Bi0.33Zr2(PO4)3, a negative thermal expansion material with a Nasicon-type structure.

A new negative thermal expansion ceramic material, Bi0.33Zr2(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 αa = -12.99 × 10-6 K-1, αc = -26.67 × 10-6 K-1, and αv = -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.

Langbeinite Phosphates KPbM2(PO4)3 (M = Cr, Fe): Synthesis, Structure, Thermal Expansion, and Magnetic Properties Investigation.

New langbeinite-type phosphates KPbCr2(PO4)3 and KPbFe2(PO4)3 are synthesized by solution method and characterized by powder X-ray diffraction, infrared spectra, thermogravimetric and differential thermal analysis, scanning electron microscope, and energy dispersive X-ray analysis. Rietveld refinement reveals that both of the compounds crystallize in the cubic system with P213 space group, and the calculated lattice parameters for Cr and Fe phases are 9.7332(2) and 9.8325(7) Å, respectively. The electron micrographs confirm the crystalline nature of the samples from their surface morphologies. Infrared spectra display the characteristic features of P-O and M-O vibrational bands for both of the phases. Thermal analysis of KPbCr2(PO4)3 and KPbFe2(PO4)3 indicates that they are thermally stable up to 1273 K. The axial thermal expansion is studied by high-temperature X-ray diffraction between 298 and 1073 K. The average thermal expansion coefficients of KPbCr2(PO4)3 and KPbFe2(PO4)3 are identified as 8.9 × 10-6 and 10.8 × 10-6 K-1, respectively. Magnetic study reveals both of the compounds follow Curie-Weiss behavior in the higher-temperature region, and antiferromagnetic interactions are dominant.