The Jahn–Teller effect in Cr⁵⁺-doped PbTiO₃: a multi-frequency electron paramagnetic resonance study

Abstract

Electron paramagnetic resonance (EPR) spectra of Cr5+ defects incorporated on Ti(4+) sites in powdered ceramics of PbTiO3 were investigated in the temperature range 50-400 K at 9 GHz (X), 34 GHz (Q) and 94 GHz (W band). The [Formula: see text] Jahn-Teller effect stabilizes the vibronic ground state of the 3d(1) electron of the Cr(5+) ion and leads to a tetragonally distorted defect- O6 octahedron with the point symmetry D(4h). The spontaneous electrical polarization present in the ferroelectric phase of PbTiO3 appears as a further perturbation producing an additional g-tensor contribution by the quadratic field effect. Its symmetry is dependent on the orientation of the electrical polarization with respect to the Jahn-Teller distortion axis, the tetragonal axis of the defect- O6 octahedron. If the polarization of a domain is anti- or parallel to this axis, the local tetragonal symmetry of the Cr5+ ion persists whereas it is reduced by a perpendicular orientation. Anisotropic EPR spectra of tetragonally and orthorhombic distorted Cr(5+)O6(12-) are detected at low temperatures. Increasing the temperature, the peaks of the two spectra are broadened and a motionally averaged isotropic spectrum appears at 200 K.

Studies of EPR g factors for tetragonal and rhombic Cr5+ centers in ferroelectric PbTiO3

“…Increasing the temperature, the peaks of the two Cr 5+ spectra are broadened and a motionally averaged isotropic spectrum (with g iso ) appears at T = 200 K [10]. In the ferroelectrical phase of PbTiO 3 , the oxygen octahedron surrounding Ti 4+ ion is elongated along C 4 axis.…”

“…In the ferroelectrical phase of PbTiO 3 , the oxygen octahedron surrounding Ti 4+ ion is elongated along C 4 axis. However, the observed g factors of t-Cr 5+ center in PbTiO 3 suggests that the substitutional Cr 5+ is arounded by a tetragonally-compressed octahedron (rather than the tetragonally-elongated octahedron in the host crystal) [10]. It is known that the ground state of 3d 1 ion in a tetragonally-elongated octahedron is the doublet 2 E. The degenerate ground state is unstable, so a T 2 ⊗ e Jahn-Teller effect may occur to change the oxygen octahedron from tetragonal elongation in the host PbTiO 3 to tetragonal compression in the t-Cr 5+ center, The cause is that the ground state of 3d 1 ions in a tetragonally-compressed octahedron is the stable singlet 2 B 2 (|d xy >).…”

“…It is known that the ground state of 3d 1 ion in a tetragonally-elongated octahedron is the doublet 2 E. The degenerate ground state is unstable, so a T 2 ⊗ e Jahn-Teller effect may occur to change the oxygen octahedron from tetragonal elongation in the host PbTiO 3 to tetragonal compression in the t-Cr 5+ center, The cause is that the ground state of 3d 1 ions in a tetragonally-compressed octahedron is the stable singlet 2 B 2 (|d xy >). The OCr 5+ center is attributed to the tetragonal t-Cr 5+ center being further perturbated by the spontaneous electrical polarization perpendicular to C 4 axis in the ferroelectric phase of PbTiO 3 [10]. At low temperature, the Jahn-…”

“…The physical and chemical properties of perovskites may be selectively changed by doping with transition-metal and rare-earth ions. For understanding the properties caused by these doping ions, many electron paramagnetic resonance (EPR) spectra of PbTiO 3 doped with transition-metal and rare-earth impurities were made [5][6][7][8][9][10] because the EPR technique is a powerful tool to study the defect model and defect structure of paramagnetic impurity centers in crystals. Recently, the EPR spectra of Cr 5+ defects incorporated on Ti 4+ site of ferroelectric PbTiO 3…”

“…were measured at the temperature range 50-400 K [10]. From the measurements, two Cr 5+ centers, one is the tetragonal (D 4h ) center t-Cr 5+ and another is the rhombic center O-Cr…”