Dopant Valence Research Articles

Effect of dopant valence state of Mn-ions on the microstructures and nonlinear properties of microwave sintered ZnO-V2O5 varistors

In this study, the effect of dopant valence state of Mn-species on the microstructure and conduction behavior of microwave sintered ZnO-V2O5 ceramics was analyzed. Microwave sintering can markedly enhance the densification rate of ZnO-V2O5 ceramics, regardless the valence state of Mn-species incorporated. But only the samples doped with low valences Mn-ions (Mn2+ or Mn2.66+) exhibit large nonlinear coefficient (α = 21.9) with low leakage current density, whereas the ZnO-V2O5 ceramics added with high valence Mn-ions (Mn4+) show inferior nonohmic behaviors with high leakage current density under the same sintering conditions. Restated, the valence state of Mn-species incorporated in ZnO-V2O5 ceramics, insignificantly alters the grain growth behavior but markedly modifies their nonlinear electrical properties.

Change of uranium lattice site in LiNbO3 induced by thermal reduction treatment

The doping of lithium niobate (LiNbO3) with uranium is of high interest due to the possible infrared laser action of U3+. On the other hand LiNbO3:U crystals grown from melt contain uranium only as U6+. It has been recently shown that by thermal reduction the valence state can be altered to U3+/U4+. Studies on the lattice site of uranium show that U occupies exclusively the Nb site in as-grown material, while after thermal reduction at 1000°C for 16 h under high vacuum ca. 20% of the U atoms have been transferred to the Li site. These findings confirm the assumption that the dopant valence state is the determining factor for the impurity lattice site: dopants with valence states + 1 to + 4 are located on the Li site, those with valence states + 5 or + 6 on Nb sites.

The effects of dopant valence on the structure and electrical conductivity of LaInO 3

The effects on the structure and electrical conduction behavior of strontium and zirconium doped LaInO 3 was studied in this paper. XRD and Raman spectra both confirm that Sr and Zr cations are substitutionally introduced to the La- and In-sites to a certain limitation respectively. Cation vacancies may be the most likely point defects for ZrO 2-doped LaInO 3 perovskite oxide (LaIn 0.9Zr 0.1O 3+ δ ), which leads to the decrease of oxygen-ion conductivity, while Sr doping (La 0.9Sr 0.1InO 3− δ ) produces oxygen vacancies and will greatly enhance the electrical conductivity of LaInO 3. When Sr and Zr were co-doped with the same concentration (La 0.9Sr 0.1In 0.9Zr 0.1O 3), the effects of Sr doping will be partially eliminated by Zr doping resulting in a relatively low electrical conductivity. When Sr-doping level increases further (La 0.8Sr 0.2In 0.9Zr 0.1O 3− δ ), its conduction behaves the same as that of La 0.9Sr 0.1InO 3− δ since they are dominated by the same acceptor doping (10 mol%). Both La 0.8Sr 0.2In 0.9Zr 0.1O 3− δ and La 0.9Sr 0.1InO 3− δ show three different conduction regions, that is, p-type at high P O 2 , n-type at low P O 2 and P O 2 -independent conduction at intermediate oxygen partial pressure while only p-type conduction appears for La 0.9Sr 0.1In 0.9Zr 0.1O 3 at high P O 2 , and LaIn 0.9Zr 0.1O 3+ δ shows an entirely different conduction behavior in the measured oxygen partial pressures.

Electrical Properties of Doped Tin Dioxide Thin Films Deposited Using Femtosecond Pulsed Laser Ablation.

AbstractTin dioxide thin films are widely used as chemical sensors. The mechanism for gas sensitivity depends on the chemisorption characteristics of the oxide surface and on the electronic characteristics of thefilm. Chemical doping and the film microstructure can influence the electronic properties. In this paper tin dioxide thin films doped with rare earth and transition metals were deposited on the (1012 -oriented sapphire substrates by femtosecond pulsed laser deposition. The resulting films were single crystalline with a thickness of about 15 nm and 30 nm. The response of the films to reducing gases was measured in a gas reactor at high temperature. The electrical transport properties of the films were determined by Hall effect measurements within the gas reactor. The valence and ionic radius of dopants have strong influence on the mobility and concentration of electrical carriers (i.e. electrons) and thus affect the response of the thin films to reducing gases. A model correlating the dopant characteristics to the electrical properties was developed.

Oxygen Diffusion in Single- and Poly-Crystalline Zinc Oxides

18O diffusion coefficients were measured in zinc oxide ceramics using a secondary ion mass spectrometer. The results are interpreted as indicating extrinsic behavior. The values of the lattice diffusion coefficients with higher valence dopants compared with zinc ions are greater than lower valence dopant such as lithium ions. Using the data at deeper depth, the grain boundary diffusivity of oxide ions was also evaluated. Although the lattice diffusion coefficients varied by two orders of magnitude, the products of grain boundary width and grain boundary diffusion coefficient were less sensitive to the type of dopants.

Valence and atomic size dependent exchange barriers in vacancy-mediated dopant diffusion

First-principles pseudopotential calculations of dopant-vacancy exchange barriers indicate a strong dependency on dopant valence and atomic size, in contrast to current models of vacancy-mediated dopant diffusion. First-row elements (B, C, N) are found to have exchange barriers which are an order of magnitude larger than the assumed value of 0.3 eV (the Si vacancy migration energy).

Crystallite Size and Microstrain of Thermally Aged Low‐Ceria‐ and Low‐Yttria‐Doped Zirconia

The final phases of zirconia powders depend on the synthesis method employed, and the amounts of stabilizer present. In this study, ceria‐ and yttria‐doped zirconia powders were prepared by urea hydrolysis and subsequent hydrothermal treatment. The amount of tetragonal (t) vs monoclinic (m) phase in the powders increased with increasing stabilizer content, while the tetragonal phase size decreased and the microstrain of t crystals remained unchanged. The thermal degradation behavior of the metastable t phase in zirconia containing a low CeO2 or YO1.5 doping level was explored during aging treatment by means of X‐ray line profile broadening analysis. Both ultrafine yttria‐ and ceria‐doped zirconia powder pellets exhibit isothermal t→m transformation after aging at 900°C for various times. It is argued that a crystallite size effect, rather than the dopant valence, dictates the occurrence of the t→m transformation in ultrafine zirconia powders. The change in crystallite sizes of both t and m phases during aging depends significantly on the amount of stabilizer, aging time, and mechanism of t→m phase transformation. However, the change of microstrain in both t and m phases is related to the amount of stabilizers present and the matrix constraints.

Influence of dopant valency for Er in ErBa2Cu3O7−d on pair breaking, hole filling and hole doping mechanisms

A systematic study of (Er1−xMx)Ba2Cu3O7−d with M=Hf4+, Pr3+→4+, Ca2+ has been caried out with an objective of investigating the effect of dopant valency and magnetic moment on hole filling, pair breaking and hole doping mechanisms in influencing the superconductivity. It is found that non magnetic Hf-substitution up to x=0.2 shows that the reduction in Tc is mainly due to hole filling, whereas the substitution of Pr (up to x=0.2) displays suppression of Tc due to both hole filling and pair breaking mechanisms, as Pr possesses fluctuating valence and magnetic moment. In the case of non magnetic Ca-substitution, the Tc decreases due to hole doping and pair breaking mechanisms. The comparative results on these three systems will be discussed in the light of pair breaking, hole doping and hole filling mechanisms through empirical relations.

Physical Properties of Mixed Conductor Solid Oxide Fuel Cell Anodes of Doped CeO2

Samples of doped with oxides such as and were prepared. Their conductivities and expansions on reduction were measured at 1000°C, and the thermal expansion coefficients in the range 50 to 1000°C were determined. The ionic and electronic conductivity were derived from curves of total conductivity vs. oxygen partial pressure. For both types of conductivity a dependence on dopant valency was observed. The electronic conductivity was independent of dopant radius in contrast to the ionic which was highly dependent. These measured physical properties are compared with the ideal requirements for solid oxide fuel cell anodes. Not all requirements are fulfilled. Measures to compensate for this are discussed.

X-ray absorption near-edge studies of substitution for Cu in YBa2(Cu1-xMx)3O7- delta (M=Fe, Co, Ni, and Zn).

We report x-ray absorption near-edge measurements of YBa{sub 2}(Cu{sub 1{minus}{ital x}M{ital x}}){sub 3}O{sub 7{minus}{delta}} ({ital M}=Fe, Co, Ni, and Zn). Our study is primarily to determine several important material parameters, i.e., the location of dopants, the valence of dopants, structural changes, modifications of electronic densities of states, and the distribution of holes, which are thought to be key factors for understanding mechanisms causing the suppression of superconductivity via metal doping. Our results indicate that each metal substituent has a preference for a specific Cu site or combination of sites. We find that Fe and Co preferentially substitute for the Cu(1) atom at the linear-chain site; Ni resides in both Cu(1) and Cu(2) (the plane site); Zn occupies only the plane position Cu(2). In all cases, the metal-oxygen bond lengths (Fe-O, Co-O, Ni-O, and Zn-O) and valence states of dopant (mainly Fe{sup 3+}, Co{sup 3+}, Ni{sup 2+}, and Zn{sup 2+}) show little dependence on dopant concentration. Metal doping also has little effect on the valence of copper. However, each dopant affects to a different degree the oxygen 2{ital p} states. The changes of the oxygen 2{ital p} states for Fe-, Co-, and Ni-doped samples may be related to a reduction ofmore » some oxygen hole states. This reduction is attributed to a redistribution of charge carriers in the chain layer and possibly an inhibition of charge transfer from the chains to the planes. However, Zn substitutions show no observable change in its valence and oxygen hole states, indicating a different mechanism for suppression of superconductivity. Several possible explanations are offered that may elucidate the large suppression of {ital T}{sub {ital c}} in the Zn-doped case.« less

A computer modeling study of defect and dopant states in SnO 2

We present results of a computer modeling study of the technologically important material SnO 2. We concentrate on the properties of defects and dopants and show first that vacancy energies are high, leading to limited deviations from stoichiometry. Second, we find that both vacancy and interstitial (self-compensating) modes of solution may have low energies for low valence dopants. Oxidation of p-type doped material is found to be endothermic, hence poor conductivity is expected. In contrast we calculate that n-type doped samples can be exothermically reduced, and would consequently predict high conductivity for such materials.

The Effects of Metal Doping: Yba2(Cu1-XMX)3O7-δ (M=;Fe, Co, Ni, and Zn)

AbstractIn this study, we describe X‐ray absorption measurements designed to determine the influence of subsitutents on the location of dopants, the valence of dopants, structural changes, and modifications of electronic densities of states. Several possible mechanisms are offered which may elucidate Tc suppression due to metal dopings.