Thermodynamic experiments as well as Raman scattering have been used to stu
dy the magnetic instabilities in the spin-tetrahedra systems Cu2Te2O5X2, X
= Cl and Br. While the phase transition observed in the Cl system at T-N =
18.2 K is consistent with 3D antiferromagnetic ordering, the phase transiti
on at T-o = 11.4 K in the Br system has several unusual features. We propos
e an explanation in terms of weakly coupled tetrahedra with a singlet-tripl
et gap and low-lying singlets.