Microstructural Observations in Barium Calcium Magnesium Niobate

Microstructural studies on (1 − x )Ba(Mg_1/3Nb_2/3)O₃– x Ca(Mg_1/3Nb_2/3)O₃ (BCMN) complex perovskite compounds, which are mixtures of Ba(Mg_1/3Nb_2/3)O₃ (BMN) and Ca(Mg_1/3Nb_2/3)O₃ (CMN), were conducted using scanning electron microscopy, transmission electron microscopy, and X-ray diffractometry. Pure BMN and CMN both have a 1:2 ordered structure, via the chemical ordering of B-site cations; however, the tilting of oxygen octahedra is involved in pure CMN, whose structure has a 1:2 ordered monoclinic unit cell that is characterized by (±1/6,±1/6,±1/6)-type superlattice reflections in electron diffraction patterns along the [110] zone axis that is based on a simple cubic perovskite. Studies of the morphologic differences have indicated two types of inhomogeneities in a mixture of the BCMN system: (i) a rather large-scale segregation (i.e., grain sizes of several micrometers), where the grains are separated compositionally as being barium-rich or calcium-rich, and (ii) fine-scale lamellar-type segregations 20 nm wide and 200 nm long. The segregation that is caused by Ba and Ca ions can be identified by the difference of superlattice modulations from high-resolution transmission electron microscopy lattice images.     

Preparation and Unit-Cell Parameters of Single Crystals of Ba2Ti9020

Ba₂Ti₉O₂₀ crystallizes in the monoclinic system with α= l.4818(5) nm, b = 1.4283(6), and c = 0.7109(2) with β = 98.37°±0.07°. The most likely space group is P 2₁/ m , Z = 4 with a calculated density 4.58 g/cm³. The powder pattern was indexed. The Ba₂Ti₉O₂₀ crystals form as stellated groups when melts of BaCl₂+ 20 to 50% TiO₂ cool from 1275°C.     

New Phases in the CaO–M2O3–CuO (M = Nd, Gd, Y) Systems at 1000°C

New layered cuprates with the general stoichiometry Ca_{2+ x} M_{2− x} Cu₅O₁₀ have been characterized for M = Nd, Gd, and Y. These insulating phases have structures closely related to that of NaCuO₂ and contain one-dimensional chains constructed from edge-sharing [CuO₄] square planes. The X-ray patterns of these ternary cuprate phases are complicated by strong reflections from both incommensurate and commensurate supercells which result from the ordering of the Ca²⁺ and M³⁺ ions in the structure. The stoichiometry range, and resultant Cu valences in these cuprates depend upon the nature of the trivalent ion. Ca-rich and trivalentrich compositions can be prepared for M = Nd and Gd, but only Ca-rich compositions were formed in the Y system. No other ternary phases are observed under the conditions of this study, 1000°C in air, and the solid-state equilibria are dominated by the single ternary phase identified above.     

Size and Charge Effects of Dopant M on the Unit-Cell Parameters of Monoclinic Zirconia Solid Solutions Zr0.98M0.02O2 –δ (M = Ce, La, Nd, Sm, Y, Er, Yb, Sc, Mg, Ca)

The crystal structure of monoclinic phase [ P 2¹/ c Z = 4] has been refined by the Rietveld analysis of X-ray powder diffraction data to study the size and charge effects of dopant M ^{n +} on the unit-cell parameters of monoclinic ZrO₂−2 mol% MO _{n /2} solid solutions ( n = 4 for M = Ce; n = 3 for M = La, Nd, Sm, Y, Er, Yb, Sc; and n = 2 for M = Mg and Ca). For trivalent dopant ( n = 3), the unit-cell parameters a _m , b_m , c_m and unit-cell volume increase and β m decreases with an increase of dopant size. Unit-cell volume increases with increasing of dopant charge n .     

Internal Oxidation of Ce3+-BaTiO3 Solid Solutions

The reoxidation process in highly Ce³⁺-doped BaTiO₃ ceramics was studied using TEM. Samples of two different types of solid solutions, Ba_1−XCe³⁺ _X Ti_1−X/4( V _Ti) X/4 O₃ and Ba_1−XCe³⁺ _X Ti⁴⁺_{1− X} Ti³⁺ _X O₃, were prepared by sintering oxide mixtures in air and in a reducing atmosphere, respectively. The solid solutions were reoxidized by annealing in air at high temperatures (1000°—1100°C). As a result of internal oxidation of Ce³⁺ and Ti³⁺, fluorite CeO₂ and monoclinic Ba₆Ti₁₇O₄₀ phases were precipitated in the perovskite matrix. In Ba_1−XCe³⁺ _X Ti_1−X/4( V _Ti)X/4O3 solid solution precipitates nucleate heterogeneously at grain boundaries and at extended defects inside the grains, whereas in Ba_1−XCe³⁺_XTi⁴⁺_1−XTi³⁺_XO₃ solid solution precipitates are nucleated mainly homogeneously inside reoxidized perovskite grains. The form of the precipitates and their orientational relationship with the matrix, as well as the mechanism of internal oxidation, are discussed.     

The System HfO2-TiO2

The system HfO₂-TiO₂ was studied in the 0 to 50 mol% TiO₂ region using X-ray diffraction and thermal analysis. The monoclinic ( M ) ⇌ tetragonal ( T ) phase transition of HfO₂ was found at 1750°± 20°C. The definite compound HfTiO₄ melts incongruently at 1980°± 10°C, 53 mol% TiO₂. A metatectic at 2300°± 20°C, 35 mol% TiO₂ was observed. The eutectoid decomposition of HfO_2,ss) ( T ) → HfO_2,ss ( M ) + HfTiO_34,ssss occurred at 1570°± 20°C and 22.5 mol% TiO₂. The maximum solubility of TiO₂ in HfO_2,ss,( M ) is 10 mol% at 1570°± 20°C and in HfO_2,ss ( T ) is 30 mol% at 1980°± 10°C. On the HfO₂-rich side and in the 10 to 30 mol% TiO₂ range a second monoclinic phase M of HfO₂( M ) type was observed for samples cooled after a melting or an annealing above 1600°C. The phase relations of the complete phase diagram are given, using the data of Schevchenko et al. for the 50% to 100% TiO₂ region, which are based on thermal analysis techniques.     

Structural and Dielectric Properties of Ag(Nb0.8Ta0.2)1−xSbxO3 (x≤0.08) Ceramics

Sb₂O₅ were selected to substitute (Nb_0.8Ta_0.2)₂O₅ and the effects of Sb substitution on the dielectric properties of Ag(Nb_0.8Ta_0.2)O₃ ceramics were studied. The perovskite Ag(Nb_0.8Ta_0.2)_{1− x} Sb _x O₃ ceramics showed no obvious change with x value being no more than 0.08, and the pseudoperovskite unit cell parameters a = c , b and monoclinic angle β decrease with Sb concentration increasing. The dielectric properties of Ag(Nb_0.8Ta_0.2)_{1− x} Sb _x O₃ ceramics were found to be affected greatly by the substitution of Sb for Nb/Ta. The ɛ value of Ag(Nb_0.8Ta_0.2)_{1− x} Sb _x O₃ ceramics sintered at their densified temperature increased from 480 to 825 with x from 0 to 0.08, the tan δ value decreased sharply from 0.0065 to 0.0023 (at 1 MHz) with x increasing from 0 to 0.04, and then kept a stable lower tan δ value ∼0.0024 with x to 0.08. The temperature coefficient of capacitance values continuously decreased from a positive value of 1450 ppm/°C for x =0 to a negative value of −38.52 ppm/°C for x =0.08.     

Thermal Expansion, Compressibility, and Polymorphism in Hafnium and Zirconium Titanates

Using X-ray diffraction techniques, thermal expansion and compressibility were measured on the orthorhombic compounds HfTiO₄, Hf_1.26Ti_0.74O₄, and ZrTiO₄ (both quenched and cooled slowly from 1300°C). The thermal expansion of HfTiO₄ is highly anisotropic; the thermal expansion coefficients along the crystallographic axes are α _a =+(8.7±0.5)×10⁻⁶°C⁻¹, α _b =−(5.2±0.5)×10⁻⁶°C⁻¹, and α _c =+ (5.3±0.5)×10⁻⁶°C⁻¹. The thermal expansion of Hf_1.26Ti_0.74O₄ was similar to that of HfTiO₄ but that of ZrTiO₄ was markedly less anisotropic. The compressibilities of HfTiO₄ and ZrTiO₄ also differed markedly. All compounds investigated, however, behaved similarly in exhibiting a polymorphic transition to a high-pressure phase having the monoclinic baddeleyite (ZrO₂) structure. The polymorphism can be explained qualitatively on the basis of crystal structure.     

Comprehensive Linkage of Defect and Phase Equilibria through Ferroelectric Transition Behavior in BaTiO3-Based Dielectrics: Part 1. Defect Energies Under Ambient Air Conditions

Defect and phase equilibria have been investigated via the ferroelectric phase transition behavior of pure and equilibrated nonstoichiometric BaTiO₃ powder samples. Through fabricating the BaTiO₃ materials under highly controlled conditions to preserve the equilibrium conditions with respect to Ba/Ti ratio, annealing temperature ( T ), and oxygen partial pressure ( P _O2), systematic variations in the phase transition temperature can be noted with respect to Ba/Ti ratio and T . From the data extracted, we can then determine solubility limits. Equilibrating the defect reactions at the solubility limits provides a direct approach to identify and calculate the defect energetics. The phase transition temperature decreased with increasing concentration of the TiO₂ partial-Schottky defects (BaTi_1−δO_3−2δ) and the BaO partial-Schottky defects (Ba_1−δTiO_3−δ), and showed discontinuous changes in the two-phase region. The formation enthalpy and entropy for the partial-Schottky defect reactions was evaluated to be 2.32±0.1 eV and 10.15±0.7 k _B for the BaO partial-Schottky defect, and 2.89±0.1 eV and 8.0±1.5 k _B for the TiO₂ partial-Schottky defects equilibrated under air annealing conditions.     

Phase Relations In The Pseudo-Ternary System La2O3–SrO–Fe2O3

The phase relations in the pseudo-ternary system La₂O₃–SrO–Fe₂O₃ have been investigated in air. Isothermal sections at 1100° and 1300°C are presented based on X-ray diffraction and thermal analysis of annealed samples. Extended solid solubility was observed for the compounds Sr _{n +1− v} La _v Fe _n O_{3 n +1−δ} ( n =1, 2, 3, and ∞) and Sr_{1− x} La _x Fe₁₂O₁₉, while only limited solubility of La in Sr_{4− z} La _z Fe₆O_13±δ was observed. At high Fe₂O₃ content, a liquid with low La₂O₃ content was stable at 1300°C.     

Vaporization Studies of the La2O3–Ga2O3 System

The vaporization of the samples of the compositions Ga₂O₃+ LaGaO₃, LaGaO₃+ La₄Ga₂O₉, and La₄Ga₂O₉+ La₂O₃ was investigated using Knudsen effusion mass spectrometry in the temperature range 1494–1937 K. The partial pressures of the gaseous species O₂, Ga, GaO, Ga₂O, and LaO were determined over the samples investigated. The equilibrium partial pressures were used for the calculation of the thermodynamic activities of the components at 1700 K. Gibbs energies of formation of LaGaO₃( s ) and La₄Ga₂O₉( s ) at 1700 K from the component oxides were derived from the thermodynamic activities as −46.4 ± 4.7 and −99.2 ± 7.9 kJ·mol⁻¹, respectively. The results were compared with the literature data obtained using other methods.     

Elastic Constants of Single Crystal Calcium Molybdate (CaMoI4)

The seven elastic compliances and seven elastic constants of single crystal calcium molybdate were determined by a resonance method. The compliance s₁₆, which is zero for higher symmetry tetragonal crystals, need not be zero for crystals of lower tetragonal symmetry and contributes significantly to the orientation dependence of the elastic moduli of calcium molybdate. The values of the compliances and their standard errors (based on 41 measurements) in units of 10⁻¹¹m²/N are s₁₁= 0.974 + 0.005, s₃₃= 0.958 ± 0.004, s₄₄= 2.720 ± 0.009, s₆₆= 2.471 ± 0.022, s₁₂=−0.380 ± 0.010, s₁₃=−0.230 ± 0.009, and s₁₆= 0.433 ± 0.010.     

Low-Temperature Phase Equilibria by the Flux Method and the Metastable–Stable Phase Diagram in the ZrO2–CeO2 System

Low-temperature phase equilibria ranging from 1000° to 1200°C in the ZrO₂–CeO₂ system were investigated by annealing compositionally homogeneous ZrO₂–CeO₂ solid solutions in a Na₂B₂O₇.1 NaF flux. The 5 mol% CeO₂ samples decomposed into monoclinic ( m ) and tetragonal ( t ) phases during annealing at 1100°2 and 1120°C, and the t -phase transformed diffusionlessly into monoclinic ( m ') symmetry during quenching. A eutectoid reaction, t → ( m + c ), was confirmed to occur at 1055°± 10°C, where the equilibrium compositions of the t -, m -, and c -phases were 11.2 ± 2.8, 0.9 ± 0.9, and 84 ± 1 mol% CeO₂, respectively. The equilibrium phase boundaries were almost independent of the annealing time and/or the flux:sample ratio, which indicates that the flux accelerates the reaction rate withouts affecting the equilibration. The previous data are discussed using metastable–stable phase diagrams. The discrepancies of the low-temperature phase diagram in the literature are attributable to either regarding the metastable phase boundaries as stable ones or ignoring the sluggish kinetics.     

Phase Diagram of the Pseudobinary System Ga2O3-Bi2-x3+Bix5+O3+x2-

The pseudobinary system Ga₂O₃-Bi_{2- x} ³⁺Bi _x ⁵⁺O_{3+ x} ^2- was studied in view of its importance for growth of SrGa₁₂O₁₉ crystals from a bismuth oxide flux. A subsolidus transition of γ*-Bi₂O₃ to a β'-phase and a strictly stoichiometric 1:2 phase with 33.3 mol% bismuth oxide were found. The single-crystal data for the compound indicated space group P2₁2₁2, with lattice constants a=0.79180±0.0003 nm, b =0.8288±0.0003 nm, and c =0.5889±0.0003 nm; the measured density was 7.1±0.3 g/cm³ and the cell content (Z) 2.     

Thermodynamic Stability of UMoO6 by the Transpiration Method

The equilibrium vaporization of UMoO₆(s) in dry air was studied by the transpiration method in the temperature interval 1110°≤ T (K) ≤ 1250°. Apparent pressure of the trimer measured over the two-phase mixture UMoO₆+ U₃O₈, with dry air as the carrier gas, was used to calculate the partial pressure of the trimer, (MoO₃)₃(g). In accordance with the vaporization reaction (UMoO₆) = 1/3(U₃O₈) + 1/6(MoO₃)₃ (g) + 1/6O₂ (g), the free energy of formation of UMoO₆)(s) is given by (δ G° in kJ-mol⁻¹) δ G° (UMoO₆) = (−1962 ± 10) + (0.463 ± 0.008) T     

Domain Structure-Second Harmonic Generation Correlation in Potassium Niobate Thin Films Deposited on a Strontium Titanate Substrate

The experimental measurement of second harmonic generation (SHG) and theoretical correlation of the SHG output to domain microstructure is presented for ferroelectric KNbO₃ (110) thin films on a SrTiO₃ (100) substrate. From X-ray diffraction, four possible domain variants in the film growth plane were determined to be KNbO₃ [1 1 0]∥SrTiO₃ [001], [00 1 ], [010], or [0 1 0] denoted as variants X +, X −, Y +, and Y −, respectively. From theoretical modeling of the SHG output from the film, the ratios of nonlinear coefficients d_o/d₃₂ and d ₂₄/d₀ in the film were determined to be 2.19 ± 0.12 and 0.46 ± 0.26, respectively, in good agreement with the bulk KNbO₃ crystals. The term d₀ is defined as d₁₅+ (d₃₁+ d₃₃)/2 . The SHG can also distinguish between X+ and X– , as well as Y+ and Y– , domains, which was not possible by X-ray diffraction. The ratio of net area fraction (| A ^X+− A ^X−|/| A ^Y+− A ^Y−|) was determined to be 2.76 ± 1.06 in the probe area (2.25π mm²) where A ^x+, A ^x-, A ^Y+, and A ^Y− are the area fractions of domain variants X+, X−, Y+ , and Y− , respectively, in the film growth plane.     

Vaporization Thermodynamics and Enthalpy of Formation of Aluminum Silicon Carbide

The vaporization thermodynamics of aluminum silicon carbide was investigated using Knudsen effusion mass spectrometry. Vaporization occurred incongruently to give Al( g ), SiC( s ), and graphite as reaction products. The vapor pressure of aluminum above (Al₄SiC₄+ SiC + C) was measured using graphite effusion cells with orifice areas between 1.1 × 10⁻²and 3.9X10⁻⁴ cm². The vapor pressure of aluminum obtained between 1427 and 1784 K using an effusion cell with the smallest orifice area, 3.9X10⁻⁴ cm², is expressed as
log p (Pa) =−(18567 ± 86) ( K/T ) + (12.143 ± 0.054)
The third-law calculation of the enthalpy change for the reaction Al₄SiC₄( s ) = 4Al( g ) + SiC( hex ) + 3C( s ) using the present aluminum pressures gives Δ H °(298.15 K) = (1455 ± 79) kJ·mol⁻¹. The corresponding second-law result is Δ H °(298.15 K) = (1456 ± 47) kJ·mol⁻¹. The standard enthalpy of formation of Al₄SiC₄( s ) from the elements calculated from the present vaporization enthalpy (third-law calculation) and the enthalpies of formation of Al( g ) and hexagonal SiC is Δ H °_f= -(221 ± 85) kJ·mol⁻¹. The standard enthalpy of formation of Al₄SiC₄( s ) from its constituent carbides Al₄C₃( s ) and SiC( c, hex ) is calculated to be Δ H °(298.15 K) = (38 ± 92) KJ·mol⁻¹.     

Effect of V2O5 Doping on the Sintering and Dielectric Properties of M-Phase Li1+x−yNb1−x−3yTix+4yO3 Ceramics

The effect of the addition of V₂O₅ on the structure, sintering and dielectric properties of M -phase (Li_{1+ x − y} Nb_{1− x −3 y} Ti _x +4 _y )O₃ ceramics has been investigated. Homogeneous substitution of V⁵⁺ for Nb⁵⁺ was obtained in LiNb_{0.6(1− x )}V_{0.6 x} Ti_0.5O₃ for x ≤ 0.02. The addition of V₂O₅ led to a large reduction in the sintering temperature and samples with x = 0.02 could be fully densified at 900°C. The substitution of vanadia had a relatively minor adverse effect on the microwave dielectric properties of the M -phase system and the x = 0.02 ceramics had [alt epsilon]_r= 66, Q × f = 3800 at 5.6 GHz, and τ_f= 11 ppm/°C. Preliminary investigations suggest that silver metallization does not diffuse into the V₂O₅-doped M -phase ceramics at 900°C, making these materials potential candidates for low-temperature cofired ceramic (LTCC) applications.     

Low-Temperature Synthesis and Properties of LaMnO3±d and La0.67R0.33MnO3±d (R = Ca, Sr, Ba) from Citrate Precursors

LaMn_{1− y} ³⁺Mn _y ⁴⁺O_3±d and La_0.67R_0.33Mn_{1− y} ³⁺Mn _y ⁴⁺O_3±d (R = Ca, Sr, Ba) phases were synthesized at 350°C by using very reactive, amorphous precursors obtained from the stoichiometric citrate solutions. The chemical process was optimized with respect to the solution concentration, pH, and additives. The precursor reactions were investigated as a function of the cation stoichiometry and the additive by simultaneous thermal and thermogravimetric analysis and X-ray diffraction. The reaction pathway was found to be independent of the cation stoichiometry, but related to the acid or base additive. The annealing temperature was systematically increased in the 350–1200°C interval and the La_0.67Sr_0.33MnO_3±d properties (i.e., crystal sizes, Mn average valence, Curie temperature, magnetization, magnetic susceptibility) were measured and found to vary consistently as a function of it.     

Low-Loss Microwave Dielectric Ceramics Using (Mg1−xMnx)2TiO4 (x=0.02–0.1) Solid Solution

Composite ceramics in a solid solution of (Mg_{1− x} Mn _x )₂TiO₄ ( x =0.02–0.1) have been prepared by the mixed oxide route. Formation of the solid solution was confirmed by the X-ray diffraction, the EDX analysis, and the measured lattice parameters, which varied linearly from Mg₂TiO₄ ( a = b = c =8.4410 Å) to (Mg_0.9 Mn_0.1)₂TiO₄ ( a = b = c =8.4445 Å). The XRD analysis also confirmed the co-existence of a cubic-structured (Mg_{1− x} Mn _x )₂TiO₄ and an ilmenite-structured second phase (Mg_{1− x} Mn _x )TiO₃. The composition expected to have a maximum Q × f (276 200 GHz at 10.5 GHz) is (Mg_0.95Mn_0.05)₂TiO₄ with ɛ_r∼15.69 and τ_f∼−52.6 ppm/°C. The existence of the second phase, however, would lead to no significant variation in the dielectric properties of the specimen because it possesses compatible properties compared with that of the main phase.