ON METAL OXYGEN COORDINATION - A STATISTICAL-METHOD TO DETERMINE COORDINATION NUMBER-I - CALCIUM

The environment around the Ca cation for compounds involving bonded oxygen has been studied for 254 inorganic structures containing a total of 368 polyhedra. Selection was made on the basis of the accuracy of the structural data. The coordination number (CN) was assigned using the criteria of maximum gap in the Ca-O distances and balanced bond-valence sums for Ca, but 32 cases were still difficult to assign unambiguously. A series of variables related to the Ca-O distances were calculated and averaged for each value of CN. The Gaussian curves representing the distribution of these variables for each CN overlap strongly. By way of contrast, the volume of the coordination polyhedra (Vol) showed well separated curves. Statistical analysis was applied to the set of structures with known CN, with seven variables and then with Vol alone, which seems to discriminate between the various CN groups equally well. A strong linear dependence was found for CN versus Vol. A method is proposed to assign CN in uncertain cases based on the equation: CN = 0.197 (2)Vol + 2.83 (5). Application of this equation to the unassigned cases compares favourably with discriminant analysis using the larger set of variables.