IntroductionAs a high-quality light source, the performance of phosphor-converted white light emitting diodes (pc-WLEDs) is closely related to the performance of the selected luminescent materials. The traditional method of combining blue LED chips and yellow phosphors to generate white light has problems of low color rendering index and high color temperature. To address the pressing requirements of lighting and display technologies, the development of novel red phosphors with high efficiency and excellent thermal stability is urgent. Perovskite tellurites have attracted extensive attention due to their elemental diversity and structural tunability. Based on the double perovskite Ca₃TeO₆, a series of NaLaCaTeO₆:xEu³⁺ phosphors that can be effectively excited by near-ultraviolet light have been designed and synthesized by elemental design. Through a co-substitution strategy of [Na⁺–La³⁺] substitution of [Ca²⁺–Ca²⁺] ion pair, the problem of low Eu³⁺ doping concentration in Ca₃TeO₆:Eu³⁺ is effectively improved by regulating the matrix structure. Meanwhile, the luminous intensity and thermal stability of the sample were also significantly improved.MethodsSeries of NLCT:xEu³⁺ (0.05 ≤ x ≤ 1.0) samples were successfully synthesized by a high-temperature solid-state reaction. The stoichiometric amounts of raw materials, including Na₂CO₃ (105.99 g/mol, 99.9%), CaCO₃ (100.09 g/mol, 99.9%), TeO₂ (159.6 g/mol, 99.99%), La₂O₃ (325.84 g/mol, 99.99%) and Eu₂O₃ (351.93 g/mol, 99.99%) purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., were weighed and grounded into homogeneous powder for 30 min. Afterward, the obtained powder was put into crucibles and heated at 1473 K for 12 h in a muffle furnace. Eventually, the target substances were obtained after cooling down and grinding again. A red LED device was assembled using the NLCT:0.6Eu³⁺ phosphor with a 395 nm chip. The WLED device was fabricated by connecting tricolor phosphors including BaMgAl₁₀O₁₇:Eu²⁺ (BAM:Eu²⁺, blue), (Ba, Sr)₂SiO₄:Eu²⁺ (BSS:Eu²⁺, green), along with the prepared NLCT:0.6Eu³⁺ (red) with a 395 nm chip.Results and discussionThe results of XRD patterns showed that [Na⁺–La³⁺] substituted the [Ca²⁺–Ca²⁺] ion pair, which did not affect the structure of the sample. The diffraction peaks of the samples with x = 0.1–0.6 did not show obvious impurity peaks, indicating that the synthesized samples were pure phases. Eu³⁺ ions tended to occupy the site of La in the matrix rather than Te. DRS results also confirmed that Eu³⁺ ions were successfully introduced into the NLCT matrix. The dominant absorption peak at 395 nm matches well with InGaN-based LED, and the emission intensity reaches the maximum at x = 0.6. The primary mechanism among Eu³⁺ ions in NLCT:xEu³⁺ samples was proved to the dipole-dipole interaction.The luminous intensity of the prepared NLCT:0.6Eu³⁺ red phosphor is about 1.98 and 3.97 times than that of CT:0.2Eu³⁺ and Y₂O₃:Eu³⁺, confirming the feasibility of the co-substitution strategy. The internal quantum yield and color purity of NLCT:0.6Eu³⁺ are 67.6% and 99.87%, respectively. The CIE color coordinates of this sample (0.658 1, 0.341 0) are closer to the ideal red point (0.670, 0.330) than that of the purchased commercial red phosphor Y₂O₃:Eu³⁺. Compared with CT:0.2Eu³⁺ (about 77.5%), the thermal stability of NLCT:0.6Eu³⁺ is significantly improved. When the temperature rises to 420 K, the emission intensity at 616 nm remains 91.7% of that at 300 K. Furthermore, the sample exhibits excellent color stability by the CIE coordinates shifting only slightly from (0.658 2,0.340 8) to (0.646 6,0.351 9) with increasing temperature. The CCT and R_a of the prepared WLED device are 6215 K and 80.6, respectively, and the CIE coordinates (0.317 6, 0.334 0) are located in the white light region.ConclusionsA series of innovative NaLaCaTeO₆:xEu³⁺ phosphors were synthesized using a high temperature solid-state reaction method. Under 395 nm excitation, the NaLaCaTeO₆:Eu³⁺ samples exhibit a strong red emission at 616 nm (⁵D₀→⁷F₂) of Eu³⁺ ions. Through a co-substitution strategy where [Na⁺–La³⁺] replaced the [Ca²⁺–Ca²⁺] ion pair, the optimized doping concentration of the NaLaCaTeO₆:xEu³⁺ is elevated to 60% (molar fractior), leading to exceptional color purity and high internal quantum efficiency reaching 99.87% and 67.60%, respectively. Furthermore, compared to the Ca₃TeO₆:0.2Eu³⁺ sample pre-substitution, the luminous intensity and thermal stability of NaLaCaTeO₆:0.6Eu³⁺ phosphor witness a notable increase. Even at 420K, the emission intensity remains at 91.7% compared to 300K. The resultant WLED displays a color rendering index R_a of 80.6 and CIE chromaticity coordinates of (0.317 6, 0.334 0), underscoring the broad application potential of NaLaCaTeO₆:0.6Eu³⁺ in solid-state lighting.