Phase-field determination of NaSICON materials in the quaternary system Na2O-P2O5-SiO2-ZrO2: II. Glass-ceramics and the phantom of excessive vacancy formation

This work focuses on a very narrow region in the quaternary system Na₂O-P₂O₅-SiO₂-ZrO₂ to explore the occasionally proposed deficiency in zirconium and oxygen content of Na⁺ super-ionic conductor (NaSICON) materials. In addition, this region is known for the formation of glass-ceramics, but a systematic study of such materials has not been carried out yet. For this purpose, 2 series of compositions were defined and synthesized: Na_3.4Zr_2-3x/4Si_2.4-x/4P_0.6+x/4O_12-11x/8 and Na_3.4Zr_2-3x/4Si_2.4+x/4P_0.6+1.5x/4O_12-x/16. They only differ in the silicate and phosphate content. In the first series the molar content is constant, n_Si + n_P = 3. The latter series allows an excess of the 2 cations to meet the composition Na_3.1Zr_1.55Si_2.3P_0.7O₁₁ or alternatively re-written as Na_3.4Zr_1.7Si_2.52P_0.77O_l2, which was formerly regarded as a superior material to the frequently reported composition Na₃Zr₂Si₂PO_l2.

Several characterization techniques were applied to better understand the relationships between phase formation, processing, and properties of the obtained glass ceramics in the context of the quasi-quaternary phase diagram. The investigations gave clear evidence that a glass phase is progressively formed with increasing x. Therefore, compounds with x > 0.2 have to be regarded as glass-ceramic composites. The resulting NaSICON materials revealed a very limited Zr deficiency with charge compensation by Na ions and a non-detectable amount of oxygen vacancies verified by neutron scattering and atomistic simulations.

Hence, this work is the first systematic investigation of pretended Zr-deficient NaSICON materials, which clearly show the chemistry of a 2-phase region. The 2 investigated series are directed toward a region that is orthogonal to the series Na₃Zr_3-ySi₂P_yO_11.5+y/2 reported in the first part of this series of publications.