Pyrochlore phases

Figure 6.2 Ionic transport number for oxide ion conductivity in the pyrochlore phases Lu2Ti207, Lu2.096Tii.904O6.952> and Lu2286Tii.7i406.857- [Data adapted from A. V. Shlyakhtina, J. C. C. Abrantes, A. V. Levchenko, A. V. Knot ko, O. K. Karyagina, and L. G. Shcherbakova, Solid State Ionics, 177, 1149-1155 (2006).]...

The ferroelectric Pb(Mgy3Nb2/3)03 (PMN) ceramic has been the snbject of extensive investigations due to its high dielectric coefficient and high electrostrictive coefficient, which renders it suitable for use in capacitors and electrostrictive actuators. However, the successful exploitation of this material is limited by the difficulty of producing a single-phase material with the perovskite structnre. Conventional solid state synthesis techniques invariably resnlt in the formation of one or more pyrochlore phases, which exhibit poor dielectric properties. 

Recent work by Wang et al." has shown that formation of undesirable pyrochlore phases can successfully be avoided through the use of mechanical alloying. In their study, it was found that mechanical alloying of the constituent oxides for 20 hours resulted in the formation of a single phase PMN perovskite powder with an average particle size of 20 to 30 nm. 

Pb(Nbo,67 Mgo,33)03 and its PbTi03-doped modifications are useful for electros-trictive transducer applications . Preparation of this material is complicated by the appearance of an unwanted pyrochlore phase. 

On the other hand, nonferroelectric pyrochlores may serve as technologically useful dielectrics in other applications, such as temperature-stable dielectrics or microwave dielectrics. It is important to understand the structural relationship between the perovskite and pyrochlore phases in the Pb-based systems and to elucidate the thermodynamic and kinetic factors that may influence the phase equilibrium. Understanding these will allow a more systematic method of tailoring dielectric compositions to applications. 

Contains about 25 wt% of U-238 to form the stable pyrochlore phase. The U-238 will also dilute U-235 (a fissile material which is a decay product from Pu-239) in the future to minimize the possibility of a criticality ... 

A. R. Boccaccini, S. Atiq and R. W. Grimes, Hot-Pressed Glass Matrix Composites Containing Pyrochlore Phase Particles for Nuclear Waste Encapsulation, Adv. Eng. Mat. 5, 501-508 (2003). 

The polycrystalline film consists of the tetragonal phase of PZT with peaks arising at approximately 20 = 22 , 31°, 39°, 45°, and 55°. Tliese peaks correspond to the (100), (110), (111), (200), and (211) orientations, respectively. A bro peak centered around 26 = 30° was also evident. This may correspond to the metastable pyrochlore phase. The pyrochlore phase is lead deficient, such that Pb, labeled x, in Pbx(Zro.4gTio.s2)03, is less than one, consistent with the loss of lead oxide during the anneal at 650°C. Lead oxide is known to evaporate from films during the thermal annealing conditions described here (17,18). 

The pure PbMgo.33Nbo.67O3 perovskite phase was crystallized at 1253 K from xerogel obtained by cogelation of Pb-acetate, Mg- and Nb-ethoxides in methoxyethanol [69]. Its purity was controlled by addition of excess Pb and Mg components that prevented formation of the lead niobate pyrochlore phase. 

Calcined samples indicated formation of carbonate groups at above 500"C at 700°C, crystalline phases started forming (apparently traces of both a pyrochlore phase and the desired tantalate). Heating at 850 C without soaking led to the crystallization of phase pure SrBi2Ta209. The particles approached sphericity with a porous and agglomerated form, and had an average size of about 60 nm. 

Kim et al. [208] synthesized perovskite-type Pb(Mgi/3Nb2/3)03 powders by an emulsion method with (a) an aqueous solution of lead nitrate, magnesium nitrate hexahydrate and niobium oxalate as the water phase, (b) Span 80 as the surfactant, (c) paraffin oil as an emulsifying agent and (d) kerosene as the oil phase. The emulsion of short stability, prepared by high speed mixing, was added drop by drop into petroleum heated at 170 C. The product particles were washed in toluene and cleaned at 150"C. Careful calcination yielded dominantly a pyrochlore phase at 600 C, but only the desired perovskite-type niobate at 800 C. 

Die. Zn-Sb spinels stand out from the ZnO DF. Pyrochlore phase appears white at triple matrix. points of grains. 

Etched with citric acid. Au coating. REM. SEM. Good contrast between ZnO. Zn-Sb spinel, and pyrochlore phase. 

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