Convergent real-space cluster expansion for configurational disorder in ionic systems

Clear evidence for element partitioning effects in a Ti-6Al-4V alloy by the first-principles phase field method

Ti-6 wt% Al-4 wt% V (Ti64) is an α + β titanium alloy, in which the alloying components strongly affect the mechanical properties. In this report, element partitioning effects in Ti64 are investigated by using the first-principles phase field (FPPF) method, which has recently been proposed by our group. In the FPPF method, the local free energy is calculated using a cluster expansion method in combination with density functional theory and the temperature effect is incorporated using potential renormalization theory. We have succeeded in identifying enrichment of Al (V) in the α (β) phase, i.e., the clear evidence for the element partitioning effects of Al and V, without using any thermodynamical parameter. The transformation of the β phase and the α phase in microstructure is investigated by varying the V and Al concentrations by a small amount. Our results are in excellent agreement with the recent experimental results, showing the validity of the FPPF method for ternary alloys.

Enhancing the Photocatalytic Performance of MXenes via Stoichiometry Engineering of Their Electronic and Optical Properties

Combining both density functional theory and the cluster expansion method, we investigate 3 binary MXene alloy systems of semiconducting Ti₂CO₂, Zr₂CO₂, and Hf₂CO₂, where the transition metals substitute one another (i.e., Ti_{2(1- x)}Zr_{2 x}CO₂, Ti_{2(1- x)}Hf_{2 x}CO₂, and Zr_{2(1- x)}Hf_{2 x}CO₂). We show that this group of MXene alloys forms the solid-solution phase across all compositions. Special quasirandom structures are generated to model the solid-solution phase of these alloys, using which we demonstrate how their structural, mechanical, electronic, and optical properties are tuned via stoichiometry engineering. These alloys exhibit outstanding mechanical strength and stability. They possess indirect band gaps of 1.25-1.80 eV. For Ti_{2(1- x)}Zr_{2 x}CO₂ and Ti_{2(1- x)}Hf_{2 x}CO₂, they display higher absorbance in the solar spectrum than their constituent Zr₂CO₂ and Hf₂CO₂, respectively. Most of the MXene alloys also show appropriately aligned band edges for water splitting. We predict the Ti_{2(1- x)}Zr_{2 x}CO₂ alloy with x = 0.2778 to be the most promising water-splitting photocatalyst among the MXenes studied here, outperforming its constituents, Ti₂CO₂ and Zr₂CO₂, when solar absorbance performance and band-edge alignments are simultaneously considered. This work demonstrates that alloying can be used to effectively tune photocatalytic performance.

Density of Configurational States from First-Principles Calculations: The Phase Diagram of Al-Na Surface Alloys

The structural phases of Al(x)Na(1-x) surface alloys have been investigated theoretically and experimentally. We describe the system using a lattice-gas Hamiltonian, determined from density functional theory, together with Monte Carlo (MC) calculations. The obtained phase diagram reproduces the experiment on a quantitative level. From calculation of the (configurational) density of states by the recently introduced Wang-Landau MC algorithm, we derive thermodynamic quantities, such as the free energy and entropy, which are not directly accessible from conventional MC simulations. We accurately reproduce the stoichiometry, as well as the temperature at which an order-disorder phase transition occurs, and demonstrate the crucial role, and magnitude, of the configurational entropy.