Ferroelectric materials are widely used in detection, conversion, storage and information processing. The development of energy-saving, environmental protection, multi-functional, miniaturized lead-free multi-functional ceramics becomes inevitable. However, lead-free ferroelectric ceramics as dielectric ceramic capacitors with a low energy storage density and a poor energy storage efficiency seriously hinder their practical application. The development of ceramic capacitors with a high energy storage density and a high energy storage efficiency thus greatly expands the applications of ferroelectric ceramics in the field of energy storage. Doping rare-earth elements is a simple, direct and effective modification method. Rare-earth elements with special electronic structure and stable luminescence properties are expected to give new luminescent properties and improve the electrical properties of ferroelectric ceramics. Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃ (BCTZ) lead-free piezoelectric ceramics have a small dielectric loss and superior electrical characteristics. Rare-earth element Er³⁺ is widely used in laser and lighting due to its excellent green luminous intensity. In this paper, the effect of Er³⁺ doping amount on the microstructure, ferroelectric, energy storage and photoluminescence properties of BCTZ ceramics and their internal relations was investigated to broaden the applications in photoelectric materials.

Er³⁺ doped Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃ (BCTZ:x%Er³⁺, x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0, in mole fraction) ceramics were prepared by a high-temperature solid-state reaction method. CaCO₃ (99.9%), ZrO₂ (99.9%), TiO₂ (99.9%), BaCO₃ (99.9%), Er₂O₃ (99.9%) were used as raw materials. The raw materials were firstly mixed and ground in a ball mill for 12 h, and then pre-burned at 1 100 ℃ for 6 h after drying The pre-burned materials were naturally cooled to room temperature, granulated and pressed. The pressed samples were sintered at 1 300 ℃ for 6 h. The crystal structure of ceramic samples was analyzed by a model D8 ADVANCE X-ray diffractometer (XRD, Germany). The surface morphology of the samples was characterized by a model JEM-7800F scanning electron microscope (SEM, JEOL Co., Japan). The electrical properties of ceramics were tested by a model TF Analyzer 3000E (Germany), and the both sides of the ceramics were plated with silver electrodes before testing. The luminescent properties of ceramics were determined by a model FS5 fluorescent spectrometer (Edinburgh Co., UK).

Rare-earth Er³⁺ doped BCTZ ceramics all have a well-crystallization with a single perovskite structure and little impurities. The results show that the mean grain size of the ceramics decreases monotonically with the increase of Er³⁺ doping concentration, from 7.59 μm to 3.82 μm, which is decreased by 49.67%.

All the ceramics exhibit superior ferroelectric properties. The ferroelectric domains are overturned and the density are different because of the lattice distortion and grain size change caused by the addition of different contents of Er³⁺, resulting in unstable polarization states. Therefore, the Er³⁺-doped BCTZ ceramics have a larger ΔP(P_max-P_r) rather than pure BCTZ ceramic. The released energy density (W_rec) and energy storage efficiency (η) of the ceramics are increased by 29.81% and 122.79%, respectively.

The worse the lattice symmetry, the better the photoluminescent (PL) performance. At a great doping amount of Er³⁺, Er³⁺ replace the A-position Ba²⁺ and Ca²⁺ positions, gradually occupying the gap position or replacing the B-position ions, resulting in more lattice distortion, reducing the lattice symmetry, and significantly enhancing the photoluminescent intensity. When the doping amount is 0.8% (in mole), PL strength reaches the maximum value. As the doping amount is greater than 0.8%, a cross relaxation phenomenon occurs due to the increase of luminescent ions and the decrease of luminescent ion spacing, resulting in fluorescence quenching and a decreased luminous intensity. The relative change in PL strength of BCTZ:0.8%Er³⁺ ceramic is enhanced by 173%, compared with BCTZ:0.2%Er³⁺ ceramic.

The mean grain size of the lead-free multifunctional ferroelectric ceramics decreased monotonously, and the microstructure became more compact and uniform with the increase of Er³⁺ doping concentration. All the ceramics had the superior ferroelectric properties. Er³⁺ doping could destroy the long-range order in BCTZ structure and the interaction between electric domains, so that the released energy density (W_rec) and energy storage efficiency (η) of the ceramics were increased by 29.81% and 122.79%, respectively, as Er³⁺ doping concentration increased. In addition, BCTZ:x%Er³⁺ ceramics exhibited an intense green emission at 548 nm under excitation of 487 nm near-ultraviolet light, and the luminous intensity was relatively adjustable up to 173.09%. The rich physical and photoelectric properties of BCTZ:x%Er³⁺ ceramics could lay a foundation for the development of new multi-functional materials with energy saving, environmental protection and low energy consumption.