Zircon to scheelite phase transition induced by pressure and magnetism in TbCrO4

The scheelite form of the TbCrO4 oxide has been obtained by treating TbCrO4-zircon at 4 GPa and 823 K. X-ray and neutron diffraction data reveal that the high-pressure polymorph of TbCrO4 crystallizes with tetragonal symmetry space group I4(1)/a and lattice parameters a=5.036 74(10) angstrom and c=11.3734(4) angstrom. Although bisdisphenoids [TbO8] and tetrahedra [CrO4] are present in both scheelite and zircon polymorphs, the remarkable changes observed in both polyhedra appear to support the reconstructive model for the zircon-scheelite first-order phase transition. Specific heat and neutron diffraction measurements confirm the antiferromagnetic ordering previously proposed from magnetic susceptibility measurements, in which both Tb3+ and Cr5+ sub-lattices are involved. The magnetic structure has been determined and can be described on the basis of the coincidence between the chemical and magnetic cells, k=(0,0,0), where the magnetic moments of the Tb3+ and Cr5+ cations are antiferromagnetically aligned along the c axis of the structure, with T-N=29 K. Their values at 2 K are 8.97(3)mu(B) and 1.089(9)mu(B), respectively.