The crystallographic and magnetic characteristics of Sr2CrO4 (K2NiF4-type) and Sr10(CrO4)6F2 (apatite-type)

Abstract : Solid-state reaction between SrCO3, Cr2O3 and SrF2 has produced the apatite phase Sr10(CrO4)6F2 and Sr2CrO4 which adopts the K2NiF4-type structure. The reaction outcome was very sensitive to the heating rate with rapid rise times favouring the formation of Sr2CrO4, which has been synthesised at ambient pressure for the first time. Powder X-ray diffraction and electron diffraction confirmed that Sr2CrO4 adopts a body centred tetragonal cell (space group I4/mmm) with lattice parameters a=3.8357(1) Å and c=12.7169(1) Å, while a combination of neutron and X-ray diffraction verified Sr10(CrO4)6F2 is hexagonal (space group P63/m) with lattice parameters a=9.9570(1) Å and c=7.4292(1) Å. X-ray photoelectron spectroscopy and magnetic measurements were used to characterise the oxidation states of chromium contained within these phases.