Protonation-state engineering enables low-temperature and phase-selective synthesis of lanthanide tellurites and high-entropy derivatives

Abstract

Phase selection in multicomponent oxides is frequently constrained by sluggish solid-state diffusion and competing reaction pathways. Here, acid–base control of solution-precipitated lanthanide tellurite precursors is used as a kinetic strategy for the low-temperature, phase-selective formation of Ln₂Te₆O₁₅ and Ln₂Te₄O₁₁ (Ln = lanthanide). Varying the LnCl₃:Na₂TeO₃:HCl ratio defines four nominal precursor regimes: non-protonated Ln₂(TeO₃)₃, partially protonated Ln(TeO₃)(HTeO₃), highly protonated Ln(HTeO₃)₃, and an over-protonated H₂TeO₃/TeO₂-rich boundary. Across the productive 2:3:0–1:3:3 window, increasing nominal protonation lowers the crystallization temperature of La₂Te₆O₁₅ from 600 to 420 °C and the minimum temperature for XRD-phase-pure La₂Te₄O₁₁ formation from 840 to 540 °C after 6 h. Excessive protonation disrupts the connected Ln–Te–O precursor network and yields TeO₂-rich products. The approach is applicable to all 14 nonradioactive lanthanides examined and enables equimolar 14-cation high-entropy derivatives with homogeneous elemental distributions. Rietveld refinements support the targeted cubic and monoclinic structures, while Yb³⁺/Er³⁺ codoping links the precursor-controlled phase pathway to structural order and upconversion emission. To our knowledge, the resulting series includes the first single-lanthanide preparations of Ln₂Te₆O₁₅ for Ln = Ce, Er, Tm, Yb, and Lu, as well as the first 14-cation (La–Lu)₂Te₆O₁₅ and (La–Lu)₂Te₄O₁₁ derivatives. These results establish precursor protonation as a practical kinetic handle for phase-selective synthesis of compositionally complex oxides at reduced temperatures.