We performed electrical resistivity and ${}^{13}\mathrm{C}$ nuclear magnetic resonance (NMR) measurements on $\lambda \text{−}{\left(\mathrm{BEDSe}\text{−}\mathrm{TTF}\right)}_2{\mathrm{FeCl}}_4$ under various pressures to obtain insights into the effect of $\pi \text{−}d$ interactions on the antiferromagnetic (AF) phase. The resistivity shows that the insulating state is maintained up to 2.0 GPa. ${}^{13}\mathrm{C}$ NMR measurements reveal that the AF transition occurs at 28, 31, and 23 K under 0.5, 1.0, and 2.0 GPa, respectively, indicating that a single AF phase in $\lambda \text{−}{\left(\mathrm{BEDSe}\text{−}\mathrm{TTF}\right)}_2{\mathrm{FeCl}}_4$ is observed under various pressures, whereas two AF phases were observed in $\lambda \text{−}{\left(\mathrm{BEDSe}\text{−}\mathrm{TTF}\right)}_2{\mathrm{GaCl}}_4$. The difference in phase diagrams between the magnetic anion salt, $\lambda \text{−}{\left(\mathrm{BEDSe}\text{−}\mathrm{TTF}\right)}_2{\mathrm{FeCl}}_4$, and the nonmagnetic anion salt, $\lambda \text{−}{\left(\mathrm{BEDSe}\text{−}\mathrm{TTF}\right)}_2{\mathrm{GaCl}}_4$, is attributed to $\pi \text{−}d$ interactions. These results provide insight into a possible microscopic role of $\pi \text{−}d$ interactions in the Fe salt. The magnetic properties of $\lambda \text{−}{\left(\mathrm{BEDSe}\text{−}\mathrm{TTF}\right)}_2{\mathrm{FeCl}}_4$ are described by the isolated cluster comprising two dimers and two anions with positive and negative $\pi \text{−}d$ interactions, suggesting antiferromagnetic coupling between two dimers in the cluster. We also discuss the possible AF structures of $\lambda \text{−}{\left(\mathrm{BEDSe}\text{−}\mathrm{TTF}\right)}_2{\mathrm{FeCl}}_4$.

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• Electrical conductivity
• Electronic structure
• Exchange interaction
• High-pressure studies
• Magnetic phase transitions
• 2-dimensional systems
• Antiferromagnets
• Strongly correlated systems
• Nuclear magnetic resonance
• Pressure techniques
• Resistivity measurements