Anisotropic thermopower in the charge density wave quasi two-dimensional compounds (PO2)4(WO3)2m(m=4, 6)

Thermopower and Hall effect have been measured between 4.2 K and 300 K on the monophosphate tungsten bronzes (PO2)4(WO3)2m with m=4 and 6. The Peierls transitions can be clearly observed. In the normal state, both compounds behave as nearly compensated metals in agreement with band structure calculations. The anisotropic thermopower found in the charge density wave state of the m=6 compound gives information on the carrier pockets left by the gap openings along different crystallographic directions.