Magnetic properties of PdAs${}_2$O${}_6$: A dilute spin system with an unusually high Néel temperature

The crystal structure and magnetic ordering pattern of PdAs${}_2$O${}_6$ were investigated by neutron powder diffraction. While the magnetic structure of PdAs${}_2$O${}_6$ is identical to that of its isostructural $3d$ homologue NiAs${}_2$O${}_6$, its Néel temperature (140 K) is much higher than that of NiAs${}_2$O${}_6$ (30 K). This is surprising in view of the long distance and indirect exchange path between the magnetic Pd${}^{2+}$ ions. Density-functional calculations yield insight into the electronic structure and the geometry of the exchange-bond network of both PdAs${}_2$O${}_6$ and NiAs${}_2$O${}_6$, and provide a semiquantitative explanation of the large amplitude difference between their primary exchange interaction parameters.

Figure 1

Crystal structure of $A$As${}_2$O${}_6$ ($A=$ Pd,Ni) showing the three-dimensional network of $A{\text{O}}_6$ and As${\text{O}}_6$ octahedra. The brown (dark gray) colored balls denote $A$ atoms, and the cyan (light gray) colored balls represent As atoms. Small black colored balls at the corners of the octahedra are O atoms.

Figure 2

Temperature dependence of the specific heat and the inverse magnetic susceptibility of PdAs${}_2$O${}_6$.

Figure 3

Neutron powder diffraction data of PdAs${}_2$O${}_6$ collected at 5 and 200 K. The crystal structure was refined in the hexagonal space group $P\overline{3}1m$. The calculated patterns (red lines) are compared to the observed ones (black circles). In the lower part of each diagram, the difference pattern (blue) as well as the positions of the nuclear reflections of PdAs${}_2$O${}_6$ and the impurity phase PdO (black bars) are shown. For the powder pattern collected at 5 K, the positions of the magnetic reflections of PdAs${}_2$O${}_6$ are also shown. The inset shows the magnetic Bragg reflections of PdAs${}_2$O${}_6$, obtained from the difference between the diffraction patterns at 5 and 200 K.

Figure 4

Magnetic structure of PdAs${}_2$O${}_6$. Shown are the isolated PdO${}_6$ octahedra between the AsO${}_6$ layers. The magnetic moments of the palladium atoms are aligned ferromagnetically within the hexagonal $ab$ plane. Along the $c$ direction, the moments are coupled in an antiparallel fashion, forming the sequence $+-+-$.

Figure 5

LDA band structure (left panel) and density of states (right panel) for NiAs${}_2$O${}_6$ (solid lines) and PdAs${}_2$O${}_6$ (dashed lines). The two band structures have their Fermi levels (0 eV) lined up. The bands are plotted along the high-symmetry directions of the hexagonal Brillouin zone and the densities of states have their dominating characters labeled.

Figure 6

The $x^2-y^2$ (left panel) and $3z^2-r^2$ (right panel) Wannier orbitals, which span the LDA Pd $e_g$ bands of PdAs${}_2$O${}_6$. Plotted are orbital shapes (constant-amplitude surfaces) with lobes of opposite signs colored red (dark) and blue (light). For clarity, the central PdO${}_6$ octahedron is given a blue skin and all AsO${}_6$ octahedra a red skin. Pd atoms are green, As atoms are yellow, and O atoms are violet.

Figure 7

Paths of the dominant hoppings. Dashed green: between $x^2-y^2$ orbitals (see the left-hand side of Fig. 6) on third-nearest $A$ neighbors. Solid red: between $3z^2-r^2$ orbitals (see the right-hand side of Fig. 6) on third-nearest $A$ neighbors. The colors of the atoms are as in Fig. 1.

Figure 8

Temperature dependence of the magnetic susceptibility (multiplied by temperature) for NiAs${}_2$O${}_6$ (left panel) and PdAs${}_2$ O${}_6$ (right panel). The circles correspond to experimental data (taken from Ref. 3 for NiAs${}_2$O${}_6$), and the solid lines correspond to calculated data based on the model described in the text.