A large variety of transport properties have been observed at the interface between the insulating oxides SrTiO3 and LaAlO3 such as insulation, 2D interface metallicity, 3D bulk metallicity, magnetic scattering, and superconductivity. The relation between the structure and the properties of the SrTiO3/LaAlO3 interface can be explained in a meaningful way by taking into account the relative contribution of three structural aspects: oxygen vacancies, structural deformations (including cation disorder), and electronic interface reconstruction. The emerging phase diagram is much richer than for related bulk oxides due to the occurrence of interface electronic reconstruction. The observation of this interface phenomenon is a display of recent advances in thin film deposition and characterization techniques, and provides an extension to the range of exceptional electronic properties of complex oxides.

Structure–Property Relation of SrTiO3/LaAlO3 Interfaces

The behavior of the $Pm\overline{3}m\text{\ensuremath{-}}R\overline{3}c$ phase transition in $\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$ (${T}_{\mathrm{C}}=813\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ from differential scanning calorimetry measurements) has been studied using temperature-dependent measurements of the crystal structure, dielectric relaxation, specific heat, birefringence, and the frequencies of the two soft modes (via Raman spectroscopy). While all these experiments show behavior near ${T}_{\mathrm{C}}$ consistent with a second-order Landau transition, there is extensive evidence for additional anomalous behavior below $730\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Below this temperature, the two soft mode frequencies are not proportional to each other, the spontaneous strain is not proportional to the square of the $\mathrm{Al}{\mathrm{O}}_{6}$ rotation angle, and anomalies are seen in the birefringence. Twin domains, which are mobile above $730\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, are frozen below $730\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. These anomalies are consistent with biquadratic coupling between the primary order parameter of the transition ($\mathrm{Al}{\mathrm{O}}_{6}$ rotation) and a second process. From the dielectric results, which indicate a smooth but rapid increase in conductivity in the temperature range $500--800\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, we propose that this second process is hopping of intrinsic oxygen vacancies. These vacancies are essentially static below $730\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and dynamically disordered above $730\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The interaction between static vacancies and the displacive phase transition is unfavorable. A similar anomaly may be observed in other aluminate perovskites undergoing the same transition.

Transformation processes in LaAlO3: neutron diffraction, dielectric, thermal, optical and Raman studies

Recent experiments on several oxide perovskites reveal that they undergo tilt phase transitions to higher-symmetry phases on increasing pressure and that $d{T}_{c}/dPl0$, contrary to a general rule previously proposed for such zone-boundary transitions. We show that the negative slope of the phase boundary is a consequence of the octahedra in these perovskites being more compressible than the extra-framework cation sites. Conversely, when the octahedra are stiffer than the extra-framework cation sites, the phase transition temperatures increase with increasing pressure, $d{T}_{c}/dPg0$.

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General Rules for Predicting Phase Transitions in Perovskites due to Octahedral Tilting

The influence of transformation twins on the seismic-frequency elastic and anelastic properties of perovskite: dynamical mechanical analysis of single crystal LaAlO3

The dynamic response of ferroelastic twins to an alternating stress has been studied in situ at high temperature using a stroboscopic x-ray diffractometer and combined dynamical mechanical analyzer (XRD-DMA). The XRD-DMA is designed to allow x-ray rocking curves to be collected while the sample is undergoing simultaneous dynamical mechanical analysis in three-point-bend geometry. The detection of diffracted x-rays is synchronized with the applied load, so that rocking curves corresponding to different parts of the dynamic load cycle can be obtained separately. The technique is applied to single-crystal LaAlO3, which undergoes a cubic to rhombohedral phase transition at 550 °C, leading to the generation of characteristic “chevron” twins. The rocking-curve topology is calculated as a function of crystal orientation for each chevron type. Systematic changes in the rocking curves during heating and cooling under dynamic load demonstrate a clear preference for chevrons containing {100}pc walls perpendicular to...

Application of real-time, stroboscopic x-ray diffraction with dynamical mechanical analysis to characterize the motion of ferroelastic domain walls

The structural changes of stoichiometric LiNbO3 at temperatures up to 1470 K have been investigated by neutron powder diffraction. At Tc ≈ 1460 K, LiNbO3 transforms from a paraelectric high-temperature (R3c) to a ferroelectric low temperature structure (R3c). The phase transition is of coupled order-disorder and displacive type. In the high-temperature phase, Li is highly disordered over two off-centre positions, whereas Nb takes a centrosymmetric position within O6 octahedra. An analysis of anisotropic displacement parameters shows that the probability density function of Li is extended along c, that of the oxygens is more or less perpendicular to the Nb-O bonds, and that of Nb is slightly extended in the a - b plane at high temperatures. An increase of the tilt angle of the NbO6 octahedra away from an h.c.p. arrangement towards a perovskite structure was observed. The octahedra become almost regular at high temperatures. Comparison with an earlier investigation of congruent lithium niobate shows a quite similar behaviour, however, all parameters of the latter are closer to those of the perovskite structure at same temperatures. Anomalies in the behaviour of the order parameters, a more complicated disorder and a high mobility of Li at high temperatures are explained by the competition of the order-disorder and displacive character of the phase transition.

A neutron powder investigation of the high-temperature structure and phase transition in stoichiometric LiNbO3

The structure of LaAlO3 has been investigated around the phase transition at T(c) ≃ 800 K by neutron powder diffraction in vacuum and by X-ray powder diffraction under nitrogen atmosphere as well as by a very high resolution synchrotron experiment in air. The results were analysed in frame of the Landau theory using the fluctuation-dissipation theorem to relate the susceptibility to the atomic displacement parameters. The room temperature structure is a rhombohedrally distorted perovskite structure, space group R3c, which undergoes a transition to the ideal perovskite structure, space group Pm3m, at high temperatures. The order parameter is a rotation of the O6-octahedron described by one χ(O)-parameter. This parameter and the spontaneous strain (c/a - √6), as well as the relevant atomic displacement parameter U(op) 11(O) in the order parameter system, show a critical behaviour in agreement with a second order phase transition. Although the critical exponents of the order parameter and strain show the expected coupling behaviour, there is a striking difference of the transition temperature: the metric becomes cubic roughly 30 K below the proper T(c). This is related to spontaneous formation of domains imposing the average cubic symmetry via internal stresses.

A powder diffraction study of the phase transition in LaAlO3

Owing to some recent confusion in the literature the room temperature structure of LaAlO3 was redetermined by X-ray diffraction using a Guinier camera equipped with an image plate system. Different samples of pure LaAlO3 as well as a sample doped with 0.2 percent by weight ThO2 were used in the investigation. The space group of the structure, which is a rhombohedrally distal ted perovskite structure, due to a rotation of the oxygen octahedra, is clearly c. The lattice constants of pure LaAlO3 an a = 5.3655(2) Angstrom and c = 13.112(1) Angstrom with a cia-ratio of 2,444(1). The influence of doping on the lattice parameters and the tilting of the Oc-octahedron is discussed.

Room temperature structure of LaAlO3

The high temperature structure above 700 °C of the oxygen superionic conductor La0.9Sr0.1Ga0.8Mg0.2O2.85 has been analyzed by means of a combined neutron and high resolution synchrotron powder study. The splitting of reflections and the absence of any superstructure reflections leads to the conclusion that the correct space group is R3¯m. This is the first example of this space group appearing within the subgroup family of the ideal parent perovskite structure with Pm3¯m symmetry.



Uber die Kristallstruktur von Sr/Mg-dotiertem Lanthangallat bei hohen Temperaturen. Der erste Perowskit-Typ mit R3¯m-Symmetrie



Die Kristallstruktur des schnellen Sauerstoffionenleiters La0.9Sr0.1Ga0.8Mg0.2O2.85 wurde in situ bei Temperaturen oberhalb von 700 °C mit Pulverbeugungsmethoden unter Nutzung von Neutronen- und Synchrotronstrahlung untersucht. Die beobachtete Reflexaufspaltung und die Abwesenheit jeglicher Uberstrukturreflexe last auf einen Perowskit-Typ mit der Raumgruppe R3¯m schliessn. Dies ware unseres Wissens das erste Beispiel einer Perowskitstruktur mit einer solchen Symmetrie.

On the High‐temperature Crystal Structure of Sr/Mg‐doped Lanthanum Gallate. Evidence for the First Perovskite‐type Structure with R3¯m Symmetry

Solid super-ionic conductors are important materials for technical applications, e.g. in fuel cells. Recently, a new type of oxygen ionic conductor was reported, based on perovskite-type LaGaO3, allowing operating temperatures at 800 °C [1]. Oxygen vacancies in the anion lattice, necessary to enhance the ionic conduction, can be introduced by doping LaGaO3 with aliovalent oxides like SrO and MgO. In order to understand the conduction mechanism in La1-xSrxGa1-yMgyO3-0.5(x+y) samples, it is essential to determine the phase relationships as a function of temperature and composition. As shown in our earlier work, this is not feasible with simple XRD measurements using a laboratory diffractometer [2]. Only the combination of scattering experiments using synchrotron radiation and neutrons allows a complete structural description as a function of temperature. In a final step this study aims at a correlation of structural features with anomalies found in the conductivity data as a function of temperature and composition.

On the High‐Temperature Crystal Structure of Sr/Mg‐Doped Lanthanum Gallate. Evidence for the First Perovskite‐Type Structure with R $\bar 3$m Symmetry.

H. Lehnert

Papers

A neutron powder investigation of the high-temperature structure and phase transition in stoichiometric LiNbO3

A powder diffraction study of the phase transition in LaAlO3

Room temperature structure of LaAlO3

On the High‐temperature Crystal Structure of Sr/Mg‐doped Lanthanum Gallate. Evidence for the First Perovskite‐type Structure with R3¯m Symmetry

On the High‐Temperature Crystal Structure of Sr/Mg‐Doped Lanthanum Gallate. Evidence for the First Perovskite‐Type Structure with R $\bar 3$m Symmetry.