M. Savinov

Dielectric properties and phase transitions in K1−xLixTa1−yNbyO3:Cu

Abstract The ε’(T, f) and tan [δ(T, f)] (10–300 K, 100 Hz – 1 MHz) behavior is studied in K1−xLixTa1−yNbyO3:Cu (0·012 %wt. in batch) with estimated x1 = 0·0014, y = 0·024 (KLTN 0·14/2·4) and x2 = 0·0028, y2 = 0·013 (KLTN 0·28/1·3). In KLTN 0·14/2·4 a strong dipole relaxation is found in the 40–80 K region, with ε’(T)max = 16 000 at 47 K for 100 Hz, and a phase transition dominated by the soft mode coupled with a relaxation freezing mode is displayed at 39 K. Below, at 15·5 K, a nondispersive ε’(T) cusp-shaped maximum with ε’(T)max≈25 000 was found. KLTN 0·28/1·3 reveals a similar behavior, namely, Li+-related dielectric dispersion with ε’(T)max = 8300 at 47 K for 100 Hz, and a phase transition at ∼35 K. At lower temperatures, ε’ increases with a weak shoulder at ∼ 27 K. The so-called “277” species of KLTN:Cu (1900 ppm), with x = 0·001, y = 0·025 stated by content analysis, whose unusual properties were marked recently[1,2], reveal a sharp phase transition, accompanied by dielectric dispersion with a very high ε’(T)max ≈86000 and increased conductivity in the transition region.

Polar state in a SrTiO3-KTaO3 solid solution

Dielectric properties of ceramic samples of a 0.85SrTiO3-0.15KTaO3 solid solution have been studied. The temperature dependence of the dielectric permittivity exhibits maxima with a strong frequency dispersion, which is described by the Vogel-Fulcher law with a characteristic temperature Tg=26 K. These features are apparently connected with transition to a polar glasslike state.

Dielectric permittivity and phase transitions in the SrTiO3-KTaO3 system

The dielectric permittivity and losses of the (SrTiO3)0.85(KTaO3)0.15 solid solution have revealed an unusual behavior at temperatures of 5–300 K and frequencies ranging from 100 Hz to 1 MHz. The dielectric permittivity exhibits a broad maximum at ≈40 K, which obeys the Curie-Weiss law in the high-temperature wing, and the onset of low-temperature relaxation contributions. Besides, while for T>40 K it is a fully ergodic, uniformly ordering system, the uniform polar ordering is replaced with decreasing temperature by two low-temperature phase transitions to a glass-like state. The proposed mechanism for the dielectric relaxation and the observed phase transformations involves reorientation and ordering in the system of electron and hole polarons, which form in the course of charge compensation of the heterovalent ions Sr2+, K+ and Ti4+, Ta5+ distributed randomly in the sublattices.

Dielectric Permittivity Study of KTaO3 Weakly Doped by 6Li Isotope

The temperature-frequency behaviour of the dielectric permittivity (9–300 K; 100 Hz–1 MHz, and at 100 GHz) of K0.984Li0.016TaO3 containing the natural ratio of the Li isotopes (7.56% of 6Li and 92.44% of 7Li, KLT-7) and only the 6Li isotope (KLT-6) were studied. Both materials reveal the characteristic low temperature dielectric relaxation obeying the Arrhenius law and associated with π/2-reorientations of the Li+ ⟨100⟩ off-centres. However, the activation energy for 6Li+ relaxation appeared to be smaller and the relaxation time pre-exponent is larger than for 7Li+ centres, which agrees well with theoretical analysis.

Long-Range Displacive to Short-Range Order-Disorder Crossover in Weakly Concentrated KTayNb1-yO3

Recently we reported on an reentrant dipole glass-like phase formation in K0.9986Li0.0014Ta0.976Nb0.024O3 (V. Trepakov et al.: Phys. Rev. B 63 (2001) 172203). Here we present the detailed low-frequency dielectric permittivity and Raman scattering data for nearly the same composition but without Li admixture, namely for KTa0.982Nb0.018O3 (KTN 1.8). We found that upon cool down, KTN 1.8 exhibits soft-mode-driven long-range displacive dynamics with phase transition at TC = 27 K. Below TC the randomness of the Nb distribution leads to a pronounced crossover to the order-disorder polar microregion dynamics which is also accompanied, nevertheless, by TO1 mode stiffness upon cooling. The application of a dc-bias returns the long-range displacive behavior to the system.

Cu− and Fe− doping in properties of KTaO3 and KTaO3:Li crystals

Abstract To deepen the understanding of the Cu and Fe doping effect in dielectric properties of KTaO3:Li crystals, detail studies of the ε’ (T, f) and tan [δ(T, f)] (9–300 K, 100 Hz–1 MHz) behavior with a series of nominally pure KTaO3, Cu− and Fe−doped KTaO3 and KTaO3:Li (0·1% ÷3% mol. in the batch) crystals were performed. Below 70 K, all pure, Cu− and Fe−doped KTaO3 reveal a universal low-temperature dielectric relaxation seen well in T, f behavior of losses obeying the Arrhenius law with activation energy Δ = 79·3 − 86·2 meV and τo ≈ 6·5 × 10−14 s. Several Fe containing KTaO3 specimens displayed an additional low-temperature Arrhenius Debye-type dielectric relaxation with Δ = 74·1 eV and τo ≈ 2 × 10−14 s. While doping by Fe and especially Cu has insignificant effect on KTaO3 properties, in KTaO3:Li it strongly increases low temperature relaxations and dipole ordering effects. This result is consistent with a model of coupled relaxations involving the off center Li+ and another relaxation dipole defect. This “other dipole defect” can be associated with double-well Cu2+(Ta5+) and Cu+(K+) off centers. Contaminating by Nb impurities dramatically modifies KTaO3:Li properties.

Reentrant Dipole Glass-Like Ordering in Weakly Concentrated KTaO3:Nb

Recently we reported that a very small admixture of Li☎ dipole impurities into moderately concentrated KTaO3:Nb leads to an unusual sequence of ferroelectric-type phase transitions with a reentrant glass phase formation at the lowest temperatures [1], Here, we present dielectric permittivity and Raman scattering study for nearly the same composition as used in Ref. [1], but without Li admixture, for KTa0.982Nb0.018,O3 (KTN1.8). It is found that the presence of strong Li dipole defects is not an essential condition for the reentrant glass formation. Above Tc (27 K) KTN1.8 behaves as a conventional soft mode displacive type ferroelectric in paraelectric phase. However, below Tc a crossover to the order-disorder polar microregion dynamics and a state at which a reentrant glass phase coexists with long-range ordering occurs.

Properties of K1-xLixTaO3 solid solutions; first-principles computations and comparison with experiments

Experiments on K0.957Li0.043TaO3 samples indicate two different relaxation processes (π and π/2). First-principles computations clarify the natures of these relaxations, and yield good agreement with experiment.

Dielectric constant and ordering effects of pure and doped KTaO3 quantum paraelectric; Cu-codoping enhancement effect

Abstract ∊′(T, f) and tan[δ(T, f)] spectra (10–300 K, 120 Hz-1 MHz) of pure KTaO3 (KTO), KTO: Li (0.1–3%), KTO: Fe 1%, KTO: Cu 0.83% below 70 K reveal a Debye-like universal Arrhenius relaxation with Δ ≈ 80–85 meV, τ0 ∼ 5 × 10−14s. In KTO: Li + Fe and KTO: Li + Cu this contribution increases, confirming dipole-pair ordering effects with the leading role of Li+ in pair relaxation (V. Trepakov et al., J. Phys.: Condens. Matter 7, 3785 (1995)). A dramatic Cu-codoping dielectric enhancement effect in KTO: Li + Cu was observed. In KTO: Li 3% + Cu 0.2% in region 50–60 K ∊′(T) reveals a maximum of ∼ 105, accompanied by temperature hysteresis and dielectric loops. Ferroelectric-type phase transition with a critical dynamic central peak, induced by Cu2+(Ta)-VO centers as a slowest relaxator or oscillator in KTO: Li, is considered as an ∊′ enhancement cause. A possibility of incommensurate phase induced by Cu2+-VO centers and cluster-induced cooperative behavior is discussed too.

New dipole relaxators in weakly reduced KTaO3:Li crystals: Dielectric spectroscopy

Abstract The temperature-frequency behaviour of ϵ′ (T, f) and tan[δ(T, f)] (13 – 300 K, 120 Hz - 8 MHz) of KTaO3:Li (0.1 wt%) single crystals before and after a weak reduction was studied. Below 70 K before reduction, the samples reveal a shoulder in ϵ′ f (T) and a maximum in tan[δ(T)] obeying Arrhenius law with an activation energy δ ≈ 1000 K. This is associated with an ordering in clusters of paired reorienting dipolar defects, composed of off-center Li+ (the leading defect) and another polar impurity, or of a small number of ‘pure’ clusters of Li+ ions and(or) reorientations of the hole polaron (O- - center), localized near an impurity ion Fe3+(Fe2+). After reduction, the ϵ′ f (T) shoulder changes into a maximum, and four new low-temperature Arrhenius-like relaxations emerge. The new relaxations are associated with: (i) charge transfer relaxation in paired oxygen vacancies V 0 - V 0 (with or without trapped electrons) or paired (Ta3+ - V 0) - (Ta3+ - V 0) centers, (ii) cooperative relaxations in cluste...