High-symmetry Phase Research Articles

Nature of the structural symmetries associated with hybrid improper ferroelectricity inCa3X2O7(X=Mnand Ti)

In hybrid improper ferroelectric systems, polarization arises from the onset of successive nonpolar lattice modes. In this work, measurements and modeling were performed to determine the spatial symmetries of the phases involved in the transitions to these modes. Structural and optical measurements reveal that the tilt and rotation distortions of the MnO6 or TiO6 polyhedra relative to the high symmetry phases driving ferroelectricity in the hybrid improper Ca3X2O7 system (X=Mn and Ti) condense at different temperatures. The tilt angle vanishes abruptly at T$_T$ ~ 400 K for Ca3Mn2O7 (and continuously for X=Ti) and the rotation mode amplitude is suppressed at much higher temperatures T$_R$ ~1060 K. Moreover, Raman measurements in Ca3Mn2O7 under isotropic pressure reveal that the polyhedral tilts can be suppressed by very low pressures (between 1.4 and2.3 GPa) indicating their softness. These results indicate that the Ca3Mn2O7 system provides a new platform for strain engineering of ferroelectric properties in film based systems with substrate induced strain.

Optical control of vibrational coherence triggered by an ultrafast phase transition

Femtosecond time-resolved x-ray diffraction is employed to study the dynamics of the periodic lattice distortion (PLD) associated with the charge-density-wave (CDW) in K0.3MoO3. Using a multi-pulse scheme we show the ability to extend the lifetime of coherent oscillations of the PLD about the undistorted structure through re-excitation of the electronic states. This suggests that it is possible to enter a regime where the symmetry of the potential energy landscape corresponds to the high symmetry phase but the scattering pathways that lead to the damping of coherent dynamics are still controllable by altering the electronic state population. The demonstrated control over the coherence time offers new routes for manipulation of coherent lattice states.

Improvement of dielectric and ferroelectric properties in bismuth sodium titanate based relaxors through Bi non-stoichiometry

Influence of Bi-non-stoichiometry is studied on the dielectric and ferroelectric properties of 0.72Bi0.5+xNa0.5TiO3-0.22SrTi0.875Nb0.1O3-0.06BaTiO3 (BNBSNT) lead-free relaxors. All compositions exhibit high symmetry phase structure with dynamic polar nano regions. An enhanced degree of disorder is revealed and more polarization can be induced in Bi deficiency, which is related to the difficulty in the formation of a neutral defect complex between cations and oxygen vacancies. The relaxation of the two anomalies on temperature dependent permittivity is greatly affected by Bi non-stoichiometry especially that around the first dielectric anomaly temperature that can be deconvoluted by phenomenological models. The materials display a remarkable stability of high ε′ (∼3000) over a wide temperature range (>320 °C) with considerably low loss (≤0.003) in the ergodic relaxor state. Significant improvement on energy storage properties as well as electrostrictive strain (Wrec = 0.86 J/cm3, η = 83%; S = 0.235%, strain hysteresis of 7.34% at a low electric field of 80 kV/cm for bipolar P-E loops) is achieved due to chemical disorder and strong random field caused by Bi3+ deficiency.

First-principles study of structural and vibrational properties of SrHfO3 compared to SrZrO3

The structural and vibrational properties of SrHfO3 compared to those of the SrZrO3 were investigated using first-principles calculations. The phonon dispersion curves, reported in the high-symmetry cubic perovskite phase, point out the coexistence of structural antiferrodistortive instabilities at the R and M zone-boundary points and a ferroelectric instability at the zone center. Different possible intermediate phases are characterized by comparing their internal energies, giving the same hierarchy of phases as for the SrZrO3. The main features of the ferroelectric instability are also discussed, and, contrary to what was found for SrZrO3, it seems very unlikely to induce the ferroelectricity in the SrHfO3Pnma ground state either by compressive or tensile epitaxial strain.

In situ investigation of stress-induced martensitic transformation in granular shape memory ceramic packings

Stress-induced martensitic transformation can occur in granular shape memory materials when individual particles experience high stresses and transform from a high-symmetry austenite phase to a low-symmetry martensite phase. This involves a highly heterogeneous distribution of the driving force and very low mechanical constraint for martensite nucleation, so the transformation behavior can be dramatically different from the well-documented case of monolithic solids. In this work, we investigate the stress-induced martensitic transformation in granular shape memory ceramic packings, which consist of single-crystal micro-particles of ZrO2-12 at%CeO2 and ZrO2-15 at%CeO2. Through in situ neutron diffraction, we study how the phase fraction, lattice strain, and integral peak broadness evolve during external loading, unloading, and subsequent heating. Several peculiar features are discovered, including (i) a continuous mode of transformation with a wide range of transformation loads, (ii) co-evolution of the packing structure, contact deformation, and martensitic transformation, and (iii) a strong correlation between the peak broadening and the transformed phase fraction. In addition, we show the first direct evidence of reversible stress-induced martensitic transformation in granular materials. We finally discuss the mechanism for martensite nucleation and growth in granular packings and show how that leads to the observed transformation characteristics.

High-voltage metal-insulator-metal capacitor based on crystalline HfAlOx film grown by atomic layer deposition

A metal–insulator–metal (MIM) capacitor was developed herein with an atomic layer deposition-fabricated hafnia aluminate (HfAlOx) dielectric layer. A preparation flow combining pre- and post-deposition treatment yielded a device with increased capacitance density and excellent dielectric integrity that can be employed in front-end and back-end of line implementations. With an equivalent oxide thickness scalable to 2 nm and beyond, the MIM capacitor can be implemented in either RF or analog/mixed applications with a functional voltage up to 3.3 V as a decoupling element, or for memory, bypass and coupling needs. Reduction of the equivalent oxide thickness was achieved by engineering the phase composition of the dielectric layer. A k-value of ∼30 was obtained via intentional crystallization of HfAlOx into a high-symmetry phase. The role of the bottom electrode (TiN) predeposition treatment in the dielectric layer crystallization process and, consequently, in the electrical performance of the MIM capacitor is emphasized.

Synthesis and characterization of ZrO2 and YSZ thin films

Abstract Zirconia-based ceramics possess a wide range of relevant applications due to its unique mechanical, electrical and optical properties. On the one hand, ZrO2 has a high mechanical and chemical resistance, which is suitable as protective coating. On the other hand, yttria-stabilized zirconia (YSZ) is one of the main electrolyte materials for solid oxide fuel cells (SOFC) devices due to its high ionic conductivity at high temperatures. In this work, ZrO2 and YSZ thin films synthesized by sol-gel method and deposited by dip coating is studied. ZrO2 films are synthesized as coatings for Zircaloy rod spacer grids used for critical heat flux tests, while YSZ films are synthesized for non-electrolyte supported SOFC. Morphological characterization of these films is performed by SEM, while TEM and XRD provide insight on the crystallographic properties. Electrical properties of these materials are measured by conductivity experiments. Highly dense films with good adherence to substrates and thickness above 200 nm are achieved by this method. The nano grain size allows to obtain high symmetry phase stabilization, which enhance the desired properties in both materials for the proposed applications.

Engineering SrSnO3 Phases and Electron Mobility at Room Temperature Using Epitaxial Strain.

High-speed electronics require epitaxial films with exceptionally high carrier mobility at room temperature (RT). Alkaline-earth stannates with high RT mobility show outstanding prospects for oxide electronics operating at ambient temperatures. However, despite significant progress over the last few years, mobility in stannate films has been limited by dislocations because of the inability to grow fully coherent films. Here, we demonstrate the growth of coherent, strain-engineered phases of epitaxial SrSnO3 (SSO) films using a radical-based molecular beam epitaxy approach. Compressive strain stabilized the high-symmetry tetragonal phase of SSO at RT, which, in bulk, exists only at temperatures between 1062 and 1295 K. We achieved a mobility enhancement of over 300% in doped films compared with the low-temperature orthorhombic polymorph. Using comprehensive temperature-dependent synchrotron-based X-ray measurements, electronic transport, and first principles calculations, crystal and electronic structures of SSO films were investigated as a function of strain. We argue that strain-engineered films of stannate will enable high mobility oxide electronics operating at RT with the added advantage of being optically transparent.

Inverse-direct magnetocaloric effect crossover in Ni47Mn40Sn12.5Cu0.5 Heusler alloy in cyclic magnetic fields

The results of direct studies of the magnetocaloric effect (MCE) of the Ni47Mn40Sn12.5Cu0.5 Heusler alloy in cyclic magnetic fields are presented. It is shown that in cyclic magnetic fields, the MCE value near the magnetostructural phase transition depends on the rate of temperature scanning. A higher rate of change in the sample temperature causes the MCE to have a greater value. The observed effect is explained by irreversible phase transition, induced by a magnetic field, from a low-symmetry martensitic phase to a high-symmetry austenitic phase. Moreover, it is observed only within the temperature hysteresis loop of the magnetostructural phase transition. Such materials, despite the considerable MCE values under a single application of the magnetic field, are unpromising due to a strong change in the magnetocaloric properties in cyclic magnetic fields.

Stabilizing n-Type Cubic GeSe by Entropy-Driven Alloying of AgBiSe2 : Ultralow Thermal Conductivity and Promising Thermoelectric Performance.

The realization of n-type Ge chalcogenides is elusive owing to intrinsic Ge vacancies that make them p-type semiconductors. GeSe crystallizes into a layered orthorhombic structure similar to SnSe at ambient conditions. The high-symmetry cubic phase of GeSe is predicted to be stabilized by applying 7 GPa external pressure or by enhancing the entropy by increasing to temperature to 920 K. Stabilization of the n-type cubic phase of GeSe at ambient conditions was achieved by alloying with AgBiSe2 (30-50 mol %), enhancing the entropy through solid solution mixing. The interplay of positive and negative chemical pressure anomalously changes the band gap of GeSe with increasing the AgBiSe2 concentration. The band gap of n-type cubic (GeSe)1-x (AgBiSe2 )x (0.30≤x≤0.50) has a value in the 0.3-0.4 eV range, which is significantly lower than orthorhombic GeSe (1.1 eV). Cubic (GeSe)1-x (AgBiSe2 )x exhibits an ultralow lattice thermal conductivity (κL ≈0.43 W m-1 K-1 ) in the 300-723 K range. The low κL is attributed to significant phonon scattering by entropy-driven enhanced solid-solution point defects.

Real-Time Observation of a Coherent Lattice Transformation into a High-Symmetry Phase

Excursions far from their equilibrium structures can bring crystalline solids through collective transformations including transitions into new phases that may be transient or long-lived. The direct spectroscopic observation of far-from-equilibrium rearrangements provides fundamental mechanistic insight into chemical and structural transformations and a potential route to practical applications, including ultrafast optical control over material structure and properties. However, in many cases, photoinduced transitions are irreversible or only slowly reversible, or the light fluence required exceeds material damage thresholds. This requirement precludes conventional ultrafast spectroscopy, in which optical excitation and probe pulses irradiate the sample many times, each measurement providing information about the sample response at just one probe delay time following excitation, with each measurement at a high repetition rate and with the sample fully recovering its initial state in between measurements. Using a single-shot, real-time measurement method, we are able to observe the photoinduced phase transition from the semimetallic, low-symmetry phase of crystalline bismuth into a high-symmetry phase whose existence at high electronic excitation densities is predicted based on earlier measurements at moderate excitation densities below the damage threshold. Our observations indicate that coherent lattice vibrational motion launched upon photoexcitation with an incident fluence above 10 mJ/cm2 in bulk bismuth brings the lattice structure directly into the high-symmetry configuration for several picoseconds, after which carrier relaxation and diffusion restore the equilibrium lattice configuration. Published by the American Physical Society 2018

Crystal and Magnetic Structures of La2CoPtO6 Double Perovskite.

We investigated the crystal structure and magnetic structure in cobaltite-platinate double perovskite of La2CoPtO6, employing various techniques of X-ray diffraction, neutron diffraction, and the extended X-ray absorption fine structure analysis. It is shown that the crystal symmetry is maintained as P21/n in the temperature range of <500 °C, whereas the lattice constants and the Co–Co distances undergo a continuous structural evolution toward the high-symmetry phases with increasing temperature. The Co–O bonds were overall longer and had a larger degree of structural and thermal disorders than the Pt–O bonds. As for the magnetism, an antiferromagnetic order is stabilized in the Co2+ sublattice at temperatures below 28 K. It is demonstrated that owing to the substantial distortions (quantified by a distortion parameter Σ > 0.03 Å) of the Co–Co networks, the system is not subject to spin frustration effect. Details in the magnetic structure are determined; at 12 K, the Co magnetic moment was (0.8, 0, 2.7) Bohr magneton, the magnetic propagation vector was (−0.5, 0, 0.5), and the magnetic symmetry was preferably Γ1(Ag).

Growth and ferroelectric properties of yttrium-doped hafnium oxide/indium-tin oxide polycrystalline heterostructures with sharp and uniform interfaces

We report on the growth of polycrystalline yttrium-5%-doped HfO2 (YHO) films with sharp and uniform interfaces over a large area on indium-tin oxide (ITO) layers and their ferroelectric properties. On substrates of thermally oxidized silicon, YHO/ITO bilayers were deposited in amorphous form by means of pulsed-laser deposition and were subsequently crystalized by two-step post-deposition annealing (PDA) at low and high temperatures. The crystallized YHO/ITO heterostructures had ultraflat surfaces: atomic-force-microscopy images exhibited a root-mean-square roughness of 0.15 nm over an area of 100 μm2. The roughness was much smaller than that of single-component polycrystalline films of ITO. X-ray diffraction and transmission electron microscopy (TEM) suggested the formation of YHO in higher symmetry phases including ferroelectric orthorhombic (o) YHO. TEM also confirmed crystallization of YHO layers in a pseudo-coherent manner from the YHO/ITO interfaces and resultant sharp and uniform YHO/ITO interfaces. These results are attributable to good lattice matching between the o-YHO layer and the ITO layer. It is suggested that this lattice matching was realized by the reduction of ITO during the crystallization in the two-step PDA process. The reduction might occur in the vicinity of the YHO/ITO interface via H2O incorporated in the amorphous bilayers. Pt/YHO/ITO junctions fabricated by using the two-step PDA method showed a polarization–electric field (P–E) hysteresis loop with remanent polarizations (Pr) of no less than 13 μC/cm2 at YHO thicknesses of 12 and 6 nm. These results indicate that the two-step PDA method can be used to fabricate high-quality YHO/ITO heterostructures in terms of surface and interface morphology, as well as ferroelectric properties.

Effect of a Rare-Earth Ion on the Structural Instability in RFe3(BO3)4 Crystals

The dynamics of the crystal lattice of RFe3(BO3)4 (R = Pr, Nd, Sm, Gd, Tb, Dy, and Ho) compounds in the high-symmetry R32 phase has been calculated. Significant changes in spectra of compounds with various rare-earth ions have been obtained only near the edge Λ point of the Brillouin zone (qΛ = 1/3(−2b1 + b2 + b3, where b1, b2, and b3 are the reciprocal lattice vectors) for acoustic oscillation branches. A decrease in the frequency of an acoustic mode at the point Λ has been revealed in all studied compounds. This frequency depends on the type of rare-earth ion and decreases from a compound with Pr to a compound with Ho down to imaginary values. Such a behavior of the frequency of the unstable acoustic mode is in good agreement with experimental data on the dependence of the temperature of the R32 → P3121 structural phase transition on the type of rare-earth ion in ferroborates.

Ferroelectric yttrium doped hafnium oxide films from all-inorganic aqueous precursor solution

We report a unique aqueous solution deposition method to prepare yttrium doped hafnium oxide (Y:HfO2) thin films using all-inorganic reagents. The composition and chemical bonding features of the films were investigated using X-ray photoelectron spectroscopy. The Y:HfO2 film was integrated into metal-insulator-semiconductor (MIS) structure capacitors for electrical measurements. A transition of the polarization behavior from apparent ferroelectric-type to linear dielectric-type was observed for films with thickness increasing from 25 nm to 80 nm, which is correlated to the dominant crystal structure change from high-symmetry phase to monoclinic phase evidenced by grazing incidence X-ray diffraction analysis.

Octahedral tilting instabilities in inorganic halide perovskites

Dynamic instabilities, stabilized by anharmonic interactions in cubic and tetragonal halide perovskites at high temperature, play a role in the electronic structure and optoelectronic properties of halide perovskites. In particular, inorganic and hybrid perovskite materials undergo structural phase transitions associated with octahedral tilts of the metal-halide octahedra. We investigate the structural instabilities present in inorganic $\mathrm{Cs}M{X}_{3}$ perovskites with Pb or Sn on the metal site and Br or I on the $X$ site. Defining primary order parameters in terms of symmetry-adapted collective displacement modes and secondary order parameters in terms of symmetrized Hencky strain components, we unravel the coupling between octahedral tilt modes and macroscopic strains as well as the role of $A$-site displacements in perovskite phase stability. Symmetry-allowed secondary strain order parameters are enumerated for the 14 unique perovskite tilt systems. Using first-principles calculations to explore the Born-Oppenheimer energy surface in terms of symmetrized order parameters, we find coupling between octahedral tilting and $A$-site displacements is necessary to stabilize $Pnma$ ground states. Additionally, we show that the relative stability of an inorganic halide perovskite tilt system correlates with the volume decrease from the high-symmetry cubic phase to the low-symmetry distorted phase.

Nature of the octahedral tilting phase transitions in perovskites: A case study of CaMnO3

The temperature-induced antiferrodistortive (AFD) structural phase transitions in ${\mathrm{CaMnO}}_{3}$, a typical perovskite oxide, are studied using first-principles density functional theory calculations. These transitions are caused by tilting of the ${\mathrm{MnO}}_{6}$ octahedra that are related to unstable phonon modes in the high-symmetry cubic perovskite phase. Transitions due to octahedral tilting in perovskites normally are believed to fit into the standard soft-mode picture of displacive phase transitions. We calculate phonon-dispersion relations and potential-energy landscapes as functions of the unstable phonon modes and argue based on the results that the phase transitions are better described as being of order-disorder type. This means that the cubic phase emerges as a dynamical average when the system hops between local minima on the potential-energy surface. We then perform ab initio molecular dynamics simulations and find explicit evidence of the order-disorder dynamics in the system. Our conclusions are expected to be valid for other perovskite oxides, and we finally suggest how to predict the nature (displacive or order-disorder) of the AFD phase transitions in any perovskite system.

Understanding the formation of the metastable ferroelectric phase in hafnia-zirconia solid solution thin films.

Hf1-xZrxO2 (x ∼ 0.5-0.7) has been the leading candidate of ferroelectric materials with a fluorite crystal structure showing highly promising compatibility with complementary metal oxide semiconductor devices. Despite the notable improvement in device performance and processing techniques, the origin of its ferroelectric crystalline phase (space group: Pca21) formation has not been clearly elucidated. Several recent experimental and theoretical studies evidently showed that the interface and grain boundary energies of the higher symmetry phases (orthorhombic and tetragonal) contribute to the stabilization of the metastable non-centrosymmetric orthorhombic phase or tetragonal phase. However, there was a clear quantitative discrepancy between the theoretical expectation and experiment results, suggesting that the thermodynamic model may not provide the full explanation. This work, therefore, focuses on the phase transition kinetics during the cooling step after the crystallization annealing. It was found that the large activation barrier for the transition from the tetragonal/orthorhombic to the monoclinic phase, which is the stable phase at room temperature, suppresses the phase transition, and thus, plays a critical role in the emergence of ferroelectricity.

Stable Ferroelectric Behavior of Nb-Modified Bi0.5K0.5TiO3-Bi(Mg0.5Ti0.5)O3 Lead-Free Relaxor Ferroelectric Ceramics

Crystal structure, dielectric, ferroelectric, piezoelectric, and electric field-induced strain properties of lead-free Nb-modified 0.96Bi0.5K0.5TiO3-0.04Bi(Mg0.5Ti0.5)O3 (BKT-BMT) piezoelectric ceramics were investigated. Crystal structure analysis showed a gradual phase transition from tetragonal to pseudocubic phase with increasing Nb content. The optimal piezoelectric property of small-signal d 33 was enhanced up to ∼ 68 pC/N with a lower coercive field (E c) of ∼ 22 kV/cm and an improved remnant polarization (P r) of ∼ 13 μC/cm2 for x = 0.020. A relaxor-like behavior with a frequency-dependent Curie temperature T m was observed, and a high T m around 320°C was obtained in the investigated system. This study suggests that the ferroelectric properties of BKT-BMT was significantly improved by means of Nb substitution. The possible shift of depolarization temperature T d toward high temperature T m may have triggered the spontaneous relaxor to ferroelectric phase transition with long-range ferroelectric order without any traces of a nonergodic relaxor state in contradiction with Bi0.5Na0.5TiO3-based systems. The possible enhancement in ferroelectric and piezoelectric properties near the critical composition x = 0.020 may be attributed to the increased anharmonicity of lattice vibrations which may facilitate the observed phase transition from a low-symmetry tetragonal to a high-symmetry cubic phase with a decrease in the lattice anisotropy of an undoped sample. This highly flexible (at a unit cell level) narrow compositional range triggers the enhancement of d 33 and P r values.

Coupling between a Charge Density Wave and Magnetism in an Heusler Material.

The prototypical magnetic memory shape alloy Ni_{2}MnGa undergoes various phase transitions as a function of the temperature, pressure, and doping. In the low-temperature phases below 260K, an incommensurate structural modulation occurs along the [110] direction which is thought to arise from the softening of a phonon mode. It is not at present clear how this phenomenon is related, if at all, to the magnetic memory effect. Here we report time-resolved measurements which track both the structural and magnetic components of the phase transition from the modulated cubic phase as it is brought into the high-symmetry phase. The results suggest that the photoinduced demagnetization modifies the Fermi surface in regions that couple strongly to the periodicity of the structural modulation through the nesting vector. The amplitude of the periodic lattice distortion, however, appears to be less affected by the demagnetization.