3786
H. J. VAX HOOK
Oxygen Stoichiometry in the Compound BaFeO, - x
by H. J. Van Hook Raytheon Research Division, W a l t h a m , Massachusetts
(Received J u n e 18, 1064)
The compound BaFeOB has been prepared from several different starting materials. At least two crystallographic forins have beer1 observed: (1) a high-temperature form of unidentified crystal structure with a liniited range in oxygen stoichiometry near the coniposition BaIf’e02.so; this phase inelts congruently a t 1370” in air; (2) a low-temperature phase similar to hexagonal a-BaTi03. This phase has an extrsordinary range in oxygen content with no detectable change in crystal structure between BaFe02.47and BaFe02.92. The thermal stability of the hexagonal phase is increased by oxygen pressure; decomposition to the high form occurs a t 915” in air, a t 1250’ under 10 atm. of oxygen pressure. At 50 atm. the hexagonal (low) phase melts a t 1412’ without transforming to high BaFe03-x. Density measurenients on hexagonal BaFe03-, indicate a linear decrease with increasing oxygen deficiency from p = 6.11 g./cc. a t x = 0.10 to p = 5.71 g./cc. a t x = 0.50.
I. Introduction The oxidation states of iron normally encountered in the synthesis of oxide compounds a t elevated temperatures are the divalent and trivalent states. The existence of higher valence states of iron in certain oxide compounds, however, was postulated some time ago. In 1715 Stahl’ first prepared potassium orthoferrate (K2Fe04)which he obtained by reaction of powdered iron and potassium nitrate. To account for the indicated oxygen fraction, Stahl assumed the iron was hexavalent. I n more recent times Scholder and cow o r k e r ~ ~have - ~ synthesized ferrates containing all of the alkali and alkaline earth elements except calcium and magnesium. Each alkali-iron oxide combination may yield several ferrate phases, for example, Sa2Fe03, NazFeOl, and NarFeO4. The ferrates are very soluble in dilute acid solution. They tend to react with the H 2 0 and C 0 2 in normal atmospheres a t room temperature and thus require special handling. The thermal stability increases generally with atomic nuniber of the alkaline element. Chemical analysis for “active oxygen” in the powdered material by the thiosulfate-iodide reaction has indicated oxidation states of Fe4+, Fe5+, and Fe6+, assuming that oxygen and the alkaline element maintain their CUStornary valence. Barium orthoferrate is among the more stable of the T h e Journal of Physical Chemistry
alkali ferrates; its preparation and physical properties were first reported by Erchak, et U Z . , ~ in 1946. XRay diffraction studies indicated a cubic perovskitetype structure, and weight loss experiments indicated a composition BaFe02,e2in a phase produced by reaction of BaC03 and Fe203above 900” in oxygen. Several weak diffraction lines were noted which led the authors to adopt a unit cell twice the length of the elementary perovskite cell. The larger unit cell (a0 = 8.06 A,) contains three oxygen vacancies per formula unit BaeFes 021. Malinofsky and Kedesdy6 prepared barium orthoferrate by similar methods; they found a better fit between their diffraction patterns and one corresponding to the hexagonal, high-temperature form of barium titanate. The authors related the cubic and tetragonal polymorphs of BaTiO3 with high temperature BaFeOs-, phases. Prokopalo, et aZ.,’ also noted
(1) G. E. Stahl, “Opusculum Chymioo-Physioo-Medicum,” HataeMagdeburgiae, 1715. (2) R. Scholder, H. v. Bunsen, F. Kindervater, and W. Zeiss, 2. anorg. allgem. Chem., 282, 268 (1955). (3) R. Scholder, H. v. Bunsen, and W. Zeiss, ibid.,283,330 (1956). (4) R. Scholder, F. Kindervater, and W. Zeiss, ibid., 283, 338 (1956). (5) M. Erchak, I. Fankuchen, and R. Ward, J . Am. Chem. SOC., 68,2085 (1946). (6) W.W.Malinofsky and H. Kedesdy, ibid.. 76, 3090 (1954).
OXYGEXSTOICHIOMETRY
I N THE COMPOUND
BaFe03-,
structural similarities between barium orthoferrate and barium titanate and prepared solid solutions between these compourids. Cubic perovskite solid solutions were also investigated recently in the series LaE’e03-SrFe03-.8 and SrTi03-SrFe03-,. The data obtained on electrical and magnetic properties of these phases are consistent with the view that positive carriers, probably Fe4+ ions, are produced. There is, however, some reluctance to assume Fe4+, or higher, oxidation states of iron in the alkali ferrates since no concrete evidence for this valence is found in the many ferrjtes presently known. White and RoyJ1 have synthesized compositions in the series CrOza t high oxygen pres“Fe02” from CrOz t o Cro.sFeo.eOz sures (50,000 bars) and low temperatures, but the existence of Fe4+ is ambiguous since Cr6+ is quite stable, a t least in pure Cr03, under these pressuretemperature conditions and could account for the oxygen content. The same uncertainty in iron valence exists in the solid solution series Mnl-zFez02 swing to the variable valence of manganese. The alkali and alkaline earth ferrates are unique in that a high oxygen content is produced in compounds with a single transition metal element. Therefore, it may be possible to determine, without ambiguity, whether the postulated Fe4+ion is stable in the crystalline state. Since the thermal stability of the ferrates parallels the increase in stability of the corresponding alkali and alkaline easth peroxides, it is necessary to establish whether peroxide-type bonding, either in the crystal structure or by surface adsorption, could account for the high oxygen content. Since all previous studies were made using powdered material with high surface area, the possibility of adsorption cannot be discounted until single crystals are available for study. The purpose of this investigation has been to prepare barium orthoferrate in single crystal and polycrystalline form and to study the range of oxygen stoichiometry in the phase and its effect on certain physical properties. $l1O
11. Experimental Procedure BaFeOs-, was prepared by the reaction of C.P. Fez03with BaC03,Ba02,or Ba(OH)z+3Hz0in the proper niolar proportions a t l l O O o in air. Ingots of single crystalline BaI
