Uniaxial compaction effects on residual microstrain, microstructure, and Co2+ EPR anisotropy in Co-doped and Co/Ca-co-doped ZnO

The effects of ex situ uniaxial hydraulic compaction (after unloading) on residual microstrain, microstructure, and Co 2+ EPR anisotropy are investigated in ZnO:Co and ZnO:Co,Ca prepared by solid-state reaction. X-ray diffraction reveals a composition-dependent lattice response to compaction, with a slight decrease in unit-cell volume in ZnO:Co and a slight increase in ZnO:Co,Ca, consistent with heterogeneous residual-strain fields. Compaction reduces the coherent-domain size and increases microstrain in both materials (crystallite size from 129(7) to 78(2) nm for Zn 0.97 Co 0.03 O and from 151(6) to 73(3) nm for Zn 0.94 Co 0.03 Ca 0.03 O), suggesting strain accommodation and refinement of diffracting domains, with a stronger effect in the Ca co-doped sample. Raman spectra further support an anisotropic response, with the most noticeable shifts/broadenings in modes associated with the basal plane. Electron microscopy reveals compaction-driven aggregate rearrangement, with the average aggregate diameter changing from 1.80(16) to 2.03(7) μm in ZnO:Co, while remaining nearly unchanged (1.18(2)–1.20(2) μm) in ZnO:Co,Ca. EPR shows that the Co-related center is a sensitive probe of symmetry lowering induced by residual strain: spectral simulations (S = 3/2) indicate the development of axial and rhombic ligand-field distortions under prolonged compaction, enhanced by Ca co-doping, with the largest impact on g ⊥ (basal-plane-related distortions). Finally, saturation behavior indicates altered spin-lattice relaxation, consistent with increased defect/strain-related disorder around Co 2+ after compaction.