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A high-pressure diffraction study of single crystals of CaSi₂O₅ (Annual Report 1996) shows that it undergoes a phase transition from a triclinic phase to a monoclinic phase at just 0.2 GPa at room temperature. The structure of the monoclinic phase was determined from single-crystal X-ray intensity data and is of the titanite aristotype, space group A2/a. Unlike the triclinic phase, which contains SiO₅ polyhedra in addition to SiO₄ tetrahedra and SiO₆ octahedra, the monoclinic phase only contains tetrahedra and octahedra. The transition is zone-boundary, first-order in character with a 2.9% volume change.

The phase transition between the monoclinic and triclinic phases of CaSi₂O₅ is the first direct observation of a displacive-like transition involving a coordination change of silicon to be observed in a crystalline oxide or silicate. Comparison of the two structures shows that the overall topology and connectivity of the polyhedral framework is maintained, with one major exception: one oxygen atom (denoted O₂ox) from every second SiO₆ polyhedra in the octahedral chains of the monoclinic phase is moved away to a non-bonded distance of 2.831(3)Å, thereby breaking one eighth of the octahedral-tetrahedral bridges in the structure. In order to maintain local charge balance at the displaced oxygen the entire framework distorts and the Ca atoms are displaced significantly so as to bring a second Ca atom into bonded contact with O₂ox (Fig. 3.3-4). Within the silicon-oxygen polyhedron that loses its sixth oxygen at the transition the Si atom becomes displaced towards the oxygen opposite that which is removed, and the coordination polyhedron becomes a square-based pyramid (Fig. 3.3-4). All five of the remaining Si-O bond distances become shorter.
 

Fig. 3.3-4: A polyhedral representation of the changes in the structure of CaSi2O5 at the monoclinic-triclinic phase transition. In the monoclinic phase the O2 oxygen atom forms the bridge between a SiO4 tetrahedron and a SiO6 octahedron, and is also bonded to the left-hand Ca (spheres). In the triclinic phase, the O2 oxygen is removed to 2.83Å away (thin line) from the Si atom, forming an SiO5 pyramid. Other displacements create an additional bond between the O2 oxygen and the right-hand Ca.

Silicon coordinated by five oxygen atoms appears to play a crucial role in determining transport properties in silicate melts at high pressures, diffusion rates in silicate perovskite, and has been implicated in the process of amorphisation of high-pressure silicates that often occurs upon pressure release. The mechanism of transformation of SiO₆ octahedra into SiO₅ groups has important consequences for the kinetics of these processes and the observations made on CaSi₂O₅ may be applicable to all of them. As in CaSi₂O₅ the driving force for the SiO₆->SiO₅ transformation will be the improvement in the local charge balance at the oxygen atoms in the structure.