IUCr journals

Symmetry survives pressure

'Theoretical investigation of metastable Al2SiO5 polymorphs'
A.R. Oganov, G.D. Price, J.P. Brodholt (2001)

Acta Cryst. A57, 548-557

[Sillimanite] Isosymmetric transition in sillimanite (Al2SiO5). Top:sillimanite, bottom: its high-pressure isosymmetric successor. The new phase has 5-coordinate Al (light grey) and Si (dark grey). The 5-coordinate Si, so rare in inorganic compounds at low pressures, may be important in the Earth’s mantle.

Most phase transitions involve changes in crystal symmetry. However, there are examples of isosymmetric transitions, where the full space group is preserved and atoms occupy the same Wyckoff positions in both phases. Such transitions have interesting properties: being necessarily first-order, they disappear above the critical temperature and become fully continuous (just like liquid-gas transitions!).

Isosymmetric transitions involve changes in either the electronic (Ce, SmS), or atomic (KTiOPO4, PbF2) structure, and often are metastable.

The number of known isosymmetric transitions is rapidly increasing.

Using quantum-mechanical simulations, we predicted a metastable isosymmetric transition in sillimanite (Al2SiO5 – see Figure) at pressures of ~30-40 GPa. Detailed structural study revealed that the transition occurs when non-bonding Al…O and Si…O contacts become too short (<2.25 Å for Si…O), and transform into bonds. The existence of suggested critical bond distances would be useful in defining coordination numbers of atoms and explaining properties and phase transitions of condensed matter.

Forces in crystals obey space group symmetry, and tend to preserve it. Most phase transitions, however, change crystal symmetry, usually by a group-subgroup scheme. Isosymmetric transitions remain a rare, but fundamentally important phenomenon. Their study can lead to new important discoveries.

Highlights of article submitted by Artem R. Oganov
U. College London