Role of intrinsic disorder in the structural phase transition of magnetoelectric EuTiO3

Abstract

Up to now, the crystallographic structure of the magnetoelectric perovskite EuTiO 3 has been considered to remain cubic down to low temperature. Here we present high-resolution synchrotron x-ray powder-diffraction data showing the existence of a structural phase transition, from cubic P m-3m to tetragonal I 4/mcm, involving TiO 6 octahedra tilting, in analogy to the case of SrTiO 3 . The temperature evolution of the tilting angle and of the full width at half maximum of the (200) cubic reflection family indicate a critical temperature T c = 235 K. This critical temperature is well below the recent anomaly reported by specific-heat measurement at T A ∼ 282 K. By performing atomic pair distribution function analysis on diffraction data, we provide evidence of a mismatch between the local (short-range) and the average crystallographic structures in this material. Below the estimated T c , the average model symmetry is fully compatible with the local environment distortion, but the former is characterized by a reduced value of the tilting angle compared to the latter. At T = 240 K, data show the presence of local octahedra tilting identical to the low-temperature one, while the average crystallographic structure remains cubic. On this basis, we propose that intrinsic lattice disorder is of fundamental importance in the understanding of EuTiO 3 properties.

EuTiO3magnetic structure studied by neutron powder diffraction and resonant x-ray scattering

“…High resolution data at 12 K were found consistent with the tetragonal structural model proposed in Ref. 13. The TiO 6 octahedral tilt angle value obtained at 12 K (φ = 3.89 ± 0.06) follows the temperature evolution suggested by Allieta et al in Ref.…”

“…Specific heat measurements suggest a structural instability at T A = 282 K, 14 while high resolution x-ray powder diffraction is able to resolve a peak splitting due to the cubic to tetragonal transition only at T B = 235 K. 13 In order to reconcile this discrepancy Allieta et al propose the presence of short-range correlation in the transient regime between T A and T B , a position supported by results from their pair distribution function analysis below T A . 13 At T N = 5.3 K the localized 4f moments on the Eu ++ (S = 7/2, L = 0) sites order in a G-type antiferromagnetic arrangement, 15 that is, the six nearest neighbors have opposite spins while the 12 next-nearest neighbors have parallel spins. Concomitant with the onset of antiferromagnetism, the dielectric constant decreases abruptly (∼3.5%) and shows a strong enhancement as a function of the applied magnetic field (∼7% at B ∼ 1.5 T), providing evidence for magnetoelectric coupling.…”

“…10,11 At room temperature bulk EuTiO 3 possesses a cubic crystal structure described by the prototype perovskite P m3m space group. 12 A structural transition 13,14 to the tetragonal I 4/mcm space group has been recently reported. The transition temperature is still controversial.…”

“…18 In bulk magnetoelectrics the coupling between various degrees of freedom is realized at a microscopic level, 19 hence a detailed knowledge of the crystallographic and magnetic structure is vital to any further investigation and modeling. Following the recent report of a structural transition, 13 we reexamined the low temperature magnetic structure by the combined use of neutron and x-ray magnetic diffraction. Both techniques have been successfully applied to study the complex magnetic structure of several magnetoelectric multiferroic materials, [20][21][22][23][24][25] …”

Electric field modulation of the tetragonal domain orientation revealed in the magnetic ground state of quantum paraelectric EuTiO3

“…This revelation immediately obliges us to revisit our understanding of the low-temperature properties of EuTiO 3 , since tetragonal symmetry pushes the material closer to a ferromagnetic/ferroelectric (FM/FE) phase boundary 7 , as well as permitting the breakage of spatial inversion symmetry and introducing an anisotropy into any magneticallyordered phase. Preliminary work suggests more complex low-temperature behaviour compared with SrTiO 3 , with XRD analysis even suggesting that tetragonal EuTiO 3 may be intrinsically disordered 13 . Theoretical studies show that EuTiO 3 is elastically harder than SrTiO 3 , which is also conducive to disorder 14 .…”