Mn+1AXnphases in theTi−Si−

Abstract

Thin films of Mn+1AXn layered compounds in the Ti-Si-C system were deposited on MgO(111) and A12O3(0001) substrates held at 900°C using dc magnetron sputtering from elemental targets of Ti, Si, and C. We report on single-crystal and epitaxial deposition of Ti3SiC2 (the previously reported MAX phase in the Ti-Si-C system), a previously unknown MAX phase Ti4SiC3 and another type of structure having the stoichiometry of Ti5Si2C3 and Ti7Si2C5. The latter two structures can be viewed as an intergrowth of 2 and 3 or 3 and 4 M layers between each A layer. In addition, epitaxial films of Ti5Si3Cx were deposited and Ti5Si4 is also observed. First-principles calculations, based on density functional theory (DFT) of Tin+1SiCn for n =1,2,3,4 and the observed intergrown Ti5Si2C3 and Ti7Si2C5 structures show that the calculated difference in cohesive energy between the M AX phases reported here and competing phases (TiC, Ti3SiC2, TiSi2, and Ti5Si3) are very small. This suggests that the observed Ti5Si2C3 and Ti7Si2C5 structures at least should be considered as metastable phases. The calculations show that the energy required for insertion of a Si layer in the TiC matrix is independent of how close the Si layers are stacked. Hardness and electrical properties can be related to the number of Si layers per Ti layer. This opens up for designed thin film structures the possibility to tune properties.

The M+1AX phases: Materials science and thin-film processing

“…The diffraction pattern from these intergrown structures is instead indexed based on a hexagonal lattice with a c axis 1.5 times the basic hexagonal c axis, i.e., 45.63 Å and 60.62 Å for Ti 5 Si 2 C 3 and Ti 7 Si 2 C 5 , respectively. In such a structure, the observed 000l reflections in the -2 diffractogram are indexed with l = 3n (n = 1, 2, 3…) [45]. Thus, the description of the "523" and "725" phases as a combination of half-unit cells of 312 and half-unit cells of 211 or 413, respectively, is not completely accurate, but a close approximation and sufficient for most purposes.…”

“…This conclusion inspired Holm et al [88] [89], and it was logical to proceed with an attempt to synthesize Ti 4 SiC 3 . It worked [45], and this result was the initial inspiration for many of the research efforts aimed at predicting and discovering new MAX phases (see section 4.3). Not much later, Ti 4 GeC 3 was synthesized too [46].…”

“…Palmquist et al [45] first demonstrated the MAX-phase "intergrown structures". Film-growth experiments showed the existence of two previously unknown types of MAX phases, Ti 5 Si 2 C 3 and Ti 7 Si 2 C 5 , or "523" and "725" phases.…”

“…The "523" and "725" phases were observed as minority phases together with, e.g., Ti 3 SiC 2 . The c axes of the unit cells were determined to 30.4 Å for Ti 5 Si 2 C 3 and 40.4 Å for Ti 7 Si 2 C 5 , respectively [45]. In the literature, the "523" and "725" phases are referred to as both "intergrown structures" and "new types of MAX phases".…”

Magnetic MAX phases from theory and experiments; a review

“…In the latter examples the focus is primarily on achieving single-phase films of high density, where steps towards making high quality single crystal samples have mostly been taken since 2010. A routine method to synthesize these materials in thin film form is PVD, such as cathodic arc [102], or, most often, magnetron sputtering [98,103,104] which, combined with a suitable substrate and co-deposition from elemental targets at a relatively high temperature, can yield epitaxial single phase thin films of high crystal quality. It is therefore easier to obtain phase-pure samples with crystals oriented in a single direction and this allows for a more detailed analysis of the materials properties.…”