Les disilicates de terres rares Ln2Si2O7: Precisions sur leur nouvelle forme cristalline X et leur polymorphisme sous pression

The new X form of rare earth disilicates Ln₂Si₂O₇ (Ln=Lu, Yb, Tm) is described by means of X-ray powder data. The X-ray indexation is made using single microcristal electronic diffraction. The investigation of structural relation between Ln₂Si₂O₇ and M₂A₂O₇ compounds as also the X-ray powder data of X-Ln₂Si₂O₇ show that this X form is isostructural with tetragonal Er₂Ge₂O₇. The rare earth coordination polyhedra and the (Si₂O₇)^6? configuration are compared with those actually known in neighbouring phases and discussed in terms of pressure effect.