Bright upconversion emission, increased T_c, enhanced ferroelectric and piezoelectric properties in Er-doped Ca Bi ₄ Ti ₄ O ₁₅ multifunctional ferroelectric oxides

Er³⁺-doped CaBi₄Ti₄O₁₅ (CBT) bismuth layer structured ferroelectric ceramics were synthesized by the solid state method. Photoluminescence (UC), dielectric, ferroelectric, and piezoelectric properties were systematically studied for the first time. The Er³⁺-doped CBT sample showed a bright up-conversion UC while simultaneously obtaining an increased Curie temperature (T_c), enhanced ferroelectric and piezoelectric properties. The UC properties of Er³⁺-doped CBT were investigated as a function of Er³⁺ concentration and incident pump power. A bright green (556 nm) and a weak red (674 nm) emission bands were obtained under excitation (980 nm) at room temperature, which correspond to the transitions from ⁴S _3/2, and ⁴F_9/2 to ⁴I_15/2, respectively. The dependence of UC emission intensity on pumping power indicated that three-photon and two-photon processes are involved in the green and red UC emission, respectively. Studies on dielectric properties indicated that the introduction of Er increased the T_c with relatively smaller values of dielectric loss of CBT, thus making this ceramic suitable for sensor applications at higher temperatures. Ferroelectric and piezoelectric measurements showed that the Er³⁺-doped ceramics showed an increase in remnant polarization and piezoelectric constant. As a multifunctional material, Er-doped CBT ferroelectric oxide showed great potential in sensor, optical-electro integration, and coupling device applications.