DFG project (1998-2000)
CHARGE TRANSFER PROCESSES AND PHOTOSENSITIVE PARAMAGNETIC CENTERS IN BARIUM CALCIUM TITANATE
G.I.Malovichko, V.G.Grachev, O.F.Schirmer
The large electro-optic coefficients and high holographic sensitivity of crystals from barium titanate (BT) family make them promising candidates for various applications. For instance, BaTiO3:Rh is infrared sensitive material showing fast photorefractive response and large self-pumped phase-conjugate reflectivities. However, the use of it in photorefractive devices operating at room temperature meets a difficulty since BT has the tetragonal-orthorhombic phase transition at about 280 K. Barium calcium titanate crystals, Ba0.77Ca0.23TiO3 (BCT) gives a chance along with the overcoming this drawback to get even better characteristics than BT by the optimization of defect related properties.
FIGURE 1. Fragments of EPR spectra of BCT for different pump energies. Magnetic field B||<100>, microwave frequency 9.85 GHz, T=20 K.
The nominally pure and Rh doped BCT crystals were investigated with the help of optical absorption spectroscopy, light induced absorption change measurements and electron paramagnetic resonance (EPR). The group of broad overlapped EPR lines was registered in all studied crystals before the sample illumination. Analyzing their angular dependencies we could identify two different centers of iron trace impurity. The first of them is low-symmetry center Fe3+ (S=5/2, g»2.00, dominated parameter of axial crystal field b20»0.15 cm-1), the second Fe3+ center has nearly cubic surrounding. The lines of four other paramagnetic defects were found after illumination at low temperatures by the light of xenon lamp with different filters. They were identified with Ti3+ group (S=1/2, g-factors about 1.91-1.93), Rh2+ (g||=2.03, g^=2.30, A£0.0015 cm-1), O- -like defects (g ~ 2.013-2.03) and non-controlled impurity Pt3+ (S=1/2, g||=1.96, g^=2.45, A||=0.0015, A^=0.02 cm-1 for isotope 195Pt). These defects are stable at temperatures 4.2-50K, but they disappear after heating to 240-300 K. All found intrinsic and extrinsic defects are participants of charge transfer processes and their correlated light induced EPR and absorption changes are elucidated. At the light energy hn about 1.3 eV the holes abandon Ti4+ creating the paramagnetic electronic Ti3+. At hn>2.3 eV hole O- centers appear. This leads to the simultaneous increase of Ti3+ concentration. It means that O2- ions capture the holes and create hole O-centers. Nearly located non-controlled or intentionally introduced impurities (Sr, K, Na, ...) or Ca itself may serve as pins for the fixation of the electrons and holes. Additional correlated changes were found at the light energy hn>3.2 eV (band-band transition), when EPR lines of Rh2+, Pt3+ appear and intensities of Ti3+, O- and Fe3+ groups further essentially increase.
The dominated charge transfer from Ti4+ to O2- is accompanied with the parallel processes, involving both intrinsic ions and impurities.
FIGURE 2. Light induced changes of absorption coefficient for BCT:Rh crystal at 150 K.
Models of defects participating in light induced processes are proposed. The similarity and difference of the obtained results for poled and unpoled, doped and undoped, reduced and oxidized BCT samples are discussed and compared with the published data for BT crystals.
FIGURE 3. Models of paramagnetic defect centers derived from EPR data: a – Fe43+ with oxygen vacancy, VO; b – Fe33+ with CaBa; c – localized electron or Ti3+; d – localized hole or O- center.
For details see: G.Malovichko, V.Grachev, R.Pankrath, O.Schirmer. Photosensitive centers and charge transfer processes in barium calcium titanate. Ferroelectrics, 258, 169-176 (2001).