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Is it possible to synthesise chromium(III) chromate/dichromate? The intended formulas are $\ce{Cr2(CrO4)3}$ and $\ce{Cr2(Cr2O7)3}$ and they feature both trivalent and hexavalent chromium. However, heating a mixture of chromium(III) oxide and chromium(VI) oxide seems to just produce chromium(IV) oxide and oxygen instead:

$$\ce{Cr2O3 + 3CrO3 -> 5CrO2 + O2}$$

There is a compound $\ce{Fe4[Fe(CN)6]3}$ which is a salt featuring trivalent iron in the cation, and divalent iron in the anion. Therefore, why not a salt featuring trivalent chromium in the cation, and hexavalent chromium in the anion?

How would chromium(III) chromate's properties differ from other "ordinary" chromates?

Search engines don't find any useful results for chromium chromate, but I suspect that to be the idiosyncratic trouble with the search engine itself (it just returns a list of various chromate salts, and doesn't seem to understand that both the "chromium" and "chromate" parts are supposed to be found in the single compound). I don't consider those results: [1], [2] to be useful, as they don't seem to have been human-generated and they contain no useful information.

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You're assumption is correct, chromic chromate, $\ce{Cr2(CrO4)3}$ does exist. Some other names listed at NIH.gov are chromium(III) chromate and dichromium tris(chromate), and the diagram below is from there.

Chromic Chromate

As one would expect, it's a strong oxidizer and an environmental health hazard. See more at Haz-Map for more information.

Chromic chromate, $\ce{Cr2(Cr2O7)3}$, as shown at EndMemo, might exist, or might be present as a complex species in solution. It is mentioned in these papers on chromium plating by C. Kasper and by J. P. Hoare.

See also N. V. Mandich's paper on chromium chemistry.

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    $\begingroup$ I don't think this compound was ever synthesized or possibly be ever synthesized. $\endgroup$
    – Nilay Ghosh
    Commented Apr 3 at 2:55
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    $\begingroup$ @NilayGhosh, which compound? Cr2(CrO4)3 exists, but there are papers on the dichromate quoted above, but existing only in solution. $\endgroup$
    – DrMoishe Pippik
    Commented Apr 3 at 13:43
  • $\begingroup$ I have found a paper mentioning the synthesis of the compound. I have added an answer. $\endgroup$
    – Nilay Ghosh
    Commented May 20 at 11:43
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    $\begingroup$ I don't think this is correct. Judging from the quality of the drawing and reported poor solubility in water (see my answer), I doubt any of this data from PubChem, related sources — let alone EndMemo and further sources — can be trusted. The beauty of Acta Chemica Scandinavica is they published trustworthy and high-quality data, but didn't bother with OCR, so their articles are normally not parsed by search engines and you need to know what and how to look for. $\endgroup$
    – andselisk ♦
    Commented May 20 at 18:17
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I found a paper which mentions the synthesis of the compound. The abstract is as follows:

Chromium chromate has been prepared from chromic chloride solution and silver chromate by double decomposition. The substance forms a brown glassy mass in the dry state, which is soluble in water. A study of the physical properties of its aqueous solution, such as the measurement of equivalent conductivity, freezing point, pH value, magnetic susceptibility and absorption spectra, as well as of its chemical properties shows that it is not a simple chromic chromate, $\ce{Cr2(CrO4)3.3H2O}$ but contains a complex chromato-chromate anion and should be represented as $\ce{Cr^{III}[Cr^{III}(CrO4)3].3H2O}$

CHROMIUM CHROMATE By Prodosh Chandra Roy Chaudhury, INORGANIC CHEMISTRY LABORATORY, UNIVERSITY COLLEGE OF SCIENCE, CALCUTTA, 1939 (link)

An informal discussion on this compound can be found in this ScienceMadness forum.

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Yes for chromate: chromium oxide (5/12) $\ce{Cr5O12}$ has been synthesized via thermal decomposition of chromium trioxide in welded platinum or gold capsules at 240–260 °C and pressure 2–3 kbar during 3–7 days. Its crystal structure has been determined using powder x-ray diffraction [1] (for CIF data and 3D view see ICSD #24299). $\ce{Cr5O12}$ adopts the structure type of $\ce{Al2(WO4)3}$ and indeed can be summarized with the formula $\ce{Cr2(CrO4)3}.$

$\ce{Cr5O12},$ synthesized at high pressure, crystallizes in the ortho­hombic system, space group $Pbcn$ (No. 60), with the unit-cell dimen­sions $a = \pu{12.044 Å},$ $b = \pu{8.212 Å}$ and $c = \pu{8.177 Å}$ and four formula units in the unit cell. The structure contains pairs of $\ce{CrO6}$ octahedra joined by sharing edges. By sharing corners the pairs of octahedra are linked with $\ce{CrO4}$ tetrahedra to form a threedimensional frame­work.

The $\ce{Cr5O12}$ structure can alternatively be described as a distorted cubic close-packed array of oxygen atoms with all the metal atoms in the interstices. There are 48 oxygen atoms in the unit cell with 12 chromium atoms in tetrahedral and 8 chromium atoms in octahedral holes. The coordination numbers as well as the interatomic distances indicate that the former chromium atoms are hexavalent and that the latter are trivalent.

Physical properties:

The $\ce{CrO_x}$ phase forms black non-ferromagnetic well developed octahedral crystals, readily distinguishable from the other chromium oxides. The crystals are stable in air, insoluble in water but slightly soluble in hot concentrated sulphuric acid.

[…] the compound can be regarded as a high-pressure phase, probably metastable at atmospheric pressure.

Crystal structure

Figure 1

Figure 2

Note

I'm not aware about the existence of chromium(III) dichromate, but dichromium bis(chromate) tetrachromate $\ce{Cr2(CrO4)2(Cr4O13)}$, or $\ce{Cr8O21}$ [2] (ICSD #71297) does exist and is arguably even more interesting from the structural perspective.

Reference

  1. Wilhelmi, K.-A.; Hassel, O.; Sillén, L. G.; Gronowitz, S.; Hoffman, R. A.; Westerdahl, A. The Crystal Structure of $\ce{Cr5O12}.$ Acta Chem. Scand. 1965, 19, 165–176. DOI: 10.3891/acta.chem.scand.19-0165.
  2. Norby, P.; Christensen, A. N.; Fjellvåg, H.; Nielsen, M. The Crystal Structure of $\ce{Cr8O21}$ Determined from Powder Diffraction Data: Thermal Transformation and Magnetic Properties of a Chromium-Chromate-Tetrachromate. J. Solid State Chem. 1991, 94 (2), 281–293. DOI: 10.1016/0022-4596(91)90193-L.
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