Crystalline La and Nd carbonates can be formed from poorly-ordered nanoparticulate precursors, termed amorphous lanthanum carbonate (ALC) and amorphous neodymium carbonate (ANC). When reacted in air or in aqueous solutions these precursors show highly variable lifetimes and crystallization pathways. We have characterized these precursors and the crystallization pathways and products with solid-state, spectroscopic and microscopic techniques to explain the differences in crystallization mechanisms between the La and Nd systems. ALC and ANC consist of highly hydrated, 10-20 nm spherical nanoparticles with a general formula of REE2(CO3)3·5H2O (REE = La, Nd). The stabilities differ by ∼2 orders of magnitude, with ANC being far more stable than ALC. This difference is due to the Nd(3+) ion having a far higher hydration energy compared to the La(3+) ion. This, together with temperature and reaction times, leads to clear differences not only in the kinetics and mechanisms of crystallization of the amorphous precursor La- and Nd-carbonate phases but also in the resulting crystallite sizes and morphologies of the end products. All crystalline La and Nd carbonates developed spherulitic morphologies when crystallization occurred from hydrous phases in solution at temperatures above 60 °C (La system) and 95 °C (Nd system). We suggest that spherulitic growth occurs due to a rapid breakdown of the amorphous precursors and a concurrent rapid increase in supersaturation levels in the aqueous solution. The kinetic data show that the crystallization pathway for both La and Nd carbonate systems is dependent on the reaction temperature and the ionic potential of the REE(3+) ion.
20We report the physical properties of -Fe 2 O 3 (hematite), synthesized by dry-21 heating (350-1000 ¼C) of a new, poorly ordered iron oxyhydroxide precursor 22 compound that we name carbonated 2-line ferrihydrite. This precursor was 23 characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy, 24 electron microscopy and thermogravimetric analysis, whereas the -Fe 2 O 3 was 25 studied with X-ray diffraction, scanning and transmission electron microscopy and 26 magnetic techniques. -Fe 2 O 3 synthesized at 350 ¡C consisted of single-nanocrystal 27 particles (length!width 20±6 nm (L)!15±4 nm (W)), which at room temperature 28 exhibited very narrow hysteresis loops of low coercivities (< 300 Oe). However, -29 Fe 2 O 3 synthesized at higher temperatures (1000 ¡C) was composed of larger 30 nanocrystalline particle aggregates (352±109 nm (L)!277±103 nm (W)) that also 31showed wide-open hysteresis loops of high magnetic coercivities (~ 5 kOe). We 32 suggest these synthesis-temperature-dependent coercivity values are a consequence of 33 2 the subparticle structure induced by the different particle and crystallite size growth 34 rates at increasing annealing temperature. 35 36 1.
The crystallization of amorphous dysprosium carbonate (ADC) has been studied in air (21-750°C) and in solution (21-250°C). This poorly ordered precursor, Dy 2 (CO 3 ) 3 Á4H 2 O, was synthesized in solution at ambient temperature. Its properties and crystallization pathways were studied by powder X-ray diffraction, Fourier transform infrared spectroscopy, scanning and transmission electron microscopy, thermogravimetric analysis, and magnetic techniques. ADC consists of highly hydrated spherical nanoparticles of 10-20 nm diameter that are exceptionally stable under dry treatment at ambient and high temperatures (\550°C). However, ADC transforms in solution to a variety of Dy-carbonates, depending on the temperature and reaction times. The transformation sequence is (a) poorly crystalline metastable tengerite-type phase, Dy 2 (CO 3 ) 3 Á2-3H 2 O; and (b) the orthorhombic kozoite-type phase DyCO 3 (OH) at 165°C after prolonged times (15 days) or faster (12 h) at 220°C. Both the amorphous phase and the kozoite-type phase DyCO 3 (OH) are paramagnetic in the range of temperatures measured from 1.8 to 300 K.
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