Orbital ordering and magnetism of fluoride and chloride compounds at high pressures

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Although the layred compounds K_2CuF_4 and La_2CuO_4 have a K_2NiF_4-type crystal structure, the former is a ferromagnet while the latter is an antiferromagnet. Such a difference of magnetism is due to the antiferrodistortive and ferrodistortive orbital orderings of 3d-holes, respectively. Under a high pressure, we expect that K_2CuF_4 turns into an antiferromagnet because of a change of orbital ordering from antiferrodistortive to ferrodistortive.From the magnetic susceptibility measurements at low temperatures under hydrostatic high pressures, we found a magnetic change from ferromagnetism to antiferromagnetis around 9 GPa. To confirm that this magnetic change was surely due to a change of orbital ordering, we performed X-ray diffraction and phonon-Raman scattering experiments under high pressures. As a result, we found that the crystal structure changed around 9 GPa. This structurql transition was found to bring about a change of orbital ordering from antifferrodistortive to ferrodistortive.