Origin of the monoclinic structure of anhydrous Na2IrCl6

Determine why anhydrous Na2IrCl6 was stabilized in a monoclinic structure rather than adopting the cubic structure expected for the A2IrCl6 family.

Background

The paper discusses an earlier report of anhydrous Na2IrCl6 crystallizing in a monoclinic P21/n structure and exhibiting canted antiferromagnetism. This structure was unexpected because related A2IrCl6 compounds generally form perfectly cubic vacancy-ordered double perovskites. Before the cubic anhydrous phase reported in the paper, the structural origin of the monoclinic phase had not been explained.

References

The reason why the anhydrous Na$_2$IrCl$_6$ got stabilized in a monoclinic structure, defying all expectations, however remained unexplained.

— Na$_2$Ir$^\text{IV}$Cl$_6$: The Missing Member of the Perfectly Cubic Vacancy Ordered $A_2$IrCl$_6$ Family, Another Potential $J_{eff}$ = $\frac{1}{2}$ Ground State Candidate  (2610.01371 - Soren et al., 1 Oct 2026) in Section 1, Introduction