O3 Single Crystals Research Articles

Direct determination of a topmost atomic layer of (100) surfaces on (La0.3Sr0.7)(Al0.65Ta0.35)O3 single crystals annealed in air by high‐resolution scanning transmission electron microscopy

Herein, we investigated the surface of an (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT) single crystal via scanning transmission electron microscopy (STEM) after annealing in air. We found amorphous mounds and atomically flat surfaces formed on the surface of the (100) LSAT single crystal subsequent to heat treatment. These surface mounds mainly constituted SrO, including ~50 at.% Al. The atomically flat surface showed a B‐site termination determined by the direct surface observation at the atomic scale through HAADF‐STEM. The atomic arrangements on the surface were accommodated by surface‐mound formation and La evaporation. Consequently, the ordered structure disappeared because of the increase in the Ta content at the terminated B‐site layer.

Homogenous Sn-doped K(Ta,Nb)O3 single crystals and its high piezoelectric response

Perovskite K(Ta,Nb)O3 (KTN) single crystal has drawn great interests for its outstanding electro-optic performance and excellent piezoelectric response. However, growth of compositionally uniform KTN single crystals has always been a great challenge for the great segregation difference between Nb and Ta. In this work, we propose a thermal field optimization strategy to resolve this challenge. Homogenous Sn doped KTN (Sn:KTN) single crystal with significantly reduced composition gradient (0.003 mol/mm, 1/4–1/8 of other KTN system), minimal TC variation (13 °C) and excellent piezoelectric and dielectric response (d33 = 373 pC/N and ε33T = 5206) has been successfully achieved. We found that the functional properties of Sn:KTN were greatly affected by the near-room temperature tetragonal-cubic phase transition. From the intrinsic aspect, longitudinal lattice deformation becomes much easier, resulting in maximum piezoelectric (d33∗), dielectric (ε33T∗), elastic (s33E∗)and electromechanical coupling (k33∗) coefficients along polar direction [001]C. From the extrinsic aspect, both domain wall density and domain wall mobility are greatly improved for the reduced lattice distortion, which also contribute a lot to the functional properties. This work provides a simple and practical route for designing and growing high quality crystals, and more importantly, reveals the fundamental mechanism of the phase transitions/boundaries on the functional properties.

Thermodynamic test of Li2WO4 single crystals with low molybdenum doping: Enthalpy, interconnections, heat capacity

The thermodynamic test of Li2WO4 single crystals doping by low molybdenum content with special emphasis on investigation of interconnections, enthalpy, heat capacity to improve device portfolio was performed. We reduced doping content in 7 times for Li2W1-xMoxO4 in this investigation. The formation enthalpy of Li2WO4 was estimated using interconnections. The heat capacity was measured for Li2W1-0.025Mo0.025O4 single crystal (T = 320–997 K) by differential scanning calorimetry. There were no phase transitions of Li2W1-0.025Mo0.025O4 single crystal that made single crystals with low doping by molybdenum promising for application. The single crystal of lithium tungstate doped by 1.25% molybdenum (content was Li2W0.9875Mo0.0125O4) was grown by LTG CZ for the first time. The reaction enthalpies of Li2W0.9875Mo0.0125O4 single crystal were investigated by reaction calorimetry. The interconnections of formation enthalpies and lattice enthalpies of Li2W1-xMoxO4 in the range (x = 1–0.0125) from x were established. It indicated the growth of phenakite structure in the range. It was shown that under conditions of low temperature gradients it was possible to grow perfect single crystals of lithium tungstate with low doping of molybdenum.

Investigation of the dielectric relaxation mechanisms for Pb(Fe1/2Nb1/2)O3 single crystal based on the universal relaxation law

Large-sized multiferroics Pb(Fe1/2Nb1/2)O3 (PFN) single crystal was grown by Bridgeman method in order to understand the giant dielectric response by investigating the electrical conductivity and dielectric properties. The synchrotron radiation soft x-ray absorption spectrum revealed the valence states of Fe in PFN crystal. PFN exhibits semiconductor behaviors in the thermal active electrical conductivity investigation. Besides, the frequency-dependent dielectric constants were measured in the frequency range from 100 Hz to 1 MHz to study the dielectric relaxation behaviors. Two relaxation processes were found and described by Havriliak-Negami equation. The origin of dielectric relaxation in PFN with semiconductor characters was discussed based on the universal relaxation law for a dipolar system.

The complexity of structural phase transitions in Pb(Hf0.92Sn0.08)O3 single crystals

AbstractPb(Hf1−xSnx)O3 single crystals with x = 0.08 were characterized using single‐crystal X‐ray diffraction and Raman scattering, in a wide temperature range. The information concerning the structure of two intermediate phases (IMs), situated between low‐temperature antiferroelectric A1 and high‐temperature paraelectric PE phases, was obtained. The lower temperature IM, A2, is characterized by incommensurate displacive modulations in the Pb sublattice. The higher temperature IM, is characterized by tilting of oxygen octahedra, and serious disorder coming from lead ions represented by X‐ray diffuse scattering. Optical phonons and phase transitions in Pb(Hf1−xSnx)O3 single crystals were investigated by temperature‐dependent Raman spectra. It was found that several soft modes control the phase transition between two antiferroelectric phases indicating its displacive character, whereas, in the paraelectric phase, both soft modes and Rayleigh scattering were observed.

Strong piezoelectric properties and electric-field-driven changes in domain structures in a PbZr0.87Ti0.13O3 single crystal

PbZr1-xTixO3 perovskite materials, known as PZT, are highly applicable, mainly due to their extraordinary piezoelectric properties. Recently, a technology of PZT crystals growth has been elaborated and opened new investigation possibilities for these compounds. In this paper, we demonstrate a highly piezoelectric response in PbZr0.87Ti0.13O3 single crystal. Under the action of an electric field, the piezoelectric coefficient d33 turns out to be over 2500 pm V-1. The optical studies performed have proved a significant influence of the domain dynamics on such high-efficiency piezoelectric response. Monoclinic M and rhombohedral R phases have been observed in the lead zirconate-titanate crystals with a low Ti content. This coexistence of phases with different symmetries and dense domain walls are the primary sources of high piezoelectric response. We have observed that strong electric fields may act on randomly oriented polarization vectors so that the total piezoelectric activity disappears, and the unusual isotropization point near 250 °C appears.

The ‘dark phase’ in Sr2Ir1−x Rh x O4 revealed by Seebeck and Hall measurements

It was found that, although isovalent, Rh substituted for Ir in Sr2IrO4 may trap one electron inducing effective hole doping of Ir sites. Transport and thermoelectric measurements on Sr2Ir1−x Rh x O4 single crystals presented here reveal the existence of an electron-like contribution to transport, in addition to the hole-doped one. As no electron band shows up in ARPES measurements, this points to the possibility that this hidden electron may delocalize in disordered clusters.

Heat capacity of lithium tungsten oxide doped by molybdenum (Li2W0.9Mo0.1O4) in the temperature range 319–910 K by DSC

For the first time the heat capacity of Li2W0.9Mo0.1O4 single crystals in the temperature range 319–910 K was studied and measured by differential scanning calorimetry. The temperature dependence of the heat capacity has been well described by a polynomial Cop,m (T) = 41.253 + 0.41785⋅T - 4.6786⋅10−4⋅T2 + 1.9945⋅10−7⋅T3. On the basis of smoothed heat capacity the enthalpy and entropy increments have been calculated in the temperature range 319–910 K. No phase transitions were observed throughout the temperature range.

Ferroelastic domain hierarchy in the intermediate state of PbZr0.98Ti0.02O3 single crystal

PbZrO3-based antiferroelectric crystals are of great interest in both fundamental and applied research, not only because of the antiferroelectric feature at room temperature but also because of the existence of a peculiar intermediate state at elevated temperatures. Here, we report a detailed description of domain structure change at the temperature-induced antiferroelectric-to-ferroelectric phase transition. A complex process of different types of domains is revealed to appear at different stages of the phase transition. A hierarchical ferroelastic domain structure forms in the stabilized intermediate state, where the dense domain walls show potential impact on the physical properties of the crystal.

Incommensurately modulated structures in Pb(Zr1−xSnx)O3 single crystals by x-ray diffraction

Pb(Zr1−xSnx)O3 single crystals were characterized using x-ray diffraction as a function of temperature and composition. The information on the structure of two intermediate phases, situated between antiferroelectric and paraelectric phases in the composition–temperature phase diagram, has been obtained. The lower-temperature intermediate phase is characterized by incommensurate displacive modulations in the Pb sublattice. The higher temperature intermediate phase is characterized by distortions of the oxygen sublattice primarily in the form of anti-phase tilts of the oxygen octahedra. The structured diffuse scattering was also observed in the paraelectric phase along Γ–M and M–R lines pointing to the high-temperature correlated disorder of Pb ions and octahedral rotations, respectively.

Formation enthalpy and thermodynamic functions of Li2W1-xMoxO4 single crystals (x = 0.025 and 0.05)

A single crystal of lithium tungstate doped by 2.5% molybdenum was grown by low-temperature-gradient Czochralski technique (LTG CZ) with weight control for the first time. The solution enthalpy of Li2W0.975Mo0.025O4 was measured by solution calorimetry in aqueous KOH solvent. Based on experimental data obtained and reference values, the standard formation enthalpy of Li2W0.975Mo0.025O4 single crystal was calculated as following: ΔfH0 (Li2W0.975Mo0.025O4, s, 298.15 K) = − 1599.77 ± 1.88 kJ mol−1. The dependence of lattice enthalpy for Li2W1-xMoxO4 single crystals on tolerance factor for x = 1; 0.15; 0.1; 0.05; 0.025 was constructed. It was shown that dependence of lattice enthalpy on tolerance factor was linear. For low doping of molybdenum, a relation that connected the lattice energy in Li2WO4-Li2MoO4 system with molybdenum content was derived as following: U = A + B · xMo · rMo. The heat capacity of Li2W0.95Mo0.05O4 single crystal was measured in two temperature ranges: (193–370) K; (320–915) K. Heat capacity in both temperature ranges was well described by cubic polynomials. It was shown that there were no phase transitions in both temperature ranges. It is important from a practical point of view for promising application of investigated compounds.

Complex structure and Mössbauer effect observed in the course of phase transitions in PbZr0.72Sn0.28O3 single crystal.

The Mössbauer effect was used to study the structural transitions in a PbZr0.72Sn0.28O3 single crystal. Two kinds of quadrupole splittings were registered and connected with two different environments of the Sn ion occupying the center of SnO6 octahedra. What is responsible for the existence of these two environments is the disorder in tilts of oxygen octahedra and antiparallel shifts of pairs of lead ions. Both disorders decide on structural transitions which the PbZr0.72Sn0.28O3 single crystal undergoes. The two kinds of quadruple splittings that have been observed do not disappear at temperatures far above phase transitions. This indicates that the structure of the paraelectric phase is locally non-centrosymmetric and confirms pre-transitional effects previously reported for Sn doped PbZrO3 single crystals.

Multiple Vacancies on (111) Facets of Single-Crystal NiFe2 O4 Spinel Boost Electrocatalytic Oxygen Evolution Reaction.

Oxygen evolution reaction (OER) as the rate-determining reaction of water splitting has been attracting enormous attention. At present, only some noble-metal oxide materials (IrO2 and RuO2 ) have been reported as efficient OER electrocatalysts for OER. However, the high cost and scarcity of these noble-metal oxide materials greatly hamper their large-scale practical application. Herein, we synthesize 100% (111) faceted NiFe2 O4 single crystals with multiple vacancies (cation vacancies and O vacancies). The (111) facets can supply enough platform to break chemical bonds and enhance electrocatalytic activity, due to its high density of atomic steps and kink atoms. Compared to NiFe2 O4 (without vacancies), the as-synthesized NiFe2 O4 -Ar (with vacancies) exhibits a dramatically improved OER activity. The NiFe2 O4 -Ar-30 shows the lowest onset potential (1.45 V vs RHE) and the best electrocatalytic OER activity with the lowest overpotential of 234 mV at 50 mA cm-2 . Furthermore, based on the theoretical calculations that the introduction of multiple vacancies can effectively modulate the electronic structure of active centers to accelerate charge transfer and reaction intermediates adsorption,which can reduce the reaction energy barrier and enhance the activity of electrochemical OER.

Surface-Dependent Activation of Model α-Al2 O3 -Supported P-Doped Hydrotreating Catalysts Prepared by Spin Coating.

Requirements for improved catalytic formulations is continuously driving research in hydrotreating (HDT) catalysis for biomass upgrading and heteroatom removal for cleaner fuels. The present work proposes a surface-science approach for the understanding of the genesis of the active (sulfide) phase in model P-doped MoS2 hydrotreating catalysts supported on α-Al2 O3 single crystals. This approach allows one to obtain a surface-dependent insight by varying the crystal orientations of the support. Model phosphorus-doped catalysts are prepared via spin-coating of Mo-P precursor solutions onto four α-Al2 O3 crystal orientations, C(0001), A(11 0), M(10 0) and R(1 02) that exhibit different speciations of surface -OH. 31 P and 95 Mo liquid-state NMR are used to give a comprehensive description of the Mo and P speciation of the phospho-molybdic precursor solution. The speciation of the deposition solution is then correlated with the genesis of the active MoS2 phase. XPS quantification of the surface P/Mo ratio reveal a surface-dependent phosphate aggregation driven by the amount of free phosphates in solution. Phosphates aggregation decreases in the following order C(0001)≫M(10 0)>A(11 0), R(1 02). This evolution can be rationalized by an increasing strength of phosphate/surface interactions on the different α-Al2 O3 surface orientations from the C(0001) to the R(1 02) plane. Retardation of the sulfidation with temperature is observed for model catalysts with the highest phosphate dispersion on the surface (A(11 0), R(1 02)), suggesting that phosphorus strongly intervene in the genesis of the active phase through a close intimacy between phosphates and molybdates. The surface P/Mo ratio appears as a key descriptor to quantify this retarding effect. It is proposed that retardation of sulfidation is driven by two effects: i) a chemical inhibition through formation of hardly reducible mixed molybdo-phosphate structures and ii) a physical inhibition with phosphate clusters inhibiting the growth of MoS2 . The surface-dependent phosphorus doping on model α-Al2 O3 supports can be used as a guide for the rational design of more efficient HDT catalysts on industrial γ-Al2 O3 carrier.

Improving strain in single crystal by composition-gradients design

Domain switching is one of the essential contributions for strain in ferroelectric materials. In this work, we utilize anisotropic composition gradients (CGs) to induce anisotropic orientation of both defects and spontaneous dipoles, aiming to enhance the contribution of domain switching on strain in KTa1–xNbxO3 single crystal. In this way, a remarkable improvement (over 60%) of strain is obtained in KTa0.58Nb0.42O3 single crystal along the smallest CG direction, achieving a large unipolar strain, i.e. 0.29%, at low driving electric field 10 kV cm–1. It is attributed to the preferred orientation of both defects and spontaneous dipoles along the largest CG directions, then improving the contribution of domain switching on strain along the smallest CG direction. Particularly, owing to the existence of Ein as recoverable forces caused by both flexoelectric fields and defects pinning effect, KTN show nearly zero remnant strain (srem) along with the small CG directions, corresponding to the double P-E loops. Moreover, the V–PFM images confirm that CGs can influence the microdomain structures. Thus, designing special anisotropic CGs materials is expected to be a novel method to improve the strain properties and a potential way to flexibly design next-generation anisotropic piezoelectric materials.

Structure of Pb(Fe2/3W1/3)O3 single crystals with partial cation order

Despite intensive studies on the complex perovskite Pb(Fe2/3W1/3)O3 (PFWO) relaxor, understanding the exact nature of its multifunctional properties has remained a challenge for decades. In this work we report a comprehensive structural study of the PFWO single crystals using a combination of synchrotron X-ray diffraction and high-resolution electron microscopy. The set of {h + ½, k + ½, l + ½} superlattice reflections was observed for the first time based on single-crystal synchrotron X-ray experiments (100–450 K) and transmission electron microscopy investigations, which indicates some kind of B-cation ordering in PFWO which had been thought to be totally disordered. It was found that (1) the crystal structure of PFWO should be described by a partly ordered cubic perovskite (i.e. Fm − 3m), (2) the weak ferromagnetic properties and excess magnetic moment of PFWO can be understood based on non-random distribution of Fe cations between the 4a and 4b sites, and (3) the Pb displacement disorder is present in this material and the cations are probably displaced along the <100> directions. The X-ray diffraction results of this investigation show that partial cation ordering indeed exists in PFWO, which makes it necessary to revisit the generally accepted interpretations of the results obtained up to date. In agreement with X-ray diffraction study the main results of TEM study include: (1) a long range order that can be described with the Fm − 3m symmetry is reliably detected, (2) the coherence length of that long range order is in the order of 1–2 nm and (3) no remarkable chemical inhomogeneity is found in the tested PFWO crystal, excluding the possibility of a compositional ordering arising from substitutional defects in the perovskite structure.

Intrinsic piezoelectricity in (K,Na)NbO3-based lead-free single crystal: Piezoelectric anisotropy and its evolution with temperature

Searching for lead-free piezoelectric materials with a large piezoelectricity and excellent thermal stability has been a major concern in both scientific research and practical applications. To understand the mechanism of high piezoelectricity and its temperature-dependent behavior in lead-free materials, we focus here on the tetragonal (K,Na,Li)(Nb,Ta)O3 single crystal and investigate the intrinsic d33* along arbitrary directions as well as its evolution with temperature. The synergistic influence of several factors (narrow tetragonal temperature interval, sharp tetragonal-cubic phase transition, and large PS below TC) leads to a strong anisotropy in the d33* profile, while a high d33* is obtained around θ = 45° over a wide temperature range. This work comprehensively reveals the physical mechanism of piezoelectric anisotropy in lead-free materials, which provides vital information to design high-performance lead-free piezoelectric materials through orientation engineering and lattice manipulation, which is expected to benefit a wide range of piezoelectric materials.

Local properties and phase transitions in Sn doped antiferroelectric PbHfO3 single crystal

Pb(Hf1−xSnx)O3 single crystals with x = 0.23 were characterized using single-crystal x-ray diffraction in the wide temperature range. The information on the structure of two intermediate phases, situated between low temperature antiferroelectric and high temperature paraelectric phases, has been obtained. The lower-temperature intermediate AFE2 phase is characterized by incommensurate displacive modulations in the Pb sublattice. The higher temperature intermediate IM phase is characterized by rotations of oxygen octahedra, primarily in the form of anti-phase tilts, which are also present in the lower-temperature AFE2 phase. For the same crystal, 119Sn Mossbauer effect in the temperature range from 300 K to 600 K has been used to study phase transitions mechanism. Two kinds of quadruple splitting have been found. It implies that two different environments of the central Sn ion exist. The observed two kinds of quadruple splitting do not disappear in the whole investigated temperature range which confirm that even far above TC the structure of paraelectric phase is locally non-centrosymmetric.

Temperature–field phase diagram and spontaneous polarization in Pb(Sc1/2Ta1/2)O3 single crystal

The temperature–field phase diagram and the temperature dependence of the spontaneous polarization in Pb(Sc1/2Ta1/2)O3 (PST) have been investigated by measuring the permittivity in the dc biasing field and observing the p–E hysteresis loop, respectively. X-ray diffraction studies to clarify the B-site cation ordering have also been performed. We have confirmed the PST single crystal used in our experiment to be a disordered PST. We have also determined that the ferroelectric critical endpoint (CEP) on the temperature–field phase diagram in the dc biasing field applied along the [111]c direction (in the cubic coordinate) was at 19.3 °C and 4.85 kV cm−1. Our experimental results of the ferroelectric CEP and the spontaneous polarization in disordered PST are discussed in this paper on the basis of the Landau-type phenomenological theory.

Local Structure Analysis around Ti in Lead Zirconate Titanate by X‐Ray Fluorescence Holography

To clarify the mechanism of ferroelectricity in lead zirconate titanate (PZT), the local structure around Ti in a Pb(Zr0.7Ti0.3)O3 single crystal is investigated using X‐ray fluorescence holography. The raw hologram data are contaminated by periodic noise due to the fluctuation of the incident X‐ray intensity. The periodical noise is successfully removed using an inverse Fourier transformation method. From the filtered holograms, the atomic images around Ti can be reconstructed clearly. The reconstructions of B‐site cations demonstrate an outward displacement of the neighboring Ti from the ideal B‐site position, indicating an expansion of the local lattice around Ti. In addition, the splitting of Pb images is found, and the relative displacement between Ti and Pb atoms is deduced. These results contribute to the understanding of the local structure of PZT and can reveal the microscopic origins of very high dielectric and piezoelectric responses, which are essential for designing new and improved ferroelectric materials.