This experimental study examines the effects of Sm (samarium) and Ni (nickel) co-doping on the structural, optical, and electrical properties of Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) ceramics. The work was motivated by prior demonstrations that Ni2+ substitution in alkaline niobates can reduce bandgaps through Ni2+–oxygen-vacancy defect dipoles, enhancing visible-light absorption and photoconductivity while maintaining polarization. The present study introduces Ni2+ on the B-site together with Sm3+ on the A-site to investigate whether similar bandgap engineering and photoferroelectric behavior occur in the technologically important PMN-PT system. Samples were prepared by a solid-state route, and the series includes air-sintered perovskite-dominant and N2-annealed pyrochlore-dominant specimens. Measurements addressed phase composition, microstructure, optical absorption, photoconductivity, charge carrier trapping, phase transitions, and piezoelectric energy-harvesting metrics. Results indicate that Ni2+ acts differently in PMN-PT than in alkaline niobates; nevertheless, Sm/Ni co-doping successfully tunes phase transitions, photoconductivity, charge trapping, and optical absorption while preserving reasonable piezoelectric performance. The study provides experimental insight into how Ni2+ affects the band structure of Sm‑PMN‑PT perovskite and pyrochlore phases.
Bandgap engineering of polar oxide perovskites is of interest because reducing the optical bandgap while retaining strong ferroelectricity can enable multifunctional devices for energy harvesting, sensing, actuation, and bulk photovoltaic effects. In ABO3 perovskites the valence band maximum is typically dominated by O 2p states and the conduction band minimum by B-site transition-metal d states; this bonding leads to large spontaneous polarization but generally wide optical bandgaps (> 3 eV) that limit visible-light absorption.
Previous theoretical and experimental work showed that substituting B-site Nb5+ with Ni2+ and introducing oxygen vacancies can dramatically lower bandgaps in alkaline niobates (e.g., KNbO3, (K,Na)NbO3) through formation of Ni2+–oxygen-vacancy defect dipoles, producing enhanced visible absorption and photoconductivity while retaining finite remanent polarizations. However, the behavior of Ni2+ in Pb-based PMN-PT had not been explored prior to this study. PMN-PT near the morphotropic phase boundary is a widely used piezoelectric material, and co-doping with Sm3+ has been reported to enhance piezoelectric performance; therefore Sm/Ni co-doping was selected to probe the potential for combining optical bandgap tuning with useful piezoelectric properties in PMN-PT.
Because the Sm/Ni-doped PMN-PT system is compositionally complex and involves strongly correlated Ni 3d and Sm 4f electrons, the authors pursued an experimental characterization of structure–property relationships rather than first-principles modeling.
Ceramics were synthesized by a conventional solid-state method using high-purity oxide and carbonate precursors: PbO, MgO, Nb2O5, TiO2, Sm2O3, and NiO. Stoichiometric mixtures were ball milled in ethanol, dried, calcined at 860 °C for 4 h, and then ball milled again. A 5 wt% polyvinyl alcohol binder was used to form green pellets (10 mm diameter) that were uniaxially pressed to ~40 MPa. Binder burn-out was performed at 550 °C for 4 h, followed by sintering at 1200–1250 °C for 4 h in a muffle furnace. Powder beds of identical composition were used during sintering to limit Pb loss. Selected samples underwent an additional anneal in N2 at 1000 °C for 2 h.
A total of six specimen types were prepared. The A family corresponds to 71PMN-29PT compositions doped with 2.5 mol% Sm on the A-site; the air-sintered A_per samples were predominantly perovskite, while N2-annealed A_pyr samples became pyrochlore-dominant. The B and C families incorporate an additional 5 mol% Ni dopant introduced by distinct processing routes intended to differentiate formation of Ni2+–oxygen-vacancy (Ni-VO) defect dipoles from formation of stoichiometric Pb(Ni1/3Nb2/3)O3-PbTiO3 (PNN-PT) solid-solution-like phases.
Key processing steps emphasized control of stoichiometry and phase stability. High-purity reagents and careful ball-milling produced homogenous precursor powders. Calcination at 860 °C and sintering at 1200–1250 °C were used to form dense ceramics; powder beds minimized Pb volatilization. N2 annealing at 1000 °C was applied to selected samples to induce the pyrochlore phase from an initial perovskite-dominant microstructure. The study explicitly varied the way Ni was introduced across sample families to probe whether Ni enters as isolated Ni2+ with associated oxygen vacancies or as part of a PNN-like solid solution.
Phase analysis showed that air sintering produced predominantly perovskite PMN-PT in the A family (A_per), while N2 annealing converted the major phase to pyrochlore in A_pyr specimens. Introducing 5 mol% Ni produced different structural and optoelectronic outcomes depending on the doping route and thermal treatment; the authors designed B and C series to separate Ni-VO-type defect scenarios from stoichiometric PNN-PT-like incorporation.
Optical absorption and photoconductivity measurements revealed that Sm/Ni co-doping alters optical absorption and photoconductive response compared with Sm-only PMN-PT. The paper reports that Ni2+ behaves differently in PMN-PT than previously observed in alkaline niobates, but that co-doping still permits tuning of optical absorption and photoconductivity. Charge-carrier trapping characteristics and phase-transition behavior were also modified by Sm/Ni co-doping.
Importantly, despite these electronic and optical modifications, the co-doped ceramics retained reasonable piezoelectric energy-harvesting performance, indicating that bandgap engineering in PMN-PT via Sm/Ni co-doping can be compatible with useful electromechanical functionality.
The authors interpret their data as providing experimental insight into how Ni2+ influences the PMN-PT band structure in both perovskite and pyrochlore phases; however, detailed atomistic mechanisms in this complex, strongly correlated system were not addressed with first-principles calculations in this work.
Sm/Ni co-doping in PMN-PT ceramics enables tuning of phase composition, optical absorption, photoconductivity, charge trapping, and phase-transition behavior. Ni2+ plays a distinct role in PMN-PT relative to alkaline niobates, but co-doping nonetheless provides a route to modulate optoelectrical properties while retaining reasonable piezoelectric energy-harvesting capability. The experimental dataset and samples designed to distinguish Ni-VO defect dipoles from PNN-PT-like incorporation afford a platform for further investigation; raw data for the study are deposited in a FAIR repository as reported by the authors.