Crystal Chemistry of Cation Order–Disorder in Pseudobrookite-Type MgTi2O5

Synthesis, stability and coloring properties of yellow–orange pigments based on Ni-doped karrooite (Ni,Mg)Ti2O5

“…Previous analyses by Yang&Hazen (Y&H) strongly suggest that the cation disorder in MgTi 2 O 5 continues to increase at higher temperatures, and that this cation disorder state may not be quenchable above 1400 • C. 56 Therefore, in order to determine if a higher solid solubility of Ni in Mg 1−x Ni x Ti 2 O 5 (x = 0.8 or even x = 1.0) could be attained in frozen (quenched) compositions having a higher cation disorder, the samples fired directly to 1500 • C/3 h (x = 0.6, 0.8 and 1.0) were pelletized and after being fired again to 1400 • C (2 h soaking) they were quenched rapidly dropping the pellets into liquid nitrogen. The corresponding XRD patterns of quenched samples ( Supplementary Fig.…”

“…In undoped karrooite (MgTi 2 O 5 ) it has been observed that an increase in cation disorder results in a shortening of M1-O distances and a lengthening of M2-O distances (since a higher fraction of the larger Mg 2+ ions are occupying the smaller M2 sites), producing an overall increase of the unit cell volume. [54][55][56] The analyses performed Table 1 Evolution of cell parameters and cell volume with Ni doping (x) and firing temperature (1200 • C-direct and 1400 • C-subsequent firing) in karrooite Mg 1−x Ni x Ti 2 O 5 compositions. by Y&H have confirmed the existence of a high V difference between the ordered and disordered forms of undoped karrooite, indicating a cell volume of 363.52Å 3 for an ordered karrooite quenched from 600 • C and 365.00Å 3 for a disordered karrooite quenched from 1400 • C. 56 Therefore, any change in the ordering of Mg 2+ and Ti IV ions induced by Ni doping would result as well in a change of unit cell volume.…”

“…[54][55][56] The analyses performed Table 1 Evolution of cell parameters and cell volume with Ni doping (x) and firing temperature (1200 • C-direct and 1400 • C-subsequent firing) in karrooite Mg 1−x Ni x Ti 2 O 5 compositions. by Y&H have confirmed the existence of a high V difference between the ordered and disordered forms of undoped karrooite, indicating a cell volume of 363.52Å 3 for an ordered karrooite quenched from 600 • C and 365.00Å 3 for a disordered karrooite quenched from 1400 • C. 56 Therefore, any change in the ordering of Mg 2+ and Ti IV ions induced by Ni doping would result as well in a change of unit cell volume. An increase of cation ordering would result as well in a further contraction of unit cell volume.…”

Magnesium dititanate (MgTi2O5) with pseudobrookite structure: a review

“…They have also reported changes in the interatomic distances and angles at high temperatures [36] and at high pressures [37]. Using these data, Ghiorso et al derived the thermodynamic parameters of cation disordering [38].…”

Comparative Crystal Chemistry of Dense Oxide Minerals

“…Hazen and Yang (1997) synthesized karooite single crystals with a range of ordered states, and found that the bulk modulus varies by as much as 6%, based on the distribution of Ti and Mg between the two octahedral positions. Subsequent structural studies at room pressure (Yang and Hazen 1998) and high pressures to 7.5 GPa revealed the underlying structural causes for this variation. In all structures, TiO 6 octahedra (octahedral bulk modulus = 250 GPa) are observed to be much less compressible than MgO 6 octahedra (octahedral bulk modulus = 170 GPa).…”