Magnetic Properties of Layer-Type Compounds TlGdS$\text{}_{2}$ and TlGdSe$\text{}_{2}$

Ternary thallium lanthanide dichalcogenides TlLnX$\text{}_{2}$ (X = S, Se, or Te; Ln = lanthanide, except Ce and Pr) crystallize in the rhombohedral structure of α-NaFeO$\text{}_{2}$ type ($R\overline{3}m$). Their crystal lattice consists of the layers of Ln$\text{}^{3+}$ ions separated by three layers of the non-magnetic ions (-Ln-X-Tl-X-Ln-). The magnetization was measured in the field range 0-14 T. The molecular field constants λ$\text{}_{m}$ were estimated by fitting the Brillouin function to the experimental magnetization plots. The difference between the λ$\text{}_{m}$ values for the thallium gadolinium sulphide and the selenide corresponds to the different character of Gd-S and Gd-Se bonds and gives rise to the different J$\text{}_{1}$ and J$\text{}_{2}$ exchange integrals.