Arsenic(III) oxide intercalate with ammonium chloride: crystal structure revision and thermal characterization

According to the crystal structure determination by Edstrand & Blomqvist [Ark. Kemi (1955), 8, 245-256], intercalate NH4Cl center dot As2O3 center dot 0.5H(2)O (Y-NH4Cl) is not isostructural with compound KCl center dot As2O3 center dot 0.5H(2)O. This is very unlikely because both NH4Br center dot 2As2O3 and KBr center dot 2As(2)O(3) as well as NH4I center dot 2As(2)O(3) and KI center dot 2As(2)O(3) are isostructural. Hence, intercalate Y-NH4Cl has been studied using single-crystal X-ray diffraction in addition to attenuated total reflection Fourier transform infrared (ATR-FTIR) and N-15 solid-state magic-angle spinning nuclear magnetic resonance (ssNMR) spectroscopies. These techniques indicate that revising the previous crystal structure model is necessary. Compound Y-NH4Cl crystallizes in space group P6/mmm with unit-cell parameters a = 5.25420 (10)angstrom and c = 12.6308 (3)angstrom and is isostructural with KCl center dot As2O3 center dot 0.5H(2)O. The presence of two symmetry-independent ammonium cations in the structure has been unequivocally confirmed using 15N ssNMR spectroscopy. The 15N ssNMR spectrum of intercalate Y NH4Cl has been compared with analogous spectra of NH4Br center dot 2As(2)O(3) and NH4I center dot 2As(2)O(3) which allowed for a probable assignment of signals to ammonium cations occupying particular sites in the crystal structures. Thermogravimetry, differential scanning calorimetry and variable-temperature ATR-FTIR spectra have revealed that intercalate Y-NH4Cl is dehydrated between 320 and 475 K. Upon cooling or standing in moist air water is re-absorbed. Dehydration leads to significant shortening of the c unit-cell parameter as revealed by powder X-ray diffraction [c = 12.1552 (7)angstrom at 293 K]. Compound Y-NH4Cl decomposes on prolonged heating above 490 K to arsenic(III) oxide and ammonium chloride.