material gives rise to a range of interesting properties such as superconductivity, [1] high electron mobility, [2,3] ferromagnetism [4,5] and 2D electron gases. [6] In the multitude of the properties exhibited by SrTiO 3 , pyroelectricity, ferroelectricity, and piezoelectricity are markedly absent as these properties are symmetry prohibited in centrosymmetric crystal lattices. The polar phase required for exhibiting these properties can, however, be induced artificially by modification of the lattice. Large biaxial strain, [7] light pulses, [8,9] and elemental substitutions [10] can convert SrTiO 3 into a ferroelectric material, whereas strain gradients can induce a polarization via the flexoelectric effect [11,12] or oxygen vacancies migration. [13] Electron diffraction measurements furthermore show that the top TiO 2 surface layer of SrTiO 3 undergoes surface relaxation and oxygen ions move outward from the surface relative to the titanium ions, leading to a polarization of this layer. [14][15][16] This is supported by shell model and density functional theory calculations. However, the calculations also predict that the SrO layers underneath display a polarization opposite to that of the TiO 2 surface layer, [17][18][19] leaving the question of the predicted net polarization and a possible pyroelectricity at the surface wide open.Symmetry-imposed restrictions on the number of available pyroelectric and piezoelectric materials remain a major limitation as 22 out of 32 crystallographic material classes exhibit neither pyroelectricity nor piezoelectricity. Yet, by breaking the lattice symmetry it is possible to circumvent this limitation. Here, using a unique technique for measuring transient currents upon rapid heating, direct experimental evidence is provided that despite the fact that bulk SrTiO 3 is not pyroelectric, the (100) surface of TiO 2 -terminated SrTiO 3 is intrinsically pyroelectric at room temperature. The pyroelectric layer is found to be ≈1 nm thick and, surprisingly, its polarization is comparable with that of strongly polar materials such as BaTiO 3 . The pyroelectric effect can be tuned ON/OFF by the formation or removal of a nanometric SiO 2 layer. Using density functional theory, the pyroelectricity is found to be a result of polar surface relaxation, which can be suppressed by varying the lattice symmetry breaking using a SiO 2 capping layer. The observation of pyroelectricity emerging at the SrTiO 3 surface also implies that it is intrinsically piezoelectric. These findings may pave the way for observing and tailoring piezo-and pyroelectricity in any material through appropriate breaking of symmetry at surfaces and artificial nanostructures such as heterointerfaces and superlattices. PyroelectricitySrTiO 3 has been the object of immense attention for over half a century owing to its multifunctional nature and popularity as a template for epitaxial growth of artificial nanostructures such as heterostructures, superlattices, and vertically aligned nanostructures. It is a classic example whe...

A protocol for characterizing relaxation of anisotropic strain in thin films of 10 mol% Eu-or Sm-doped ceria is described. The method is based on comparison of Raman spectra and X-ray diffraction patterns from substratesupported films, displaying in-plane compressive strain (initial state), with analogous data from 2 mm diameter self-supported films (i.e., membranes), prepared by partial substrate removal (final state). These membranes are found to be relaxed, i.e., approximately unstrained, but with increased unit cell volume. The effective (i.e., 2-state) Grüneisen parameter of the F 2g Raman active mode for these films is calculated to be 0.4 ± 0.1, which is ≈30% of the literature value for the corresponding ceramics under isostatic pressure. On this basis, it is found that the observed red-shift of the F 2g mode frequency following isothermal strain relaxation of the doped ceria thin films cannot be determined solely by the increase in average unit cell volume. The study presented here may shed light on the suitability of Raman spectroscopy as a technique for characterizing strain in lanthanide-doped ceria thin films.

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Gd‐doped ceria (CGO), one of the most extensively studied oxygen ion conductors, is a low dielectric constant/low mechanical compliance material exhibiting large nonclassical electrostriction. The electromechanical response of the micro‐electromechanical devices with CGO films as an active material described previously can not be attributed exclusively to electrostriction. Here it is shown that, below 1 Hz, in addition to electrostriction (second‐harmonic response), there is a strong contribution of the electro‐chemomechanical effect (ECM, first harmonic response). ECM is the change in mechanical dimensions of ionic and mixed ionic‐electronic conductors as a result of a change in chemical composition induced by an electric field. In batteries, the presence of ECM is highly detrimental. In ceria at room temperature, it was considered to be negligible because of slow oxygen diffusion. This work demonstrates ECM actuation at ambient temperature and moderate electric field (<5 V µm−1). ECM‐induced strain is attributed to reversible oxidation/ reduction of TiO2 layers at the Ti‐CGO interface. At 25 °C, the ECM bending strain is 1.2 × 10−6, increasing exponentially with temperature. These data suggest that with a proper choice of materials, ECM‐type response can be a viable mechanism for mechanical actuation at ambient and also at slightly elevated temperatures.

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Dimensional change in a solid due to electrochemically driven compositional change is termed electro-chemo-mechanical (ECM) coupling. This effect causes mechanical instability in Li-ion batteries and solid oxide fuel cells. Nevertheless, it can generate considerable force and deformation, making it attractive for mechanical actuation. Here a Si-compatible ECM actuator in the form of a 2 mm diameter membrane is demonstrated. Actuation results from oxygen ion transfer between two 0.1 µm thick Ti oxide\Ce 0.8 Gd 0.2 O 1.9 nanocomposite layers separated by a 1.5 µm thick Ce 0.8 Gd 0.2 O 1.9 solid electrolyte. The chemical reaction responsible for stress generation is electrochemical oxidation/reduction in the composites. Under ambient conditions, application of 5 V DC produces actuator response within seconds, generating vertical displacement of several µm with calculated stress ≈3.5 MPa. The membrane actuator preserves its final mechanical state for more than 1 h following voltage removal. These characteristics uniquely suit ECM actuators for room temperature applications in Si-integrated microelectromechanical systems.

Doped ceria is known for decades as an excellent ionic conductor used ubiquitously in fuel cells and other devices. Recent discovery of a giant electrostriction effect has brought world-wide interest to this class of materials for actuation applications in micromechanical systems. From this aspect, the electromechanical response has to be studied as a function of external parameters, such as frequency, temperature, and electrode material. In this work, we fabricated circular membranes based on Gd-doped ceria (CGO) with Ti electrodes and studied their electromechanical response using a sensitive interferometric technique. The self-supported membranes are flat at room temperature and reversibly buckle upon heating, indicating that the membranes are under in-plane tensile strain. We have found that the electromechanical response is strongly frequency dependent. Significant hysteresis is observed in the displacement-vs.-voltage curves, which is deleterious for micromechanical applications but can be eliminated by tuning the phase of the excitation voltage. The electromechanical response of the system increases with temperature. Finite Element Modeling is applied to evaluate the electrostriction coefficient of the CGO material. At low frequencies, the M12 electrostriction coefficient is about 5 × 10−18 m2/V2, which is in line with the previous reports.

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