The bulk modulus of a crystal is often closely related to the compressibility of its constituent elements, such as cation-centred coordination polyhedra. The compressive behaviour of regular polyhedra with a fixed coordination number was studied thoroughly some decades ago and was found to follow a simple universal rule. However, the compressive behaviour of large cations with irregular and poorly defined coordination polyhedra has not been studied systematically because of methodological challenges and biases. Here, an original approach to defining and analysing polyhedral compressibility is proposed, based on the concepts of effective coordination number (ECoN) and weighted mean bond length. This approach is demonstrated using K+ coordination polyhedra as an example.
