Publication date: 15th December 2025
This research utilizes advanced solid-state nuclear magnetic resonance (ssNMR) spectroscopy to study local structural imperfections and electronic distortions, specifically pseudo Jahn-Teller effects, in double-doped perovskite alloys Cs2Na1-xAgxIn0.93Cr0.07Cl6. By means of 133Cs magic-angle spinning (MAS) NMR and static 35Cl WURST-QCPMG NMR experiments, atomic-scale heterogeneity induced by Na+/Ag+ substitution and paramagnetic Cr3+ incorporation was investigated. The 133Cs MAS NMR investigations reveal a linear correlation between the isotropic chemical shifts and the amount of Ag+, reflecting their influence on the diamagnetic lattice. Introduction of Cr3+ results in symmetric broadening of spectral lines due to pseudo-contact paramagnetic effects as well as three distinct components of multi-exponential T1 (133Cs) relaxation were uncovered: ultra-fast (T1 < 1 s), intermediate (T1 = 35–74 s), and slow (T1 = 100–700 s). The prevalence of the ultra-fast component in Ag-rich samples points to a more uniform dispersion of Cr3+ in smaller domains, whereas Na-rich samples show Cr3+ clustering into larger segregated areas. Two crystallographic phases were identified by 35Cl WURST-QCPMG NMR experiment: phase A (δiso = 226 ppm, CQ = 22.5 MHz, ηQ = 0) corresponding to Cs₂NaInCl₆, and phase B (δiso = 253 ppm, CQ = 26.7 MHz, ηQ = 0.01) associated with Cs2AgInCl6. The relative proportions of these phases exhibit a nonlinear dependence on the Na/Ag composition. Ag-rich samples display reduced signal-to-noise ratios in Na-rich regions because Cl- nuclei near Cr- ions become "invisible," indicating better dispersion of paramagnetic centers. 35Cl WURST-QCPMG NMR and 133Cs MAS NMR experiemnts revealed existence of pseudo Jahn-Teller distortions, which help explain the observed structural features observed by XRPD. In summary, unprecedented insights into paramagnetic species distribution and local symmetry breaking in doped perovskites were provided by the synergy of 133Cs/35Cl ssNMR. Highlighting not only the fundamental role of dopant chemistry in tuning optoelectronic behavior but emphasizing a powerful and versatile tool (ssNMR spectroscopy) for characterizing the intricate complexity and unraveling the subtle structural and electronic variations inherent in these sophisticated systems.
This work was supported by Czech Science Foundation (Grant No. GA 24-10199S)
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