Carsten Richter

Dielectric to pyroelectric phase transition induced by defect migration

Subjecting strontium titanate single crystals to an electric field in the order of 106 V m−1 is accompanied by a distortion of the cubic crystal structure, so that inversion symmetry vanishes and a polar phase is established. Since the polar nature of the migration-induced field-stabilized polar (MFP) phase is still unclear, the present work investigates and confirms the pyroelectric structure. We present measurements of thermally stimulated and pyroelectric currents that reveal a pyroelectric coefficient pMFP in the order of 30 μC K−1m−2. Therefore, a dielectric to pyroelectric phase transition in an originally centrosymmetric crystal structure with an inherent dipole moment is found, which is induced by defect migration. From symmetry considerations, we derive space group for the MFP phase of SrTiO3. The entire electroformation cycle yields additional information about the directed movement and defect chemistry of oxygen vacancies.

Large piezoelectricity in electric-field modified single crystals of SrTiO3

Defect engineering is an effective and powerful tool to control the existing material properties and produce completely new ones, which are symmetry-forbidden in a defect-free crystal. For example, the application of a static electric field to a single crystal of SrTiO3 forms a strained near-surface layer through the migration of oxygen vacancies out of the area beneath the positively charged electrode. While it was previously shown that this near-surface phase holds pyroelectric properties, which are symmetry-forbidden in centrosymmetric bulk SrTiO3, this paper reports that the same phase is strongly piezoelectric. We demonstrate the piezoelectricity of this phase through stroboscopic time-resolved X-ray diffraction under alternating electric field and show that the effective piezoelectric coefficient d33 ranges between 60 and 100 pC/N. The possible atomistic origins of the piezoelectric activity are discussed as a coupling between the electrostrictive effect and spontaneous polarization of this near-sur...

Crystallization dynamics and interface stability of strontium titanate thin films on silicon

Nonstoichiometric SrTiO3 thin films were fabricated by different thin-film deposition methods. The impact on the oxide/silcon interface stability as well as the crystallization onset temperature is investigated.

Picometer polar atomic displacements in strontium titanate determined by resonant X-ray diffraction

Physical properties of crystalline materials often manifest themselves as atomic displacements either away from symmetry positions or driven by external fields. Especially the origin of multiferroic or magnetoelectric effects may be hard to ascertain as the related displacements can reach the detection limit. Here we present a resonant X-ray crystal structure analysis technique that shows enhanced sensitivity to minute atomic displacements. It is applied to a recently found crystalline modification of strontium titanate that forms in single crystals under electric field due to oxygen vacancy migration. The phase has demonstrated unexpected properties, including piezoelectricity and pyroelectricity, which can only exist in non-centrosymmetric crystals. Apart from that, the atomic structure has remained elusive and could not be obtained by standard methods. Using resonant X-ray diffraction, we determine atomic displacements with sub-picometer precision and show that the modified structure of strontium titanate corresponds to that of well-known ferroelectrics such as lead titanate.It is a challenge to measure changes in the crystal structures in picometer scale and the associated phase. Here the authors demonstrate the lattice expansion and polar distortions of oxygen deficient SrTiO3 using a resonance X-ray scattering technique.

Probing structural distortions with new high-precision resonant X-ray diffraction approach

Matthias Zschornak1, Carsten Richter2, Dmitri Novikov3, Erik Mehner1, Melanie Nentwich1, Semën Gorfman4, Juliane Hanzig1, Hartmut Stöcker1, Tilmann Leisegang1, Dirk C. Meyer1 1Institute Of Experimental Physics, TU Bergakademie Freiberg, Freiberg, Germany, 2European Synchrotron Radiation Facility (ESRF), Grenoble, France, 3Deutsches Elektronen-Synchrotron (DESY), Hamburg, Germany, 4Department of Physics, University of Siegen, Siegen, Germany E-mail: matthias.zschornak@physik.tu-freiberg.de

Analysis of modulated Ho2PdSi3 crystal structure at Pd K and Ho L absorption edges using resonant elastic X-scattering.

Replacing Si atoms with a transition metal in rare earth disilicides results in a family of intermetallic compounds with a variety of complex magnetic phase transitions. In particular, the family R 2PdSi3 shows interesting magnetic behavior arising from the electronic interaction of the R element with the transition metal in the Si network, inducing the specific structure of the related phase. Within this series, the highest degree of superstructural order was reported for the investigated representative Ho2PdSi3, although several competing superstructures have been proposed in literature. The diffraction anomalous fine structure (DAFS) method is highly sensitive to the local structure of chosen atoms at specific positions within the unit cell of a crystalline phase. In combination with x-ray absorption fine structure (XAFS), this sophisticated synchrotron method has been applied in the present work to several selected reflections, i.a. a satellite reflection. Extensive electronic modeling was used to test the most relevant structure proposals. The [Formula: see text] superstructure has been strongly confirmed, although a small amount of disorder in the modulation is very probable.

Evaluation of structure models of Ho2PdSi3 using DAFS, inter alia at a satellite reflection

The compounds R2PdSi3, with R = rare earth, exhibit a very interesting magnetic behavior with two phase transitions. Substituting one in four Si atoms by Pd in HoSi2 results in a modulation of the aristotype. There are several different variants discussed in literature about the nature of the modulation of this rare-earth compound. Two of the latest models were compared: a 2 × 2 × 1 layer and a 2 × 2 × 8 stack. The chosen method is Diffraction Anomalous Fine Structure (DAFS) and was applied both experimentally and by simulation at different absorption edges and reflections, i. a. a satellite reflection, aiming on finding the correct crystal structure.

Anisotropy in anomalous scattering in TiO2 and the influence of point defects

Effects in the atomic structure of BaFe2As2 by pressure and chemical substitution E. M. Bittar,a E. Granado,b,a L. Mendonça-Ferreira,c F. Garcia,a G. de M. Azevedo,d C. Adriano,b T. M. Garitezi,b L. F. Bufaiçal,e P. G. Pagliuso,b aLaboratório Nacional de Luz Síncrotron, Campinas, SP, (Brazil). bInstituto de Física “Gleb Wataghin”, UNICAMP, Campinas, SP, (Brazil). cInstituto de Física e Matemática, UFPel, Pelotas, RS, (Brazil). dInstituto de Física, UFRGS, Porto Alegre, RS, (Brazil). eInstituto de Física, UFG, Goiânia, GO, (Brazil). E-mail: eduardo. bittar@lnls.br

Influence of defects on the polarization dependent DAFS of rutile TiO2

C583 and physical systems [1, 2]. An external stimulus is applied to the system, by periodically varying a parameter affecting it (for example concentration, pH, light flux, temperature or pressure). The response of the system, which is also periodic, is first averaged into one period, then analyzed offline by means of phase-sensitive detection, i.e. by separating the signals of the different frequency terms. A high signal-tonoise ratio is gained by the averaging procedure and the phase-sensitive detection, recovering small signals buried under large ambient noise, besides achieving high time-resolution for kinetic studies [1]. We conceived the application of this technique to Crystallography for two main reasons: to select contributions to the diffraction pattern arising from specific groups of atoms in the crystal cell and to achieve a time-dependent characterization of the crystallized system. To this aim, we developed a theory to explain the diffraction response of a crystal subjected to a periodically varying external perturbation, where the effect of the variation of different structural parameters on the diffraction intensity is accounted for [3]. We showed that the interference contribution of the substructure composed by the atoms actively responding to the stimulus may be separated by analyzing the diffraction signal at a frequency which is double with respect to that of the external stimulus. This new technique has been called Modulation Enhanced Diffraction (MED). Experiments to verify the MED potentialities have been first simulated and then carried out at synchrotron sources. The experiments carried out on powder samples, by periodically varying the X-ray beam energy or the pressure exerted by a gas on the sample, will be described. The data analysis involves two steps: first the phase sensitive detection is applied to a set of diffraction patterns of one modulation cycle to obtain a demodulated pattern, then a phasing procedure is applied to it. A special procedure, making use of the Patterson deconvolution technique [4], has been developed and successfully used to phase the demodulated diffraction patterns and obtain the substructure of the active moiety.