Extrinsic origin of the insulating behavior of polygrain icosahedral $\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Re}$ quasicrystals

Polygrain icosahedral $i\text{−}\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Re}$ quasicrystals are known to exhibit dramatically different electronic transport properties to other Al-based quasicrystals. By performing comparative experimental and theoretical studies of the electronic transport and electronic structure of polygrain and monocrystalline $i\text{−}\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Re}$ samples, we show that the extraordinarily high electrical resistivity and the metal-to-insulator transition in the polygrain material are not intrinsic properties of the quasiperiodic lattice, but are of extrinsic origin due to the high porosity and the oxygen-rich weakly insulating regions in the material. We also compare theoretical electronic structures and experimental electrical resistivities of monocrystalline $i\text{−}\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Re}$ and $i\text{−}\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Mn}$ quasicrystals and show that there are no significant differences between these two isomorphous compounds, suggesting that $i\text{−}\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Re}$ is on common ground with other Al-based quasicrystals. We present a structural model of $i\text{−}\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Re}$.

Figure 1

(a) Electrical resistivities $\rho$ of the polygrain and monocrystalline $i\text{−}\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Re}$. (b) Electrical conductivity $\sigma ={\rho }^{-1}$ of the polygrain sample (solid line is the fit $\sigma ={\sigma }_0+a{T}^{1.35}$). (c) Thermopowers [Seebeck coefficient $S\left(T\right)$] of the two samples.

Figure 2

(Color online) (a) ${}^{27}\mathrm{Al}$ NMR spin-lattice relaxation rate of the polygrain and monocrystalline $i\text{−}\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Re}$. The inset shows the ${}^{27}\mathrm{Al}$ magnetization-recovery curves on a normalized time scale $\tau ∕T_1$ at $200\mathrm{K}$ and $20\mathrm{K}$ of both samples superimposed. (b) Electronic DOSs of the $\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Mn}$ and $\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Re}$ theoretical quasicrystalline approximants. The lower curves show the partial $\mathrm{Re}\text{−}d$ and $\mathrm{Mn}\text{−}d$ contributions.

Figure 3

Secondary-electron images of (a) monocrystalline and (b) polygrain $i\text{−}\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Re}$. A connected pathway through the porous geometry of the polygrain sample is indicated in (b) by a dotted line (the bridges are located between the large dots).

Figure 4

Secondary-electron images of three representative bridges [(a), (b), and (c)] in the porous structure of the polygrain $i\text{−}\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Re}$. The concentration line profiles of the constituent elements over the bridges are also shown.

Figure 5

(Color online) Unit cell of the structural model of $i\text{−}{\mathrm{Al}}_{70.8}{\mathrm{Pd}}_{21.5}{\mathrm{Re}}_{7.7}$. Selected bonds are shown to emphasize the geometrical structure. Light bonds indicate the first shell of a Mackay-like cluster (in the center of the unit cell) and the first shells (icosahedra) of two Bergman clusters. Dark bonds indicate the second shells (pentagonal dodecahedra) of the Bergman clusters.

Figure 6

Electrical resistivities of flux-grown monocrystalline $i\text{−}{\mathrm{Al}}_{72}{\mathrm{Pd}}_{19.5}{\mathrm{Mn}}_{8.5}$ (squares) and $i\text{−}{\mathrm{Al}}_{74}{\mathrm{Pd}}_{17}{\mathrm{Re}}_9$ (circles) quasicrystals.