La1-x Eu x B4O6(OH)2Cl (0 ≤ x ≤ 0.54): Strong 4f-4f excitations due to the noncentrosymmetric and oxychloride coordination of Eu3+

Highlights: • La_1-xEu_xB₄O₆(OH)₂Cl were prepared by boric acid flux method. • Eu³⁺-contents in La_1-xEu_xB₄O₆(OH)₂Cl were determined by external standard method. • 4f-4f transitions of Eu³⁺ were enhanced due to the oxychloride coordination. • Once combined with a 395nm LED chip, it emitted a bright pink light. • In-situ high temperature PL spectra proved its high thermal stability. A strategy to utilize the nonlinear optical borate as the photoluminescent host is proposed in order to gain enhanced 4f-4f transitions because there exists a high probability of Eu³⁺ locating in a noncentrosymmetric coordination, serving as the near ultraviolet LED pumped red phosphor. With such a rationale, La_1-xEu_xB₄O₆(OH)₂Cl have been prepared by boric acid flux method, which transfer to TD-α-La_1-xEu_xB₃O₆ by high temperature annealing. In the meantime, HS-α-La_1-yEu_yB₃O₆ (y=0, 0.075, 0.15, 0.3, 0.4) are synthesized directly by high temperature solid state reactions. The Eu³⁺-dependent cell volume is employed to obtain equation (1): V1=421.0(3)-26(1)y. Accordingly, the x values of TD-α-La_1-xEu_xB₃O₆ can be deduced and the cell volume of La_1-xEu_xB₄O₆(OH)₂Cl can be simulated into equation (2): V2=695.1(1)-37.9(8)x. In the following, all the x values for 11 samples of La_1-xEu_xB₄O₆(OH)₂Cl are determined. The noncentrosymmetric coordination as well as the mix-coordination atoms lead to strong 4f-4f transitions of Eu³⁺, and only a slight concentration quenching effect is observed despite the existence of hydroxyl groups. Once combined with a 395nm LED chip, it emits a bright pink light, and the thermal stability is satisfied according to the in-situ high temperature photoluminescent study.