Structural Investigation of Monoclinic-Rhombohedral Phase Transition in Na3Zr2Si2PO12 and Doped NASICON

Aliovalent doping of the zirconium site in Na3Zr2Si2PO12 (NASICON) was performed with a range of +3 (Al, Y, Fe) and +2 (Co, Ni, Zn) valent cations. The monoclinic-rhombohedral phase transition was analyzed with high-temperature insitu X-ray diffraction, and differential scanning calorimetry (DSC). From the lattice parameters extracted at room temperature up to 300 degrees C it was determined that the high-temperature rhombohedral phase distorts to the low-temperature monoclinic phase through a shear deformation of the unit cell. DSC confirmed the phase transition and demonstrated that the phase transition temperature was lowered by doping the NASICON structure. Furthermore, the distortion of the lattice was less severe for all doped samples. Ultimately aliovalent substitution for zirconium stabilized the higher symmetry rhombohedral phase of NASICON, with yttrium doping providing the lowest phase transition temperature and the smallest distortion of the lattice through the phase change.