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DOI: 10.1039/A903489G (Paper) Reference Section for: J. Mater. Chem., 1999, 9, 2631-2636

Powder neutron diffraction studies of three low thermal expansion phases in the NZP family: K0.5Nb0.5Ti1.5(PO4)3, Ba0.5Ti2(PO4)3 and Ca0.25Sr0.25Zr2(PO4)3

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David A. Woodcock, Philip Lightfoot and Ron I. Smith

Abstract

We present the results of variable temperature neutron powder diffraction studies of three low thermal expansion materials belonging to the NZP family; K0.5Nb0.5Ti1.5(PO4)3, Ba0.5Ti2(PO4)3 and Ca0.25Sr0.25Zr2(PO4)3. Each of these structures has half occupancy of the crystallographic site for the large cation (MI = K, Ba or Ca/Sr), but differ in that the vacancies on this site are disordered in the former two (space group <$$>\[ R\overline 3 c \]<$$>) but ordered in the latter (space group <$$>\[ R\overline 3 \]<$$>). The thermal expansion behaviour is quantified from parameters obtained by Rietveld structure refinement, and is described in terms of rotations and distortions of linked MO6 (M = Nb, Ti, Zr) and PO4 polyhedra. The driving force for the anisotropic low thermal expansion behaviour is found to be in the thermal expansivity of the MI–O bonds. This behaviour is intricately linked to the order/disorder behaviour of the MI sites, as shown by a comparison of the three title structures, together with those of NaTi2(PO4)3 in which the MI sites are completely filled, and Sr0.5Ti2(PO4)3, in which the MI sites are half-occupied in an ordered manner.

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