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The title compounds were prepared and characterized analytically, structurally, and spectroscopically. They contain isolated BiX 6 (X = I. Br, Cl) anions. Their optical absorption spectra show excitonic bands in the UV-visible spectral region. These bands occur at higher energies than the corresponding bands of R 3 M 2 X 9 (R = Me m NH 4-m ; m = 1,2, 3, 4).
Papavassiliou et al. (Sun,) studied this question.