A recently observed crystal structure is characterised by reference to the calculated diffraction patterns. The "X'' crystal is related to the beta-Sn crystal structure via an atom displacement and a reduction in symmetry from tetragonal to orthorhombic. A further crystal polymorph is identified which retains the tetragonal symmetry of the beta-Sn structure but involves a related atom displacement. The unit cells and space groups are reported. The two crystals are labelled as o-X and t-X respectively. The transformation mechanisms from the beta-Sn crystal structure to the two structures are identified. No routes to these crystals from other polymorphs are found. (C) 2017 The Authors. Published by Elsevier B.V.