  Single crystals of a new twinned hexagonal perovskite compound Sr8Os6.3O24
have been synthesized, and structural and magnetic properties have been
determined. The compound crystallizes in a hexagonal cell with lattice
parameters a = 9.6988(3) {\AA} and c = 18.1657(5) {\AA}. The structure is an
eight-layered hexagonal B-site deficient perovskite with the layer sequence
(ccch)2 and represents the first example of a hexagonal structure among 5d
oxides adopting a twin option. The sample shows spontaneous ferromagnetic
magnetization below 430 K with a small saturation moment of 0.11 {\mu}B/Os ion.
This is the highest Curie temperature (TC) reported for any bulk perovskite
containing only 5d ions at the B site.
