Optical and EPR investigations of Np4+ in single crystals of ZrSiO4.

Abstract

Polarized optical spectroscopy has been used to investigate the electronic structure of tetravalent neptunium incorporated as a dilute impurity in single crystals of the tetragonal host ${\mathrm{ZrSiO}}_{4}$. Thirty-one levels were assigned from 30 intense polarized optical transitions and used to obtain a fit to a parametric Hamiltonian to within an rms deviation of 34 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Thirteen other levels with comparable intensities and good polarization characteristics were observed. Inclusion of these levels using the parameters from the 31-level fit, however, resulted in an increase of the rms deviation to approximately 100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The EPR spectra of the ground state of ${\mathrm{Np}}^{4+}$:${\mathrm{ZrSiO}}_{4}$ were measured and fit to an axial spin Hamiltonian to obtain the parameters: \ensuremath{\Vert}${g}_{?}$\ensuremath{\Vert}=0.8\ifmmode\pm\else\textpm\fi{}0.6, \ensuremath{\Vert}${g}_{\ensuremath{\perp}}$\ensuremath{\Vert}=2.59\ifmmode\pm\else\textpm\fi{}0.02, A=-2400\ifmmode\pm\else\textpm\fi{}1200 MHz, and B=4750\ifmmode\pm\else\textpm\fi{}30 MHz. These results are consistent with the calculated ${\ensuremath{\Gamma}}_{6}$ ground state.