Structural transition and possible orbital order in a trirutile compound

The interplay between orbital, lattice and spin degrees of freedom is a fundamental ingredient for exotic phenomena such as colossal magnetoresistance (CMR), metal-insulator transitions (MIT), multiferroicity or superconductivity, which can be tuned by external parameters. Prominent examples are the manganates R_1-xA_xMnO₃, which exhibit CMR for R=La accompanied by MIT transition upon heating above ferromagnetic ordering at an optimal doping level (A) and multiferroicity for smaller R, or lower dimensional cuprates, which exhibit superconductivity upon doping the antiferromagnetic parent compounds. In both cases the ions Mn³⁺ and Cu²⁺ are Jahn-Teller active and emphasize the importance of orbital degrees of freedom. The related phenomena accompanying Jahn-Teller distortion is the orbital ordering (OO), which in analogous to spin ordering, is a symmetry breaking due to the localized orbital occupation. Whereas LaMnO₃ and La₂CuO₄ are compounds exhibiting OO in partially filled e_g levels, LaTiO₃ is an example for partially filled t_2g levels. In this work the lower temperature structural transition in the trirutile CrTa₂O₆ with a rare Jahn-Teller active ion Cr²⁺ is investigated in terms of the OO by neutron diffraction and theoretical modeling.