Holmium polysulfides: crystal growth, structure and properties

Crystals of holmium polysulfides, up to 3x2x1.5 mm in size, were grown by s pontaneous crystallization from melted KI. A diversity in morphology was ob served from thin transparent plates to polyhedrons with mirror-like surface s. For crystals of different forms, the composition was determined, and den sity as well as refractory index was measured. Several phases crystallized, two of which were characterized using single-crystal X-ray diffraction as HoS1.885(5)(I), tetragonal, P4/nmm, a = 3.820(1), c = 7.840(3) Angstrom, V = 114.40(6) Angstrom (3), Z = 2, d(eat) = 6.542 g/cm(3) and HoS1.8863(8)(II ), monoclinic P2(1)/m, a = 10.961(2), b = 11.465(2), c = 10.984(2) Angstrom , beta =91.27(3)degrees, V = 1380.0(4) Angstrom (3), Z = 24, d(cal) = 6.486 g/cm(3). Raman spectra reveals the characteristic band, corresponding to S --S- pair vibrations in the polyanionic layer. Crystals are transparent in the 0.65-25 mum spectral range, the bands with a fine structure in the tran sparency region being related to Ho3+. The shortwave absorption edge is des cribed by an exponential law (Urbach's rule) due to tails of state density for conduction and valence bands. (C) 2001 Elsevier Science BN. All rights reserved.