📄 chemistry

Computational discovery and proof-of-concept synthesis of super-ordered quinary MAB phases (s-MAB) with combined in-plane and out-of-plane chemical ordering

This study combines computational screening and proof-of-concept synthesis to discover and validate a new family of thermodynamically stable, super-ordered quinary MAB phases (s-MAB) that uniquely feature simultaneous in-plane and out-of-plane chemical ordering, thereby expanding the structural diversity and mechanical properties of layered borides.

Original authors: Martin Dahlqvist, Rana Eissa, Sohan K Ghosh, Florian Chabanais, Per O Å Persson, Johanna Rosen

Published 2026-09-16

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Martin Dahlqvist, Rana Eissa, Sohan K Ghosh, Florian Chabanais, Per O Å Persson, Johanna Rosen

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Materials scientists have long been fascinated by a special class of hard, heat-resistant compounds known as layered borides. Imagine a stack of thin sheets, where layers of metal atoms are interleaved with layers of boron, a lightweight element known for its strength. For decades, researchers have learned to arrange the metal atoms within these sheets in specific patterns to tune the material's properties, such as its ability to conduct electricity or resist wear. Some of these materials feature a flat, honeycomb-like arrangement of boron atoms that allows the metal atoms to sort themselves into neat rows within a single layer. Others show a different kind of order, where different types of metal atoms separate into distinct layers stacked on top of one another. Until now, however, no one had successfully combined both of these sorting behaviors into a single material. The question remained whether it was possible to create a structure where the metal atoms were organized both within the sheets and between the sheets simultaneously, creating a highly complex, multi-layered architecture.

A team of researchers at Linköping University in Sweden has now taken a major step toward answering that question. Using powerful computer simulations, they predicted the existence of a new family of materials they call "super-ordered" borides. These are not just simple stacks of metal and boron; they are intricate structures where five different elements arrange themselves in a precise, repeating pattern. The researchers focused on specific chemical recipes involving combinations of transition metals like chromium, molybdenum, and tungsten, along with elements like yttrium, aluminum, and boron. By running thousands of calculations to test which combinations would hold together without falling apart, they identified twenty-six specific compositions that appear to be thermodynamically stable. This means that, according to their models, these materials should be able to exist in the real world without spontaneously breaking down into simpler, more common compounds.

The simulations revealed that for these new structures to form, the atoms must follow strict rules. The metal atoms need to separate based on their size and chemical preferences. In the predicted structures, larger atoms like yttrium and smaller atoms like chromium or molybdenum sort themselves into the outer layers, while different transition metals like niobium or tantalum settle into the inner layers. This creates a sandwich-like arrangement where the outer layers have their own internal pattern, and the inner layers are distinct from them. The researchers found that this specific type of double ordering was energetically favorable, meaning the atoms "prefer" this organized state over a messy, random mix. They also discovered that these new materials would likely be metallic, conducting electricity, and that they would be mechanically stronger than their simpler, less-ordered cousins, with a stiffness about twenty percent higher.

To see if these computer predictions matched reality, the team moved from the digital realm to the laboratory. They chose one of the most promising candidates from their list, a compound made of chromium, yttrium, niobium, aluminum, and boron, and attempted to create it using a standard method of heating powdered ingredients together in a vacuum furnace. The process was delicate, requiring precise temperatures and timing to encourage the atoms to find their correct places without forming unwanted byproducts. After heating the mixture to 1375 degrees Celsius, they analyzed the resulting powder. The initial tests showed that a new, layered material had indeed formed, with a crystal structure that closely matched the computer's prediction.

The most convincing evidence came from looking at the material under an extremely powerful microscope capable of seeing individual atoms. The images revealed the distinct layers the researchers had hoped to find. They could see clear bands of niobium-rich material separated from bands containing a mix of chromium and yttrium. Furthermore, within the chromium and yttrium layers, the atoms were not randomly scattered but showed signs of the predicted in-plane ordering. While the final product was a mixture of different phases rather than a perfect single crystal, the presence of these specific, ordered grains confirmed that the complex structural motif was physically achievable. This proof-of-concept experiment suggests that the path to creating these highly ordered, five-element materials is open.

The significance of this work extends beyond just adding a new entry to a list of known materials. By demonstrating that such complex chemical ordering is possible, the researchers have expanded the toolkit available for designing advanced materials. These super-ordered borides could serve as a new platform for creating even more specialized materials, potentially including two-dimensional sheets that could be peeled off from the bulk structure for use in electronics or other technologies. The study shows that by carefully choosing which atoms to mix and how to arrange them, scientists can unlock structural possibilities that were previously thought to be too complex to stabilize. The successful synthesis of the chromium-yttrium-niobium compound proves that the theoretical map drawn by the computer simulations corresponds to a real, navigable territory in the world of materials science.

Technical Summary: Computational Discovery and Proof-of-Concept Synthesis of Super-Ordered Quinary MAB Phases

Problem Statement
Layered transition metal borides, known as MAB phases, exhibit structural diversity and tunable properties. While in-plane chemical ordering has been established in hexagonal i-MAB phases (e.g., Mo $_{4/3}$ Sc $_{2/3}$ AlB $_2$ ) and out-of-plane ordering is known in related tetragonal borides and o-MAX phases, the simultaneous realization of both ordering modes within a single hexagonal MAB structure remains unexplored. Existing orthorhombic MAB phases typically form solid solutions rather than ordered structures when alloyed. The challenge lies in designing a "super-ordered" (s-MAB) phase that combines in-plane ordering of metal atoms within outer layers and out-of-plane segregation of distinct metal layers, all while maintaining thermodynamic stability against competing disordered configurations and decomposition products.

Methodology
The study employs a combined computational and experimental approach:

Computational Screening: First-principles calculations based on Density Functional Theory (DFT) were performed using the VASP code. The authors systematically evaluated the thermodynamic stability of quinary 314 ( $M_1^{4/3}M_2^{2/3}M_3$ $M_{1}^{4/3} M_{2}^{2/3} M_{3}$ AlB $_4$ $_{4}$ ) and 416 ( $M_1^{4/3}M_2^{2/3}M_3^2$ $M_{1}^{4/3} M_{2}^{2/3} M_{3}^{2}$ AlB $_6$ $_{6}$ ) compositions.
- Elements: $M_1$ and $M_3$ were selected from transition metals (Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe), while $M_2$ was restricted to Sc or Y to promote in-plane size-driven ordering.
- Structural Variants: Three stacking sequences corresponding to space groups $P6/mmm$, $P6_3/mmc$ , and $R\bar{3}m$ were modeled to represent different out-of-plane ordering motifs.
- Stability Analysis: Formation enthalpies ( $\Delta H_{cp}$ ) were calculated relative to an equilibrium simplex of competing phases, including ternary/quaternary MABs, disordered solid solutions, and binary borides. Configurational entropy was included at 2000 K to rigorously test the stability of ordered structures against entropy-driven disorder.
- Property Prediction: Electronic band structures, densities of states, and elastic moduli (Young's, shear, bulk) were calculated for selected candidates.
Experimental Synthesis: Guided by the predictions, a proof-of-concept synthesis was attempted for Cr $_{4/3}$ Y $_{2/3}$ NbAlB $_4$ . Powders of Cr, CrB $_2$ , NbB $_2$ , Al, and Y were mixed, cold-pressed, and sintered in a vacuum furnace at temperatures ranging from 1350 to 1550 °C.
Characterization: The resulting material was analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX), and high-resolution scanning transmission electron microscopy (HRSTEM) with HAADF imaging and EDX mapping.

Key Contributions and Results

Discovery of Stable s-MAB Phases: The computational screening identified 26 s-MAB compositions predicted to be thermodynamically stable ( $\Delta G_{cp} < 0$ ) at 2000 K. Of these, 25 are also dynamically stable (confirmed via phonon dispersion spectra).
- Composition Trends: The most stable candidates typically feature $M_2 = \text{Y}$ , $M_1$ from Groups 6–8 (Cr, Mo, W, Mn, Fe), and $M_3$ from Group 5 (V, Nb, Ta).
- Stability vs. Disorder: Despite modest formation enthalpies relative to competing phases (ranging from -16 to 0 meV/atom), the fully ordered s-MAB structures are energetically favored over both partially and completely disordered configurations by 29–198 meV/atom at 2000 K. This suggests that chemical ordering is robust even at high synthesis temperatures.
- Competing Phases: The stability of s-MAB phases is strongly influenced by the presence of competing i-MAB, other s-MAB, and MB $_2$ phases, highlighting the complexity of the five-element chemical space.
Property Enhancements: Electronic structure calculations indicate that s-MAB phases are metallic. Mechanical property calculations for selected 314 s-MAB compositions (e.g., Cr $_{4/3}$ Y $_{2/3}$ NbAlB $_4$ ) show that Young's, shear, and bulk moduli are approximately 20% higher than their corresponding i-MAB reference phases (e.g., Cr $_{4/3}$ Y $_{2/3}$ AlB $_2$ ). This enhancement is attributed to the thicker metal-boride slabs introduced by the additional transition metal boride layer.
Experimental Validation: The synthesis of Cr $_{4/3}$ Y $_{2/3}$ NbAlB $_4$ resulted in a multiphase material containing grains with the predicted layered morphology.
- XRD: Observed lattice parameters ( $a \approx 5.35$ Å, $d_{00l} \approx 10.73$ Å) align closely with calculated values for the predicted structures.
- Microscopy: STEM-HAADF imaging and STEM-EDX mapping provided local evidence consistent with the s-MAB structural motif. Specifically, the data confirmed out-of-plane chemical ordering (separation of Nb-rich layers from Cr/Y-containing layers) and in-plane ordering (segregation of Cr and Y within the outer metal layers).

Significance and Claims
The paper claims to have successfully introduced and validated a new class of layered borides: super-ordered MAB phases (s-MAB). The primary significance lies in demonstrating that the simultaneous combination of in-plane and out-of-plane chemical ordering is a viable design motif for expanding the structural and compositional space of layered borides.

The authors assert that their work provides a "theory-guided map" of candidate s-MAB phases for future synthesis. They suggest that these chemically ordered structures, with their distinct metal layers and directional bonding environments, offer a platform for tuning electronic, mechanical, and magnetic properties. Furthermore, the paper posits that these phases may serve as precursors for two-dimensional derivatives (MBenes) with greater compositional flexibility than currently available 2D borides, although the actual etching or exfoliation of s-MAB phases is noted as a future task not yet demonstrated. The study emphasizes that the stability of these complex phases is highly dependent on comprehensive screening against a wide array of competing phases, including disordered configurations.

Technical Summary: Computational Discovery and Proof-of-Concept Synthesis of Super-Ordered Quinary MAB Phases

More like this