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

虽然层状化合物K_2CuF_4和La_2CuO_4具有k_2nif_4型晶体结构，但前者为铁磁体，后者为反铁磁体。这种磁性的差异分别是由于三维空穴的反铁畸变和铁畸变轨道有序造成的。在高压下，我们预计K_2CuF_4会变成反铁磁体，因为轨道的顺序从反铁扭曲变为铁扭曲。在流体静力高压下的低温磁化率测量中，我们发现在9 GPa左右，磁性从铁磁性转变为反铁磁性。为了证实这种磁变化肯定是由于轨道顺序的改变，我们在高压下进行了x射线衍射和声子-拉曼散射实验。结果，我们发现晶体结构在9 GPa左右发生了变化。这种结构跃迁使轨道的顺序从抗铁畸变变为铁畸变。

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.