Hexagonal Rare Earth Research Articles

A low-temperature self-flux route to prepare hexagonal rare earth fluorides and manipulation of upconversion luminescence in rodlike NaYbF4:Tm3+/Fe3+ crystals

The distorted lattice, ET between Tm3+ and Fe3+ and no ET between Yb3+ and Fe3+ make 89Yb1Tm10Fe crystals obtained in low-temperature self-flux system exhibit the strongest emission with its absolute UCQY of 0.46% and of 0.56% with and without the magnetic field of 15 kOe, respectively.

The Kondo Effect in CexLaLuScY (x = 0.05-1.0) High-Entropy Alloys.

In the search for electronic phenomena in high-entropy alloys (HEAs) that go beyond the independent-electron description, we have synthesized a series of hexagonal rare earth (RE)-based HEAs: CexLaLuScY (x = 0.05-1.0). The measurements of electrical resistivity, magnetic susceptibility and specific heat have shown that the CexLaLuScY HEAs exhibit the Kondo effect, which is of a single impurity type in the entire range of employed Ce concentrations despite the alloys being classified as dense (concentrated) Kondo systems. A comparison to other known dense Kondo systems has revealed that the Kondo effect in the CexLaLuScY HEAs behaves quite differently from the chemically ordered Kondo lattices but quite similar to the RE-containing magnetic metallic glasses and randomly chemically disordered Kondo lattices of the chemical formula RE1xRE21-xM (with RE1 being magnetic and RE2 being nonmagnetic). The main reason for the similarity between HEAs and the metallic glasses and chemically disordered Kondo lattices appears to be the absence of a periodic 4f sublattice in these systems, which prevents the formation of a coherent state between the 4f-scattering sites in the T→ 0 limit. The crystal-glass duality of HEAs does not bring conceptually new features to the Kondo effect that would not be already present in other disordered dense Kondo systems. This study broadens the classification of HEAs to correlated electron systems.

Molten Salt Synthesis of Micro-Sized Hexagonally Shaped REMnO3 (RE = Y, Er, Tm, Yb)

Yttrium manganite (YMnO3) is a compound belonging to the hexagonal rare earth manganites family, which demonstrates multiferroic properties. This material can be prepared by several synthetic approaches, with the most common one being the solid-state synthesis. In this work, we show a possibility of preparing this material via molten salt synthesis using the NaCl-KCl mixture as the reaction medium and yttrium and manganese nitrates as the starting materials. We demonstrate that, by varying the reaction temperature and the nitrates-to-chlorides ratio, it is feasible to synthesize hexagonally shaped particles of microscopic dimensions. A similar synthesis procedure can be successfully applied for the preparation of other hexagonal manganites—ErMnO3, TmMnO3, and YbMnO3.

Phase-Transition-Driven Regional Distribution of Rare-Earth Ions for Multiplexed Upconversion Emissions.

Phase transition of the polymorphs is critical for controlled synthesis and property modulation of functional materials. Upconversion emissions from an efficient hexagonal sodium rare-earth (RE) fluoride compound, β-NaREF4, which is generally obtained from the phase transition of the cubic (α-) phase counterpart, are attractive for photonic applications. However, the investigation of the α → β phase transition of NaREF4 and its effect on the composition and architecture is still preliminary. Herein, we investigated the phase transition with two kinds of α-NaREF4 particles. Instead of a uniform composition, the β-NaREF4 microcrystals exhibited regionally distributed RE3+ ions, in which the RE3+ with a smaller ionic radius (smaller RE3+) sandwiched the RE3+ with a larger ionic radius (larger RE3+). We unravel that the α-NaREF4 particles transformed to β-NaREF4 nuclei with no controversial dissolution, and the α → β phase transition toward NaREF4 microcrystals included nucleation and growth steps. The component-dependent phase transition is corroborated with RE3+ ions from Ho3+ to Lu3+ and multiple sandwiched microcrystals were obtained, in which up to five kinds of RE components were distributed regionally. Moreover, with rational integration of luminescent RE3+ ions, a single particle with multiplexed upconversion emissions in wavelength and lifetime domains is demonstrated, which provides a unique platform for optical multiplexing applications.

Hexagonal Lu1-xInxFeO3 Room-Temperature Multiferroic Thin Films.

The hexagonal rare earth ferrites h-RFeO3(R = rare earth element) have been recognized as promising candidates for a room-temperature multiferroic system, and the primary issue for these materials is how to get a stable hexagonal structure since the centrosymmetric orthorhombic structure is generally stable for most RFeO3 at room-temperature, while the hexagonal phase is only stable under some strict conditions. In the present work, h-Lu1-xInxFeO3 (x = 0-1) thin films were prepared on a Nb-SrTiO3 (111) single-crystal substrate by a pulsed laser deposition (PLD) process, and the multiferroic characterization was performed at room temperature. With the combined effects of chemical pressure and epitaxial strain, the stable hexagonal structure was achieved in a wide composition range (x = 0.5-0.7), and the results of XRD (X-ray diffraction) and SAED (selected area electron diffraction) indicate the super-cell match relations between the h-Lu0.3In0.7FeO3 thin film and substrate. The saturated P-E hysteresis loop was obtained at room temperature with a remanent polarization of about 4.3 μC/cm2, and polarization switching was also confirmed by PFM measurement. Furthermore, a strong magnetoelectric coupling with a linear magnetoelectric coefficient of 1.9 V/cm Oe was determined, which was about three orders of magnitude larger than that of h-RFeO3 ceramics. The present results indicate that the h-Lu1-xInxFeO3 thin films are expected to have great application potential for magnetoelectric memory and detection devices.

Electronic structure of hexagonal REN (RE = Sc, Y, and Lu) materials

Structural properties and band structures of bulk and monolayer hexagonal (BN-type) rare earth nitrides have been studied from first principles. The bulk materials are expected to exhibit bigger lattice parameters than those of monolayers. The band structures and band gaps of 3D and 2D systems also differ from each other. Band gaps of ScN and YN bulk materials are rather pressure resistant, whereas the band gaps of all monolayers may be well tuned by strain. The monolayer rare earth nitrides exhibit splittings of valence bands in the K point in the Brillouin zone, driven by spin–orbit coupling, which are similar to those present in transition metal dichalcogenides. The 2D rare earth nitrides may be interesting candidate materials for some applications in optoelectronics and spintronics.

Density Functional Theory‐Based Calculations for 2D Hexagonal Lanthanide Metals

AbstractScrutiny of the stability, properties, and applications of 2D metals belonging to s‐, p‐, and d‐block series has acquired intense research interest in the past few years. The present report is solely focused to systematically explore the stability and properties of 2D hexagonal (HX) lanthanides employing density functional theory calculations. To probe the dynamical stability of these materials, the phonon dispersion calculation is performed. The mechanical stability is analyzed on the basis of the 2D bulk modulus and elastic constant values. Further, to unravel the electronic properties of the 2D metals, electronic band structure, electron localization function, and the work function values are estimated. Moreover, en route to explore their magnetic properties, spin polarized density of states calculation is carried out and the ground state magnetic behavior is studied by considering ferromagnetic and antiferromagnetic spin configurations. Result explicitly demonstrates that 11 lanthanide metals are dynamically stable as an atomically thin 2D HX films. All of them are conducting in nature exhibiting ferromagnetic behavior in their ground state. These results offer the fundamentals of stability and properties of 2D HX lanthanides which can lay the foundation for exploring their diverse future applications.

Assessment of PBE+U and HSE06 methods and determination of optimal parameter U for the structural and energetic properties of rare earth oxides.

Rare earth oxides are attracting increasing interest as a relatively unexplored group of materials with potential applications in heterogeneous catalysis and electrocatalysis; therefore, a credible and universal computational approach is needed for modeling their reactivity. In this work, we systematically assessed the performance of the PBE+U method against the results of the hybrid HSE06 method with respect to the description of structural parameters and energetic properties of the selected hexagonal lanthanide sesquioxides and the cubic fluorite-type cerium dioxide. In addition, we evaluated the performance of PBE+U in describing the electronic structure and adsorption properties of the CeO2(111) and Nd2O3(0001) surfaces. The HSE06 method reproduces rather well the lattice parameters and selected energetic properties with respect to the experimental values. The PBE+U method is able to reproduce the results of HSE06 or the experimental values only if the U parameter is selected from an appropriate range of values. The U value around 3 eV gives the best description of the lattice parameters of most bulk oxides. 2 eV-3 eV is also found to be the optimal range of U for the reaction energies of bulk La2O3, Ce2O3, Nd2O3, Er2O3, and Ho2O3. U = 1 eV gives the best results for Pr2O3, Pm2O3, Eu2O3, Tm2O3, and Lu2O3, whereas Gd2O3 could not be accurately described by the PBE+U method. The U values (∼3 eV) found optimal for most bulk oxides also work well in the calculations of adsorption of small molecules on Nd2O3(0001) and CeO2(111), although larger U values are required to obtain sufficient localization of 4f electrons.

Anomalous lattice compression in the hexagonal La2O3– A high pressure X-ray diffraction, Raman spectroscopy and first principle study

High pressure behavior of hexagonal rare earth sesquioxide La2O3 have been investigated using X-ray diffraction, Raman spectroscopy and first principle calculations. Though the hexagonal structure is stable up to 26.5 GPa, incompressibility along the a axis is visible above 9.7 GPa. Rietveld structure refinements in conjunction with the Stephans anisotropic strain broadening model revealed the onset of an atypical bond compression and modification in the relative intensities of the diffraction peaks at ∼5.6 GPa. A significant increase in the intensity of 100 reflection and a clear reduction in the intensity of 103 reflection above 5.6 GPa suggests that pressure induces sliding of the adjacent La–O layers in opposite directions rather than compressing along the a axis. Further, a negative correlation of in-plane strain with pressure above 5.6 GPa indicate the layer sliding is accompanied by a reduction of in-plane strain in the hexagonal structure. Raman studies show the lifting of the degeneracy of the two stretching modes, in-plane Eg and out of plane A1g, and the change of slope of ω vs. P curve for the two in-plane Eg modes at 3.6 GPa. First principle density functional theory calculation substantiates our experimental observations that the pressure induced anisotropic strain distribution is driving the system into low energy configurations. The Birch –Murnaghan equation of state fit to the experimental pressure-volume data yielded a bulk modulus value of B0 = 102(5) GPa with B′0 = 9.8(1.0) for P ≤ 9.7 GPa, in good agreement with the 114 GPa of density functional theory calculations.

Electrochemical detection of trace heavy metal ions using a Ln-MOF modified glass carbon electrode

A novel electrochemical sensor was designed based on the metal-organic framework (MOF) modified glassy carbon electrode, which can detect trace cadmium (Cd2+) and lead (Pb2+) ions with high sensitivity. A hexagonal lanthanide MOF, ZJU-27 was synthesized by the solvothermal method from Yb(NO3)3·5H2O with 1,3,5-benzenetrisbenzoic acid (H3BTB) in the presence of 2-FBA and then used to modify the glassy carbon electrode. Excellent repeatability, high stability and practical application in real samples for the detection of Cd2+ and Pb2+ were demonstrated by the electrochemical sensor. The sensitivity of the sensor toward Pb2+ was calculated with a detection limit of 0.228 ppb.

Ferroelectricity and magnetoelectric coupling in hexagonal Lu0.5In0.5FeO3 ceramics

Recently, hexagonal rare earth ferrites (h-RFeO3) have attracted much scientific attention due to their room-temperature multiferroicity. The primary issue for h-RFeO3 is how to manipulate from nonpolar orthorhombic (Pbnm) structure to polar hexagonal (P63cm) structure, and this could be realized by the wet-chemical method, epitaxial stain engineering, or introducing chemical pressure. Another key issue for h-RFeO3 is the experimental determination of its macroferroelectricity and magnetoelectric (ME) coupling because the semiconducting nature seriously obstructs the experimental characterization. In the present work, owing to the suppressed leakage current density with decreasing temperature, saturated electrical hysteresis loops were observed in A-site substituted h-Lu0.5In0.5FeO3 ceramics at 198 K, directly demonstrating the presence of the macroferroelectricity. Room-temperature switchable ferroelectric polarization was further confirmed by the piezoresponse force microscopy. Furthermore, noticeable ME coupling with a linear ME voltage coefficient of 0.67 mV/cm Oe was observed at room temperature.

Submicron-sized phosphors based on hexagonal rare earth oxycarbonate for near-infrared excitation and emission

ABSTRACTNeodymium-doped rare-earth oxycarbonates, Ln2O2CO3: x mol% Nd3+ (Ln = La, Gd, and Y, 0 ≤ x ≤ 5), were synthesized by a simple flux method using a 0.476Li2CO3–0.270Na2CO3–0.254K2CO3 eutectic mixture. The suitability of these oxycarbonates as stable, nontoxic bioimaging phosphors that exhibit near-infrared (NIR) emission under NIR excitation was investigated. Gd2O2CO3: x mol% Nd3+ and Y2O2CO3: x mol% Nd3+ were obtained in single-phase form with a hexagonal type-II structure. The particle sizes of the Gd2O2CO3: x mol% Nd3+ and Y2O2CO3: x mol% Nd3+ were on a submicron scale suitable for bioimaging, but that of La2O2CO3: x mol% Nd3+ was outside the suitable range for bioimaging. Gd2O2CO3: x mol% Nd3+ and Y2O2CO3: x mol% Nd3+ showed characteristic emissions corresponding to the f–f transitions of Nd3+. The strongest emission peak was observed at around 1064 nm under excitation at 823 nm in the samples containing 2 mol% of Nd3+. The emission intensity of Gd2O2CO3: 2 mol% Nd3+ was 1.25 times higher than that of Y2O2CO3: 2 mol% Nd3+. The emission intensity of both phosphors was also significantly higher than that of the indocyanine green in practical use as a bio-imaging reagent.

An Electronically Driven Improper Ferroelectric: Tungsten Bronzes as Microstructural Analogs for the Hexagonal Manganites.

Since the observation that the properties of ferroic domain walls (DWs) can differ significantly from the bulk materials in which they are formed, it has been realized that domain wall engineering offers exciting new opportunities for nanoelectronics and nanodevice architectures. Here, a novel improper ferroelectric, CsNbW2 O9 , with the hexagonal tungsten bronze structure, is reported. Powder neutron diffraction and symmetry mode analysis indicate that the improper transition (TC = 1100 K) involves unit cell tripling, reminiscent of the hexagonal rare earth manganites. However, in contrast to the manganites, the symmetry breaking in CsNbW2 O9 is electronically driven (i.e., purely displacive) via the second-order Jahn-Teller effect in contrast to the geometrically driven tilt mechanism of the manganites. Nevertheless CsNbW2 O9 displays the same kinds of domain microstructure as those found in the manganites, such as the characteristic six-domain "cloverleaf" vertices and DW sections with polar discontinuities. The discovery of a completely new material system, with domain patterns already known to generate interesting functionality in the manganites, is important for the emerging field of DW nanoelectronics.

NaYF4:Yb3+/Er3+ Upconversion Nanoparticles for Infrared Photodynamic Therapy of Tumors

Luminescence spectra of oleylamine- (OM) cubic or pure hexagonal OM-NaYF4:Yb3+/Er3+ nanoparticles demonstrated that these light-upconverting nanoparticles produced upconversion luminescence at ∼670 nm under excitation at 980 nm. When coupled with a photosensitizer (phthalocyanine), the resulting drug formulation was suitable for infra-red photodynamic therapy (PDT) of tumors (1) as proven by pre-clinical testing with xeno-transplanted human tumors to athymic nude (nu/nu) mice. Laser 980 nm irradiation penetrating deeper into the skin and tissue did not cure xenotransplanted tumors in nude mice i.v. injected with photosenzitizer-free nanoparticles, whereas tumor remissions frequently occurred after i.v. injected phthalocyanine-contaning light-upconverting nanoparticles. Prior to photosensitizer incorporation, the OM-NaYF4:Yb3+/Er3+ nanoparticles were coated by a silica shell to enable further amino-functionalization and render the particles with biocompatibility and stability in aqueous media, preventing particles from aggregation. By varying the reaction time and Ln3+/Na+/F- ratio, the nanocrystal size was controlled within the range 16-270 nm (2). In vitro cytotoxicity and intracellular nanoparticle uptake were determined with human cervix carcinoma mRoGFP HeLa cells using confocal microscopy. In conclusion, infra-red PDT was established.Supported from GACR (15-01897S) and Czech Ministry of Education (LF14001).(1) Nekvasil, M., Zadinova, M., Tahotna, L., Žackova, M., Pouckova, P., Ježek, P. An optimum modality for photodynamic therapy of tumors: gels containing liposomes with hydrophobic photosenzitizers. Drug. Dev. Res. 68, 235-252, 2007.(2) Kostiv U, Janouskova O, Slouf M, Kotov N, Engstova H, Smolkova K, Ježek P, Horak, D. Silica-modified monodisperse hexagonal lanthanide nanocrystals: Synthesis and biological properties Nanoscale, 2015.

Direct visualization of magnetoelectric domains

The coupling between the magnetic and electric dipoles in multiferroic and magnetoelectric materials holds promise for conceptually novel electronic devices. This calls for the development of local probes of the magnetoelectric response, which is strongly affected by defects in magnetic and ferroelectric ground states. For example, multiferroic hexagonal rare earth manganites exhibit a dense network of boundaries between six degenerate states of their crystal lattice, which are locked to both ferroelectric and magnetic domain walls. Here we present the application of a magnetoelectric force microscopy technique that combines magnetic force microscopy with in situ modulating high electric fields. This method allows us to image the magnetoelectric response of the domain patterns in hexagonal manganites directly. We find that this response changes sign at each structural domain wall, a result that is corroborated by symmetry analysis and phenomenological modelling, and provides compelling evidence for a lattice-mediated magnetoelectric coupling. The direct visualization of magnetoelectric domains at mesoscopic scales opens up explorations of emergent phenomena in multifunctional materials with multiple coupled orders.

Synthesis and Structural and Electrical Investigations of a Hexagonal Y1–xGdxInO3 (0.0 ≤ x ≤ 1.0) System Obtained via Metastable C-Type Intermediates

Detailed structural and electrical investigations were carried out on an A-site disordered hexagonal Y(1-x)Gd(x)InO3 (0.0 ≤ x ≤ 1.0) series synthesized by a self-assisted gel-combustion route. The phase relations show profound temperature dependence. The metastable C-type modification could be stabilized for all the compositions, which on further heating get converted to stable hexagonal polymorphs. The conversion temperature (C-type to hexagonal) was found to increase with an increase in Y(3+) content. The system was observed to be single-phasic hexagonal at 1250 °C throughout the composition range. Interestingly, the increase in planar bonds of InO5 polyhedra was found to be twice that of the apical bonds on Gd(3+) substitution. Careful Raman spectroscopic studies highlighted a definitive though subtle structural change from x = 0.7 onward. The same observation is also corroborated by the dielectric studies. Electric field-dependent polarization measurements showed the ferroelectric hysteresis loop for pure YInO3. The system transforms from ferroelectric in YInO3 to almost paraelectric for GdInO3. In the present study, XRD, Raman, and electrical characterizations in conjunction reveal that to tune the electrical properties of the hexagonal rare earth indates, the variation in tilting of InO5 polyhedra has to be influenced, which could not be brought about by isovalent A-site substitution.

Phase‐ and Size‐Controllable Synthesis of Hexagonal Upconversion Rare‐Earth Fluoride Nanocrystals through an Oleic Acid/Ionic Liquid Two‐Phase System

Herein, we introduce a facile, user- and environmentally friendly (n-octanol-induced) oleic acid (OA)/ionic liquid (IL) two-phase system for the phase- and size-controllable synthesis of water-soluble hexagonal rare earth (RE = La, Gd, and Y) fluoride nanocrystals with uniform morphologies (mainly spheres and elongated particles) and small sizes (<50 nm). The unique role of the IL 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF(6)) and n-octanol in modulating the phase structure and particle size are discussed in detail. More importantly, the mechanism of the (n-octanol-induced) OA/IL two-phase system, the formation of the RE fluoride nanocrystals, and the distinctive size- and morphology-controlling capacity of the system are presented. BmimPF(6) is versatile in term of crystal-phase manipulation, size and shape maintenance, and providing water solubility in a one-step reaction. The luminescent properties of Er(3+)-, Ho(3+)-, and Tm(3+)-doped LaF(3), NaGdF(4), and NaYF(4) nanocrystals were also studied. It is worth noting that the as-prepared products can be directly dispersed in water due to the hydrophilic property of Bmim(+) (cationic part of the IL) as a capping agent. This advantageous feature has made the IL-capped products favorable in facile surface modifications, such as the classic Stober method. Finally, the cytotoxicity evaluation of NaYF(4):Yb,Er nanocrystals before and after silica coating was conducted for further biological applications.

Crystal structure of hexagonal RE(CO3)OH

Hexagonal rare earth carbonate hydroxides, RE(CO 3 )OH, where RE = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er, were hydrothermally synthesized from formic acid and hydroxide gels of rare earth elements. The crystals exhibited bicephalous hexagonal prisms with lengths of several tens of micrometers. The crystal structures of a series of hexagonal RE(CO 3 )OH were solved using the single crystal CCD-XRD intensity data sets. The space groups of the synthetic hexagonal RE(CO 3 )OH crystals are all P -6. The present study has cast doubt upon the space group P -62 c previously reported for the natural Ce(CO 3 )OH, hydroxylbastnäsite-(Ce). The cell parameters decreased linearly with decreases in the ionic radii of the rare earth elements. La(CO 3 )OH showed the largest unit cell ( a = 12.6752(6), c = 10.0806(10) Å), while Er(CO 3 )OH showed the smallest ( a = 11.8977(4), c = 9.6978(8) Å). The rare earth atoms are in ninefold coordination with oxygen atoms to form a tricapped trigonal prism. The structure consists of layers of 2 ∞ [(OH)RE 3/3 ] 2+ ions linked by carbonate ions. Raman spectra indicate the presence of carbonate and hydroxide groups. An evolutionary shift was observed from La to Er towards higher frequency, which was associated with a decreasing RE—O bond length.

Synthesis and photoluminescence properties of hexagonal Lanthanide(iii)-doped NaYF4 microprisms

NaYF4:Ln3+ (Ln = Eu, Sm, Pr and Yb/Er) hexagonal microprisms have been successfully synthesized using a mild hydrothermal method. Field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM) have been employed to investigate the formation of NaYF4:Ln3+ hexagonal microprisms and a possible growth mechanism was proposed. The down- and up- conversion luminescence of hexagonal NaYF4:Ln3+ (Ln = Eu, Sm, Pr and Yb/Er) microprisms have been characterized.

LnPO4: RE3+ (La = La, Gd; RE = Eu, Tb) nanocrystals: solvo-thermal synthesis, microstructure and photoluminescence

Through altering the solvents, we have obtained the Eu3+/Tb3+ ions-doped LnPO4 (Ln = La, Gd) phosphors with different particle sizes, microstructures and morphologies via a facile solvo-thermal technology. X-ray powder diffraction (XRD), transmission electron microscope (TEM), and scanning electron microscope (SEM) have shown that the products using different solvents have various structures and morphologies. With the increase of DMA/water volume ratio, the microstructure has changed from hexagonal phase to monoclinic one, and the morphology from nanorod to nanoparticle, revealing the decreased oriented growth. The presence of DMA is an important factor in guiding the anisotropic growth of hexagonal lanthanide phosphates. Besides, N-methyl-2-pyrrolidone has been used as solvent to induce the Eu3+/Tb3+ ions-doped LnPO4 (Ln = La, Gd) phosphors with different morphologies and structures. Finally, the photoluminescence behaviors of these nanocrystals have been investigated, which are dependent on their microstructures and morphologies.