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Are there any examples of alloys or ceramics that have higher density than their densest elemental constituent? If so, do they have unusual or difficult synthesis processes? If not, is it provable that this is not possible, or demonstrably unlikely.

Since homogeneous metals are essentially crystals in their solid phase it seems plausible that, in theory, there are elements that could fit within the crystal structure without altering it to increase their density.

This would be particularly interesting for metals that are used in applications primarily for their density: e.g., lead, tungsten, and uranium.

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Yes, for both alloys and ceramics it is possible have a density that is higher than their densest elemental constituent. This is because every lattice structure of the denser elemental constituent contains interstitial void spaces, or holes, typically tetrahedral holes (surrounded by 4 atoms) or octahedral holes (surrounded by 6 atoms). These holes exist despite the crystal having an optimal packing structure, thus a maximum density for the pure solid.

The densest elemental solids are typically made up of larger atoms toward the bottom of the periodic table, that have larger atomic radii due to electron-electron repulsion, but which have much higher atomic mass due to their nuclei. This produces larger interstitial voids that can fit smaller atoms without significantly disturbing the lattice constants of the larger, denser component. An increase in mass without concomitant increase in volume will produce a denser composite.

Specific examples are difficult to find, as densities are not always reported. One common material which is denser than the corresponding "densest elemental constituent" is steel. Steel (maximum density: 8.05 g·cm⁻³) is an alloy of iron (density 7.874 g·cm⁻³) and carbon (diamond density: 3.5 g·cm⁻³), although occasionally some chromium (density: 7.19 g·cm⁻³) and nickel (density: 8.91 g·cm⁻³) may also be included.

Another possible, although less clear-cut example is the alloy heavymet, which is used in ATLAS. It is composed of nickel (9.9 atom %), copper (4.0 atom %), and tungsten (86.1 atom %). Heavymet has a density of 19.3 g·cm⁻³, which despite the less dense diluents, may be slightly larger than tungsten's density which is 19.25 g·cm⁻³.

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  • $\begingroup$ Excellent answer so far. Can we put a theoretical upper limit on the size and/or classes of atoms that can fill the interstitial voids of heavier metals? The alloy examples you cite are helpful but not dramatic. I'm wondering if there are or could be examples where the alloy density is significantly greater than the elemental density. $\endgroup$
    – feetwet
    Feb 7, 2015 at 20:57
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In a somewhat different vein are compounds where the density of hydrogen is greater than that of pure liquid hydrogen. Water is one such compound, and several metal hydrides that are being considered for hydrogen storage also qualify. All these compounds sport 0.11 to 0.15 gram hydrogen per cubic centimeter,versus only about 0.071 for pure liquid hydrogen.

In such cases we are dealing with hydrogen atoms that are relatively large for their mass on their own, but lacking inner core electrons they can be compressed with favorable energy balance to gain stronger covalent bonds with other relatively small atoms (water) or ionic bonds small ions (magnesium hydride and other hydrifldes listed in the answer referenced above).

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