Nature of the Color of Borates with “Anti-Zeolite” Structure

Abstract

Crystals of the Mn _xBa₁₂(BO₃)_{8-2 x}F_{8 x} phase were grown from a high-temperature solution. This new fluoride borate is built of positively charged [Ba₁₂(BO₃)₆]⁶⁺ blocks, the so-called "anti-zeolite" pattern. Using X-ray single-crystal diffraction, the bulk atomic arrangement in the centrosymmetric tetragonal unit cell in I4/ mcm could be elucidated. Crystals of the (MnF₆)^4- group-containing solid solution Mn _xBa₁₂(BO₃)_{8-2 x}F_{8 x} are dark brown in color in contrast to the differently colored crystals of (LiF₄)^3- group-containing "anti-zeolite" LiBa₁₂(BO₃)₇F₄ ( P4₂ bc). According to the electron spin resonance and optical spectroscopic investigation, the absorption spectrum of LiBa₁₂(BO₃)₇F₄ crystals results from the absorption of light by both exciton and free charge carriers and can be tuned by varying the initial composition of the high-temperature solution.