Low Temperature Phases of Na2Ti3Cl8 Revisited

The low temperature phases of Na2Ti3Cl8 and their phase transitions were investigated by powder neutron diffraction, heat capacity, and magnetic susceptibility measurements between 1.6 K and room temperature. Aside from the previously known high temperature -phase (Rm) and low temperature -phase (R3m), a new intermediate temperature -phase was detected. It has a k = (1/4,1/4,0) superstructure and its molar volume and T product are half way between the - and -phases. The -phase is observed between 210 K and 190 K on cooling in powder samples. Its formation is kinetically hindered in crystals. Upon heating the transition occurs at 227 K. From the -phase, a transitions is observed on heating. Only in heat capacity measurements, a shoulder of the peak indicates an intermediate formation of the -phase. Strong antiferromagnetic interactions between the Ti2+ ions result in the formation of triangular trimers (Ti-3 clusters). In the -phase all Ti2+ ions are part of trimers. For the -phase a structural model is proposed, where half of the Ti2+ ions form trimers. No long-range magnetic order was observed in Na2Ti3Cl8 down to 1.6 K.