A magnetic circular dichroism and optically detected magnetic resonance investigation of Fe²⁺and Fe³⁺centres in KTaO₃

Abstract

A cubic and an axial Fe 2+ centre, Fe 2+ Ta and Fe 2+ Ta -V O , are found in reduced Fe-doped KTaO 3 crystals in conjunction with their trivalent partners. These four defects are characterized by optically detected magnetic resonance (ODMR) via the magnetic circular dichroism (MCD) of the absorption. The g-values are determined by and interpreted using crystal-field terms. For all centres, the optical absorption bands are identified by the tagged-MCD method. The main MCD bands of the divalent ions are attributed to the well known 5 T 2 → 5 E crystal-field transition in the 10 000 cm −1 region. Bands at higher energies are interpreted as intervalence transfer transitions to conduction band-like states.

Modeling local structure using crystal field and spin Hamiltonian parameters: the tetragonal FeK3+–OI2−defect center in KTaO3crystal

“…A controversy exists in the literature concerning the sign of D for the Fe 3+ K -O 2− I center. The sign of D = 4.46 cm −1 has unequivocally been determined experimentally as positive [9,10], and thus represented as such by other authors, see, e.g., [4,6,7,[13][14][15]. However, in a recent paper Zheng et al [16] have claimed that Bykov et al [9] did not determine the sign of D. The authors [16] have compared their negative theoretical values of D with the alleged experimental value D expt = −4.46 cm −1 , just disregarding the bulk of experimental evidence [4,6,7,9,10,[13][14][15].…”

“…Bykov et al [9] have determined that D does not depend on temperature for the former center, whereas it reduces linearly with temperature for the latter center. The site assignments were confirmed by the optically detected magnetic resonance and the magnetic circular dichroism [6,11]. Structural models proposed by Laguta et al [7] for the two axial Fe 3+ centers have been confirmed by the authors' ENDOR studies and additionally by the electric-field-induced orientation studies by Sochava et al [12] for the Fe 3+ K -O 2− I center.…”

“…The parametric relations for δ 1 and δ 2 are obtained by the diagonalization of the energy matrices within the full 3d 5 configuration. Comparing the experimental [4,6,7,9,[13][14][15] values D = 4.46 cm −1 for Fe 3+ K -O 2− I and the only available value a = 0.0030 cm −1 [58] for Fe 3+ K at the cubic site in KTaO 3 or a ∼ = 0.0090 cm −1 (see table I in [59]) for Fe 3+ at the cubic and tetragonal sites in BaTiO 3 it may be safely assumed that the relation: 4D…”

“…A perfect KTaO 3 crystal has cubic structure resulting in cubic site symmetry around the cations, namely the 6-fold cubic coordination for Ta 5+ sites, whereas it has 12-fold cubic coordination for K + sites (see figure 1) [2,3]. Trivalent Fe 3+ ions doped in KTaO 3 crystal can substitute for both Ta 5+ and K + ions [4][5][6]. However, in addition to the two cubic sites, two axial (tetragonal) defect sites have been observed in EMR spectra [4,7].…”

“…To this end we utilize the superposition model of CF parameters [17][18][19][20][21][34][35][36] and the microscopic SH theory [22][23][24][25][26]. Since experimental work [6,13,39] favors the idea that the interstitial defect oxygen is responsible for the strong axial character of this center, we attempt to corroborate this model. Note that EMR and dielectric measurements on samples with different Fe 3+ concentrations have confirmed the decisive role of the complex Fe 3+ K -O 2− I [40,41].…”