There are many nice educational Java applets, but using those in web pages has become a problem. Students encounter high security barriers to enable Java on their computers. It is often impossible if one does not have administrator rights.
The alternative is to use HTML5 and javascript, but that has a learning curve. Fortunately, for visualizing molecules most of the work has been done. I am very impressed with 3Dmol by David Koes, especially with how little html code is required to show an interactive 3D model.
The figure on the left is not an animated gif. It is javascript code that is running in your browser. You can control it with your mouse or trackpad. You can zoom in or out, you can translate the cluster from the middle.
The atomic positions are in this XYZ file that I made
in Mathematica starting from LatticeData for face-centered cubic. I then edited the atoms
to emphasize the ABC stacking of close-packed layers. The same file can also be visualized
by a server at the University of Pittsburgh like this:
3dmol.csb.pitt.edu/view...stick:radius~0.05;sphere:radius~0.5&surface=opacity:0.8
(with parameters for sticks and for a surface, easily modified).
(Screenshot in case your browser does not show anything.)
