Pressure-dependent properties of muoniated radicals in an electron acceptor crystal studied using avoided level crossing muon spectroscopy

Muon spin spectroscopy is a powerful tool for probing magnetism and electron configurations in materials. In the electron acceptor TCNQ, implanted polarised positive muons form radicals with a delocalised unpaired electron. Using avoided level crossing (ALC) spectroscopy under pressures up to 6 kbar, we observed upward shifts in resonance fields, suggesting an increase in muon hyperfine coupling constant $A_{\mu }$. First-principles calculations with quantum corrections identified two muon addition sites: a bridging carbon and a terminal nitrogen. Statistical analysis of relaxed structures from random muon positions showed how site basins of attraction change with pressure, revealing a shift in site populations. Simulated ALC spectra using DFT parameters reproduced the observed shifts. This indicates the pressure-dependent resonance shift arises not from a direct change in $A_{\mu }$, but from redistribution between muon sites due to modifications in the local energy landscape.