Stress-induced cubic-tetragonal transformation in partially stabilized ZrO2: Raman spectroscopy study

Regularities of the cubic-tetragonal transformation (C→t′) in partially stabilized zirconia were studied with Raman spectroscopy and high-temperature Raman spectroscopy techniques. New ‘low temperature’ mechanism of tetragonal nanoparticles formation in a volume of cubic solid solution was revealed in ZrO₂-Gd₂O₃ (Eu₂O₃) (6-8 mol%) single crystals. This mechanism includes nucleation of the tetragonal nanoparticles due to diffusionless C→t′ phase transformation at the first stage and gradual decrease of the stabilizer concentration inside t′-domains after subsequent low-temperature annealing. Predominant orientation of tetragonal domains due to the stress-induced C→t′ transformation was registered in ZrO₂-Gd₂O₃ (8 mol%) single crystals.     

Monoclinic-tetragonal phase transition in zirconium and hafnium dioxides: A high-temperature raman scattering investigation

Zirconium dioxide ZrO₂ and hafnium dioxide HfO₂ are investigated using high-temperature Raman spectroscopy in the temperature range 300–2080 K, including the regions of the monoclinic-tetragonal phase transitions revealed in these materials. An analysis is made of the specific features observed in the evolution of the high-temperature Raman spectra of both the monoclinic (m) and tetragonal (t) modifications of ZrO₂ and HfO₂ with variations in the temperature. The polarized Raman spectra of the metastable tetragonal phases in solid solutions based on zirconia and hafnia are used to identify the symmetry of vibrations in the spectra of the tetragonal modifications of pure zirconium and hafnium dioxides, which exist at high temperatures.     

Evidences of the evolution from solid solution to surface segregation in Ni‐doped SnO2 nanoparticles using Raman spectroscopy

Ni‐doped SnO₂ nanoparticles, promising for gas‐sensing applications, have been synthesized by a polymer precursor method. X‐ray diffraction (XRD) and transmission electron microscopy (TEM) data analyses indicate the exclusive formation of nanosized particles with rutile‐type phase (tetragonal SnO₂) for Ni contents below 10 mol%. The mean crystallite size shows a progressive reduction with the Ni content. Room‐temperature Raman spectra of Ni‐doped SnO₂ nanoparticles show the presence of Raman active modes and modes activated by size effects. From the evolution of the A_1g mode with the Ni content, a solubility limit at ∼2 mol% was estimated. Below that content, Raman results are consistent with the occurrence of solid solution (ss) and surface segregation (seg.) of Ni ions. Above ∼2 mol% Ni, the redshift of A_1g mode suggests that the surface segregation of Ni ions takes place. Disorder‐activated bands were determined and their integrated intensity evolution with the Ni content suggest that the solid‐solution regime favors the increase of disorder; meanwhile, that disorder becomes weaker as the Ni content is increased. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of Hydrolysis Conditions on the Direct Formation of Nanoparticles of Ceria–Zirconia Solid Solutions from Acidic Aqueous Solutions

The effect of the cation concentration, hydrolysis temperature, and composition in the CeO₂–ZrO₂ system on the direct precipitation of ceria–zirconia solid solutions and the structure of the precipitates from acidic aqueous solutions of (NH₄)₂Ce(NO₃)₆ and ZrOCl₂ by hydrolysis under hydrothermal conditions were investigated. Nanometer-sized (8–10 nm) ceria–zirconia solid solution particles in a composition range of 0 to 60 mol% ZrO₂ were directly precipitated from the solutions with total metal cation concentration less than 0.2 mol/dm³ by simultaneous thermal hydrolysis at 150–240°C. The crystalline phase of the precipitates gradually changed from cubic and/or tetragonal to monoclinic with increasing the cation concentration of the solution from 0.2 to 0.8 mol/dm³ at the starting composition of 50 mol% ZrO₂ under hydrolysis condition of 150°C for 48 h, which was attributed to decrease in the supply of hydrolyzed Ce component caused by decrease in the hydrolysis ratio of (NH₄)₂Ce(NO₃)₆. Ceria–zirconia solid solutions containing large amount of ZrO₂ maintained high specific surface area and small-sized crystallite after heat-treatment at 900–1000°C for 1 h.     

Raman spectroscopic study of phase transitions in undoped morphotropic PbZr1−xTixO3

Several PbZr_1−xTi_xO₃ (PZT) compositions in the proximity of the morphotropic phase boundary (MPB) were examined by means of Raman spectroscopy in the 15–800 K temperature range. Previous studies performed by other researchers using various techniques evidenced that, in the phase diagram of PZT, areas with rhombohedral/monoclinic and tetragonal/monoclinic phases coexist across the MPB. For these compositions, either long‐range or short‐range symmetry ordering of the monoclinic phase should be considered, so that no true rhombohedral–monoclinic–tetragonal phase boundary exists. In addition, the onset of antiferrodistortive phase transitions between high‐T and low‐T perovskite phases has been predicted by ab initio calculations and experimentally reported. In the present work, low‐T and high‐T Raman scattering spectra were collected on undoped PbZr_1−xTi_xO₃ with compositions x = 0.42, 0.45, 0.465, 0.48 and 0.53 in an attempt to clarify the current open issues on the phase diagram of PZT. Spectra clearly belonging to the respective phases were observed in the rhombohedral, monoclinic and tetragonal areas, thus confirming that monoclinic ordering is long‐range only for a narrow range of compositions. Raman measurements at cryogenic temperatures allowed detecting all predicted low‐T phases, including the tetragonal one. These results are in good agreement with both ab initio calculations and experimental results obtained by other authors on the same compositions. Copyright © 2010 John Wiley & Sons, Ltd.     

Molecular dynamics simulation of zirconia melting

The melting point for the tetragonal and cubic phases of zirconia (ZrO₂) was computed using Z-method microcanonical molecular dynamics simulations for two different interaction models: the empirical Lewis-Catlow potential versus the relatively new reactive force field (ReaxFF) model. While both models reproduce the stability of the cubic phase over the tetragonal phase at high temperatures, ReaxFF also gives approximately the correct melting point, around 2900 K, whereas the Lewis-Catlow estimate is above 6000 K.     

Processing and characterization of ultra-thin yttria-stabilized zirconia (YSZ) electrolytic films for SOFC

Sub-micron yttria-stabilized zirconia (YSZ) electrolyte layer was prepared by a liquid state deposition method and with an average thickness of 0.5 μm to improve the performance of the anode-supported solid oxide fuel cell (SOFC). The YSZ precursors, containing yttrium and zirconium species and an additive, poly-vinyl-pyrrolidone (PVP), were spin-coated on a Ni/YSZ anode substrate. Several properties, including crystalline phases, microstructures, and current–voltage (I–V) characteristics, were investigated. The thin film of 4 mol% Y₂O₃-doped ZrO₂ (4YSZ) consisted of cubic, tetragonal, and a trace of monoclinic phases, and showed a crack-free layer after sintering at 1300 °C. The anode supported SOFC, which consists of the Ni–YSZ anode, 4YSZ electrolyte, and Pt/Pd cathode, showed power densities of 477 mW/cm² at 600 °C, and 684 mW/cm² at 800 °C. Otherwise, the surface cracks of the other YSZ-coated samples (e.g. 8YSZ) can be repaired by a multi-coating method.     

Analysis of low temperature specific heat in the ferromagnetic state of the Ca-doped manganites

X-ray diffraction and Raman data on the pressure induced phase transitions of a nanometric zirconia, ZrO₂, are analyzed via a classical phenomenological Landau approach of the bulk. It is concluded that the initial tetragonal structure (D _4h ¹⁵), which is a metastable bulk state of zirconia at ambient conditions, evolves continuously towards the ideal cubic fluorite structure (O _h ⁵) via an intermediate tetragonal form (D _4h ¹⁴). The proposed phenomenological model describes consistently all experimental peculiarities, including the hybridization and softening of the low-frequency Raman active modes along with lattice-parameter anomalies.Received: 29 July 2003, Published online: 15 October 2003PACS: 64.70.Nd Structural transitions in nanoscale materials - 61.50.Ks Crystallographic aspects of phase transformations; pressure effects - 61.10.Nz X-ray diffraction     

Unique sharp photoluminescence of size-controlled sonochemically synthesized zirconia nanoparticles

The present study explores the features of tetragonally stabilized polycrystalline zirconia nanophosphors prepared by a sonochemistry based synthesis from zirconium oxalate precursor complex. The sonochemically prepared pristine zirconia, 3 mol%, 5 mol% and 8 mol% yttrium doped zirconia nanophosphors were characterized using thermo-gravimetric analysis (TGA), X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM) with energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS) and photoluminescence spectroscopy (PL). The reaction mechanism of formation of zirconia nanophosphors is discussed in detail. The probable sonochemical formation mechanism is being proposed. Stabilization of tetragonal phase of pristine zirconia even at room temperature was effectively established by controlling the particle size using ultrasonic waves. Improved phase purity and good surface morphology of the nanophosphors is being achieved via sonochemical route. FE-SEM micrographs reveal that the nanoparticles have uniform spherical shape and size. The narrow particle size distribution (∼15–25 nm) of the zirconia nanoparticles was found from FE-SEM statistical analysis and further confirmed by TEM. Zirconia nanophosphors exhibit a wide energy band gap and which was found to vary with yttrium dopant concentration. The highlight of the present study is the synthesis of novel nanocrystalline ZrO₂ and Y-ZrO₂ phosphor which simultaneously emits extremely sharp as well as intense UV, violet and cyan light on exciting the host atom. The yttrium ion dopant further enhances the photoluminescence property of zirconia. These nanocrystalline phosphors are likely to have remarkable optical applications as light emitting UV-LEDs, UV lasers and multi color displays.     

Degradation mechanism of scandia-stabilised zirconia electrolytes: Discussion based on annealing effects on mechanical strength,ionic conductivity,and Raman spectrum

In the present study, the mechanical strength as well as the ionic conductivity of the scandia- and ytterbium oxide-codoped zirconia electrolyte is investigated during annealing. Based on the annealing effect on both properties, the degradation mechanism is discussed. The X-ray diffraction and Raman spectroscopic analyses confirmed that annealing enhanced the tetragonal phase content in the samples which initially contained the tetragonal phase. The samples with the rhombohedral phase showed no phase changes. The samples initially with the tetragonal phase showed a significant decrease in the conductivity but an increase in the strength whereas the samples with the rhombohedral phase generally showed no change and the ones with the cubic phase showed slight decreases in both properties. From above results, the conductivity degradation of the sample with the tetragonal phase was attributed to two causes: the formation of low conductive but transformable t-phase from the high conductive t′-phase and the formation of t′-phase from the t″-phase. The annealing effect was little for the samples initially with rhombohedral phase and complex for the ones with cubic phase.     

Lattice dynamics and low‐temperature Raman spectroscopy studies of PMN–PT relaxors

In this work we present a Raman scattering study of a specific region of the morphotropic phase boundary (MPB) of the [Pb(Mg_1/3Nb_2/3)O₃]_1−x (PbTiO₃)_x relaxor system. We performed low‐temperature measurement for the x = 0.4 composition in the 20–300 K temperature range, and a detailed analysis of Raman spectra of x = 0.4 and x = 0.37 compositions at 180 K. The analysis of Raman spectra indicates a structural phase transition at around 170 K for x = 0.4. The comparison of Raman data from x = 0.4 and x = 0.37 compositions suggests different phases for these samples at 180 K. These results are in accordance with the tetragonal to monoclinic structural phase transition observed in the PMN–PT MPB and contribute to improve the knowledge of the MPB of this solid solution. Additionally, we have performed the lattice dynamics phonon calculation of the (1 − x) PMN–xPT relaxor in order to best understand its complex Raman spectral properties. The normal mode analyses (at q ∼ 0) were performed by considering tetragonal symmetry for the (1 − x) PMN–xPT system and using the rigid ion model and mean field approximation. Our calculated wavenumber values are in good agreement with experimental and calculated results reported for PbTiO₃ thus providing a reliable assignment of the various Raman modes. The low wavenumber modes are interpreted as arising from a lifting of the degeneracy of the vibrational modes related to Mg, Nb and Ti sites. Copyright © 2009 John Wiley & Sons, Ltd.     

Raman tensor analysis of (K0.5Na0.5)NbO3–LiSbO3 lead‐free ceramics and its application to study grain/domain orientation

A quantitative polarized Raman analysis of ferroelectric grain/domain orientation in LiSbO₃ (LS‐modified) (K_0.5Na_0.5)NbO₃ (KNN) ceramics is presented, based on the analysis of the complex orientation dependence in space of their Raman‐active modes. Complete sets of Raman tensor elements of A_g, and E_g phonon modes for orthorhombic/tetragonal structures of KNN have been determined. Through this spectroscopic algorithm, quantitative information could be extracted in terms of three Euler angles in space for KNN samples consisting of mixed phases, thus enabling quantitative visualization of the local distribution of grains/domains in the solid angle. As an application of the method, we quantitatively examined the unknown crystallographic grain orientation patterns on the surfaces of pure KNN and of KNN‐0.05LS ceramics. These two samples were useful to clarify a polymorphic phase transition from the orthorhombic to the tetragonal phase taking place in the LS‐modified KNN system. Thus, we demonstrated that polarized Raman spectroscopy is a valuable and efficient tool for nondestructive three‐dimensional assessments of grain/domain orientation in ferroelectric materials with complex polymorphic structures. We believe that the data shown here represent a typical scenario encountered in grain/domain orientation assessments of piezoelectric perovskites. Copyright © 2012 John Wiley & Sons, Ltd.     

Characterization of laser sealed coatings of yttria partially stabilized zirconia

The present paper reports an experimental investigation based on X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and wavelength dispersive spectroscopy (WDS) analyses of phases formed after laser sealing of plasma sprayed coatings of 8.5 wt% yttria partially stabilized zirconia (YPSZ). X-ray diffraction and X-ray step-scanning analyses showed that the plasma sprayed and sealed coatings consisted mainly of t′ phase with a very small amount of monoclinic phase (m phase) in the plasma sprayed coatings. It was also found that the small amounts of m and cubic phases (c phase) present in the sealed coatings were dependent on laser processing specific energies (specific energy is equal to laser power/traverse speed x beam diameter). It was also found that rhombohedral (r) phase formed after laser sealing of coatings at higher specific energies. A direct relationship between c/a ratio of transformable tetragonal phase (t phase) produced by thermal treatment of sealed coatings and nontransformable tetragonal phase (t′ phase) and the amount of Yttria was obtained. A new equation was derived, which describes the relationship between Yttria concentration and the c/a ratio of tetragonal phases (t or t′).     

Phase transformation of ZrO2 nanoparticles produced from zircon

This article focuses on the phase transformation of zirconia (ZrO₂) nanoparticles produced from zircon using a bottom-up approach. The influence of mechanical milling and thermal annealing on crystalline phase transformation of ZrO₂ nanoparticles was explored. It was found that the iron oxide, as an inherent impurity present in ZrO₂ nanoparticles, produced from zircon stabilises the cubic phase after calcination at 600°C. The stabilised cubic phase of ZrO₂ nanoparticles was disappeared and transformed into partial tetragonal and monoclinic phases after mechanical milling. The phase transformation occurred on account of the crystal defect induced by high-energy mechanical milling. The destabilisation of cubic phase into monoclinic phase was observed after the thermal annealing of ZrO₂ nanoparticles at 1000°C. The phase transitions observed are correlated to the exclusion of iron oxide from the zirconia crystal structure.     

Effect of lanthanum substitution on the Raman spectra of barium titanate thin films

Thin films of Ba_1−xLa_xTiO₃ on platinum substrates were synthesized using the sol–gel method for x values of 0.0, 0.03, 0.05, and 0.10, and the effect of trivalent La³⁺ substitution on the structural and dielectric properties was studied. Using X‐ray diffraction, structural analysis of these compositions revealed a slight increase in the tetragonal distortion of the unit cell with increase in La content. Accordingly, an increase in the tetragonal to cubic transition temperature T_T/C was detected by temperature‐dependent Raman spectroscopy in the range of 70–500 K. Unlike the results from Raman scattering for the La‐doped BaTiO₃ films, the dielectric measurements showed broad and diffused dielectric maxima, making the estimation of the transition temperature merely qualitative. Copyright © 2006 John Wiley & Sons, Ltd.     

Synthesis,structural and conductive properties of Nd doped garnet-type Li7La3Zr2O12 Li-ion conductor

Garnet type solid electrolyte Li₇La₃Zr₂O₁₂ (LLZO) is the most promising candidate among solid electrolytes for all solid state Li batteries. In this work, small amount of Nd doping (5%–20%) to the garnet structure is proposed to improve its ionic conductivity. Nd doped garnet type solid electrolytes for Li-ion batteries were synthesized through a conventional solid state reaction method. The effect of Nd doping on the microstructure, morphology and ionic conductivity of the LLZO was studied by powder X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, Field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS) and Electrochemical impedance spectroscopy (EIS) methods. Instead of La whose valence is similar to that of Nd, XRD and Raman analyzes revealed that Nd takes the place of the higher valence element Zr. In order to compensate the valence difference, the ratio of Li increases in the structure. On the other side, results showed that Nd doped LLZO samples formed as a mixture of both tetragonal and cubic phases. According to EIS measurements, among the prepared samples, 5% Nd doped LLZO exhibits the highest ionic conductivity of 2.47 × 10⁻⁶ S cm⁻¹ at room temperature.     

Structural disorder and phase transitions in ZrO2-Y2O3 system

The phase transitions in the ZrO₂-YzO₃ system have been investigated as a function of temperature using Raman scattering. The cubic phase was found to be structurally disordered and the spectrum has been compared with a one phonon density of states derived from a rigid ion model. The frequencies and symmetries of the modes were obtained for the cubic, tetragonal and monoclinic phases and comparisons have been made between the various temperatures and compositions.     

In situ Raman and photoluminescence study on pressure‐induced phase transition in C60 nanotubes

Single crystalline C₆₀ nanotubes having face‐centered‐cubic structure with diameters in the nanometer range were synthesized by a solution method. In situ Raman and photoluminescence spectroscopy under high pressure were employed to study the structural stabilities and transitions of the pristine C₆₀ nanotubes. A phase transition, probably because of the orientational ordering of C₆₀ molecules, from face‐centered‐cubic structure to simple cubic structure occurred at the pressure between 1.46 and 2.26 GPa. At above 20.41 GPa, the Raman spectrum became very diffuse and lost its fine structure in all wavenumber regions, and only two broad and asymmetry peaks initially centered at 1469 and 1570 cm^–1 were observed, indicating an occurrence of amorphization. This amorphous phase remained to be reversible until 31.1 GPa, and it became irreversible to the ambient pressure after the pressure cycle of 34.3 GPa was applied. Copyright © 2012 John Wiley & Sons, Ltd.     

Er-Y共掺杂ZrO2的Raman光谱和Er3+的荧光光谱

采用柠檬酸溶胶凝胶法制备Er-Y共掺杂的具有单斜、四方和立方结构的ZrO2基复合氧化物．用XRD和Raman对复合氧化物结构和不同晶相环境中Er3+的荧光光谱进行了表征．结果表明，掺杂Er3+在三种不同晶相Y-ZrO2中表现出不同的荧光特征．随着基体ZrO2由单斜相逐渐向四方和立方相转变，其荧光谱峰位置对称性提高，峰分裂数减少，谱带逐步向单峰转变．在632.8 nm激发下的Raman图谱有Er3+的荧光干扰，在514.5 nm下则以荧光光谱为主体，不能获得Raman信号，在325 nm激发下无荧光干扰．XRD和紫外Raman二种方法对物相表征结果的差别是由于样品体相和表面不一致，表层容易生成比内层结构对称性更低的物相造成的．