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

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 (3) over bar c) but ordered in the latter (space group R (3) over bar). 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.