Single-crystal in situ high-P X-ray diffraction was performed at P = 0.0001, 3.2, 4.4, 6.2, and 7.4 GPa on synthetic Ca0.2Sr0.8Al2Si2O8 feldspar (An20SrF80). Data collections confirmed the displacive first-order triclinic I1̅-monoclinic I2/c phase transition at P ~4.3 GPa found in a previous high-P investigation and defined the first-order monoclinic-monoclinic transformation at P ~7.3 GPa as an I2/c-P21/c symmetry change. The structural modifications induced by the increase of pressure inside the stability fields of I1̅ and I2/c configurations as well as the structural behavior of the two phase transitions were detailed. The I1̅-I2/c transition is similar to the displacive ferroelastic I1̅-I2/c phase transition observed for the same composition with increasing temperature and is related to the increase of the M-site coordination number, with the consequent regularization of Ca/Sr polyhedra and framework. Variations of the T-O-T bond angles are observed, whereas the O-T-O angles do not change significantly.
The I2/c-P21/c transition involves a significant modification of the M-polyhedra. In the M(0)-polyhedron, the two OC atoms, that were not coordinated in I1̅ and I2/c space groups, are now bonded more strongly than either OB or OD atoms [the M(0)-OC(0i) and M(0)-OC(zi) distances are 2.54 and 2.57 Å, respectively], whereas in the M(i)-polyhedron, it is the second OA(20) oxygen that becomes coordinated and, at the same time, one of the bonds to OD atoms is broken. Moreover, a significant deformation of the framework is obtained due to the decrease of the symmetry from the loss of the two-fold axis and of half of the centers of symmetry. The transition induces, besides a significant distortion in the T-O-T bond angles, a deformation of the internal O-T-O angles within the tetrahedra. The T2(00) tetrahedron, with the OA(20)-T2(00)-OC(0i) angle of 91° and the OC(0i)-T2(00)-OD(0i) angle of 140°, is the most deformed.