Inorganic Single-Ion Magnet DyO ⁺ in the Apatite-Type Structure of (Ca,Sr) ₁₀ (VO ₄ ) ₆ (OH) ₂

Dysprosium-doped calcium-strontium vanadate(V) hydroxyapatites (Ca1-ySry)₁₀(VO₄)₆(OH)₂:Dy, y = 0-0.4, were synthesized by the solid-state reaction at temperatures between 900 and 1000 °C. Dy³⁺ substitutes for Ca²⁺ at the 6h Wyckoff site (Ca2) and displaces strongly toward the isolated oxygen anion imbedded in the trigonal channel. This results in the formation of dysprosyl ion DyO⁺ with a short bond length of 2.15 Å. In zero external magnetic field and below 65 K, the compounds exhibit slow relaxation of the magnetization. With increasing strontium content y, the energy barrier for remagnetization grows from 614 cm⁻¹ to 699 cm⁻¹, the magnetization blocking temperature changes from 3 to 5.5 K, and the magnetization hysteresis at T = 2 K extends from 12 to 16 kOe. The photoluminescence bands exhibit a large crystal field splitting that increases with y. The electronic energy level diagram of Dy³⁺ obtained from the luminescence data agrees well with the measured magnetic properties. That is the first example of the DyO⁺ single-ion magnet (SIM) in a nonphosphate compound. This provides an opportunity for comparative studies to reveal new relations between crystal structure details and SIM parameters.