Calcium aluminate glasses are important materials where AlO 4/2 − is the only network former. Aluminum in crystals or glasses between CaO and Al 2 O 3 can have different environments as a function of the CaO/Al 2 O 3 ratio. Using X-ray absorption at the Al K - and L -edges, Raman and 27 Al NMR spectroscopies, we have determined the structural surroundings of Al in glasses, crystals, and melts in this binary system. Aluminum is in octahedral coordination at high-Al 2 O 3 content (>80 mol%) and essentially in fourfold coordination with 4 bridging O atoms (BOs) at Al 2 O 3 contents between 30 and 75 mol%. At around 25 mol% Al 2 O 3 , Al is in tetrahedral coordination with two BOs. The presence of higher-coordinated species at high-Al 2 O 3 contents and their absence at low Al 2 O 3 imply different viscous flow mechanisms for high- and low-concentration Al 2 O 3 networks.

The structure of crystals, glasses, and melts along the CaO-Al2O3 join: Results from Raman, Al L- and K-edge X-ray absorption, and 27Al NMR spectroscopy

Liquid Al2O3 has been supercooled more than 500 K below its melting point (Tm = 2327 K) using aerodynamic levitation and laser heating techniques. High energy synchrotron x-ray measurements were performed over a temperature range of 1817 ≤ T (K) ≤ 2700 and stroboscopic neutron diffraction at 1984 K and 2587 K. The diffraction patterns have been fitted with Empirical Potential Structure Refinement (EPSR) models and compared to classical molecular dynamics (MD) simulation results. Both sets of models show similar trends, indicating the presence of high populations of AlO4 and AlO5 polyhedral units predominantly linked by triply shared oxygen atoms. EPSR reveals that the mean Al–O coordination number changes linearly with temperature with nAlO = 4.41 – [1.25 × 10-4] (T – Tm), with a 2.5 A cutoff. Both EPSR and MD simulations reveal a direction of the temperature dependence of the aluminate network structure which moves further away from the glass forming ideal i.e. towards nAlO = 3 during supercooling. Furthermore, we provide new experimental data and models for amorphous alumina grown by sequential infiltration synthesis of a polymer template. The amorphous solid form likely has a larger Al–O coordination number than the liquid, consistent with expectations for the hypothetical glass.

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The structure of amorphous and deeply supercooled liquid alumina

We have measured the isotropic chemical shifts (δiso) and the spin-lattice relaxation times (T1) for 17O and 25Mg in MgO from room temperature up to 1300° C. The 17O chemical shifts increase linearly from 47 ppm at room temperature to 57 ppm at 1300° C, and over the same temperature range the 25Mg chemical shift increases linearly from 25 to 27 ppm. These changes are not the result of changes in the bulk magnetic susceptibility of the samples, but may be due to increased orbital overlap which is the result of the increase in thermal vibration of the ions with temperature. In the case of 25Mg, the shift to lower shielding with increasing temperature is opposite to that expected from simple bond length versus chemical shift trends established for the oxides at room temperature. If this is a general phenomenon, high-temperature NMR data may be biased to lower shielding.

Bonding and dynamical phenomena in MgO: A high temperature 17O and 25Mg NMR study

When Frank Hawthorne (1988) edited the Reviews in Mineralogy volume on “Spectroscopic Methods in Mineralogy and Geology,” all the experiments presented had been performed at room pressure and room temperature because, at that time, vibrational and X-ray techniques were already quite difficult at ambient conditions so more sophisticated sample environments were not a priority. However, it has now become somewhat easier to perform experiments in situ at high temperatures (HT), high pressures (HP) or under combined high temperature and pressure (HP-HT). These types of experiments are becoming routine on crystals, glasses and liquids (see Shen and Wang 2014, this volume).

High-temperature experiments are important because most of the physical properties of high-temperature liquids, such as magmas and melts, are related to their atomic structure. Consequently, it is important to probe the local environment of the atoms in the sample under the conditions noted above (e.g., HT). However, at very high temperatures (~≥ 1200 °C) it is difficult to use conventional furnaces because of a number of experimental difficulties associated with their use: temperature regulation, thermal inertia and spatial obstruction of the sample. Due to the progress made in the development of lasers and X-ray, neutron and magnetic sources it is now possible to perform experiments in situ at HT, HP and HT-HP on samples of millimeter or micron size.

In this chapter, we discuss some of these noncommercial methods used in performing experiments at HT, and outline the best choices for heating systems with regard to the experimental requirements. Different commercial heating systems are available such as the systems available from Linkam® ( http://www.linkam.co.uk/ ) or Leica® ( http://www.leica-microsystems.com/ ) for example. These two systems are well adapted to performing experiments at HT including Raman (Neuville et al. 2014, this volume) and IR spectroscopy (Della Ventura et al. 2014, this volume) …

In situ High-Temperature Experiments

Aluminum 5-fold coordination coexisting with 4-and 6-fold coordination structurally characterizes amorphous aluminum oxide. For nearly 30 years now, 27Al MAS NMR has enabled to detect and later on to quantify this feature thanks to advances in high-resolution instrumentation. The Introduction shortly reviews the results of investigations of 5-coordinate aluminum in amorphous alumina through NMR analysis. Aluminum oxide is not a glass-forming oxide. A convenient way to obtain the amorphous state is by thin film deposition. We present here 27Al NMR analysis of a series of thin films of aluminum oxide prepared by metalorganic chemical vapor deposition (MOCVD) in the temperature range 360 ≤ Td ≤ 720 °C. In this range, low Td yield OH-containing films, while high Td yield nanocrystallites-containing films. The variation of the [4]Al, [5]Al and [6]Al content with Td is presented and discussed. It is correlated with the Td dependence of mechanical and corrosion protection properties. These properties are optimal when the structural disorder is to the utmost. Al coordination dependence on film thickness and the formation of metallic aluminum during the deposition process are also presented.

https://hal.archives-ouvertes.fr/hal-01169598/document

Temperature Dependent 4-, 5- and 6-Fold Coordination of Aluminum in MOCVD-Grown Amorphous Alumina Films: From Local Coordination to Material Properties

J. P. Coutures

Papers

Etude par RNA 27Al d'almuminates liquides dans le domaine 1600-2100oC