Tuning the giant magnetoelastic transition in Ba_3BiIr_2O_9 and Ba_3BiRu_2O_9

摘要

We have experimentally investigated the effects of pressure on the magnetoelastic transitions associated with the opening of spin-gaps in Ba_3BiIr_2O_9 and Ba_3BiRu_2O_9. 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) × 10~2 J mol~(-1) for Ba_3BiIr_2O_9 and -7.1(5) × 10~2 J mol~(-1) for Ba_3BiRu_2O_9. The transition in Ba_3BiRu_2O_9 is significantly more robust than in Ba_3BiIr_2O_9, 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.