Morphotropy

Morphotropy 

a change in the crystal structure of a chemical compound upon replacement of one of its atoms by an atom of an element from a neighboring group in the periodic table of elements. Morphotropy is the opposite of isomorphy, in which such substitution does not lead to any change in the structure.

Morphotropy is determined by the difference in the crystal-chemical radii of the atoms involved in the replacement, which leads to a change in the coordination number and, therefore, in the crystal structure. For example, cesium fluoride has a cubic face-centered lattice with a coordination number of 6 for fluorine, whereas cesium chloride (in which a fluorine atom with a radius of 1.36 angstroms [Å] has been replaced by a chlorine atom with a radius of 1.81 Å) has a cubic lattice with a coordination number of 8 for chlorine, in which the chlorine atom is located in the center of a cube composed of cesium atoms.