Abstract
Layered Ruddlesden–Popper (RP) oxides provide a versatile platform for topochemical anion modification, enabling access to metastable mixed-anion phases with tuneable structural and electronic properties. In this work, we present a comprehensive investigation of how mixed
B
-site (indium and antimony) chemistry influences topochemical fluorination of Ba
2
In
0.5
Sb
0.5
O
4
using polyvinylidene fluoride (PVDF). Powder X-ray diffraction (PXRD) reveals anisotropic lattice expansion, dominated by elongation perpendicular to the perovskite layers, with minimal change in the in-plane lattice parameters. Fluorination proceeds through a composition-dependent multiphase regime in which several tetragonal (
I
4
/mmm
) RP components with progressively expanded
c
and slightly contracted
a
lattice parameters coexist, representing a distribution of fluorination states, followed by convergence to a single dominant fluorine-rich tetragonal phase at high fluorine content. Coupled X-ray/neutron Rietveld refinement establishes the end member as the anion-saturated oxyfluoride Ba
2
In
0.5
Sb
0.5
O
2
F
4
, and locates the three fully occupied anion sites of the tetragonal
I
4
/mmm
framework. The preferred O/F distribution, with oxide at the equatorial site and fluoride at the apical and interstitial sites, is supported by bond-valence-sums (BVSs) analysis of the refined interatomic distances and the corresponding global instability indices (GIIs), together with charge balance, since O and F cannot be reliably distinguished from their neutron scattering lengths alone. X-ray absorption near-edge structure (XANES) measurements support the retention of the formal In
3+
and Sb
5+
oxidation states upon fluorination.