Synthesis and magnetic properties of the high-pressure scheelite-type GdCrO4 polymorph

The scheelite-type polymorph of GdCrO 4 has been obtained from the corresponding zircon-type compound under high pressure and temperature conditions, namely 4 GPa and 803 K. The crystal structure has been determined by X-ray powder diffraction. This GdCrO 4 scheelite crystallizes in a tetragonal symmetry with space group I 4 1 / a (No. 88, Z =4), a =5.0501(1) A, c =11.4533(2) A and V =292.099(7) A 3 . The thermal decomposition leads to the formation of the zircon–polymorph as intermediate phase at 773 K to end in the corresponding GdCrO 3 distorted perovskite-structure at higher temperatures. Magnetic susceptibility and magnetization measurements suggest the existence of long-range antiferromagnetic interactions which have been also confirmed from specific heat measurements. Neutron powder diffraction data reveal the simultaneous antiferromagnetic Gd 3+ and Cr 5+ ordering in the scheelite-type GdCrO 4 with a T N ∼20 K. The magnetic propagation vector was found to be k =(0 0 0). Combined with group theory analysis, the best neutron powder diffraction fit was obtained with a collinear antiferromagnetic coupling in which the m Cr 5 + and m Gd 3 + magnetic moments are confined in the tetragonal basal plane according to the mixed representation Γ 6 ⊕ Γ 8 .