Stepwise Local Structural Evolution of Ni ²⁺ in Na ₂ O–MgO–Al ₂ O _{3 Enoch Mori, Naoya Hatakeyama, Kana Tomita, Tetsuo Kishi, Takuma Kurakami, Ryunosuke Enoeda, Daisuke Miyabe, Daiju Matsumura, Tetsuji Yano}

ABSTRACT

We investigate how composition governs the local structure of Ni ²⁺ , which is used as a structural probe for Mg ²⁺ in Na ₂ O–MgO–Al ₂ O ₃ –SiO ₂ (NMAS) glasses. Ni K‐edge X‐ray absorption near‐edge structure (XANES)/extended X‐ray absorption fine structure (EXAFS) and optical absorption spectroscopy show that varying MgO and Al ₂ O ₃ contents drive a systematic shift in Ni ²⁺ coordination between four‐ and fivefold. To quantify the optical spectra, we apply stretched non‐negative matrix factorization (stretched‐NMF), which enables the identification of three polyhedral components to square pyramid (SP), trigonal bipyramid (TBP), and tetrahedral (Td) sites. Their fractions exhibit compositional dependence across three regimes defined by Na ₂ O/Al ₂ O ₃ and Na ₂ O/MgO ratios. In region 1 (Na ₂ O/Al ₂ O ₃ ≦ 1; Na ₂ O/MgO ≦ 1.5), SP and TBP remain nearly constant (∼70% and ∼30%) with shared charge compensation by Na ⁺ , Mg ²⁺ , and Ni ²⁺ for Al polyhedra. In region 2 (Na ₂ O/Al ₂ O ₃ < 1; Na ₂ O/MgO > 1.5), increasing Na ⁺ availability initiates Td formation with SP and TBP approaching ∼50% each, suggesting TBP as a precursor to Td. In region 3 (Na ₂ O/Al ₂ O ₃ > 1; Na ₂ O/MgO ≧ 1.5), Na ⁺ fully compensates Al polyhedral and excess Na ⁺ stabilizes NiO ₄ ²⁻ , yielding a clear SP → TBP → Td progression. Stretched‐NMF analysis, which yields consistent results in XAFS, provides a three‐dimensional insight into how the Ni ²⁺ coordination environment evolves.