Single crystal ESR and optical studies of VO(HPO₄)·0.5H₂O

We report single crystal ESR and optical results on VO(HPO₄)·0.5H₂O. Blue, blue-green, and green transparent crystals resulted from the hydrothermal reaction between VOSO₄·3H₂O and phenylphosphonic acid. Single crystal x-ray diffraction and chemical analysis confirmed the identity of the material as VO(HPO₄)·0.5H₂O. Crystals as large as 1mm × 1mm × 0.08mm were grown, making them suitable for ESR and optical experiments. Single crystals of VO(HPO₄)·0.5H₂O were mounted on a two circle goniometer and the six necessary angular measurements were recorded in order to diagonalize the g-tensor and provide the direction cosines. The crystals were indexed such that the crystallographic axes were known with respect to the goniometer. The V⁺⁴ (d¹) are paired antiferromagnetically in discreet dimers. The ESR signal results from the thermal population of an excited triplet state. The intensity of the ESR signal as a function of temperature was determined and fit to a thermally accessible triplet equation yielding a singlet-triplet separation of 60.8 cm^-1. Optical absorption in the visible and near infrared on single crystals was also employed to assist in the characterization of the nonaxial the g tensor of the individual vanadyl monomers.