Structure of the two-dimensional incommensurate charge-density wave in (PO₂)₄(WO₃)₈ at 20 K

Titelangaben

Lüdecke, Jens ; Jobst, Andreas ; Geupel, Sandra ; van Smaalen, Sander:
Structure of the two-dimensional incommensurate charge-density wave in (PO₂)₄(WO₃)₈ at 20 K.
In: Physical Review B. Bd. 64 (2001) Heft 10 . - No. 104105.
ISSN 0163-1829
DOI: https://doi.org/10.1103/PhysRevB.64.104105

Volltext

Link zum Volltext (externe URL):

Abstract

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 scattering. The CDW is found to correspond to a 2−q state for all temperatures below the first phase transition at Tc1=80 K. The second phase transition at Tc2=52 K is found to involve the development of mixed second-order satellites within the primary CDW. The incommensurately modulated structure has 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) Å, b=6.591(2) Å, and c=17.415(16) Å. The two-dimensional CDW is characterized by the modulation wave vectors q1=[0.330(2),0.292(2),0.000(2)] and q2=(−0.330,0.292,0). The (3+2)D superspace group was found as P212121(α,β,0)(−α,β,0). The primary CDW comprises the satellite reflections (hklm1m2) with (m1m2) equal to (±1,0) and (0,±1). Below Tc2 additional satellite reflections develop, that correspond to (m1m2)=±(1,−1). Accordingly, harmonic modulation amplitudes (1,0), (0,1), and (1,−1) were incorporated into the structure model. Refinement against 4970 observed reflection intensities resulted in a fit with R=0.039. The periodic lattice distortion shows that the CDW is located at the centers of the WO3 slabs. The CDW is 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.