[butlerm]

The only way to produce a higher density metal at standard pressures and temperatures would be to get it to go through some sort of allotropic transformation to a crystal structure that is more tightly packed, but the ordinary solid phase of most metals is usually (relatively) close to its limits due to atomic structure already. Most of these alternatives are only stable at high pressures or unusual temperatures or both, and they may very well be less dense than the common form.