ABSTRACT MXenes combine widely tunable electronic properties with promising chemical agility, yet Ti 3 C 2 T x , the most studied member of this family, exhibits puzzling electronic and magnetic anomalies near room temperature. Here, we show that these effects arise from self‐formed titanium suboxide domains, predominantly Ti 3 O 5 , stabilized within the MXene galleries. Employing comprehensive in situ structural, spectroscopic, thermodynamic, and transport analyses, we identify subtle but reproducible transitions at K and K. Raman microscopy and X‐ray diffraction were used to track Ti 3 O 5 polymorphism, while thermodynamic and transport signatures coincide with known transformations. Electron paramagnetic resonance reveals a marked anomaly in the Dysonian diffusion parameter without a considerable change in spin susceptibility, indicating reconfigured electronic pathways rather than carrier loss. These results reframe Ti 3 C 2 T x as an inherently composite MXene‐oxide system whose functional response is governed by minority suboxide phases. Beyond resolving a field‐wide controversy, controlled oxidation emerges as a design lever to engineer phase‐change behavior for thermal latching, opto‐thermal switching, and robust, near‐room‐temperature MXene devices.
Márkus et al. (Wed,) studied this question.