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Semiconducting CsMo4−x O12(x≍0.13): Room temperature crystal structure and resistivity anisotropy of a new alkali molybdenum bronze

Published online by Cambridge University Press:  31 January 2011

S. C. Abrahams ,
P. Marsh ,
L. F. Schneemeyer ,
C. E. Rice  and
S. E. Spengler
S. C. Abrahams
Affiliation:
AT&T Bell Laboratories, Murray Hill, New Jersey 07974
P. Marsh
Affiliation:
AT&T Bell Laboratories, Murray Hill, New Jersey 07974
L. F. Schneemeyer
Affiliation:
AT&T Bell Laboratories, Murray Hill, New Jersey 07974
C. E. Rice
Affiliation:
AT&T Bell Laboratories, Murray Hill, New Jersey 07974
S. E. Spengler
Affiliation:
AT&T Bell Laboratories, Murray Hill, New Jersey 07974
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Abstract

Cesium molybdenum bronze, CsMo4xO12, is a recently discovered highly anisotropic semiconductor that crystallizes in the monoclinic system with space group C2/m and four formulas in the unit cell. The lattice constants at 295 K are a = 19.063(5), b = 5.5827(23), c = 12.1147(23) Å, and β = 118.94(2)°. The integrated intensities of 13.529 reflections within a full sphere of reciprocal space with radius (sin θ)/λ≤0.75 Å−1 were measured on a CAD-4 diffractometer with MoKα radiation, resulting in 12 553 reflections above background. Correction and averaging in C2/m gave 1923 significant and independent structure factors, for an internal unweighted agreement factor of 0.0288. The crystal structure was solved from the Patterson function and Fourier series and refined by the method of least squares. The Cs atoms occupy two different sites; both, as well as the nine independent O atom sites, are fully occupied, but the four independent Mo atom sites contain significant Schottky defects, with x = 0.132(8) in the chemical formula. All metal atoms undergo significant anharmonic motion. The final agreement factor R = 0.0269, Two Mo atoms are tetrahedrally, the other two octahedrally, coordinated. The average tetrahedral Mo–O distance is 1.761 Å, the octahedral distance is 1.950 Å. Bond length–bond strength considerations suggest that Mo6+ ions (almost) completely occupy the tetrahedral sites whereas the octahedral sites are (almost) half occupied by Mo6+, half by Mo5+ ions, resulting in charge balance. Infinite polyhedral sheets of corner-sharing Mo–O octahedra and tetrahedra form normal to the a axis, separated by linear arrays of Cs+ ions. The resistivity in this and other low-dimensional alkali molybdenum bronzes is a minimum (10−1–10−2 Ω cm) in the direction along which both corner-sharing octahedra, and the shortest contacts between cations arranged in infinite chains, form. The resistivity normal to the polyhedral sheets is about 2 orders of magnitude greater, at ∼ 10 Ω cm.

Type
Articles
Information
Journal of Materials Research , Volume 2 , Issue 1 , February 1987 , pp. 82 - 90
Copyright
Copyright © Materials Research Society 1987

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