According to the crystal structure determination by Edstrand & Blomqvist [Ark. Kemi (1955), 8, 245-256], intercalate NH4Cl·As2O3·0.5H2O ({\bf Y_{{NH}_{4}Cl}}) is not isostructural with compound KCl·As2O3·0.5H2O. This is very unlikely because both NH4Br·2As2O3 and KBr·2As2O3 as well as NH4I·2As2O3 and KI·2As2O3 are isostructural. Hence, intercalate {\bf Y_{{NH}_{4}Cl}} has been studied using single-crystal X-ray diffraction in addition to attenuated total reflection Fourier transform infrared (ATR-FTIR) and 15N solid-state magic-angle spinning nuclear magnetic resonance (ssNMR) spectroscopies. These techniques indicate that revising the previous crystal structure model is necessary. Compound {\bf Y_{{NH}_{4}Cl}} crystallizes in space group P6/mmm with unit-cell parameters a = 5.25420 (10) Å and c = 12.6308 (3) Å and is isostructural with KCl·As2O3·0.5H2O. 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 {\bf Y_{{NH}_{4}Cl}} has been compared with analogous spectra of NH4Br·2As2O3 and NH4I·2As2O3 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 {\bf Y_{{NH}_{4}Cl}} 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) Å at 293 K]. Compound {\bf Y_{{NH}_{4}Cl}} decomposes on prolonged heating above 490 K to arsenic(III) oxide and ammonium chloride.
Keywords: arsenic(III) oxide; crystal structure; intercalates; solid-state NMR.