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.