Radius of sphere tangent to eight spheres in a close packing

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Assuming I have a cube of size a which already filled with 8 spheres with radius of a/4. I would like to find the radius of the sphere tangent to all this 8 and fill the hole at the center of cube. Next, I would like to find a smaller sphere which fill the holes again (similar to Apollonian packing). The question is what is the radius of spheres and how much the void space can be reduced in this packing.