Phase-pure perovskite

Metal (I) - oxygen - metal (II) bridging bond formation occurs as a consequence of a redox reaction between two metal oxides or hydroxides as mentioned above. It is particularly interesting to apply these principles to the synthesis of complex oxides such as perovskites. Phase pure perovskites, e.g. 

PMN-PT) are notorious for the difficulty to obtain as phase pure perovskite. To fabricate phase pure PMN it is necessary to prepare magnesium niobate (columbite) first, and then to heat a mixture of columbite with other oxide species. PbO is usually added in a small excess over exact stoichiometry that is a complicated and tedious process. 

The principle was further extended to other perovskites, which are even more difficult to synthesize, such as (l-x)Pb(Mg, 3Nb2 3)03 xPb(Zn, 3Nb2/3)0 (PMZN), where a fraction x of Mg is replaced by Zn [23,24]. As shown in Fig. 7.10, we obtained phase pure perovskite up to x=0.7 in one step from a stoichiometric mixture comprising those components used for PMN and basic zinc carbonate. During heating, perovskite formed as a result j j... 

Calcination of the oxalate coprecipitates readily yielded phase-pure perovskite-type complex metal oxides. The required calcinations for the microemulsion-derived mixed oxalates were 100-250"C below the temperatures used for oxalates prepared in homogeneous aqueous solutions. 

A-site cations, could affect the electrical conductivity through changes in charge carrier mobility. The electrical conductivity was shown to change when substituting La with Mg and Ba, but this was mostly due to the formation of secraidary phases [72]. Apart from Sr, only substitution with Ca resulted in a phase-pure perovskite, but without an increase in conductivity compared to LSCM [72], Furthermore, the introduction of Ca is expected to result in the instability of LSCM under reducing conditions [73]. 

Perovskites AB)/3C2/303 (A = Ba, Sr, B = Zn, Mg, Co, Ni C = Nb, Ta) are promising compounds for microwave applications. It is important to synthesize these complex oxides as pure perovskite phases because the slightest admixture of a second phase hinders drastically the dielectric properties of ceramics, which sinter only at very high temperatures (1400 to 1500°Q. The precursor chemistry resembles greatly that of BaTi03 formation by alkoxide or alkoxide-hydroxide routes. Below we summarize the 3 approaches to the synthesis of these perovskites by the sol-gel method ... 

As shown in Fig. 7.7, a pure perovskite phase was obtained after calcining the milled mixture at 850°C for 4 h in air. On the other hand, a cubic pyrochlore (P3N4) phase was predominant with the coexistence of perovsWte and PbO (litharge) on the calcined products from non-milled mixture. When MgO was used instead of Mg(OH)2, no significant change of... 

Composition of the lower mantle A variety of compositions have been proposed for the lower mantle including almost pure perovskite, chondrite, and pyrolite. However, most models of the Earth assume that the upper and lower mantle have the same composition. Recent attempts to directly estimate the composition of the lower mantle have used best-fit curves of the thermoelastic properties of the Earth to a PREM model mantle made up of the phases Mg-perovskite, Ca-perovskite, and magnesiowustite. The recent calculations by Li and Zhang (2005) indicate a pyrolitic composition for the lower mantle, with Mg/Si atomic ratios between 1.29 and 1.39, slightly higher than those for the pyrolite models in Table 3.1 (Mg/Siatomic = 1.24—1.25). 

A primary characterization of perovskite-type oxides must include textural analysis and X-ray identification of the phase(s) present. For a more detailed characterization, structural analysis for establishing the lattice position of cations and surface analysis (by means of techniques such as XPS) for defining the surface concentration and oxidation states of cations are desirable. Consequently, information provided by these techniques will furnish the essential criteria for comparing the different preparation methods. For convenience, we will classify the methods used to date for the preparation of pure perovskite phases according to the scheme proposed by Courty and Marcilly (29) for the whole field of mixed oxides. Table I gives a survey of methods used as a function of the phenomena on which they are based. 

When no excess Na2C03 was added (x = 2), corresponding to Equation (1), the impurity K NbeO was detected see Figure 2, (a). When more Na2C03 was added (x > 2) pure perovskite (Na, K)Nb03 phases were obtained. A similar phenomenon was observed for the synthesis of KNbO3.03]... 

Example 3 Synthesis of multi-component complex compounds A nmnber of materials such as some lead-containing complex oxides are very difficult to prepare in the form of pine phase. Powder of Pb(Znj,.Mgi jt)i/3Nb2/303 was obtained by soft-mechanochemical procedure, that is, by milling of a stoichiometric mixture of PbO, Mg(OH)2, Nb20s and 2Zn(OH)2 H2O in a multi-ring-type mill up to 3 h. Partial formation of desired phase with perovskite structure already took place during milling. Subsequent heat treatment at 1000°C for 1 h yielded pure perovskite phase. 

A pure perovskite phase with the composition SrBi2Ta209 and a crystal size <50 nm was formed at 1073 K from a xerogel precursor, obtained by gelation from Ta(EtO)5 and Sr-acetate and BiO-nitrate solution in methoxyethanol using an acetic acid catalyst [70]. Acetate ions act as bidentate ligands in this solution, equalizing the hydrolysis/condensation rates of Ta and other ions. 

For the low-temperature synthesis of LaA103, Taspinar and Tas (1997) proposed a self-propagating combustion synthesis from aqueous solution containing urea and the respective nitrate salts. A pure perovskite phase was obtained at 1023 K, which is one of the lowest temperatures reported for the powder s)mthesis of LaA103 so far. Han et al. (2006) used a combustion method for the s)mthesis of Eu " -activated YAIO3 and GdA103 by redox reactions between the respective metal nitrates and glycine in a preheated furnace at 773 K. 

Pure perovskite phase Bao.7Sro.3Ti03 nanoparticles were synthesized by the molten-salt method in NQ-KCl flux at a low temperature of 850°C. The molten salts accelerated the formation of the Bao.ySrojTiOs, and the calcination temperature played an important role in the development of BST particle morphology [79]. 

The first synthesis of mesoporous mbced oxides was reported by Schwickardi et al. [64]. These authors reported the synthesis of a series of phase-pure spinels (ZnCr204, CoCr204, and NLA.I2O4) and LaFeOs perovskites with disordered pore structure using activated carbon as the hard templates and metal nitrates as... 

Khorasani-Motlagh, M., Noroozifer, M., and Ahanin-Jan, A. (2012) Ultrasonic and microwave-assisted co-precipitation synthesis of pure phase LaFeOs perovskite nanocrystals. /. Iran. Chem. Soc., 9, 833-839. 

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