High Temperature Behaviour of Polyoxometalates Containing Lanthanides
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High Temperature Behaviour of Polyoxometalates Containing Lanthanides Hajime Kinoshita1, Marcus Brewer1, Caytie E. Talbot-Eeckelaers 2, Nik Reeves1, Roy Copping2, Clint A. Sharrad2 and Iain May2 1 Department of Engineering Materials, The University of Sheffield, Mappin Street, Sheffield, S1 3JD, U.K. 2 Department of Chemistry, The University of Manchester, Oxford Road, Manchester, M13 9PL, U.K. ABSTRACT The possibility of a simple heating process of POM to obtain tungsten bronze was investigated for nuclear waste immobilisation via DTA/TG and high temperature XRD. Heating process up to 900 oC caused the decomposition of structure for both systems. Cooling process seemed to have little effect on the final product for the K11[Nd(PW11O39)2]·xH2O, whereas the cooling profile showed a significant effect on the K13[Nd(SiW11O39)2]·xH2O. Nd formed two types of tungsten bronzes, namely Nd2WO6 and Nd4W3O15 in K11[Nd(PW11O39)2]·xH2O and K13[Nd(SiW11O39)2]·xH2O, respectively. INTRODUCTION Recent studies suggest a possibility of a heating process of polyoxometalates (POM) at a temperature significantly lower than the classical methods to obtain tungsten bronzes, MxWO3 (M: alkali, alkaline earth and rare earth, x: between 0 and 1) containing lanthanide [1], which may be applied for nuclear waste immobilisation [2]. The majority of oxide bronzes are tungsten bronzes of general form MxWO3 which exhibit a cubic structure based on WO6 octahedra as shown in Figure1 (a). It is known that one of the tungsten bronzes, ZrW2O8 possesses a negative thermal expansion property [3]. Such a property can be highly advantageous for the nuclear waste forms. Until recently the synthesis of tungsten bronzes containing lanthanides has required extensive heating e.g. around 1050 ºC for 100 hours [4]. Such a time and energy consuming process could be simplified by using POM as precursors which would be thermally decomposed at lower temperatures and for shorter times yielding the same results [1, 2]. POM are best described as symmetrical and compact groups of edge and corner shared M’O6 octahedra [5] with a general composition of [MxM’yOz]n- where M (the heteroatom) is a positive metal or non-metal and M’ (the addenda) is W or Mo in many cases. With the addition of further heteroatoms, preferably atoms with large radii and high coordination numbers, two or more of the core POM units can be linked as shown in Figure 1 (b). It suits the properties of the lanthanide and actinide elements and hence they are ideal for integrating into POM structures. Present work investigates the possibility of a simple heating process of POM to obtain tungsten bronze. The high temperature behaviour of various POM containing lanthanide elements was studied to investigate 1) the change of specimen upon heating, 2) phases formed in high temperature, and 3) effect of cooling profile on the final products.
(a)
(b) Ln M P W W
Figure 1. WO6 octahedra based structures: (a) tungsten bronze MxWO3 with a cubic structure [1] and (b) POM units integrated with lanthanide atom [Ln1(P1
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