The Crystal Structure of Molybdenite; From the Journal of the American Chemical Society, Vol. XLV, No. 6., pp.1466–1471
Easton, PA: The American Chemical Society, 1923. First Edition. Offprint, 8vo (234 × 161mm), pp. [1], 6, [1]. Original printed green wrappers, staple-bound, toned and lightly rubbed to the extremities, very good. Signed by Pauling on the front wrapper. Pauling's first published scientific paper, and a formative one: the crystallographic habits of mind it demanded (precise geometric reasoning, the disciplined interpretation of structural evidence) would prove constitutive of the scientific sensibility he brought to everything that followed. Published during his graduate work at the Gates Chemical Laboratory, California Institute of Technology, under the supervision of Roscoe G. Dickinson (among the first American chemists to apply the X-ray diffraction methods pioneered by W. H. and W. L. Bragg to the systematic determination of crystal structures) this paper was produced at a moment when that program remained sufficiently novel that each successfully resolved structure constituted both a methodological and a scientific achievement. The Gates Laboratory was among a small number of institutions worldwide capable of carrying out such work with precision. Read April 24, 1923, the paper established the complete atomic arrangement of molybdenite (MoS₂), resolving a previously undescribed structural type and demonstrating for the first time the precise geometric relationship between molybdenum and sulfur atoms within the hexagonal unit cell. The layered architecture so revealed (in which planes of sulfur atoms sandwich each plane of molybdenum atoms with only weak cohesion between successive sulfur layers) directly accounts for the mineral's longstanding utility as one of the most effective solid lubricants known, a role it continues to fulfill in high-temperature industrial machinery, aerospace components, and dry lubricant coatings applied wherever conventional oils cannot function. Signed copies of Pauling's graduate-period offprints are of considerable rarity. Item #1304
The investigation proceeded by means of X-ray spectral photographs and Laue photographs of naturally cleaved molybdenite crystals 0.23 mm thick, with measurements made between planes of different classes and comparisons between estimated and calculated intensities tabulated in Tables I and II. The unit cell was determined to have d = 12.30 Å and an axial ratio of 3.90, containing 2 MoS in a hexagonal unit; a unit having d = 6.15 Å was shown to be impossible on the basis of minimum wave-length constraints. Reference to the tabulation of equivalent point coordinates in space groups isomorphous with D³ , C , D³ , and D identified six possible arrangements of the molybdenum and sulfur atoms in the hexagonal unit, of which arrangements 3 and 5 were eliminated as impossible and arrangements 1, 2, and 4 were eliminated by the Laue photographic data, leaving arrangement 6 as the only one consistent with all observations, with the positional parameter u = 0.621 ± 0.004. In the structure so determined, each sulfur atom is equidistant from three molybdenum atoms, and each molybdenum atom is surrounded by six equidistant sulfur atoms at the corners of a small triangular prism whose altitude is 3.17 ± 0.10 Å and whose edge is 3.15 ± 0.02 Å, with the distance from molybdenum to its nearest sulfur neighbors 2.41 ± 0.06 Å, in good agreement with Bragg's hypothesis of constant atomic radii taking the radius of molybdenum as 1.36 Å. The authors note that the great distance between the two sulfur atoms marked A and B in Figure 2, which at 3.40 Å substantially exceeds the value of 2.10 Å required by constant-radii contact, is undoubtedly connected with the excellent basal cleavage of molybdenite, since the planes of weakest cohesion are precisely those sulfur-sulfur planes across which cleavage occurs.
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