Metamagnetic transition in EuSe2:: A new, metastable binary rare-earth polychalcogenide

The reaction of Eu with molten lithium polyselenide produced a new rare-earth dichalcogenide, EuSe2. This compound cannot be obtained via direct combination of the elements and is therefore an example of how alkali metal chalcogenide fluxes can access metastable or kinetically stable phases. The crystals of EuSe2 are very dark red and air stable for several months. The structure was determined with single-crystal X-ray diffraction. The compound crystallizes in the tetragonal space group, I4/mcm (#140) with a = 6.391(1) Angstrom and c = 7.848(1) Angstrom, Z = 4, R1 = 3.9%, and wR2 = 10.0%. It adopts the CuAl2 structure type, and contains rows of staggered diselenide dumbbells running parallel to the c-axis, separated by rows of square antiprismatically coordinated Eu atoms. EuSe2 is a semiconductor with a band gap of 1.43 eV and decomposes at 569 degrees C to the more stable cubic EuSe. EuSe2 displays metamagnetic behavior below 8 K. Variable temperature magnetic susceptibility and magnetization measurements were performed on single-crystal samples to study this phenomenon. A model is proposed in which all Eu spins in the ab-plane are coupled ferromagnetically, whereas alternate sheets are coupled antiferromagnetically to form a bulk antiferromagnet. The Se-Se bond stretch exhibits a Raman shift of 256 cm(-1).