Optical and magnetic studies of thulium nicotinate dihydrate

Abstract

The series of compounds with stoichiometric formula [Ln(C 5 H 4 NCO 2 ) 3 (H 2 O) 2 ] 2 , in which Ln is a trivalent lanthanide ion, have a dimeric molecular crystal structure with space group P2 1 /c.Optically clear single crystals of the thulium compound, with typical dimensions 3 mm x 2 mm x 1 mm, have been grown from aqueous solution. The structure is such that Tm 3+ ions occur in dimeric pairs with separation 0.4346 nm. The two ions in the pair are related by inversion symmetry, but there is no point symmetry at each thulium ion site. The crystals have been studied by optical absorption (partly with the use of a tuneable dye laser) at 490 nm wavelength; magnetic resonance spectroscopy at 35, 17, 10 and 0.02-0.06 GHz ; and magnetic susceptibility measurements. In spite of the absence of site symmetry, the results show that the ground state of each Tm 3+ ion, 4f 12 , 3 H 6 is a non-Kramers doublet with a single g value of 13.59(5), Close to the maximum possible value 12 g J = 13.97. The ground state must therefore be predominantly M J = | ± 6 >. The axis of the unique g value lies about 10° from the inter-ion vector. The magnetic properties are dominated by interaction between the two Tm ions within each dimer ; in zero applied field this splits the fourfold ground state of the pair by 0.927(15) cm -1 , i.e. 27.79(45) GHz, almost identical with the value of 0.926(72) cm -1 corresponding to pure dipolar interaction. Although the lower level is that with parallel spins, there is no evidence of longer range cooperative ordering down to 0.55 K.