Effect of Trivalent Rare Earth, Dy3+ Substitution for Ba2+ on Low Temperature Magnetic and High Temperature Thermoelectric Properties of Type-I Clathrate, Ba8Al16Si30

Si-based type-I clathrates with Ba-filler being partially replaced with Dy having the compositions Ba8-xDyxAlySi46-y have been synthesized. Their low temperature magnetic behavior together with high temperature thermoelectric properties have been investigated to understand charge and heat transport. Substitution of nonmagnetic Ba2+ with magnetic Dy3+ changes the magnetic behavior from diamagnetic to paramagnetic, which upon cooling transforms to an antiferromagnetic state at similar to 12 K. The room temperature paramagnetic moment increases from 5.5 mu(B) for x = 0.5 to 6.68 mu(B) for x = 1.0 clearly showing the role of magnetic Dy3+. The antiferromagnetic state has been found to be unstable below 6 K and transforms to a ferromagnetic state. These transformations are reflected in the isothermal magnetic hysteresis, which also reveals the presence of metamagnetic transitions at 8 kG and 30 kG magnetic fields. The high temperature Seebeck coefficient shows an n-type behavior with a low value varying from similar to 3 mu VK-1 at 330 K to 105 mu VK-1 at 865 K. The electrical resistivity on the other hand varies from similar to 2.8 mu Omega m to 13.9 mu Omega m. The Seebeck coefficient and electrical conductivity have been found to be strong functions of Dy and Al contents with both the properties increasing with increasing Al content. This leads to an enhanced power factor of similar to 1 mWm(-1) K-2 at 865 K for Ba7.5Dy0.5Al14.7Si31.3, an increase by about an order of magnitude compared to the base alloy. The concomitant decrease in thermal conductivity from 3.8 Wm(-1) K-1 to 2.3 Wm(-1) K-1 results in an increased figure-of-merit from 0.17 to 0.36 at 865 K, an increase of 112% compared to the base clathrate with no Dy.