Origin of ferroelectricity in cubic phase of Hf substituted BaTiO₃

Abstract

The origin of ferroelectricity in the cubic phase of BaTi1−x Hf x O3 has been investigated. The presence of well-defined ferroelectric polarization versus electric field (PE) hysteresis loop in the samples with global cubic symmetry suggests the presence of ‘local polar regions’, induced possibly due to the huge difference in the electronegativity and also difference in the ionic radii of Hf+4 and Ti+4 ions, which may lead to local structural disorder. The presence of polar regions is also supported through the appearance of A1(TO) polar mode in Raman spectra which in principle should be absent in the samples with cubic symmetry. The results are discussed in terms of disorder-induced local dipoles due to the electronegativity difference between Hf and Ti ions.

Exploring the Interrelation between Urbach Energy and Dielectric Constant in Hf-Substituted BaTiO3

“…Two possible inferences could be responsible for a decrease in the dielectric constant: As θ­(Ti–O–Ti)→180°, the dipole moments corresponding to in-plane Ti–O bonds tend to cancel each other, and therefore, the dipole moment reduces with Hf substitution. The non-centrosymmetric TiO 6 octahedra, in which the Ti 4+ ion is displaced relative to its central position along the c -axis, possess a certain dipole moment . However, in BTHO samples, Hf replaces Ti-sites randomly, thereby increasing the effective distance between two successive TiO 6 octahedra, which weakens the dipole–dipole interaction corresponding to neighboring TiO 6 octahedra, leading to comparatively reduced polarization. It should be noted that HfO 6 octahedra may not necessarily occupy all the alternating positions with respect to the TiO 6 octahedra, but their (HfO 6 ) random occurrence within the lattice can affect the net dielectric behavior by weakening the local dipolar interactions which collectively may result as a decrease in the dielectric constant with a higher Hf content. …”

“…In addition, BTO exhibits crystal symmetries of four polymorphs in different temperature regions, namely, rhombohedral, orthorhombic, tetragonal, and cubic phases, from lower to higher temperatures (in a temperature range of ∼100 to 400 K). Interestingly, all these phases are ferroelectric, irrespective of the cubic phase . Moreover, the properties possessed by BTO can be tuned by substituting at A- or B-sites, ,− which makes it an eligible candidate for exploring the correlation between E U and ε r , because being a dielectric material, BTO can be expected to exhibit significant variation in ε r as a result of doping along with introduction of considerable disorder in the host systemaltering E U .…”

“…Earlier efforts have been successfully made by researchers to understand the behavior of Urbach energy (∼electronic disorder) through suitable modelling and also to correlate it with optical band gap. − Moreover, attempts in the direction to realize the interrelation of the dielectric constant (ε r ) with the electrical, magnetic, and absorption-related properties for various materials have also been contributed by various research groups. − In this, BTO has been a popular material for study of dielectric behavior of solids. ,,, However, reports that study the correlation of Urbach energy ( E U ) with ε r are limited. In the present work, barium titanate (BTO) has been subjected to Hf incorporation at Ti-sitesBaTi 1– x Hf x O 3 (BTHO), for which optical and dielectric measurements have been performed at various compositions.…”

Role of Laser Excitation Wavelength and Power in the Fano Resonance Scattering in RFe0.50Cr0.50O3 (R = Sm, Er, and Eu): A Brief Raman Study

“…Polycrystalline RFe 0.50 Cr 0.50 O 3 samples have been synthesized by the wet chemical method, and details about the sample synthesis are reported elsewhere. ,,, The phase purity of the prepared samples has been ensured by the X-ray diffraction experiments with a Bruker D8 diffractometer equipped with a Cu target (Cu Kα = 1.54 Å), in the angular range of 2θ = 20–80° (see the Supporting Information). The vibrational properties of the synthesized samples have been investigated using a Horiba Raman microspectrometer equipped with excitation laser sources of wavelengths 633 and 785 nm and grating having 1800 grooves/mm accompanied by a charge-coupled device detector in backscattered mode. − A 50× magnification glass-compensating objective was used, and the focused laser beam spot size on the sample surface was ∼1.5 μm for the 785 nm laser, whereas ∼1.9 μm for the 633 nm laser; also, the resolution was ∼0.35 cm –1 , and the laser power was varied in the range of ∼0.4 to 8 mW. The acquisition time for data collection was set to 50 s with accumulation set to 2 during the Raman measurements.…”

Investigations on the Electronic Structure of the Strongly Correlated Electron System Cr-Doped PrFeO3

“…The optical properties of the prepared series of samples have been investigated using an Agilent Carry 60 UV–vis spectrophotometer, − and the variation of the optical band gap as a function of Cr content is tabulated in Table . The vibrational properties of the system have been investigated using a LABRAM HR dispersive spectrometer equipped with a 633 nm excitation laser source having a CCD detector in the backscattered mode. ,− …”