Structure refinements of Mg₂TiO₄, MgTiO₃ and MgTi₂O₅ by time-of-flight neutron powder diffraction

Abstract

Rietveld refinements using time-of-flight neutron diffraction data are reported for two forms of Mg2TiO4 (Mr=160-51), MgTiO3 (Mr=120.20) and two samples of MgTi205 (Mr = 200" 10). The compounds were synthesized at 1673K and subsequently annealed and quenched from other temperatures. All data were collected at room temperature on a 10 m powder diffractometer at a nominal scattering angle of 150 ° 20. Mg2TiO4 annealed at 973 K has the spinel structure, space group Fd3m, with a= 8.4376(5),~, V=600.71(ll)A 3, Z=8. Mg2TiO4 prepared at 773 K is tetragonal, space group P4122, a = 5.9748 (5), c = 8.414 (7) A, V= 300.37 (7) A 3, Z = 4. Final weighted-profile R values are 0.0376 and 0-0327 for the cubic and tetragonal spinels, respectively. In the cubic form, Mg and Ti are disordered in a single octahedral site to form a nearly perfect inverse spinel, although there may be considerable short-range order. The tetragonal structure is a slight distortion of the cubic one, with two inequivalent octahedral sites over which the Mg and Ti are highly, but completely, ordered. MgTiO3 annealed_at 1073 K has the ilmenite structure, space group R3, a = 5.05478 (26), c = 13.8992 (7)/~, V = 307.56 (4) A 3, Z = 6. The final Rwp is 0.0257. Mg and Ti are completely ordered between two octahedral sites, and probably remain so at temperatures up to Mg-Ti distribution in both samples is disordered, with the 1773 K sample substantially more disordered. The lattice parameters of MgTi205 are sensitive to the degree of disorder. These results have been combined with thermochemical data obtained on the same specimens to derive an understanding of the effects of order-disorder on the phase-stability relations of these compounds.

Structural and electrical properties of Mg2TiO4

“…XRD spectrum was indexed using Powder X 18) software shows that the system exhibits a cubic spinel structure with space group Fd 3 -m and lattice constant 8.456 Å. These results are consistent with the earlier reported, 19) where they studied the structure refinement of Mg2TiO4 by time-of-flight neutron diffraction and confirmed that cubic Mg2TiO4 had a completely inverse spinel structure.…”

“…Mg2TiO4 has an inverse cubic spinel structure belonging to the space group Oh 7 (Fd 3 -m). 19) The unit cell of the Mg2TiO4 consists of eight formulae unit (Z = 8), the smallest Bravais cell contains only 14 atoms (Z = 2). The Ti and Mg atoms occupy the tetrahedral (8a) and octahedral (16d) sites.…”

Magnesium dititanate (MgTi2O5) with pseudobrookite structure: a review

“…The structure refinement of MgTi 2 O 5 by time-of-flight neutron powder diffraction was conducted by Wechsler [32].…”

Microwave and terahertz dielectric properties of MgTiO3–CaTiO3 ceramics

“…Although significant progress has been made in the development of reliable measurement systems to extract materials' dielectric properties in the THz band [9,10], systematic research on the development of materials with appropriate dielectric properties has barely started. MgTiO 3, exhibiting a rhombohedral structure with space group R3c [11], is a widely studied microwave material which shows a relatively low permittivity (ε r ~ 17), high quality factor (Qf ~ 160000 at 7 GHz) and negative temperature coefficient of resonant frequency (τ f ~ 50 ppm/°C) [12]. In addition to a high permittivity and quality factor, a near-zero temperature coefficient of resonant frequency is also required for commercial applications.…”