S. Adams et al., PROTON ORDERING IN THE PEIERLS-DISTORTED HYDROGEN MOLYBDENUM BRONZE H0.33MOO3 - STRUCTURE AND PHYSICAL-PROPERTIES, Acta crystallographica. Section B, Structural science, 49, 1993, pp. 958-967
The intercalation of hydrogen into the layered structure of MoO3 produ
ces four hydrogen molybdenum bronze phases H(x)MoO3 (0 < x < 2). The c
orrelation between the structure and the physical properties of these
low-dimensional conductors has been investigated by X-ray diffraction
and conductivity measurements. Powder diffraction studies revealed pha
se transitions as a function of temperature and hydrogen content. A ne
w proton-distribution model describes the lattice distortions resultin
g from the intercalation in the whole composition range. Superstructur
e reflections were detected in precession photographs of single crysta
ls of the phases I (x = 0.3) and III (x = 1.6). A single-crystal struc
ture determination was performed for H0.33MoO3, which exhibits a 3a x
6c superstructure at ambient temperature. Structural and experimental
data for this particular composition are: P21/b 11, a = 11.70 (1), b =
14.070 (5), c = 22.40 (2) angstrom, a = 90.0 (1)-degrees, V = 3687 (8
) angstrom, Z = 72, D(x) = 4.68 (1) Mg m-3, lambda(Mo Kalpha) = 0.7107
angstrom, mu = 0.593 cm-1, F(000) = 4622.6, R(F) = 0.10 for 1223 uniq
ue reflections. Valence-sum calculations revealed that all the protons
of H0.33MoO3 are located in periodically arranged 6-(OH)-clusters. Th
e long-range proton ordering breaks down at T(c) = 380 K giving rise t
o a second-order phase transition. The identification of this transiti
on as a Peierls distortion explains many properties of phase 1: conduc
tivity measurements show a metal to non-metal transition at T(c) with
an unusual temperature dependence of sigma in the ordered phase. The m
ultiplication of the unit cell along the c direction as well as T(c) d
epend on the hydrogen content x. The critical exponent of the order pa
rameter beta = 0.36 is compatible with an incommensurate superstructur
e. Frohlich conductivity as a result of charge-density-wave depinning
is observed in field-dependent conductivity measurements.