Structure of the two-dimensional incommensurate charge-density wave in (PO2)(4)(WO3)(8) at 20 K - art. no. 104105

The incommensurate charge-density waves (CDW's) in the m=4 phosphate bronze (PO2)(4)(WO3)(2m) have been studied by synchrotron-radiation x-ray scatter ing. The CDW is found to correspond to a 2-q state for all temperatures bel ow the first phase transition at T-c1 = 80 K. The second phase transition a t T-c2 = 52 K is found to involve the development of mixed second-order sat ellites within the primary CDW. The incommensurately modulated structure ha s been determined at T=20 K. The structure remains orthorhombic down to T = 20 K. At 20 K the lattice parameters are a = 5.294(1) Angstrom, b = 6.591( 2) Angstrom, and c = 17.415(16) Angstrom. The two-dimensional CDW is charac terized by the modulation wave vectors (q) over right arrow (1) =[0.330(2), 0.292(2),0.000(2)] and (q) over right arrow (2)=(-0.330,0.292,0). The (3+2) D superspace group was found as P2(1)2(1)2(1)(alpha,beta ,0)(-alpha,beta ,0 ). The, primary CDW comprises the satellite reflections (hklm(1)m(2)) with (m(1)m(2)) equal to (+/-1,0) and (0,+/-1). Below T-c2 additional satellite reflections develop, that correspond to (m(1)m(2)) = +/-(1, - 1). According ly, harmonic modulation amplitudes (1,0), (0, 1), and (1,- 1) were incorpor ated into the structure model. Refinement against 4970 observed reflection intensities resulted in a fit with R = 0.039, The periodic lattice distorti on shows that the CDW is located at the centers of the WO3 slabs. The CDW i s interpreted in terms of the formation of clusters of six tungsten atoms, that are linked together to form larger clusters of 12 to 18 atoms.