Crystal Structure Modeling with VESTA

Building Crystal Structures with VESTA

VESTA provides an intuitive interface for creating crystal structures by specifying lattice parameters, space groups, and atomic positions. Let's walk through the process of manually constructing an FCC aluminum crystal structure.

Creating an FCC Aluminum Structure

Follow these steps to create an FCC aluminum bulk structure in VESTA:

1. Initialize a new structure: Navigate to File → New Structure to open the structure creation dialog.

2. Define the unit cell: In the Unit Cell tab, configure the following parameters:

   • Set the lattice constants a, b, and c to 4.05 Å
   • Select Cubic for the crystal system
   • Choose space group No. 225 (Fm-3m), which corresponds to the FCC structure

3. Add aluminum atoms: In the Structure Parameters section:

   • Click the New button to create a new atom
   • Set the Symbol to Al
   • Enter Al1 as the Label
   • Confirm the settings to generate the FCC aluminum bulk structure

4. Prepare for export: Before exporting to .xyz or .vasp format, it's important to trim edge atoms along all three crystallographic directions (a, b, and c) and save the work in VESTA's native format (.vesta). And This ensures clean periodic boundary conditions and prevents artifacts in subsequent calculations.

Creating an Aluminum Slab Structure

Building a surface slab model from a bulk FCC aluminum structure involves several key steps: scaling the unit cell, optimizing the geometry, applying crystallographic transformations to expose the desired surface, and adding vacuum space. This section provides a detailed workflow for creating slab structures with different surface orientations.

Step 1: Scaling the Unit Cell

Starting with the FCC aluminum bulk structure, we first need to scale the unit cell to create a larger supercell:

1. Open the .vasp file in VESTA (the .vasp format is preferred as it contains complete cell size information)
2. Navigate to Edit → Edit Data → Unit Cell
3. Click on the Transformation tab
4. In the rotation matrix P, enter the desired scaling factors (e.g., diagonal elements of 2, 2, 2 to double the cell in each direction)
5. Save the scaled structure and export it as both .vasp and .xyz files

At this stage, the bulk structure remains unoptimized. It's recommended to perform geometry optimization using a DFT code such as ABINIT with optcell = 1 to relax both the atomic positions and cell parameters. Save the optimized structure in both .vasp and .xyz formats.

Step 2: Surface Orientation Transformation

To create a slab with a specific surface exposed along the z-axis, we apply crystallographic transformation matrices. Open the optimized .vasp file in VESTA and navigate to Edit → Edit Data → Unit Cell → Transformation. Enter one of the following rotation matrices in the rotation matrix P field:

For the (001) surface (45° rotated from the original FCC structure):

$$ \begin{pmatrix} -0.5 & 0.5 & 0 \\ -0.5 & -0.5 & 0 \\ 0 & 0 & 1 \end{pmatrix} $$

Technical Note: The \\\\ (four backslashes) in the matrix source code is required for proper rendering. In Markdown, backslashes need to be escaped, so \\\\ renders as \\, which LaTeX then interprets as a line break in the matrix.

For the (110) surface:

$$ \begin{pmatrix} 0.5 & 0 & -0.5 \\ -0.5 & 0 & -0.5 \\ 0 & 1 & 0 \end{pmatrix} $$

For the (111) surface:

$$ \begin{pmatrix} -0.5 & -0.5 & -1 \\ -0.5 & 0.5 & -1 \\ 1 & 0 & -1 \end{pmatrix} $$

Note: For a detailed explanation of how these transformation matrices are derived, refer to this YouTube tutorial.

Step 3: Trimming Edge Atoms and Adding Vacuum

After applying the transformation, the desired crystallographic plane is now oriented along the z-axis. Next, trim the edge atoms in the x and y directions only to create clean periodic boundaries. The atoms along the z-axis should be preserved as they will form the exposed surface.

Export the trimmed structure as both .vasp and .xyz files. Then:

1. Open the .xyz file and copy all atomic Cartesian coordinates
2. Paste these absolute coordinates into the .vasp file, replacing the original coordinates
3. Update the atom count in the .vasp file header to match the .xyz file
4. Modify the z-axis lattice parameter in the .vasp file to add the desired vacuum thickness (e.g., add 15-20 Å for typical surface calculations)
5. Save the final structure

You now have a complete aluminum slab structure in .vasp format, ready for surface calculations or adsorption studies.