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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 .

Spinel-type (S.G.= Fd 3 m ) ZnFe 2 O 4 fine particles with sizes from 4 to 19 nm prepared by solvothermal and microwave-assisted solvothermal methods have been studied by neutron powder diffraction at room temperature. The cation distribution corresponding to mixed spinel structure (Zn 2+ 1− x Fe 3+ x )[Fe 3+ 2− x Zn 2+ x ]O 4 along with the unit cell parameter has been estimated after Rietveld refinement of the obtained neutron diffraction data for all the samples. It has been found that the inversion degree parameter ( x ) takes values between 0.11 and 0.20 depending not only on the particle size but also on the synthesis conditions as well. All the samples behave as superparamagnetic with an effective magnetic moment per particle ( μ SP ) from 7.0×10 2 to 7.7×10 3 μ B . The sample obtained by microwave assistance displays a different magnetic behavior as the ZFC and FC magnetic susceptibility and the magnetization versus applied field hysteresis loop measured at 5 K suggest. This is related with the dipole interactions that are a consequence of the higher inversion degree and μ SP .