Issue 10, 1999

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

Abstract

We present the results of variable temperature neutron powder diffraction studies of three low thermal expansion materials belonging to the NZP family; K 0.5 Nb 0.5 Ti 1.5 (PO 4 ) 3 , Ba 0.5 Ti 2 (PO 4 ) 3 and Ca 0.25 Sr 0.25 Zr 2 (PO 4 ) 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 MO 6 (M = Nb, Ti, Zr) and PO 4 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 NaTi 2 (PO 4 ) 3 in which the MI sites are completely filled, and Sr 0.5 Ti 2 (PO 4 ) 3 , in which the MI sites are half-occupied in an ordered manner.

Article information

Article type
Paper

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

D. A. Woodcock, P. Lightfoot and R. I. Smith, J. Mater. Chem., 1999, 9, 2631 DOI: 10.1039/A903489G

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