Urea route to homoleptic cyanates--characterization and luminescence properties of [M(OCN)2(urea)] and M(OCN)2 with M=Sr, Eu.

A novel approach for the synthesis of urea complexes and homoleptic cyanates of alkaline earth metals and europium is described. Direct reaction of urea with elemental Sr or Eu in closed ampoules at temperatures above 120 degrees C yields [M(OCN)2(urea)] with M=Sr, Eu. According to single-crystal X-ray diffraction, the isotypic complexes exhibit a layer structure ([Eu(OCN)2(urea)]: P2(1)/c, a=7.826(2), b=7.130(1), c=12.916(3) A, beta=99.76(3) degrees , Z=4, V=710.3(2) A3). Furthermore, they were characterized by vibrational spectroscopy, thermal analysis, magnetic measurements, and photoluminescence studies. Thermal treatment of the compounds [M(OCN)2(urea)] to 160-240 degrees C affords evaporation of urea and the subsequent formation of solvent-free homoleptic cyanates of Sr and Eu. The crystal structures of Sr(OCN)2 and Eu(OCN)2 were determined from X-ray powder diffraction data and refined by the Rietveld method. Both compounds crystallize in the orthorhombic space group Fddd and adopt the Sr(N3)2 type structure (Sr(OCN)2: a=6.1510(4), b=11.268(1), c=11.848(1) A, V=821.1(2) A3; Eu(OCN)2: a=6.1514(6), b=11.2863(12), c=11.8201(12) A, V=820.63(15) A3). The cyanates are stable up to 450 degrees C. Above 500 degrees C beta-Sr(CN)2 and Eu2O2(CN)2 are formed. Excitation and emission spectra of [Eu(OCN)2(urea)], [Sr(OCN)2(urea)]:Eu2+, Eu(OCN)2, Sr(OCN)2:Eu2+ at different temperatures are reported. A strong green emission for all examined Eu-containing compounds due to a 4f(6)5d(1)-4f(7) transition is observed at low temperatures. The luminescence properties are discussed in detail and are comparable to those of thiocyanates. Compared to the latter, a blue shift of the emission bands is observed due to the higher ionicity.