Manganese(II)-doped
cesium–lead–chloride (Mn2+:CsPbCl3) perovskite nanocrystals have recently
been developed as promising luminescent materials and attractive candidates
for white-light generation. One approach to tuning the luminescence
of these materials has involved anion exchange to incorporate Br–, but the effects of anion exchange on Mn2+ speciation in doped metal-halide perovskites is not well understood
at a microscopic level. Here, we use a combination of X-band electron
paramagnetic resonance (EPR) and photoluminescence spectroscopies
to monitor the Mn2+ dopants in Mn2+:CsPbCl3 nanocrystals during Cl– → Br– anion exchange. Analytical measurements show that
the nanocrystals retain their Mn2+ over the course of Cl– → Br– anion exchange and
they continue to show strong Mn2+
d–d luminescence but, surprisingly, the Mn2+ EPR intensities
all but vanish. Further results suggest that Mn2+ ions
migrate during anion exchange to form clusters that are still luminescent
but show no EPR signal due to antiferromagnetic superexchange coupling.
Monte Carlo simulation and analysis of the Mn2+:CsPb(Cl1–x
Br
x
)3 lattice at various halide compositions (x) bolsters this interpretation by indicating a propensity for Mn2+–Cl– units to cluster as the Br– content increases, increasing the probability of the
nearest-neighbor Mn2+–Mn2+ interactions.
The driving force for this clustering is retention of the stronger
Mn–Cl bonds compared to Mn–Br bonds. In addition, modeling
predicts spinodal decomposition to form Mn2+-enriched domains
even at the end point compositions of x = 0 and 1,
with Mn2+ ordering in next-nearest-neighbor positions driven
by Coulomb interactions and lattice-strain minimization. These results
have important implications for both fundamental studies and applications
of doped and alloyed metal-halide perovskites.
We explore static spherically symmetric black hole solutions allowing a bulk U(1) vector field in the khronometric formulation of Hořava gravity by way of EinsteinAEther. We examine analytic solutions and study numerical results in the limit that the khronon does not backreact on the metric.
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