A new double perovskite Mn2MnTeO6 has been obtained by high pressure phase transformation of a corundum-related precursor. It is antiferromagnetic below 36 K and develops a magnetic structure with magnetic moments of 4.8 µB and 3.8 µB for Mn 2+ at the A and B sites respectively. This new polymorph accounts for a recently reported decrease in the bandgap of Mn3TeO6 under pressure that may lead to useful light-harvesting properties. The ABO3 perovskite structure has proven to be very versatile, since it can accommodate several cations at both A and B sites. The consequent accessibility to a wide range of magnetic and electrical properties have attracted much attention. Perovskites show increasing distortion with decreasing ionic radius of the A-site cation, down to tolerance factors t = (rA+rO)/(√2(rB+rO)) of 0.75, where rA, rB and rO are the ionic radii of the A, B and O ions 1. However, the use of high pressures enables small cations such as Mn 2+ (r = 0.96 Å) 2 to be accommodated at the A-sites, allowing complex electronic and magnetic interactions. For example Mn 2+ V 4+ O3 perovskite shows the coexistence of localized d 5 Mn 2+ and itinerant d 1 electron V 4+ cations 3. Further variety is achieved in high pressure double perovskites (DPvs) Mn2BB'O6 with ordering of two cations on the B sites; Mn2BSbO6 (B = Sc, Cr, and Fe) 4-6 , Mn2BReO6 (B = Mn, Fe and Co) 7-9 and Mn2(Fe0.8Mo0.2)MoO6. 10 Tellurium based oxides M2MTeO6 (M = Mn, Co and Ni) have structures based on the corundum arrangement at ambient pressure (see Supplementary Figure 1) and show a variety of complex magnetic orders. Mn3TeO6-I crystallises in the Mg3TeO6-type structure. 11 Below 24 K it shows the coexistence of an elliptical helix and a sinusoidal spin density wave, both being incommensurate with the crystal structure. This phase has shown a dielectric response below 21 K and so is a type-II multiferroic 12. Co3TeO6 has 5 independent Co sites, providing a rich magnetic phase diagram 13 and Ni3TeO6 is a non-hysteretic colossal magnetoelectric material with a collinear antiferromagnetic (AFM) structure stabilised on stacked honeycomb layers 14. A recent study of light-harvesting properties of Mn3TeO6-I discovered an irreversible 39% bandgap reduction during in-situ measurements under pressure 15. We have explored the possibility of transforming ambient pressure Mn3TeO6-I phase into a recoverable double perovskite using high pressure and high temperature conditions, and we report here the new double perovskite Mn2MnTeO6, which is likely to be responsible for the reported bandgap engineering. Mn3TeO6-I, prepared at 1270 K by the solid state synthesis method of reference 11 was packed into a Pt capsule, pressed at 8 GPa and heated at 1173 K using a Walker-type module in a multianvil press. After 20 minutes, the temperature was quenched and the pressure was slowly released. Synchrotron X-ray diffraction (SXRD) powder data were collected at room temperature using the BL04 beamline at ALBA, Barcelona. Neutron powder diffraction (NPD) data were taken on 6 combined high pressure samples (≈ 120 mg) at 50 K, 1.5 K and several intermediate temperatures using the D1B beamline at the ILL, Grenoble. Zero-field cooled and field cooled (ZFC and FC) magnetic susceptibilities and a magnetisation-field loop at 2 K were measured using a PPMS Dynacool from Quantum Design. Preliminary laboratory X-ray diffraction evidenced the transition from Mn3TeO6-I into a new double perovskite, Mn3TeO6-II. This was found to have a monoclinic P21/n rock-salt B-site ordered DPv structure from Rieveld fitting of 300 K SXRD data (Fig. 1a). A few % of δ-Mn3O4 (CaMn2O4-type) and Pt from the capsule are included in the refinements. The SXRD results are summarised in Table 1 and the structure is shown in Fig. 1b.