A-site Substitution Research Articles

Variation in the ordering of Ba(Zn1/3Ta2/3)O3 with A-site substitutions

The ordering of Ba(Zn1/3Ta2/3)O3 is altered by A-site substitution of La or K. The ordering behavior is studied using X-ray diffraction, electron microscopy, and Raman spectroscopy. Ba1-x La x (Zn(1-+x)/3Ta(2-x)/3)O3 shows a transformation from 1:2 to 1:1 ordering with the increasing La content. Ba1-x K x (Zn(1-x)/3Ta(2-x)/3)O3 displays a monotonous decrease in 1:2 order with the K content. The ordered domains of BLZT first shrink in size with x, then transform into 1:1 ordered domains, and the sizes of new domains increase with x again. The effect of ordering on the quality factor of BLZT is discussed.

Changes in the Crystal Structure and Dielectric Properites of the Complex Perovskite Sr(Ga0.5Ta0.5)O3 with A-Site Cation Substitution

A-site cation substitution with Ba2+ or Ca2+ ions was made for the complex perovskite Sr(Ga0.5Ta0.5)O3 phase to modify its dielectric properties. The largely sized Ba2+ ions could be incorporated in the A-site sublattice up to 55mol% without cation ordering. On the contrary, the A-site Sr2+ ions were completely replaced by the Ca2+ ions. The crystal symmetry of the (Sr, Ca)(Ga0.5Ta0.5)O3 perovskites correspondingly changed from cubic to orthorhombic with increasing Ca content. Partial B-site cation ordering occurred when 10-50mol% Ca2+ ions were incorpolated in the A-site Sr2+ ions. The A-site cation substitution caused a very slight change in dielectric constant (er) at microwave frequencies, suggesting a little effect of the substitution on the ionic polarization of the perovskite structure. A substantial lowering in microwave Q-value observed for the solid solutions containing 10-20mol% Ba2+ or Ca2+ ions might be related to the lack of long-range structure ordering. Temperature coefficient of er at 1MHz changed from positive high values of the Ca2+-containing perovskites to nearly zero for compounds with 20-40mol% Ba2+ ions. This change was discussed in relation to the structual strain of (Ga, Ta)O6 octahedra in the compounds.

Effect of A-Site Substitution and Firing Temperature on Microstructure and Electrical Properties of BaTiO3 Semiconducting Ceramics Fired by Reduction-Reoxidation Method

The effects of A-site substitution and firing temperature on the microstructure and electrical properties of BaTiO3 semiconducting ceramics fired by the reduction-reoxidation method were experimentally investigated. Ba ions were substituted with Ca or (Sr0.5Pb0.5). Various microstructures were obtained by varying the firing temperature in reducing atmosphere. Using these samples, the recovery of the characteristics of positive temperature coefficient of resistivity (PTCR) was evaluated. In particular, the minimum grain size required for the reoxidation of only grain boundaries during annealing in oxygen atmosphere in the cooling process was investigated.

Variations of Structure and Dielectric Properties on Substituting A-site Cations for Sr2+ in (Na1/2Bi1/2)TiO3

Changes in structure and phase transition behavior of (Na1/2Bi1/2)1−xSrxTiO3 were investigated using x-ray diffraction (XRD) analysis and dielectric measurements. A decrease in the degree of lattice distortion was observed when the structure was rhombohedral (x < 0.26). Further substitution of Sr2+ (x ≥ 0.26) caused (Na1/2Bi1/2)1−xSrxTiO3 to show no macroscopic lattice distortion. However, all investigated samples (0.1 ≤ x ≤ 0.5) were ferroelectric at room temperature regardless of the lack of lattice distortion found in XRD patterns. Further XRD investigations revealed that nonuniform strains caused by nonuniform polarized islands were responsible for this behavior. A-site cation substitution of Sr2+ also resulted in the decrease of Tmax (the temperature where dielectric constant is maximum). Typical relaxor ferroelectric phase transition behavior, associated with the shift of Tmax, with respect to the measuring frequencies, was found above x = 0.18. This could be also ascribed to the decrease of Tmax—this reveals the increase in the shift of Tmax with frequency, with associated decrease in ionic displacement and space charge contributions and associated increase in the relative contribution to the polarization by microdomains and domain walls at temperatures near Tmax.

Anomaly in the Infrared Active Phonon Modes and Its Relationship to the Dielectric Constant of (Ba1-xSrx)(Mg 1/3Ta 2/3)O 3 Compound

A characteristic change in the dielectric properties with A-site substitution is known in complex perovskite compounds, (Ba, Sr)(B', B")O3, which are expected to be applied to dielectric resonators. Recently, a phase transformation upon tilting of oxygen octahedra has been found in some of the compounds. However the oxygen tilting transformation has not been considered to affect the dielectric constant explicitly. To clarify the relationship between the dielectric constant and the phase transformation, dielectric properties of a solid-solution system, (Ba1-xSrx)(Mg1/3Ta2/3)O3, have been studied. The origin of the dielectric constant, the polar mode of the lattice vibration, has been investigated using infrared reflectivity spectroscopy and the Kramers-Kronig analysis. It was found that the phase transformation due to the tilting of the oxygen octahedra clearly affects polar phonon modes of the compound. The origin of the change in the dielectric properties with Sr concentration is discussed referring to the behavior of polar phonon modes.

On the loss of PbO during sintering of (Pb 1 − xCa x)(Zr 1 − ySn y) O 3 ceramics

The effect of A-site and B-site ion substitution on the loss of PbO from (Pb 1 − x Ca x )(Zr 1 − y Sn y )O 3 (PCZS) ceramics was investigated. The reduced unit cell and the change of chemical bond affects the vaporization of PCZS. Sn substitution has pronounced effect on the weight loss of PbO in PCZS.

Scapolite cell-parameter trends along the solid-solution series

Scapolites are tetragonal framework aluminosilicates with the general formula M4T 12024A, the major constituents being M = Na + Ca, T = Si + AI, and A = Cl + C03 + S-bearing anions, along with minor Fe, Sr, Ba, K, and H. Substitutions among the M, T, and A sites are independent and constrained only by net charge balance and maximum site occupancy. The cell parameters vary primarily with framework chemistry and Al-Si order and are not a strong function of the M- or A-site substitutions. End-member marialite has a = 12.06(1) and c = 7.551(5) A, and meionite has a = 12.20(1) and c = 7.556(5) A. Changes in cellparameter trends in the series occur at AI3.6Sis.4024 and AI4.7Si7.3024 and are correlated with the locations of P421n-141m phase transitions. The scapolite group can be divided into three isomorphous series: 9.0 > Si > 8.4,8.4 > Si > 7.3, and 7.3 > Si > 6.0, each with its own patterns of Al-Si order. Because scapolites with the same Si content have variable cell parameters, there might be variable degrees of Al-Si order or a minor, secondary influence of the M- or A-site substituents.

Electrochemical durability of Ca-based alloys

In order to improve the cycling durability of Ca-based alloy in KOH solution as hydride electrode in Ni-H battery, we investigated the substitution effects by using Al and Co to substitute nickel as well as alkaline earth metals and rare earth elements to calcium in CaNi 5. It was found that rare earth and alkaline earth substitution can significantly improve the cycling durability during electrochemical charging and discharging. Comparing with A-site substitution in AB 5, B-site substitution by Al or Co shows little improvement on the durability. The structural and metalographical analyses have been done to understand the substitution effects.

Ordering Behavior of Lead Magnesium Niobate Ceramics with A‐Site Substitution

The ordering behavior of lanthanum‐modified lead magnesium niobate ceramics was investigated over a wide range of lanthanum content (from 0 to 50 at.%). The ordering of magnesium and niobium ions in the B site was examined by X‐ray diffraction, and the superlattice reflection was associated with the A(B′1/2B″1/2)O3‐type partial ordering structure. Moreover, the ordering degree increased pronouncedly with respect to lanthanum content of the sample and almost reached unity at [La] = 50 at.%. Finally, a comparison of the ordering behavior was performed among several lead magnesium niobate (PMN) samples with different A‐site substitutional ions. The mechanism used to enhance the ordering of PMN by A‐site substitution is then described by the effect of high valence.

Quantum Ferroelectricity in (Cd, Pb)2(Nb, Ta)2O7

Quantum ferroelectricity has been induced for the first time in a pyrochlore structure. Starting with the known ferroelectric Cd2Nb2O7, (Tc∼185 K), the transition temperature can be lowered to 0 K by a combination of Pb substitutions on the A-site and Ta substitutions on the B-site. The dielectric constants of the resulting materials have several of the typical characteristics of known quantum ferroelectrics which are all perovskites. There are, however, differences in the dielectric behavior compared to the perovskites and to renormalization-group theory results.

Substitution of Bi and Nb Ions in Lead Zirconate-Titanate

Solid-state substitution of Bi and Nb ions in the perovskite crystal structure of Pb(Ti0.47Zr0.53)O3 was investigated by sample weight changes at 1150°C in an “equilibrium” PbO atmosphere. A model is discussed that includes the effect of charge neutralization, requiring weight losses for A-site substitutions and weight gains for B-site substitutions. Experiments support the substitution of Bi as Bi3+ on the Pb2+ A site and Nb as Nb5+ on the (Ti,Zr)4+ B site of lead zirconate-titanate. For each mole of BiO1.5 added to a specimen, 1.5 moles of PbO are lost; for each mole of NbO2.5 added, 0.5 moles of PbO are gained. These results agree with the proposed model and with ionic size arguments. The weight changes were time-dependent during the first 30 to 40 h at 1150°C. This behavior is accounted for by the presence of transient second phases.