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The Wikipedia page for ferrous oxide states that $ceFeO$ can be prepared by the thermal decomposition of iron(II) oxalate, with the following reaction:

$$ceFeC2O4 → FeO + CO2 + CO$$

And that the procedure is conducted under an inert atmosphere to avoid the formation of ferric oxide.

Sicius' Eisengruppe: Elemente der achten Nebengruppe: Eine Reise durch das Periodensystem states that $ceFeC2O4$ should "slowly" be heated in a vacuum and then rapidly cooled down.

Both of these resources, however, omit critical information as to the preparation process. These questions remain:

• At which temperature does this reaction really start occurring, and what is the ideal temperature range to carry out this thermal decomposition?




• How "fast" should it be cooled down? And what are appropriate methods to do so?

I'd be very thankful for any additional information on the topic.

In appears that the synthetic procedure for preparation of iron(II) oxide from iron(II) oxalate has been first described by Günther et al. [1] and subsequently summarized in Brauer's Handbook of Preparative Inorganic Chemistry [2, p. 1497]:

Iron (II) Oxide

I.

$$ceunderset143.8FeC2O4 = underset71.8FeO + underset28.0CO + underset44.0CO2$$

Thermal decomposition of $ceFeC2O4$ yields pure $ceFeO$ only under specific conditions.
The decomposition is carried out in a quartz vessel (Fig. 332) whose lower section is kept at $pu850 °C$ by means of an electric furnace.
The joint is surrounded by a water-cooled lead coil or a rubber hose.
The nascent gases should be removed as quickly as possible; for this reason, the reactor is connected to two parallel mercury pumps and a good forepump; the gas is carried into two liquid-nitrogen-cooled traps containing activated charcoal.

Fig. 332. Preparation of iron (II) oxide.

The starting $ceFeC2O4$ $(pu0.5 - 0.8 g.)$ is placed in the small bulb above the quartz vessel, and the water of crystallization is completely vaporized by heating in vacuum for 12 hours at $pu200 °C.$
The bulb is turned in the joint, and the $ceFeC2O4$ drops into the heated lower section of the reactor where it is rapidly decomposed to $ceFeO,$ $ceCO$ and $ceCO2$ (the decomposition is complete in about 20 seconds). The product $ceFeO$ is retained by a quartz wool plug, which must be loose enough to prevent a buildup of pressure during the decomposition.

The furnace is now removed and the hot quartz tube is chilled as rapidly as possible in cold water, since $ceFeO$ is unstable in the range of $pu300-560 °C$ and decomposes according to:

$$ce4 FeO = Fe3O4 + Fe$$

(this decomposition proceeds most rapidly at about $pu480 °C,$ but
ceases below $pu300 °C).$
The above procedure yields a jet-black product, readily soluble in dilute acids; it is rapidly oxidized in air, but does not ignite.

Alternatively, iron(III) oxide can be reduced by iron to iron(II) oxide [2, p. 1498]:

II. The preparation from stoichiometric quantities of commercial $ceFe2O3$ and reduced iron can also be recommended.
The mixture and a few drops of water are sealed into a preevacuated quartz tube, heated for about three days at $pu900 °C,$ and quenched in cold water.

$$ceFe + Fe2O3 ->[pu900 °C] 3 FeO$$

Iron(II) oxide can also be produced from iron(III) oxide reduction by carbon monoxide at the temperature range between $pu500 °C$ and $pu600 °C$ (adapted from [3, p. 415]):

$$
beginalign
ce3 Fe2O3 + CO &->[pu400 °C] 2 Fe3O4 + CO2 \
ceFe2O3 + CO &->[pu500-600 °C] 2 FeO + CO2 \
ceFe2O3 + 3 CO &->[pu700 °C] 2 Fe + 3 CO2
endalign
$$

as well as by thermal decomposition of magnetite [3, p. 415]:

$$ce2 Fe3O4 ->[>pu1538 °C] 6 FeO + O2$$

References

1. Günther, P. L.; Rehaag, H. Über Die Thermische Zersetzung von Oxalaten II. Mitteilung. Darstellung von Reinem Ferrooxyd. Z. Anorg. Allg. Chem. 1939, 243 (1), 60–68. https://doi.org/10/cqzgk8. (in German)


2. Handbook of Preparative Inorganic Chemistry, 2nd ed.; Brauer, G., Ed.; Academic Press: New York; London, 1965; Vol. 2.


3. R. A. Lidin, V. A. Molochko, and L. L. Andreeva, Reactivity of Inorganic Substances, 3rd ed.; Khimia: Moscow, 2000. (in Russian)

The paper linked below indicates that the proper temperature is north of 535°C. This decomposition should be carried out with great caution though as some of the products might enflame, or even initiate a thermite reaction if carried out in the presence of Aluminum (as in the experimental setup used for the paper hereafter).

Source: https://pubs.rsc.org/en/content/articlelanding/2006/jm/b514565a#!divAbstract