Access to metastable complex ion conductors viamechanosynthesis: preparation, microstructure and conductivity of (Ba,Sr)LiF3 with inverse perovskite structure

Abstract

Highly metastable Ba_1−xSr_xLiF₃ (0 < x ≤ x_max ≈ 0.4) with an inverse perovskite structure analogous to that of BaLiF₃ was synthesized by soft mechanical treatment of BaF₂ and LiF together with SrF₂ at ambient temperature. Ex as well as in situX-ray powder diffraction (XRPD) measurements show that heat treatment at 393 K initiates the decomposition of the mixed phase into BaLiF₃, LiF and (Sr,Ba)F₂. Structural details of the metastable compound (Ba,Sr)LiF₃ were investigated by ultrafast ¹⁹F magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. Interestingly, five magnetically inequivalent F sites were identified which correspond to fluorine anions coordinated by a variable number of Ba and Sr cations, respectively. Details from XRPD and NMR spectroscopy are discussed with respect to the formation mechanisms and thermal stability of the as prepared fluorides. Impedance spectroscopy is used to characterize (long-range) ionic transport properties. Results are compared with those obtained recently on mechanosynthesized BaLiF₃.