Extrinsic Point Defects in Low-Positive Thermal Expansion Al₂W₃O₁₂ and Their Effects on Thermal and Optical Properties

Abstract

A 2 M 3 O 12 -type ceramics are potentially useful in a variety of applications due to their peculiar thermal and mechanical properties. In addition, their intrinsic coefficients of thermal expansion can be finely tuned through different mechanisms. Despite the great influence of extrinsic point defects on physical properties, only a few reports have dealt with their relationship to thermal expansion and thermal conductivity. Extrinsic oxygen vacancies in orthorhombic Al 2 W 3 O 12 , in different concentrations, were formed through heat treatments in argon or hydrogen atmospheres. X-ray powder diffraction, diffuse reflectance spectroscopy, and Raman and electron paramagnetic resonance spectroscopies were used to study the as-formed vacancies, and X-ray photoelectron spectroscopy was employed to propose a charge compensation mechanism. It was found that the intrinsic coefficient of thermal expansion of orthorhombic Al 2 W 3 O 12 was severely affected by extrinsic oxygen vacancies. Thermal expansion was decreased up to 40% (from 25 to 400 °C) with respect to the extrinsic-point-defect-free counterpart. Unit-cell volumes of defective orthorhombic Al 2 W 3 O 12 were larger, while their W−O bonds were weaker, likely leading to higher lattice flexibility and enhanced low-energy transverse acoustic modes. Extrinsic oxygen vacancies could be an additional mechanism for fine-tuning the intrinsic coefficients of thermal expansion in A 2 M 3 O 12 -type ceramics and in other framework structures built through two or threefold linkages.

Extrinsic Oxygen Vacancies Formation during Crystallization of Al2W3O12 by Calcination in Air

“…This feature provides a large range of compounds with a tunable coefficient of thermal expansion (CTE) and unusual physical and chemical properties. − Al 2 W 3 O 12 adopts an orthorhombic structure at room temperature ( Pbcn space group) consisting of an open framework of corner-sharing AlO 6 octahedra and WO 4 tetrahedra . In addition, the formation of extrinsic oxygen vacancies in this family seems to add extra lattice flexibility, permitting fine-tuning of CTE, and might corroborate the average atomic volume model, recently extended and proposed for anisotropic materials …”

“…As the heat treatment temperature increases, the Al 2 W 3 O 12 powders change color from dark gray (A500) to white (A620) suggesting that the extrinsic oxygen vacancy content is decreasing. A 2 M 3 O 12 -type materials are known wide-band-gap semiconductors that do not absorb light within the visible range, ,, therefore, the color of the powders could be an indicator of the presence of point defects.…”

“…, who introduced the oxygen vacancies into In 0.6 (HfMg) 0.7 Mo 3 O 12 phase (belonging to A 2 M 3 O 12 -family) by heat treatment in He atmosphere. The oxygen vacancies enhanced the negative CTE when compared to the extrinsic defect-free material calcined in air, from −0.47 × 10 –6 to −1.5 × 10 –6 K –1 , between 323 and 573 K. More recently, Moreno Diaz et al found that Al 2 W 3 O 12 showed a lowering of about 33 and 40% in the linear CTE when oxygen vacancies were introduced through heat treatment in Ar or H 2 , respectively, of previously well-crystallized Al 2 W 3 O 12 , in comparison to the extrinsic defect-free phase. Therefore, the oxygen-defective Al 2 W 3 O 11.36 phase, obtained by heating in the H 2 atmosphere, presented a near-zero thermal expansion with a CTE of 0.90 × 10 –6 K –1 from room temperature to 673 K.…”

“…Point defects play an essential role in the physical properties of materials, influencing a wide range of applications such as thermal and chemical sensors, optical devices, and transparent infrared windows. ,− In semiconductors and insulators, these defects can affect electrical, thermal, optical, and photochemical properties by modifying the band gap energy and, hence, its radiation absorption, by forming intermediate energy states within the band gap. ,, TiO 2 nanofibers showed a formation of oxygen vacancies when heat-treated in 100% Ar atmosphere, significantly lowering the optical band gap from 3.33 to 2.18 eV. MgFe 2 O 4 photocatalysts calcined in air displayed, on the other hand, an increase of the band gap as the calcination temperature and oxygen vacancies concentration increased, going from 1.99 eV at 650 °C to 2.17 eV at 750 °C.…”

“…A survey of the literature shows that multiple studies approached the crystallization of different ceramic phases through calcination, including Al 2−δ Sc δ W 3 O 12 , Al 2 W 3 O 12 , , Sc 2 W 3 O 12 , Cr 2 x Fe 2–2 x Mo 3 O 12 , Al 2 x Cr 2–2 x Mo 3 O 12 , Al 2 x Fe 2–2 x Mo 3 O 12 , BaMoO 4 , La 2 Mo 3 O 12 , TiO 2 , , and Ga 2 O 3 . However, to the best of the author’s knowledge, no efforts have been made to understand the mechanism of crystallization from amorphous powder and its relation to the introduction of extrinsic oxygen vacancies through calcination in air, especially within the A 2 M 3 O 12 family.…”