非化学计量和掺杂对(Na_(1/2)Bi_(1/2))_(0.92)Ba_(0.08)TiO_3陶瓷电性能的影响

研究了非化学计量和掺杂对无铅压电陶瓷 (Na1 2 Bi1 2 ) 0 .92 Ba0 .0 8TiO3的压电性能及去极化温度的影响 .研究发现A位非化学计量可以提高陶瓷的压电性能 ;B位掺杂对材料电学性能的影响规律类似于Pb(Ti,Zr)O3系压电陶瓷的相关规律 ;由于非化学计量和掺杂会影响到A位离子对B位离子与氧离子形成的BO6 八面体的耦合作用 ,影响到畴的稳定性 ,从而影响到 (Na1 2 Bi1 2 ) 0 .92 Ba0 .0 8TiO3陶瓷的去极化温度 ;所研究的陶瓷样品的去极化温度越低 ,压电系数越高 .     

The La2S3-LaS2 system: Thermodynamic and kinetic study

A detailed thermodynamic study of the LaS₂-La₂S₃ system in the temperature range 350-1000 °C was performed, starting from high quality crystals LaS₂ as the highest polysulfide in the system, and using a sensitive static tensimetric method with a quartz Bourdon gauge and a membrane as a null-point instrument. The p_S-T-x diagram obtained has shown that the phase region covering the composition between LaS₂ and La₂S₃, which was previously described as a single grossly nonstoichiometric phase, consists of three discrete stoichiometric phases, LaS_2.00, LaS_1.91, and LaS_1.76, where compositions could be determined with an accuracy of ±0.01 f.u. The thermodynamic characteristics of evaporation of the polysulfides as well as standard heat of LaS₂ formation were calculated. The role of kinetics in the formation of ordered superstructures of sulfur-poorer polysulfides with different formal concentration of vacancies is considered.     

Oxygen nonstoichiometry and chemical stability of Nd2−xSrxNiO4+δ

Nonstoichiometric variation of oxygen content in Nd_2−xSr_xNiO_4+δ (x=0, 0.2, 0.4) and decomposition P(O₂) were determined by means of high temperature gravimetry and coulometric titration. The measurements were carried out in the temperature range from 873 to 1173 K and the P(O₂) range from 10⁻²⁰ to 1 bar. Nd_2−xSr_xNiO_4+δ shows the oxygen excess and the oxygen deficient composition depending on P(O₂), temperature, and the Sr content. To evaluate the characteristics of oxygen nonstoichiometric behavior, partial molar enthalpy of oxygen was calculated. The value of partial molar enthalpy of oxygen slightly approaches zero as δ increases in the oxygen excess region while that is independent of δ in the oxygen deficient region. Discussion was made by comparing data of this study with nonstoichiometric and thermodynamic data of La_2−xSr_xNiO_4+δ: Nd_2−xSr_xNiO_4+δ show more oxygen excess than La_2−xSr_xNiO_4+δ in the higher P(O₂) region, while the nonstoichiometric behavior in the oxygen deficient composition is almost the same. The variation of partial molar enthalpy of oxygen with δ for Nd_2−xSr_xNiO_4+δ in the oxygen excess region is much smaller than that of La_2−xSr_xNiO_4+δ. The oxygen nonstoichiometric behavior of Nd_2−xSr_xNiO_4+δ is more ideal-solution-like than that of La_2−xSr_xNiO_4+δ.     

Synthesis and properties of Sr-free Bi cuprate superconductors

Both the oxygen content and T^onset _c of the Bi₂Ca_2.5(Ln, Ln⁺)_0.5Cu₂O _y system monotonically increases as the mean ionic radius of (Ln, Ln⁺) increases. The structural modulations in all of the samples are almost commensurate with a period equal to 9b. In Bi_2-xPbCa_2.5Nd_0.5Cu₂O _y , T^onset _c decreases with increasing Pb concentration. In order to investigate the effect of post annealing using a hot isostatic press (HIP), Bi₂Ca_2.5La_0.25Pr_0.25Cu₂O _y was processed under different HIP conditions. This result suggested that HIP annealing causes a phase change from the 232 structure to the 221 structure.     

A Structural Study of the Homogeneity Domain of LaNi5

LaNi₅ is a very important intermetallic compound for hydrogen storage applications. Its homogeneity range at 1200°C has been investigated by single-crystal X-ray diffraction, electron probe microanalysis, and mass density measurements. At the nickel-rich end the hexagonal structure (CaCu₅ type) extends to the composition LaNi_5.45. The excess of nickel is accommodated by random substitutions of lanthanum atoms by nickel dumbbells and by displacements of nickel atoms which are presumably correlated with the La atom substitutions. Interatomic distances suggest that the Ni atom displacements occur in directions toward vacant La atom sites, i.e., toward the center of Ni dumbbells. At the nickel-poor end the structure extends to the composition LaNi_4.88. The excess of lanthanum is presumably accommodated by the simultaneous occurrence of lanthanum atoms and vacancies on nickel atom sites. Both models are consistent with mass density and electron probe microanalysis measurements. Hydrogenation thermodynamic properties have been measured as a function of composition.     

Some sources of defects in α-HgI2, the room temperature γ-ray detector

In the course of a systematic optimization of the materials properties of -HgI₂ crystals for room temperature - and X-ray detectors, we have investigated possible sources of defects and discuss briefly the possibility to supress them. Due to the particular structure of -HgI₂, large amount of impurities, particularly hydrocarbons, can be absorbed invan der Waals layers and lattice channels. Purification by sublimation does not work due to the affinity of hydrocarbons to iodine and their easy re-absorption in the sublimate. Lattice filtering of the large concentration of hydrocarbons contained even in suprapure iodine has been performed using the close spaced lattice of CuI. Oxidation of hydrocarbons by reaction of oxygen with HgI₂ is another possibility for their removal.Mass spectrometric investigations of a molecular beam of -HgI₂ has solved the long disputed problem of the existence of nonstoichiometry: both excess of Hg or excess of I are possible. The removal of nonstoichiometry can be achieved by suitable thermal treatment only in the case of pure crystals. In the presence of hydrocarbons, the non-stoichiometric defects are fixed showing the predominant importance of hydrocarbons for -HgI₂.Investigation of the evaporation of -HgI₂ with mass spectrometry at low temperatures (150>T>40 °C) shows a strong change of the enthalpy of evalporation at 67 °C whereas DSC does not show any peak at this temperature. It seems probable that this is due to a surface reconstruction which influences the evaporation but not the thermal bulk lattice effects which are detected by DSC.Dedicated to Prof. Dr.K. L. Komarek at the occasion his 60th birthday.     

Oxygen nonstoichiometry (δ) of TiO2−δ-revisited

Oxygen nonstoichiometry (δ) of “undoped” polycrystalline TiO_2−δ has been measured as a function of oxygen partial pressure in the widest ever examined range of 10^-18?P_O2/atm?10^-1 at elevated temperatures (1073?T/K?1473) by thermogravimetry and coulometric titrometry combined and compared with all the reported values. Isothermal variation of nonstoichiometry against P_O2 is explained with a defect model involving quadruply ionized titanium interstitials, electrons, holes, and unidentified acceptors which may be background impurity acceptors or cation vacancies. The equilibrium constants for intrinsic electronic excitation reaction and redox reaction are determined from the nonstoichiometry measured and compared exhaustively with all the reported values. The relative partial molar enthalpy and entropy of oxygen are evaluated as functions of nonstoichiometry and the inner workings of their variations discussed.     

Influence of Oxygen Nonstoichiometry on the Spectral Properties of Solid Solutions LaNb2−2xTa2xVO9−δ (x=0-0.4) and LaTa2−2xNb2xVO9−δ (x=0-0.1)

Phase equilibria in the LaVO₄-Nb₂O₅-Ta₂O₅ system were analyzed. New solid solutions LaTa_2−2xNb_2xVO_9−δ (x=0-0.1) and LaNb_2−2xTa_2xVO_9−δ (x=0-0.4) were detected in this system. The structures of the vanadate-niobate LaNb₂VO₉ and vanadate-tantalate LaTa₂VO₉ are not known. The structures of the vanadate-tantalate LaTa₂VO₉ and LaTa₂VO₉-based solid solutions are similar to the structure of LaTa₇O₁₉, which refers to the hexagonal crystal system. The influence of the oxygen nonstoichiometry δ(x) on crystallochemical characteristics and spectral properties of these solid solutions were examined by the X-ray phase analysis, IR and radio spectroscopic methods. A correlation between the nonstoichiometry δ(x) and the volume of a unit cell V(x) of solid solutions LaTa_2−2xNb_2xVO_9−δ was found. The IR spectrum of LaTa₂VO_9−δ transformed in going from δ=0 to δ≠0. Two types of VO₄ tetrahedra were formed in solid solutions LaNb_2−2xTa_2xVO_9−δ depending on δ(x).     

Co掺杂的YBa2Cu2O6+δ的氧非计量和氧渗透性能研究

研究钴离子部分取代铜离子对YBa2Cu3O6+δ的氧非计量值δ和氧渗透率的影响.对于钴替代的样品,氧非计量的绝对值变大,且其数值不再随温度和氧分压的变化发生显著变化.YBa2Cu2CoO6+δ样品在中、高温具有可观的氧渗透率.对于厚度为1.2 mm的致密YBa2Cu2CoO6+δ样品,在850℃时,只要在样品两端施加较小的氧分压差(PO2=21.2 kPa、 PO2=101 Pa),其氧渗透率即可达57 μmol/cm2 s, 明显高于YBa2Cu3O6+δ的氧渗透率(31 μmol/cm2 s).YBa2Cu2CoO6+δ的高氧渗透率在结构上可被归结为位于晶胞基面上的氧离子和氧空位的均匀分布.