Thermal expansivity of geikielite and ilmenite utilizing in-situ synchrotron X-ray diffraction at high temperature

The unit-cell parameters and volumes of geikielite(MgTiO_(3))and ilmenite(FeTiO_(3))were investigated at high temperatures up to 700 K and ambient pressure,using in-situ angle-dispersive synchrotron X-ray diffraction.No phase transition was detected over the experimental temperature range.Using(Berman in J Petrol29:445-522,1988.10.1093/petrology/29.2.445)equations to fit the temperature-volume data,the volumetric thermal expansion coefficients at ambient conditions(α_(V0))of MgTiO_(3) and FeTiO_(3) were obtained as follows:2.55(6)×10^(-5)K^(-1)and 2.82(10)×10^(-5)K^(-1),respectively.We infer that the larger effective ionic radius of Fe^(2+)(Ⅵ)(0.78 A)than that of Mg^(2+)(Ⅵ)(0.72?)renders FeTiO_(3)has a larger volumetric thermal expansivity than MgTiO_(3).Simultaneously,the refined axial thermal expansion coefficients under ambient conditions areα_(a0)=0.74(3)×10^(-5)K^(-1)andα_(c0)=1.08(5)×10^(-5)K^(-1)for the aaxis and c-axis of MgTiO_(3),respectively,andα_(a0)=0.95(5)×10^(-5)K^(-1)andα_(c0)=0.92(12)×10^(-5)K^(-1)for the aaxis and c-axis of FeTiO_(3),respectively.The axial thermal expansivity of MgTiO_(3) is anisotropic,but that of FeTiO_(3) is nearly isotropic.We infer that the main reason for the different axial thermal expansivity between MgTiO_(3) and FeTiO_(3) is that the thermal expansion mode of the Mg-O bond in MgTiO_(3) is different from that of the Fe-O bonds in FeTiO_(3).

Thermodynamic Modeling of MgO-Al_(2)O_(3)-TiO_(2) System

This scientific paper gives consideration to the information on the structure of the triple component system,in particular MgO-Al_(2)O_(3)-TiO_(2) that serves as a basis for the production of thermal resistance materials.The structure of such binary systems as MgO-Al_(2)O_(3),Al_(2)O_(3)-TiO_(2),MgO-TiO_(2) was described and the data available for the MgO-Al_(2)O_(3)-TiO_(2) system were given.Thermodynamic data on all the system compounds were also presented and used for the computation of a change in the free Gibbs energy in the temperature range form 800 K to 1900 K for the basic exchange reactions.It was established that the triangulation of the MgO-Al_(2)O_(3)-TiO_(2) system was changed in the three temperature intervals:in the temperature range lower than 1537 K TiO_(2) existed as the polymorphous modification,i.e.anatase;in the temperature range from 1537 K to 2076 K TiO_(2) existed as a polymorphous modification in form of rutile and tialite was stable;and at the temperatures above 2076 K the availability of stochiometric compound of Al4TiO_(8) was possible.In the temperature range lower than 1537 K the two-phase equilibra of Al_(2)O_(3)-MgTi2O_(5),Mg-MgALCU MgTiO_(3)-MgAl_(2)O_(4),and Mg_(2)TiO_(4)-MgAl_(2)O_(4) were stable;in the temperature range from 1537 K to 2076 K the two-phase equilibria of MgTigO_(5)-AlgTiO_(5),MgTiO_(3)-Al_(2)TiO_(5),MgTiO_(3)-Al_(2)O_(3),MgTiO_(3)-MgAl_(2)O_(4),and Mg_(2)TiO_(4)-MgAl_(2)O_(4) were stable,and above 2076 K the MgTi_(2)O_(5)-Al_(2)TiO_(5),MgTi_(2)O_(5)-Al_(4)TiO_(8),Al_(4)TiO_(8)-MgTiO_(3),Al_(4)TiO_(8)-Mg_(2)TiO_(4),Al_(4)TiO_(8)-Mg0,and Al_(4)TiO_(8)-MgAl_(2)O_(4) systems were stable.