Single crystal Raman and infrared spectra of natural and synthetic erythrite Co3(AsO4)2.8H2O are reported, and compared to the spectra polycrystalline, synthetic annabergite (Ni3(AsO4)2.8H2O) and hörnesite (Mg3(AsO4)2.8H2O). Factor group analysis and single crystal considerations have been used to interpret the experimental data. The Raman spectra of erythrite reveal ν1 arsenate stretching vibration at 850 cm-1 (Ag) with the corresponding infrared band at 821 cm-1 (Bu). The ν3 antisymmetric vibration is split into three components, observed at 796 (Ag), 788 (Ag) and 803 (Bg) cm-1. The ν2 symmetric bending modes are observed at 375 (Ag) and 385 (Bg) cm-1. The ν4 bending modes are predicted to split into three bands which are observed at 441 (Ag), 446 (Bg) and 457 (Ag) cm-1. Lattice vibrations are found at 112 (Ag), 124 (Bg), 145(Ag), 157 (Bg), 165 (Ag), 179 (Ag), 189 (Ag), 191(Bg), 201 (Bg), 210 (Ag), 227 (Ag), 250 (Ag), 264(Ag), 264 (Ag), 280 (Bg), 302 (Bg), 321 (Bg), and 338 cm-1 (Ag). Hydroxyl stretching modes are observed at 3050, 3218, 3333, 3449 and 3479 cm-1, in the infrared spectrum. Raman active hydroxyl bands are detected at 3009 (Bg), 3052 (Ag), 3190 (Bg) 3203 (Bg), 3281 (Ag) and 3310 (Bg), 3436 (Bg) and 3443 (Ag) cm-1. Infrared hydroxyl bands at 3050, and 3218 cm-1 are from water type II, short hydrogen bonding distances, and the bands at 3449 and 3479 cm-1 are due to water I, long hydrogen bonding distances. Water bending modes are detected in the infrared spectrum at 1571, 1621, 1641, and 1682 cm-1, but due to the inherent weak Raman scattering cross section of water these could not be detected in the Raman spectra
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