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Multiferroic materials capable of robust magnetoelectric coupling at room temperature are currently being explored for their possible multifunctional device applications. Highly (100)-oriented Pb(Fe0.5Ta0.5)x(Zr0.53Ti0.47)1−x (PZTFTx) thin films (x = 0.2 and 0.3) with a thickness of about 300 nm were grown on La0.67Sr0.33CoO3 (LSCO)-buffered MgO 100-oriented substrates via the pulsed laser deposition method. An analysis of their X-ray diffraction patterns suggests the stabilization of the orthorhombic phase in the thin films at room temperature. Dielectric spectroscopic measurements of the metal–insulator–metal (Pt/PZTFTx/LSCO) thin-film capacitors as a function of temperature revealed a diffuse ferroelectric-to-paraelectric phase transition around Tm ~520 and 560 K for the x = 0.2 and 0.3 thin films, respectively. Well-saturated electrical hysteresis loops with large remanent (Pr) and saturation (Ps) polarizations were observed in these capacitors, which indicates the establishment of intrinsic ferroelectric ordering in the thin films at room temperature. These thin films retained ferromagnetic/ferrimagnetic ordering up to 300 K and showed saturation magnetization values of 8.3 (x = 0.2) and 6.1 (x = 0.3) emu/cm3 at room temperature. The magnetoelectric coupling constants of 2040 mV/cmOe (x = 0.2) and 850 mV/cmOe (x = 0.3), respectively, were obtained at an in-plane bias field at room temperature. The present study demonstrates that PZTFTx thin films are multiferroic at room temperature with large magnetoelectric couplings, and these materials may be suitable for use in magnetic sensors and spintronic device applications. [ABSTRACT FROM AUTHOR]

Structure, phonon, and energy storage density in Sr2+-substituted lead-free ferroelectric Ba1−xSrxTiO3 (BSTx) for compositions x = 0.1, 0.3, and 0.7 were investigated using X-ray diffraction, Raman, and ferroelectric polarization measurements as a function of temperature. The samples were tetragonal for x = 0.1 with a large c/a ratio. The tetragonal anisotropy was decreased upon increasing x and transforming to cubic for x = 0.7. The changes in structural and ferroelectric properties were found to be related to the c/a ratios. The temperature-dependent phonon spectroscopy results indicated a decrease in tetragonal–cubic phase transition temperature, Tc, upon increasing x due to a reduction in the lattice anisotropy. The intensity of ~303 cm−1 E(TO2) mode decreased gradually with temperature and finally disappeared around the tetragonal ferroelectric to cubic paraelectric phase at about 100 ℃ and 40 ℃ for x = 0.1 and 0.3, respectively. A gradual reduction in the band gap Eg of BSTx with x was evident from the analysis of UV-visible absorption spectra. The energy storage density (Udis) of the ferroelectric capacitors for x = 0.7 was ~0.20 J/cm3 with an energy storage efficiency of ~88% at an applied electric field of 104.6 kV/cm. Nearly room temperature transition temperatures TC and reasonably fair energy storage density of the BSTx capacitors were found. [ABSTRACT FROM AUTHOR]

Magnetoelectric (ME) materials with ferroelectric and magnetic transition temperatures near room temperature are of current interest in research due to their technological importance and potential applications. Herein, we report the coupled magnetic ordering and phonons in gallium ferrite (GaFeO3), a near room temperature ME perovskite, using magnetization and inelastic light scattering measurements as a function of temperature. Temperature-dependent magnetization behavior studied in a wide temperature range of 5–395 K at both field-cooled (FC) and zero-field-cooled (ZFC) conditions using different static magnetic fields confirms the magnetic transition temperature Tc at 220 K. On temperature dependent Raman spectroscopy, no significant changes in Raman spectra were observed at an elevated temperature that can be attributed to the absence of structural phase transition. Instead of expected anharmonic behavior, several librational (rigid rotation) and stretching modes of rigid BO6 units were found to be hardening below Tc, suggesting the significant magnetoelastic coupling contributions to phonon frequencies. The asymmetric stretching mode at 753 cm−1 is found to have larger spin-phonon coupling contribution (λ∼2.93), while the lattice mode at 153 cm−1 and external librational mode at 240 cm−1 had the lowest effect (λ∼0.88). [ABSTRACT FROM AUTHOR]

Vanadium sesquioxide V2O3, a transition metal oxide, is an important metal transition insulator due to its potential applications in novel electronic and memory devices. V2O3 thin films of thickness around 230 nm were grown on Si/SiO2/Ti/Pt substrates at deposition temperature of 723 K in a controlled Ar:O2 atmosphere of 35:2.5 sccm employing Direct Current (DC) magnetron sputtering. X-ray diffraction studies confirmed single phase of the material stabilized in corundum rhombohedral R 3 ¯ C phase. X-ray photoelectron spectroscopic results revealed chemical oxidation states are of V3+ and O2− and have nearly stochiometric elemental compositions in the films. Magnetization studies down to 10 K predicts a canted antiferromagnetic transition around 55 K. Out of 7 expected Raman active modes (2A1g + 5Eg), two A1g Raman active modes at 242 and 500 cm−1 were observed at ambient R 3 ¯ C phase. Temperature dependent Raman spectroscopic studies carried out from 80 to 300 K identified a monoclinic to rhombohedral phase transition at ~143 K. [ABSTRACT FROM AUTHOR]

Herein we report the evolution of the crystal structure of K3Gd5(PO4)6 in the temperature range from 20 K to 1073 K, as observed from variable temperature X-ray diffraction and Raman spectroscopic studies. K3Gd5(PO4)6 has an open tunnel containing a three dimensional structure built by [Gd5(PO4)6]3− ions which in turn are formed of PO4 tetrahedra and GdOn (n = 8 and 9) polyhedra. The empty tunnels in the structure are occupied by K+ ions and maintain charge neutrality in the lattice. Evolution of unit cell parameters with temperature shows a systematic increase with temperature. The average axial thermal expansion coefficients between 20 K and 1073 K are: αa = 10.6 × 10−6 K−1, αb = 5.5 × 10−6 K−1 and αc = 16.4 × 10−6 K−1. The evolution of distortion indices of the various coordination polyhedra with temperature indicates a gradual decrease with increasing temperature, while those of Gd2O9 and K2O8 polyhedra show opposite trends. The overall anisotropy of the lattice thermal expansion is found to be controlled largely by the effect of temperature on GdOn polyhedra and their linkages. Temperature dependent Raman spectroscopic studies indicated that the intensities and wavenumbers of most of the Raman modes decrease continuously with increasing temperature. Anharmonic analyses of Raman modes indicated that the lattice, rigid translation and librational modes have larger contributions towards thermal expansion of K3Gd5(PO4)6 compared to high frequency internal modes. The temperature and field dependent magnetic measurements indicated no long range ordering down to 2 K and the observed effective magnetic moment per Gd3+ ion and the Weiss constant are 7.91 μB and 0.38 K, respectively. [ABSTRACT FROM AUTHOR]

A series of lead-free perovskite solid solutions of (1 − x) Na0.5 Bi0.5 TiO3( NBT)- x BaSnO3( BSN), for 0.0 ≤ x ≤ 0.15 have been synthesized using a high-temperature solid-state reaction route. The phase transition behaviors are studied using dielectric and Raman spectroscopic techniques. The ferroelectric to relaxor phase transition temperature ( TFR) and the temperature corresponding to maximum dielectric permittivity ( Tm) are estimated from the temperature-dependent dielectric data. Dielectric studies show diffuse phase transition around ~335°C in pure NBT and this transition temperature decreases with increase in x. The disappearance of x-dependence of A1 mode frequency at ~134 cm−1 for x ≥ 0.1 is consistent with rhombohedral-orthorhombic transition. In situ temperature dependence Raman spectroscopic studies show disappearance and discontinuous changes in the phonon mode frequencies across rhombohedral ( x < 0.1)/orthorhombic ( x ≥ 0.1) to tetragonal transition. [ABSTRACT FROM AUTHOR]

The compound 0.9[KNbO3]-0.1[(BaNi1/2Nb1/2O3−δ] (KBNNO), a robust eco-friendly (lead-free) ferroelectric perovskite, has diverse applications in electronic and photonic devices. In this work, we report the dielectric, ferroelectric, and structural phase transitions behavior in the KBNNO compound using dielectric, X-ray diffraction, and Raman studies at ambient and as a function of temperature. Analyses of X-ray diffraction (XRD) data at room temperature (rtp) revealed the orthorhombic phase (sp. Gr. Amm2) of the compound with a minor secondary NiO cubic phase (sp. Gr. Fm3m). A direct optical band gap Eg of 1.66 eV was estimated at rtp from the UV–Vis reflectance spectrum analysis. Observation of non-saturated electric polarization loops were attributed to leakage current effects pertaining to oxygen vacancies in the compound. Magnetization studies showed ferromagnetism at room temperature (300 K) in this material. XRD studies on KBNNO at elevated temperatures revealed orthorhombic-to-tetragonal and tetragonal-to-cubic phase transitions at 523 and 713 K, respectively. Temperature-dependent dielectric response, being leaky, did not reveal any phase transition. Electrical conductivity data as a function of temperature obeyed Jonscher power law and satisfied the correlated barrier-hopping model, indicating dominance of the hopping conduction mechanism. Temperature-dependent Raman spectroscopic studies over a wide range of temperature (82–673 K) inferred the rhombohedral-to-orthorhombic and orthorhombic-to-tetragonal phase transitions at ~260, and 533 K, respectively. Several Raman bands were found to disappear, while a few Raman modes such as at 225, 270, 289, and 831 cm−1 exhibited discontinuity across the phase transitions at ~260 and 533 K. [ABSTRACT FROM AUTHOR]

Herein we report the high-temperature crystal chemistry of K 2 Ce(PO 4 ) 2 as observed from a joint in situ variable-temperature X-ray diffraction (XRD) and Raman spectroscopy as well as ab initio density functional theory (DFT) calculations. These studies revealed that the ambient-temperature monoclinic (P2 1 /n) phase reversibly transforms to a tetragonal (I4 1 /amd) structure at higher temperature. Also, from the experimental and theoretical calculations, a possible existence of an orthorhombic (Imma) structure with almost zero orthorhombicity is predicted which is closely related to tetragonal K 2 Ce(PO 4 ) 2 . The high-temperature tetragonal phase reverts back to ambient monoclinic phase at much lower temperature in the cooling cycle compared to that observed at the heating cycle. XRD studies revealed the transition is accompanied by volume expansion of about 14.4%. The lower packing density of the high-temperature phase is reflected in its significantly lower thermal expansion coefficient (α V = 3.83 × 10 -6 K -1 ) compared to that in ambient monoclinic phase (α V = 41.30 × 10 -6 K -1 ). The coexistences of low- and high-temperature phases, large volume discontinuity in transition, and large hysteresis of transition temperature in heating and cooling cycles, as well as drastically different structural arrangement are in accordance with the first-order reconstructive nature of the transition. Temperature-dependent Raman spectra indicate significant changes around 783 K attributable to the phase transition. In situ low-temperature XRD, neutron diffraction, and Raman spectroscopic studies revealed no structural transition below ambient temperature. Raman mode frequencies, temperature coefficients, and reduced temperature coefficients for both monoclinic and tetragonal phases of K 2 Ce(PO 4 ) 2 have been obtained. Several lattice and external modes of rigid PO 4 units are found to be strongly anharmonic. The observed phase transition and structures as well as vibrational properties of both ambient- and high-temperature phases were complimented by DFT calculations. The optical absorption studies on monoclinic phase indicated a band gap of about 2.46 eV. The electronic structure calculations on ambient-temperature monoclinic and high-temperature phases were also carried out.