Disorder Tolerance in LED Phosphor Host CaAlSiN₃: Correlated Cation Disorder with Minimal Electronic Penalty

In functional multinary nitrides, intrinsic cation disorder typically has severe electronic consequences, introducing gap states and driving band gap reduction through local bonding irregularities. CaAlSiN₃ (CASN), a benchmark nitride phosphor host for phosphor-converted light-emitting diodes, therefore presents a paradox: despite substantial Al/Si disorder generated during high-temperature synthesis, it maintains its exceptional optical performance. Understanding the fundamental effects of intrinsic disorder in this system could therefore provide broader insight into both its performance as a phosphor and the electronic structure of disorder-tolerant nitrides. Experiments typically treat the Al/Si disorder in CASN as random, whereas prior theoretical studies have been limited to a narrow set of ordered configurations, leaving its thermodynamic origin and electronic consequences unresolved. Here, we combine cluster-expansion Monte Carlo simulations with hybrid density functional theory to characterize the Al/Si disorder in CASN across a range of temperatures. Despite the emergence of disorder, the electronic structure remains resilient, with a maximum band gap reduction of only 3.8% and minimal carrier localization. This contrasts with related ternary nitrides, such as ZnGeN₂ and ZnSnN₂, in which less correlated cation disorder directly perturbs both band edges, driving pronounced band gap collapse and carrier localization. We attribute this tolerance to two features of CASN's crystal chemistry: a Ca d-dominated conduction-band minimum decoupled from the disordered sublattice and correlated disorder that suppresses the most electronically damaging local environments. These results establish CASN as a model system for disorder tolerance in multinary nitrides and suggest broader design principles for electronically robust phosphor hosts.