Critical Properties of Weak Ferroelectric LGO Crystals Doped with Cr³⁺ Ions

Angular diagrams of Cr ³⁺ EPR spectra in Li ₂ Ge ₇ O ₁₅ (LGO) crystals confirm that magnetic ions substitute for Ge ⁴⁺ and together with interstitial lithium ions form Cr ³⁺ −Li ⁺ paired centers. On approaching ferroelectric phase transition point EPR lines of Cr ³⁺ broaden and split below T _C = 283.4 K. Temperature dependence of the local order parameter η(T) determined from EPR lines splitting, shows power like behavior with critical exponent β = 0.31 of three-dimensional Izing model. Results of previous investigations evidence that on going away from T _C the crossover from fluctuation regime to classical mean field one is characteristic for macroscopic and local LGO properties. For dependence η(T) in LGO:Cr such crossover is not detected. Absence of the crossover to classical mean field behavior is attributed to the defect nature of Cr ³⁺ −Li ⁺ centers which weaken the order parameter correlations. Influence of Cr ³⁺ −Li ⁺ centers on LGO properties is discussed on the basis of microscopic model of structural phase transitions.

Keywords:

• Phase transition
• weak ferroelectric
• structural defect
• electron paramagnetic resonance

PACS::

• 77.80.Bh
• 61.72.Hh
• 76.30.Fc

Acknowledgements

The author expresses sincere thanks to A. Kudzin for helpful discussions and to M. Volnianskii for single crystal growth.