Detailed structural analysis has been undertaken with the use of neutron diffraction on a variety of Li ionic conductors as demonstrated by studies of Li argyrodites. Using a combination of simulations and structural refinements against X-ray and NPD data, the structure of the Li-ion conducting argyrodites, Li6PS5X where X = Cl, Br, and I, were determined [151]. The Li content and location, along with the mixed occupancies of the X and Li or X and S sites was modelled. For X = Cl and Br, Cl or Br was found on two S sites, whilst I was found on an independent
wholly I-containing site. For X = Cl only one Li site was found, while for X = Br and I two Li sites were revealed with a distribution of Li that differs depending on X. This highlighted that the halide plays a critical role in the distribution of atoms, including the location and occupancy of Li. Ionic conductivity was found to be highest for the X = Br samples, suggesting the influence Br has on the atomic distribution to be favourable for this property [151].

These materials were also investigated using variable-temperature neutron dif­fraction, where the starting reagents were reacted under similar conditions to those used in the laboratory synthesis. This was to determine the optimal reaction tem­peratures and conditions for favourable properties and whether any intermediate highly-conducting phases were present. The notion was to explore whether syn­thesis temperatures could be lowered, potentially reducing manufacturing costs, or whether intermediate phases were able to provide superior ionic-conduction prop­erties. Analysis of neutron diffraction data showed that argyrodite formation begins at relatively low temperatures around 100 °C, well below the reported synthesis temperature of 550 °C, but at temperatures around 550 °C the reagents become amorphous or nano-crystalline with all reflections from the sample disappear (Fig. 7.17). Notably, on cooling the desired phase re-condenses and on inspection it is found that the anion ordering, leading to the most conductive phase, is actually found in the re-condensed phase rather than the initially-formed phase [152]. These types of systematic studies on bulk formation shed light on which phases and synthetic routines may provide the best ionic conduction.

Structurally disordered solid-state electrolytes have been investigated using INS. Work [153] exploring low-energy vibrational dynamics of the 11B2O3-7Li2O

Highly conductive phase formation

§m loss of long ■; range order

Anion ordering

Phase formation

20 (degrees)

Fig. 7.17 Collated NPD patterns of a heating and cooling sequence applied to Li6PS5Br. Although an argyrodite phase forms at relatively low temperatures, it is found to be less conducting than the phase formed after the loss of long-range order
system showed a boson peak between 2 and 10 meV. It was found that with increasing Li content the intensity and position of the boson peak changed, sug­gesting the presence of intermediate glass structures. More importantly, this information can be used to generate a master curve, which suggests a universal distribution of vibrational density-of-states that is composition independent, even though the structure changes markedly. Furthermore, increasing the Li content in these glasses results in chemical structure-induced densification, as fewer low- density B-containing groups are found. It is argued that the densification may arise from the same microscopic origin as the boson peak.