Soft antiphase tilt of oxygen octahedra in the hybrid improper multiferroicCa3Mn1.9Ti0.1O7

Abstract

We report a single crystal neutron and x-ray diffraction study of the hybrid improper multiferroic Ca3Mn1.9Ti0.1O7 (CMTO), a prototypical system where the electric polarization arises from the condensation of two lattice distortion modes. With increasing temperature (T ), the out-of-plane, antiphase tilt of MnO6 decreases in amplitude while the in-plane, inphase rotation remains robust and experiences abrupt changes across the first-order structural transition. Application of hydrostatic pressure (P ) to CMTO at room temperature shows a similar effect. The consistent behavior under both T and P reveals the softness of antiphase tilt and highlights the role of the partially occupied d orbital of the transition metal ions in determining the stability of the octahedral distortion. Polarized neutron analysis indicates the symmetry-allowed canted ferromagnetic moment is less than 0.04 µB/Mn site, despite a substantial out-of-plane tilt of the MnO6 octahedra.PACS numbers: 75.58.+t,81.40.Vw,61.05.F-Multiferroic compounds with spontaneous elastic, electrical, magnetic orders are considered as the key materials to achieve cross-control between magnetism and electricity in solids with small energy dissipation [1,2]. The functional properties including colossal magnetoelectric effect could be used in solid-state memories and sensors [3]. The desired multifunctional behavior requires common microscopic origin of the long-range order such that one order parameter is strongly coupled to the conjugate field of the other one. So far, the majority of attention has focused on exploring materials with magnetic origin, where the underlying microscopic mechanisms are primarily classified into three types: symmetric spin exchange interaction Σ ij (S i · S j ) [4, 5], antisymmetric spin-exchange interaction S i × S j [6, 7], and spindependent p − d hybridization due to spin-ligand interaction (e il · S i ) 2 e il [8]. The material-by-design efforts focusing on magnetic oxides has been productive; the ferroelectricity is induced either through epitaxial strain engineering or chemical substitution of stereochemical inactive ions with lone-pair-active cations [9,10].However, this approach requires a strong coupling between ferroelectricity and magnetism. The microscopic mechanism with spin origin also implicitly suggests a low operating temperature because of the magnetic frustration. On the other hand, perovskites in the form of ABO 3 and their derivatives are favorably chosen for functional materials due to their high susceptibility toward polar structural instability and the intimate coupling between the ferroelectric polarization and the magnetic, orbital, and electronic degrees of freedom. Recently, a novel mechanism termed as "hybrid improper ferroelectric" has been proposed to search for materials with spontaneous ferroelectricity. The central idea is that the polar mode is driven by the condensation of two nonpolar lattice modes, which represent oxygen octahedral rotation (X + 2 ) and tilt (X − 3 ), respectively [11][12][13]. It i...

A new electron diffraction approach for structure refinement applied to Ca3Mn2O7

“…9) is quite clear, as is the relative lack of change in octahedral twist about the c axis (X þ 2 ). Our results are in good agreement with those of more sophisticated studies such as that of Ye et al (2018).…”

Enhanced hybrid improper ferroelectricity in Fe/Nb cosubstituted Ca3Mn2O7 ceramics

“…In Figure 2D, the pronounced phase coexistence phenomenon can be observed at 373 K, where the contents of A2 1 am and Acaa phases are 39.9(5) wt% and 60.1(7) wt%, respectively. The single-crystal diffraction measurement indicated that a polar Aba2 phase exists at 450 K. 34 However, the Rietveld refinement results performed at 473 K show that the nonpolar Acaa structure provides a much better goodness-of-fit χ 2 than that of polar Aba2 phase, demonstrating the presence of Acaa phase (see Figure 1 and Table 2). Figure 2E shows that the lattice parameters increase until the discontinuities appear above 373 K. Then the parameters a, b, and cell volume indicate a sharp increase, while the parameter c starts to decrease with increasing temperature.…”

“…Unfortunately, however, it remains challenging to observe the switchable polarization at room temperature, and its inferior remanent polarization (~1 μC/cm 2 for polycrystalline and ~6.4 μC/cm 2 for single-crystal Ca 3 Ti 2 O 7 ) also limits the investigation and application. 9,34,39,40 Therefore, systematic studies of Ca 3 [Mn 1x (Fe 0.5 Nb 0.5 ) x ] 2 O 7 (x = 0.15, 0.25, and 0.50) ceramics have been conducted to tune the crystallographic symmetry and ferroelectricity. The microstructures and distribution of grain size of dense Ca 3 [Mn 1x (Fe 0.5 Nb 0.5 ) x ] 2 O 7 ceramics are shown in Figure 4.…”

Magneto-electric multiferroics: designing new materials from first-principles calculations

“…Several theoretical and experimental efforts are still devoted to understand HIF in perovskites and related materials and to rationalize rules maximizing the polarization 57,101,[152][153][154][155][156]126,128,141,142,146,[149][150][151] . One key point is to understand the phase transition of these materials:…”