A space-charge model is applied to describe the equilibrium effects of segregation of double-donor oxygen vacancies to grain boundaries in dry and wet acceptor-doped samples of the perovskite oxide BaZrO 3 . The grain boundary core vacancy concentrations and electrostatic potential barriers resulting from different vacancy segregation energies are evaluated. Density-functional calculations on vacancy segregation to the mirrorsymmetric Σ3 (112) [110] tilt grain boundary are also presented. Our results indicate that oxygen vacancy segregation can be responsible for the low grain boundary proton conductivity in BaZrO 3 reported in the literature. 1,2 It has become apparent that boundaries between grains in the material are the prime source of inhibited overall proton conductivity. 3-5Experiments have established that the grain boundary (GB) proton resistivity is an intrinsic effect, not caused by the segregation of secondary phases at the GB.6,7 Two different explanations have instead been put forward 8 , in which the GB effects are believed to originate in either a structural distortion in the GB region 3 , or in the appearance of positively charged GBs, caused by a change in chemical composition and leading to Schottky barriers and the depletion of mobile protonic charge carriers. 6,9,10At present the latter explanation model is the predominating one 6,9,10 , but the details of the GB cores are neither well understood nor sufficiently explored.In this letter we present an investigation concerning the equilibrium segregation of oxygen vacancies to GBs in dry and hydrated acceptor-doped BZO and the electrostatic potential barrier such segregation causes. Densityfunctional theory (DFT) is used to evaluate the energy of vacancy segregation to a Σ3 tilt GB and a space-charge model is applied to describe the equilibrium barrier and core vacancy concentration resulting from different segregation energies. We show that segregation energies on the order that we find can cause potential barriers consistent a) joakim.nyman@chalmers.se b) edit@chalmers.se c) Corresponding author: goran.wahnstrom@chalmers.se with experimental findings on GB proton conductivity in BZO.The system is modeled using the periodic supercell technique and all DFT calculations are performed within the plane-wave approach as implemented in the Vienna ab-initio simulation package (VASP) 11,12 . Electron-ion interactions are described by the projector augmented wave method 13 and a generalized gradient approximation (GGA) exchange-correlation functional due to Perdew and Wang (PW91) 14 is used. More computational details have been reported elsewhere.15 In the present work all calculations are performed non-spin-polarized with a plane-wave cutoff of 400 eV (constant volume) or 520 eV (volume relaxations), and a 4 × 2 × 1 Monkhorst-Pack k-point grid is used to sample the Brillouin zone.Theoretical and experimental work on the currently considered Σ3 (112) [110] tilt GB has previously been performed on strontium titanate 16 and this particular GB is ...