Molybdenum bronze

In chemistry, molybdenum bronze is a generic name for certain mixed oxides of molybdenum with the generic formula A
xMo
yO
z where A may be hydrogen, an alkali metal cation (such as Li⁺, Na⁺, K⁺), and Tl⁺. These compounds form deeply coloured plate-like crystals with a metallic sheen, hence their name. These bronzes derive their metallic character partially occupied 4d-bands.^[1] The oxidation state in K_0.28MoO₃ are K⁺¹, O²⁻, and Mo^+5.62. MoO₃ is an insulator, with an unfilled 4d band.

These compounds have been much studied since the 1980s due to their markedly anisotropic electrical properties, reflecting their layered structure. The electrical resistivity can vary considerably depending on the direction, in some cases by 200:1 or more. They are generally non-stoichiometric compounds. Some are metals and some are semiconductors.

Preparation

The first report of a "molybdenum bronze" was by Alfred Stavenhagen and E. Engels in 1895. They reported that electrolysis of molten Na
2MoO
4 and MoO
3 gave indigo-blue needles with metallic sheen, which they analysed by weight as Na
2Mo
5O
7.^[2] The first unambiguous synthesis of alkali molybdenum bronzes was reported only in 1964, by Wold and others.^[3] They obtained two potassium bronzes, "red" K
0.26MoO
3 and "blue" K
0.28MoO
3, by electrolysis of molten K
2MoO
4+MoO
3 at 550 °C and 560 °C, respectively. Sodium bronzes were also obtained by the same method. It was observed that at a slightly higher temperature (about 575 °C and above) only MoO
2 is obtained.^[3][4]

Another preparation technique involves crystallization from the melt in a temperature gradient. This report also called attention to the marked anisotropic resistivity of the purple lithium bronze Li
0.9Mo
6O
17 and its metal-to-insulator transition at about 24 K.^[5]

Hydrogen bronzes H
xMoO
3 were obtained in 1950 by Glemser and Lutz, by ambient-temperature reactions.^[6] The hydrogen in these compounds can be replaced by alkali metals by treatment with solutions of the corresponding halides. Reactions are conducted in an autoclave at about 160 °C.^[7]

Crystals of K_0.28MoO₃, also called "potassium-molybdenum blue bronze".

Classification

Molybdenum bronzes are classified in three major families:^[4][7]

• Red bronzes with limiting composition A
  0.33MoO
  3, that is, AMo
  3O
  9:^[7]
  • Lithium molybdenum red bronze Li
    0.33MoO
    3 Reau and others.^[7][8]
  • Potassium molybdenum red bronze K
    0.26Mo
    1.02O
    3^[3] or K
    0.3MoO
    3^[8][9]
  • Cesium molybdenum red bronze Cs
    0.33MoO
    3^[8]
  • Potassium molybdenum red bronze K
    0.23Mo
    1.01O
    3 a semi-conductor.^[3]
• Blue bronzes, with limiting composition A
  0.30MoO
  3, that is, A
  3Mo
  10O
  30.^[7] Their the electronic properties generally do not depend on the metal A.^[1]
  • Potassium molybdenum blue bronze K
    0.28Mo
    1.02O
    3^[3] or K
    0.3MoO
    3^[8][9]
  • Rubidium molybdenum blue bronze Rb
    0.3MoO
    3^[3][9]
  • Thallium molybdenum blue bronze Tl
    0.3MoO
    3^[10]
• Purple bronzes, generally with limiting formula A
  0.9Mo
  6O
  17. Their the electronic depend strongly on the metal A.^[1]
  • Lithium molybdenum purple bronze Li
    0.9Mo
    6O
    17
  • Sodium molybdenum purple bronze Na
    0.9Mo
    6O
    17
  • Potassium molybdenum purple bronze K
    0.9Mo
    6O
    17
  • Rubidium molybdenum purple bronze Rb
    0.9Mo
    6O
    17
  • Thallium molybdenum purple bronze Cs
    0.9Mo
    6O
    17^[11]

The hydrogen molydbdenum bronzes have similar appearances but different compositions:

• Hydrogen molybdenum orthorombic blue bronze H
  xMoO
  3, 0.23 < x < 0.4 ^[12]
• Hydrogen molybdenum monoclinic blue bronze H
  xMoO
  3, 0.85 < x < 1.4 ^[12]
• Hydrogen molybdenum red bronze H
  xMoO
  3, 1.55 < x < 1.72 ^[12]
• Hydrogen molybdenum green bronze H
  2MoO
  3 or MoO
  2.H
  2O ^[6][12]

Other molybdenum bronzes with anomalous electrical properties have been reported, which do not fit in these families. These include

• Tetragonal KMo
  4O
  6^[13][14]
• K
  xMoO
  2−δ.^[15]

See also

• sodium tungsten bronze

References

1. 1 2 3 M. Onoda, K. Toriumi, Y. Matsuda, M. Sato "Crystal structure of lithium molybdenum purple bronze Li
   0.9Mo
   6O
   17" Journal of Solid State Chemistry, volume 66, issue 1, pages 163–170 doi:10.1016/0022-4596(87)90231-3
2. ↑ A. Stavenhagen, E. Engels (1895) "Ueber Molybdänbronzen" Berichte der deutschen chemischen Gesellschaft, volume 28, pages 2280-2281. doi:10.1002/cber.189502802213
3. 1 2 3 4 5 6 A. Wold, W. Kunnmann, R. J. Arnott, and A. Ferreti (1964), "Preparation and properties of sodium and potassium molybdenum bronze crystals". Inorganic Chemistry, volume 3, issue 4, pages 545-547. doi:10.1021/ic50014a022
4. 1 2 Martha Greenblatt (1996), "Molybdenum and tungsten bronzes: Low-dimensional metals with unisial properties". In C. Schlenker ed., "Physics and Chemistry of Low-Dimensional Inorganic Conductors" Book, Springer, 481 pages. ISBN 9780306453045
5. ↑ M. Greenblatt, W. H. McCarroll, R. Neifeld, M. Croft, J. V. Waszczak (1984), "Quasi two-dimensional electronic properties of the lithium molybdenum bronze, Li
   0.9Mo
   6O17". Solid State Communications, volume 51, issue 9, pages 671–674. doi:10.1016/0038-1098(84)90944-X
6. 1 2 Oskar Glemser, Gertrud Lutz (1950) "Über ein Hydroxydhydrid des Molybdäns". Naturwissenschaften, volume 37, issue 23, pages 539-540. doi:10.1007/BF00589341
7. 1 2 3 4 5 Kin Chin, Kazuo Eda, Noriyuki Sotani, M.Stanley Whittingham (2002), "Hydrothermal synthesis of the blue potassium molybdenum bronze, K
   0.28MoO
   3" Journal of Solid State Chemistry, volume 164, issue 1, pages 81–87. doi:10.1006/jssc.2001.9450
8. 1 2 3 4 P.P. Tsai, J.A. Potenza, M. Greenblatt, H.J. Schugar(1986), "Crystal structure of Li
   0.33MoO
   3, a stoichiometric, triclinic, lithium molybdenum bronze". Journal of Solid State Chemistry, volume 64, issue 1, pages 47–56 doi:10.1016/0022-4596(86)90120-9
9. 1 2 3 M. H. Whangbo and L. F. Schneemeyer (1986), "Band electronic structure of the molybdenum blue bronze A
   0.30MoO
   3 (A = K, Rb)". Inorganic Chemistry, volume 25, issue 14, pages 2424–2429. doi:10.1021/ic00234a028
10. ↑ B.T. Collins, K.V. Ramanujachary, M. Greenblatt, and J.V. Waszczak (1985), "Charge-density wave instability and nonlinear transport in Tl
    0.3MoO
    3, a new blue molybdenum oxide bronze". Solid State Communications, volume 56, issue 12, pages 1023–1028. doi:10.1016/0038-1098(85)90863-4
11. ↑ E. Canadell and M.-H. Wangbo (1996), "Fermi surfaces instabilities in oxides and bronzes". In C. Schlenker ed. (1996), "Physics and Chemistry of Low-Dimensional Inorganic Conductors" Book, Springer, 481 pages. ISBN 9780306453045
12. 1 2 3 4 J.J. Birtill and P.G. Dickens (1979), "Thermochemistry of hydrogen molybdenum bronze phases H
    xMoO
    3". Journal of Solid State Chemistry, volume 29, issue 3, pages 367–372. doi:10.1016/0022-4596(79)90193-2
13. ↑ K. V. Ramanujachary, D. M. Greenblatt, E. B. Jones, W. H. McCarroll (1993), "Synthesis and characterization of a new modification of the quasi-low-dimensional compound KMo
    4O
    6" Journal of Solid State Chemistry, volume 102, issue 1, pages 69–78 doi:10.1006/jssc.1993.1008
14. ↑ Margareth Andrade, Mariana Lanzoni Maffei, Leandro Marcos Salgado Alves, Carlos Alberto Moreira dos Santos, Bento Ferreira, Antonio Fernando Sartori (2012), "Microstructure and metal-insulator transition in single crystalline KMo
    4O
    6". Materials Research, volume 15, issue 6 doi:10.1590/S1516-14392012005000132
15. ↑ L. M. S. Alves, V. I. Damasceno, C. A. M. dos Santos, A. D. Bortolozo, P. A. Suzuki, H. J. Izario Filho, A. J. S. Machado, and Z. Fisk (2010), "Unconventional metallic behavior and superconductivity in the K-Mo-O system". Physical Review B, volume 81, issue 17, paper 174532 (5 pages) doi:10.1103/PhysRevB.81.174532