Barium phase transformations

Pure barium is a silvery-white metal, although contamination with nitrogen produces a yellowish color. The metal is relatively soft and ductile and may be worked readily. It is fairly volatile (though less so than magnesium), and this property is used to advantage in commercial production. Barium has a bcc crystal stmcture at atmospheric pressure, but undergoes soHd-state phase transformations at high pressures (2,3). Because of such transformations, barium exhibits pressure-induced superconductivity at sufftciendy low temperatures (4,5). 

Zirconium hydroxide is precipitated by bases at lower pH than the hafnium compound. Zr and Hf are obviously unable to form true hydroxides, and these compounds are more correctly formulated as MO2 XH2O. Amorphous hydrous zirconia and hafhia (a-phase) transform to microcrystalline forms (/f-phase) with noticeable heat evolution. They lose water up to the composition MO2 H2O at 140 °C (Zr) or 155 °C (Hf). Hydrous zirconia has excellent absorptive capacity, particularly for oxygen-containing anions. For example, the concentration of S04 anions over hydrous zirconia is so low that no precipitate forms on the addition of barium salts to the filtrate. While the hydroxides of composition M(OH)4 are not stable, in alkaline solutions, M(OH)s are present and even M(0H)6 anions have been reported in very concentrated alkalis. Salts of these anions, such as Na2Hf(OH)6, can be isolated. 

The inherent stability of aluminas can be further improved by addition of other oxides (ref. 32). Base metals can act as promoters and in an ideal situation would fulfill a dual role. Fig. 3 shows the change in surface areas for boehmite derived activated aluminas as a function of temperature. It may be seen that addition of barium retards phase transformation and consequent loss of surface area to well above 1000°C. 

Strobel et al. [51] prepared barium carbonate nanoparticles (50-100 nm) by flame spray pyrolysis. The rapid quenching during the preparation process resulted in the unprecedented formation of pure monoclinic BaCOa- The as-prepared material was characterized by electron microscopy, XRD as well as by TG and DSC analyses (Figures 6 and 7). At ambient conditions the metastable monoclinic phase transformed easily into the thermodynamically stable orthorhombic BaCOs (witherite) within a few days. 

Frey MH, Payne DA (1996) Grain-size effect on structure and phase transformations for barium titanate. Phys Rev 6 54 3158-3168... 

In the next few paragraphs, we will discuss the processes involved in synthesis ofbarium titanate. This is an extremely important material for electronic industry due to its outstanding ferroelectric, piezoelectric and dielectric properties. Chemistry ofBa and Ti and crystal structure of BaTiOg are relatively simple. Phase transformations of barium titanate have been deeply studied and well documented. Thus, it may be considered as a convenient model oxide material. It is not surprising that a lot of information about application of Pechini method is available for this compound. Here we will attempt to create the complete scenario of barium titanate synthesis by PC method, and then we will expand this knowledge to discuss synthesis of other important multicomponent oxide materials. 

Experiments were performed using a visible optical parametric amplifier based on noncollinear phase-matching in /3-barium borate, followed by a pulse compressor using chirped dielectric mirrors. This optical source provides ultrabroadband pulses, with bandwidth extending from 500 to 720 nm, compressed to an almost transform-limited duration of 5-6 fs. The pump-... 

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