Synthesis and Thermal Expansion of MZr₄P₆O₂₄ (M=Mg, Ca, Sr, Ba)

MZr₄P₆O₂₄ (M=Mg, Ca, Sr, Ba), which belongs to a new, low‐thermal‐expansion family of materials known as [NZP] or [CTP], were synthesized by solid‐state reaction (oxide mixing) and sol‐gel methods, and their sinterability in two cases was investigated and compared. Thermal‐expansion measurements were made on sintered samples by dilatometry, high‐temperature X‐ray diffractometry, and the laser speckle technique. An anisotropy in axial thermal‐expansion coefficients was observed in these materials. CaZr₄P₆O₂₄ and SrZ₄P₆O₂₄ showed reverse anisotropy as well as opposite bulk thermal expansion; i.e., CaZr₄P₆O₂₄ had negative bulk thermal expansion and SrZr₄P₆O₂₄ positive bulk thermal expansion in the temperature range 25° to 500°C. The microstructure of the sintered samples was also examined by scanning electron microscopy.