High-pressure phases and transitions of the layered alkaline earth nitridosilicates SrSiN(2) and BaSiN(2).

We investigate the high-pressure phase diagram of SrSiN(2) and BaSiN(2) with density-functional calculation. Searching a manifold of possible candidate structures, we propose new structural modifications of SrSiN(2) and BaSiN(2) attainable in high-pressure experiments. The monoclinic ground state of SrSiN(2) transforms at 3 GPa into an orthorhombic BaSiN(2) type. At 14 GPa a CaSiN(2)-type structure becomes the most stable configuration of SrSiN(2). A hitherto unknown Pbcm modification is adopted at 85 GPa and, finally, at 131 GPa a LiFeO(2)-type structure. The higher homologue BaSiN(2) transforms to a CaSiN(2) type at 41 GPa and further to a Pbcm modification at 105 GPa. Both systems follow the pressure-coordination rule: the coordination environment of Si increases from tetrahedral through trigonal bipyramidal to octahedral. Some high-pressure phases are related in structure through simple group-subgroup mechanisms, indicating displacive phase transformations with low activation barriers.