Optical and magnetic resonance spectra of inorganic molecular crystals-uranium borohydride [U(BH4)4] in hafnium borohydride [Hf(BH4)4]

The electronic absorption spectra of U(BH₄)₄ as a guest in Hf(BH₄)₄ host crystals are reported. Optical spectra, from the charge transfer or f‐to‐d cutoff at approximately 2900 Å to the low energy limit due to host molecular vibrations at approximately 2.0 μm, have been obtained at 300, 77, and 2°K for mixed crystals of U(BH₄)₄/Hf(BH₄)₄ and U(BD₄)₄/Hf(BD₄)₄. From temperature effects, deuteration shifts, and intensity patterns, over 20 possible forced electric dipole crystal field electronic origins have been identified. Several molecular vibrations have been identified built on these origins and, based on physically reasonable arguments, are tentatively associated with specific vibrational symmetries and motions of M(BH₄)₄ type molecules. Attempts to obtain EPR spectra between 2 and 77°K are discussed. Zeeman spectra in fields of up to 93.5 kG for the region 4000–7500 Å have been used to aid in the identification of both ground and excited electronic crystal field states. Calculations, based on a free ion and cubic crystal field parameterized Hamiltonian, are employed to assign electronic origins. The spectra are best fit to a crystal field model Hamiltonian whose two parameters $A_{60}{\mathrm{\hspace{0.17em}〈\; r}}^6〉$ and $A_{40}{\mathrm{\hspace{0.17em}〈\; r}}^4〉$ have their signs determined from considerations of 12 negative point charges at the hydrogen positions. Implications of these crystal field and vibrational analyses for actinide spectroscopy are discussed.