A. Tkach

X-ray absorption fine structure studies of Mn coordination in doped perovskite SrTiO3

The coordination of Mn in doped SrTiO3 ceramics having nominal compositions SrTi0.98Mn0.02O3 and Sr0.98Mn0.02TiO3 was analyzed using x-ray absorption fine structure (XAFS) measurements. As expected, Mn4+ substitution for Ti4+ leads to Mn occupancy of the octahedral B-sites of ABO3 perovskite lattice with a Mn–O bond distance of 1.902 A (compared to 1.953 A for Ti–O) and no significant local distortions around the Mn atoms. In contrast, for the composition Sr0.98Mn0.02TiO3, Mn segregates to both the A-sites (as Mn2+) and the B-sites (predominantly as Mn4+). Extended XAFS confirms strong (≈0.77u2002A) displacements of Mn2+ cations off the ideal A-site positions along ⟨001⟩ directions with a significant distortion of several coordination shells around the dopant atoms.

Low-temperature dielectric response of NaTaO3 ceramics and films

In this work, NaTaO3 ceramics are prepared by conventional mixed oxide method and NaTaO3 films are deposited by RF magnetron sputtering on Si/SiO2/TiO2/Pt substrates. The dielectric response of the obtained NaTaO3 ceramics and films is analyzed as a function of frequency and temperature. Between 1 kHz and 1u2009MHz, the dielectric permittivity of NaTaO3 ceramics is frequency independent and increases on cooling up to ∼324, which is the highest value ever reported for NaTaO3 ceramics. In contrast, NaTaO3 films exhibit a dielectric relaxation between ∼20 and 30u2009K, following the Arrhenius law with activation energy ∼51u2009meV and pre-exponential term ∼10−15 s and attributed to polaron hopping.

Enhancement of tetragonality and role of strontium vacancies in heterovalent doped SrTiO3

The effect of Sr vacancies on the behavior of strontium titanate with trivalent dopants (La3+, Gd3+, and Y3+) substituting Sr2+ ions is reported. A remarkable shift of the antiferrodistortive transition temperature Ta is revealed by Raman spectroscopy for just a small content of dopant. It is shown that a unique linear dependence of Ta versus tolerance factor is obtained when Sr-vacancies are taken into account. A vacancy size value of ∼1.54u2002A is estimated, which is ∼7% larger than Sr2+ radius. This size difference enables explaining the unexpected increase of lattice parameter with increasing Bi3+ content in Sr1−1.5xBixTiO3.

Spin cluster glass and magnetoelectricity in Mn-doped KTaO3

In ceramics of KTaO3 doped with 3 at.u2009% of Mn the dielectric response is dominated by the polydispersive behavior of Mn2+ centered polar regions, whereas the magnetic and magnetoelectric (ME) behaviors reflect an intimate coupling between A-site substituted Mn2+ ions and minute amounts of Mn3O4 precipitates mediated by the polar host material. This becomes apparent by the common onset at Tc≈42u2002K of the ordering of ferrimagnetic Mn3O4 and of a spin cluster glass, which is characterized by memory and rejuvenation effects. The composite magnetic system exposed to external magnetic and electric dc fields shows large third order ME susceptibility with a sharp anomaly at Tc and 1/T2 dependence as T→0.

Mechanisms of magnetoelectricity in manganese-doped incipient ferroelectrics

We report magnetization measurements and magnetic resonance data for SrTiO3 doped by manganese. We show that the recently reported coexistent spin and dipole glass (multiglass) behaviours are strongly affected by the distribution of Mn ions between the Sr and Ti sites. Motivated by this finding we calculate the magnetic interactions between Mn impurities of different kinds. Both LSDA+U and many-body perturbation theory evidence that magnetic and magnetoelectric interactions are mediated by MnB4+ ions substituting for Ti. We propose two microscopic magnetoelectric coupling mechanisms, which can be involved in all magnetoelectric systems based on incipient ferroelectrics. In the first one, the electric field modifies the spin susceptibility via spin-strain coupling of MnB4+. The second mechanism concerns Mn pairs coupled by the position-dependent exchange interaction.

Lattice dynamics in Ba0.7Sr0.3TiO3: study by THz and IR spectroscopy and ab initio simulations

We report on the experimental and theoretical study of the lattice dynamics in Ba0.7Sr0.3TiO3 in 20–900u2009K temperature range. Complex dielectric response in the terahertz and far infrared range was evaluated from the time-domain THz transmission spectra and infrared reflectivity by means of damped harmonic oscillator models in classical and factorized form of the dielectric function. The anisotropic dielectric function in the ferroelectric phases was extracted using the Bruggeman model of the effective medium approximation. The fitted parameters of the temperature-dependent Slater-type vibrations (soft mode and central mode, CM) were compared with those obtained from ab initio simulations. The role of the lattice phonons and CM in the dielectric anomaly is discussed.

Lithium-induced dielectric relaxations in potassium tantalate ceramics

This work reports the effect of lithium doping on the dielectric and polar properties of potassium tantalate. Experimental data were obtained in K1−xLixTaO3 ceramics with x = 0, 0.02, 0.05 and 0.10 by measuring both the dielectric permittivity from 102 to 108 Hz, and polarization under an ac electric field driven at 2.5 Hz, for temperatures from 10 to 300 K. The dielectric permittivity exhibits all the relaxations reported for K1−xLixTaO3 single crystals. Two dielectric relaxations observed at 40–125 K are ascribed to the individual hopping by 90° and 180° of dipoles created by the off-centre Li ions. Another relaxation observed at 100–200 K is related to 180°-flips of Li pairs for x = 0.02 and of polar clusters of interacting Li ions for x = 0.05 and 0.10. In addition to that, an additional relaxation not reported before is presented at 135–235 K for x = 0.10 and attributed to 90°-reorientation of Li polar clusters. Both the change from an Arrhenius to a Vogel–Fulcher dependence with increasing lithium content, and the emergence of slim P (E) hysteresis loops around the relaxation temperatures show that the relaxation dynamics in K1−xLixTaO3 can be understood if a crossover from a dipolar glass to a relaxor-like behaviour is assumed.

Dielectric relaxation of Sr1–1.5xBixTiO3 sol-gel thin films

The dielectric response of Sr1–1.5xBixTiO3 films (0.002u2009≤u2009xu2009≤u20090.167), prepared by sol-gel and deposited on Si/SiO2/TiO2/Pt substrates, is analyzed as a function of frequency and temperature. The hysteretic behavior of the polarization versus the electric field is studied as well. Between 100 Hz and 1 MHz, the real part of the dielectric permittivity ɛ′ exhibit a relaxation between ∼60 and 260 K, shifting to high temperatures with increasing the Bi content. In the imaginary part of the dielectric permittivity ɛ′′ of these films two relaxations are induced by Bi doping below the temperature of the ɛ′ relaxation. The first relaxation observed in films with 0.002u2009≤u2009xu2009≤u20090.10 follows the Arrhenius law with an activation energy of Uu2009=u200964–80 meV and a preexponential term τ0u2009=u2009(0.3–10.8)u2009×u200910−14u2009s almost independent on the Bi content and is ascribed to the individual hopping of dipoles created by the off-center Bi ions. The second relaxation observed in the films with 0.04 ≤u2009xu2009≤u20090.167 is described by the Vogel–Ful...

Low temperature dielectric characterization of Mg-doped SrTiO3 thin films prepared by sol-gel

Voltage dependence of dielectric constant e′ of ferroelectrics and low dielectric loss tanu2009δ of incipient ferroelectrics make them attractive for tuning elements in microwave circuits. In this letter, field dependence of the low-temperature dielectric permittivity and polarization of Mg-doped SrTiO3 films is studied. Incorporation of Mg on both Sr and Ti sites decreases the e′ and relative tunability nr of sol-gel derived SrTiO3 films, whereas polarization is reduced by Ti site substitution only. tanu2009δ of the studied films is ≤0.012, decreasing at low temperatures down to 0.001 when Ti is substituted by 5% of Mg.

Dielectric response of polycrystalline Sr1−1.5xBixTiO3 thin films under direct current bias

The effect of dc electric field Edc on the dielectric behavior of sol-gel derived Sr1−1.5xBixTiO3 films on Si/SiO2/TiO2/Pt substrates is systematically studied between 10 and 300 K. The rounded peak in the temperature dependence of the dielectric constant e′(T) of all Sr1−1.5xBixTiO3 films is strongly suppressed under dc bias. The observed dielectric behavior is attributed to the formation of polar dipoles by the introduction of Bi into Sr site. A high tunability (∼40%) and communication quality factor (∼10u2009000) under 100 kV/cm make moderately Bi-doped SrTiO3 films a promising material for tunable devices.