Structural phase transition and quasi-layered active-ion distribution suppress concentration quenching in Tb³⁺-activated KBi(MoO₄)₂

Conventional Tb³⁺-doped phosphors typically suffer from concentration quenching once the doping level exceeds a critical threshold. Consequently, the development of Tb³⁺ phosphors with intrinsic resistance to concentration quenching has become a key research focus. In this work, we successfully synthesized KBi(MoO₄)₂: xTb³⁺ (x = 0–100 at%) (denoted as KBM: xTb³⁺) phosphors via a high-temperature solid-state reaction. Remarkably, no concentration quenching was observed across the entire doping range. This anti-quenching behavior originates from the large Tb³⁺–Tb³⁺ interionic distance (> 5 Å) inherent to the quasi-layered crystal structure, which effectively suppresses multipole-interaction-mediated energy migration. At full Tb³⁺ substitution (x = 100 at%), the material undergoes a structural phase transition from the monoclinic KBM phase to the triclinic α-KTb(MoO₄)₂ (α-KTM) phase. The α-KTM phosphor exhibits excellent thermal stability (activation energy = 0.6129 eV) and a single-exponential decay profile, whereas KBM: xTb³⁺ (x < 100%) display double-exponential decay behaviors, attributed to dual energy transfer pathways. These findings provide new insights into the luminescence mechanisms of high-concentration rare-earth-doped systems and offer guidance for designing next-generation anti-quenching phosphors.