A threshold electric field for acoustic emission from ferroelectric Pb5Ge3O11

Acoustic emission has been observed in ferroelectric Pb₅Ge₃O₁₁ and TGS near saturation polarisation. An abrupt electric field threshold for AE has been discovered in Pb₅Ge₃O₁₁ (but not in TGS) which is related to the onset of domain wall movement.     

Phase transition and dielectric properties of Li2-x NaxGe4O9 crystals (0.2 ≤ x ≤ 0.3)

The dielectric nonlinearity of ferroelectric Li_2-x Na_xGe₄O₉ crystals (0.2 ≤ x ≤ 0.3) is measured in the region of the phase transition temperatures. The ?(T) dependences for various values of the applied dc electric field E ₌ and the ?(E ₌)_T dependences at a constant temperature are studied in the Li_2-x Na_xGe₄O₉ crystals with T _c > 300 K and T _c < 300 K. It is shown that the Landau theory for second-order phase transitions describes the dielectric properties of the crystals with T _c < 300 K and does not describe the behavior of the crystals with T _c > 300 K. The results obtained lead to the conclusion that the different properties of the crystals with T _c > 300 K are likely to be related to the changed structure of these crystals caused by a change in the ratio between the Li and Na atoms.     

Observation of the ferroelectric phase transition in Pb5Ge3O11 by the chemical potential changes

We observed the transition from the ferroelectric (FE) to paraelectric (PE) phase in the semiconducting, ferroelectric Pb₅Ge₃O₁₁ single crystal with the use of the contact electrode method. To this purpose a thin, metallic layer was placed onto the Pb₅Ge₃O₁₁ crystal surface, forming the contact electrode. At opposite ends of the contact electrode, silver wires were glued and a voltage was applied to the contact electrode in such a way that the electric current could flow only through the attached electrode. The electric resistance R(T) of the electrode was measured as a function of temperature. Two series of measurements were performed. In one of them the ferroelectric c-axis of the investigated crystal was perpendicular to the contact electrode. In the second one the c-axis was parallel to the attached electrode. We used gold as the contact electrode material. The anomaly in the R(T) in a form of a kink at T _kink?=?452?K was found for both c-axis orientations. The measured value of T _kink, appearing in the temperature dependence of contact electrode resistance, corresponds exactly to the phase transition temperature T _C from the FE to PE phase of the investigated Pb₅Ge₃O₁₁ material. This result demonstrates that the contact electrode method, primarily proposed exclusively to find critical temperatures of metallic samples, also works well in the case of ferroelectric and semiconducting materials like Pb₅Ge₃O₁₁. We ascribe the effect of the resistance kink in the temperature dependence of the contact electrode R(T) to thermal excitations of the electrons with different rates below and above T _C due to different electronic activation energies in the FE and PE phases of the investigated Pb₅Ge₃O₁₁ crystal. It, however, means that the phase transition in the electronic subsystem of the Pb₅Ge₃O₁₁ transfers into the electron gas of the contact electrode via the chemical potential relation µ _sample?=?µ _electrode due to the contact between the sample and the electrode. The magnitude of the kink, observed in the R(T) dependence, was higher on heating than on cooling. The additional measurement of the thermally stimulated current (TSC) was carried out on the non-polarised Pb₅Ge₃O₁₁ sample. In this series of measurements, the sample was covered with gold layers sputtered on the two opposite surfaces of the crystal. The TSC anomaly occurred, related to the residual pyroelectric effect, several degrees below the Curie temperature, T _C, and does not disturb the detection of the critical point with the use of the contact electrode method.     

The temperature dependence of the ALO-modes in Pb5Ge3O11

The temperature dependence of the LO-TO splitting of the A-modes of Pb₅Ge₃O₁₁ between room temperature and T_c=450K was determined from the Raman spectrum. The finite splitting of two A-modes above T_c leads to the conclusion that the space group of Pb₅Ge₃O₁₁ in the paraelastic phase deviates from P6 which was determined by X-ray and other physical methods.     

Raman scattering from the ferroelectric mode in the Pb5(Ge1−x,Six)3O11 system

We have observed the temperature dependence of the ferroelectric soft mode in the single crystals of Pb₅(Ge_1?x,Si_x)₃O₁₁ (x = 0.05 and 0.10) by means of the Raman scattering technique. The increasing of the amount of the silicon impurity lowers the soft mode frequency and influences the small change on the damping. The damping constant seems to be dominantly dependent upon the frequency. The experimental results have been well explained by considering the change of the short range harmonic force due to the silicon impurity.     

The effect of off-center Cu2+ ions on the phase transition in lead germanate crystals

The properties of Pb₅Ge₃O₁₁: Cu²⁺ crystals near the temperature of the ferroelectric phase transition are discussed in terms of a phenomenological approach. Assuming a quadratic interaction with the order parameter, the effect of Cu²⁺ is considered a result of static distortions, which result from the off-center position of the copper impurity ions in the lead germanate structure. In this approximation, Cu²⁺ jumps between off-center positions do not affect the dynamic properties of the crystal matrix near T _C.     

Spontaneous phase transition from relaxor to macrodomain ferroelectric state in single-crystal PbSc0.5Nb0.5O3-BaSc0.5Nb0.5O3 solid solutions

The cooling of Pb_1?x Ba_xSc_0.5Nb_0.5O₃ solid solutions with x≤0.04 leads to a spontaneous transition from a relaxor to a macrodomain ferroelectric state, accompanied by anomalous variation of the dielectric and optical properties of the material. As the barium content in the system increases, the relaxor state becomes more stable and eventually “freezes” at x≈0.05. The crystals with x=0.06 exhibited the appearance of a macrodomain ferroelectric phase induced both by an external electric field with a strength of 1.5 kV/cm and by an internal electric field formed in the course of dielectric aging.     

Structural and dielectric properties of Pb5(Ge,Si)O11

The ferroelectric lead germanate (Pb₅Ge₃O₁₁) and its isomorphous compounds are important because of their uses as pyroelectric and electro-optic devices. Comparison of inter-planar d-spacings of Pb₅Ge_3−x Si _x O₁₁ (x=0, 0.3, 0.7 and 1.00) suggests that there is no change in basic structure of Pb₅Ge_3−x Si _x O₁₁ when Si is substituted for Ge in small quantity (x<1). The dielectric properties of the Si-substituted compounds have been studied as a function of temperature (30 to 200°C). The ferroelectric-paraelectric phase transition has been observed at 185°C. The Si doping causes (a) Curie point to shift towards low temperature, (b) peak value of the dielectric constant to decrease and (c) phase transition diffuse. The fast increase in dielectric constant of pure Pb₅Ge₃O₁₁ with temperature (beyond transition temperature) may be attributed to the development of space charge polarization in the system.     

Heat capacity of the Pb<Subscript>5</Subscript>(Ge<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Si<Subscript>x</Subscript>)<Subscript>3</Subscript>O<Subscript>11</Subscript> ferroelectric system

The temperature dependences of the molar heat capacity at constant pressure, C_p, of Pb₅(Ge_1?xSi_x)₃O₁₁ crystals with x=0, 0.39, and 0.45 in the range 5–300 K, as well as of their permittivity, dielectric losses, and the pyroelectric effect, have been measured. Experimental data on the temperature behavior of the heat capacity are presented in the form of a sum of two Debye and one Einstein terms, C_p(T)=0.405C_D1(Θ_D1=160 K, T)+0.53C_D2(Θ_D2=750 K, T)+0.046C_E(Θ_E=47 K, T). Besides a peak in the region of the ferroelectric Curie point T_c=450 K for crystals with x=0, the temperature dependences of the heat capacity did not reveal any other pronounced anomalies.     

Ferroelectric phase transitions in Pb0.96Ba0.04Sc0.5Nb0.5O3 and Pb0.94Ba0.06Sc0.5Nb0.5O3 solid-solution single crystals

The optical properties of Pb_0.96Ba_0.04Sc_0.5Nb_0.5O₃ (PBSN-4) and Pb_0.94Ba_0.06Sc_0.5Nb_0.5O₃ (PBSN-6) single-crystal solid solutions were studied for the first time. It was shown that the spontaneous phase transition occurring in PBSN-4 with no electric field present is accompanied by a sharp minimum in optical transmission, which indicates the percolation nature of the transition. No sharp changes were observed in the temperature dependence of optical transmission in PBSN-6 single crystals with no electric field applied. However, a very weak electric field, ～0.4 kV/cm, is sufficient to induce the ferroelectric state in PBSN-6 single crystals. It was shown that the destruction of the induced ferroelectric state is a first-order phase transition which is accompanied by an anomalously narrow peak in the small-angle light scattering intensity (or by a minimum in optical transmission) and occurs through the percolation mechanism.     

Nonlinear properties of Pb5Ge3O11:Gd3+ in an electric field: Paramagnetic resonance

This paper discusses the field and temperature dependences of the shift in position of EPR signals in an external electric field, which is linearly related to the polarization in the paraelectric phase of Pb₅Ge₃O₁₁:Gd³⁺. Fiz. Tverd. Tela (St. Petersburg) 39, 1643–1644 (September 1997)     

Influence of the defect structure on the electrical conductivity of Pb5Ge3O11 single crystals at high temperatures

The impedance spectra of Pb₅Ge₃O₁₁ single crystals are measured in the frequency range from 5 Hz to 13 MHz at temperatures of 600 to 800 K in dry air and in a dry or wet nitrogen gas. It is found that the temperature and the gas composition significantly affect the electrical properties of the compound. The data obtained are used to discuss the origin of crystal lattice defects and their influence on charge transfer. It is concluded that the conduction is mixed in character (p-type electronic and ionic due to oxygen ions). The proton conduction is shown experimentally to be feasible. The possible mechanisms of proton transport in Pb₅Ge₃O₁₁ are discussed.     

Effective charge of divalent lead: Application to the assignment of infrared modes in ferroelectric Pb5Ge3O11

A correlation between Szigeti effective charge and the electronic polarizability is found for the eight compounds containing divalent lead where experimental data are available. Results in Pb₅Ge₃O₁₁ contribute to remove some ambiguity about the assignment of polar vibrational modes as external or internal to GeO₄ and Ge₂O₇ entities in ferroelectric lead germanate. Complete TO-LO splittings are also deduced from infrared reflection spectra obtained at room temperature. Results are compared with previous controversial Raman measurements of the polar character of the modes including the soft mode.     

Specific features of the electron paramagnetic resonance spectrum in the vicinity of the convergence of the transitions of gadolinium centers in Pb5(Ge1 − x Si x )3O11

An anomalous electron paramagnetic resonance spectrum of the transitions ?1/2 ? +1/2 of four Gd³⁺-Si dimer clusters in the Pb₅(Ge_{1 ? x} Si _x )₃O₁₁ crystals doped with gadolinium has been found in the vicinity of the orientation of the magnetic field along the optic axis of the crystal. It has been assumed that this spectrum is caused by rapid transitions between the spin packets of the initial resonances due to the crossrelaxation. A computer simulation of the spectrum has been carried out. The results obtained adequately describe the experiment.     

Dynamic scaling hysteresis behavior and field-induced domain transition of 0.16PIN-0.62PMN-0.22 PT single crystals

Due to the good dielectric, piezoelectric and ferroelectric properties, the pseudo-ternary ferroelectric single crystals Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PIN-PMN-PT) have been widely concerned and studied. In this study we found that the dynamic hysteresis loop area <A> with electric field E₀ and frequency f₀ of the 0.16 Pb(In_1/2Nb_1/2)O₃-0.62 Pb(Mg_1/3Nb_2/3)O₃-0.22PbTiO₃ (0.16PIN-0.62PMN-0.22 PT) single crystals satisfies the relational equation <A> ∝ E₀^βf₀^α. In the <A> ∝ E₀^β relationship, both linear stages are found correlated with the low electric field in the first stage E₀<E_c and the high electric field in the third stage E₀ > 3E_c. The hysteresis loop area <A> with frequency f₀ relationship satisfies the equation <A> ∝ f₀^α with roughly linear trend and the hysteresis loop area <A> decreases with increasing frequency f₀. Such electric field dependent dynamic hysteresis scaling can be observed by polarized light microscopy.     

Fabrication and properties of PLT/PLZT/PLT structures obtained by RF magnetron sputtering

Ferroelectric Pb_0.84La_0.16Ti_0.96O₃/Pb_0.96La_0.04(Zr_0.52Ti_0.48)_0.99O₃/Pb_0.84La_0.16Ti_0.96O₃ (PLT/PLZT/PLT) structures were fabricated on platinum-coated silicon wafers by RF magnetron sputtering. A Pb_0.84La_0.16Ti_0.96O₃ layer was used as a seed layer to improve the crystallization and enhance the ferroelectric properties of the PLZT film. With the PLT seed layers, the films showed excellent ferroelectric properties in terms of large remnant polarization (2P_r) of 52.7 μC/cm², lower coercive field (2E_c) of 130 kV/cm for an applied field of 500 kV/cm. Moreover, the PLT/PLZT/PLT structures exhibited good fatigue endurance after 10¹⁰ switching cycles, which was attributed to the double-sided PLT layers. They improved the electrical fatigue by eliminating the pyrochlore phase, reduced the strong (111) orientation, and assimilated the oxygen vacancies from the PLZT layer.     

Low-frequency relaxation processes in Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript> ferroelectric crystals

Measurements and analysis of the temperature and frequency dependences of permittivity and losses and of the electrical resistivity of Pb₅Ge₃O₁₁ ferroelectric crystals at temperatures of 100 to 600 K and frequencies of 0.1 to 100 kHz are reported. The dielectric characteristics of the crystals exhibit, in addition to clearly pronounced anomalies near the Curie point T_C=450 K, less distinct anomalous features of the relaxation character in the range 230–260 K. The data obtained on the effect of various factors (degree of crystal polarization, crystal annealing at different temperatures and in different environments, etc.) on the low-temperature anomalies serve as a basis for discussing the possible mechanisms responsible for these anomalies. It is concluded that the low-temperature dielectric anomalies originate from thermal carrier localization in defect levels in the band gap, which entail the formation of local polarized states.     

Variation of the soft-mode frequency in ferroelectric semiconductors in a static electric field

The action of a static electric field on the soft-mode frequency in narrow-gap ferroelectric semiconductors, as a result of the contribution of the electron subsystem, is investigated within the framework of the vibronic mechanism. It is shown that the frequency of the soft mode varies directly as the square of the electric field strength under the action of the field, and the soft-mode frequency is determined in the vibronic model by the electron distribution function, whose parameters also vary under the action of the electric field. The effect is stronger for crystals with a one-dimensional electron spectrum than for those with an isotropic three-dimensional spectrum; according to estimates for ferroelectric semiconductors of the type Pb_1–xGe_xTe, a frequency variation of the order of the soft-mode frequency itself is attained in fields of the order of tens of volts per centimeter.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 33–37, January, 1988.     

Polarization-optical study of the Barkhausen effect and domain structure of lead germanate

A method of localized polarization-optical measurement of dielectric hysteresis loop parameters on microscopical areas of ferroelectric samples combined with simultaneous observation of their domain structure is proposed. A sensitive two-beam differential polarization-optical hysteresisgraph is built for performing the experiments. Examples are given of the study of slow domain structure relaxation processes and dielectric hysteresis curves of lead germanate Pb₅Ge₃O₁₁.     

Specific features of dielectric anomalies in Pb<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te(Ga) near a ferroelectric phase transition

The dielectric properties of Pb_{1 ? x} Ge _x Te(Ga) (x = 0.02, 0.03, 0.05) were studied in the temperature range 77–150 K at frequencies of 10⁴–10⁶ Hz. It is revealed that the ferroelectric phase transition temperature T _c and the permittivity ? of Pb_{1 ? x} Ge _x Te(Ga) increase substantially with the Ge content. The temperature dependence of the permittivity of Pb_{1 ? x} Ge _x Te shows two peaks; the main peak is at the ferroelectric phase transition temperature T _c , and an additional peak is at T ₁ > T _c .