Ordering of (Cr,V) Layers in Nanolamellar (Cr 0.5 V 0.5 ) n +1 AlC n Compounds

J. Phys.: Condens. Matter

Rosén

2016

109

This review presents MAX phases (M is a transition metal, A an A-group element, X is C or N), known for their unique combination of ceramic/metallic properties, as a recently uncovered family of novel magnetic nanolaminates. The first created magnetic MAX phases were predicted using density functional theory through evaluation of phase stability, and subsequently synthesized as heteroepitaxial thin films. All magnetic MAX phases reported to date, in bulk or thin film form, are based on Cr and/or Mn, and they include (Cr,Mn)2AlC, (Cr,Mn)2GeC, (Cr,Mn)2GaC, (Mo,Mn)2GaC, and (V,Mn)3GaC2, Cr2AlC, Cr2GeC and Mn2GaC. A variety of magnetic properties have been found, such as ferromagnetic response well above room temperature and structural changes linked to magnetic anisotropy. In this paper, theoretical as well as experimental work performed on these materials to date is critically reviewed, in terms of methods used, results acquired, and conclusions drawn. Open questions concerning magnetic characteristics are discussed, and an outlook focused on new materials, superstructures, property tailoring and further synthesis and characterization is presented.

Section: A Methodsmentioning

confidence: 99%

Section: Additional Research Routes For Magnetic Nanolaminatesmentioning

confidence: 99%

Magnetic MAX phases from theory and experiments; a review

Ingason

J. Phys.: Condens. Matter

Rosén

2016

109

“…Recently, two research groups reported that the transition metal layers in Cr 2 TiAlC 2 and V 2 CrAlC 2 phases were, for the most part, ordered. 24,25 Spurred by this work, we very recently reported the discovery of a new Mo-containing ordered phase-isostructural with Ti 3 SiC 2 4,26 -Mo 2 TiAlC 2 . 27 In this phase, the Ti atoms are sandwiched between two Mo-layers that, in turn, are adjacent to the Al planes resulting in a Mo-Ti-Mo-Al-Mo-Ti-Mo stacking order.…”

Section: Introductionmentioning

confidence: 99%

Experimental and theoretical characterization of ordered MAX phases Mo2TiAlC2 and Mo2Ti2AlC3

Anasori

Journal of Applied Physics

Halim

et al. 2015

Self Cite

264

180

Herein, we report on the phase stabilities and crystal structures of two newly discovered ordered, quaternary MAX phases-Mo 2 TiAlC 2 and Mo 2 Ti 2 AlC 3 -synthesized by mixing and heating different elemental powder mixtures of mMo:(3-m)Ti:1.1Al:2C with 1.5 m 2.2 and 2Mo: 2Ti:1.1Al:2.7C to 1600 C for 4 h under Ar flow. In general, for m ! 2 an ordered 312 phase, (Mo 2 Ti)AlC 2 , was the majority phase; for m < 2, an ordered 413 phase (Mo 2 Ti 2 )AlC 3 , was the major product. The actual chemistries determined from X-ray photoelectron spectroscopy (XPS) are Mo 2 TiAlC 1.7 and Mo 2 Ti 1.9 Al 0.9 C 2.5 , respectively. High resolution scanning transmission microscopy, XPS and Rietveld analysis of powder X-ray diffraction confirmed the general ordered stacking sequence to be Mo-Ti-Mo-Al-Mo-Ti-Mo for Mo 2 TiAlC 2 and Mo-Ti-Ti-Mo-Al-Mo-Ti-TiMo for Mo 2 Ti 2 AlC 3 , with the carbon atoms occupying the octahedral sites between the transition metal layers. Consistent with the experimental results, the theoretical calculations clearly show that M layer ordering is mostly driven by the high penalty paid in energy by having the Mo atoms surrounded by C in a face-centered configuration, i.e., in the center of the M nþ1 X n blocks. At 331 GPa and 367 GPa, respectively, the Young's moduli of the ordered Mo 2 TiAlC 2 and Mo 2 Ti 2 AlC 3 are predicted to be higher than those calculated for their ternary end members. Like most other MAX phases, because of the high density of states at the Fermi level, the resistivity measurement over 300 to 10 K for both phases showed metallic behavior. V C 2015 AIP Publishing LLC.

“…In particular, solid solutions on the M-site include, e.g., (Ti,M)2AlC where M = V, Nb, Ta, Cr, [8][9][10][11] (V,M)2AlC where M = Nb, Ta, Cr, 9,12 (Cr,Mn)2AC where A = Al, Ga, Ge, 3,[13][14][15][16][17][18][19] (Ti,V)3AC2 where A = Al, Ge, 20,21 and (V,M)4AlC3 where M = Ti, Nb. 20,21 Adding a fourth element has also been demonstrated by realization of TiCr2AlC2, 22,23 V1.5Cr1.5AlC2, 12 TiMo2AlC2, 24,25 and Ti2Mo2AlC3, 24 which all are recent discoveries of chemically ordered quaternary MAX phases, with atomic layers composed of a single element only. This raises the question why certain combinations of M elements form layered chemically ordered MAX phases, while other combinations result in a solid solution.…”

Section: Introductionmentioning

confidence: 99%

Order and disorder in quaternary atomic laminates from first-principles calculations

Phys. Chem. Chem. Phys.

Rosén

2015

We report on the phase stability of chemically ordered and disordered quaternary MAX phases -TiMAlC, TiM2AlC2, MTi2AlC2, and Ti2M2AlC3 where M=Zr,Hf (group IV), M=V,Nb,Ta (group V), and M=Cr,Mo,W (group VI). At 0 K, layered chemically ordered structures are predicted to be stable for M from group V and VI. By taking into account the configurational entropy, an order-disorder temperature Tdisorder can be estimated. TiM2AlC2 (M=Cr,Mo,W) and Ti2M2AlC3 (M=Mo,W) are found with Tdisorder >1773 K and are hence predicted to be ordered at the typical bulk synthesis temperatures of 1773 K. Other ordered phases, even though metastable at elevated temperatures, may be synthesized by non-equilibrium methods such as thin film growth. Furthermore, phases predicted not to be stable in any form at 0 K can be stabilized at higher temperatures in a disordered form, being the case for group IV, for MTi2AlC2 (M=V,Cr,Mo), and for Ti2M2AlC3 (M=V,Ta). The stability of the layered ordered structures with M from group VI can primarily be explained by Ti breaking the energetically unfavorable stacking of M and C where M is surrounded by C in a face-centered cubic configuration, and by M having a larger electronegativity than Al resulting in fewer electrons available for populating antibonding Al-Al orbitals. The results show that these chemically ordered quaternary MAX phases allow for new elemental combinations in MAX phases, which can be used to add new properties to this family of atomic laminates and in turn prospects for tuning these properties.