Ferroelectric transition induced dipole moments in probe ions in Li₂Ge₇O₁₅ crystals doped with Mn⁴⁺ and Cr³⁺

Abstract

In ferroelectric phase of Li₂Ge₇O₁₅ (LGO) crystals doped with Mn⁴⁺ and Cr³⁺ ions the effect of external electric field on the R-lines in luminescence spectra of probe ions is studied. From the observed linear in field pseudo-Stark splitting of R-lines the existence of phase transition induced electric dipole moments in probe ions was established which are symmetry forbidden in paraelectric phase and appear only in ferrophase. The symmetry analysis of the phenomenon is given. The results allow to conclude that in pure LGO ferrophase the large electric dipole moments perpendicular to ferroelectric axis “c” are induced in octahedral GeO₆ sites which are ordered and alternatively opposite in sign in four GeO₆ sites included in unit cell which provides the full cancellation of net induced moment in plane ⊤ _c .

Key Words:

• Lithium heptagermanate Li₂Ge₇O₁₅
• ferroelectric transition
• probe ions Cr³⁺
• Mn⁴⁺
• dipole moments
• luminescence spectra
• pseudo-Stark splitting