Influence of the lone electron pair in the phase transitions observed in group XV sesquioxides under pressure

The different polymorphs of sesquioxides formed by group XV elements (As, Sb and Bi) have recently attracted an increasing interest with the aim to understand the effect of the lone electron pairs in their molecular character. At high pressures, the different polymorphs formed by these sesquioxides have shown a completely different behavior. As2O3 is one of the most compressible solid inorganic compounds and crystallizes in a cubic structure with strong molecular character that becomes amorphous around 20 GPa. In turn, isostructural Sb2O3 shows two 2nd-order phase transitions driven by dynamical instabilities, below 10 GPa, and a first-order phase transition above 20 GPa. The completely different behavior of both compounds with the same structure points out that the lone electron pair effect could play an important role in the presence of slight changes of compressibility. On the other hand, intermediate symmetric structures such as orthorhombic Sb2O3 and tetragonal Bi2O3 seem to be more prone to undergo different compressibility behaviors. In particular, the anomalous compressibility of Sb2O3 has revealed an electronic change associated to a transition of order higher than 2, which is hidden by the larger compressibility of the structural voids in the crystal lattice. Finally, Bi2O3 that crystallizes in a monoclinic structure, is the least symmetric structure and with the smaller cationic lone electron pair effect. This compound undergoes amorphization around 20 GPa and does not exhibit any anomalous compressibility associated to a first- or second-order phase transition. In summary, this work will show some guidelines in the stability of sesquioxide polymorphs and how the stereochemically active lone electron pair distribution affects the stability of the different structures, paying special attention on the intermediate symmetric structures where the most striking results have been observed.Peer Reviewe