Atomic Cartography of High‐Entropy Cs ₂ <i>B</i> Cl ₆ Perovskite‐Inspired Materials: The Vital Role of Solid‐State NMR Spectroscopy in Identifying Elemental Disorder

The burgeoning field of high-entropy materials (HEMs) has sparked significant interest by leveraging synergistic "cocktail effects" from inexpensive and abundant elements to access unprecedented physical, optical, and chemical properties. While standard characterization techniques, such as diffraction and energy-dispersive X-ray spectroscopy, provide valuable insights, they often fall short in elucidating the intricate atomic-level disorder and the presence of nanoscale phase separation or persistent nanodomains. To overcome these limitations, this work introduces a robust and rapid analytical method based on solid-state ¹³³Cs nuclear magnetic resonance (NMR) spectroscopy, capable of directly probing atomic-level mixing in these complex materials. This technique is demonstrated by exploring a series of Cs₂BCl₆ (where B represents various combinations of 1 to 8 elements at the B-site) perovskite-inspired materials synthesized via multiple routes. Although X-ray diffraction and EDX suggest successful HEM formation across all methods, ¹³³Cs NMR analysis reveals the prevalence of phase separation and preferred elemental clustering. A high-energy mechanochemical synthetic approach is proven to drive atomic-level mixing of up to eight elements. These results demonstrate the need for a synergistic approach that combines local atomic sensitivity using NMR methods with long-range order diffraction methods to solve chemical structure and comprehensively assess rational design strategies for halogen-containing materials.