Composition-dependent intrinsic defect structures in pyroclore RE2B2O7 (RE = La,Nd, Gd; B = Sn,Hf, Zr)

Point defects are closely correlated with various properties of pyrochlore oxides and therefore play a key role on their engineering applications. Here, the native point defect complexes in RE₂B₂O₇ (RE = La, Nd, Gd; B = Sn, Hf, Zr) under stoichiometric and nonstoichiometric compositions are studied by first-principles calculations. The O Frenkel defect complex is predicted to be the predominant defect structure in stoichiometric zirconates and hafnates, whereas the cation antisite defect complex is the predominant one in stannates. In the case of BO₂ excess, the formation of the B-RE antisite defect together with the RE vacancy and the oxygen interstitial is energetically favorable, whereas the RE-B antisite defect together with the oxygen vacancy and the RE interstitial is preferable under the RE₂O₃ excess environments. Additionally, the formation energies of the native defect complexes are greatly affected by the B-site and/or RE-site cations. The strategy on tailoring the intrinsic defect structures of these pyrochlore oxides is proposed. It is expected to guide the experiments on the defect-related property optimization through stoichiometric and nonstoichiometric compositions, so as to meet the specific engineering requirements and promote their commercial applications.     

Selectively enhanced oxygen vacancies in undoped polycrystalline ZnO as a consequence of Multi-Step Sintering

With a motivation to investigate the simultaneous effect of sintering temperature and sintering time in tandem on defect distributions in polycrystalline undoped ZnO a new technique called Multi-step sintering (MSS) has been deliberately designed. Systematic investigations on structural parameters suggest the rigorous defect propagation along a-axis of the unit cell. We report the unwavering nature of Zinc interstitials (Zn_i) and the complexes of Zn_i with oxygen vacancies (V_o) with sintering temperature. The enhanced and broad green emission in Photoluminescence spectroscopy reveals the existence of surplus V_o in the sample. The band gap narrowing at 800 °C and enhanced Raman modes in Raman scattering experiment confirms the existence of excess oxygen vacancies and make MSS successful technique to tailor the intrinsic defects without any extrinsic doping.     

Stable anatase phase with a bandgap in visible light region by a charge compensated Ga–V (1:1) co-doping in TiO2

A series of charge compensated Ga–V co-doped TiO₂ samples (Ti_(1-x)(Ga_0.5V_0.5)_xO₂) have been synthesized by a modified sol-gel process. X-ray diffraction pattern shows that the anatase to rutile (A→R) onset temperature (T_O) shifts to a higher temperature, whereas the complete phase transformation temperature (T_C) shifts to a low-temperature region as compared to pure TiO₂, due to Ga–V incorporation. Ga–V co-doping helps in the transformation of some smaller sized Ti⁴⁺ to a relatively larger Ti³⁺. In the anatase phase, oxygen content also increases with increasing doping concentration, which along with the larger size of Ti³⁺ results in lattice expansion and thereby delays the T_O. In the rutile phase, oxygen vacancy increases with increasing doping concentration, which results in lattice contraction and accelerates phase transition. Grain growth process is hindered in the anatase phase (crystallites size reduces from ~15 nm (x = 0.00) to 8 nm (0.10)), whereas it is accelerated in the rutile phase as compared to pure TiO₂. In both phases bandgap (E_g) reduces to the visible light region (anatase: E_g = 3.16 eV (x = 0.00) to 2.19 eV (x = 0.10) and rutile: 3.08 eV (x = 0.00) to 2.18 eV (x = 0.10)) in all co-doped samples. The tail of the absorption edge reveals lattice distortion and increase of Urbach energy proofs the same due to co-doping. All these changes (grain growth, phase transition, and optical properties) are due to lattice distortion created by the combined effect of substitution, interstitials, and oxygen vacancies due to Ga–V incorporation in TiO₂.     

Unveiling surface stability and oxygen vacancy segregation of Yb2SiO5 and Lu2SiO5 by first-principles calculations

RE₂SiO₅ (RE = Yb and Lu) are significant environmental barrier coating (EBC) materials, in which surface and oxygen vacancy play crucial roles in their structural stability and functionality. In this work, the structural configuration and thermodynamics of (1 0 0), (0 1 0), and (0 0 1) surfaces of RE₂SiO₅ are investigated by first-principles calculations. The (0 0 1) surface is preferred energetically, which is attributed to the weak bond broken environment and large rare-earth polyhedron distortion on this surface. Moreover, the formation energies of various oxygen vacancies on the stable (0 0 1) surface are estimated and the optimal location for oxygen vacancies is held by the [SiO₄] tetrahedron. The oxygen vacancies are more likely to segregate on the surface because of the lower formation energies on the surfaces compared with those in the bulk. These findings are expected to enable the development of RE₂SiO₅-based EBCs by tuning grain size and/or thin film growth orientation.     

Defect engineering in semiconducting oxides: Control of ZnO surface potential via temperature and oxygen pressure

The technological usefulness of a semiconductor often depends on the types, concentrations, charges, spatial distributions, and mobilities of the atomic‐scale defects it contains. For semiconducting metal oxides, defect engineering is relatively new and involves complex transport and reaction networks. Surface‐based methods hold special promise in nanostructures where surface‐to‐volume ratios are high. This work uses photoreflectance augmented by X‐ray photoelectron spectroscopy to show that the surface potential V_S for Zn‐terminated ZnO(0001) can be manipulated over a significant range 54.97–79.08 kJ/mol (0.57–0.82 eV) via temperature and the partial pressure of O₂. A defect transport model implies this variation in V_S should affect the injection rate of oxygen interstitials by a factor of three. Such injection plays an important role in controlling the concentrations of oxygen vacancies deep in the bulk, which often prove troublesome as trapping centers in photocatalysis and photovoltaics and as parasitic emitters in light‐emitting devices. © 2015 American Institute of Chemical Engineers AIChE J, 62: 500–507, 2016     

Unveiling surface stability and oxygen diffusion of rare-earth zirconate pyrochlores by density functional theory

The surface structures, bond variations, and segregation of oxygen vacancies play crucial roles in the structural stability and functionality of nanocrystalline rare-earth zirconate pyrochlores. In this work, the stabilities of (1 0 0), (1 1 0), and (1 1 1) surfaces of pyrochlore A₂Zr₂O₇ (A = La, Ce, Pr, Nd, Pm, Sm, Eu, or Gd) are investigated by first-principles calculations. Surface reconstruction occurs on (1 1 0) surface with a transition of ZrO₆ octahedron to ZrO₄ tetrahedron, leading to their large relaxation energies. In combination with the small amount of broken bonds during the surface formation process, the (1 1 0) surfaces are identified having the lowest surface formation energies than the (1 0 0) and (1 1 1) surfaces. Moreover, the reconstructed (1 1 0) surface has characteristics of the segregation of oxygen vacancies. The surface oxygen vacancies have the low migration barriers (<1.2 eV), which are comparable with those in bulk and ensure the long-distance diffusion of oxygen vacancies in A₂Zr₂O₇. These discoveries provide fundamental insight to the surface structure and related oxygen vacancy behavior, which are expected to guide the optimization of the surface related properties for nanocrystalline rare-earth zirconates.     

Electric field dependence of ferroelectric stability in BiFeO3 thin films co-doped with Er and Mn

Bi_0.9Er_0.1Fe_1−xMn_xO₃ (BEFM_xO, x = 0.00–0.03) films are synthesized by a sol–gel technique. The BEFO film exhibits a conduction mechanism based on electron tunneling. The high applied electric field causes dissociation of the defect complex, and the resulting oxygen vacancies contribute to fake polarization. Consequently, the BEFO film has poor polarization stability at high applied electric fields. Coexistence of two phases (with space groups R3c:H and R3m:R) and reduced concentrations of oxygen vacancies and Fe²⁺ in BEFM_xO are achieved by co-doping with Er and Mn. The presence of bulk-based conduction in the BEFM_xO films then leads to ferroelectric domain switching contributing to the real polarization and to excellent ferroelectric stability. In addition, the BEFM_0.02O film shows a typical symmetrical butterfly curve, the highest remnant polarization of ~109 μC/cm², and the highest switching current of ~1.66 mA. It also has the smallest oxygen vacancy concentration and thus the smallest amount of defect complex, which means that there are fewer pinning effects on ferroelectric domains and therefore excellent ferroelectric stability. This excellent ferroelectric stability makes the BEFM_xO films obtain good stability and reliability in the application of ferroelectric memory devices.     

Tritium trapping by oxygen and lithium vacancies in Li4TiO4 from first-principles calculations

The Li₄TiO₄ ceramic is a promising solid-state tritium breeder material for the production of tritium fuel in the designs of fusion reactors. Tritium extraction efficiency is one of the key factors that determines the performance of the breeder material. Vacancies are known to trap tritium and adversely affect tritium extraction. Understandings of tritium-trapped defect configurations in Li₄TiO₄ are limited so far. In this work, the oxygen/lithium vacancy (V_O/Li) and the vacancy-tritium defect complex (V_O/Li + T) in Li₄TiO₄ are studied by first-principles density functional theory. The atomic configurations, formation energies and electronic structures of various charged defect species are obtained. We find that 2+ and 0 are the dominate charge states of V_O, 1- is the dominate charge state of V_Li, 1+ is the dominate charge state of the defect complex (V_O + T), and 0 is the dominate charge state of (V_Li + T). Tritium atoms trapped in V_O are bonded to Ti atoms, and those trapped in V_Li are bonded to O atoms.     

Microwave dielectric properties,low-temperature sintering mechanism,and defects behaviour of temperature stable (1-x)Li2Zn3Ti4O12-xSr3(VO4)2 ceramics

(1-x)Li₂Zn₃Ti₄O₁₂-xSr₃(VO₄)₂ (0.1 ≤ x ≤ 0.4) microwave dielectric ceramics were fabricated by solid-state sintering technology. The impact of SV addition on the microstructure, dielectric properties, sintering process, and defects behaviour was studied. The formation of SrTiO₃ and the glass phase were observed via XRD and TEM, and the latter resulted in a decrease in the sintering temperature. The variations in microwave dielectric properties were consistent with the empirical mixture rules calculated by XRD refinement, and a near-zero τ_f value was obtained. The Li, Zn and V elements of the glass phase and the liquid phase sintering model were deduced via DSC, TEM and Raman spectroscopy. Then, the defect behaviour, such as oxygen vacancies, Ti³⁺, and V⁴⁺, was investigated by XPS and complex impedance spectroscopy. It was found that the generation and migration of defects occurred much more easily in 0.7LZT-0.3 SV than in LZT, resulting in a higher dielectric loss. Finally, the 0.7Li₂Zn₃Ti₄O₁₂-0.3Sr₃(VO₄)₂ ceramic sintered at 900 °C exhibited excellent microwave dielectric properties of ε_r = 17.8, Q × f = 41,891 GHz, and τ_f = ?4.4 ppm/°C and good compatibility with silver electrode, showing a good potential application for LTCC.     

The origin of the red emission in n-ZnO nanotubes/p-GaN white light emitting diodes

In this article, the electroluminescence (EL) spectra of zinc oxide (ZnO) nanotubes/p-GaN light emitting diodes (LEDs) annealed in different ambients (argon, air, oxygen, and nitrogen) have been investigated. The ZnO nanotubes by aqueous chemical growth (ACG) technique on p-GaN substrates were obtained. The as-grown ZnO nanotubes were annealed in different ambients at 600°C for 30 min. The EL investigations showed that air, oxygen, and nitrogen annealing ambients have strongly affected the deep level emission bands in ZnO. It was concluded from the EL investigation that more than one deep level defect is involved in the red emission appearing between 620 and 750 nm and that the red emission in ZnO can be attributed to oxygen interstitials (O_i) appearing in the range from 620 nm (1.99 eV) to 690 nm (1.79 eV), and to oxygen vacancies (V_o) appearing in the range from 690 nm (1.79 eV) to 750 nm (1.65 eV). The annealing ambients, especially the nitrogen ambient, were also found to greatly influence the color-rendering properties and increase the CRI of the as - grown LEDs from 87 to 96.     

Phase transformation and growth mechanism of RF sputtered ferroelectric lead scandium tantalate (PbSc0.5Ta0.5O3) films

Lead scandium tantalate (PbSc_0.5Ta_0.5O₃, PST), an order/disorder ferroelectric, is a potential candidate for electrocaloric cooling and pyroelectric infrared (IR) detector. In this work, we report the phase transformation kinetics from two series of samples containing pure amorphous and mixture of amorphous and pyrochlore to desired perovskite phase using postdeposition rapid thermal processing (RTP) as well as growth mechanism of RF sputtered PST thin films using excess lead target on platinized silicon (Pt/Ti/SiO₂/Si) substrates. We find that small changes in the temperature ramp have a large effect on the degree of perovskite conversion (ferroelectric phase), orientation (crystallographic texture), and long-range order parameter (< S₁₁₁ >). Through isothermal annealing, we obtained optimal perovskite phase at ≥700°C temperature. The phase transformation is characterized by spontaneous formation of center-type in-plane radial rosette-like structures revealed by scanning electron microscopy. The PST perovskite crystallites were found to coexist with pyrochlore in RTP annealed films. The volume fractions for perovskite and pyrochlore phase were obtained from the analysis of “rosettes” and respective X-ray diffraction intensities which helped to determine various parameters associated with phase kinetics (n, k, and activation energy, E_a) and accompanying growth. The effective activation energies of perovskite transition and growth were found to be 332 ± 11 kJ/mol (345 ± 11 kJ/mol) and 114 ± 10 kJ/mol (122 ± 10 kJ/mol), respectively, for pure amorphous only (and mixed amorphous and pyrochlore) phase following nucleation-growth controlled Avrami's equation. A linear growth rate (n∼1) for the perovskite phase indicates predominant interface-controlled process and diffusion-limited phenomena thus inhibiting rosette size owing to reactant depletion and soft impingement at the grain boundary. However, the growth behavior is isotropic in two-dimension parallel to the plane of the substrates for both sample series. Lead loss was severe for in-situ growth and RTP combined with conventional furnace annealing than those of RTP only films, which were closer to stoichiometric albeit with excess lead and marginal oxygen vacancies (V_o).     

Modulation of microwave dielectric properties in melilite-structured SrREGa3O7 (RE = La,Pr) ceramics

Gallium-based SrREGa₃O₇ (RE = La, Pr) melilite ceramics were prepared and selected to modify their microwave dielectric properties. Sintered at 1425 °C for 6 h, SrLaGa₃O₇ (SLGO) and SrPrGa₃O₇ (SPGO) ceramics exhibited high relative densities of 98.33 and 95.23%, low ε_r values of 11.8 and 10.9, Q×f values of 32,500 GHz (at 12.1 GHz) and 26,400 GHz (at 12.7 GHz), negative τ_f values of ?32 and ?54 ppm/°C. As a compensator, CaTiO₃ can tune the τ_f values of SLGO and SPGO to near zero (+2 and ?4 ppm/°C). In SrREGa₃O₇ melilite ceramics, the ε_r and τ_f values are mainly dependent on ionic polarizability, crystal structure and the “rattling” effect. The micromorphology, XPS, Raman spectrum and A-site bond valence (V_RE) of SrREGa₃O₇ (RE = La, Pr) microwave dielectric ceramics have also been comprehensively reported.     

Preparation of new heteronuclear (NH4)RE[FeII(CN)6]·nH2O complexes (RE = La,Ce, Pr,Nd, Sm,Gd, Dy,Y, Er,Lu) and their low-temperature decomposition for perovskite-type oxide

New heteronuclear (NH₄)RE^III[Fe^II(CN)₆]·nH₂O complexes (RE = La, Ce, Pr, Nd, Sm, Gd, Dy, Y, Er, Lu) were synthesized and their thermal decomposition products were investigated. The crystal structure of (NH₄)RE[Fe^II(CN)₆]·nH₂O would be a hexagonal unit cell (space group: P6₃/m), which was the same as that of La[Fe^III(CN)₆]·5H₂O. The hydration number n = 4 was estimated by TG results for all the RE complexes. The lattice constants depended on the ionic radius of the RE³⁺ ion for the heteronuclear complexes. The single phase of the perovskite type materials was directly obtained by decomposition of the heteronuclear complexes for RE = La, Pr, Nd, Sm, and Gd. A mixture of CeO₂ and Fe₂O₃ was formed for RE = Ce because of its oxidation to Ce⁴⁺. In the case of RE = Dy, Y, Er, and Lu complexes, the perovskite type materials formed at higher temperature via. mixed oxides such as RE₂O₃ and RE₄Fe₅O₁₃ due to the small RE³⁺ ionic radius.     

DFT+U study and in-situ TEM investigation of high-entropy titanate pyrochlore (Lu0.25Y0.25Eu0.25Gd0.25)2Ti2O7

High-entropy pyrochlore has become a promising immobilization matrix due to its ability to immobilize multiple nuclides especially for high-level radioactive waste. This work adopts first principle method to analyze the formation possibility of (Lu_0.25Y_0.25Eu_0.25Gd_0.25)₂Ti₂O₇ (HTP-1). By calculating the Gibbs free energy of possible chemical reactions, we predict that the driving force to synthesize HTP-1 is greater than the driving force to form single-component pyrochlore at temperatures above 1225 K. The characterization results show that the HTP-1 sample is successfully prepared by the solid-state reaction at the predicted temperature. To investigate the radiation resistance of HTP-1, the sample is irradiated by 800 keV Kr²⁺ ions, and the microstructure evolution is characterized by in-situ TEM. HTP-1 sample achieves complete amorphous at 0.26 dpa, indicating that its radiation resistance is between Eu₂Ti₂O₇ and Y₂Ti₂O₇, which is proved by the calculation results of x_O48f and antisite defect formation energy.     

Potential of pyrochlore structure materials in solid oxide fuel cell applications

Pyrochlore structure material (A₂B₂O₇) has gained interest in diverse applications like catalysis, nuclear waste encapsulation, sensors, and various electronic devices due to the unique crystal structure, electrical property, and thermal stability. This review deals with the ionic/electronic conductivity of numerous pyrochlore structure materials (titanates, zirconates, hafnates, stannates, niobates, ruthenates, and tantalite based pyrochlore) as electrolyte and electrode materials for solid oxide fuel cells (SOFCs). The impact of cation radius ratio (r_A/r_B) on the lattice constant and oxygen ‘x’ parameter of different pyrochlore structure materials obtained by various synthesis methods are reported. Higher ionic conductivity is essential for better ion transport in an electrolyte, and mixed ionic and electronic conductivity in electrode is essential for attaining higher efficiency in a typical SOFC. Gd_xTi₂O_7-δ, Gd_2-xCa_xTi₂O_7-δ, Nd_2-yGd_yZr₂O₇, Y₂Zr₂O₇, Y₂Zr_2-xMn_xO_7-δ, SmDy_1-xMg_xZr₂O_7-x/2, Gd_2-xCa_xTi₂O_7-δ pyrochlore are reported as electrolytes for fuel cell applications. Some pyrochlore material (La_2-xCa_xZr₂O₇, Sm_2-xM_xTi₂O₇ (M = Mg, Co, and Ni) pyrochlore) shows protonic conductivity at lower temperatures and ionic conductivity at higher temperature condition. Also, the mixed ionic-electronic conductivity behavior is reported in electrode materials for SOFC such as R₂MnTiO₇ (R = Er and Y), R₂MnRuO₇ (R = Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y), R₂Ru₂O₇ (R = Bi, Pb and Y), Y_2-xPr_xRu₂O₇, Ni-(Gd_0.9Ca_0.1)₂Ti₂O_7-δ, (Gd_0.9Ca_0.1)₂Ti₂O_7-δ, Gd₂(Ti_0.8Ru_0.2)₂O_7-δ, (Sm_0.9Ca_0.1)₂Ti₂O_7-δ and (Y_0.9Ca_0.1)₂Ti₂O_7-δ pyrochlore. The detailed study of the electronic behavior of these pyrochlore system confirms the necessity of defect structure with high oxygen mobility, lower activation energy, ionic radii ratio criterion should satisfy, and possess notable ion-ion interaction. Ionic conductivity in pyrochlore is increased by enhancing the oxygen migration through 48f-48f site with the formation of oxygen vacancy. Vacancy formation can be achieved by adding a suitable dopant that creates oxygen vacancy by charge compensation mechanism or as anion Frenkel defects. Similarly, the electrical conductivity is improved while adding suitable dopant (Ce, Pr, Ru, etc.) due to disordered structure and anti-Frenkel defect formation which leads to oxygen vacancy formation and thus improves conductivity.     

Zn1−xSixO: Improved optical transmission and electrical conductivity

Si addition in ZnO lattice significantly improves electrical conductivity. The extra charge of Si⁴⁺ ion (in comparison to Zn²⁺) attracts more oxygen in the lattice and reduces oxygen vacancies. Reduction of oxygen vacancies (defects) reduces strain in the lattice. Transparency of visible light (<3.0 eV) improves due to reduction of these defects in the wide bandgap (~3.3 eV: UV) of ZnO. Extra charge of Si⁴⁺ enhances carrier density in the ZnO lattice. Improved carrier density, reduced strain facilitate transport of carriers and therefore conductivity increases. Si incorporation also makes the samples moisture resistant. The material becomes more robust to operate in adverse humid conditions. An ideal transparent conductive oxide (TCO) should be conductive, transmit visible light and able to sustain humid conditions. All these properties are observed in Zn_(1−x)Si_xO material.     

Accommodation,Accumulation, and Migration of Defects in Ti3SiC2 and Ti3AlC2 MAX Phases

We have determined the energetics of defect formation and migration in M_n+1AX_n phases with M = Ti, A = Si or Al, X = C, and n = 3 using density functional theory calculations. In the Ti₃SiC₂ structure, the resulting Frenkel defect formation energies are 6.5 eV for Ti, 2.6 eV for Si, and 2.9 eV for C. All three interstitial species reside within the Si layer of the structure, the C interstitial in particular is coordinated to three Si atoms in a triangular configuration (C–Si = 1.889 Å) and to two apical Ti atoms (C–Ti = 2.057 Å). This carbon–metal bonding is typical of the bonding in the SiC and TiC binary carbides. Antisite defects were also considered, giving formation energies of 4.1 eV for Ti–Si, 17.3 eV for Ti–C, and 6.1 eV for Si–C. Broadly similar behavior was found for Frenkel and antisite defect energies in the Ti₃AlC₂ structure, with interstitial atoms preferentially lying in the analogous Al layer. Although the population of residual defects in both structures is expected to be dominated by C interstitials, the defect migration and Frenkel recombination mechanism in Ti₃AlC₂ is different and the energy is lower compared with the Ti₃SiC₂ structure. This effect, together with the observation of a stable C interstitial defect coordinated by three silicon species and two titanium species in Ti₃SiC₂, will have important implications for radiation damage response in these materials.     

Understanding the Behavior of Native Point Defects in ZrC by First‐Principles Calculations

Phase stability and macroscopic performances of ZrC are closely related to the behavior of native point defects. In this study, structures and stabilities of native point defects in ZrC as well as diffusion of C‐related defects are investigated by first‐principles calculations. It is shown that the carbon vacancy (V_C) and interstitials (C_is) are the dominant native point defects in ZrC. Six types of C_i configurations: two C‐trimers, one C‐tetrahedron, and three C‐dimers are identified with low defect formation energies. The V_C has a high migration energy (4.39 eV) which suggests its low mobility in ZrC. The C_is have low diffusion energy barriers (from 0.26 to 1.29 eV) which lead to their high mobility. In addition, the impact of defects (V_C, V_Zr, Zr_C, and C_Zr) on bonding strengths of neighboring Zr–C bonds is discussed. Especially near V_C, the 1NN (nearest neighboring) and 2NN Zr–C bonds are strengthened but 4NN Zr–C bonds are weakened. Interestingly, the 3NN Zr–C bonds are almost not affected by the presence of V_C. These results may be closely related to the short‐range interactions and ordering of V_C in nonstoichiometric ZrC.     

Crystal structure,chemical bonding and infrared reflectivity of microwave dielectric SrIn2O4 ceramics

In this work, the orthorhombic structured SrIn₂O₄ ceramics with a space group Pnam were synthesized by a solid-state reaction method. A high relative density (96.5%) coupled with excellent microwave dielectric properties (ε_r ∼ 12.3, Q × f = 96,900 GHz, τ_f ∼ −61.6 ppm/°C) were obtained as sintered 1300 °C for 4 h. The bond valence analysis demonstrates that the large sized cation Sr²⁺ exhibits a compressed state, while the In³⁺ exhibits a weaken rattling effect. The P-V-L chemical bond theory analysis indicates that the In-O bonds play a key role in affecting the dielectric loss. The thermally conductivity activation energy E_dc (0.94 eV) of SrIn₂O₄ was obtained by the dielectric spectroscopy, indicating that the E_dc was contributed to the double ionized oxygen vacancies. Furthermore, the intrinsic dielectric properties (ε_r ∼ 11.2, Q × f = 148,900 GHz) of SrIn₂O₄ were obtained by infrared reflectivity spectroscopy.     

Relationship between bond characteristics and microwave dielectric properties of REVO4 (RE = Yb,Ho) ceramics

Two novel low-ε_r REVO₄ (RE = Yb, Ho) microwave dielectric ceramics with the symmetry of the zircon structure, space group I4₁/amd, were prepared using the solid-state method. Dense REVO₄ (RE = Yb, Ho) ceramics sintered at 1200 °C and 1160 °C performed ε_r ～ 12.3 ± 0.1 and 13.3 ± 0.1, Q × f ～ 28,200 ± 300 GHz and 24,100 ± 300 GHz, τ_f ～ ?18.8 ± 0.5 ppm/°C and ?17.4 ± 0.5 ppm/°C, along with thermal expansion coefficient (α_L) of 9.0 ppm/°C and 8.1 ppm/°C, respectively. Bond valence results indicated that the slightly rattling RE³⁺ cations at the A-site and compressed V⁵⁺ at the B-site occurred in both ceramics. The positive deviations (Δε_r) of porosity corrected ε_r(Corr) from those calculated by the Clausius-Mosotti equation ε_r(C-M), 8.1% for YbVO₄ and 17.7% for HoVO₄, were observed, implying that the rattling effect of RE³⁺ in dodecahedral A-site were greater than those of compressed V⁵⁺ in tetrahedral B-site. Rattling effect also led REVO₄ (RE = Yb, Ho) to develop higher ε_r, and smaller τ_ε and τ_αm, then closer to zero τ_f values than other zircon-structured REVO₄ (RE = Ce, Nd, Sm, Eu) ceramics with large negative τ_f. The differences in sintering temperature and microwave dielectric performance of both ceramics were discussed using the packing fraction, full width at half maximum (FWHM) of Raman modes and Phillips-Van Vechten-Levine (P–V-L) theory.