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

The scheelite-type polymorph of GdCrO4 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 GdCrO4 scheelite crystallizes in a tetragonal symmetry with space group I 41/a (No. 88, Z =4), a =5.0501(1) Å, c =11.4533(2) Å and V =292.099(7) Å3. The thermal decomposition leads to the formation of the zircon–polymorph as intermediate phase at 773 K to end in the corresponding GdCrO3 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 Gd3+ and Cr5+ ordering in the scheelite-type GdCrO4 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 mCr5+mCr5+ and mGd3+mGd3+ magnetic moments are confined in the tetragonal basal plane according to the mixed representation Γ6 ⊕ Γ8.