Magnetic properties of honeycomb-based spin models in verdazyl-based salts

We successfully synthesized the verdazyl-based salts ${(m-\mathrm{MePy}-\mathrm{V})}_2{\mathrm{Fe}}_2{\mathrm{OCl}}_6$ and ${[m-\mathrm{MePy}-\mathrm{V}-{(p-\mathrm{Cl})}_2]}_2{\mathrm{Fe}}_2{\mathrm{OCl}}_6·{\left({\mathrm{CH}}_3\mathrm{CN}\right)}_2$ with different ${\mathrm{Fe}}_2{\mathrm{OCl}}_6^{2-}$ structures. The exchange interactions between ${\mathrm{Fe}}^{3+}$ ions in the ${\mathrm{Fe}}_2{\mathrm{OCl}}_6^{2-}$ anions were evaluated to be nearly equivalent and to form an $S=5/2$ antiferromagnetic (AF) dimer in each salt. We obtained the magnetic susceptibility of $S=1/2$ spins on the radical by subtracting the contribution of $S=5/2$ spins in the anion from the observed magnetic susceptibility. For (${m-\mathrm{MePy}-\mathrm{V})}_2{\mathrm{Fe}}_2{\mathrm{OCl}}_6$, ab initio molecular orbital (MO) calculations indicated that the $S=1/2$ AF dimers are weakly connected by ferromagnetic interactions, thereby forming a honeycomb lattice. The experimentally measured magnetic susceptibility and specific heat indicate a corresponding nonmagnetic gapped behavior. For $[m-\mathrm{MePy}-\mathrm{V}-{(p-\mathrm{Cl})}_2{]}_2{\mathrm{Fe}}_2{\mathrm{OCl}}_6·{\left({\mathrm{CH}}_3\mathrm{CN}\right)}_2$, MO calculations indicated that four types of dominant interactions form an unconventional $S=1/2$ honeycomb-based 3D spin model. The magnetic susceptibility and specific heat indicate a phase transition to a 3D order at approximately 4.8 K.

Figure 1

(a) Molecular structure of (${m-\mathrm{MePy}-\mathrm{V})}_2{\mathrm{Fe}}_2{\mathrm{OCl}}_6$. (b) ${\mathrm{Fe}}_2{\mathrm{OCl}}_6^{2-}$ anion forming a bent structure. (c) Crystal structure in the $ac$ plane. Hydrogen atoms are omitted for clarity. Molecular pairs associated with exchange interactions (d) $J_{\mathrm{a}}$ and (e) $J_{\mathrm{b}}$. The dashed lines indicate N-N and N-C short contacts. (f) Crystal structure of (${m-\mathrm{MePy}-\mathrm{V})}_2{\mathrm{Fe}}_2{\mathrm{OCl}}_6$ in the $bc$ plane. The lines represent $J_{\mathrm{a}}$ and $J_{\mathrm{b}}$ forming the honeycomb lattice.

Figure 2

(a) Molecular structure of $[m-\mathrm{MePy}-\mathrm{V}-{(p-\mathrm{Cl})}_2{]}_2{\mathrm{Fe}}_2{\mathrm{OCl}}_6$. (b) ${\mathrm{Fe}}_2{\mathrm{OCl}}_6^{2-}$ anion forming a linear structure. Molecular pairs associated with exchange interactions (c) $J_1$, (d) $J_2$, (e) $J_3$, and (f) $J_4$. The dashed lines indicate C-C short contacts. Crystal structure of $[m-\mathrm{MePy}-\mathrm{V}-{(p-\mathrm{Cl})}_2{]}_2{\mathrm{Fe}}_2{\mathrm{OCl}}_6·{\left({\mathrm{CH}}_3\mathrm{CN}\right)}_2$ in the (g) $bc$ and (h) $ab$ planes. Hydrogen atoms are omitted for clarity. The lines represent $J_1, J_2, J_3$, and $J_4$ forming the honeycomb-based 3D lattice.

Figure 3

$S=1/2$ honycomb-based 3D spin model composed of $J_i$ ($i$ = 1–4) in $[m-\mathrm{MePy}-\mathrm{V}-{(p-\mathrm{Cl})}_2{]}_2{\mathrm{Fe}}_2{\mathrm{OCl}}_6·{\left({\mathrm{CH}}_3\mathrm{CN}\right)}_2$.

Figure 4

Temperature dependence of magnetic susceptibility ($\chi =M/H$) of (${m-\mathrm{MePy}-\mathrm{V})}_2{\mathrm{Fe}}_2{\mathrm{OCl}}_6$ at 0.5 T. The solid lines in (a) and (b) represent the calculated results for the $S=5/2$ Fe dimer and $S=1/2$ dimer coupled by $J_1$. The open triangles denote raw data, and ${\chi }_{\mathrm{v}}$ shows the contribution of $S=1/2$ spins on the radical, which is obtained by subtracting the Fe dimer from the raw data. The corrected ${\chi }_{\mathrm{v}}$ is adjusted to account for the paramagnetic term due to impurities.

Figure 5

Temperature dependence of specific heat $C_{\mathrm{p}}$ of (${m-\mathrm{MePy}-\mathrm{V})}_2{\mathrm{Fe}}_2{\mathrm{OCl}}_6$.

Figure 6

Temperature dependence of magnetic susceptibility ($\chi =M/H$) of $[m-\mathrm{MePy}-\mathrm{V}-{(p-\mathrm{Cl})}_2{]}_2{\mathrm{Fe}}_2{\mathrm{OCl}}_6·{\left({\mathrm{CH}}_3\mathrm{CN}\right)}_2$ at 0.05 T. The solid line represents the calculated result for the $S$ =5/2 Fe dimer. The open triangles denote raw data and ${\chi }_{\mathrm{v}}$ shows the contribution of $S=1/2$ spins on the radical, which was obtained by subtracting the Fe dimer from the raw data. The inset shows the temperature dependence of $\chi T$.

Figure 7

Temperature dependence of specific heat $C_{\mathrm{p}}$ of $[m-\mathrm{MePy}-\mathrm{V}-{(p-\mathrm{Cl})}_2{]}_2{\mathrm{Fe}}_2{\mathrm{OCl}}_6·{\left({\mathrm{CH}}_3\mathrm{CN}\right)}_2$. The inset shows the temperature dependence of $C_{\mathrm{p}}/T$ . The arrows indicate the phase-transition temperature.