We report structural and magnetic properties of the spin-$\frac{1}{2}$ quantum antiferromagnet $\mathrm{Cu[}{\mathrm{C}}_6{\mathrm{H}}_2{\left(\mathrm{COO}\right)}_4\mathrm{][}{\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{NH}}_3\mathrm{]}{}_2$ by means of single-crystal x-ray diffraction, magnetization, heat capacity, and electron-spin-resonance (ESR) measurements on polycrystalline samples, as well as band-structure calculations. The triclinic crystal structure of this compound features ${\mathrm{CuO}}_4$ plaquette units connected into a two-dimensional framework through anions of the pyromellitic acid $\mathrm{[}{\mathrm{C}}_6{\mathrm{H}}_2{\left(\mathrm{COO}\right)}_4{\mathrm{]}}^{4-}$. The ethylamine cations $\mathrm{[}{\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{NH}}_3{]}^{+}$ are located between the layers and act as spacers. Magnetic susceptibility and heat capacity measurements establish a quasi-two-dimensional, weakly anisotropic, and nonfrustrated spin-$\frac{1}{2}$ square lattice with the ratio of the couplings $J_a/J_c\simeq 0.7$ along the $a$ and $c$ directions, respectively. No clear signatures of the long-range magnetic order are seen in thermodynamic measurements down to 1.8 K. However, the gradual broadening of the ESR line suggests that magnetic ordering occurs at lower temperatures. Leading magnetic couplings are mediated by the organic anion of the pyromellitic acid and exhibit a nontrivial dependence on the Cu-Cu distance, with the stronger coupling between those Cu atoms that are farther apart.

• Received 29 July 2014
• Revised 9 January 2015

DOI:https://doi.org/10.1103/PhysRevB.91.054409