium versus lithium. One of the highest ionic conductivities in any solid-state material is found in Na beta'' alumina Na2MgAl10O17 (BASE), which has a structure consisting of Al2O3 spinel planes separated by conducting planes. Although BASE has been known for decades, the details of the diffusion mechanism have never been verified experimentally. We have recently carried out preliminary QENS experiments using LET and IRIS which show two dynamic components, both of which are potentially consistent with long-range diffusion. To complement these measurements, we now wish to measure the phonon density of states at high and low temperatures using the MARI spectrometer.