Frustrated and Allowed Structural Transitions: The Theory-Guided Discovery of the Modulated Structure of IrSi

To the experienced molecular chemist, predicting the
geometries
and reactivities of a system is an exercise in balancing simple concepts
such as sterics and electronics. In this Article, we illustrate how
recent theoretical developments can give this combination of concepts
a similar predictive power in intermetallic chemistry through the
anticipation and discovery of structural complexity in the nominally
MnP-type compound IrSi. Analysis of the bonding scheme and DFT-Chemical
Pressure (CP) distribution of the reported MnP-type structure exposes
issues pointing toward new structural behavior. The placement of the
Fermi energy below an electronic pseudogap indicates that this structure
is electron-poor, an observation that can be traced via the 18–n rule to the structure’s Ir–Ir connectivity.
In parallel with this, the structure’s CP scheme highlights
facile paths of atomic motion that could enable a structural response
to this electronic deficiency. Combined, these analyses suggest that
IrSi may adopt a more complex structure than previously recognized.
Through synthesis and detailed structural investigation of this phase,
we confirm this prediction: single-crystal X-ray diffraction reveals
an incommensurately modulated structure with the (3+1)­D superspace
group P21/n(0βγ)­00 and q ≈ −0.22b* + 0.29c*. The structural modulations increase
the average number of Ir–Ir bonds to nearly match the 18–n expectations of the phase through Ir–Ir trimerization
along negative CPs with the incommensurability arising from the difficulty
of contracting and stretching the Ir–Ir contacts in a regular
pattern without expanding the structure along directions of negative
Si–Si CP. Integrating these results with prior analyses of
related systems points to a simple guideline for materials design,
the Frustrated and Allowed Structural Transitions (FAST) principle:
the ease with which competing structural phenomena can be experimentally
realized is governed by the degree to which they are supported by
the coordination of the atomic packing and electronic factors.