Thermoelectric Power of RO$_{1-x-y}$F$_{x}$FeAs

The thermoelectric power $S$($T$) has been measured on the RO$_ {1-x-y}$F$_{x}$FeAs samples with $0 \leq x \leq 0.3$ and $0 \leq y \leq 0.5$ and with the rare earth R = La, Ce, Sm and Pr. Together with Hall and lattice parameter studies, systematical $x$-dependency is observed, although rather weak for samples with $x > 0$ and $y = 0$. The $S$($T$) of the undoped samples with $x = y = 0$, however, appears to have rather different shape and amplitude. By comparing with the resistivity drop around 150 K, the change seems to be associated with the spin/lattice instability previously proposed.