EPR and dielectric spectroscopy of reorienting Cr^{3+}-Li^{+} pair centres in Li_{2}Ge_{7}O_{15} crystal

Abstract

Earlier it has been shown that in the lithium heptagermanate Li 2 Ge 7 O 15 structure the chromium doping ions substitute the germanium hosts and create Cr 3+ -Li + pair centres. In the paper EPR and dielectric spectra of Li 2 Ge 7 O 15 :Cr 3+ crystal are studied in the temperature range 300 ÷ 500 K. The thermally activated reorientation of Cr 3+ -Li + pairs has been detected through EPR line width broadening and dielectric relaxation.

Optical Properties of Ferroelectrics and Measurement Procedures

“…It may be assumed that pair centers can reorient due to thermal activation. Reorientation of the pair centers should be accompanied by: i) shortening of the configuration life time and ii) switching of defect dipole moments [35]. This is reflected in the typical temperature dependence of the imaginary part of dielectric permittivity of chromium doped LGO single crystals [35] along the a-axis of the crystal shown in fig.8.…”

“…Reorientation of the pair centers should be accompanied by: i) shortening of the configuration life time and ii) switching of defect dipole moments [35]. This is reflected in the typical temperature dependence of the imaginary part of dielectric permittivity of chromium doped LGO single crystals [35] along the a-axis of the crystal shown in fig.8. Temperature dependence of Imaginary part ε''of permittivity ε=ε'-je'' of LGO:Cr 3+ , measured along crystal aaxis at the following frequencies : 0.5 kHz (1); 1 kHz (2); 5 kHz (3); 10 kHz (4).…”

“…It may be related with the effect of electron-photon interaction. It is known that Cr 3+ doping ions in the structure of Li 2 Ge 7 O 15 :Cr 3+ crystals substitute the Ge 4+ host ions within oxygen octahedral (GeO 6 ) complexes [31][32][33][34][35][36]. The optical spectra of Cr 3+ ions shows the existence of two types of Cr 3+ centre (type I and II with different values of effective g-factor) as observed in EPR (Electron Paramagnetic Resonance) spectra of Cr 3+ ions in ferroelectric phase of the crystals Li 2 Ge 7 O 15 :Cr 3+ [32,33].…”

“…The Crystals doped with chromium impurity give EPR (Electron Paramagnetic Resonance) signals characteristics of the trivalent chromium ions [ Fig.6]. It is known that impurity Cr 3+ ions substitute the Ge 4+ host ions within oxygen octahedral in the basic structure of (LGO) crystal [31][32][33][34][35][36]. Incorporation of tri-positive chromium ions into GeO 6 -octahedra changes the local symmetry of the lattice site from monoclinic C 2 group to triclinic C 1 group.…”

Conductivity of lithium heptagermanate glasses and single crystals doped with 3d- ions

“…The models of Cr and Mn centers in Li 2 Ge 7 O 15 single crystal were proposed on the basis of EPR data ( [8][9][10][11] and the refs therein). It was shown that Cr ions produced excess to stoichiometry Li interstitials (A Li ) and formed Cr 3+ -Li + pair centers.…”