MNCL2.H2O - A QUASI-ONE-DIMENSIONAL HEISENBERG-ANTIFERROMAGNET

The magnetic properties of MnCl2·H2O, a heretofore unexamined hydrate of manganese dichloride, have been studied. The behavior is clearly distinguishable from that of the anhydrous material or either the dihydrate or the tetrahydrate. At high temperatures the susceptibility is Curie-Weiss-like, with a Weiss constant θ=-4.9±0. 3 K in χM=C/(T-θ); this may be compared with the values θ=-3.3 K for the anhydrous material, -14.5±0.3 K for the dihydrate (as determined in this work), and -1.8 K for the tetrahydrate. Departures from linearity occur below 20 K, and a broad maximum in the powder susceptibility appears at T(χmax)=3.60±0.10 K with χmax=0. 304±0.003 emu/mol. The susceptibility drops sharply below 2.23 K, and ∂χ/∂T is a maximum at 2.16±0.01 K, which is identified with the Néel temperature Tc. Certain features of the data suggest two-dimensional Heisenberg model behavior, for example the ratios T c/∥θ∥=0.44 and Tc/T(χmax)=0. 60. However, fits to χ(T) using a high-temperature series expansion for the S=5/2 two-dimensional square planar Heisenberg model are unconvincing, as are similar attempts based on a three-dimensional model. In contrast, the model of a classical Heisenberg antiferromagnetic spin chain scaled to S=5/2 permits an excellent fit, with J/k=-0.49±0.04 K the intrachain exchange (in Ĥex=-2J∑i> jŜi· Ŝj). An interchain exchange of ∥J'/k∥=0.015±0. 004 K, probably antiferromagnetic, can also be inferred, from the antiferromagnetic transition at Tc =2.16 K. The intrachain exchange is very similar to that in MnCl2·2H2O, while the interchain exchange is much weaker.