Molybdenum bronze

"Purple bronze" redirects here. Not to be confused with Hepatizon, a form of bronze which takes on a purple-colored patina.

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 from partially occupied 4d bands. ^[1] The oxidation states in K_0.28MoO₃ are K⁺¹, O²⁻, and Mo^+5.72. 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] [Note 1]} 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 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 electronic properties 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 molybdenum bronzes have similar appearances but different compositions:

• Hydrogen molybdenum orthorhombic 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  – Chemical intercalation compound

Notes

1. The article by Glemser and Lutz mentions thermal decomposition ("thermische Zersetzung" in German), and that does not seem like an ambient temperature though.

References

1. Onoda, M.; Toriumi, K.; Matsuda, Y.; Sato, M. (1987). "Crystal structure of lithium molybdenum purple bronze Li_0.9Mo₆O₁₇". Journal of Solid State Chemistry. 66 (1). Elsevier BV: 163–170. Bibcode:1987JSSCh..66..163O. doi:10.1016/0022-4596(87)90231-3. ISSN   0022-4596.
2. Stavenhagen, A.; Engels, E. (1895). "Ueber Molybdänbronzen". Berichte der Deutschen Chemischen Gesellschaft. 28 (2). Wiley: 2280–2281. doi:10.1002/cber.189502802213. ISSN   0365-9496.
3. Wold, A.; Kunnmann, W.; Arnott, R. J.; Ferretti, A. (1964). "Preparation and Properties of Sodium and Potassium Molybdenum Bronze Crystals". Inorganic Chemistry. 3 (4). American Chemical Society (ACS): 545–547. doi:10.1021/ic50014a022. ISSN   0020-1669.
4. 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. Greenblatt, M.; McCarroll, W.H.; Neifeld, R.; Croft, M.; Waszczak, J.V. (1984). "Quasi two-dimensional electronic properties of the lithium molybdenum bronze, Li_0.9Mo₆O₁₇". Solid State Communications. 51 (9). Elsevier BV: 671–674. Bibcode:1984SSCom..51..671G. doi:10.1016/0038-1098(84)90944-x. ISSN   0038-1098.
6. Glemser, Oskar; Lutz, Gertrud (1950). "Über ein Hydroxydhydrid des Molybdäns". Die Naturwissenschaften (in German). 37 (23). Springer Science and Business Media LLC: 539–540. Bibcode:1950NW.....37..539G. doi:10.1007/bf00589341. ISSN   0028-1042. S2CID   46699509.
7. Chin, Kin; Eda, Kazuo; Sotani, Noriyuki; Whittingham, M.Stanley (2002). "Hydrothermal Synthesis of the Blue Potassium Molybdenum Bronze, K_0.28MoO₃". Journal of Solid State Chemistry. 164 (1). Elsevier BV: 81–87. Bibcode:2002JSSCh.164...81C. doi:10.1006/jssc.2001.9450. ISSN   0022-4596.
8. Tsai, P.P.; Potenza, J.A.; Greenblatt, M.; Schugar, H.J. (1986). "Crystal structure of Li_0.33MoO₃, a stoichiometric, triclinic, lithium molybdenum bronze". Journal of Solid State Chemistry. 64 (1). Elsevier BV: 47–56. Bibcode:1986JSSCh..64...47T. doi:10.1016/0022-4596(86)90120-9. ISSN   0022-4596.
9. Whangbo, M. H.; Schneemeyer, L. F. (1986). "Band electronic structure of the molybdenum blue bronze A_0.30MoO₃ (A = K, Rb)". Inorganic Chemistry. 25 (14). American Chemical Society (ACS): 2424–2429. doi:10.1021/ic00234a028. ISSN   0020-1669.
10. Collins, B.T.; Ramanujachary, K.V.; Greenblatt, M.; Waszczak, J.V. (1985). "Charge-density wave instability and nonlinear transport in Tl_0.3MoO₃ a new blue molybdenum oxide bronze". Solid State Communications. 56 (12). Elsevier BV: 1023–1028. Bibcode:1985SSCom..56.1023C. doi:10.1016/0038-1098(85)90863-4. ISSN   0038-1098.
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. Birtill, J.J.; Dickens, P.G. (1979). "Thermochemistry of hydrogen molybdenum bronze phases H_xMoO₃". Journal of Solid State Chemistry. 29 (3). Elsevier BV: 367–372. Bibcode:1979JSSCh..29..367B. doi:10.1016/0022-4596(79)90193-2. ISSN   0022-4596.
13. Ramanujachary, K.V.; Greenblatt, D.M.; Jones, E.B.; McCarroll, W.H. (1993). "Synthesis and Characterization of a New Modification of the Quasi-Low-Dimensional Compound KMo₄O₆". Journal of Solid State Chemistry. 102 (1). Elsevier BV: 69–78. Bibcode:1993JSSCh.102...69R. doi:10.1006/jssc.1993.1008. ISSN   0022-4596.
14. Andrade, Margareth; Maffei, Mariana Lanzoni; Alves, Leandro Marcos Salgado; Santos, Carlos Alberto Moreira dos; Ferreira, Bento; Sartori, Antonio Fernando (2012-10-11). "Microstructure and metal-insulator transition in single crystalline KMo₄O₆". Materials Research. 15 (6). FapUNIFESP (SciELO): 998–1002. doi: 10.1590/s1516-14392012005000132 . ISSN   1980-5373.
15. Alves, L. M. S.; Damasceno, V. I.; dos Santos, C. A. M.; Bortolozo, A. D.; Suzuki, P. A.; Izario Filho, H. J.; Machado, A. J. S.; Fisk, Z. (2010-05-26). "Unconventional metallic behavior and superconductivity in the K-Mo-O system". Physical Review B. 81 (17) 174532. American Physical Society (APS). Bibcode:2010PhRvB..81q4532A. doi:10.1103/physrevb.81.174532. ISSN   1098-0121. S2CID   123147025.