Rhombohedral Lattice Distortion Research Articles

Defect Engineering Realizes Superior Thermoelectric Performance of GeTe

AbstractGeTe has been considered as a promising mid‐temperature thermoelectric (TE) candidate, but its zT value is severely limited by the excessive hole concentration and high thermal conductivity. Here, it is demonstrated that the TE properties of GeTe can be significantly improve by defect engineering of Sb‐Pb and AgCuTe codoping. The Sb‐Pb codoping is adopted to optimize the carrier concentration and manipulate the rhombohedral lattice distortion, leading to valence band convergence and enhanced power factor. The AgCuTe alloying introduces multiscale phonon scattering centers including dislocations and nano‐precipitates to reduce the lattice thermal conductivity in GeTe. Consequently, a maximum zT of 2.3 at 773 K and an average zT of 1.43 (300–773 K) are obtained in (Ge0.84Sb0.06Pb0.1Te)0.99(AgCuTe)0.01. Moreover, the fabricated thermoelectric module exhibits a high output power density of 0.59 W cm–2 and an energy conversion efficiency of 7.9% at ΔT = 500 K, suggesting hierarchical defect engineering is an effective strategy to realize high‐performance GeTe‐based thermoelectric.

Non-linear temperature dependences of pseudoelastic stress and stress hysteresis of a nanocrystalline Ni47Ti50Fe3 alloy

This paper reports a study on non-linear temperature dependences of the critical pseudoelastic stress and stress hysteresis for stress-induced martensitic transformation deformation of a nanocrystallined Ni47Ti50Fe3 alloy. Such non-linear temperature dependences were found over a wide low temperature range from -196 °C to 20 °C and they defy the predictions of the Clausius-Clapeyron law for the transformation. The pseudoelastic behaviour exhibited two unusual characteristics, including a progressively decreasing dσ/dT slope and a rapidly widening stress hysteresis with decreasing the temperature. In-situ X-ray diffraction analysis indicated that the starting microstructure of the alloy for pseudoelastic deformation was a mixture of the B2 and a crystallographically suppressed R phase (denoted R*) within -8 °C to -196 °C. The volume fraction, the rhombohedral lattice distortion and the internal atomic shuffle of the suppressed R phase increased upon cooling. These continued evolutions of the crystal structure of the R* phase and the microstructure of the alloy upon cooling are ascribed to be responsible for the abnormal non-linear temperature dependences of the pseudoelastic stresses and the widening of the stress hysteresis.

Origin of enhanced piezoelectric properties revealed through electric field driven studies in 0.94(Na0.5Bi0.5TiO3)−0.06(Ba0.85Ca0.15Ti0.9Zr0.1O3) ceramics

A single phase 0.94(Na0.5Bi0.5TiO3)–0.06(Ba0.85Ca0.15Ti0.9Zr0.1O3) solid solution (i.e., BCZT-0.06) is prepared using a conventional solid-state sintering route with enhanced piezo and ferroelectric properties as compared to Na0.5Bi0.5TiO3 (NBT). In the context of understanding the origin of enhanced piezoelectric properties in a BCZT-0.06 specimen, electric field driven studies on different length scales, viz., global, local, and electronic structure, are carried out using x-ray/neutron diffraction, Raman scattering, and UV–Vis spectroscopic techniques. An analysis on different length scales of the electric field-driven BCZT-0.06 specimen displays minimum rhombohedral lattice distortion (δr), reduced homogeneous lattice strain (δ), octahedral strain (ζ), and pronounced Ti-cation displacement along the polar [111] direction as compared to parent NBT. The enhanced ferro and piezoelectric responses observed in the BCZT-0.06 specimen have been attributed to the ease of non-180° domain re-orientation, domain switching, and domain wall motion due to reduced strain coupled with a polarization extension mechanism.

High-Performance GeTe Thermoelectrics in Both Rhombohedral and Cubic Phases.

GeTe experiences phase transition between cubic and rhombohedral through distortion along the [111] direction. Cubic GeTe shares the similarity of a two-valence-band structure (high-energy L and low-energy Σ bands) with other cubic IV-VI semiconductors such as PbTe, SnTe, and PbSe, and all show a high thermoelectric performance due to a high band degeneracy. Very recently, the two valence bands were found to switch in energy in rhombohedral GeTe and to be split due to symmetry-breaking of the crystal structure. This enables the overall band degeneracy to be manipulated either by the control of symmetry-induced degeneracy or by the design of energy-aligned orbital degeneracy. Here, we show Sb-doping for optimizing carrier concentration and manipulating the degree of rhombohedral lattice distortion to maximize the band degeneracy and then electronic performance. In addition, Sb-doping significantly promotes the solubility of PbTe, enhancing the scattering of phonons by Ge/Pb substitutional defects for minimizing the lattice thermal conductivity. This successfully realizes a superior thermoelectric figure of merit, zT of >2 in both rhombohedral and cubic GeTe, demonstrating these alloys as top candidates for thermoelectric applications at T < 800 K. This work further sheds light on the importance of crystal structure symmetry manipulation for advancing thermoelectrics.

Vanadium clusters formation in geometrically frustrated spinel oxide AlV2O4.

The spinel oxide AlV2O4 is a unique material, in which the formation of clusters is accompanied by atomic, charge and orbital ordering and a rhombohedral lattice distortion. In this work a theory of the structural phase transition in AlV2O4 is proposed. This theory is based on the study of the order-parameter symmetry, thermodynamics, electron density distribution, crystal chemistry and mechanisms of formation of the atomic and orbital structures of the rhombohedral phase. It is established that the critical order parameter is transformed according to irreducible representation k9(τ4) (in Kovalev notation) of the Fd \bar{3}m space group. Knowledge of the order-parameter symmetry allows us to show that the derived AlV2O4 rhombohedral structure is a result of displacements of all atom types and the ordering of Al atoms (1:1 order type in tetrahedral spinel sites), V atoms (1:1:6 order type in octahedral sites) and O atoms (1:1:3:3 order type), and the ordering of dxy, dxz and dyz orbitals. Application of the density functional theory showed that V atoms in the Kagomé sublattice formed separate trimers. Also, no sign of metallic bonding between separate vanadium trimers in the heptamer structure was found. The density functional theory study and the crystal chemical analysis of V-O bond lengths allowed us to assume the existence of dimers and trimers as main clusters in the structure of the AlV2O4 rhombohedral modification. The trimer model of the low-symmetry AlV2O4 structure is proposed. Within the Landau theory of phase transitions, typical diagrams of possible phase states are built. It is shown that phase states can be changed as a first-order phase transition close to the second order in the vicinity of tricritical points of the phase diagrams.

Structure–property relation to enhance the piezoelectric and ferroelectric properties in (Na0.5Bi0.5)TiO3-based non-MPB lead-free piezoelectric ceramics

Lead-free sodium bismuth titanate-based (NBT) non-MPB solid solutions for different substitutions were synthesized by solid-state reaction method and their structural relation with ferroelectric and piezoelectric properties were analyzed. Extrinsic contribution to piezoelectric coefficient more particularly non-180° domain reorientation was facilitated by enhanced mobility. Minimum strain and low rhombohedral lattice distortion improves mobility and increases the piezoelectric coefficient which in turn enhances remnant polarization and reduces coercive field. Regarding substitution in NBT, as a rule of thumb, substitution of high polarisable ion in the A-site and low polarisable ion in the B-site enhances the piezoelectric coefficient in non-MPB NBT-based systems.

The influence of electric poling on structural, thermal, dielectric and ferroelectric properties of Na0.5Bi0.5TiO3 ceramics

Two types of lead-free Na0.5Bi0.5TiO3 ceramics were manufactured by using a high-temperature solid-state reaction technique. X-ray diffraction analysis showed the formation of a single perovskite phase with the rhombohedral symmetry at room temperature. The structural, vibrational, dielectric and ferroelectric properties of these ceramics in unpoled and poled states were investigated. It was shown that a prior E-field poling process significantly influences the properties of these ceramics. This includes changes in the character of ε(T)- and tanδ(T) plots, P(E) hysteresis loops, as well as shifts of Td and TR-T. These effects are mainly attributed to the improvement of polarization/domain ordering and transformation of tetragonal regions into rhombohedral phase by applied electric field. This is accompanied by a change of rhombohedral lattice distortion and a change of unit cell constants. The electric field supported and extended the ferroelectric phase existence towards higher temperatures.

Phonon triggered rhombohedral lattice distortion in vanadium at high pressure.

In spite of the simple body-centered-cubic crystal structure, the elements of group V, vanadium, niobium and tantalum, show strong interactions between the electronic properties and lattice dynamics. Further, these interactions can be tuned by external parameters, such as pressure and temperature. We used inelastic x-ray scattering to probe the phonon dispersion of single-crystalline vanadium as a function of pressure to 45 GPa. Our measurements show an anomalous high-pressure behavior of the transverse acoustic mode along the (100) direction and a softening of the elastic modulus C44 that triggers a rhombohedral lattice distortion occurring between 34 and 39 GPa. Our results provide the missing experimental confirmation of the theoretically predicted shear instability arising from the progressive intra-band nesting of the Fermi surface with increasing pressure, a scenario common to all transition metals of group V.

Spin-lattice coupling phase transition and phonon anomalies in bismuth ferrite BiFeO3

Phase transition, phonon vibration, and magnetization in (100) BiFeO3 (BFO) multiferroic single crystals have been characterized as functions of temperature. The BFO single crystal exhibits a weak ferromagnetic hysteresis loop possibly due to appearance of ferromagnetic FeOFe orderings and a modulation of the FeOFe bond. Zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibilities suggest a weak spin-glass behavior below ∼200 K. High-resolution transmission electron microscopy (HRTEM) suggests a rhombohedral R3c symmetry (with tilted FeO6 octahedra) and lattice distortion in the surface layer. The BFO crystal undergoes a structural transition sequence of rhombohedral−tetragonal−orthorhombic− cubic at ∼620–640, ∼1000, and ∼1100 K, respectively. The spin-lattice coupling accounts for the rhombohedraltetragonal transition upon approaching the Néel temperature (TN). The low-frequency Raman-active modes ψ4,II, E(1), A1(1), and A1(2) reveal clear softening anomalies in frequency shifts and broad peaks in full-width-at-half-maximum (FWHM) around 450 K, which is attributed to the FeOFe ordering transition without change of structural symmetry.

Enhanced electrical properties in Rb-substituted sodium bismuth titanate ceramics

Influence of large ionic radius cation rubidium (Rb) substitution in the A-site of sodium bismuth titanate (NBT) on remnant polarization and dielectric constant is investigated. Substitution of 0.01 mole fraction of Rb in NBT ceramics escalates remnant polarization and dielectric constant coupled with reduction in coercive field and enhanced piezoelectric constant. Spontaneous and remnant polarization of Rb-substituted NBT ceramics is 66.3 and 59.9 µC/cm2 respectively, which is greater than other isovalent substitution cations in NBT that have been reported in the literature. Room-temperature dielectric constant and piezoelectric constant are 704.68 and 88 pC/N successively. Increase in remnant polarization, dielectric constant and piezoelectric constant along with decreased coercive field is attributed to improved mobility of domain reorientation and domain wall motion due to low homogeneous strain and minimum rhombohedral lattice distortion in Rb-substituted NBT ceramics. The present work provides a reproducible preparation of Rb-substituted ceramics to enhance ferroelectric and dielectric properties for use in functional devices which require high reliability.

Structural and electrical properties of bismuth magnesium titanate substituted lead-free sodium bismuth titanate ceramics

Bismuth magnesium titanate substituted sodium bismuth titanate lead-free piezoceramics are synthesized by solid state reaction method up to x = 0.06 mole fraction. Samples are poled and influence of substitution on poled ceramics is studied. Dielectric, ferroelectric and piezoelectric properties are analyzed in relation with intrinsic parameters strain and lattice distortion. Rhombohedral R3c symmetry is retained up to x = 0.04 mole fraction. Change to higher symmetry cubic phase is noticed for higher mole fraction of BMT substitution. Enhanced electrical properties are obtained for x = 0.04 mole fraction due to smallest rhombohedral lattice distortion and low homogeneous strain. Structural change to higher symmetry phase leads to low remnant polarization and decreased piezoelectric coefficient for higher mole fraction of bismuth magnesium titanate substitution.

Role of chemically and thermally induced crystal lattice distortion in enhancing the Seebeck coefficient in complex tellurides

Composition and crystal structure of complex materials can significantly change the Seebeck effect, i.e., heat to electrical energy conversion, which is utilized in thermoelectric materials. Despite decades of studies of various thermoelectric materials and their application, the fundamental understanding of this effect still is limited. One of the most efficient groups of thermoelectric materials is based on GeTe, where Ge is replaced by [Ag + Sb], i.e., AgxSbxGe50−2xTe50 alloys, traditionally shown as (GeTe)m(AgSbTe2)100−m (TAGS-m series). Here we report on the discovery of two unique phenomena in TAGS materials attributed to the effects from [Ag + Sb] atoms: (i) a linear relation between the Seebeck coefficient and rhombohedral lattice distortion, and (ii) resonance-like temperature-induced behavior of the contribution to the Seebeck coefficient produced by [Ag + Sb] atoms. Our findings show that heat to electrical energy conversion strongly depends on the temperature- and compositionally-induced rhombohedral to cubic transformation where [Ag + Sb] atoms play a crucial mediating role.

Role of strain and lattice distortion on ferroelectric and piezoelectric properties of bismuth magnesium zirconate substituted sodium bismuth titanate ceramics

Effect of weak ferroelectric perovskite, bismuth magnesium zirconate [Bi(Mg0.5Zr0.5)O3] substitution in lead-free sodium bismuth titanate [(Na0.5Bi0.5)TiO3] ceramics is studied. Influence of substitution on intrinsic and extrinsic contribution and impact on ferroelectric and piezoelectric properties are investigated. Improved spontaneous polarization (Ps), increased remnant polarization (Pr), decreased coercive field (Ec) and high piezoelectric coefficient (d33) are obtained for x = 0.01 mole fraction of Bi(Mg0.5Zr0.5)O3 substitution due to decrease in rhombohedral lattice distortion and homogeneous strain. Small rhombohedral lattice distortion (δr) and minimum homogeneous strain (δ) are the primary intrinsic parameters which favours the extrinsic parameters such as mobility of non-180° domain reorientation, domain switching and domain wall motion. Enhanced mobility softens the coercive field and increases remnant polarization to maximum. Reduced rhombohedral lattice distortion, low strain and enhanced mobility are the key factors for enhanced piezoelectric constant, highest remnant polarization and decreased coercive field in non-MPB (1 − x)(Na0.5Bi0.5)TiO3–xBi(Mg0.5Zr0.5)O3 solid solutions.

Structural and electrical properties of (1−x)(Na0.5Bi0.5)TiO3–xBi(Mg0.5Zr0.5)O3 lead-free piezoelectric ceramics

Sodium bismuth titanate–bismuth magnesium zirconate (NBT–BMZ) lead-free piezoelectric ceramics are synthesized by a solid-state reaction method up to x=0.04mole fraction. Effect of substitution on dielectric, ferroelectric and piezoelectric properties is studied in relation with structure and morphology. Rhombohedral R3c structure symmetry is maintained up to x=0.025mole fraction. Higher mole fraction of bismuth magnesium zirconate substitution prompts phase transition from rhombohedral to higher symmetry cubic phase. Morphology and homogeneity of grains changes on BMZ substitution. Minimum homogeneous strain and small rhombohedral lattice distortion influences the extrinsic contribution for improved dielectric constant, remnant polarization and piezoelectric coefficient. Enhanced mobility and domain reorientation softens coercive field and increases remnant polarization and piezoelectric coefficient to maximum at x=0.01mole fraction of BMZ substitution. Decrease in dielectric, ferroelectric and piezoelectric properties are due to structure change to cubic phase on higher mole fraction of BMZ substitution.

Fermi Surface of Three-Dimensional La(1-x)Sr(x)MnO3 Explored by Soft-X-Ray ARPES: Rhombohedral Lattice Distortion and its Effect on Magnetoresistance.

Electronic structure of the three-dimensional colossal magnetoresistive perovskite La(1-x)Sr(x)MnO3 has been established using soft-x-ray angle-resolved photoemission spectroscopy with its intrinsically sharp definition of three-dimensional electron momentum. The experimental results show much weaker polaronic coupling compared to the bilayer manganites and are consistent with the theoretical band structure including the empirical Hubbard parameter U. The experimental Fermi surface unveils the canonical topology of alternating three-dimensional electron spheres and hole cubes, with their shadow contours manifesting the rhombohedral lattice distortion. This picture has been confirmed by one-step photoemission calculations including displacement of the apical oxygen atoms. The rhombohedral distortion is neutral to the Jahn-Teller effect and thus polaronic coupling, but affects the double-exchange electron hopping and thus the colossal magnetoresistance effect.

Role of rubidium cation substitution in the A-site of sodium bismuth titanate ceramics

Effect of large ionic radius cation rubidium (Rb) substitution in the A-site of sodium bismuth titanate (Bi0.5Na0.5−xRbx) TiO3 ceramics is studied. Influence of substitution on intrinsic and extrinsic contribution and impact on ferroelectric properties and piezoelectric constant are investigated. Small rhombohedral lattice distortion (δr) and minimum homogeneous strain (δ) are the primary intrinsic parameters which favors the extrinsic parameters such as domain wall motion and mobility of non-180° domain switching. Enhanced mobility softens the coercive field and increases remnant polarization to maximum. Reduced lattice distortion, low strain and enhanced mobility are the significant factors for enhanced piezoelectric constant, highest remnant polarization and decreased coercive field in (Bi0.5Na0.5−xRbx) TiO3 ceramics.

Structural phase transitions of robust insulating Bi1-xLaxFe1-yTiyO3 multiferroics

In contrast to leaky BiFeO3, of which two structural phase transitions of cycloidal modulated antiferromagnetic-paramagnetic and ferroelectric-paraelectric are observed below 850 °C, chemical co-substitution to form Bi1−xLaxFe1−yTiyO3 ternary solid solution makes these multiferroic ceramics become robust insulating low dielectric loss and exhibit rich structural phase transitions. Differential thermal analysis and temperature-dependent X-ray diffraction measurements probe four first-order structural phase transitions, e.g., TH = 305 °C, TN = 365 °C, TC = 810 °C, and TS = 830 °C observed in the Bi0.98La0.02Fe0.99Ti0.01O3 system, which are reasonably attributed to Brazovskii-type cycloidal modulated antiferromagnetic-helimagnetic (at TH) and helimagnetic-paramagnetic (at TN) magnetic phase transitions, ferroelectric-paraelectric (at TC) and rhombohedral-cubic (at TS) structural phase transitions, respectively. Magnetic phase transition temperatures change a little but ferroelectric and lattice structural phase transition temperatures decrease gradually with increasing co-substitution up to composition-induced rhombohedral-(pseudo-)cubic structural phase boundary. The first-order nature of magnetic phase transition and emergence of helimagnetic phase were attributed to Ti3+ d1 magnetic disorder distribution in the Fe3+ d5-O-Fe3+ d5 chains, while the first-order nature becomes weak with increasing co-substitution, owing to decreased ferroelectric rhombohedral lattice distortion. An intermediate rhombohedral paraelectric phase is discovered intervening between ferroelectric rhombohedral and paraelectric cubic phase, of which the temperature range defined by difference between TC and TS increases with increasing co-substitution. It was found that TC and TS are able to be predicted quantitatively by reduced mass of unit cell. These findings enrich our understanding of ferroic phase transitions and advance designing novel high temperature multiferroic compounds.

An Investigation of Solvent Effect on Rhombohedral/Monoclinic/Tetragonal Phase Properties of Pb(Zr&lt;sub&gt;0.53&lt;/sub&gt;Ti&lt;sub&gt;0.47&lt;/sub&gt;)O&lt;sub&gt;3&lt;/sub&gt; Nanoparticles Prepared via Sol-Gel Method

The highest piezoelectric and ferroelectric properties of lead zirconate titanate (PZT) occur in vicinity of morphotropic phase boundary (MPB). In MPB, crystalline phase ratios play a crucial role in determining the dielectric properties. In this paper, PZT nanoparticles were synthesized by a modified sol gel procedure and the effects of different solvents of methanol, ethanol, 1-propanol, and acetic acid on tetragonal, rhombohedral, and monoclinic phase ratios near the Pb (Zr0.53Ti0.47)O3 were investigated. The X-ray diffraction results show the formation of almost pure perovskite structure in all samples. The volume fraction of tetragonal and rhombohedral phases, lattice constants, and lattice distortion were measured from the X-ray diffraction and Raman spectroscopic analysis for each solvent. Also, the domain size was compared between the solvents. A trace amount of monoclinic phase could be detected by Raman spectroscopy. The results show that acetic acid is the most appropriate solvent for synthesizing of PZT nanoparticles in which the tetragonal lattice parameters are ct=4.16 Ǻ and at=4.02 Ǻ with a distortion of 3%. The tetragonal volume fraction is estimated about 77%. The particle size and degree of agglomeration were measured according to field emission electron microscopy and indicates that 70 nm particles were formed for acid-based sample.

Structural change in polar nanoregion in alkali niobate added Pb(Zn1/3Nb2/3)0.95Ti0.05O3 single crystal and its effect on ferroelectric properties

Pb(Zn1/3Nb2/3)0.95Ti0.05O3 (PZNT) single crystals with 5 mol. % alkali niobate such as LiNbO3 (LN), NaNbO3 (NN), and KNbO3 (KN) were fabricated by using a flux method to investigate the effect of A-site cation radius on the structure and ferroelectric properties of PZNT under electric field (E-field). Their structure and properties showed different electric field dependence. Polarization versus electric field and strain versus electric field curves of PZNT-0.05LN showed E-field induced phase transition from a relaxor state to a normal ferroelectric state. However, only relaxor behavior was observed in PZNT-0.05NN and PZNT-0.05KN. The effect of A-site ion doping is attributed to the change in local lattice distortion and polar nano-region. When smaller cation such as Li ion substitutes Pb ion, the off-center displacement of Nb ion stabilizes rhombohedral lattice distortion. They, in turn, facilitate the development of macro-domains under electric field (E-field) in PZNT-0.05LN. In contrast, the substitution of Pb with larger cations such as Ni and K decreases the rhombohedral distortion of PZNT, which leads to the disappearance of unique E-field induced phase transition from rhombohedral to tetragonal phase in PZNT. Therefore, non-linear electrostrictive behavior of relaxor ferroelectrics is found in PZNT-0.05NN and PZNT-0.05KN.

A rhombohedral structure-properties relation in Pb-free Bi0.5(Na1−x K x )0.5TiO3 ceramics

A structure-properties relation in Bi0.5(Na1−x K x )0.5TiO3 ceramics was investigated for K contents ranging from 0 to 20 mol% substituting for Na in (Bi0.5Na0.5)TiO3 ceramics. A rhombohedral perovskite solid solution was confirmed up to x = 18 mol%. An inverse relation between depolarization temperature (T d ) and piezoelectric coefficient (d 33) was observed in the rhombohedral phase region. As x increased toward the morphotropic phase boundary (MPB), T d was lowered while d 33 was enhanced. Structurally, the rhombohedral lattice distortion (90−α) decreased while the lattice constant (a R ) increased with increasing x. 90−α appeared to be coupled with the compositiondependent variations in T d and d 33; the smaller 90 − α, the lower T d , and the higher d 33.