A set of effective ionic radii corresponding to different coordination numbers (CNs) and compatible with the radii system by Shannon [Acta Cryst. (1976), A32, 751-767] has been derived for ammonium: 1.40 Å (CN = IV), 1.48 Å (CN = VI), 1.54 Å (CN = VIII) and 1.67 Å (CN = XII). The bond-valence parameters r0 = 2.3433 Å and B = 0.262 Å have been determined for ammonium-fluorine bonds.

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Using a new calculation scheme, reliable and physically meaningful values of the bond-valence parameters (r(0) = 2.567 A and b = 0.421 A) have been deduced for the Bi(3+)/Br(-) ion pair from the molecular geometry and from the crystal structure of BiBr(3).

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