Ion Transport Properties of the Inverse Perovskite BaLiF3 Prepared by High-Energy Ball Milling

For many applications such as advanced energy storage systems, chemical sensors or electrochromic devices fast ionic conduction plays an important role. In many cases, see e.g., Refs. [1-3], nanocrystal-line ceramics with crystallite diameters smaller than 50 nm show an enhanced ion conductivity compared to their coarse-grained counterparts, i.e., single crystals or materials with µm-sized particles. This observation can be explained by their large fraction of structurally disordered interfacial regions providing fast migration pathways for the ions [2,3]. Mechanical treatment of µm-sized particles in a high-energy ball mill represents a simple technique to obtain large quantities of a nanocrystalline material. In addition to that it is also possible to synthesize ceramics directly by ball milling [3]. In many cases this leads to metastable, non-equilibrium compounds which cannot be prepared following conventional synthesis methods [4,5]. In the present paper, BaLiF 3 , the only known inverted perovskite of the AMF 3 compounds [6], was prepared by high-energy ball milling using a Fritsch P7 planetary mill (premium line). An equimolar mixture of LiF and BaF 2 with an overall mass of 2 g was treated in a 45 mL zirconia vial with 140 balls of the same material for 3 h at a rotational speed of 600 rpm. As verified by X-ray diffraction highly pure BaLiF 3 is obtained which shows an average crystallite size d of about 30 nm. d was estimated with the equation introduced by Scherrer [7]. This is in contrast to a sample which was synthesized via conventional solid state synthesis showing µm-sized crystallites. The latter was prepared by heating an equimolar mixture of LiF and BaF 2 for two hours at 620 K. After that the sample was fired at 1020 K for 5 hours under nitrogen atmosphere. Ionic conductivities were measured by impedance spectroscopy using an HP 4192A analyzer (5 Hz to 1 MHz) in combination with a home-built cell. The dc-conductivities were obtained from the frequency independent plateau of the corresponding impedance spectra (not shown for the sake of brevity). In Fig. 1 σ dc T vs the inverse temperature 1/T of the two samples is shown in semi-logarithmic representation. As can be clearly seen, at T = 550 K, for example, σ dc of the mechanosynthesized product is by approximately three orders of magnitude