Crystallization of RE2(OH)2CO3SO4·nH2O as a new family of layered hydroxides (RE = Gd−Lu lanthanides and Y), derivation of RE2O2SO4, photoluminescence and optical thermometry

Layered rare-earth hydroxides (LREHs) draw wide research interest because of their peculiar crystal structure, rich interlayer chemistry and abundant functionality of the RE element, but are limited to the two categories of RE₂(OH)₅A·nH₂O (A: typical of Cl⁻ or NO₃⁻) and RE₂(OH)₄SO₄·nH₂O. On the other hand, rare-earth oxysulfates (RE₂O₂SO₄) have attracted attention due to their properties of large-capacity oxygen storage, low-temperature magnetism and luminescence, but their preparation procedure mostly involves toxic SO_x gases and/or complicated procedures. In this work, RE₂(OH)₂CO₃SO₄·nH₂O as a new family of LREHs (RE = Gd‒Lu lanthanides and Y) were produced via hydrothermal reaction, from which phase-pure RE₂O₂SO₄ was derived via subsequent annealing at 800 °C in air without the involvement of SO_x. The compounds were thoroughly characterized to reveal the intrinsic influence of lanthanide contraction (RE³⁺ radius) on crystal structure, thermal behavior (dehydroxylation/decarbonation/desulfurization), vibrational property and crystallite morphology. Through analyzing the photoluminescence of Eu³⁺ and Sm³⁺ in the Gd₂O₂SO₄ typical host it is found that the 617 nm (Eu³⁺, λ_ex = 275 nm) and 608 nm (Sm³⁺, λ_ex = 407 nm) main emissions can retain as high as ∼79.6% and 85.5% of their room-temperature intensities at 423 K, with activation energies of ∼0.19 and 0.21 eV for thermal quenching, respectively. Application also indicates that both the phosphors have the potential for optical temperature sensing via the fluorescence intensity ratio (FIR) technology, whose maximum relative sensitivity reaches ∼2.70%/K for Eu³⁺ and 1.73%/K for Sm³⁺ at 298 K.