Rietveld analysis of the diffraction data was performed using TOPAS V6 29 using a fundamental parameters approach as described in literature , with the instrumental broadening being derived from a reference scan on a NIST standard of LaB6 (660a). The pattern corresponding to the solid electrolyte phase is difficult to refine (asymmetric peak shapes originating from inhomogeneous distribution of Ba/Sn within the particles, requiring multiple similar phases to refine the reflection shapes appropriately; more information see paragraph 1-3 in chapter 3.1) and was learnt from a scan of the pure electrolyte to minimize refinable parameters for the analysis of the complex phase mixtures on initiating the reduction of BiF3. Thus, only one parameter was allowed for the scaling of the electrolyte phases and marginal reflection shifts due to thermal expansion were allowed for the in-situ analysis by applying appropriate constraints on the scaling and lattice parameters of the individual electrolyte phases. Structural models, as reported in literature , were used for the different modifications of BiF3, Bi, and BiOF without adjusting the atomic positions within the refinements, but allowing for the refinement of lattice parameters. The structural model for the new orthorhombic modification o'-BiF3-x was derived as described in section 0. Two Voigt functions were used to account for angular dependent broadening effects due to crystallite size and microstrain in the individual phases; reasonable parameter limits derived from patterns with maximum phase quantities were used in order to stabilize the refinements with reduced phase fractions of the individual phases. The thermal displacement parameters of all atoms of all phases were constrained to be identical to minimize quantification errors.