Growth and Optical Properties of Li_xNa_1–xBa₁₂(BO₃)₇F₄ Fluoride Borates with “Antizeolite” Structure

Studied Li_xNa_1–xBa₁₂(BO₃)₇F₄ (P4₂bc) solid solution belongs to the new class of “antizeolite” borates with [Ba₁₂(BO₃)₆]⁶⁺ cation pattern, which contains channels filled by anionic clusters. Optical-quality crystals were grown from the compositions with different sodium–lithium ratio. The results of Rietveld refinement based on powder data demonstrate linear increase of parameter a and unit cell volume with Na/(Na + Li) ratio in cation site. Parameter c is less sensitive to the changes in stoichiometry, which is consistent with channel topology of Li_xNa_1–xBa₁₂­(BO₃)₇F₄ structure. Distinctive feature of Li_xNa_1–xBa₁₂­(BO₃)₇F₄ crystals is their deep purple color, which is due to both hole-type and electron-type centers. Crystals are characterized by linear dichroism effect.