Emergent superstructural dynamic order due to competing antiferroelectric and antiferrodistortive instabilities in bulk EuTiO3

Magnetic field driven phase transitions in EuTiO3

The influence of an external static magnetic field (up to 480 mT) on the structural properties of EuTiO₃(ETO) polycrystalline samples was examined by powder XRD at the Elettra synchrotron facilities in the temperature range 100-300 K. While the cubic to tetragonal structural phase transition temperature in this magnetic field range remains almost unaffected, significant lattice effects appear at two characteristic temperatures (∼200 K and ∼250 K), which become more pronounced at a critical threshold field. At ∼200  K a change in the sign of magnetostriction is detected attributed to a modification of the local magnetic properties from intrinsic ferromagnetism to intrinsic antiferromagnetism. These data are a clear indication that strong spin-lattice interactions govern also the high temperature phase of ETO and trigger the appearance of magnetic domain formation and phase transitions.

Magnetic and dielectric property control in the multivalent nanoscale perovskite Eu0.5Ba0.5TiO3

We report nanoscale Eu_0.5Ba_0.5TiO₃, a multiferroic in the bulk and candidate in the search to quantify the electric dipole moment of the electron. Eu_0.5Ba_0.5TiO₃, in the form of nanoparticles and other nanostructures is interesting for nanocomposite integration, biomedical imaging and fundamental research, based upon the prospect of polarizability, f-orbital magnetism and tunable optical/radio luminescence. We developed a [non-hydrolytic]sol-[H₂O-activated]gel route, derived from in-house metallic Ba_(s)/Eu_(s) alkoxide precursors and Ti{(OCH(CH₃)₂}₄. Two distinct nanoscale compounds of Ba:Ti:Eu with the parent perovskite crystal structure were produced, with variable dielectric, magnetic and optical properties, based on altering the oxidizing/reducing conditions. Eu_0.5Ba_0.5TiO₃ prepared under air/O₂ atmospheres produced a spherical core-shell nanostructure (30-35 nm), with perovskite Eu_0.5Ba_0.5TiO₃ nanocrystal core-insulating oxide shell layer (∼3 nm), presumed a pre-pyrochlore layer abundant with Eu³⁺. Fluorescence spectroscopy shows a high intensity ⁵D₀→⁷F₂ transition at 622 nm and strong red fluorescence. The core/shell structure demonstrated excellent capacitive properties: assembly into dielectric thin films gave low conductivity (2133 GΩ mm^-1) and an extremely stable, low loss permittivity of ε_eff∼25 over a wide frequency range (tan δ < 0.01, 100 kHz-2 MHz). Eu_0.5Ba_0.5TiO₃ prepared under H₂/argon produced more irregular shaped nanocrystals (20-25) nm, with a thin film permittivity around 4 times greater (ε_eff 101, tan δ < 0.05, 10 kHz-2 MHz, σ∼59.54 kΩ mm^-1). Field-cooled magnetization values of 0.025 emu g^-1 for EBTO-Air and 0.84 emu g^-1 for EBTO-Argon were observed. X-ray photoelectron spectroscopy analysis reveals a complex interplay of Eu^II/III/Ti^III/IV configurations which contribute to the observed ferroic and fluorescence behavior.

XMaS @ the ESRF

This paper describes the motivation for the design and construction of a beamline at the European Synchrotron Radiation Facility (ESRF) for the use of UK material scientists. Although originally focused on the study of magnetic materials, the beamline has been running for 20 years and currently supports a very broad range of science as evidenced by the research topics highlighted in this article. We describe how the beamline will adapt to align with the ESRF's upgrade to a diffraction limited storage ring. This article is part of the theme issue 'Fifty years of synchrotron science: achievements and opportunities'.

Relaxation dynamics and polydispersivity associated with defects and ferroelectric correlations in Ba-doped EuTiO3

We present the frequency- and temperature-dependent dielectric response of Eu_1-x Ba _x TiO₃ (0  ⩽  x  ⩽  0.5) in detail. Excluding grain boundary effects, four relaxation mechanisms were observed. Relaxation dynamics were observed to arise due to hopping conduction associated with defects, namely oxygen vacancies as well as Eu³⁺ and Ti³⁺ ions. Dielectric relaxation analysis led to the identification of Ti ions in two different environments with different relaxation rates in the overall EuTiO₃ perovskite structure. The emergence of another relaxation mechanism associated with ferroelectric order as a consequence of the formation of polar regions was also observed for higher Ba concentrations. The addition of Ba led to the identification of relaxation dynamics associated with hopping conduction between Eu ions, Ti ions (in the regions with and without oxygen vacancies) and with the formation of ferroelectric polar regions. Furthermore, the polydispersivity and relaxation times were extracted within the framework of the modified Debye model. Relaxation times have been observed to increase with a decrease in temperature while larger values of polydispersivity reveal a wide distribution of relaxation times due to the presence of lattice parameter and energy barrier distributions.

Unravelling the complex nanostructure of La0.5−xLi0.5−xSr2xTiO3 Li ionic conductors

The intricate nanostructure of La_0.5−xLi_0.5−xSr_2xTiO₃ Li-ion conductors has been elucidated. Advanced transmission electron microscopy has allowed investigation where average structure models cannot account for the changeable local atomic arrangements detected.

Static and dynamic strain coupling behaviour of ferroic and multiferroic perovskites from resonant ultrasound spectroscopy

Resonant ultrasound spectroscopy (RUS) provides a window on the pervasive influence of strain coupling at phase transitions in perovskites through determination of elastic and anelastic relaxations across wide temperature intervals and with the application of external fields. In particular, large variations of elastic constants occur at structural, ferroelectric and electronic transitions and, because of the relatively long interaction length provided by strain fields in a crystal, Landau theory provides an effective formal framework for characterizing their form and magnitude. At the same time, the Debye equations provide a robust description of dynamic relaxational processes involving the mobility of defects which are coupled with strain. Improper ferroelastic transitions driven by octahedral tilting in KMnF3, LaAlO3, (Ca,Sr)TiO3, Sr(Ti,Zr)O3 and BaCeO3 are accompanied by elastic softening of tens of % and characteristic patterns of acoustic loss due to the mobility of twin walls. RUS data for ferroelectrics and ferroelectric relaxors, including BaTiO3, (K,Na)NbO3,Pb(Mg1/3Nb2/3)O3 (PMN), Pb(Sc1/2Ta1/2)O3 (PST), (Pb(Zn1/3Nb2/3)O3)0.955(PbTiO3)0.045 (PZN-PT) and (Pb(In1/2Nb1/2)O3)0.26(Pb(Mg1/3Nb2/3)O3)0.44(PbTiO3)0.30 (PIN-PMN-PT) show similar patterns of softening and attenuation but also have precursor softening associated with the development of polar nano regions. Defect-induced ferroelectricity occurs in KTaO3, without the development of long range ordering. By way of contrast, spin-lattice coupling is much more variable in strength, as reflected in a greater range of softening behaviour for Pr0.48Ca0.52MnO3 and Sm0.6Y0.4MnO3 as well as for the multiferroic perovskites EuTiO3,BiFeO3, Bi0.9Sm0.1FeO3, Bi0.9Nd0.1FeO3, (BiFeO3)0.64(CaFeO2.5)0.36, (Pb(Fe0.5Ti0.5)O3)0.4(Pb(Zr0.53Ti0.47)O3)0.6. A characteristic feature of transitions in which there is a significant Jahn-Teller component is softening as the transition point is approached from above, as illustrated by PrAlO3, and this is suppressed by application of an external magnetic field in the colossal magnetoresistive manganite Pr0.48Ca0.52MnO3 or by reducing grain size in La0.5Ca0.5MnO3. Spin state transitions for Co(3+) in LaCoO3, NdCoO3 and GdCoO3 produce changes in the shear modulus that scale with a spin state order parameter, which is itself coupled with the order parameter(s) for octahedral tilting in a linear-quadratic manner. A new class of phase transitions in perovskites, due to orientational or conformational ordering of organic molecules on the crystallographic A-site of metal organic frameworks, is illustrated for [(CH3)2NH2]Co(HCOO)3 and [(CH2)3NH2]Mn(HCOO)3 which also display elastic and anelastic anomalies due to the influence of intrinsic and extrinsic strain relaxation behaviour.

Studies of the antiferrodistortive transition in EuTiO3

Structural studies of EuTiO3, conducted using synchrotron x-ray powder diffraction, reveal the sensitivity of this material to defects. The presence of a single tetragonal-cubic (I4/mcm-Pm-3m) transition is confirmed. Neutron diffraction measurements show EuTiO3 to have a G-type antiferromagnetic arrangement. Examinations of the symmetry-adapted tetragonal strains demonstrate that the stoichiometry impacts on the continuous nature of this. The impact of defects on the electrical conductivity and magnetodielectric effect is also described.

Creating multiferroics with large tunable electrical polarization from paraelectric rare-earth orthoferrites

The quest for materials possessing both a magnetic ordering temperature above room temperature and a large electrical polarization is an important research direction in order to design novel spintronic and memory devices. Up to now, BiFeO3 and related systems are the only known compounds simultaneously possessing such characteristics. Here, first-principles calculations predict that another family of materials, namely epitaxial films made of rare-earth orthoferrites (RFeO3), can also exhibit such desired features. As a matter of fact, applying a large enough strain to these compounds, which are nominally paraelectric and have a high magnetic transition temperature, is predicted to render them ferroelectric, and thus multiferroic. At high compressive strain, the resulting ferroelectric phase of RFeO3 systems having large rare-earth ions is even a tetragonal state characterized by a giant polarization and axial ratio. For large tensile strain, two striking inhomogenous ferroelectric phases--including one never observed before in any perovskite--are further predicted as having significant polarization. A multiphase boundary also occurs, which may lead to optimization of properties or unusual features. Finally, many quantities, including electrical polarization and magnetic ordering temperature, are tunable by varying the epitaxial strain and/or chemical pressure.