Layered mixed tin-titanium phosphates
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Layered mixed tin-titanium phosphates Camino Trobajo, Maria L. Rodr´ıguez, Marta Su´arez, Jos´e R. Garc´ıa,a) and Julio Rodr´ıguez Departamento de Qu´ımica Org´anica e Inorg´anica, Universidad de Oviedo, 33071 Oviedo, Spain
Jos´e B. Parra Instituto Nacional del Carb´on, C.S.I.C. Apartado 73, 33080 Oviedo, Spain
Miguel A. Salvad´o, Pilar Pertierra, and Santiago Garc´ıa-Granda Departamento de Qu´ımica F´ısica y Anal´ıtica, Universidad de Oviedo, 33071 Oviedo, Spain (Received 30 January 1997; accepted 22 May 1997)
Mixed crystalline tin-titanium phosphates with variable tin-to-titanium molar ratio have been prepared by precipitation of soluble salts of the metal (IV) with phosphoric acid and refluxing the amorphous solids in 17 M H3 PO4 . The new materials are characterized by chemical textural and thermal analysis and x-ray powder diffraction. The tin-titanium phosphates are solid solutions showing an isomorphic substitution of tin by titanium in the a-tin phosphate lattice and tin substitution in the g-titanium phosphate lattice. In both cases, the solubility is partial. The coexistence of both saturated phases is observed in samples of composition between the solubility limits.
I. INTRODUCTION
In recent years, the layered phosphates of tetravalent metals have been the subjects of several investigations as a consequence of their interesting properties in fields such as ion exchange, catalysis, ionic conductivity, etc.1–6 The investigation of mixed compounds of this type is also of interest because the specific properties of single components may be improved by adding other suitable chosen elements, in order to control their properties. Two structural types of layered phosphates have mainly been studied: a- and g-forms.7 Despite the chemical composition of both varieties being similar (MH2 P2 O8 ? nH2 O, n 1, 2), they are very different since the a-form is a bis(hydrogenphosphate) monohydrate8,9 and the g-form is a dihydrogenphosphate-orthophosphate dihydrate.10,11 Many layered phosphates12–15 and phosphonates16–18 with a-structure are known. The g-compounds are scarce.19–21 Mixed titanium-zirconium phosphates have earlier been prepared by Clearfield and Frianeza22 by one of the conventional methods used in the synthesis of a-zirconium phosphate: precipitation of soluble salts of the metal (IV) with phosphoric acid and reflux of the amorphous solids obtained in concentrated H3 PO4 . Later, Shakshooki et al.23 have also prepared mixed titanium-zirconium phosphates by slow phosphoric acid precipitation of the fluorocomplex of titanium and zirconium formed in HF aqueous solution, but poorly crystalline powders are obtained. In both cases, segregation of one or two metal phosphates could take place during precipitation leading to the formation a)
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754
http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 3, Mar 1998
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of mixed phases instead of one well-defined phase. Farf´an-Torres et al.
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