K2NaOsO5.5 and K3NaOs2O9: The first osmium perovskites containing alkali cations at the “A” site

Abstract K 2 NaOsO 5.5 and K 3 NaOs 2 O 9 were obtained from solid-state reactions of potassium superoxide, sodium peroxide and osmium metal at elevated oxygen pressures. K 2 NaOsO 5.5 crystallizes as an oxygen-deficient cubic double perovskite in space group Fm 3 ¯ m with a =8.4184(5) A and contains isolated OsO 6 octahedra. K 3 NaOs 2 O 9 crystallizes hexagonally in P 6 3 / mmc with a =5.9998(4) A and c =14.3053(14) A. K 3 NaOs 2 O 9 consists of face sharing Os 2 O 9 pairs of octahedra. According to magnetic measurements K 2 NaOsO 5.5 is diamagnetic, whereas K 3 NaOs 2 O 9 displays strong antiferromagnetic coupling ( T N =140 K), indicating enhanced magnetic interactions within the octahedral pair.