Substitution and proton doping effect on SrZrO3 behaviour: high‐pressure Raman study

The high‐pressure Raman studies of pure, Yb‐modified, protonated and non‐protonated SrZrO₃ dense ceramics were performed between 0.1 and 40 GPa using a diamond anvil cell. Lanthanide‐modified, protonated SrZrO₃ perovskites are potential materials for electrolytic membrane in fuel cells and electrolysers working at medium temperature. The comparison of the Raman spectra shows important differences in the pressure behaviour between the pure and Yb‐modified SrZrO₃ ceramics. SrZrO₃ exhibits a rigid structure without any structural modification, whereas for both SrZr_0.93 Yb_0.07 O_2.965 and SrZr_0.93 Yb_0.07 O_2.962 H_0.003 a sequence of structural modifications at 10, 20 and 35 GPa is revealed. The character of these structural modifications is very similar to that observed as a function of the temperature (orthorhombic Pnma 750 °C → pseudo‐tetragonal Imma 840 °C → tetragonal I4/mcm 1070 °C → cubic Pm3m), which suggests that they can be considered as the phase transitions. Despite the low level of proton content (0.3% mole/mole), significant difference between protonated and non‐protonated compounds is observed for the 700–750 cm⁻¹ doublet assigned to the Zr O octahedron stretching mode, perturbed by an oxygen atom vacancy and/or neighbouring Yb ion. The location of proton is discussed. Copyright © 2011 John Wiley & Sons, Ltd.     

Dielectric relaxation of protonic defects in hydrated and dehydrated SrZrO3:Yb

Dielectric relaxations of proton conductor Yb-doped SrZrO₃ have been measured as a function of temperature and frequency in different pretreatments with water vapor. Well defined two relaxation peaks were observed in the compounds doped with more than 1 mol% of Yb irrespective of annealing in WET (hydrate) or DRY (dehydrate) conditions. These two relaxations can be assigned to reorientations of Yb-proton dipole at an isolated [YbO₆]-octahedron and at a [YbO₆]–[YbO₆]–... cluster in the SrZrO₃ crystal (i.e. effectively negative charged Yb and positive charged proton). By DRY pretreatment, the magnitude of the dipole strength was almost fixed for the isolated Yb-proton dipole, while the magnitude became significantly decreased for the dipole associated with the Yb-cluster. It shows the proton associated with the isolated Yb is more stable than that with the Yb-cluster.     

Magnetic, Mössbauer and X-ray photoelectron spectroscopic investigations on nanocrystalline Fe3Mo3N, FeMoN2 and Ni3Mo3N nitrides

Nanocrystalline Fe₃Mo₃N, FeMoN₂ and Ni₃Mo₃N interstitial nitrides having particle sizes of 32, 50 and 80 nm respectively, were synthesized by molybdate precursor route. The nanocrystalline nature was examined by XRD and SEM. Both, the field and temperature dependence of magnetic characteristics indicate paramagnetic behavior of the materials and the possible presence of magnetic contamination in the nitride materials. We examine the Mössbauer spectroscopic investigations in order to probe the magnetic characteristics and contaminations therein. X-ray photoelectron spectroscopy (XPS) study corroborates the possible presence of magnetic oxide phases at the surface of the Fe₃Mo₃N and Ni₃Mo₃N nano-particles. In summary, this paper investigates the magnetic properties of newer nitrides with appropriate corroboration with the Mössbauer and XPS studies.     

Perovskite thin films grown by direct liquid injection MOCVD

The continuous scaling down of devices dimensions, in silicon technology, imposes to replace silicon dioxide. Among the potential candidates for new capacitors, some perovskite structure materials (such as titanate or zirconate) show interesting characteristics. The first way to develop perovskite films is to use a mixture of two β-diketonates by varying the solution's cationic ratio. However, our previous results on SrZrO₃ showed that a wide parametric study had to be carried on. Another way is to design novel heterometallic precursors that contain both cations on the same molecule. The ligands could be chosen so that peculiar evaporation and decomposition temperatures could be obtained.Thus, perovskite films (SrZrO₃) were deposited on plane Si(1 0 0) substrates by direct liquid injection MOCVD from two original heterometallic precursors Sr₂Zr₂(OⁿPr)₈(thd)₄(ⁿPrOH)₂ and Sr₂Zr₂(thd)₄(OⁱPr)₈. The oxide films were deposited at substrate temperature ranging from 550 to 900 °C. At the lowest temperatures (550 and 600 °C) the as-deposited films were amorphous. After a postannealing at 700 °C for 1 h under N₂/O₂, the films deposited at 550 °C were crystallized in the SrZrO₃ orthorhombic phase. Crystallographic and chemical structures were controlled applying grazing X-ray diffraction and infrared spectroscopy measurements. Results are discussed with respect to experimental synthesis conditions.     

First-principles study of the (0 0 1) surface of cubic SrZrO3

Density functional calculations have been used to investigate the (0 0 1) surface of cubic SrZrO₃ with both SrO and ZrO₂ termination. Surface structure and electronic structure have been obtained. The SrO surface is found to be similar to its counterpart in SrTiO₃, while there are marked differences between the ZrO₂ and TiO₂ terminations in SrZrO₃ and SrTiO₃, respectively, concerning surface relaxation and rumpling. For the ZrO₂-terminated surface of SrZrO₃, the covalency of the interaction between the outmost Zr and the O beneath is enhanced as a result of their bond contraction. The band gap reduction and the presence of the surface states are also discussed in relation with the behavior of the electrostatic potential.     

Electron paramagnetic resonance study of impurities and point defects in oxide crystals

In this topic review the results of the X-band electron paramagnetic resonance (EPR) measurements of Mn, Co, Cr, Fe ions in YAlO₃ (YAP) crystals and Fe ions in LiNbO₃ (LNO) crystals and of chromium doped Bi₁₂GeO₂₀ (BGO) and Ca₄GdO(BO₃)₃ single crystals, are presented. It is well known that the oxide crystals (for example:YAP, LNO, BGO) are one of the most widely used host materials for different optoelectronic applications. The nature of point defect of impurities and produced in the oxide crystal after irradiation by bismuth ions and after irradiation by the ²³⁵U ions with energy 9.47 MeV/u and fluency 5?×?10¹¹?cm^?1 is discussed. The latter is important for applications of these oxide crystal as laser materials.     

Phase transitions in the H+-conducting perovskite ceramics by the quasi-elastic neutron and high-pressure Raman scattering

H⁺-containing lanthanide-doped perovskites A(Ba, Sr etc.)B(Zr, Ce, Ti etc.)O₃ are potential ceramic membranes for fuel cell and medium temperature water electrolysis (300–800 °C). The comparison studies of the hydrated and non-hydrated Yb-doped BaZrO₃ and SrZrO₃ were performed by thermal expansion, medium–high temperature neutron and room-temperature high-pressure Raman scattering. Neutron diffraction and elastic/quasi-elastic studies carried out for BaZrO₃ ceramic show the presence of the protons, their successive diffusion above ∼600 °C, and then their departure above 750 °C (under vacuum). Phase transitions and their modification by proton insertion are discussed. A high-pressure Raman study of SrZrO₃ performed at room temperature in the diamond anvil cell reveals the presence of two pressure-induced phase transitions at about 5 and 22 GPa and confirms that proton insertion modifies the phase transition sequences. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.     

Mössbauer spectra and magnetic properties of iron nitrides

The iron nitrides Fe₃N, Fe₄N and Fe₈N (impure) have been prepared by treating iron in NH₃/H₂ gas mixtures followed by appropriate heat treatment. Fe₈N has also been prepared by mechanical alloying. Mössbauer spectra are analysed in terms of iron sites with different nitrogen environments, and results are compared with magnetization measurements and band-structure calculations. Moments and hyperfine fields on sites with nitrogen neighbours are reduced by p-d hybridization, whereas the sites farthest from nitrogen have the lowest charge and the highest moments. There is no evidence of an exceptionally high magnetization in any of these compounds.     

Constituted oxides/nitrides on nitriding 304, 430 and 17-4 PH stainless steel in salt baths over the temperature range 723 to 923 K

The progressively developed oxides and nitrides that form on nitriding 304, 430 and 17-4 PH stainless steel are analysed by X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) in this study. The experimental results show that the Cr contents and matrix structures (ferrite, austenite and martensite) play an important role in forming FeCr₂O₄, Cr₂O₃ and Fe₂O₃ oxides as well as nitrides. After a short immersion time, oxides of Cr₂O₃ and FeCr₂O₄ form in nitride films on 304 stainless steel samples. Fe₂O₃ oxide will subsequently form following an increasing immersion time. For the 430 stainless steel, Cr₂O₃ predominately forms after a short dipping time which hinders the growth of the nitride layer. As a result, this sample had the thinnest nitride film of the three for a given immersion time. After the formation of oxides, both CrN and Cr₂N were detected near the surface of the nitride films of three samples while Cr₂N phases formed in the deeper zone. The greatest amount of Fe₂O₃ oxide among the three samples was obtained on the nitriding 17-4 PH stainless steel which also had a high intensity count of N 1s.     

Band alignments at SrZrO3/Ge(0 0 1) interface: Thermal annealing effects

High-κ dielectrics SrZrO₃ were prepared on Ge(0 0 1) substrate using pulse laser deposition, and band alignments and thermal annealing effects were studied with high resolution X-ray photoemission spectroscopy. Valence and conduction band offsets at this interface were measured to be 3.26 eV and 1.77 eV, respectively. Interfacial Ge oxide layers were found at the interface. After annealing at 600 °C, the interfacial Ge oxide layers were eliminated, and the valence band offset increased to 3.50 eV, but the amorphous SrZrO₃ became polycrystalline in the meantime.     

Optical waveguides in laser crystals

This article reviews the recent research on different types of planar and channel crystalline optical waveguides, fabrication methods such as liquid phase epitaxy, pulsed laser deposition, thermal bonding, reactive ion or ion beam etching, wet chemical etching, ion in-diffusion, proton exchange, ion beam implantation, and femtosecond laser writing, as well as waveguide laser operation of rare-earth and transition-metal ions in oxide crystalline materials such as Al₂O₃, Y₃Al₅O₁₂, YAlO₃, KY(WO₄)₂, and LiNbO₃. To cite this article: M. Pollnau, Y.E. Romanyuk, C. R. Physique 8 (2007).     

Strain induced ferroelectricity in the SrZrO3 / SrTiO3 superlattice: First principles study

The lattice distortion strain induced ferroelectricity in SrZrO₃/ SrTiO₃ superlattice is studied using first principles density functional theory. Within the tetragonal symmetry , the lattice distortion from the lattice mismatch in the superlattice structure is determined through energy minimization. This kind of lattice distortion leads to the formation of spontaneous polarization in the superlattice, although neither SrZrO₃ nor SrTiO₃ is ferroelectric. From analysis of the relative displacements of the cations and anions, we have found that the SrZrO₃ cell may make a greater contribution to the polarization in the SrZrO₃/ SrTiO₃ superlattice than the SrTiO₃ cell. An extremely large polarization of 42.7 μC/cm² has been predicted.     

Combustion synthesis of nitrides: mechanistic studies

Combustion synthesis (CS) is characterized by extremely high heating rates (up to 10⁶ K/s) and temperatures (up to 4000 K), as well as short times of reaction completion (usually less than 1 s, sometimes even 10⁻³–10⁻² s). The above unique conditions lead to materials microstructure formation mechanisms that cannot be observed in the isothermal or quasi-isothermal cases. In turn, evolution of the reaction media microstructure influences characteristics (temperature, velocity) of the combustion wave. In this work, such microstructural effects are demonstrated on the examples of CS in different gas–solid systems (i.e., Si–N₂, Al–N₂, and Nb–N₂). It is shown that dilution of the reaction media by inert high surface area precursors with phase compositions similar to the CS product leads to a significant increase in combustion velocity in the Si–N system, while the temperature remains constant. Also, it is revealed that dispersion of the metal (Nb) particles in the preheating zone might be responsible for constancy of the combustion velocity as a function of the size of the initial reactants. Finally, a complex two-stage mechanism of AlN formation explains the non-monotonic shape of the temperature–time profile observed during CS in this system.     

Magnetism without magnetic ions in non-magnetic perovskites SrTiO3, SrZrO3 and SrSnO3

Using the full-potential linearized augmented plane-wave (FP-LAPW) method with the generalized gradient approximation (GGA) for the exchange-correlation potential, we studied spin polarization induced by replacement of oxygen atoms by non-magnetic 2p impurities (B, C and N) in non-magnetic cubic SrMO₃ perovskites, where M=Ti, Zr and Sn. The results show that the magnetization may appear because of the spin–split impurity bands inside the energy gap of the insulating SrMO₃ matrix. Large magnetic moments are found for the impurity centers. Smaller magnetic moments are induced on the oxygen atoms around impurities. It is shown that SrTiO₃:C and SrSnO₃:C should be magnetic semiconductors while other compounds in this series (SrTiO₃:B, SrTiO₃:N and SrZrO₃:C) are expected to exhibit magnetic half-metallic or pseudo-half-metallic properties.     

Structural,elastic and optoelectronic properties of Sr-based perovskite-type oxides SrXO3 (M = Th,Zr) via first-principles calculations

In this paper, we present a detailed theoretical investigation on the structural, elastic, electronic and optical properties of the perovskite oxides SrThO₃ and SrZrO₃ by using the pseudo-potential plane wave (PP-PW) method. The computed lattice constants of SrXO₃ (X = Th and Zr) are in excellent agreement with the available experimental data. SrThO₃ and SrZrO₃ are direct (Γ–Γ) and indirect (Γ–R) band gap semiconductors, respectively. Under pressure effect a crossover between the indirect band gap (R–Γ) and the direct band gap (Γ–Γ) curves occurs at about 35 GPa for SrZrO₃, resulting in the energy minimum of direct gap (Γ–Γ) for this compound. The covalence in the Zr–O and Th–O bonds arises due to the hybridization between O–p and Zr–d (Th–d) states. Under pressure effect, the threshold energy becomes slightly greater (smaller) for SrZrO₃ (SrTO₃) for 3.21 (2.28) eV and the main peaks are shifted towards higher energies. Although the positions of all peaks shifted under pressure, they still have the same type as those at zero pressure, with decreasing the intensity of the main peaks.     

Oxygen diffusion in SrZrO3

Oxygen diffusion coefficients in SrZrO₃ polycrystals were determined using the isotopic exchange method with ¹⁸O as oxygen tracer. Diffusion treatments were performed at different temperatures between 1173 K and 1473 K. Oxygen diffusion profiles were established by secondary ion mass spectroscopy (SIMS). Classical diffusion equations were used to fit experimental results and to determine bulk diffusion (D_vol) and surface exchange (k) coefficients of oxygen in SrZrO₃ polycrystalline materials. From these values, bulk diffusion and grain boundary diffusion coefficients as well as oxygen surface exchange coefficients were determined. The activation energy of oxygen diffusion in the bulk is 2.1 eV, while for the diffusion in the grain boundary, 1.8 eV was found. The surface exchange reaction has an activation enthalpy of 1.2 eV.     

57Fe Mössbauer study of Fe nitrides

Summary Magnetic properties of Fe nitrides have been re-examined by⁵⁷Fe M?ssbauer spectroscopy. Hyperfine magnetic fields for α″-Fe₁₆N₂ are 30, 31 and 39T at 298K, but the averaged hyperfine field is 33T and nearly equal to the value of pure α-Fe. σ-Fe₂ N is an antiferromagnet below 9K having a small magnetic moment less than 0.1 μ_B, although γ′-Fe₄N and ε-Fe_3–2N are ferromagnets. ZnS-type FeN is non-magnetic at 4.2K. M?ssbauer spectra obtained from NaCl-type FeN are complex and some Fe atoms in this nitride show a surprisingly large hyperfine magnetic field of 49T. Paper presented at the ICAME-95, Rimini, 10–16 September 1995.     

Muon spectroscopy for studying magnetism and protons and lithium dynamics in spinel manganese oxides

Polarised positive muons can be implanted into any type of material and rapidly thermalize, then the local magnetic environment dictates the evolution of muon spin vectors and provokes the muon depolarisation. The muon spin relaxation (μSR) technique provides interesting information on magnetism and spin dynamics in spinel lithium manganates insertion compounds. In this work, we compare the behaviour of muons into a lithium-rich spinel manganese oxide and its lithium extracted product. The chemical extraction of lithium from Li_1.33Mn_1.67O₄, where all the manganese is Mn^IV, is essentially a lithium by proton ion exchange process to give a protonated manganese oxide with spinel structure, H⁺–MnO₂. Muons clearly have showed the presence of protons in H⁺–MnO₂, and the movement of lithium ions or protons at increasing temperatures in both samples. Muons are quasi-static in these compounds, and they are located both in ‘regular’ lithium and proton sites and also in interstitial sites of the spinel structure, these latter being used during diffusion of lithium ions. Below 50 K, static muons behave as in a paramagnet, where Mn magnetic spins are slowing down and ordering near 6 and 14 K in the protonated and lithiated spinels, respectively.     

Curie temperatures and modified de Gennes factors of rare earth nitrides

The Curie temperatures T_C of the nitrides of the rare earths (Gd, Tb, Dy, Ho, and Er), including binary systems, were investigated. T_C was found to be approximately proportional to the de Gennes factor, ξ=(g−1)²J(J+1), where g is the Landé g-factor and J is the total angular momentum quantum number of a trivalent rare earth (RE). This proportionality was significantly improved by introducing a modified de Gennes factor, ξ_bi. The conventional de Gennes factor ξ indicates the exchange interaction given by the inner product of the effective spin components of ions of the same kind, whereas our modified de Gennes factor ξ_bi also considers interactions between different kinds of ions and statistical factors calculated on the basis of the binomial distribution. The good proportionality obtained between T_C and ξ_bi indicates that the spin components of RE ions interact with each other. This interaction is considered to be responsible for the ferromagnetism of these nitrides (including binary systems). These considerations were supported by the synthesis of and magnetization measurements on Gd_xEr_1−xN (x=0, 0.25, 0.5, 0.75, 1) samples.     

Electronic structure of nitrides PuN and UN

The electronic structure of uranium and plutonium nitrides in ambient conditions and under pressure is investigated using the LDA + U + SO band method taking into account the spin–orbit coupling and the strong correlations of 5f electrons of actinoid ions. The parameters of these interactions for the equilibrium cubic structure are calculated additionally. The application of pressure reduces the magnetic moment in PuN due to predominance of the f ⁶ configuration and the jj-type coupling. An increase in the occupancy of the 5f state in UN leads to a decrease in the magnetic moment, which is also detected in the trigonal structure of the UN _x β phase (La₂O₃-type structure). The theoretical results are in good agreement with the available experimental data.