Bi_0.33Zr₂(PO₄)₃, a negative thermal expansion material with a Nasicon-type structure

A new negative thermal expansion ceramic material, Bi_0.33Zr₂(PO₄)₃, with a sodium zirconium phosphate structure, has been investigated and the results are presented. A three-dimensional structural framework is formed by the interconnection of ZrO₆ octahedra and PO₄ tetrahedra through their vertices. The occupation of a Bi ion in a type I site, located between two ZrO₆ octahedra along ZrO₆ < PO₄ > ZrO₆ ribbons induced the structure to crystallize in a trigonal system with a P3̄c1 space group. The average thermal expansion coefficients between 300 and 1073 K are found to be α = -12.99 × 10^-6 K^-1, α = -26.67 × 10^-6 K^-1, and α_v = -52.25 × 10^-6 K^-1. The large negative thermal expansion is highly associated with the peak shift towards a higher angle in temperature variable X-ray diffraction patterns, coupled rotation and distortion of ZrO₆ octahedra and PO₄ tetrahedra, and elongation or shrinkage in the Zr-O-P bond angles. Dilatometer results authenticate the negative thermal expansion behavior. The average bulk thermal expansion is registered as -0.73 × 10^-6 K^-1.