Molecular self-organization of Ho$^{3+}$ impurity ions in synthetic forsterite

The structure of paramagnetic centers formed by impurity Ho$^{3+}$ ions in synthetic forsterite is studied by submillimeter EPR spectroscopy in the frequency range 65–200 GHz. It is found that Ho$^{3+}$ enters into the Mg$^{2+}$ sublattice in the form of single ions and dimer centers. The concentration of dimer centers considerably exceeds the concentration of single ions, which points to the molecular self-organization of Ho$^{3+}$ impurity ions into dimers during the growing of the crystals from melt. Possible structures of the dimer center are discussed. The parameters of the effective spin Hamiltonian describing the behavior of the electron-nuclear sublevels of the two lowest electronic levels of the Ho$^{3+}$ $^5$I$_8$ ground multiplet are determined for a single ion and a dimer center.