Praseodymium diselenide is an inorganic compound of praseodymium and selenium with the formula PrSe2. It is a rare-earth polyselenide containing praseodymium in the +3 oxidation state. Its crystal structure contains both isolated selenide ions and Se22− diselenide units.
Preparation Praseodymium diselenide can be prepared from pre-annealed praseodymium–selenium powders by chemical transport using iodine as transport agent. Single crystals were obtained over two weeks in a temperature gradient of about 800 to 600 °C.
Properties Structure PrSe2 crystallizes in the monoclinic crystal system with the CeSe2 structure type, space group P21/a, with four formula units per unit cell. Its lattice parameters are a = 8.396(2) Å, b = 4.184(1) Å, c = 8.431(2) Å and β = 90.08(4)°. The structure consists of alternating puckered double layers containing praseodymium and selenium atoms and planar selenium layers. Selenium atoms in the planar layers form Se22− dumbbells arranged in a herringbone pattern, with an Se–Se distance of 2.467(1) Å. The bonding can therefore be represented approximately as Pr3+(Se2−)(Se22−)1/2, giving an average selenium oxidation state of −1.5.
Non-stoichiometry Praseodymium selenides form a series of selenium-deficient phases between Pr2Se3 and PrSe2. Compositions such as PrSe1.9 and PrSe1.85 are known and show ordered or modulated selenium-vacancy arrangements. In PrSe1.85, the modulation originates from a selenium site-occupancy wave coupled to a charge-density wave in the planar selenium layer. Magnetic measurements across the PrSe2−x series are consistent with essentially trivalent praseodymium.
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