Ferromagnetism vs. antiferromagnetism of the dimorphic HoCrO4 oxide

This paper reports the specific conditions used in the preparation of the dimorphic phases of HoCrO₄ oxide. The scheelite form has been obtained from the room pressure stable HoCrO₄–zircon heated at 823 K at 40 kbar. The structures of both the polytypes have been refined from X-ray powder diffraction data using the Rietveld method. The zircon type of HoCrO₄ oxide crystallizes with tetragonal symmetry, S.G. I4₁/amd and lattice parameters a = 7.119(10) and c = 6.2557(5) Å; while the scheelite–HoCrO₄ derivative shows tetragonal symmetry, S.G. I4₁/a and lattice parameters a = 5.0017(1) and c = 11.2664(2) Å. Magnetic susceptibility measurements show that both zircon and scheelite forms of HoCrO₄ oxide present a very different magnetic behaviour at low temperatures. The zircon form behaves as a ferromagnet with a Curie temperature of 17.6 K, while the scheelite form is antiferromagnetic with a Néel temperature of 7.6 K. A metamagnetic transition for the scheelite form with a critical field of 1 T at 2 K has been also observed. The changes in the sign of the interactions have been also analyzed by considering the possible super-exchange mechanisms and the differences found in the Ho–O–Cr distances and bond angles in going from zircon to scheelite structural type.