Local structure distortion and spin Hamiltonian parameters for Cr4+ ions in Cr4+:α-Al2O3 crystals

The relations between the spin Hamiltonian (SH) parameters and crystal structure of Cr4+:α-Al2O3 crystals have been established. On the basis of this, the SH parameters including zero-field splitting parameter D and Zeeman g-factors (g|| and g⊥) for Cr4+ ions in Cr4+:α-Al2O3 crystals, taking into account the spin–spin (SS), spin-other-orbit (SOO) and orbit–orbit (OO) magnetic interactions in addition to the spin–orbit (SO) magnetic interaction, are theoretically investigated using complete diagonalization method (CDM). The theoretical results are in excellent agreement with the experimental ones when the upper three O2− ions rotate 0.94° toward [1 1 1] axis and the lower three O2− ions rotate 0.92° toward it. Hence, the local structure distortion effect plays an important role in explaining the spectroscopic properties of Cr4+ ions in Cr4+:α-Al2O3 crystals. This study shows that for Cr4+:α-Al2O3 the contributions arising from SS, SOO, and OO interactions to the zero-field splitting (ZFS) parameter D are appreciable, whereas those to g|| and g⊥ are quite small.