Structure and properties of the low-density phase ι-Al2O3from first principles

Abstract

Of the various phases of transition alumina, iota-alumina (ι-Al 2 O 3 ) is the least well known. It is considered to be the end member of the mullite series in the limit of zero Si content. The structural details of ι-Al 2 O 3 are not available and its physical properties are totally unknown.Based on an appropriately modified structure of a high alumina content mullite phase close to the 9-1 mullite, we have successfully constructed a structural model for ι-Al 2 O 3 . The simulated x-ray diffraction (XRD) pattern of this model agrees well with a measured XRD pattern obtained from samples that claim to belong to ι-Al 2 O 3 . ι-Al 2 O 3 is a highly disordered ultra-low-density phase of alumina with a theoretical density of 2854 kg/m 3 . The calculated total energy per Al 2 O 3 is much higher than α-Al 2 O 3 and the other well-known disordered phase, γ-Al 2 O 3 .Using this theoretically constructed model, we have calculated the elastic, thermodynamic, electronic, and spectroscopic properties of ι-Al 2 O 3 and compared it with those of α-Al 2 O 3 and γ- only the weighted average of these spectra can be directly compared with the measured ones which are currently unavailable.

Intrinsic Mechanical Properties of 20 MAX‐Phase Compounds

“…In the Reuss approximation, a uniform stress distribution is assumed, giving the lower limit through the elastic compliance tensor S ij . The average of these two limits, known as the Hill approximation, provides a realistic representation of polycrystalline bulk properties of many ceramic and metallic crystals, including those containing complex defect structures . To obtain the elastic constants of the 20 MAX phases, their crystal structures were first fully relaxed using the Vienna ab initio simulation package .…”

Assessing thermochemical properties of materials through ab initio quantum-mechanical methods: the case of α-Al2O3

“…Several computational studies on aluminum oxide polymorphs have been reported in recent years, which mainly apply standard DFT methods. [22][23][24][25][26][27][28][29][30] In this paper, we apply a recently developed technique based on the so-called quasi-harmonic approximation (QHA): a simple and effective method that is able to address the aforementioned issue in the case of corundum. 31,32 The thermal expansion coefficient, α(T ), heat capacities, C V (T ) and C P (T ), entropy, S(T ) and bulk modulus, K(T ) of corundum are calculated in its entire range of thermal stability (i.e.…”

Elastic and electronic properties of Ti2Al(C N1−) solid solutions

“…The combination of VASP and OLCAO method in studying properties of different types of complex materials systems including metal [2,33,34], ceramics [35][36][37][38], and polymeric biomolecules [39,40] and liquid and amorphous solids [41,42] have been well demonstrated.…”