Pb decay and M4 transition probabilities in neutron-deficient lead isotopes

The decay of 6.1 sec ^203mPb has been examined with a high-resolution Ge(Li) spectrometer, and a 5.1 keV M2 transition was found to compete with the 825.2 keV M4 isometric transition. The M4 transition probability, thus corrected, is now found to be consistent with those of the other M4 transitions in the lead region.     

Diffuse ferroelectric phase transition in Li2Pb2Dy2W2Ti4V4O30

Tungsten–bronze polycrystalline ceramic Li₂Pb₂Dy₂W₂Ti₄V₄O₃₀ was prepared by mixed-oxide method. The preparation conditions were optimized by thermogravimetry analysis and repeated firing. Room-temperature structural analysis confirms the formation of the compound in single phase (orthorhombic). Studies of dielectric parameters () of the compound with temperature at different frequencies suggest the existence of phase transition in the compound. The existence of ferroelectric property (based on dielectric anomaly) of the material was confirmed by polarization study.     

The CoO/Co3O4 phase transition studied by PAC

The hyperfine environment of¹¹¹Cd nuclei formed in the radioactive decay of¹¹¹In ions implanted into CoO was studied via the PAC method. The quadrupole coupling constants measured in isochronal and isothermal annealing treatments at various oxygen pressures were related to the initial stages of the CoO→Co₃O₄ phase transition and interpreted as the successive trapping of cation vacancies at the¹¹¹In probe. The phase transition was confirmed by X-ray diffraction, RBS and a PAC measurement on¹¹¹Cd in spinel type Co₃O₄.     

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.     

Structural phase transition due to La substitution in Bi4Ti3O12

The structure of Bi_4?x La _x Ti₃O₁₂ (BLaTx) as a function of La content from x = 0.00 to 1.25 was studied by Raman spectroscopy and X-ray diffraction (XRD). It was observed that the Raman modes evolve discontinuously at about x = 1.00, which indicates a structural phase transition. Specifically, the B _2g and B _3g Raman modes shown evidences to coalesce into E _g modes with increasing La content. The evolution of Raman modes corresponds to the weakening of ferroelectric orthorhombic lattice distortion with increasing La substitution when x ≤ 1.00 and the final transition into the paraelectric tetragonal structure at higher La content. XRD and electrical measurement results confirm this phase transition by the observation that diffraction peaks from (200) and (020) planes of BLaT0.75 coalesce into one peak in BLaT1.25 and by the corresponding disappearance of remanent polarisation. The impact of La substitution on electrical properties of BLaTx is discussed briefly.     

Configurational entropy-induced phase transition in spinel LiMn2O4

The spinel-type LiMn$_{2}$O$_{4}$ is a promising candidate as cathode material for rechargeable Li-ion batteries due to its good thermal stability and safety. Experimentally, it is observed that in this compound there occur the structural phase transitions from cubic ($Fd\bar{3}m)$ to tetragonal ($I4_{1}/{amd}$) phase at slightly below room temperature. To understand the phase transition mechanism, we compare the Gibbs free energy between cubic phase and tetragonal phase by including the configurational entropy. Our results show that the configurational entropy contributes substantially to the stability of the cubic phase at room temperature due to the disordered Mn$^{3+}$/Mn$^{4+}$ distribution as well as the orientation of the Jahn-Teller elongation of the Mn$^{3+}$O$_{6}$ octahedron in the the spinel phase. Meanwhile, the phase transition temperature is predicted to be 267.8 K, which is comparable to the experimentally observed temperature. These results serve as a good complement to the experimental study, and are beneficial to the improving of the electrochemical performance of LiMn$_{2}$O$_{4}$ cathode.     

First order magnetic phase transition in GeCo2O4

The antiferromagnetic compound GeCo₂O₄ exhibits a magnetic phase transition characterized by thermal hysteresis of the susceptibility versus temperature curve and by a diffuse neutron scattering with a small correlation length. The data are compared to the expected first-order phase transition of the n ≥ 4 component vector models.     

Photodielectric effect in polycrystalline layers of Pb3O4

The results of the investigation into the influence of light excitation on the dielectric characteristics of polycrystalline layers of lead orthoplumbate Pb₃O₄ are reported. The temperature dependences of the photoelectric response of the studied samples in the frequency range 10²–10⁵ Hz are presented. The parameters and activation energy of dielectric relaxation are determined. The manifestation of the photodielectric effect is interpreted in terms of the formation of a space charge in the bulk of the semiconductor. The role of electrons of the lone pair belonging to the Pb²⁺ cations in the processes under investigation is discussed.     

High-pressure phase transition of Bi2Fe4O9

The high-pressure behaviour of Bi2Fe4O9 was analysed by in situ powder and single-crystal x-ray diffraction and Raman spectroscopy. Pressures up to 34.3(8) GPa were generated using the diamond anvil cell technique. A reversible phase transition is observed at approximately 6.89(6) GPa and the high-pressure structure is stable up to 26.3(1) GPa. At higher pressures the onset of amorphization is observed. The crystal structures were refined from single-crystal data at ambient pressure and pressures of 4.49(2), 6.46(2), 7.26(2) and 9.4(1) GPa. The high-pressure structure is isotypic to the high-pressure structure of Bi2Ga4O9. The lower phase transition pressure of Bi2Fe4O9 with respect to that of Bi2Ga4O9 (16 GPa) confirms the previously proposed strong influence of cation substitution on the high-pressure stability and the misfit of Ga3+ and Fe3+ in tetrahedral coordination at high pressure. A fit of a second-order Birch–Murnaghan equation of state to the p–V data results in K0 = 74(3) GPa for the low-pressure phase and K0 = 79(2) GPa for the high-pressure phase. The mode Grüneisen parameters were obtained from Raman-spectroscopic measurements.     

Mechanisms of EPR line broadening in Pb5Ge3O11 near a structural transition

The polar angular dependence of the anomalous linewidth of three EPR transitions in the Gd³⁺ trigonal center has been used to determine the broadening mechanism. It is shown that the dominant mechanism of Gd³⁺ EPR signal broadening in the vicinity of the ferroelectric transition is the critically growing spread of the b ₂₁ and c ₂₁ parameters accounting for the odd-order fields of remote charge-compensating defects. Fiz. Tverd. Tela (St. Petersburg) 40, 321–326 (February 1998)     

First-order magnetic phase transition in layered Sr3YCo4O10.5 + δ-type cobaltites

JETP Letters - In layered Sr3YCo4O10.5 + δ-type cobaltites with different oxygen contents, we have observed a first order magnetic phase transition from the high-temperature...     

Long-term relaxation of the current in a naturally disordered Pb3O4 semiconductor

Charge transfer has been studied in metal-dielectric-metal structures based on polycrystalline layers of lead orthoplumbate Pb₃O₄ with a binder—cyanoethyl ester of polyvinyl alcohol, deposited on a glass substrate with a conducting ITO film. Time dependences of the current have been investigated in the temperature range T = 300–370 K and in the range of dc electric field strengths E = 2 × 10⁵?9 × 10⁵ V/m. Flowing of the relaxation polarization current leads to charge accumulation in the near-electrode region. Experimental dependences agree with the relay race mechanism of charge transfer with the participation of deep local levels. Microparameters of charge transfer are determined upon varying the experimental conditions.     

Electric-field-induced phase transition in single-crystal lead zinc niobate

The behavior of lead zinc niobate, a relaxor ferroelectric, in electric fields has been studied by dielectric, optical transmittance, and small-angle light scattering techniques. A field-induced transition from an averaged cubic to macrodomain ferroelectric phase has been found. A comparison is made with the properties of the phase induced in lead magnoniobate, a classical relaxor. Fiz. Tverd. Tela (St. Petersburg) 40, 527–530 (March 1998)     

Raman scattering of high-temperature phase transition in KH2PO4

Raman scattering was applied to study the high-temperature phase transition (near 175°C) in KH₂PO₄. Drastic temperature-dependent changes were observed to take place in the normal modes of B₁ symmetry between 1000–3400 cm^?1. The disintegration of the dominant broad feature near 2500 cm^?1 when temperature rises beyond 150°C suggests that the alteration of the hydrogen-bond network is closely connected with this high-temperature phase transition.     

Vortex-to-polarization phase transformation path in ferroelectric Pb(ZrTi)O3 nanoparticles

Phase transformation in finite-size ferroelectrics is of fundamental relevance for understanding collective behaviors and balance of competing interactions in low-dimensional systems. We report a first-principles effective Hamiltonian study of vortex-to-polarization transformation in Pb(Zr0.5Ti0.5)O3 nanoparticles, caused by homogeneous electric fields normal to the vortex plane. The transformation is shown to (1) follow an unusual macroscopic path that is symmetry nonconforming and characterized by the occurrence of a previously unknown structure as the bridging phase, and (2) lead to the discovery of a striking collective phenomenon, revealing how ferroelectric vortex is annihilated microscopically. Interactions underlying these behaviors are discussed.