Low Temperature Phases of Na2Ti3Cl8 Revisited

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.