Entropic Control of Bonding, Guided by Chemical Pressure: Phase Transitions and 18- n + m Isomerism of IrIn 3

As with other electron counting rules, the 18- rule of transition metal-main group (T-E) intermetallics offers a variety of potential interatomic connectivity patterns for any given electron count. What leads a compound to prefer one structure over others that satisfy this rule? Herein, we investigate this question as it relates to the two polymorphs of IrIn : the high-temperature CoGa -type and the low-temperature IrIn -type forms. DFT-reversed approximation Molecular Orbital analysis reveals that both structures can be interpreted in terms of the 18- rule but with different electron configurations. In the IrIn type, the Ir atoms obtain largely independent 18-electron configurations, while in the CoGa type, Ir-Ir isolobal bonds form as 1 electron/Ir atom is transferred to In-In interactions. The presence of a deep pseudogap for the CoGa type, but not for the IrIn type, suggests that it is electronically preferred. DFT-Chemical Pressure (CP) analysis shows that atomic packing provides another distinction between the structures. While both involve tensions between positive Ir-In CPs and negative In-In CPs, which call for the expansion and contraction of the structures, respectively, their distinct spatial arrangements create very different situations. In the CoGa type, the positive CPs create a framework that holds open large void spaces for In-based electrons (a scenario suitable for relatively small T atoms), while in the IrIn type the pressures are more homogenously distributed (a better solution for relatively large T atoms). The open spaces in the CoGa type result in quadrupolar CP features, a hallmark of low-frequency phonon modes and suggestive of higher vibrational entropy. Indeed, phonon band structure calculations for the two IrIn polymorphs indicate that the phase transition between them can largely be attributed to the entropic stabilization of the CoGa -type phase due to soft motions associated with its CP quadrupoles. These CP-driven effects illustrate how the competition between global and local packing can shape how a structure realizes the 18- rule and how the temperature can influence this balance.