Temperature Rhombohedral Phase Research Articles

Role of V–V dimerization on electronic and magnetic properties of ilmenite-type CoVO3

Structural, electronic and magnetic properties of ilmenite-type CoVO3 have been explored via the generalized gradient approximation + effective Hubbard U eff correction, in the framework of density functional theory. Our results indicate that high temperature rhombohedral phase is metallic with oxidation states and electronic configurations Co2+ (), V4+ (), respectively, while low temperature triclinic phase, induced from spin-Peierls transition in the V–V dimerization manner, is insulating, maintaining charge and electronic states unchanged. Furthermore, the A-type antiferromagnetic ordering, where the ferromagnetic honeycomb layers are anti-aligned along the stacking axis, is identified to be the magnetic ground state for the low temperature phase, in nice agreement with experimental findings, analogous to CoTiO3. The unexpected intralayer ferromagnetic couplings can be attributed to the intraorbital t2g −t2g exchange coupling, which was assumed to be small earlier and ignored, but actually large in honeycomb cobaltates with 3d 7 electronic configuration. In addition, the computed magnetic moment on Co2+ ion ranges from 2.5 to 2.7 μ B, Hubbard U eff dependent, close to ideal S = 3/2 state, rather than the anticipated J eff = 1/2 state. Furthermore, the supplemental calculations, taking spin-orbit coupling (SOC) into account, uncover faint orbital moments of 0.21–0.27 μ B at the Co site, illustrating the insignificance of SOC. Except for the inevitable trigonal distortions, the excessive structural distortion triggered by the formation V–V dimerization, i.e. the breaking of trigonal symmetry around Co2+, further lifts the degeneracy of t 2g orbitals and increases crystal field splitting, driving it away from potential candidates for realizing Kitaev model physics.

Probing of Bi doped GeTe thermoelectrics leads to revelation of resonant states

Quest for lead free high performance mid temperature thermoelectric materials has led to extensive research on SnTe and GeTe based materials. Among various strategies implemented to improve the transport properties of GeTe, electronic structure engineering holds a prominent place. Herein, we reinvestigate the impact of substitutional doping of Bi in GeTe on its electronic structure in both rhombohedral and cubic phase. We uncover that Bi introduces resonance levels in GeTe acting as a first n-type resonant dopant in both rhombohedral and cubic phases. We also report for the first time that Bi resonance states undergo Rashba splitting in low temperature rhombohedral phase. Bi doping acts in a multifunctional way by tuning the carrier concentration, causing conduction band convergence and phonon scattering in addition to improving the band effective mass by distorting the density of states near the Fermi level to enhance the thermoelectric performance of the material. The convergence of L, Z and Σ valence bands in rhombohedral phase due to Bi doping leads to manifestation of Rashba effect in p-type material which was not reported till date. Thus, we propose that the chemical potential tuning can result in p-type as well as n-type Bi doped GeTe for thermoelectric application.

Effect of low temperature structural phase transitions in BaTiO3 on electrical transport through a metal–ferroelectric–metal multilayer of AuCr/BaTiO3/Nb:SrTiO3

In this paper we report an investigation of electronic transport through the metal–ferroelectric–metal (MFM) multilayer consisting of AuCr/BaTiO3/Nb:SrTiO3 over a temperature range of 100 K–300 K where BaTiO3 (BTO) shows a series of structural phase transitions leading to change of magnitude as well as the orientation of the polarization . We observed that the bias dependent barrier heights associated with the interfaces carry strong signature of the phase transitions in the BTO layer which lead to a strong temperature dependent asymmetric transport, when cooled down below room temperature. Specifically, it is observed that the temperature dependence is closely correlated to low temperature transitions in the BTO layer as revealed through the temperature dependent x-ray diffraction (XRD), capacitance as well as resistivity behavior of the BTO layer. There is substantial enhancement of the asymmetry in the device current that occurs at or close to temperatures T 2 ∼ 190 K where BTO shows a crystallographic phase change to the low temperature rhombohedral phase. The temperature dependent changes occur due to barrier modulation at the interfaces of AuCr/BaTiO3 as well as BaTiO3/Nb:SrTiO3 that softens on cooling due to inhomogenities present there. The change in barrier on change of the bias direction has been observed below T 2 which arises from alignment of the polarization in-plane or out-of-plane as determined by tensile or compressive character of the in-plane strain in the BTO film. We also discuss the effect of space charge determined by the oxygen vacancies in the interface region, regulated by the applied bias.

A tunable elastic metamaterial beam with flat-curved shape memory alloy resonators

When realizing an elastic metamaterial beam, beam-like resonators are regarded as the simplest forms of distributed resonators and their influences on the 1st flexural wave bandgap have been well studied. In this letter, to study the relation between the curvature of the beam-like resonators and the high-order bandgaps (specifically, the 2nd and 3rd bandgaps) and make the bandgaps tunable, we propose a metamaterial beam with a periodic array of two-way shape memory alloys (SMAs). The considered aging-treated Ni-rich Ti-Ni SMAs remember a curved shape with a central angle of 180° at high temperature (austenite phase, A phase) and a flat shape at low temperature (rhombohedral phase, R phase) without the bias springs required for conventional one-way SMAs. Our analyses show that, while reducing the performance of the 1st bandgap, the central frequency of the 2nd (3rd) bandgap of the metamaterial beam with flat-curved SMAs can be decreased by 30.1% (18.3%) with an increase in the bandgap width by 35.9% (19.7%) compared to that with the flat-flat SMAs. In addition, the first torsional mode-induced flexural bandgap is widened and lowered when using the 180° curved beam resonators. The proposed SMA-based metamaterial utilizing the two-way shape memory effect thus offers a flexible and diverse route for tuning the bandgaps.

Tailored phase transition and electric properties of Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 ferroelectric ceramics by ZnO modification

Microstructure, phase transition and electric properties of Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 (PZTN 95/5) ferroelectric ceramics modified with x wt% ZnO additive (where x = 0, 0.1, 0.2, 0.3 and 0.4) were systematically investigated. It was found that ZnO modification brought out an improvement of densification and an increase in grain size of PZTN 95/5 ceramics. Results of dielectric and pyroelectric measurements demonstrated that, the low temperature rhombohedral-high temperature rhombohedral ferroelectric phase transition temperature (TLH) gradually decreased with increasing x to 0.2, while the Curie temperature (Tc) gradually increased. With further increasing ZnO content, however, TLH and TC almost showed no change. Additionally, the excellent ferroelectric, pyroelectric and piezoelectric properties were all achieved by less than 0.2 wt% ZnO modification. It is suggested that, due to superior room–temperature pyroelectric properties, the 0.2 wt% ZnO-modified PZTN 95/5 ceramics may be a promising material for applications in uncooled pyroelectric infrared detectors.

Phase transformations in the relaxor Na1/2Bi1/2TiO3 studied by means of density functional theory calculations

AbstractThe relaxor material Na1/2Bi1/2TiO3 (NBT) is an important basis for the development of lead‐free piezoceramics, but still many features of this material are not well understood. Here, we study the kinetics of phase transformations by octahedral tilts and A‐cation displacements in NBT by means of density functional theory calculations, employing ab initio molecular dynamics and nudged elastic band calculations. Our results show that the energetic differences between the low temperature rhombohedral, intermediate orthorhombic and other metastable phases are close to the room temperature thermal energy. Therefore, it is likely that above room temperature, several octahedral tilt patterns are present simultaneously on the local scale, just because of thermal vibration of the oxygen ions. Octahedral tilt transformations and A‐cation displacements show similarly high energy barriers, however, since the vibrational frequency of oxygen is higher, tilt transformations occur more frequently. Further, tilt transformations in which the oxygen octahedra get deformed the least are more probable to occur. We also find that the chemical A‐cation order affects energy barriers, influences the coupling between rotational and displacive modes and determines the stability of certain octahedral tilt orders. We conclude that the so‐called polar nanoregions in this material result from local octahedral tilt transformations and subsequent A‐cation displacements, which are driven by thermal vibration and are mediated by the underlying chemical order.

Domain wall motion and electromechanical strain in lead-free piezoelectrics: Insight from the model system (1 − x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 using in situ high-energy X-ray diffraction during application of electric fields

The piezoelectric compositions (1 − x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 (BZT-xBCT) span a model lead-free morphotropic phase boundary (MPB) between room temperature rhombohedral and tetragonal phases at approximately x = 0.5. In the present work, in situ X-ray diffraction measurements during electric field application are used to elucidate the origin of electromechanical strain in several compositions spanning the tetragonal compositional range 0.6 ≤ x ≤ 0.9. As BCT concentration decreases towards the MPB, the tetragonal distortion (given by c/a-1) decreases concomitantly with an increase in 90° domain wall motion. The increase in observed macroscopic strain is predominantly attributed to the increased contribution from 90° domain wall motion. The results demonstrate that domain wall motion is a significant factor in achieving high strain and piezoelectric coefficients in lead-free polycrystalline piezoelectrics.

Solid Solutions of Lead Metaniobate—Stabilization of the Ferroelectric Polymorph and the Effect on the Lattice Parameters, Dielectric, Ferroelectric, and Piezoelectric Properties

Ferroelectric orthorhombic lead metaniobate (PbNb2O6) is known to be metastable with respect to the thermodynamically stable nonferroelectric rhombohedral polymorph. The high‐temperature tetragonal to low temperature rhombohedral phase transition is reconstructive and thereby sluggish; ferroelectric PbNb2O6 is obtained by quenching from the stable phase field of the tetragonal polymorph. We report on the stabilization of the ferroelectric tungsten bronze polymorphs of PbNb2O6 by minor chemical substitution in the series [(1 − x) PbNb2O6–xBiTiNbO6], [(1 − x)PbNb2O6–xNa0.5Bi0.5Nb2O6], [(1 − x)PbNb2O6–xK0.5Bi0.5Nb2O6], and [(1 − x)PbNb2O6–xCaTiO3]. The high‐temperature tungsten bronze polymorph is entropy stabilized with respect to the stable rhombohedral polymorph, and we propose that the tungsten bronze is further entropy stabilized by chemical substitution, reducing the transition temperature of the rhombohedral polymorph and further disfavoring the kinetics of the undesired phase transition. Optimized solid‐state synthesis and processing to obtain dense ceramics were developed for the solid solutions, and the dielectric, ferroelectric, and piezoelectric properties of the PbNb2O6 solid solutions are reported. Curie temperature is suppressed with chemical substitution in all the systems. Lattice cell parameters display systematic variation with composition, reducing the molar volume, and the lattice parameter ratio 2b/a with increasing degree of substitution, reflecting a suppression of the polarization along the (010) direction due to chemical substitution. The piezoelectric properties improved with increasing substitution level probably due to the ease of poling of the materials with lower Tc. However, some improvements seen with 2% CaTiO3 were not accompanied by Tc decrease.

Pressure-induced frustration in charge ordered spinel AlV2O4

AlV2O4 is the only spinel compound so far known that exists in the charge ordered state at room temperature. It is known to transform to a charge frustrated cubic spinel structure above 427 ° C. The presence of multivalent V ions in the pyrochlore lattice of the cubic spinel phase brings about the charge frustration that is relieved in the room temperature rhombohedral phase by the clustering of vanadium into a heptamer molecular unit along with a lone V atom. The present work is the first demonstration of pressure-induced frustration in the charge ordered state of AlV2O4. Synchrotron powder x-ray diffraction studies carried out at room temperature on AlV2O4 subjected to high pressure in a diamond anvil cell show that the charge ordered rhombohedral phase becomes unstable under the application of pressure and transforms to the frustrated cubic spinel structure. The frustration is found to be present even after pressure recovery. The possible role of pressure on vanadium t2g orbitals in understanding these observations is discussed.

Ferroelectric and antiferrodistortive phase transition in the multiferroic (Bi0.8Ba0.2)(Fe0.8Ti0.2)O3: A high temperature neutron powder diffraction study

Presented here are the results of Rietveld analysis of powder neutron diffraction data, which reveal that both the ferroelectric and antiferrodistortive phase transitions occur together at T ∼ 925 K during heating, leading to a change in crystal structure from room temperature rhombohedral phase in the R3c space group with tilted oxygen octahedra to the high temperature paraelectric and paraelastic cubic phase in Pm3−m space group. The presence of superlattice peaks due to antiphase rotation of oxygen octahedra in the rhombohedral phase until the transition to the cubic phase precludes the possibility of an intermediate R3m space group, conjectured previously on the basis of an x-ray powder diffraction study. Discontinuous change in the unit-cell volume, tilt angle, and bond lengths at the transition temperature TC, and phase coexistence of R3c and Pm3−m over 100 K range across TC, suggest that the transition is of first order. It is argued that the R3c to Pm3−m phase transition is of trigger type involving simultaneous condensation of Γ4- and R4+ modes.

Structural origins of relaxor behavior in a 0.96(Bi1/2Na1/2)TiO3–0.04BaTiO3 single crystal under electric field

Diffuse x-ray scattering intensities from a single crystal of 0.96(Bi1/2Na1/2TiO3)–0.04(BaTiO3) have been collected at room temperature with and without application of an electric field along the [100] direction. Distinct features in the diffuse scattering intensities indicate correlations on a nanometer length scale. It is shown that locally correlated planar-like structures and octahedral tilt-domains within the room temperature rhombohedral R3c phase are both electrically active and are irreversibly affected by application of an electric field of 4.3 kV/mm. The field dependence of these nanoscale structures is correlated with the relaxor behavior of the material by macroscopic permittivity measurements.

Ion dynamics in combustion synthesized Na 3Cr 2(PO 4) 3 crystallites

Phase pure Na 3Cr 2(PO 4) 3 crystallites of 41 nm are synthesized by novel solution combustion technique. Characterization methods like X-ray diffraction, Fourier transform infrared spectroscopy, UV–vis spectroscopy, Thermo gravimetric, differential thermal analysis and scanning electron microscopy with energy dispersive X-ray analysis are utilized to validate the structure and phase purity. These crystallites are stable in room temperature rhombohedral phase of R 3 ¯ c symmetry. Fascination of the material is because of its structural stability and high conductivity. Due to its structural stability, present study depicts the ion dynamics in the crystal lattice by analyzing the conductivity relaxation process. Interpretations from impedance representation fortify these conclusions and the dc conductivity activation energy is found to be 0.70 ± 0.02 eV. Almost same value of activation energy for conductivity relaxation from modulus and conductivity representations show that while conducting and relaxing, ion has to overcome the same energy barrier. The conductivity and modulus isotherms are shown to be collapsed to a master curve upon appropriate scaling of the axes; indicating the temperature independent relaxation mechanism. It is evident that the scaling of conductivity and modulus spectra provides the same information on ion dynamics of this material.

High-temperature phase transitions in the improper ferroelectric KIO3

The ac conductivity (σac) and dielectric permittivity (ϵ) are determined in the temperature range 300 K < T< 520 K at some selected frequencies (0.5–10 kHz) for the polycrystalline samples of KIO3 compound. The results indicated that the compound behaves as an improper ferroelectric and undergoes a ferroelectric phase transition from a high temperature rhombohedral phase I to a low temperature monoclinic phase II at T c = (486 ± 1) K. A second structural phase transition was observed around 345 K. The conductivity varies with temperature range and for T > 428 K intrinsic conduction prevails. Different activation energies in the different temperature regions were calculated. The frequency dependence of σ(ω) was found to follow the universal dynamic response [σ(ω)∝(ω) s(T)]. The thermal behaviour of the frequency exponent s(T) suggests the hopping over the barrier model rather than the quantum mechanical tunneling model for the conduction mechanism.

Orbital Correlations in the PseudocubicOand RhombohedralRPhases ofLaMnO3

The local and intermediate structure of stoichiometric LaMnO3 has been studied in the pseudocubic and rhombohedral phases at high temperatures (300-1150 K). Neutron powder diffraction data were collected and a combined Rietveld and high real space resolution atomic pair distribution function analysis was carried out. The nature of the Jahn-Teller (JT) transition around 750 K is confirmed to be orbital order to disorder. In the high-temperature orthorhombic (O) and rhombohedral (R) phases, the MnO6 octahedra are still fully distorted locally. More importantly, the intermediate structure suggests the presence of local ordered clusters of diameter approximately 16 A ( approximately 4 MnO6 octahedra) implying strong nearest-neighbor JT antiferrodistortive coupling. These clusters persist well above the JT transition temperature even into the high-temperature R phase.

Pyroelectric properties of (1−x)Pb(Mg1∕3Nb2∕3)O3-xPbTiO3 and (1−x)Pb(Zn1∕3Nb2∕3)O3-xPbTiO3 single crystals measured using a dynamic method

The pyroelectric coefficients of 0.72Pb(Mg1∕3Nb2∕3)O3-0.28PbTiO3 (PMN-28PT), 0.67Pb(Mg1∕3Nb2∕3)O3-0.33PbTiO3 (PMN-33PT), and 0.92Pb(Zn1∕3Nb2∕3)O3-0.08PbTiO3 (PZN-8PT) single crystals, oriented and poled along three different crystallographic directions, [001]C, [011]C, and [111]C, have been measured using a dynamic method. The suffix C indicates reference to the prototypic cubic coordinate system. Large coefficients of up to 1070μCm−2K−1 have been observed for the [111]C oriented, rhombohedral PMN-28PT and PMN-33PT samples, where the measurement direction corresponds to the polar axis of the crystal. For each composition and orientation two different poling procedures are compared, a commonly used field cooling method and a low temperature poling protocol. It was found that in otherwise pseudoorthorhombic samples of PZN-8PT, a room temperature rhombohedral phase could be stabilized by application of a field along the [111]C direction while heating from −70°C.

Characterisation of the SOFC material, LaCrO3, using vibrational spectroscopy

Abstract LaCrO3 is reported to undergo a low to high temperature (HT) phase transition from orthorhombic (Pnma) to rhombohedral (R-3c), at ca. 255 °C. The phases involved in the low temperature phase transition of LaCrO3 have been determined using Raman spectroscopy at temperatures from −196 to 300 °C. There are nine Raman bands observed from a total of 24 predicted modes, seven of which are assigned from comparison with the Raman profile and relative band positions observed and calculated for the isostructural compound, YMnO3, as follows: 131(B2g), 150(B3g), 174(Ag), 252(B1g), 279(Ag), 441(Ag) and 590(Ag) cm−1. A phase transformation was observed at ca. 260 °C from the change in the Raman profile. The high temperature rhombohedral phase of LaCrO3 had four bands which are assigned as follows: 58(Eg), 161(Eg), 288(A1g) and 434(Eg, Eg) cm−1, from comparison with the Raman profile and relative band positions observed for the isostructural compound, NdAlO3. The Fourier transform infrared (FTIR) spectrum of LaCrO3 showed a total of eight bands discernible at room temperature from 25 predicted modes for the orthorhombic structure. The mode assignments were determined by comparison with the Raman profile and relative band positions observed and calculated for the isostructural compound, SmAlO3, as follows: 138(B2u), 166(B3u), 197(B1u), 240(B3u), 266(B2u), 332(B2u), 357(B2u), 381(B3u), 425(B3u), 446(B1u), 471(B3u), 493(B3u), 573(B1u), 606(B3u) and 670 (B1u) cm−1.

MD simulation of the dynamics of chlorocyclohexane guest molecules in the thiourea inclusion compound

We present the results of molecular dynamics simulations of the chlorocyclohexane/thiourea inclusion compound within the high temperature rhombohedral and low temperature monoclinic phases of the composite crystal. By defining a molecular reference frame by means of two vectors n→ and p→, we have been able to analyze separately the reorientations of the guest molecules as a whole in the laboratory reference frame and the reorientations of the guest molecules about an intramolecular reference axis. In the rhombohedral phase, we found that the chlorocyclohexane guest molecules reorient as a whole over six favored sites around the C3 and the C2 symmetry axes of the crystallographic site (D3 point group). In the low temperature phase, the symmetry elements of phase I are lost and the guest molecules sit in general positions with a dynamic disorder among two orientations. Orientational distribution functions and space–time correlation functions (intermediate incoherent neutron scattering laws) for the dynamics of n→ and p→ have been analyzed in both structural phases. These results are discussed in relation to incoherent quasi-elastic neutron scattering (QENS) experiments and compared to those previously obtained for the cyclohexane/thiourea inclusion compound. Marked differences between both systems have been found for the reorientations of guest molecules. These simulations will be exploited to interpret QENS experiments in view of elucidating the phase transition mechanisms in these compounds.

Processing and characterization of solution deposited Pb 1.1(Zr 0.58Fe 0.2Nb 0.2Ti 0.02)O 3 thin films

High quality ferroelectric thin films with the stoichiometry Pb 1.1(Zr 0.58Fe 0.2Nb 0.2Ti 0.02)O 3 (PZFNT) have been processed via chemical solution deposition on textured (1 1 1) Pt/Ti/SiO 2/Si substrates. X-ray diffractometry and scanning electron microscopy investigations show that full transformation from pyrochlore to perovskite phase is accomplished after annealing at 750 °C for 20 min. The temperature dependence of the dielectric permittivity (ε r) measured at different frequencies shows two maxima. The first one occurs in the vicinity of 110 °C and belongs to the low-to-high temperature rhombohedral phase transition. The second maximum is attributed to Curie point, and is found to be strongly dependent on frequency, which indicates relaxor type behaviour. The remnant polarization and coercive field are determined from ferroelectric hysteresis measurements as 6.5 μC/cm 2 and 25 kV/cm, respectively. Values of ε r are measured as 1490 and 1350 at 0.5 and 10 kHz respectively, and are much higher than the value reported for bulk ceramic material. The results are discussed in terms of microstructure and doping effects on the properties.

Field-enhanced piezoelectric deformation during the high temperature/low temperature rhombohedral (FERh/FERL) phase transformation for tin modified lead zirconate titanate ceramics

An unusual field-enhanced piezoelectric deformation near the FERH/FERL structural phase transformation was observed in a tin modified lead zirconate titanate solid solution. In addition to the typical field-induced domain reorientation and the piezoelectric strain, this additional field-enhanced deformation only observed near the phase transformation increases linearly with external electric field strength. A 78% increase in field-enhanced strain was observed at a field strength of 32 kV/cm. Comparison of the dielectric susceptibility at low and high field conditions suggests that the observed unusual behavior is created by a field-induced lattice softening during the structural phase transformation. Experimental observations on the field-induced softening phenomena are reported.

Effect of A-Site Substitution of Calcium on Zr-Rich Lead Zirconate Titanate

Effect of calcium substitution on the A-site of Zr-rich lead zirconate titanate Pb(Zr 0.94 Ti 0.06 )O 3 and Pb(Zr 0.92 Ti 0.08 )O 3 is studied. It is found that the rhombohedral structure of Zr-rich lead zirconate titanate changes to tetragonal and finally to cubic with increasing amount of calcium substitution. Both the dielectric constant and the Curie temperature decrease with the substitution of calcium on the A-site of the ABO 3 perovskite structure. A pyroelectric current peak is observed which is resulted from the phase transition from the low temperature rhombohedral ferroelectric phase F R ( LT ) to the high temperature F R ( HT ) one. Small amount of calcium substitution increases the peak value and also shifts the peak position to higher temperatures. However, further increase in the calcium content lowers the peak value and finally smears out the peak.