to hexagonal close-packed (hcp), but transition temperature depends on metal purity. Further heating to transforms the metal into a body-centered cubic (bcc) phase. Heating to plus compression to 40 kbar results in a double-hexagonally close-packed structure (dhcp). Higher pressure of the order of hundreds or thousands of kilobars induces a series of phase transformations, in particular with a tetragonal phase appearing at about 900 kbar. In one study, the dhcp phase could be produced without compression, using a nonequilibrium annealing regime with a rapid temperature change between about and , confirming the transient character of this samarium phase. Also, thin films of samarium obtained by vapor deposition may contain the hcp or dhcp phases at ambient conditions. Samarium and its sesquioxide are paramagnetic at room temperature. Their corresponding effective magnetic moments, below 2 Bohr magnetons, are the third-lowest among lanthanides (and their oxides) after lanthanum and lutetium. The metal transforms to an antiferromagnetic state upon cooling to 14.8 K. Individual samarium atoms can be isolated by encapsulating them into fullerene molecules. They can also be doped between the C60 molecules in the fullerene solid, rendering it superconductive at temperatures below 8 K. Samarium doping of iron-based superconductors – a class of high-temperature superconductor – allows enhancing their transition temperature to 56 K, which is the highest value achieved so far in this series. Chemical properties Freshly prepared samarium has a silvery luster. In air, it slowly oxidizes at room temperature and spontaneously ignites at . Even when stored under mineral oil, samarium gradually oxidizes and develops a grayish-yellow powder of the oxide-hydroxide mixture at the surface. The metallic appearance of a sample can be preserved by sealing it under an inert gas such as argon. Samarium is quite electropositive and reacts slowly with cold water and quite quickly with hot water to form samarium hydroxide: Samarium dissolves readily in dilute sulfuric acid to form solutions containing the yellow to pale green Sm(III) ions, which exist as complexes: Samarium is one of the few lanthanides that exhibit the +2 oxidation state. ions are blood-red in aqueous solution. Compounds Oxides The most stable oxide of samarium is the sesquioxide Sm2O3. Like many samarium compounds, it exists in several crystalline phases. The trigonal form is obtained by slow cooling from the melt. The melting point of Sm2O3 is high (2345 °C), so it is usually melted not by direct heating, but with induction