Highly ordered MAX Phases of the Lanthanide Elements

[NNadir]

The paper I'll discuss in this post is this one: Atomically Layered and Ordered Rare-Earth i-MAX Phases: A New Class of Magnetic Quaternary Compounds (Quanzheng Tao, et al, Chem. Mater., 2019, 31 (7), pp 2476–2485. The paper is an international collaboration with scientists in the Sweden, the United States, Germany, Israel and France.

Happily for me, my son will be interning at one of these labs this summer. (I sent him the paper, but he has finals coming up and will need to read it later.)

The MAX phases are machinable ceramics with interesting properties, among the most interesting, to me at least, is resistance to chemical attack, high thermal resistance, and in some cases, considerable resistance to amorphization in neutron fluxes. These materials were originally discovered by Nowotny in 1980. The most well known of these compounds is Ti3SiC2.

Here from the internet is a picture of the structure of these highly ordered layered compounds:



From the discussion in the text of the paper under discussion:

In the sixties, Nowotny et al. discovered, in powder form, roughly 50 MAX phases. The original set of M elements was Ti, Nb, Zr, Hf, V, Mo, Ta, and Cr.35 These phases were then mostly ignored for the next two decades. Interest in these phases was revitalized in the mid-nineties36 and has since remained high.12 In 2013 we discovered the first magnetic MAX phase and added Mn to the original set of M elements.37 In 2017 and 2018, we discovered the i-MAX phases, which added Sc, Y, and W to the mix.16?18 The only study on a RE MAX phase was on the superconductivity of Lu2SnC.38 Herein, we add 11 new elements and essentially double the known MAX-phase M elements by realizing a new class of REbased magnetic i-MAX phases.

One of the authors of this paper is Michel Barsoum, who has been the main driver of research into these remarkable compounds.

He has written a monograph on these remarkable materials. It is here: MAX Phases: Properties of Machinable Ternary Carbides and Nitrides

I expect that these compounds will play a huge role in technology in the case where humanity is able to recover from climate change and other consequences of vast popular stupidity.

Personally I am only familiar with MAX phases of the 8 first elements, and of those, have been mostly interested in those of the earth abundant elements Ti and Zr. However I often wondered about whether they could be made with lanthanides and actinides. Now they have been made with the former.

This paper is about magnetic properties, mostly at very cold temperatures and not so much about mechanical or thermal properties, which are of more interest to me. Nevertheless, this discovery is wonderful and I hope to read more about these new lanthanide "i-MAX" materials in the future.

From the paper's introduction:

Rare earth (RE)-containing solids exhibit a variety of magnetic and electronic ground states through hybridization between their 4f and conduction electrons.1?3 Heavy fermion compounds, mostly based on Ce and Yb, are prototype systems for the study of quantum critical4 and collective quantum states.5 Other degrees of freedom, like orbital and valence, yield a variety of states of matter.6,7 For example, Sm exhibits valence instabilities that play a significant role in the exotic, strongly correlated, behavior in Sm compounds such as SmOs4Sb12 8 and SmB6.9 While the collective magnetic response of RE ions is also influenced by the crystal structure and chemical environment, we propose to probe for such effects in the Mn+1AXn phases, where M is an early transition metal (that as shown here can be partially substituted with a RE), A is an A group element (mostly 13 and 14), and X is C and/or N. These so-called MAX phases are a class of atomically laminar materials. Most common examples are carbides and nitrides,10 combining the characteristics of metals and ceramics.11 The rich chemistry of these materials renders them promising for applications ranging from structural materials at extreme conditions,12 ohmic contact materials for semiconductors,13 and as precursors for their 2D counterparts, that is, MXenes.14,15

Recently, we discovered quaternary (M1 2/3,M2 1/3)2AC phases that we coined i-MAX because the M elements are in-plane ordered. The M1 atoms sit on a honeycomb lattice, and the M2 atoms sit on a triangular lattice. The A layers form Kagomé-like patterns. This discovery has greatly expanded the number of M elements and their combinations that can be incorporated in the MAX phases. Examples include (Mo2/3Sc1/3)2AlC, (Mo2/3Y1/3)2AlC, (V2/3Zr1/3)2AlC,16,17 and (W2/3Sc1/3)2AlC.18 ...

...Herein, we report the synthesis of 11 new i-MAX phases with the general formula (M1 2/3,M2 1/3)2AlC, where M1 is Mo and M2 is a RE element, namely, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu. In all cases, the resulting structure is layered with chain-like arrangement of the RE element (Figure 1a). When the metal layers are imaged from the top, it is clear that the RE atoms (depicted in cyan in Figure 1b) form a 2D triangular lattice. The here demonstrated frustrated quasi-2D sheets of RE elements are expected to lead to complex magnetic properties via intra- and interplane exchanges caused by the well-known Ruderman?Kittel?Kasuya?Yosida indirect coupling mechanism of localized inner 4f-shell electron spins through the conduction electrons.

For now, let's just look at the beautiful pictures from the paper:



The caption:

Figure 1. Crystal structure of space group C2/c i-MAX phase. (a) 3D perspective side view of the C2/c i-MAX structure with (b) corresponding top view of building block, (Mo2/3RE1/3)2C layer. STEM images of (Mo2/3Tb1/3)2AlC along (c) [100], (d) [010], (e) [110], and (f) [001] zone axes, with corresponding SAED. Schematics to the right of each panel represent the corresponding atomic arrangements assuming the structure is the space group C2/c (15). Atomic resolution EDX mapping of (Mo2/3Tb1/3)2AlC along (g) [110] and (h) [100] zone axes. (i) Rietveld refinement of XRD pattern of (Mo2/3Tb1/3)2AlC at room temperature. (j) Rietveld refinement of NPD pattern of (Mo2/3Tb1/3)2AlC at 100 K. Scale bar (c–h), 1 nm.

The caption:

Figure 2. Crystal structure of the i-MAX phases. STEM images of (Mo2/3Ce1/3)2AlC along (a) [010], (b) [100], and (c) [110] zone axes, with corresponding SAED. Schematics to the right of each panel represent the corresponding atomic arrangements assuming the structure is the space group C2/m (12). STEM images of (Mo2/3Gd1/3)2AlC along (d) [110] and (e) [100] zone axes. Schematics to the right of each panel represent the corresponding atomic arrangements assuming the structure is the orthorhombic space group Cmcm (63). Scale bar, 1 nm.

The caption:

Figure 3. Summary of crystal structures of the i-MAX phases. Schematic of (Mo2/3RE1/3)2AlC crystallized in (a) monoclinic C2/m, (b) monoclinic C2/c, and (c) orthorhombic Cmcm. (d) RE = Ce and Pr crystallize in C2/m, and RE = Nd, Tb, Dy, Ho, Er, Tm, and Lu crystallize in C2/c. Two polymorphs are found for RE = Sm and Gd.

The caption:

Figure 4. Structural parameters and magnetic properties. Measured (solid circles determined from XRD at RT) and calculated lattice parameters (red crosses and blue triangles) (a) a, (b) b, (c) c, and (d) angle ? for (Mo2/3RE1/3)2AlC crystallized in C2/c. Theoretical data are obtained by treating 4f electrons in the core (4f-core, red crosses) or as part of the valence band (4f-band, blue triangles). For the 4f-band we used U = 9 eV and J = 2 eV. Data points for the 4f-band are given as averaged value obtained for the lowest energy spin states, within 2 meV/atom, and corresponding min and max values are represented by the blue shaded area. The ionic radii of trivalent RE cations are included in panel (c) for comparison.

The caption:

Figure 5. Magnetic properties. (a) Magnetization and specific heat cp versus T for (Mo2/3Nd1/3)2AlC. (b) Magnetization and resistivity versus T for (Mo2/3Gd1/3)2AlC

The caption:

Figure 6. Magnetic properties of (Mo2/3Tb1/3)2AlC. (a) M/H versus T as a function of H and cp versus T at 0 T. (b) cp versus T as a function of H. (c) M versus H . Vertical arrows indicate field-induced transitions. (d) Magnetic scattering at 8 K obtained by subtracting the pattern at 100 K and its refinement. (e) Neutron magnetic scattering at 22 K obtained by subtracting the pattern at 100 K and its refinement. (f) Schematic of magnetic structures at 8 and 22 K. Al atoms are not shown for clarity.

The caption:

Figure 7. Magnetic properties of (Mo2/3Er1/3)2AlC. (a) M/H versus T (left axis) and cp versus T (right axis). (b) M versus H at different temperatures. (c) NPD above TN at 22 K. (d) Neutron magnetic scattering at 1.5 K. (e,f) Schematics of k1 and k2magnetic structures.

Some cool experimental details:

Neutron Powder Diffraction. Neutron powder diffraction (NPD) measurements of (Mo2/3Tb1/3)2AlC were carried out using the HB-2A high-flux powder diffractometer at the High Flux Isotope Reactor at Oak Ridge National Laboratory (ORNL, USA). A 5 g powder sample was loaded in an Al cylindrical sample holder. Measurements were performed with a ? = 2.41 Å neutron wavelength produced by the (113) reflections from a vertical focusing Ge monochromator. (Mo2/3Tb1/3)2AlC was measured at 1.5, 8, 15, 22, and 100 K. The data were collected using a 3He detector bank covering a 2? range of 7?133° in steps of 0.05°. The magnetic structures were determined using the following procedure: The additional peaks below the ordering temperature were fitted by a Gaussian function to find the peak position and corrected for zerooffset. The propagation vectors were searched using FullProf “ksearch”. The symmetry analysis was performed by the FullProf “BasIreps”. Finally, the spin configuration was determined by considering all possible magnetic models.

Cool stuff overall.

Have a nice day tomorrow.

[mr_lebowski]

1. It's cool, but all kinda obvious at the same time ... I mean, of COURSE!

[NNadir]

2. I know but...

...at three in the morning, in a bout of insomnia, it did seem interesting and exciting. On the other hand, I did get back to sleep after writing the post, which is unusual.