Neutron pair distribution function analysis of cation order in B-site deficient twinned hexagonal perovskite Ba8ZnTa6O24

Twinned hexagonal perovskite Ba 8 ZnTa 6 O 24 exhibits partially B-cation order but exceptionally low microwave dielectric loss, surpassing those of its fully ordered shifted hexagonal perovskite counterparts, on which the underlying structural origin has not been fully understood. This study employs neutron pair distribution function (nPDF) analysis combined with reverse Monte Carlo (RMC) modeling to elucidate the local B-site cation and vacancy ordering in Ba 8 ZnTa 6 O 24 . Refinements against nPDF data, utilizing both small unit cell and large supercell (8 ×8 ×4) models, consistently confirm that corner-sharing octahedral (CSO) sites are exclusively fully occupied by Ta 5 + cations and face-sharing octahedral (FSO) sites are partially occupied by both Zn 2+ and Ta 5+ cations, accommodating the B-site vacancies. The large supercell RMC simulations confirm the specific B-site atomic and vacancy arrangement avoiding strong electrostatic repulsion by preventing high-charge Ta 5+ -Ta 5+ pairs in the FSO layers and establishes charge neutrality over short ranges, which consequently could account for the observed ultra-low dielectric loss. This work provides atomic-scale cation-ordering insights into the low microwave loss of twinned hexagonal perovskites, offering guidance for designing high-performance microwave dielectrics.