A stable layered inorganic solid at high temperature: Heat treatment of Eu-doped hexacelsian without phase transformation

This study demonstrated the formation of stable layered inorganic solids toward higher temperature based on the structure of hexacelsian, a layered aluminosilicate having the formula of BaAl₂Si₂O₈ in which Ba²⁺ and aluminosilicate layers are alternately stacked. In this trial, Eu-doped hexacelsian was helpful because the reduction of Eu3+ to Eu2+ generally induces the phase transformation of hexcelsian to celsian, monoclinic BaAl2Si2O8 without layered structure. In addition, the heat treatment of raw materials of hexacelsian at above 1000 ​°C has generally formed hexacelsian and celsian. When powder and its compact Eu-doped hexacelsian were heat-treated at above 1100 ​°C under a reducing atmosphere and at 1200 ​°C under air, respectively, the phase transformation did not occur as evidently revealed by X-ray diffraction patterns. In addition, the valence state of Eu in Eu-doped hexacelsian changed based on coloration under UV irradiation and fluorescence spectra. Therefore, the present results indicated that hexacelsian, which can accommodate Eu as a potential probe for calcination atmosphere, is a stable layered inorganic solid toward a relatively higher temperature. This condition generally breaks layered structures of layered clay minerals and perovskites.