Soft tilt and rotational modes in the hybrid improper ferroelectric Ca3Mn2O7

Abstract

Raman spectroscopy is employed to probe directly the soft rotation and tilting modes, which are two primary order parameters predicted in the hybrid improper ferroelectric material Ca3Mn2O7. We observe a giant softening of the 107-cm −1 octahedron tilting mode by 26 cm −1 , on heating through the structural transition from a ferroelectric to paraelectric orthorhombic phase. This is contrasted by a small softening of the 150-cm −1 rotational mode by 6 cm −1 . In the intermediate phase, the competing soft modes with different symmetries coexist, bringing about many-faceted anomalies in spin excitations and lattice vibrations. Our work demonstrates that the soft rotation and tilt patterns, relying on a phase-transition path, are a key factor in determining ferroelectric, magnetic, and lattice properties of Ca3Mn2O7.

Interplay of spin, lattice, vibration, and charge degrees of freedom: Magneto‐dielectricity in Ca3Mn2O7

“…While the drop of β KWW (R 1 ) slows down and reverts back (minimum at 120 K ≤ T N ), that of β KWW (R 2 ) speeds up below 115 K (~T W under 6T field). These trends further corroborate our earlier inferences 17 ; that in the spin disordered state (>T N ), antiferrodistortive correlations among both the R 1,2type dipoles are enhanced by the applied H-field. Moreover, in the AFM state, the FE-correlations between R 1 -type dipoles (hosted in the bulk-AFM phase) are field-suppressed, whereas those between R 2 -type dipoles are further enhanced (especially below T W , upon the WFM-nanophase emergence).…”

“…Iodimetric titration confirmed a close‐to‐ideal (Ca 3 Mn 2 O 6.963 ) stoichiometry of the system . AFM‐ T N = 123 K found for our specimen is not lower than that reported for the single crystal; affirming negligible influence of the defects. Raman scattering measurements on Ca 3 Mn 2 O 7 system have been performed with HR800 Jobin‐Yvon spectrometer, at close by temperatures across magnetic ordering and reconfiguration, over 5‐300 K range, using He‐Ne laser of wavelength 632.8 nm.…”

“…Raman active optical modes predicted for the system with Cmc21 space group are 18A 1 +17A 2 +16B 1 +18B 2 . The Raman Figure 5A over 5-300 K. The two most prominent phonon modes have been recognized as A 1 (15) and A1 (17) for the Cmc21 phase (=A21ma structure with 'c'-long axis). 21 Lorentzian peak-fit analysis ( Figure 5B,C) has been carried out for the respective modes, to obtain thermal evolution of the phonon modes.…”

“…This explains that the MnO 6 octahedra-distortion-related changes in the Mn-O-Mn bond angles induce changes in the magnetic configuration. Here, F I G U R E 4 Fitted SPXRD pattern for Ca 3 Mn 2 O 7 at 340 K. Inset: SXRD patterns obtained at 300 K and above, depicting the introduction of the tetragonal phase [Color figure can be viewed at wileyonlinelibrary.com]F I G U R E 5 A, Obtained Raman spectra from room temperature down to 7 K. B and C, Lorentzian peak-fit for A 1(15) and A 1(17) modes, respectively. D, Temperature dependence of Raman shift (left pane) and line-width (right pane) obtained for the A 1 (15) mode.…”

Mapping the structural transitions controlled by the trilinear coupling in Ca3-xSrxTi2O7

“…The family of HIF compounds reveals an amazingly rich set of structure transitions, as was demonstrated on Ca3-xSrxTi2O7 [4], Sr3Zr2O7 [7], and Ca3Mn2O7 [8], recently. These materials belong to the Ruddlesden-Popper phases described by the general formula An+1BnO3n+1 (n = 2).…”