Low Temperature Phases of Na 2 Ti 3 Cl 8 Revisited

The low temperature phases of Na 2 Ti 3 Cl 8 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 α‐phase ( R 3 m ) and low temperature γ‐phase ( R 3 m ), a new intermediate temperature β‐phase was detected. It has a k = (1/4,1/4,0) superstructure and its molar volume and χ T product are half way between the α‐ and γ‐phases. The β‐phase is observed between 210 K and 190 K on cooling in powder samples. Its formation is kinetically hindered in crystals. Upon heating the β→α transition occurs at 227 K. From the γ‐phase, a γ→α transitions is observed on heating. Only in heat capacity measurements, a shoulder of the peak indicates an intermediate formation of the β‐phase. Strong antiferromagnetic interactions between the Ti 2+ ions result in the formation of triangular trimers (Ti 3 clusters). In the γ‐phase all Ti 2+ ions are part of trimers. For the β‐phase a structural model is proposed, where half of the Ti 2+ ions form trimers. No long‐range magnetic order was observed in Na 2 Ti 3 Cl 8 down to 1.6 K.