The description and identification of corrosion products formed on archaeological iron artefacts need various approaches at different observation scales. Among analytical techniques available to document phase structure at the microscopic range, Raman spectroscopy offers sensitivity and discrimination between iron corrosion products with an easy implementation. Results obtained for iron artefacts corrosion in soils and atmosphere are presented. Corrosion forms observed for anoxic and aerated soils on one hand and indoor atmosphere on the other are documented. Beyond the identification and organisation of corrosion products through hyperspectral imaging, Raman micro-spectroscopy could also provide quantitative phase proportions which will be needed in the proposition of reactivity diagnosis indicators.

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International audienceRaman structural imaging can extract original information in order to answer new questions raised in the recent studies on iron and low alloy steel corrosion. Up to now, this technique has allowed the extraction of the qualitative distribution of the compounds constituting the corrosion product layers. We propose here a methodology to extract quantitative parameters from Raman hyperspectral maps, executed by a home-developed software: LADIR-CAT. Specifically developed for iron corrosion quantitative component imaging, the approach and program operation are described. The LADIR-CAT is applied on ancient corroded iron samples originating from the Amiens cathedral (France) to establish the description and the composition of the corrosion system through quantitative compound imaging

Substitution of Sn for As in α-As2Te3 enhances the thermoelectric performances with a maximum ZT of 0.8 at 523 K.

Metastable β-As2Te3 (R3̅m, a = 4.047 Å and c = 29.492 Å at 300 K) is isostructural to layered Bi2Te3 and is known for similarly displaying good thermoelectric properties around 400 K. Crystallizing glassy-As2Te3 leads to multiphase samples, while β-As2Te3 could indeed be synthesized with good phase purity (97%) by melt quenching. As expected, β-As2Te3 reconstructively transforms into stable α-As2Te3 (C2/m, a = 14.337 Å, b = 4.015 Å, c = 9.887 Å, and β = 95.06°) at 480 K. This β → α transformation can be seen as the displacement of part of the As atoms from their As2Te3 layers into the van der Waals bonding interspace. Upon cooling, β-As2Te3 displacively transforms in two steps below T(S1) = 205-210 K and T(S2) = 193-197 K into a new β'-As2Te3 allotrope. These reversible and first-order phase transitions give rise to anomalies in the resistance and in the calorimetry measurements. The new monoclinic β'-As2Te3 crystal structure (P2(1)/m, a = 6.982 Å, b = 16.187 Å, c = 10.232 Å, β = 103.46° at 20 K) was solved from Rietveld refinements of X-ray and neutron powder patterns collected at low temperatures. These analyses showed that the distortion undergone by β-As2Te3 is accompanied by a 4-fold modulation along its b axis. In agreement with our experimental results, electronic structure calculations indicate that all three structures are semiconducting with the α-phase being the most stable one and the β'-phase being more stable than the β-phase. These calculations also confirm the occurrence of a van der Waals interspace between covalently bonded As2Te3 layers in all three structures.

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