Defect‐enhanced F^– ion conductivity in layer‐structured nanocrystalline BaSnF₄ prepared by high‐energy ball milling combined with soft annealing

Abstract

Fast ion conductors play one of the most important roles in solid state ionics as there is a great demand for their application in safe and powerful electrochemical energy storage systems. For such materials, it is known that the synthesis conditions may have significant impact on the final properties of the materials prepared. In this contribution, we made use of mechanosynthesis, carried out via high‐energy ball milling, to influence the ionic transport parameters of tetragonal, i.e., layer‐structured, BaSnF4. X‐ray powder diffraction (XRD) revealed that mechanical treatment of the binary fluorides BaF2 and SnF2 leads to a powder pointing to a nanocrystalline fluoride with (distorted) cubic symmetry. Differential scanning calorimetry (DSC) as well as preliminary in situ XRD measurements were used to follow the transformation towards the tetragonal modification with the composition BaSnF4. Broadband impedance spectroscopy was used to measure the overall electrical conductivity of the ternary fluoride. Remarkably, the layered form shows a room temperature conductivity of 7 × 10–4 S cm–1. Further emphasis was put on the characterization of the dielectric properties of the material, which was investigated by using different electrode materials to distinguish artefacts from intrinsic properties. Since we found a strong dependence of the real part of the permittivity on the electrode materials applied (carbon paste or sputtered Pt), we tend to assign the huge increase in permittivity, which was recently interpreted as giant dielectric constant, to interfacial polarization effects rather than to intrinsic properties. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Mismatch in cation size causes rapid anion dynamics in solid electrolytes: the role of the Arrhenius pre-factor

“…The introduction of defects, that is, in a more general sense describable with structural disorder, represents a powerful instrument with which we can control the chemical and physical properties of a material. [12][13][14][15][16][17] Ion dynamics sensitively depend on local arrangements. The present work is aimed at describing the interrelations between local disorder and short-range ion dynamics using a suitable model substance that may also be of interest as a fast ion conducting solid electrolyte.…”

Structure and ion dynamics of mechanosynthesized oxides and fluorides

“…Joint milling of BaF 2 and MgF 2 or SnF 2 leads to the formation of the ternary compounds BaSnF 4 and BaMgF 4 [18,19,55]. While the first shows a layered structure, BaMgF 4 crystallizes in the orthorhombic Cmc2 1 structure and consists of strands of corner-sharing MgF 6 -octahedra that are separated by the Ba-ions [56].…”

“…During milling BaF 2 and SnF 2 are partly converted into amorphous products; moreover, X-ray diffraction [61] indicates the formation of a metastable Sn-rich BaF 2 phase with cubic symmetry that is transformed into tetragonal BaSnF 4 without any by-phases expected for some nonreacted BaF 2 (see Figure 15a and b), cf. [18].…”

“…The fact that the mixture treated for several hours in a planetary mill easily transforms into tetragonal BaSnF 4 could serve as a strong argument that Ba and Sn cations in fact form a metastable, highly disordered compound. When annealed at moderate temperatures, defects may partly be preserved facilitating ion transport in nanocrystalline, defect-rich BaSnF 4 against the coarsegrained counterpart [18] that is available via high-temperature ceramic synthesis routes. The conductivity of the tetragonal modification of BaSnF 4 is much higher than that of the "cubic" form (see Figure 15c).…”

“…Hence, the product obtained is characterized by the preserved properties belonging to the corresponding high-temperature or high-pressure phases, respectively. High-energy ball milling is well suited to prepare inorganic ceramic materials such as sulfides, oxides and fluorides [10][11][12][13][14][15][16][17][18][19][20][21]. The variation of the milling conditions, that is, milling time, rotation speed, vial sets, number and kind of balls (size, material), offers a rich playground of possibilities to find the best route to prepare yet unknown materials.…”