Anisotropic thermoelectric properties associated with dimensional crossover in quasi-one-dimensional SrNbO_3.4+d (d∼0.03)

We have grown large single crystals of SrNbO_3.4+d (d∼0.03) with n=5 in the homologous series Sr_nNb_nO_3n+2 by using a traveling solvent floating-zone method and measured resistivity, thermopower, and thermal conductivity along all crystallographic axes. The thermoelectric properties are found to be highly anisotropic, which reflects a quasi-one-dimensional electronic structure. In particular, the thermopower along the b axis is -170 μV/K at 300 K, which is 1 order of magnitude higher than the -15 μV/K along the a axis and -25 μV/K along the c axis. A possible origin of the high anisotropy in the thermopower is discussed in terms of dimensional crossover associated with a structural modification.