Bi 9 Fe 5 Ti 3 O 27 is an eight-layered material belonging to the family of bismuth layered structured ferroelectromagnets. The polycrystalline sample of this compound was prepared by a standard solid-state reaction technique. The formation of the compound in an orthorhombic crystal structure was confirmed by an X-ray diffraction (XRD) technique (lattice parameters: a=5.5045. Detailed studies of surface morphology of the compound using scanning electron microscopy (SEM) exhibit that the compound has domains of plate shaped grains. Studies of dielectric and electric properties in a wide temperature range (30-500°C) at different frequencies (100 Hz-1 MHz) exhibit an anomaly at 291±2°C, which is related to ferroelectric to paraelectric phase transition as suggested by hysteresis loop at room temperature. The values and nature of temperature variation of dc conductivity exhibit the NTCR behavior of the compound.

Polycrystalline sample of Ba 3 Sr 2 DyTi 3 V 7 O 30 was prepared at 950°C using a high-temperature solid-state reaction technique. X-ray structural analysis indicated the formation of a single-phase orthorhombic structure with lattice parameters: a = 12⋅2719 (39) Å, b = 8⋅9715(39) Å and c = 19⋅7812(39) Å. Microstructural study showed densely packed uniform distribution of grains over the surface of the sample. The a.c. impedance plots were used as tools to analyse the electrical response of the sample as a function of frequency at different temperatures (30-500°C). These plots revealed the presence of grain boundary effect, from 200°C onwards. Complex impedance analysis showed non-Debye type of dielectric relaxation. The Nyquist plots showed the negative temperature coefficient of resistance character of Ba 3 Sr 2 DyTi 3 V 7 O 30 . A hopping mechanism of electrical transport processes in the system is evident from the modulus analysis. The activation energy of the compound (calculated both from loss and modulus spectrum) is the same, and hence the relaxation process may be attributed to the same type of charge carrier.

Abstract:Polycrystalline samples of Ba 4 SrRTi 3 V 7 O 30 (R=Sm and Dy), members of the tungsten-bronze family, were prepared using a high-temperature, solid-state reaction technique and studied their electrical properties (using complex impedance spectroscopy) in a wide range of temperature (31-500 • C) and frequency (1 kHz -1 MHz). Preliminary structural (XRD) analyses of these compounds show the formation of single-phase, orthorhombic structures at room temperature. The scanning electron micrographs (SEM) provided information on the quality of the samples and uniform distribution of grains over the entire surface of the samples. Detailed studies of the dielectric properties suggest that they have undergone ferroelectric-paraelectric phase transition well above the room temperatures (i.e., 432 and 355 • C for R= Sm and Dy, respectively, at frequency 100 kHz). Measurements of electrical conductivity (ac and dc) as a function of temperature suggest that the compounds have semiconducting properties much above the room temperature, with negative temperature coefficient of resistance (NTCR) behavior. The existence of ferroelectricity in these compounds was confirmed from a polarization study.

The polycrystalline sample of Ba 2 Sr 3 SmTi 3 V 7 O 30, a member of the tungsten bronze structural family, was prepared by a high-temperature solid-state reaction technique. Preliminary X-ray diffraction analysis suggests the formation of a single-phase compound with orthorhombic structure. Detailed studies of the dielectric constant and tangent loss as a function of frequency (100 Hz to 1 MHz) and temperature (32°–500°C) show that this compound has a diffused-type of ferroelectric phase transition at 230°C. Study of the surface morphology by SEM showed uniform grain distribution on the surface of the sample with less porosity. The activation energy, calculated from the plot of temperature dependence of AC conductivity, of the compound was found to be 0.11 eV and 0.14 eV at 500 kHz and 1 MHz respectively. The nature of the variation of conductivity and value of activation energy suggest that the conduction process is of a mixed-type.

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