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Showing papers on "Orthorhombic crystal system published in 2020"


Journal ArticleDOI
TL;DR: In this paper, the perovskite structure of CSPbI3 nanocrystals with narrow size distributions was studied and the size-dependent properties of the nanostructures were investigated.
Abstract: CsPbI3 nanocrystals with narrow size distributions were prepared to study the size-dependent properties. The nanocrystals adopt the perovskite (over the nonperovskite orthorhombic) structure with i...

160 citations


Journal ArticleDOI
TL;DR: In this article, a lead-free antiferroelectric (AFE) ceramics have attracted increasing attention in recent years for application in high-power capacitors owing to both environmental friendliness and high energy density.
Abstract: Lead-free antiferroelectric (AFE) ceramics have attracted increasing attention in recent years for application in high-power capacitors owing to both environmental friendliness and high energy density. However, the relevant research progress has been seriously restricted by the limited amount of AFE candidate materials with low cost and excellent properties, which significantly rely on the AFE phase stability and crystal symmetry. In this work, NaNbO3–Bi(Mg0.5Ti0.5)O3 (NN–BMT) perovskite solid solutions were reported to obviously exhibit AFE phase structure dependent energy-storage performances, evolving from Wrec ∼ 1.08 J cm−3 and η ∼ 19% at x = 0.05 with an orthorhombic P phase (Pbam) under 25 kV mm−1 to 3.12 J cm−3 and 74%, respectively, at x = 0.08 with an orthorhombic R phase (Pnma) under 30 kV mm−1 owing to the transition of square-like double hysteresis loops into slim and double-like ones and the increased testable electric fields. Most interestingly, doping 0.5 mol% transition-metal oxides (CuO, CeO2 and MnO2) was found to evidently improve the sintering behaviour, bulk resistivity and defect structure, thus leading to largely enhanced dielectric breakdown strength. In particular, the MnO2 doped 0.92NN–0.08BMT sample exhibits a large Wrec of ∼ 5.57 J cm−3 and a high η of ∼ 71% as well as excellent charge–discharge performance (CD = 636.7 A cm−2, PD = 63.7 MW cm−3 and t0.9 ∼ 85 ns), determined by means of the detailed analysis of the grain size distribution, impedance and X-ray photoelectron spectra. The results demonstrate that NN–BMT bulk ceramics could be very competitive lead-free AFE materials for energy-storage capacitors in pulsed power devices.

153 citations


Journal ArticleDOI
TL;DR: The results exemplify the feasibility to tailor the active lattice oxygen of perovskite by modulating the distortion of BO6 in ABO3, which ultimately influences their reaction performance in chemical looping processes.
Abstract: Modulating lattice oxygen in metal oxides that conducts partial oxidation of methane in balancing C-H activation and syngas selectivity remains challenging. This paper describes the discovery of distorting FeO6 octahedra in La1-xCexFeO3 (x = 0, 0.25 0.5, 0.75, 1) orthorhombic perovskites for the promotion of lattice oxygen activation. By combined electrical conductivity relaxation measurements and density functional theory calculations studies, this paper describes the enhancement of FeO6 octahedral distortion in La1-xCexFeO3 promoting their bulk oxygen mobility and surface oxygen exchange capability. Consequently, La0.5Ce0.5FeO3 with the highest FeO6 distortion achieves exceptional syngas productivity of ∼3 and 8 times higher than LaFeO3 and CeFeO3, respectively, in CH4 partial oxidation step with simultaneous high CO2 conversion (92%) in the CO2-splitting step at 850 °C. The results exemplify the feasibility to tailor the active lattice oxygen of perovskite by modulating the distortion of BO6 in ABO3, which ultimately influences their reaction performance in chemical looping processes.

152 citations


Journal ArticleDOI
24 Mar 2020
TL;DR: D density functional theory was used to determine the structural, electronic, and optical properties of the cubic, tetragonal, and orthorhombic temperature-dependent phases of CsPbBr3 perovskite using the full-potential linear augmented plane wave method.
Abstract: Cesium lead bromide (CsPbBr3) perovskite has recently gained significance owing to its rapidly increasing performance when used for light-emitting devices. In this study, we used density functional...

87 citations


Journal ArticleDOI
TL;DR: In this paper, the first high-entropy photocatalyst is synthesized by mechanical alloying via the high-pressure torsion (HPT) method followed by high-temperature oxidation.
Abstract: The introduction of high-entropy oxides (HEOs), i.e. compounds containing oxygen and five or more cations in their crystal structure, has led to interesting functional properties in recent years. In this study, the first high-entropy photocatalyst is synthesized by mechanical alloying via the high-pressure torsion (HPT) method followed by high-temperature oxidation. The synthesized oxide contains 60 mol% of AB2O7 monoclinic perovskite and 40 mol% of A6B2O17 orthorhombic perovskite, where A represents Ti, Zr and Hf and B represents Nb and Ta. This two-phase oxide with an overall composition of TiHfZrNbTaO11 and a d0 electronic configuration shows an appreciable light absorbance in the visible-light region with a bandgap of 2.9 eV and appropriate valence and conduction bands for water splitting. The material successfully produces hydrogen by photocatalytic water splitting, suggesting the potential of HEOs as new low-bandgap photocatalysts.

84 citations


Journal ArticleDOI
TL;DR: Insight is provided into the role of electrodes on the performance of hafnium oxide-based ferroelectrics, mechanisms driving wake-up and fatigue, and a non-destructive means to characterize the phase changes accompanying polarization instabilities are demonstrated.
Abstract: Ferroelectric hafnium zirconium oxide holds great promise for a broad spectrum of complementary metal-oxide-semiconductor (CMOS) compatible and scaled microelectronic applications, including memory, low-voltage transistors, and infrared sensors, among others. An outstanding challenge hindering the implementation of this material is polarization instability during field cycling. In this study, the nanoscale phenomena contributing to both polarization fatigue and wake-up are reported. Using synchrotron X-ray diffraction, the conversion of non-polar tetragonal and polar orthorhombic phases to a non-polar monoclinic phase while field cycling devices comprising noble metal contacts is observed. This phase exchange accompanies a diminishing ferroelectric remanent polarization and provides device-scale crystallographic evidence of phase exchange leading to ferroelectric fatigue in these structures. A reduction in the full width at half-maximum of the superimposed tetragonal (101) and orthorhombic (111) diffraction reflections is observed to accompany wake-up in structures comprising tantalum nitride and tungsten electrodes. Combined with polarization and relative permittivity measurements, the observed peak narrowing and a shift in position to lower angles is attributed, in part, to a phase exchange of the non-polar tetragonal to the polar orthorhombic phase during wake-up. These results provide insight into the role of electrodes in the performance of hafnium oxide-based ferroelectrics and mechanisms driving wake-up and fatigue, and demonstrate a non-destructive means to characterize the phase changes accompanying polarization instabilities.

70 citations


Journal ArticleDOI
TL;DR: In this paper, the Gaussian process regression model is developed to find statistical correlations between T C and lattice parameters among lanthanum manganites, and nearly 100 lattices, cubic, pseudocubic, orthorhombic, and rhombohedral, with T C ranging from 40'K to 375'K are explored for this purpose.

69 citations


Journal ArticleDOI
TL;DR: Repetitive electrical pulse stimulation of blue-phase liquid crystals promotes their reconfiguration into stable non-cubic structures with promising electro-optical responses for display technologies.
Abstract: Natural self-assembled three-dimensional photonic crystals such as blue-phase liquid crystals typically assume cubic lattice structures. Nonetheless, blue-phase liquid crystals with distinct crystal symmetries and thus band structures will be advantageous for optical applications. Here we use repetitive electrical pulses to reconfigure blue-phase liquid crystals into stable orthorhombic and tetragonal lattices. This approach, termed repetitively applied field, allows the system to relax between each pulse, gradually transforming the initial cubic lattice into various intermediate metastable states until a stable non-cubic crystal is achieved. We show that this technique is suitable for engineering non-cubic lattices with tailored photonic bandgaps, associated dispersion and band structure across the entire visible spectrum in blue-phase liquid crystals with distinct composition and initial crystal orientation. These field-free blue-phase liquid crystals exhibit large electro-optic responses and can be polymer-stabilized to have a wide operating temperature range and submillisecond response speed, which are promising properties for information display, electro-optics, nonlinear optics, microlasers and biosensing applications.

68 citations


Journal ArticleDOI
TL;DR: In this paper, the authors reveal the coexistence of a metastable and the main orthorhombic crystal structure in stoichiometric Ag2Se, and the formation of the metastable structure was found to be detrimental to the transport properties.
Abstract: Ag2Se is considered as an attractive candidate for use in room-temperature thermoelectric applications owing to its unique transport properties, such as glass-like thermal conductivity and good electrical conductivity. However, understanding the correlation between composition (Ag/Se ratio), defect structure, and transport properties is an important prerequisite to optimize its figure of merit (ZT). Using in-depth microscopic analysis, this study reveals the coexistence of a metastable and the main orthorhombic crystal structure in stoichiometric Ag2Se. The formation of the metastable structure was found to be detrimental to the transport properties of bulk Ag2Se. We were able to successfully inhibit its formation and stabilize the main orthorhombic structure via small anion (Se and S) excess. The compositions Ag2SeChy (y ≤ 0.01; Ch = Se, S) yielded 40–70% rise in carrier mobility with a value of 2510 cm2 V−1 s−1 at 300 K and extremely low lattice-thermal-conductivity (0.2–0.1 W m−1 K−1 over 300–375 K). This combination of transport properties yielded a room-temperature power factor of 3.2 mW m−1 K−2 and a nearly flat ZT value of ∼1.0 over the 300–375 K temperature range. Additionally, a record-high conversion efficiency (ηmax) of 3.7% was theoretically obtained for single-leg Ag2Se for a small temperature gradient of ∼80 K.

65 citations


Journal ArticleDOI
TL;DR: V2O5 nanoparticles were prepared by ultrasound assisted method and characterized using XRD, FTIR, SEM, and TEM as mentioned in this paper, which revealed an orthorhombic V2O-5 phase with an average crystallite size of 5...
Abstract: V2O5 nanoparticles were prepared by ultrasound-assisted method and characterized using XRD, FTIR, SEM, and TEM. XRD pattern revealed an orthorhombic V2O5 phase with an average crystallite size of 5...

65 citations


Journal ArticleDOI
TL;DR: In this paper, the Gaussian process regression model is used as a machine learning tool to find statistical correlations between the MMCE and lattice parameters among lanthanum manganites.
Abstract: Efficient solid-state refrigeration techniques have drawn increasing attention due to their potential for improving energy efficiency of refrigeration temperature control systems without using harmful gas as in conventional gas compression techniques. Research on magnetocaloric lanthanum manganites with a large maximum magnetic entropy change near room temperature shows promising results for further developments of magnetic refrigeration devices. By incorporating chemical substitutions, oxygen content modifications, and various synthesis methods, these manganites experience lattice distortions from perovskite cubic structures to pseudocubic, orthorhombic, and rhombohedral structures. Further changes in lattice parameters can also be achieved by the introduction of strain due to lattice mismatches. Empirical results and previous models through thermodynamics and first principles show that changes in lattice parameters correlate with those in MMCE, but correlations are merely general tendencies and obviously not universal. In this work, the Gaussian process regression model is developed as a machine learning tool to find statistical correlations between the MMCE and lattice parameters among lanthanum manganites. More than 100 lattices, cubic, pseudocubic, orthorhombic, and rhombohedral, with the MMCE ranging from 0.65 to $$8.00\,\hbox {J}\,\hbox {kg}^{-1}\,\hbox {K}^{-1}$$ under a field change of 5 T are explored for this purpose. The modeling approach demonstrates a high degree of accuracy and stability, contributing to efficient and low-cost estimations of the magnetocaloric effect. Furthermore, the machine learning algorithm predicts close MMCE results on epitaxial films with strained lattices against experimental results, which can provide guidance on thin film structure design and help understandings of magnetic phase transformations and magnetocaloric effects.

Journal ArticleDOI
10 Mar 2020
TL;DR: The mechanochemical synthesis was designed to reduce the impact energy during the milling process, leading to a defect-free, well-crystallized sample characterized by a minimum unit-cell volume and octahedral tilting angles in the low-temperature orthorhombic perovskite framework, defined in the Pbnm space group.
Abstract: We present a mechanochemical procedure, with solvent-free, green-chemistry credentials, to grow all-inorganic CsPbBr3 perovskite. The crystal structure of this perovskite and its correlations with the physicochemical properties have been studied. Synchrotron X-ray diffraction (SXRD) and neutron powder diffraction (NPD) allowed us to follow the crystallographic behavior from 4 to 773 K. Unreported features like the observed negative thermal expansion of the b unit-cell parameter stem from octahedral distortions in the 4-100 K temperature range. The mechanochemical synthesis was designed to reduce the impact energy during the milling process, leading to a defect-free, well-crystallized sample characterized by a minimum unit-cell volume and octahedral tilting angles in the low-temperature orthorhombic perovskite framework, defined in the Pbnm space group. The UV-vis diffuse reflectance spectrum shows a reduced band gap of 2.22(3) eV, and the photocurrent characterization in a photodetector reveals excellent properties with potential applications of this material in optoelectronic devices.

Journal ArticleDOI
TL;DR: In this article, the phase stabilities and mechanical properties of Cu3Sn compounds were investigated based on their formation enthalpy and cohesive energy, and the results indicated that the phase stability of the Cu3sn intermetallic compounds follow the order of: o-Cu3sn-Cmcm where c-cu3sn is energetically unstable.

Journal ArticleDOI
13 Aug 2020
TL;DR: In this paper, a combined theoretical and experimental study of Nb under high pressure and temperature was performed using synchrotron-based fast x-ray diffraction combined with ab initio quantum molecular dynamics simulations.
Abstract: Compared to other body-centered cubic (bcc) transition metals, Nb has been the subject of fewer compression studies and there are still aspects of its phase diagram which are unclear. Here, we report a combined theoretical and experimental study of Nb under high pressure and temperature. We present the results of static laser-heated diamond anvil cell experiments up to 120 GPa using synchrotron-based fast x-ray diffraction combined with ab initio quantum molecular dynamics simulations. The melting curve of Nb is determined and evidence for a solid-solid phase transformation in Nb with increasing temperature is found. The high-temperature phase of Nb is orthorhombic Pnma. The bcc-Pnma transition is clearly seen in the experimental data on the Nb principal Hugoniot. The bcc-Pnma coexistence observed in our experiments is explained. Agreement between the measured and calculated melting curves is very good except at 40–60 GPa where three experimental points lie below the theoretical melting curve by 250 K (or 7%); a possible explanation is given. The study of materials under extreme conditions can reveal interesting physics in diverse areas such as condensed matter and geophysics. Here, the authors investigate experimentally and theoretically the high pressure-high temperature phase diagram of niobium revealing a previously unobserved phase transition from body-centered cubic to orthorhombic phase.

Journal ArticleDOI
01 Nov 2020-Vacuum
TL;DR: In this paper, the structural, elastic modulus, hardness and thermodynamic properties of orthorhombic ZrAl-type aluminides are studied by the first-principles calculations.

Journal ArticleDOI
01 Jan 2020-Carbon
TL;DR: In this paper, the authors established a carbon phase in purely sp3-bonded networks with a 14-atom orthorhombic unit cell, termed C14 carbon, by swarm structural searches.

Journal ArticleDOI
TL;DR: In this article, the authors compared the properties of different phase structures prepared by an efficient solvothermal method and provided a new efficient way to enhance the electrochemical performance of oxalate-based materials by optimizing the interaction of crystal structure and morphology.

Journal ArticleDOI
TL;DR: Transmission Kikuchi diffraction and scanning transmission electron microscopy STEM techniques are used to compare the crystallographic phase and orientation of Si and Zr doped HfO2 thin films integrated in a 22 nm fully-depleted silicon-on-insulator (FDSOI) ferroelectric field effect transistor (FeFET).
Abstract: The microstructure of ferroelectric hafnium oxide plays a vital role for its application, e.g., non-volatile memories. In this study, transmission Kikuchi diffraction and scanning transmission electron microscopy STEM techniques are used to compare the crystallographic phase and orientation of Si and Zr doped HfO2 thin films as well as integrated in a 22 nm fully-depleted silicon-on-insulator (FDSOI) ferroelectric field effect transistor (FeFET). Both HfO2 films showed a predominately orthorhombic phase in accordance with electrical measurements and X-ray diffraction XRD data. Furthermore, a stronger texture is found for the microstructure of the Si doped HfO2 (HSO) thin film, which is attributed to stress conditions inside the film stack during crystallization. For the HSO thin film fabricated in a metal-oxide-semiconductor (MOS) like structure, a different microstructure, with no apparent texture as well as a different fraction of orthorhombic phase is observed. The 22 nm FDSOI FeFET showed an orthorhombic phase for the HSO layer, as well as an out-of-plane texture of the [111]-axis, which is preferable for the application as non-volatile memory.

Journal ArticleDOI
TL;DR: The structural, optical, electrical and dielectric properties of PrCrO3 perovskite prepared through the sol-gel method were investigated in details by X-ray powder diffraction at room temperature as discussed by the authors.

Journal ArticleDOI
TL;DR: In this article, the sputtering of a diboride target composed of Hf,Ta, V, W2B5 and ZrB2 with equimolar composition leads to the formation of crystalline single-phase solid solution dibboride thin films, with a high-entropy metal-sublattice.

Journal ArticleDOI
TL;DR: In this paper, a multiphase dendritic microstructure with W-rich dendrites and V2.5Cr1.2WMoCo0.04 was fabricated by arc melting and was found to exhibit a multi-phase XRD pattern, which attests to the BCC crystal structure observed in the as-cast state being metastable.

Journal ArticleDOI
13 Oct 2020
TL;DR: In this article, the phase transition and enhanced degree of crystallinity were observed with increasing temperature, and the results highlight that the mixed-phase WO3 nanoplates would make a suitable electrode material for supercapacitors with desired electrochemical features.
Abstract: The morphology and crystal structure of electrode materials have an enormous impact on their electrochemical properties for employment in supercapacitors for various applications. In this study, the transformations of the crystal structure of WO3·H2O nanoplates were conducted by post-annealing at 200 °C and 400 °C. The morphological and structural evolution of the electrodes was studied via FEG-SEM, HRTEM, FTIR, XRD, and Raman spectroscopy. The phase transition and enhanced degree of crystallinity were observed with increasing temperature. The orthorhombic structures of the hydrate WO3·H2O (W80), the mixed-phase with mesoporous structure (W200), and finally the monoclinic phase of WO3 structures (W400) were achieved at annealing temperatures of 80 °C, 200 °C, and 400 °C respectively. The electrochemical performance of electrode W200 showed the highest specific capacitance of 606 F g−1 as compared to electrode W80 (361 F g−1), and was two-fold greater than electrode W400 (302 F g−1) at a current density of 1 A g−1. Moreover, electrode W200 exhibited excellent cyclic stability of 89% at an ultrahigh scan rate of 100 mV s−1 after 4000 cycles. The results highlight that the mixed-phase WO3 nanoplates would make a suitable electrode material for supercapacitors with desired electrochemical features.

Journal ArticleDOI
TL;DR: In this article, shock wave induced switchable phase transition (β to α and α to β) of a potassium sulfate (K2SO4) crystal is demonstrated, where the test crystals are subjected to shock waves of...
Abstract: In this research article, shock wave induced switchable phase transition (β to α and α to β) of potassium sulfate (K2SO4) crystal is demonstrated. The test crystals are subjected to shock waves of ...

Journal ArticleDOI
TL;DR: In this paper, cubic and orthorhombic phase SnS nanoparticles were used for the photodegradation of methylene blue (MB) under visible light irradiation, and the results indicated the potential application of SnS NPs in natural sunlight driven photocatalytic oxidation of industrial dyes and consequent cost-effective treatment of waste water from textile industries.

Journal ArticleDOI
TL;DR: The discovery of i-MAB phases expands the elemental space of these borides with M = Sc, Y, Zr, Hf, and Nb, realizing an increased property tuning potential of these phases as well as their suggested potential two-dimensional derivatives.
Abstract: All atomically laminated MAB phases (M = transition metal, A = A-group element, and B = boron) exhibit orthorhombic or tetragonal symmetry, with the only exception being hexagonal Ti2InB2. Inspired by the recent discovery of chemically ordered hexagonal carbides, i-MAX phases, we perform an extensive first-principles study to explore chemical ordering upon metal alloying of M2AlB2 (M from groups 3 to 9) in orthorhombic and hexagonal symmetry. Fifteen stable novel phases with in-plane chemical ordering are identified, coined i-MAB, along with 16 disordered stable alloys. The predictions are verified through the powder synthesis of Mo4/3Y2/3AlB2 and Mo4/3Sc2/3AlB2 of space group R3m (no. 166), displaying the characteristic in-plane chemical order of Mo and Y/Sc and Kagome ordering of the Al atoms, as evident from X-ray diffraction and electron microscopy. The discovery of i-MAB phases expands the elemental space of these borides with M = Sc, Y, Zr, Hf, and Nb, realizing an increased property tuning potential of these phases as well as their suggested potential two-dimensional derivatives.

Journal ArticleDOI
31 Jul 2020
TL;DR: The orthorhombic Ag2Se1‐xSx samples show better electrical transport performance and higher zT than the monoclinic samples under a comparable carrier concentration, most likely due to their weaker electron-phonon interactions.
Abstract: Self-powered wearable electronics require thermoelectric materials simultaneously with a high dimensionless figure of merit (zT) and good flexibility to convert the heat discharged by the human body into electricity. Ag2(S,Se)-based semiconducting materials can well satisfy these requirements, and thus, they are attracting great attention in thermoelectric society recently. Ag2(S,Se) crystalizes in an orthorhombic structure or monoclinic structure, depending on the detailed S/Se atomic ratio, but the relationship between its crystalline structure and mechanical/thermoelectric performance is still unclear to date. In this study, a series of Ag2Se1-x S x (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.45) samples were prepared and their mechanical and thermoelectric performance dependence on the crystalline structure was systematically investigated. x = 0.3 in the Ag2Se1-x S x system was found to be the transition boundary between orthorhombic and monoclinic structures. Mechanical property measurement shows that the orthorhombic Ag2Se1-x S x samples are brittle while the monoclinic Ag2Se1-x S x samples are ductile and flexible. In addition, the orthorhombic Ag2Se1-x S x samples show better electrical transport performance and higher zT than the monoclinic samples under a comparable carrier concentration, most likely due to their weaker electron-phonon interactions. This study sheds light on the further development of flexible inorganic TE materials.

Journal ArticleDOI
TL;DR: In this paper, complex impedance spectroscopy (CIS) and Raman spectrograms have been used for structural refinement on Na0.5(Bi1-xDyx)0.9TiO3 (xDyNBT) ceramic systems with x = 0, 2, 5 and 15%.

Journal ArticleDOI
TL;DR: The phase synthesis process, structural evolution and microwave dielectric properties of BaCu2-xCoxSi2O7 ceramics were investigated in this article, where the authors used P-V-L complex chemical bond theory and Raman spectra.
Abstract: BaCu2-xCoxSi2O7 solid solutions with orthorhombic structure (Pnma) were prepared by solid-state reaction method. The phase synthesis process, structural evolution and microwave dielectric properties of BaCu2-xCoxSi2O7 ceramics were investigated. Single BaCu2Si2O7 phase was obtained when calcined at 950 °C for 3 h and was decomposed into BaCuSi2O6 phase when calcined at 1075 °C for 3 h. The sintering process was effectively promoted when Cu2+ was replaced by Co2+ and the maximum solubility of BaCu2-xCoxSi2O7 was located between 0.15 and 0.20. P-V-L complex chemical bond theory and Raman spectra were used to explain the structure-property correlations of BaCu2-xCoxSi2O7 ceramics. The corrected dielectric constant (er-corr) of BaCu2-xCoxSi2O7 ceramics decreased monotonously with the susceptibility (Σχμ) and ionic polarizability of primitive unit cell. The quality factor (Q × f) increased with bond strength and lattice energy (Ucal), especially the lattice energy of the Si-O bond. The temperature coefficient of resonant frequency (τf) was determined by the susceptibility and lattice energy of the Cu/Co-O bond. The following optimum microwave dielectric properties were obtained at x = 0.15 when sintered at 1000 °C for 3 h: er = 8.45, Q×f =58958 GHz and τf = -34.4 ppm/°C.

Journal ArticleDOI
TL;DR: The synthesis of blue-emitting Cs-oleate capped CsCdxPb1–xBr3 NCs are reported, which exhibit a cubic perovskite phase containing Cd-rich domains of Ruddlesden–Popper phases (RP phases), highlighting the metastability of RP domains.
Abstract: Various strategies have been proposed to engineer the band gap of metal halide perovskite nanocrystals (NCs) while preserving their structure and composition and thus ensuring spectral stability of the emission color. An aspect that has only been marginally investigated is how the type of surface passivation influences the structural/color stability of AMX3 perovskite NCs composed of two different M2+ cations. Here, we report the synthesis of blue-emitting Cs-oleate capped CsCd x Pb1-x Br3 NCs, which exhibit a cubic perovskite phase containing Cd-rich domains of Ruddlesden-Popper phases (RP phases). The RP domains spontaneously transform into pure orthorhombic perovskite ones upon NC aging, and the emission color of the NCs shifts from blue to green over days. On the other hand, postsynthesis ligand exchange with various Cs-carboxylate or ammonium bromide salts, right after NC synthesis, provides monocrystalline NCs with cubic phase, highlighting the metastability of RP domains. When NCs are treated with Cs-carboxylates (including Cs-oleate), most of the Cd2+ ions are expelled from NCs upon aging, and the NCs phase evolves from cubic to orthorhombic and their emission color changes from blue to green. Instead, when NCs are coated with ammonium bromides, the loss of Cd2+ ions is suppressed and the NCs tend to retain their blue emission (both in colloidal dispersions and in electroluminescent devices), as well as their cubic phase, over time. The improved compositional and structural stability in the latter cases is ascribed to the saturation of surface vacancies, which may act as channels for the expulsion of Cd2+ ions from NCs.

Journal ArticleDOI
TL;DR: In this article, the phase stability and onset crystallization temperature of polymorphic (HfxZr1−x)O2 (HZO) thin films were investigated via high-temperature x-ray diffraction (HTXRD) for five different compositions.
Abstract: Polymorphic (HfxZr1−x)O2 (HZO) thin films exhibit ferroelectric, dielectric, and antiferroelectric properties across a wide compositional range due to the existence of orthorhombic, monoclinic, and tetragonal phases. To better understand the phase stability across the HfO2–ZrO2 compositional range, we investigate the structural evolution of HZO thin films in situ via high-temperature x-ray diffraction (HTXRD) for five different compositions [ZrO2, (Hf0.23Zr0.77)O2, (Hf0.43Zr0.57)O2, (Hf0.67Zr0.33)O2, and HfO2]. The real-time monitoring of HZO crystallization reveals a competing driving force between the tetragonal and monoclinic phase stabilities for HfO2-rich vs ZrO2-rich compositions. Additionally, we confirm an XRD peak shift toward lower 2θ with increasing temperature in ZrO2, (Hf0.23Zr0.77)O2, and (Hf0.43Zr0.57)O2 films, which we ascribe to the appearance of a metastable orthorhombic phase during heating. A monotonic trend for the onset crystallization temperature is reported for five compositions of HZO and reveals an increase in onset crystallization temperature for HfO2-rich compositions. Relative intensity fraction calculations suggest a higher fraction of monoclinic phase with increasing annealing temperature for (Hf0.67Zr0.33)O2. This study of phase stability and onset crystallization temperatures offers insight for managing the thermal budget for HZO thin films, especially for temperature-constrained processing.