The A and B cations in pyrochlore oxides can be combined in various oxidation states, including A³⁺/B⁴⁺ (e.g., rare-earth titanates and zirconates), A²⁺/B⁵⁺ (e.g., Cd₂Nb₂O₇), and A⁺/B⁵⁺ (e.g., Ag₂Sb₂O₆). Antisite defects, where A-site and B-site cations partially exchange positions, are commonly observed in rare-earth pyrochlores under extreme conditions such as ion irradiation [1] and external pressure [2-4]. The formation of antisite defects in A₂B₂O₇ oxides is strongly influenced by the ionic radius ratio (r_A/r_B) between A and B cations, as well as their electronic structures [3,4]. In particular, rare-earth zirconates, which have smaller r_A/r_B ratios compared to titanates, are more prone to antisite defect formation. Studies have shown that under high-pressure conditions, rare-earth titanates and zirconates undergo irreversible structural phase transitions closely linked to antisite defect formation [2,3]. To rare earth zirconates, after pressure release from 45 GPa and above, the HP phase transforms irreversibly into a disordered fluorite phase. In contrast, Gd₂Ti₂O₇ undergoes pressure-induced amorphization rather than forming a stable HP phase, highlighting the different structural responses of titanates and zirconates to compression. Interestingly, the pressure-induced structural transitions observed in pyrochlore oxides are comparable to those triggered by ion irradiation [3,4], suggesting a common underlying defect-driven mechanism. Pyrochlore oxides with the formula A⁺₂B⁵⁺₂O₆, such as Ag₂Sb₂O₆ (AgSbO₃), lack the additional oxygen at the 8a site. Due to the significant difference in electronic configurations between A⁺ and B⁵⁺ cations, cation exchange under high pressure is energetically unfavorable, making antisite defect formation unlikely.

Our experiments confirm that AgSbO₃ undergoes a fully reversible pressure-induced phase transition at ~13.5 GPa [5] and the high-pressure phase is stable up to more than 40 GPa. The high-pressure phase adopts a monoclinic structure, and HP phase transforms back to the cubic pyrochlore structure after pressure release. Unlike A₂B₂O₇ pyrochlores, the absence of antisite defects in AgSbO₃ plays a key role in enabling the reversible nature of the structural transformation under pressure.

References

[1] R.C. Ewing, W.J. Weber, J. Lian, J. Appl. Phys.  95, 5949-5971 (2004)

[2] J. Lian, et al, Acta Mater. 51, 1493-1502 (2003).

[3] F.X. Zhang, et al, Phys. Rev. Lett. 100, 045503 (2008).

[4] M. Lang, et al, NIM-B, 268, 2951-2959 (2010).

[5] F.X. Zhang, et al, not published work. (2025)