Rare-earth Nitrides Research Articles

FP-LAPW Calculation of Elastic, Thermal Properties and Chemical Bonding of Heavy REN (RE = Tb–Lu)

The full-potential linearised augmented plane wave (FP-LAPW) method has been used to study the elastic, thermal and bonding properties of heavy rare earth nitrides REN (RE = Tb–Lu). In the present work, band structure and density of states of TbN–LuN are studied. We have calculated the Young’s modulus E, shear modulus G, bulk modulus B, Poisson’s ratio υ, shear anisotropy factor A, and Lame’s coefficients μ & λ for all these compounds. Thermal property such as Debye temperature and average sound velocity have been estimated for all these compounds and their relation to elastic properties is discussed. To understand the bonding nature of these compounds, the electronic charge density has been calculated for all these compounds and the relation of this bonding with the mechanical properties has been studied. From the calculated ratio of bulk to shear modulus B/G, these compounds are found to be brittle in nature. The value of Poisson’s ratio suggests that these compounds are ionic and this is in agreement with the charge density plots which show the ionic bonding nature of these compounds. The G values show that these compounds are easier to shear on the plane [110] rather than on plane [100] for all the compounds except YbN.

ChemInform Abstract: Lanthanide Amidinates and Guanidinates in Catalysis and Materials Science: A Continuing Success Story

AbstractReview: 72 refs.

Nano-crystals of various lanthanide fluorides prepared using the ionic liquid bmimPF6

Lanthanide fluorides are important materials for a wide spectrum of applications. In this work, nano-sized crystalline fluorides of nearly all of the lanthanides were prepared via a one-pot reaction using the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) as both the solvent and as the fluorination agent and the appropriate lanthanide nitride as the source of the trivalent ion. The phase compositions of the samples (hexagonal, orthorhombic or a mixture of these) as well as the sizes of the nanocrystals were characterised by X-ray diffraction (XRD) and confirmed by high resolution transmission electron microscopy (HR-TEM). Fluorescence measurements were performed for all of the samples to gain insight into the optical properties of the samples.

Half-metallic ferromagnetism in the hypothetical RbN and CsN compounds: First-principles calculations

We report a first-principles study of the hypothetical RbN and CsN compounds. The spin-polarized calculations indicate that these materials are half-metallic ferromagnets and exhibit an integer magnetic moment of 2μB. The most important property in these alkaline metal nitrides is that the half-metallicity is originated from the polarization of the p-N orbitals not present in the conventional transition metal or rare earth nitrides. We find also that the half-metallicity is maintained in the zinc-blende structure on a wide range of lattice constant. This offers the possibility to growth such materials on various semiconductor substrates.

Visible photoluminescence in polycrystalline terbium doped aluminum nitride (Tb:AlN) ceramics with high thermal conductivity

Thermal management continues to be one of the major challenges in the development of high powered light sources such as solid state lasers. In particular, the relatively low thermal conductivity of standard photoluminescent (PL) materials limits the overall power output and/or duty cycle. We present a method based on current activated pressure assisted densification for the fabrication of high thermal conductivity PL materials: rare earth doped polycrystalline bulk aluminum nitride. Specifically, the ceramics are translucent and are doped with Tb3+, allowing for emission in the visible. Remarkably, the ceramics have a room temperature thermal conductivity of 94 W/(m K) which is almost seven times higher than that of the state of the art host material, Nd-doped yttrium aluminum garnet. These light emitting properties coupled with very high thermal conductivity should enable the development of a wide variety of more powerful light sources.

Rare earth co-doping nitride layers for visible light

AlN and GaN films were co-implanted with europium and praseodymium ions followed by thermal annealing treatments for the lattice recovery and ions activation. Their structural and optical properties were studied by Rutherford Backscattering Spectrometry and optical spectroscopy techniques. The evolution of the ions photoluminescence intensity with temperature was analyzed for both AlN:Eu,Pr and GaN:Eu,Pr hosts, and compared with the ones of individually doped layers (GaN:Eu, GaN:Pr, AlN:Eu and AlN:Pr). Sharp emission lines due to intra-4fn shell transitions can be observed even at room temperature for the Eu3+ and Pr3+ for both hosts.

P-f hybridization in the ferromagnetic semiconductor HoN

The electronic structure of thin film HoN has been studied using soft x-ray spectroscopy. The combination of soft x-ray emission, x-ray absorption, and photoemission techniques yields direct evidence for hybridization between the N 2p and the Ho 4f states, previously unseen in this or related rare earth nitride systems. The N 2p states extend up to 10 eV below the Fermi level to nearly twice the binding energy as previously believed. Optical spectroscopy yields a minimum direct gap of 1.48 eV. In light of these results, we identify HoN as a prime candidate for spin-diffusion and spintronics applications.

Lanthanide amidinates and guanidinates in catalysis and materials science: a continuing success story

Today the rare-earth elements play a critical role in numerous high-tech applications. This is why various areas of rare-earth chemistry are currently thriving. In organolanthanide chemistry the search for new ligand sets which are able to satisfy the coordination requirements of the large lanthanide cations continues to be a hot topic. Among the most successful approaches in this field is the use of amidinate and guanidinate ligands of the general types [RC(NR')(2)](-) (R = H, alkyl, aryl; R' = alkyl, cycloalkyl, aryl, SiMe(3)) and [R(2)NC(NR')(2)](-) (R = alkyl, SiMe(3); R' = alkyl, cycloalkyl, aryl, SiMe(3)), which can both be regarded as steric cyclopentadienyl equivalents. Mono-, di- and trisubstituted lanthanide amidinate and guanidinate complexes are all readily available. Various rare earth amidinates and guanidinates have turned out to be very efficient homogeneous catalysts e.g. for the polymerization of olefins and dienes, the ring-opening polymerization of cyclic esters or the guanylation of amines. Moreover, certain alkyl-substituted lanthanide tris(amidinates) and tris(guanidinates) were found to be highly volatile and are thus promising precursors for ALD (= atomic layer deposition) and MOCVD (= metal-organic chemical vapor deposition) processes in materials science, e.g. for the production of lanthanide nitride thin layers. This tutorial review covers the continuing success story of lanthanide amidinates and guanidinates which have undergone an astonishing transition from mere laboratory curiosities to efficient homogeneous catalysts as well as ALD and MOCVD precursors within the past 10 years.

Electronic structure of EuN: Growth, spectroscopy, and theory

We present a detailed study of the electronic structure of europium nitride (EuN), comparing spectroscopic data to the results of advanced electronic structure calculations. We demonstrate the epitaxial growth of EuN films, and show that in contrast to other rare-earth nitrides successful growth of EuN requires an activated nitrogen source. Synchrotron-based x-ray spectroscopy shows that the samples contain predominantly Eu${}^{3+}$, but with a small and varying quantity of Eu${}^{2+}$ that we associate with defects, most likely nitrogen vacancies. X-ray absorption and x-ray emission spectroscopies (XAS and XES) at the nitrogen K edge are compared to several different theoretical models, namely, local spin density functional theory with Hubbard $U$ corrections ($\text{LSDA}+U$), dynamic mean field theory (DMFT) in the Hubbard-I approximation, and quasiparticle self-consistent $GW$ (QS$GW$) calculations. The DMFT and QS$GW$ models capture the density of conduction band states better than does $\text{LSDA}+U$. Only the Hubbard-I model contains a correct description of the Eu $4f$ atomic multiplets and locates their energies relative to the band states, and we see some evidence in XAS for hybridization between the conduction band and the lowest-lying ${}^{8}S$ multiplet. The Hubbard-I model is also in good agreement with purely atomic multiplet calculations for the Eu M-edge XAS. $\text{LSDA}+U$ and DMFT calculations find a metallic ground state, while QS$GW$ results predict a direct band gap at X for EuN of about 0.9 eV that matches closely an absorption edge seen in optical transmittance at 0.9 eV, and a smaller indirect gap. Overall, the combination of theoretical methods and spectroscopies provides insights into the complex nature of the electronic structure of this material. The results imply that EuN is a narrow-band-gap semiconductor that lies close to the metal-insulator boundary, where the close proximity to the Fermi level of an empty Eu $4f$ multiplet raises the possibility of tuning both the magnetic and electronic states in this system.

The influence of nitrogen vacancies on the magnetic behaviour of rare-earth nitrides

The rare-earth nitrides are ferromagnetic semiconductors with promise for spintronic devices. Their most common dopants are nitrogen vacancies (VN), with a small enough energy of formation that they exist at of order 1% in epitaxial films. Here we report preliminary investigations of their effect on the magnetic states of two of them in the series, GdN and EuN. In the former we find an enhanced Curie temperature at very high VN concentration, and the Eu2+ ions associated with VN in the latter show strong exchange with their Eu3+ neighbours that might form the basis of a diluted magnetic semiconductor.

Evaluation of Homoleptic Guanidinate and Amidinate Complexes of Gadolinium and Dysprosium for MOCVD of Rare-Earth Nitride Thin Films

Metal−organic chemical vapor deposition (MOCVD) of thin films of two representative rare-earth nitrides is reported here for the first time. Four homoleptic, all-nitrogen-coordinated, rare-earth (RE) complexes were evaluated as precursors for the respective nitride thin film materials. Two guanidinato complexes [RE{(iPrN)2C(NMe2)}3] [RE = Gd (1), Dy (2)] and two amidinato complexes [RE{(iPrN)2CMe}3] [RE = Gd (3), Dy (4)] were compared and used either as single source precursors or together with ammonia for MOCVD of gadolinium nitride (GdN) and dysprosium nitride (DyN), respectively. The thermal properties of the precursors were studied and the fragmentation patterns were characterized by high-resolution electron impact−mass spectrometry (HR EI-MS). The obtained nitride films were investigated using a series of techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), nuclear reaction analysis (NRA), Rutherford backscattering (RBS), and X-ray photoelectron spectroscopy (XPS). The f...

Ultraviolet Light Emitting Devices Using AlGdN

ABSTRACTWe developed ultra-violet field-emission devices using rare-earth nitrides of Al1-xGdxN grown by a reactive radio-frequency magnetron sputtering technique. The Al1-xGdxN phosphor film excited by high-energy electrons shows a resolution limited, narrow intra-orbital luminescence from Gd3+ ions at 318 nm. The devise characteristics depend on injected current and acceleration voltage, which were analyzed by considering multiple excitation process of injected high-energy electrons.

Structural phase transition and elastic properties of rare earth nitrides

The structural phase transition, elastic and thermal properties of nitrides of lanthanum and ytterbium (REN, RE = La and Yb) under high pressure have been investigated by using two-body interionic potential theory with modified ionic charge (Zme) to include Coulomb screening effect by the delocalized f electrons of rare earth ion. The present theoretical approach predicted that LaN and YbN exhibit a NaCl (B1) type structure at an ambient pressure, undergo a structural phase transition from NaCl (B1) into CsCl (B2) phase at 100 GPa and 133 GPa, respectively. We have reported the second order elastic constants (SOECs), third order elastic constants (TOECs) and pressure derivatives of second order elastic constant for these compounds. The variation of elastic constants (C11, C44) with pressure is also presented. The thermal properties and its variation with pressure have been reported for these compounds for the first time.

Structural, electronic, and magnetoresponsive properties of triangular lanthanide clusters and their free‐standing nitrides

The molecular and electronic structures, stabilities, bonding features, and magnetoresponsive properties of three-membered [c-Ln(3)](+/0/-) (Ln = La, Ce, Pr, Nd, Gd, Lu) and heterocyclic six-membered [c-Ln(3)E(3)](q) (Ln = La, Ce, Pr, Nd, Gd, Lu; E = C, N; q = 0 or 1) rings have been investigated by means of electronic structure calculation methods at the DFT level. The [c-Ln(3)](+/0/-) clusters are predicted to be bound with respect to dissociation to their constituent atoms, the estimated binding energies ranging from 45.8 to 2056.4 kJ/mol. The [c-Ln(3)] rings capture easily a planar three-coordinated nitrogen atom at the center or above the center of the ring yielding the lanthanide nitride clusters [c-Ln(3)(μ(3)-N)] adopting a planar geometry, except [c-La(3)(μ(3)-N)] which exhibits pyramidal geometry. The [c-Ln(3)(μ(3)-N)] clusters are predicted to be bound, with respect to dissociation to N ((4)S) atom and [c-Ln(3)] clusters in their ground states, the binding energies ranging from 53.9 to 257.9 kcal/mol. The six-membered [c-Ln(3)E(3)](q) rings are predicted to be bound with respect to dissociation to LnE(q) monomers in their ground states with dissociation energies in the range of 173.8 to 318.0 kcal/mol. Calculation of the NICS(zz)-scan curves of the clusters predicted a "hermaphrodic" magnetic response of the [c-Ln(3)](+/0/-) and heterocyclic six-membered [c-Ln(3)E(3)](q) rings, manifested by the coexistence of successive diatropic (aromatic) and paratropic (antiaromatic) zones. The [c-La(3)](+/0/-) and [c-Lu(3)](-) are predicted to be weakly antiaromatic, the [c-Lu(3)](0/+), [c-Lu(3)C(3) ](+), and [c-Lu(3)N(3)] double (σ+π) aromatic, and the [c-Gd(3)C(3)] and [c-Gd(3)N(3)](+) rings (σ+δ)-aromatic systems.

ChemInform Abstract: Rare‐Earth Activated Nitride Phosphors: Synthesis, Luminescence and Applications

AbstractReview: 51 refs.

Rare-Earth Activated Nitride Phosphors: Synthesis, Luminescence and Applications

Nitridosilicates are structurally built up on three-dimensional SiN4 tetrahedral networks, forming a very interesting class of materials with high thermomechanical properties, hardness, and wide band gap. Traditionally, nitridosilicates are often used as structural materials such as abrasive particles, cutting tools, turbine blade, etc. Recently, the luminescence of rare earth doped nitridosilicates has been extensively studied, and a novel family of luminescent materials has been developed. This paper reviews the synthesis, luminescence and applications of nitridosilicate phosphors, with emphasis on rare earth nitrides in the system of M-Si-Al-O-N (M = Li, Ca, Sr, Ba, La) and their applications in white LEDs. These phosphors exhibit interesting luminescent properties, such as red-shifted excitation and emission, small Stokes shift, small thermal quenching, and high conversion efficiency, enabling them to use as down-conversion luminescent materials in white LEDs with tunable color temperature and high color rendering index.

Study of electronic and structural properties of half metallic rare earth mononitrides

In the present work we investigated theoretically the electronic, magnetic and structural properties of two rare-earth nitrides (REN: RE = Sm, Eu) by using self- consistent tight-binding linear muffin tin orbital (TBLMTO) method. Magnetically, both the rare earth nitrides (RENs) are stable in ferromagnetic (FM) state, while its ambient structure is found to be stable in NaCl-type (B1) structure. From the present study we predict the pressure induced structural phase transition in both RENs from the relatively open NaCl-type structure into more dense CsCl-type structure at 8.6 GPa and 14.6 GPa respectively. They form a new class of half-metallic magnets with high magnetic moments and are strong candidates for applications in spintronics and spinfiltering devices. We have therefore, calculated electronic band structures, equilibrium lattice constants, cohesive energies, bulk moduli and magnetic moments for REN compounds in both B1 and B2 phases.

Photoluminescence properties of rare earth doped α-Si 3N 4

The photoluminescence properties of Eu 2+, Ce 3+ and Tb 3+ doped α-Si 3N 4 have been studied and a possible structural model has been proposed on the basis of the Rietveld refinement of X-ray powder diffraction data. Nearly single phase rare earth doped α-Si 3N 4 was synthesized by a solid state reaction at 1600 °C in N 2–H 2 atmosphere starting from amorphous Si 3N 4 and rare earth oxides or nitrides. Because of small crystal field splitting of the 5d levels, the excitation and emission bands of Eu 2+ and Ce 3+ are positioned at higher energies as isolated ions in comparison with that in Ca-α-Sialon. Both Eu 2+- and Ce 3+-doped α-Si 3N 4 show blue band emission peaking at about 470 and 450 nm, respectively, under UV excitation. α-Si 3N 4:Tb 3+ exhibits dominant green line emission mainly arising from 5D 4→ 7F J ( J=6–3) with weak 5D 3→ 7F J ( J=6–3) transitions of Tb 3+ when excited by UV light. The thermal stability of α-Si 3N 4:Eu 2+ is comparable with that of Ca-α-Sialon:Eu 2+ and is much better than that of α-Si 3N 4:Ce 3+.

ChemInform Abstract: Chains of trans‐Edge Connected [ZM4] Tetrahedra (Z = N and O) in the Lanthanide Nitride Chlorides Ln2NCl3 and Na2Ln4ONCl9 (Ln: La—Nd).

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Elastic properties and hardness calculations of lanthanide nitrides in rocksalt structure

First principles calculations were performed to study the hardness, electronic and elastic properties of lanthanide nitrides in rocksalt structure. The calculated lattice parameters are in good agreement with experimental and other theoretical values. The calculations indicate that NdN, PmN, SmN, EuN, TbN are mechanically unstable. The lower B/ G and Poisson's ratio of HoN and ErN show that they are brittle than other lanthanide nitrides. Results reveal that the ligand field stabilization energy and lanthanide contraction play important roles in determining the hardness of lanthanide nitrides in rocksalt structure.