Sistema de monitoreo y protección

Perovskites display a large array of possible distortions which lead to a wide range of interesting and useful properties. Although perovskites have an empirical formula ABX3, any pair of ions can be included in the structure as long as they have

the appropriate ionic radii at each site and the total positive charge is equal to the negative charge from the anions. The structure can also be made more complex when multiple ions substitute the A or/and B ions. This chapter will focus on A-site substituted perovskites

Most perovskites that have interesting properties are usually complex, with different combinations of metal ions present. For example, the most extensively used perovskite material in electronic devices is PbZrxTi1-xO3 (PZT), as it has impressive

piezoelectric properties. However, the success of PZT releases more lead, mainly in the form of lead oxide, into the environment. Lead oxide is toxic and is volatile at the high temperatures used during calcinations and sintering processes. This environmental concern over the toxicity of lead has increased the need to develop alternative lead-free materials with properties approaching that of PZT. Alternative materials such as NaBiTi2O6 and KNbO3 have been developed. However, the

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of studies have aimed to improve their properties by making complex combinations such as (K,Na)BiTi2O6 and (K,Na)NbO3-LiTaO3-LiSbO3.1-6

The hydrothermal synthesis of NaBiTi2O6 was reported by Lencka et al.,

where they developed a thermodynamic model of a heterogeneous aqueous electrolyte system enabling them to predict the required conditions for the hydrothermal synthesis of NaBiTi2O6.7 They performed the synthesis by reacting

Bi(NO3)3·5H2O and TiO2 in NaOH solutions at 200 oC. On the other hand, Jing et al.

obtained NaBiTi2O6 from Bi(NO3)3·5H2O and Ti tetrabutoxide in NaOH solutions at

160-220 oC.8 They showed how the morphology was changed from spherical

agglomerates to uniform cubic particles depending on the NaOH concentration. Setinc et al. prepared NaBiTi2O6 hydrothermally from Bi(NO3)3·5H2O and TiO2 and

investigated the effect of temperature, reaction time and concentration of NaOH.9

Liu et al. showed that NaBiTi2O6 nanowhiskers with diameters of 20 nm can be

obtained by using a combination of sol-gel and hydrothermal techniques.10

Wang et al. obtained NaBiTi2O6 microcubes, by increasing the NaOH concentration

from 4 M to 10 M. Sardar and Walton studied the hydrothermal synthesis of a variety of bismuth titanate materials that crystallise from NaBiO3 and TiO2.11 By

altering the reagent ratios, pH and reaction time, different products were obtained and they successfully prepared NaBiTi2O6 in a one-step reaction when the Bi:Ti ratio

was 1:2 and 10 M NaOH were used.

The structure of NaBiTi2O6 at room temperature has been widely reported as

rhombohedral R3c.12-13 However, recent studies on both single crystals and sintered

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undergo phase transitions upon heating to tetragonal symmetry (270-300 oC) and

then cubic above 500 oC.16-20

NaLaTi2O6 is an analogue of NaBiTi2O6. It is usually prepared by the

ceramic method from the reaction of Na2CO3, La2O3 and TiO2 at 1200 oC.21-22 Its

hydrothermal synthesis has not been investigated as much as NaBiTi2O6. Shi et al.

prepared NaLaTi2O6 hydrothermally using TiN and La(NO3)3·6H2O in sodium

hydroxide solution.23 The structure of NaLaTi2O6 has not been generally agreed in

the literature. It has been reported to be cubic Pm m24, tetragonal I4/mcm25, rhombohedral R c22,26 and orthorhombic Pnma21,27-28. Garg et al. performed Rietveld refinement on both high resolution X-ray and neutron powder diffraction data of NaLaTi2O6.26 They reported a significantly better goodness-of-fit for the

rhombohedral structural model compared to the tetragonal and orthorhombic structure.

Another related perovskite is NaCeTi2O6. Wright et al. synthesised this

material by hydrothermally using TiF3 and CeCl3·7H2O in sodium hydroxide

solutions.29 They showed that substitution on the A-site (Ce3+ with Nd3+) or B-site

(Ti4+ with V4+) was easily achieved under hydrothermal conditions. They reported

the structure of NaCeTi2O6 to be orthorhombic, Pnma, the same as “synthetic

loparite” which was made by ceramic method.30

The “synthetic loparite” is NaCeTi2O6 made from Na2CO3, CeO2 and TiO2 in reducing conditions and at high

temperatures. However, this method results in minor CeO2 and rutile TiO2 being

present as impurities.

The aim of the work described in this chapter was to investigate a reproducible hydrothermal route to complex perovskite solid solutions and examine

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the degree of element mixing using a variety of structural probes. The materials discussed in this chapter are a solid solution of NaCe1-xLaxTi2O6 and A-site

substituted NaBiTi2O6. The structure of these materials was determined from X-ray

and neutron diffraction techniques with the aim of understanding the differences reported in the literature between different studies. XANES at the Ce LIII-edge was

used to confirm the oxidation state of Ce while 23Na NMR provided an insight on

how the paramagnetic effect of Ce and the A-site disorder affects the Na environment. The level of defects present in the hydrothermally prepared samples was also investigated using IR and 2H NMR. Attempts on controlling the particle

size and morphology of these materials were also performed by varying the solvents used.