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Platinum Metals Rev., 1989, 33, (1), 14

Densities of Osmium and Iridium

Recalculations Based upon a Review of the Latest Crystallographic Data

  • By J. W. Arblaster
  • Alexander Metal Company Limited, Bilston, West Midlands, England
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Much confusion exists in the literature concerning the densities of osmium and iridium. This has occurred because these values are calculated from crystallographic data, and errors in the absolute value of the X-ray wavelength scale, Avogadro’s Number, and the atomic weights of these elements have only slowly been resolved. Unfortunately density values have been published from time to time which have incorporated one or more of these errors, and these have become fixed in the literature and repeated ever since.

The introduction of new values for Avogadro’s Number and the X-ray wavelength scale conversion factor (1), both of which have been used in the present calculations, is an apt moment to review the crystallographic data for these elements and to show that osmium is the densest metal. Avogadro’s Number is now (6.0221367 ± 0.0000036) × 1023 per mol while the conversion factor from kX units to Ångström units is 1.00207789 ± 0.00000070. The equivalent Cu Kα 1 X-ray wavelength standard is then 1.5405945 ± 0.0000011 Å and an earlier version of this value (1.540598 Å) has already been adopted by the U.S. National Bureau of Standards (2) as a wavelength standard to replace the currently accepted conversion factor of 1.00202.

For iridium, which has a face-centred cubic structure, the density is calculated from:

where Ar = atomic weight

NA = Avogadro’s Number

a = lattice parameter in Ångström units

The atomic weight is well established at 192.22 ± 0.03 (3) while an average lattice parameter of 3.8392 ± 0.0005 Å at 20°C is selected from the high precision measurements given in Table I, which have been corrected to this temperature using a thermal expansion coefficient of 6.4 × 10−6 per °C (4). The calculated density is 22.562 ± 0.009 g/cm3 and the selected value is then 22.56 g/cm3 (22,560 kg/m3).

Table I

Lattice Parameter of Iridium at 20°C

Literature referenceValue, ÅTemperature corrected from, °C
Owen and Yates (5)3.839218
Swanson, Fuyat and Ugrinic (6)3.839526
Schaake (7)3.839725
Singh (8)3.839030
Schröder, Schmitz-Pranghe and Kohlhaas (9)3.838624

For osmium, which has a closer-packed hexagonal structure, the density is calculated from:

where a and c = lattice parameters parallel to the a- and c-axes, respectively.

The accepted value for the atomic weight is 190.2 ± 0.1 (3), but this value is based entirely on the isotopic abundance measurements of Nier (10) and the actual value obtained is 190.238 ± 0.005 (11). Therefore 190.24 ± 0.10 has been used in calculating the density. Lattice parameters of a = 2.7343 ± 0.0005 Å and c = 4.3200 ± 0.0005 Å, at 20°C, have been selected from the measurements in Table II which have been corrected to this temperature using thermal expansion coefficients of 4.3 × 10−6 per °C parallel to the a-axis, and 6.8 × 10−6 per °C parallel to the c-axis (4).

Table II

Lattice Parameters of Osmium, Parallel to the a- and c-axes, at 20°C, Ångström units

Literature referenceacTemperature corrected from, °C
Owen, Pickup and Roberts (12)2.73614.319018
Owen and Roberts (13)2.73554.319420
Finkel’, Palatnik and Kovtun (14)2.73464.317420*
Swanson, Fuyat and Ugrinic (6)2.73424.319826
Taylor, Doyle and Kagle (15)2.73424.320123
Mueller and Heaton (16)2.73464.320125
Schröder, Schmitz-Pranghe and Kohlhaas (9)2.73404.319916

Graphical only – actual values reported in Reference (4)

The discrepant values of Owen, Pickup and Roberts, and Owen and Roberts may be associated with the relatively low purity of the metal used (99.8 per cent) while the measurements of Finkel’, Palatnik and Kovtun, although apparently carried out on pure single crystals, nevertheless differ significantly from all other determinations on high purity materials. Hence these three values were rejected and the remainder were averaged to obtain the selected lattice parameters. A density of 22.588 ± 0.015 g/cm3 is then calculated; inclusion of all lattice parameters would lower this value by only 0.003 g/cm3 however, and would therefore not affect the selected value of 22.59 g/cm3 (22,590 kg/m3) significantly.

Crystallographic data for the platinum group metals are presented in Table III. The lattice parameters for osmium and iridium are those selected above, while values for the remaining four platinum group metals are those selected by Donohue (17), after correction to the new conversion factor (1).

Table III

Crystallographic Data for the Platinum Group Metals, at 20°C

ElementAtomic numberAtomic weight (1985)StructureLattice parametersNearest neighbour distance, nmDensity, kg/m3
a, nmc, nm
Ruthenium44101.07c.p.h.0.270550.428160.2677812,370
Rhodium45102.9055f.c.c.0.380340.2689412,420
Palladium46106.42f.c.c.0.389020.2750812,010
Osmium76190.2*c.p.h.0.273430.432000.2704822,590
Iridium77192.22f.c.c.0.383920.2714722,560
Platinum78195.08f.c.c.0.392350.2774321,450

c.p.h. = close-packed hexagonal, f.c.c. = face-centred cubic

Official value, 190.24 used in calculating the density (see text)

Having reviewed crystallographic data for both osmium and iridium, selected values of their densities at a temperature of 20°C are 22,590 kg/m3 and 22,560 kg/m3, respectively, thus confirming that osmium is the densest metal.

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References

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    E. R. Cohen and B. N. Taylor, “The 1986 Adjustment of the Fundamental Physical Constants”, CODATA Bulletin Number 63, 1986
  2. 2
    H. F. McMurdie,, M. C. Morris,, E. H. Evans,, B. Paretzkin,, J. H. de Groot,, C. R. Hubbard and S. J. Carmel, “Standard X-Ray Diffraction Powder Patterns”, National Bureau of Standards Monograph 25, Section 12, 1975
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    Commission on Atomic Weights and Isotopic Abundances, Pure Appl. Chem., 1986, 58, 1677
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    Y. S. Touloukian,, R. K. Kirby,, R. E. Taylor and P. D. Desai, “Thermophysical Properties of Matter”, Volume 12: Thermal Expansion Metallic Elements and Alloys, IFI/Plenum, New York, 1975
  5. 5
    E. A. Owen and E. L. Yates, Phil. Mag., 1933, 15, 472
  6. 6
    H. E. Swanson,, R. K. Fuyat and G. M. Ugrinic, “Standard X-Ray Diffraction Powder Patterns”, National Bureau of Standards Circular 539, Volume IV, 1955
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    H. F. Schaake, J.Less-Common Met., 1968, 15, 103
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    H. P. Singh, Acta Cryst., 1968, A24, 469
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    R. H. Schröder,, N. Schmitz-Pranghe and R. Kohlhaas, Z. Metallkunde, 1972, 63, 12
  10. 10
    A. O. Nier, Phys. Rev., 1937, 52, 885
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    P. DeBièvre,, M. Gallet,, N. E. Holden and I. L. Barnes, J. Phys. Chem. Ref. Data, 1984, 13, 809
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    E. A. Owen,, L. Pickup and I. O. Roberts, Z. Krist., 1935, A91, 70
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    E. A. Owen and E. W. Roberts, Z. Krist., 1937, A96, 497
  14. 14
    V. A. Finkel’,, M. I. Palatnik and G. P. Kovtun, Fiz. Metal. Metalloved., 1971, 32, 212 (English Translation: Phys. Met. Metallogr., 1972, 32, 231)
  15. 15
    A. Taylor,, N. J. Doyle and B. J. Kagle, J. Less-Common Met., 1962, 4, 436
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    M. H. Mueller and L. Heaton, U.S.A.E.C. Report ANL–6176, 1961
  17. 17
    J. Donohue,, “The Structure of the Elements”, John Wiley and Sons, New York, 1974

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