Electron paramagnetic resonance of ${\mathrm{Fe}}^{3+}$ in $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$

We report electron paramagnetic resonance (EPR) experiments on the dominant ${\mathrm{Fe}}^{3+}$ center in high-quality near stoichiometric $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ grown from a $\mathrm{Li}∕\mathrm{Nb}$ ratio controlled melt. Symmetry related line splittings were clearly resolved and allowed unambiguous determination of the relative signs and magnitudes of the fourth-order trigonal zero field splitting (ZFS) spin-Hamiltonian (SH) parameters. Two conflicting EPR $C_3$ symmetry SHs for this center have been previously reported. Approximate agreement was found with that obtained for near-stoichiometric $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ grown using a ${\mathrm{K}}_2\mathrm{O}$ flux [G. I. Malovichko et al., J. Phys.: Condens. Matter 5, 3971 (1993)]. Extensive superposition model calculations of ZFS-SH terms showed that small displacement from either the Li or the Nb host site allowed the experimental rank-2 parameter value to be reproduced. Hence, on the basis of the EPR ZFS values alone, it is not currently possible to distinguish between possible $C_3$ incorporation sites within either the Li or Nb octahedra.

Figure 1

Comparison of symmetry related Li and Nb sites (middle and bottom). Projection on the $xy$ plane (top).

Figure 2

Experimental EPR line positions (black open circles) and line positions simulated using spin-Hamiltonian parameters given in Table (grey solid circles).

Figure 3

Energy level diagrams for magnetic field parallel to $z$ and $x$ axes with observed EPR transitions marked.

Figure 4

Energy level diagram for the magnetic field direction of 65° in the $zx$ plane. Levels calculated with $+b_4^3$ (solid line) and $-b_4^3$ (dashed line) shown along with EPR transitions.

Figure 5

Temperature dependence of ZFS parameters $b_2^0$ and $b_4^0$.

Figure 6

Superposition model results for $b_2^0$ using the crystal structures detailed in Table . The displacement from the host lattice site for which the calculated value is in agreement with experiment is shown. The methods of calculation are discussed in the text.

Figure 7

Superposition model results for rank-4 SH terms using the crystal structure from Ref. 21. Displacements are from the center of the octahedron, the Nb and Li sites positions are marked. The methods of calculation are discussed in the text. The calculation result obtained using the Li octahedron are shown with black lines, those for the Nb octahedron in grey.