  A new perovskite KOsO3 has been stabilized under high-pressure and high
temperature conditions. It is cubic at 500 K (Pm-3m) and undergoes subsequent
phase transitions to tetragonal at 320 K (P4/mmm) and rhombohedral (R-3m) at
230 K as shown from refining synchrotron X-ray powder diffraction (SXRD) data.
The larger orbital overlap integral and the extended wavefunction of 5d
electrons in the perovskite KOsO3 allow to explore physics from the regime
where Mott and Hund's rule couplings dominate to the state where the multiple
interactions are on equal footing. We demonstrate an exotic magnetic ordering
phase found by neutron powder diffraction along with physical properties via a
suite of measurements including magnetic and transport properties, differential
scanning calorimetry, and specific heat, which provide comprehensive
information for a system at the crossover from localized to itinerant
electronic behavior.
