Lead-halide perovskites increasingly
mesmerize researchers because they exhibit a high degree of structural
defects and dynamics yet nonetheless offer an outstanding (opto)electronic
performance on par with the best examples of structurally stable and
defect-free semiconductors. This highly unusual feature necessitates
the adoption of an experimental and theoretical mindset and the reexamination
of techniques that may be uniquely suited to understand these materials.
Surprisingly, the suite of methods for the structural characterization
of these materials does not commonly include nuclear magnetic resonance
(NMR) spectroscopy. The present study showcases both the utility and
versatility of halide NMR and NQR (nuclear quadrupole resonance) for
probing the structure and structural dynamics of CsPbX
3
(X = Cl, Br, I), in both bulk and nanocrystalline forms. The strong
quadrupole couplings, which originate from the interaction between
the large quadrupole moments of, e.g., the
35
Cl,
79
Br, and
127
I nuclei, and the local electric-field gradients,
are highly sensitive to subtle structural variations, both static
and dynamic. The quadrupole interaction can resolve structural changes
with accuracies commensurate with synchrotron X-ray diffraction and
scattering. It is shown that space-averaged site-disorder is greatly
enhanced in the nanocrystals compared to the bulk, while the dynamics
of nuclear spin relaxation indicates enhanced structural dynamics
in the nanocrystals. The findings from NMR and NQR were corroborated
by
ab initio
molecular dynamics, which point to the
role of the surface in causing the radial strain distribution and
disorder. These findings showcase a great synergy between solid-state
NMR or NQR and molecular dynamics simulations in shedding light on
the structure of soft lead-halide semiconductors.