Detailed knowledge of the structural evolution of diverse aluminum (oxy)hydroxides with varying temperatures up to ~300 °C provides insights into the dehydration processes involving transitions among metastable phases on Earth's surface and in the crust. Here, we report the high-resolution solid-state 27Al NMR spectra for three different types of aluminum (oxy)hydroxides (i.e., boehmite, bayerite, and gibbsite) with varying annealing temperatures up to 300 °C, revealing the effect of distinct precursor minerals on the stability of metastable alumina. 27Al MAS NMR results allow us to obtain the quantitative fractions and NMR parameters for each phase during transformation. The results demonstrate that each aluminum (oxy)hydroxide phase follows a unique transformation path to metastable alumina. The 27Al MAS and 3QMAS NMR spectra of boehmite show that a minor but observable [4]Al signal (~2%) is detected at ~50 °C, and the [4]Al fraction gradually increases up to 300 °C (~16%), indicating that the phase transformation from boehmite to γ-/η-Al2O3 occurs at a temperature as low as ~50 °C, significantly lower than earlier estimations based on XRD. Together with the [4]Al fraction, the [5]Al fraction increases from <1% at 50 °C to ~2.3% at 300 °C, whereas the NMR results of bayerite and gibbsite do not show the presence of [5]Al. In addition, the 27Al 3QMAS NMR spectra resolved the [6]Al site in boehmite and that in γ-/η-Al2O3, which could not be uniquely determined from 1D NMR spectra. The population of bayerite abruptly decreases from 100% (at 150 °C), through ~47% (at 200 °C), to 0% (at 250 °C), indicating that the phase transition from bayerite to boehmite + γ-/η-Al2O3 occurs within a narrow temperature range. As for gibbsite, while [4]Al is not observed in the spectra up to 200 °C, the [4]Al fraction of ~2% is observed in the spectra for gibbsite annealed at 250 °C, and the [4]Al fraction increases rapidly to ~15% as the annealing temperature increases to 300 °C, suggesting that the phase transformation into γ-/η-Al2O3 occurs at ~250 °C. The results confirm that the phase transformation paths (gradual vs. dramatic) depend on the type of precursor minerals. Particularly, the onset temperature of the phase transformation from boehmite to metastable alumina (~50 °C) is lower than those from other precursor minerals (>150 °C). Furthermore, the phase transformation from boehmite to γ-/η-Al2O3 occurs gradually within broad temperature ranges from ~50 °C. This is due to their configurational disorder as evidenced by the presence of [5]Al. The observed structural evolution in aluminum (oxy)hydroxides in the low-temperature range facilitates our understanding of the nature of phase transformation and dehydration of oxides and hydroxides in the Earth's surface environments.

中文翻译：

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探究从氢氧化铝（勃姆石，三水铝石和三水铝石）到亚稳氧化铝的转变路径：高分辨率的27Al MAS NMR视图

对温度高达约300°C的多种氢氧化铝的结构演化的详细了解，为洞悉地球表面和地壳中亚稳相之间过渡的脱水过程提供了深刻见解。在这里，我们报告了三种不同类型的铝（氧）氢氧化物（即勃姆石，拜耳石和三水铝石）的高分辨率固态27Al NMR光谱，其退火温度高达300°C，揭示了不同前体的影响矿物对亚稳氧化铝的稳定性。27Al MAS NMR结果使我们能够获得转化过程中每个相的定量级分和NMR参数。结果表明，每个（羟基）氢氧化铝相都遵循独特的向亚稳态氧化铝转化的路径。勃姆石的27Al MAS和3QMAS NMR光谱表明，在〜50°C时检测到较小但可观察到的[4] Al信号（〜2％），并且[4] Al分数逐渐增加到300°C（〜16 ％），表明从勃姆石到γ-/η-Al2O3的相变发生在低至〜50°C的温度下，大大低于基于XRD的早期估计。连同[4] Al组分，[5] Al组分从50°C下的<1％增加到300°C下的〜2.3％，而三羟铝石和三水铝石的NMR结果未显示[5]的存在。铝 此外，27Al 3QMAS NMR光谱解析了勃姆石中的[6] Al位置和γ-/η-Al2O3中的[6] Al位置，这无法从一维NMR光谱中唯一确定。硅藻土的数量从100％（在150°C下）突然减少到〜47％（在200°C下）降至0％（在250°C下），这表明在较窄的温​​度范围内发生了从三钠铝石到勃姆石+γ-/η-Al2O3的相变。至于三水铝石，虽然在高达200°C的光谱中未观察到[4] Al，但在250°C退火的三水铝石的光谱中观察到〜2％的〜2％Al，而[4] Al随着退火温度升至300°C，金属含量迅速增加至〜15％，这表明在〜250°C时会发生向γ-/η-Al2O3的相变。结果证实，相变路径（渐进与剧烈）取决于前体矿物的类型。特别是，从勃姆石到亚稳态氧化铝的相变起始温度（〜50°C）低于其他前体矿物的相变起始温度（> 150°C）。此外，从勃姆石到γ-/η-Al2O3的相变在约50°C的宽温度范围内逐渐发生。这是由于[5] A1的存在证明了它们的构型紊乱。在低温范围内观察到的氢氧化铝中的结构演化，有助于我们理解地球表面环境中的氧化物和氢氧化物的相变和脱水性质。