X. Q. Liu

Dielectric abnormities of complex perovskite Ba(Fe1∕2Nb1∕2)O3 ceramics over broad temperature and frequency range

Dielectric characteristics of Ba(Fe1∕2Nb1∕2)O3 ceramics were investigated over a broad temperature and a frequency range. Two dielectric relaxations following Arrhenius law were observed at 150–400 and 406–650K. An extremely high relaxorlike dielectric peak (202 270 at 5Hz, 91 930 at 1kHz, and 37 030 at 100kHz) with very strong frequency dispersion was observed at 406–650K, and it was not relaxor ferroelectric but an oxygen defect induced dielectric abnormity. The significant drop of dielectric constant at 146–400K was also frequency dependent, and it was nearly a Debye relaxation. Between these two dielectric relaxations, there was a frequency dependent dielectric constant plateau over a wide temperature interval.

Dielectric relaxations in Ba(Fe1∕2Ta1∕2)O3 giant dielectric constant ceramics

Dielectric relaxations of Ba(Fe1∕2Ta1∕2)O3 ceramics were investigated and discussed over a broad temperature and frequency range. Two dielectric relaxations following Arrhenius law were observed at 153–382 and 440–623K, where there was a giant dielectric constant step between them. The frequency dependent rapid drop of dielectric constant at 153–382K was nearly a Debye relaxation with the intrinsic nature, while the high temperature dielectric relaxation with an extremely high dielectric constant peak and very strong frequency dispersion was attributed to the defect ordering but not a typical relaxor ferroelectric behavior. The O2 annealing almost completely suppressed the dielectric peak and subsequently extended the giant dielectric step, while the low temperature dielectric relaxation and the magnitude of such step were not obviously affected.

Giant dielectric response and relaxor behaviors induced by charge and defect ordering in Sr(Fe1∕2Nb1∕2)O3 ceramics

Dielectric spectra of Sr(Fe1∕2Nb1∕2)O3 ceramics were characterized in wide temperature (123–623K) and frequency (100Hz–1MHz) ranges. Two dielectric relaxations with strong frequency dispersion and following the Arrhenius law were detected in low and high temperature ranges, respectively, and between them there was a giant dielectric constant step. The high temperature relaxorlike dielectric peak could be almost completely removed by annealing in O2, and it should be assigned to be a defect ordering induced relaxor behavior, while the low temperature dielectric relaxation was proposed to stem from the electronic ferroelectricity. Moreover, it was speculated that the giant dielectric constant step resulted from the competing balance between the low and high temperature dielectric relaxations.

Dielectric relaxations in Ca(Fe1∕2Nb1∕2)O3 complex perovskite ceramics

Dielectric characteristics of Ca(Fe1∕2Nb1∕2)O3 ceramics were investigated in wide temperature and frequency ranges. A very high dielectric constant peak with strong frequency dispersion following the Arrhenius law was observed in the temperature range of 473–673K, and this high temperature dielectric relaxation reflected the defect ordering induced relaxor ferroelectricity. A dielectric constant step associated with a low temperature dielectric relaxation was observed in the temperature range of 200–500K, but the dielectric constant step was very low (∼50) and could only be detected in the O2-annealed samples due to the overshadowing amplitude of the high temperature dielectric relaxation. The low temperature dielectric relaxation, following the Arrhenius law, indicated the electronic ferroelectricity due to the ordering among heterovalent iron ions.

Ferroelectric phase transition and low-temperature structure fluctuations in Ba4Nd2Ti4Nb6O30 tungsten bronze ceramics

Dielectric characteristics of Ba4Nd2Ti4Nb6O30 tungsten bronze ceramics have been investigated over a broad range of temperature and frequency. The real part of the dielectric constant (e′) shows a sharp peak at 417 K indicating the ferroelectric phase transition. Two dielectric relaxations are observed in the lower temperature range, which are very obvious in the dielectric loss (tan δ) and the imaginary part of the dielectric constant (e″). The differential scanning calorimetry (DSC), Raman scattering, and ultrasonic studies of the present ceramics have also been conducted in a broad temperature range covering the ferroelectric transition and/or the dielectric relaxation temperatures. The endothermal peak in the DSC curve around the ferroelectric transition temperature confirms the first-order structural transition with the transition enthalpy of 5.72 kJ/mol. Steplike anomalies in the wave number of the Raman vibrations and abnormal drop of the intensity of the Raman spectra are observed accompanying the...

Structure and dielectric relaxation of double-perovskite La2CuTiO6 ceramics

Dielectric properties of La2CuTiO6 ceramics were investigated in a broad frequency and temperature range. There is only one dielectric relaxation in the curve of temperature dependence of dielectric properties for La2CuTiO6 ceramics. This dielectric relaxation is a thermal activated process. The bulk and grain boundary resistances can be obtained from results of the least-mean-square fitting on impedance spectra. The conduction mechanism of the present ceramics is also a thermal activated process. The activation energy of dielectric relaxation is almost same as that of electrical conductivity and this indicates the closely correlation between the dielectric relaxation and electrical conductivity. The dielectric relaxation in the present ceramics should be attributed to the mixed-valent structure (Cu+/Cu2+ and Ti3+/Ti4+), which is induced from the oxygen vacancy.

Hybrid improper ferroelectricity in Ruddlesden-Popper Ca3(Ti,Mn)2O7 ceramics

The hybrid improper ferroelectricity (HIF) has been proposed as a promising way to create multiferroic materials with strong magnetoelectric coupling by the first-principle calculation, and the experimental evidences of HIF in Ruddlesden-Poper Ca3(Ti1−xMnx)2O7 (x = 0, 0.05, 0.1, and 0.15) ceramics have been shown in the present work. The room temperature ferroelectric hysteresis loops are observed in these ceramics, and a polar orthorhombic structure with two oxygen tilting modes has been confirmed by the X-ray powder diffraction. A first-order phase transition around 1100 K in Ca3Ti2O7 was evidenced, and the temperatures of phase transitions decrease linearly with increasing of the contents of Mn4+ ions. Based on the result of first-principle calculations, the polarization should be reversed by switching through the mediated Amam phase in Ca3Ti2O7 ceramics.

Room temperature multiferroic Ba4Bi2Fe2Nb8O30: Structural, dielectric, and magnetic properties

A room temperature multiferroic compound Ba4Bi2Fe2Nb8O30 was synthesized using a solid state reaction technique. Rietveld analysis of x-ray diffraction data shows that Ba4Bi2Fe2Nb8O30 has a tetragonal (space group P4bm) tungsten bronze structure. In this structure, the Fe3+ and Nb5+ statistically occupy the octahedral center while the Ba2+ ions and the Bi3+ ions occupy the pentagonal channels and the square channels, respectively. Diffuse dielectric peaks with strong frequency dispersion in the temperature range from 150 to 300 K can be attributed to the random distribution of Fe3+ and Nb5+ at B sites. Magnetic hysteresis loop at room temperature is also obtained, which suggests that Ba4Bi2Fe2Nb8O30 is a room temperature multiferroic compound. The coupling between the relaxor behavior and magnetic ordering is verified by observing an increase of magnetization near the maximum dielectric constant temperature.

Raman spectra analysis for Ca(B1/3′B2/3″)O3-based complex perovskite ceramics

Ca[(Mg1/3Ta2/3)1−xTix]O3 (hereafter denoted as CMTT) and (Ca1−0.39xNd0.26x) [(Mg1/3Nb2/3)1−xTix]O3 (hereafter denoted as CNMNT) complex perovskite ceramics were studied by Raman spectroscopy combined with Rietveld analysis. The crystal structures changed from monoclinic structure with P21/c space group to orthorhombic structure with Pbnm space group for both systems. The main structure of the two systems was orthorhombic and the characteristic modes for the orthorhombic structure with space group Pbnm could be found in all the compositions. Raman modes at around 408 cm−1 could only be found for compositions with 1:2 long range cation ordering while modes at around 800 cm−1 could be found for all the compositions in both systems. They were concluded to be correlated with the 1:2 long range cation ordering and different short range cation ordering, respectively. Two Ag modes at around 800 cm−1 represented for 1:2 and 1:1 short range cation ordering were observed for CMTT ceramics with x=0.1–0.6 while only 1...

Ba[(Fe0.9Al0.1)0.5Ta0.5]O3 ceramics with extended giant dielectric constant step and reduced dielectric loss

Structure and dielectric characteristics of Ba[(Fe0.9Al0.1)0.5Ta0.5]O3 solid solution were investigated. The cubic crystal structure in space group Pm3¯m(221) was confirmed in the Al-substituted Ba(Fe0.5Ta0.5)O3 ceramics. The extended giant dielectric constant step and the significantly reduced room-temperature dielectric loss were obtained in the Al-substituted Ba(Fe0.5Ta0.5)O3 ceramics. These improvements of dielectric characteristics have great scientific significance for potential application of giant dielectric constant materials. XPS analysis results confirmed that the low-temperature dielectric relaxation originated from the mixed-valent structure of Fe2+/Fe3+ and the hopping of the charge carriers between them, and the improvements of dielectric characteristics in the Al-substituted Ba(Fe0.5Ta0.5)O3 ceramics were attributed to the modification of such mixed-valent structure of Fe2+/Fe3+.