Bond-length distributions for ions bonded to oxygen: alkali and alkaline-earth metals

Bond-length distributions have been examined for 55 configurations of alkali-metal ions and 29 configurations of alkaline-earth-metal ions bonded to oxygen, for 4859 coordination polyhedra and 38 594 bond distances (alkali metals), and for 3038 coordination polyhedra and 24 487 bond distances (alkaline-earth metals). Bond lengths generally show a positively skewed Gaussian distribution that originates from the variation in Born repulsion and Coulomb attraction as a function of interatomic distance. The skewness and kurtosis of these distributions generally decrease with increasing coordination number of the central cation, a result of decreasing Born repulsion with increasing coordination number. We confirm the following minimum coordination numbers: Li-[3](+), Na-[3](+), K-[4](+), Rb-[4](+), Cs-[6](+), Be-[3](2+), Mg-[4](2+), Ca-[6](2+), Sr-[6](2+) and Ba-[6](2+), but note that some reported examples are the result of extensive dynamic and/or positional short-range disorder and are not ordered arrangements. Some distributions of bond lengths are distinctly multi-modal. This is commonly due to the occurrence of large numbers of structure refinements of a particular structure type in which a particular cation is always present, leading to an over-representation of a specific range of bond lengths. Outliers in the distributions of mean bond lengths are often associated with anomalous values of atomic displacement of the constituent cations and/or anions. For a sample of Na-[6](+), the ratio U-eq(Na)/U-eq(bonded anions) is partially correlated with < Na-[6](+)-O2-> (R-2 = 0.57), suggesting that the mean bond length is correlated with vibrational/displacement characteristics of the constituent ions for a fixed coordination number. Mean bond lengths also show a weak correlation with bond-length distortion from the mean value in general, although some coordination numbers show the widest variation in mean bond length for zero distortion, e.g. Li+ in [4]- and [6]-coordination, Na+ in [4]- and [6]-coordination. For alkali-metal and alkaline-earth-metal ions, there is a positive correlation between cation coordination number and the grand mean incident bond-valence sum at the central cation, the values varying from 0.84 v.u. for K-[5](+) to 1.06 v.u. for Li-[8](+), and from 1.76 v.u. for Ba-[7](2+) to 2.10 v.u. for Sr-[12](2+). Bond-valence arguments suggest coordination numbers higher than [12] for K+, Rb+, Cs+ and Ba2+.