Bismuth.

1) Only the usual form of solid water is less dense than its liquid state: "Hexagonal ice is less dense than liquid water whereas the other ices found in equilibrium with water are all denser with phase changes occurring on the approach of the liquid and solid densities." Source and further information: http://www.lsbu.ac.uk/water/phase.html (Please see details about other ice phases further down) 2) "Water is the only known non-metallic substance to expand when it freezes." "Everyday ice and snow have a hexagonal crystal structure (ice Ih). Subjected to higher pressures and varying temperatures, ice can form in roughly a dozen different phases. Only a little less stable (metastable) than Ih is the cubic structure (Ic). At other temperatures and pressures, other forms of ice exist, including II, III, V, VI, VII, VIII, IX, and X. With care all these types can be recovered at ambient pressure. The types are differentiated by their crystalline structure, ordering and density. There are also two metastable phases of ice under pressure, both fully hydrogen-disordered; these are IV and XII. Ice XII was discovered in 1996. In 2006, XIII and XIV were discovered.[3] Ices XI, XIII, and XIV are hydrogen-ordered forms of ices Ih, V, and XII respectively. As well as crystalline forms, solid water can exist in amorphous states as amorphous solid water (ASW), low-density amorphous ice (LDA), high-density amorphous ice (HDA), very high-density amorphous ice (VHDA) and hyperquenched glassy water (HGW)." Source and further information: http://en.wikipedia.org/wiki/Ice 3) "Water, gallium, bismuth, acetic acid, antimony and silicon are some of the few materials which expand when they freeze; most other materials contract. It should be noted however, that not all forms of ice are less dense than liquid water. For example HDA and VHDA are both more dense than liquid phase pure water." Source and further information: http://science.box.sk/forum.php?thread=36270&page=1&did=multGeneral%20forum 4) "The four elements which contract on melting: (silicon, gallium, germanium and bismuth) are shaded." Source and further information: "Understanding the Properties of Matter By Michael De Podesta" http://books.google.com/books?id=h8BNvnR050cC&pg=PA278&lpg=PA278&dq=liquid+phase+is+denser+than+its+solid+phase+water+bismuth&source=web&ots=QIfhq8FIZS&sig=kx63AVVdkqXrzD8Wi0ZDd7cePic&hl=en&sa=X&oi=book_result&resnum=1&ct=result#PPA276,M1