Structure and spectroscopic properties of hydrogencarbonate containing alumosilicate sodalite and cancrinite

The preparation of the sodalite Na₈[AlSiO₄]₆·(HCO₃)₁(CO₃)_0.5(H₂O)₂ and cancrinite Na_7.6[AlSiO₄]₆·(HCO₃)_1.2(CO₃)_0.2(H₂O)₂ were carried out in a cold-sealed autoclave using a preheated mixture of γ-Al₂O₃ and precipitated silica together with NaHCO₃ and NaOH. The material was encapsulated in a small silver tube and heated for 48 hours at a pressure of 0.1 GPa. Two different types of crystals remained during this process. From these crystals the structure of both compounds were determined from single crystal diffractometer data. The sodalite Na₈[AlSiO₄]₆(HCO₃)₁(CO₃)_0.5(H₂O)₂: P-43n, a = 903.3(1) pm, V = 0.7370(1) nm³, Z = 2, R = 0.013, wR₂ = 0.027 for 242 structure factors and 63 variable positional parameters; the cancrinite Na_7.6[AlSiO₄]₆(HCO₃)_1.2(CO₃)_0.2(H₂O)₂: P6₃, a = 1272.5(2) pm, c = 517.7(1) pm, V = 0.7260(1) nm³, Z = 2, R = 0.034, wR2 = 0.066 for 845 F’s and 99 variables. Spectroscopic properties were measured with an IR microscope in transmission mode using single crystals polished to a thickness of 30 μm. Both structures are build by corner shared six rings of AlO₄ and SiO₄ tetrahedra stacked cubic close packed ABCA’B’C’ for the sodalite and hexagonal close packed ABA’B’ for the cancrinite. The mean Al-O (174.3 pm) and Si-O (161.4 pm) distances for the sodalite and 173.7 pm (Al-O) and 161.8 pm (Si-O) are in good agreement with former structure description of related materials. Hydrogencarbonate and carbonate was found statistically distributed in the sodalite β-cages and in the channels of the cancrinite structure. Whereas for the sodium atoms in the sodalite four different positions are observed, the sodium atoms in the cancrinite structure where found near the threefold axes in the cancrinite ε-cage and on one position in the channel.