The crystal structure of a new complex Ti–Cr oxide phase, K 0.82Mg 1.68(Cr 2.84Fe 0.84Ti 2.11Zr 0.08)O 12, synthesized at 13 GPa and 1400 °C, has been determined with single-crystal X-ray diffraction. It has a hexagonal symmetry with the space group P6 3/ m and unit-cell parameters a = 9.1763 ( 13 ) and c = 2.8790 ( 6 ) Å , V = 209.89 ( 8 ) Å 3 , Z = 1 . The structure is characterized by the hollandite-type double chains of edge-shared M2 octahedra occupied by trivalent and tetravalent cations (Ti+Cr+Fe+Zr); these double chains are linked to one another through shared octahedral apexes to form a framework structure containing two types of tunnels running parallel to the c-axis. One type of tunnels has a hexagonal cross-section and is occupied by large K +, whereas the other has a triangular cross-section and is occupied by Mg 2+. The K + cation is disordered between two crystallographically equivalent (2 a) sites in the tunnels and displays a U 33 displacement parameter that is significantly greater than U 11 . The new high-pressure phase reported in this study possesses many structural features similar to those for the hollandite compounds, making it a candidate for the 1-D fast ionic conductors.