Abstract
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 α-phase (R3m) and low temperature γ-phase (R3m), a new intermediate temperature β-phase was detected. It has a k = (1/4,1/4,0) superstructure and its molar volume and χT 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 Ti2+ ions result in the formation of triangular trimers (Ti3 clusters). In the γ-phase all Ti2+ ions are part of trimers. For the β-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.
