Specific heat , thermal conductivity κ, dielectric permittivity ε, electric polarization , and Raman scattering experiments are performed on () single crystals. The Cl compound undergoes a structural phase transition at evident in , ɛ(T), and and accompanied by the appearance of unique phonon lines in Raman scattering. No evident structural changes are detected in the Br compound. At , a very weak polarization loop with a axis is observed in the Cl compound. Both compounds order antiferromagnetically at comparable temperatures marked by sharp λ-type singularities. For , an intensive mode of magnetic origin appears in both compounds. At the lowest temperatures, the energy of this mode is in good agreement with both the magnon excitation observed in infrared spectroscopy and the spin gap found recently in inelastic neutron scattering of the Cl compound.