Electron Spin Resonance of ${\mathrm{Cu}}^{2+}$ in Zn${\mathrm{}\left(\mathrm{HCOO}\right)\mathrm{}}_2$·2${\mathrm{H}}_2$O: Isolated Ions and Exchange-Coupled Pairs

The ESR spectrum of ${\mathrm{Cu}}^{2+}$ introduced substitutionally into single crystals of zinc formate dihydrate, Zn${\left(\mathrm{H}\mathrm{C}{\mathrm{O}}_2\right)}_2$·2${\mathrm{H}}_2$O, has been investigated at $X$ band in the temperature range 4.2 to 77°K. At low concentrations (Cu:Zn<0.005) the spectra from isolated ${\mathrm{Cu}}^{2+}$ ions occupying each of the four metal-ion sites per unit cell have been observed and fitted to a spin Hamiltonian of axial symmetry. For the type-1 sites, which are coordinated by a slightly distorted octahedron of oxygen atoms belonging to six different carboxyl groups, the spin Hamiltonian parameters are: $g_{\mathrm{II}}=2.416\mathrm{}\pm 0.005$, $g_{\perp }=2.077\mathrm{}\pm 0.007$, $A^{63}=0.0124\mathrm{}\pm 0.0002$ ${\mathrm{cm}}^{-1\mathrm{}}$, $B^{63}=0.0023\mathrm{}\pm 0.0002$ ${\mathrm{cm}}^{-1\mathrm{}}$, $Q^{\prime 63}=0.00095\mathrm{}\pm 0.00002$ ${\mathrm{cm}}^{-1\mathrm{}}$. For the type-2 sites, which are coordinated by a slightly distorted octahedron of four oxygen atoms from water molecules and two oxygens from two different carboxyl groups, the parameters are: $g_{\mathrm{II}}=2.420\mathrm{}\pm 0.005$, $g_{\perp }=2.066\mathrm{}\pm 0.007$, $A^{63}=0.0134\mathrm{}\pm 0.0005$ ${\mathrm{cm}}^{-1\mathrm{}}$, $B^{63}=0.0026\mathrm{}\pm 0.0012$ ${\mathrm{cm}}^{-1\mathrm{}}$, $Q^{\prime 63}\approx 0.0015$ ${\mathrm{cm}}^{-1\mathrm{}}$. At both kinds of sites the symmetry axis is directed toward the oxygen belonging to the formate group which links nearest-neighbor type-1 and type-2 sites. In a sample of higher concentration (Cu: Zn = 0.034) we have observed a spectrum which we attribute to pairs of exchange-coupled ${\mathrm{Cu}}^{2+}$ ions occupying nearest-neighbor type-1 sites. From the temperature dependence of the intensity of the spectrum we have determined the isotropic exchange interaction to be antiferromagnetic and given by $\mathrm{}\left|J\right|=33\mathrm{}\pm 1$ ${\mathrm{cm}}^{-1\mathrm{}}$.