Structural anatomy and thermal transitions of barium feldspars, BaAl2Si2O8

BaAl2Si2O8 (barium feldspar-based) materials are widely used in glass and ceramic industry due to high melting temperature, chemical resistance, low thermal expansion, low dielectric constants and attractive luminescence properties. The durability of barium feldspar ceramics strongly depends on reversible phase transitions between the low-temperature modification (celsian), and series of high-temperature (HT) polymorphs known as 'hexacelsian'. Numerous works have been devoted to the study of the thermal transitions of 'hexacelsian' by X-ray powder diffraction methods and transmission electron microscopy, the use of which provides limited information about structural changes. Here, the crystal structures, thermal properties and phase transitions of HT BaAl2Si2O8 polymorphs are characterized for the first time using in-situ HT X-ray single-crystal structural study (SCXRD), and micro-Raman spectroscopy up to 1000 degrees C. The crystal structure of alpha-hexacelsian under ambient conditions is monoclinic (I2/a). The alpha-form undergoes two polymorphic transformations upon heating: into an orthorhombic (Fmmm) beta-modification above 300 degrees C and into a hexagonal (P-62m) gamma-modification above 700 degrees C. These transitions lead to significant changes in the unit-cell volume: an obstacle in the production of BaAl2Si2O8-based ceramics.