Vibrational properties of monophosphate tungsten bronzes (PO2)4(WO3)2m (m = 4, 6)

We report the vibrational properties of quasi-two-dimensional charge density wave (CDW) materials (PO 2 ) 4 (WO 3 ) 2m (m = 4, 6). The CDW transitions occur at 80 and 54 K for m = 4 and 120 and 60 K for m = 6. The polarized middle infrared response clearly demonstrates the two-dimensional character of both compounds, with strong vibrational modes and low reflectance along the interlayer direction (c) but screened vibrational modes and high reflectance in the conducting (ab) plane. The temperature dependence of the vibrational modes is weak for the m = 4 compound, whereas the spectrum of the m = 6 material displays mode splitting and additional fine structure in the CDW states. The results are discussed in terms of the CDW formation and structural changes at low temperature.