Dielectric relaxation in SrTiO<Subscript>3</Subscript>: Mn

Dielectric relaxation (activation energy U≈0.03 eV, relaxation time τ₀≈5×10^?11 s) has been observed in SrTiO₃: Mn solid solutions at low temperatures. It is assumed that the relaxation is related to reorientation of the polarons localized at defects of the {Mn _Ti ²⁺ -O^?} type and that the deviations from classical thermally activated behavior at the lowest temperatures reached are due to the quantum tunneling mode.     

Thermo-Optical Study of Short-Range Polar Order in a Ferroelectric Phase: The Ca<Superscript>2+</Superscript> Impurity-Induced Ferroelectric Phase in SrTiO<Subscript>3</Subscript>

The magnitudes and temperature dependences of the root-mean-square of polarization fluctuations, which characterizes short-range ordering below T_C, are determined for the first time for Sr_0.986Ca_0.014TiO₃ single crystals, using temperature measurements of light refraction made in a manner developed by the authors. Ab initio calculations of the equilibrium structures and total energies of three lowtemperature phases for SrTiO₃ and CaTiO₃ are performed and the symmetry of the ground state of their solid solution is identified on the basis of the obtained results. Features of short- and long-range ordering in the Ca²⁺-induced ferroelectric phase of the Sr_{1 ? x}Ca _x TiO₃ system are discussed.     

Phase transitions and metastable states in stressed SrTiO<Subscript>3</Subscript> films

The sequence of the ground states in SrTiO₃ films subjected to epitaxial strain and fixed mechanical stress in the [001] and [110] directions is calculated from first principles within the density functional theory. Under the fixed-strain conditions, an increase in the substrate lattice parameter results in the following sequence of the ground states: I4cm → I4/mcm → Ima2 → Cm → Fmm2 → Ima2(II). When moving to the fixed-stress conditions, the phase sequence changes significantly and depends on how the stress is applied. It is revealed that the simultaneous presence of two types of the lattice instability (the ferroelectric and structural ones) in strontium titanate leads to the formation of a whole system of metastable phases whose number increases abruptly under the fixed-stress conditions. The stability of these phases changes with pressure and phase transitions occur between them. The appearance of broad bistability regions in certain parts of the phase diagram enables the use of this phenomenon for developing nonvolatile phase-change memory.     

Formation of ceramic solid solutions in a SrTiO<Subscript>3</Subscript>-BiScO<Subscript>3</Subscript> system

X-ray structural, X-ray phase, and dilatometric analyses were used to explore specific features of the formation of solid solutions in the (1 − x)SrTiO₃−xBiScO₃ system with x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5. It was found that the synthesis of solid solutions from the initial Bi₂O₃, Sc₂O₃, TiO₂, and SrCO₃ components is accompanied by a considerable increase in the linear dimensions of the samples, depending on their composition. Solid solutions with x > 0.1 are formed through intermediate phases (Sr₅Bi₆O₁₄ at x = 0.2; Sr_0.78Bi_2.22O₄ at x = 0.3; and Sr_2.25Bi_6.75O_12.38 at x = 0.4 and 0.5). It was shown that the samples with x = 0.2, 0.3, 0.4, and 0.5 have two phases: one with a cubic Pm3m structure and one with a tetragonal I4/mcm structure.     

Electron gas induced in SrTiO<Subscript>3</Subscript>

This mini-review is dedicated to the 85th birthday of Prof. L.V. Keldysh, from whom we have learned so much. In this paper, we study the potential and electron density depth profiles in surface accumulation layers in crystals with a large and nonlinear dielectric response such as SrTiO₃ (STO) in the cases of planar, spherical, and cylindrical geometries. The electron gas can be created by applying an induction D₀ to the STO surface. We describe the lattice dielectric response of STO using the Landau–Ginzburg free energy expansion and employ the Thomas–Fermi (TF) approximation for the electron gas. For the planar geometry, we arrive at the electron density profile n(x) ∝ (x + d)^–12/7, where d ∝ D₀^–12/7. We extend our results to overlapping electron gases in GTO/STO/GTO heterojunctions and electron gases created by spill-out from NSTO (heavily n-type doped STO) layers into STO. Generalization of our approach to a spherical donor cluster creating a big TF atom with electrons in STO brings us to the problem of supercharged nuclei. It is known that for an atom with a nuclear charge Ze where Z > 170, electrons collapse onto the nucleus, resulting in a net charge Zn < Z. Here, instead of relativistic physics, the collapse is caused by the nonlinear dielectric response. Electrons collapse into the charged spherical donor cluster with radius R when its total charge number Z exceeds the critical value Z_c ≈ R/a, where a is the lattice constant. The net charge eZ_n grows with Z until Z exceeds Z* ≈ (R/a)^9/7. After this point, the charge number of the compact core Z_n remains ≈ Z*, with the rest Z* electrons forming a sparse TF atom with it. We extend our studies of collapse to the case of long cylindrical clusters as well.     

Symmetry and lattice parameters of SrTiO<Subscript>3</Subscript>-PbTiO<Subscript>3</Subscript> solid solutions

The lattice parameters of ceramic samples of (1 ? x)SrTiO₃-xPbTiO₃ solid solutions are measured at room temperature. It is found that the samples have cubic symmetry in the concentration range x = 0?0.3 and tetragonal symmetry for x > 0.3. The lattice parameter a is virtually independent of temperature for x < 0.8 and slightly decreases in the range x = 0.8?1.0, while the lattice parameter c increases with increasing x. The reduced cubic parameter varies nonlinearly and deviates from Vegard’s linear law as the concentration x increases.     

Synchrotron X-ray diffraction study of SrRuO<Subscript>3</Subscript>/SrTiO<Subscript>3</Subscript>/SrRuO<Subscript>3</Subscript> nano-sized heterostructures grown by laser MBE

Structural investigation using X-ray synchrotron radiation has been performed on SrRuO₃/SrTiO₃/SrRuO₃ epitaxial heterostructures, with each constituent layer a few unit cell thick grown on (001) SrTiO₃ substrate. Detailed information on the evolution of the in-plane lattice structure has been obtained, in these heterostructures, by grazing incidence diffraction measurements. The samples have been grown by low-pressure pulsed laser deposition. Under our deposition conditions, SrRuO₃ layers grow with an elongated cell perpendicular to the substrate surface. The in-plane pseudocubic lattice parameters do not match the in-plane square SrTiO₃ structure even in the case of very thin SrRuO₃ layers. Such a distortion was found to decrease with increasing the thickness of the SrTiO₃ barrier layer.     

Dielectric Properties of Nanoporous Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Films Filled with Ferroelectric SC(NH<Subscript>2</Subscript>)<Subscript>2</Subscript>

Temperature dependences of permittivity ε′ and third harmonic amplitude γ_3ω of nanocomposites obtained by embedding ferroelectric SC(NH₂)₂ in porous alumina films with pore sizes of 60 and 100 nm are studied. A substantial increase in the temperatures of ferroelectric phase transition T_c1 and T_c2 and that of phase transition T_i from incommensurate phase to paraphase are also observed. The temperatures of all phase transitions are found to rise as pore diameters shrink.     

Effects of BaTiO<Subscript>3</Subscript> and SrTiO<Subscript>3</Subscript> as the buffer layers of epitaxial BiFeO<Subscript>3</Subscript> thin films

BiFeO₃ (BFO) thin films with BaTiO₃ (BTO) or SrTiO₃ (STO) as buffer layer were epitaxially grown on SrRuO₃-covered SrTiO₃ substrates. X-ray diffraction measurements show that the BTO buffer causes tensile strain in the BFO films, whereas the STO buffer causes compressive strain. Different ferroelectric domain structures caused by these two strain statuses are revealed by piezoelectric force microscopy. Electrical and magnetical measurements show that the tensile-strained BFO/BTO samples have reduced leakage current and large ferroelectric polarization and magnetization, compared with compressively strained BFO/STO. These results demonstrate that the electrical and magnetical properties of BFO thin films can be artificially modified by using a buffer layer.     

Dielectric response of a (1000 nm)SrTiO<Subscript>3</Subscript> layer epitaxially grown on (001)La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> to temperature variation and electric field

The structure and dielectric characteristics of the (1000 nm)SrTiO₃ spacer in a (001)SrRuO₃ ‖ (001)SrTiO₃ ‖ (001)La_0.67Ca_0.33MnO₃ trilayer heterostructure grown on a (001)(LaAlO₃)_0.3+(Sr₂AlTaO₆)_0.7 substrate have been studied. Both oxide electrodes, as well as the strontium titanate layer, were cube-on-cube epitaxially grown. The unit cell parameter in the SrTiO₃ layer measured in the substrate plane (3.908±0.003 Å) practically coincided with that determined along the normal to the substrate surface (3.909±0.003 Å). The temperature dependence of the real part of the permittivity ?′ of the SrTiO₃ layer in the range 70–180 K fits the relation (?′)^?1 ～ ? ₀ ^?1 C ₀ ^?1 (T-T _C ) well, where C₀ and T_C are the Curie constant and the Curie-Weiss temperature, respectively, for bulk strontium titanate crystals and ?₀ is the free-space permittivity. The data obtained on the temperature dependence of the permittivity of SrTiO₃ films enabled us to evaluate the effective depth of electric field penetration into the manganite electrode (L _e ≈ 0.5 nm) and the corresponding capacitance (C _e ≈1×10^?6 F/cm²) of the interface separating the (001)SrTiO₃ layer from the (001)La_0.67Ca_0.33MnO₃ bottom electrode.     

Response of the capacitance and dielectric loss of the SrRuO<Subscript>3</Subscript>/SrTiO<Subscript>3</Subscript>/SrRuO<Subscript>3</Subscript> film heterostructures to variations in temperature and electric field

Three-layer epitaxial heterostructures with a 750-nm-thick intermediate strontium titanate layer between two strontium ruthenate conductive thin-film electrodes have been grown by laser deposition. Photolithography and ion etching have been used to form film parallel-plate capacitors based on the grown heterostructures. The capacitance (C) and dielectric loss tangent (tanδ) of the parallel-plate capacitors have been measured in the temperature range T = 4.2–300 K at an applied bias voltage of up to ±2.5 V and without it. At T > 100 K, the temperature dependence of the dielectric permittivity (ε) of the SrTiO₃ intermediate layer is well approximated by the Curie–Weiss law taking into account the capacitance induced by the penetration of an electric field into the oxide electrodes. At T ≈ 20 K, the dielectric permittivity ε of the SrTiO₃ intermediate layer decreases by approximately 20% in an electric field of 25 kV/cm. The dielectric loss tangent of the film capacitor heterostructures decreases monotonically with a decrease in the temperature in the range from 300 to 80 K and almost does not depend on the electric field strength. However, in the range from 80 to 4.2 K, the dielectric loss tangent increases nonmonotonically (abruptly) with a decrease in the temperature and decreases significantly in an applied electric field.     

Phase transition in YbAl<Subscript>3</Subscript>C<Subscript>3</Subscript>

¹⁷⁰Yb M?ssbauer spectroscopy, temperature dependent X-ray, magnetisation and specific heat data are presented in the hexagonal intermetallic YbAl₃C₃, in order to shed light on the isostructural transition occurring near 80 K and to investigate the electronic state of the Yb ion above and below the transition. In the low temperature phase, we find that there occurs an atomic rearrangement in the hexagonal unit cell, leading to a strong symmetry lowering at the Yb site. We show that no magnetic ordering of the Yb³⁺ moments occurs down to 0.04 K, and we discuss this finding in terms of 4f-conduction electron hybridisation and geometric frustration.     

Direct evidence for off-centering of Mn impurity in SrTiO<Subscript>3</Subscript>

Mn-doped SrTiO₃ samples (3% Mn) with different deviations from stoichiometry have been investigated using XAFS spectroscopy. It was shown the under various preparation conditions manganese atoms can sub-stitute A and B sites of the perovskite structure and are in them in different charge states. Impurity Mn⁴⁺ ions that substitute Ti sites occupy on-center position in the lattice, whereas Mn²⁺ ions that substitute Sr sites are off-center and are displaced from the lattice sites by a distance of 0.32Å.     

Rectifying and photovoltage characteristics of Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Co<Subscript>2</Subscript>O<Subscript>y</Subscript>/Nb-doped SrTiO<Subscript>3</Subscript> heterojunction

A heterojunction was fabricated by growing a layer of Bi₂Sr₂Co₂O _y thin film on the 0.7 wt% Nb-doped SrTiO₃ substrate. Such heterojunction showed good rectifying characteristics over a wide temperature range, and its transport mechanism under the forward bias can be attributed to a space charge limited conduction mechanism via defects near the interface of the heterojunction. Photovoltaic properties of the heterojunction were studied by using both continuous-wave and pulsed irradiations and the results can be well explained by the photovoltaic effect of a p–n junction.     

Electroluminescence in SrTiO<Subscript>3</Subscript>:Cr single-crystal nonvolatile memory cells

Metal–insulator–metal (M–I–M) structures involving transition-metal oxides and, more recently, also perovskite oxides with resistive switching effects have attracted substantial interest in research aimed at nonvolatile memories of nanometer dimensions. Although some models are presently under discussion, it is still not clear whether the fundamental switching mechanism is an interface or a bulk property, or a combination of both. Extended defects, such as dislocation lines and changes in the oxygen vacancy concentration, are considered responsible for the conducting state, and local reduction/oxidation processes have been proposed to be responsible for the resistive switching. In addition, the role of dopants has not been discussed in depth. Here we report on an electric-field-controlled electron trapping/detrapping process involved in the resistive switching in Cr-doped SrTiO₃. Electroluminescence (EL) measurements reveal that during resistive switching, light emission is observed only in the switching transition from high to low conductivity. The EL spectrum is typical for Cr³⁺ in an octahedral ligand field, indicating that the switching process involves trapping/detrapping of electrons at the Cr site. With increasing conductivity of SrTiO₃, we observe a change from the predominant (R-line) to the vibronically red-shifted transition, which points to a modification of the Cr-occupied lattice sites. PACS 71.30.+h; 78.60.Fi; 73.40.Rw; 78.55.-m; 85.30.Tv     

La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> thin films on SrTiO<Subscript>3</Subscript> and CaTiO<Subscript>3</Subscript> buffered Si substrates: structural,static, and dynamic magnetic properties

Nearly 50-nm thick La_0.7Sr_0.3MnO₃ (LSMO) films were grown on Si substrates using molecular beam epitaxy on (001) Si substrates over-layered by a 20 nm thick SrTiO₃ (STO) or by a 20 nm thick CaTiO₃ (CTO) film. In addition, a reference LSMO film was directly deposited on a (001) STO substrate by pulsed laser deposition. For all the samples, X-ray diffraction revealed an excellent epitaxy of the LSMO film and small mosaicity around (001), with in-plane [100] and [010] cubic axes. The LSMO/CTO films are in-plane compressed while the LSMO/STO ones are in-plane extended. The temperature dependence of their static magnetic properties was studied using a SQUID, showing a Curie temperature overpassing 315 K for all the samples. Hysteresis loops performed at room temperature (294 K) with the help of a vibrating sample magnetometer (VSM) are also discussed. At 294 K Micro-strip ferromagnetic resonance (MS-FMR) was used to investigate the dynamic magnetic properties. It allows concluding to a strong anisotropy perpendicular to the films and to a weak fourfold in-plane anisotropy with easy axes along the [110] and [1[`1]0 1\bar{1}0 ] directions. Their values strongly depend on the studied sample and are presumably related to the strains suffered by the films.     

Dielectric relaxation in BiLi<Subscript>0.6</Subscript>W<Subscript>0.4</Subscript>O<Subscript>3</Subscript> ceramics

The low-frequency process of dielectric relaxation in the new lead-free compound BiLi_0.6W_0.4O₃ prepared by conventional ceramic technology is studied. The features of dielectric relaxation are discussed in terms of a model of interaction between the domain boundaries and point defects of a crystalline lattice.     

First-principles study of bulk and surface oxygen vacancies in SrTiO<Subscript>3</Subscript> crystal

The structural and electronic properties of the neutral and positively charged oxygen vacancies (F and F⁺ centres) in the bulk and on the (001) surfaces of SrTiO₃ crystal are examined within the hybrid Hartree-Fock and density functional theory (HF-DFT) method based upon the linear combination of atomic orbital (LCAO) approach. A comparison of the formation energy for surface and bulk defects indicates a perceptible propensity for the segregation of neutral and charged vacancies to both SrO and TiO₂ surface terminations with a preference in the latter case which is important for interpretation of space charge effects at ceramic interfaces. It is found that the vacancies reveal more shallow energy levels in the band gap on surfaces rather than in the bulk, in particular, on the TiO₂ surface. The charged F⁺ centre has significantly deeper energy levels both in bulk and on the surfaces, as compared with the neutral F centre.     

Phase transition in CdHfO<Subscript>3</Subscript>

The phase transition from an orthorhombic phase (space group Pnma) to a rhombohedral phase (space group R3m) of the CdHfO₃ hafnate is investigated using methods of structural analysis. It is shown that crystal lattices of both phases contain polar structural units (octahedra, cubooctahedra). On this basis, it is assumed that the orthorhombic and rhombohedral phases of the CdHfO₃ compound are the antiferroelectric and ferroelectric phases, respectively.     

Phase separation in a manganite La<Subscript>0.95</Subscript>Ba<Subscript>0.05</Subscript>MnO<Subscript>3</Subscript> crystal

The structure and magnetic states of a crystal of lightly doped manganite La_0.95Ba_0.05MnO₃ were studied using thermal-neutron diffraction, magnetic measurements, and electrical resistance data in a wide temperature range. It is shown that, in terms of its magnetic properties, the orthorhombic crystal is characterized by two order parameters, namely, antiferromagnetic (T _N = 123.6 K) and ferromagnetic (T _C = 136.7 K). The results obtained differ in detail from known information on the manganites La_0.95Ca_0.05MnO₃ and La_0.94Sr_0.06MnO₃. Two models of the magnetic state of the La_0.95Ba_0.05MnO₃ crystal are discussed, one of which is a model of a canted antiferromagnetic spin system and another is associated with the phase separation of the manganite. Arguments are advanced in favor of the coexistence in this crystal of the antiferromagnetic phase (about 87%) with a Mn⁴⁺ ion concentration of 0.048 and the 1/16-type charge-ordered ferromagnetic phase (about 13%) with a Mn⁴⁺ ion concentration of 0.0625. The specific features of the manganite studied are due to self-organization of the La_0.95Ba_0.05MnO₃ crystal lattice caused by the relatively large barium ion size.