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Structures and phase transitions of Molybdenum Phosphates

Sarah E. Lister ,  John S. Evans 

Department of Chemistry, University of Durham, Science Labs, South Road, Durham DH1-3LE, United Kingdom

Abstract
We have an ongoing interest in metal phosphate systems whose structures can be described as containing corner-sharing polyhedra.  These systems often exhibit unusual thermal expansion, for example some materials show strong negative thermal expansion over large temperature ranges.  These interesting structure-temperature relationships are often encountered alongside displacive phase transitions from the high temperature substructure to the more complicated low temperature superstructure, sometimes via a number of intermediate phases.  It is from an understanding of the changing structure of the materials that the bulk material properties can be understood.  As part of these studies the synthesis, structure and phase transitions of several molybdenum phosphate systems with different Mo:P ratios have been investigated.

Both in-situ and ex-situ synthetic techniques have been used to study the formation of (MoVIO2)2P2O7 from a hydrated precursor.  These reveal the formation of two previously unknown MoPO(H) phases.  Also, despite its simple formula, (MoVIO2)2P2O7 has been discovered to undergo a displacive phase transition just above room temperature, having a complicated structure requiring a much larger unit cell to fully describe it compared to the simple structure reported in the literature[1].  As this work has been carried out predominantly using powder XRD data, the use of complementary techniques was necessary to gain greater understanding of the transition.  The observations gained from 31P NMR, electron diffraction and neutron diffraction will be discussed and refinement strategies outlined which enable solution of a structure with a minimum of 60 atoms in the asymmetric unit.  

1.  P. Kierkegaard (1962), Arkiv Foer Kemi, 19, 1-14

 

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Presentation: Poster at 11th European Powder Diffraction Conference, Poster session, by Sarah E. Lister
See On-line Journal of 11th European Powder Diffraction Conference

Submitted: 2008-04-29 16:22
Revised:   2009-06-07 00:48