Electron paramagnetic resonance of Cu²⁺ in KHCO₃ crystals

Electron paramagnetic resonance study of the Cu²⁺ doped into KHCO₃ single crystals has revealed that it is of axial symmetry. The g-values of Cu²⁺ are very nearly identical to some known Cu²⁺ in square-planar coordinations. The resonance linewidth is relatively narrow, such that the hyperfine patterns of Cu⁶³ and Cu⁶⁵ can be resolved. Moreover, the observation of the forbidden transitions induced by the Cu nuclear electric quadrupole interaction permits a test of the usual hypothesis that the g-factor, the hyperfine interaction, and the nuclear electric quadrupole interaction share a common set of principal axes.