Tuning the giant magnetoelastic transition in Ba₃BiIr₂O₉ and Ba₃BiRu₂O₉

We have experimentally investigated the effects of pressure on the magnetoelastic transitions associated with the opening of spin-gaps in Ba₃BiIr₂O₉ and Ba₃BiRu₂O₉. For both compounds, reducing the unit cell volume by either external physical and internal chemical pressure was found to reduce the temperature T* of the transition and, to a lesser extent, the magnitude of the associated negative thermal volume expansion. The results yield the latent heat associated with the transitions, −3.34(3) × 10² J mol⁻¹ for Ba₃BiIr₂O₉ and −7.1(5) × 10² J mol⁻¹ for Ba₃BiRu₂O₉. The transition in Ba₃BiRu₂O₉ is significantly more robust than in Ba₃BiIr₂O₉, requiring an order of magnitude higher pressures to achieve the same reduction in T*. The differing responses of the two compounds points to differences between the 4d and 5d metals and hence to the importance of spin-orbit coupling, which is expected to be much stronger in the Ir compound.