Cation ordering in MgTi2O5 (karrooite): Probing temperature dependent effects with neutrons

MgTi2O5 (karrooite) exhibits cation exchange between the two non-equivalent octahedral M1 and M2 sites. The temperature dependence of Ti-Mg disorder has been determined using in situ time-of-flight powder neutron diffraction to establish the cation population of each site. The equilibrium Ti-Mg exchange commences above 700-800 °C, and continues up to 1300 °C. At ∼1350 °C MgTi2O5 appears to undergo a reversible, displacive phase transition, although the structure can still be refined in space group 63. This transition shows discontinuities in the degree of order, the c cell parameter, and in the M-O bond lengths, quadratic elongation, and bond angle variance for the M1 octahedron, which becomes pseudo-tetrahedral. By contrast, the M2 octahedron shows no significant change. An Arrhenius plot exhibits linear behavior from 750-1300 °C, gives an exchange enthalpy of ordering of 33.6 kJ/mol, and a value of 10.7 kJ/mol for the entropy of disordering. A value of 5.92 cm3/mol is deduced for the ΔV of the high-temperature reaction of geikielite (MgTiO3) + rutile (TiO2) to form fully disordered MgTi2O5 at an extrapolated temperature of 1860 °C.