Cs2AgBiBr6 無鉛雙鈣鈦礦之分解行為研究

Abstract

Cs2AgBiBr6 作為一種無鉛雙鈣鈦礦材料，因兼具低毒性與優異的環境穩定性，被視為傳統含鉛鹵化物鈣鈦礦的潛力替代者。本研究以溶液旋塗法在氮氣手套箱中製備 Cs2AgBiBr6 薄膜，並系統性地探討其在不同熱處理及環境濕度條件下之相演化、分解產物及分解機制。 儘管 Cs2AgBiBr6 在熱力學上屬於穩定相，TGA 結果顯示其分解應由 BiBr3 的逸散所驅動。此系統中涉及 BiBr3 釋放與保留的可逆反應可表示為： 3Cs2AgBiBr6 ⇌ 3CsAgBr2 + Cs3BiBr6 + 2BiBr3↑ 在高溫或高濕的條件下，具高揮發性且易水解的 BiBr3 會離開系統，同時高遷移性的 Ag+ 離子會發生側向擴散，進而導致富銀相與富鉍相的偏析。本研究利用 XRD 為主的繞射技術，並結合 SEM、EDS、EBSD 等顯微技術，釐清了次生相在微觀尺度下的形貌、物相與晶向的演變行為。確認了最終的分解產物包括 CsAgBr2、Cs3BiBr6，以及一種未知相，本文稱之為「W相」。此結果不同於先前文獻所報導以 Cs3Bi2Br9 作為主要分解產物的結論。 值得注意的是，本研究進一步辨識出 (Cs, Ag)3BiBr6 作為分解過程中的關鍵中間相，其WDS定量結果顯示Cs與Ag間具有一致且非隨機之比例關係，且目前已可歸納出兩種同質異象物。並且，即使這些次生相生長於石英玻璃基板上，仍表現出明顯且一致的面外取向，顯示其形成過程並非單純隨機成核成長。另一方面，「W相」之本質目前仍未完全明瞭，顯示其可能涉及更複雜之局部結構或雜質效應。 本研究聚焦於 Cs2AgBiBr6 薄膜之基礎性質探討，整合了相分離分析、晶體學鑑定與機制闡明，深化了對其薄膜穩定性、分解路徑與中間相形成之理解，並為後續相關研究奠定重要基礎。

Cs2AgBiBr6, a lead-free double perovskite, is regarded as a promising alternative to conventional lead-based halide perovskites owing to its combination of low toxicity and excellent environmental stability. In this study, Cs2AgBiBr6 thin films were fabricated via solution spin-coating in an N2-filled glovebox, and their decomposition behavior under various thermal treatments and humidity conditions was systematically investigated. Although Cs2AgBiBr6 is a thermodynamically stable phase, TGA results indicate that its decomposition may be driven by the loss of BiBr3. The reversible reaction involving the release and retention of BiBr3 in this system can be expressed as: 3Cs2AgBiBr6 ⇌ 3CsAgBr2 + Cs3BiBr6 + 2BiBr3↑ Under high-temperature or high-humidity conditions, BiBr3, which is highly volatile and readily hydrolyzed, escapes from the system, while highly mobile Ag+ ions undergo lateral diffusion, leading to the segregation of Ag-rich and Bi-rich secondary phases. By combining XRD-based diffraction analyses with SEM, EDS, and EBSD, the microscale evolution of the morphology, phases, and crystallographic orientations of the secondary phases was clarified. The final decomposition products were identified as CsAgBr2, Cs3BiBr6, and an unknown phase, hereafter referred to as the “W phase.” This result differs from previous reports in the literature that describe Cs3Bi2Br9 as the major decomposition product. Notably, this work further identifies (Cs,Ag)3BiBr6 as a key intermediate phase during decomposition. Quantitative WDS analysis shows that the Cs/Ag ratio in this intermediate is consistent and non-random, and the intermediate can currently be classified into two polymorphs. In addition, even when these secondary phases form on fused silica substrates, they still exhibit a pronounced and consistent out-of-plane preferred orientation, indicating that their formation does not proceed through simple random nucleation and growth. By contrast, the exact nature of the “W phase" remains unresolved, suggesting that it may involve a more complex local structure or impurity-related effects. This work focuses on the fundamental properties of Cs2AgBiBr6 thin films by integrating phase-separation analysis, crystallographic characterization, and mechanistic elucidation. It deepens the understanding of their thin-film stability, decomposition pathway, and intermediate-phase formation, and provides an important foundation for future related studies.