Synthetic Mg 2 tio 4 qandilite was investigated to 50 and 40.4 GPa at room temperature using Raman spectroscopy and X-ray diffraction, respectively. The Raman measurements showed that cubic Mg 2 tio 4 spinel transforms to a high pressure tetragonal (I4 1 /amd, No.141) phase at 14.7 GPa. Owing to sluggish kinetics at room temperature, the spinel phase coexists with the tetragonal phase between 14.7 and 24.3 GPa. In the X-ray diffraction experiment, transformation of the cubic Mg 2 tio 4 to the tetragonal structure was complete by 29.2 GPa, ~5 GPa higher than the transition pressure obtained by Raman measurements, owing to slow kinetics. The obtained isothermal bulk modulus of Mg 2 tio 4 spinel is K T0 = 148(3) GPa when K T0 ' = 6.6, or K T0 = 166(1) GPa when K T0 ' is fixed at 4. The isothermal bulk modulus of the high-pressure tetragonal phase is calculated to be 209(2) GPa and V 0 = 270(2) Å 3 when K T0 ' is fixed at 4, and the volume reduction on change from cubic to tetragonal phase is about 9%. The calculated thermal Grüneisen parameters (γ th) of cubic and tetragonal Mg 2 tio 4 phases are 1.01 and 0.63. Based on the radii ratio of spinel cations, a simple model is proposed to predict post-spinel structures. Mg 2 TiO 4 (qandilite) is an oxospinel with excellent dielectric properties that are widely used in satellite communications, mobile phones and wireless communication systems 1,2. In addition, Mg 2 TiO 4 spinel can form a high-temperature superconducting epitaxial thin film 3,4 , and is a good candidate for thin film phosphor in optoelectronic applications due to its red emission at high temperature 5. Natural Mg 2 TiO 4 was discovered in the Kangerdlugssuaq region of East Greenland 6 and named qandilite after the Qandil Group of metamorphic rocks at Qala-Dizeh region of Iraq 7. At ambient pressure, Mg 2 TiO 4 exhibits as a tetragonal structure below 660 °C but as a cubic structure above 660 °C 8,9. The cubic phase breaks down to MgTiO 3 (geikielite) and MgO (periclase) with increasing pressure 10. Synthetic Mg 2 TiO 4 qandilite has inverse spinel structure T (Mg 2+)°(Mg 2+ , Ti 4+)O 4 9,11,12 , which means that Mg 2+ cations occupy both tetrahedral (T) and octahedral (O) sites, while Ti 4+ cations are present only in octahedral sites. The isothermal bulk modulus of cubic qandilite was reported to be 169 GPa, based on empirical calculations 13 , and 175 GPa in a diamond anvil cell study 14 , whereas the adiabatic bulk modulus of qandilite was determined to be 152 GPa by ultrasonic measurements 15. However, no phase transformation was found in previous studies, even though three major post-spinel structures, namely CaTi 2 O 4 (CT; space group Cmcm), CaMn 2 O 4 (CM; space group Pbcm), and CaFe 2 O 4 (CF; space group Pnma) have been proposed to be stable under high pressure environments 16. Unlike CM and CF structures, most CT phases have not been discovered at room temperature but instead under higher temperatures 17-19. In addition to the orthorhombic structures, a tetragonal structure (space group I4 1 /a...

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