of the crystals. Tin melts at about the lowest in group 14. The melting point is further lowered to for 11 nm particles. β-tin, the metallic form or white tin, has BCT structure and is stable at and above room temperature and is malleable. α-tin, the nonmetallic form or gray tin, is stable below and is brittle. α-tin has a diamond cubic crystal structure, similar to diamond, silicon or germanium. α-tin has no metallic properties, because its atoms form a covalent structure in which electrons cannot move freely. α-tin is a dull-gray powdery material with no common uses other than specialized semiconductor applications. γ-tin and σ-tin exist at temperatures above and pressures above several GPa. In cold conditions β-tin tends to transform spontaneously into α-tin, a phenomenon known as "tin pest" or "tin disease". Some unverifiable sources also say that, during Napoleon's Russian campaign of 1812, the temperatures became so cold that the tin buttons on the soldiers' uniforms disintegrated over time, contributing to the defeat of the Grande Armée, a persistent legend. The α-β transformation temperature is , but impurities (e.g. Al, Zn, etc.) lower it well below . With the addition of antimony or bismuth the transformation might not occur at all, increasing durability. Commercial grades of tin (99.8% tin content) resist transformation because of the inhibiting effect of small amounts of bismuth, antimony, lead, and silver present as impurities. Alloying elements such as copper, antimony, bismuth, cadmium, and silver increase the hardness of tin. Tin easily forms hard, brittle intermetallic phases that are typically undesirable. It does not mix into a solution with most metals and elements so tin does not have much solid solubility. Tin mixes well with bismuth, gallium, lead, thallium and zinc forming simple eutectic systems. Tin becomes a superconductor below 3.72 K and was one of the first superconductors to be studied. The Meissner effect, one of the characteristic features of superconductors, was first discovered in superconducting tin crystals. Chemical Tin resists corrosion from water, but can be corroded by acids and alkalis. Tin can be highly polished and is used as a protective coat for other metals, a protective oxide (passivation) layer prevents further oxidation. Tin acts as a catalyst triggering a chemical reaction of a solution containing oxygen and helps to increase the speed of the chemical reaction that results. Isotopes Tin has ten stable isotopes, the greatest number of any