Relaxor–super-paraelectric behaviour and crystal-field–driven spin-phonon coupling in pyrochlore Eu₂Ti₂O₇

Abstract

The temperature-dependent Raman spectroscopy and dielectric property of pyrochlore Eu2Ti2O7 have been investigated. The appearance of additional phonon modes below 200 K is observed suggesting a local structural change. The anomalous softening of the phonon modes and existence of crystal-field-induced short-range magnetic ordering below 150 K unveiled the possible spin-phonon coupling. The signature behaviour of a dipolar glassy-relaxor state and super-paraelectric nature has been demonstrated. Additional dielectric anomaly at lower temperature, below 40 K, related to spin-spin correlation has also been reported.

Effect of f–d and d–d Interactions on Dielectric and Optical Properties of Pyrochlore Eu2−xFexTi2O7

“…[ 27 ] The previous Raman study of ETO also showed that a structural change in ETO occurred below 200 K which is related to the TiO 6 octahedra. [ 28 ] Such octahedral distortion of TiO 6 in similar pyrochlore Ho 2 Ti 2 O 7 was considered to be the key factor for producing the net electrical polarization leading to the relaxor ferroelectricity. [ 49 ] Relaxor ferroelectric material also possesses spontaneous polarization similar to normal ferroelectrics but their domains are very small (of the size of nanolength scale), whereas normal ferroelectric domains are relatively large (of the size of microlength scale).…”

“…The previous report on dielectric constant () of ETOF0 shows a strong dielectric relaxation in the form of a broad peak below 150 K, with a maximum value of the relative permittivity () around the temperature ( T m ), which was described as relaxor superparaelectricity. [ 28 ] The superparaelectricity was introduced due to the asymmetric distribution of around T m . [ 28 ] Naturally, variation of around prefers to be symmetric (Gaussian) profile.…”

“…[ 28 ] The superparaelectricity was introduced due to the asymmetric distribution of around T m . [ 28 ] Naturally, variation of around prefers to be symmetric (Gaussian) profile. In contrast, for ETOF0 (see ref.…”

“…In contrast, for ETOF0 (see ref. [28]), we have noticed the asymmetrical variation of around the temperature T m and they are assumed to be a superposition of multiple Gaussian distributions around T m and each Gaussian profile corresponds to PNRs of different sizes employing that multiple PNRs are contributing in dielectric relaxation which is referred to as a superparaelectric state in the system. Similar dielectric relaxation was also observed in isostructural pyrochlore compounds Dy 2 Ti 2 O 7 and Ho 2 Ti 2 O 7 which was referred to as a typical ferroelectric relaxor state.…”

“…[ 27 ] Other than this, Eu 2 Ti 2 O 7 shows a diffused relaxor superparaelectric dielectric relaxation and signature of spin–phonon coupling. [ 28 ] Moreover, there is strong octahedral distortion present in the TiO 6 octahedra owing to the interstitial anionic vacant site lying close to the TiO 6 octahedra, which was probed by X‐ray diffraction (XRD) and X‐ray absorption spectroscopy (XAS) study. [ 27 ] However, due to the introduction of Fe doping, i.e., f – d and d – d interactions, octahedral distortion gets diminished owing to the anionic disorder introduced by the comparable ionic radii of Ti 4+ and Fe 3+ due to the existence of O vacancy at 8 a site adjacent to the TiO 6 octahedra on the ab ‐plane.…”