Vol. 3, No. 12, December, 1964
NOTES 1791
chromium. Previous investigations5-s have shown that, in addition to Cr03 and Cr203, the compounds Cr02 and CrOOH (I) can occur. Various hydrated oxides, usually poorly crystallized and of uncertain composition, have been described, and also a metastable "cubic y-Cr203," analogous to ~ - F e ~ 0 3The .~ crystal lattice of the CrOOH compound (I) was deduced as rhombohedral by Thamer, Douglass, and Staritzky.' Their material was obtained as brown-red crystals by decomposition of CrO3 in water under hydrothermal conditions. They also observed coexistence of Cr02 in some experiments, but the conditions favoring formation of either product were not defined. Laubengayer and McCune5 treated a precipitated hydrated chromium oxide in water or aqueous NaOH solution, a t high temperature and pressure, and obtained a bluegray compound (11). The chromium content of I1 was consistent with CrOOH, and the compound gave the same X-ray pattern as that subsequently reported for I. In the present work, treatment of CrO3, either in the presence of water or nominally in the dry state, a t 450" under a total pressure of 40,000 p s i , ha5 yielded the black, magnetic Cr02. This result is consistent with earlier reports.8-12 Substitution of Acknowledgments.-The financial support of the 0.25 M K2Cr207solution for water in two experiments U. S. Office of Naval Research is gratefully acknowlyielded the red CrOOH (I), although this has not been edged, and also the award of a National Research established as a consistent result. Treatment of Cr02, Council (Ottawa) Studentship to R. G. C. obtained as described above, in water a t 450" and 40,000 (15) V. Giitmann and K . €1. Jack, A c t a C$,yst., 4 , 246 (1951). p s i . , yielded a green product (111), which gave an (16) R. D. Peacock and D. W . A. Sharp, J. Chem. SO&,2762 (1959) X-ray pattern distinct from those of the known phases in this system. Emission spectrographic examination showed only chromium as a major constituent. Gravimetric determinations were consistent with the emCONTRIBUTION FROM THE SPERRY RAND pirical formula CrOOH for 111. The crystal lattice of RESEARCH CENTER,SUDBURY, MASSACHUSETTS I11 has been determined as orthorhombic, with a = 4.861, b = 4.292, c = 2.960k.
0ctahedr0n.l~ The broad band a t 540 cm.-l can therefore be considered the v3 frequency arising from an octahedral species of oh symmetry and is comparable to the value of 511 cm.-l observedL6for K3VFs. The suggestion was made previously2that vanadium tetrafluoride has an associated structure in which four fluorine atoms of each VFe octahedral unit are shared with adjacent octahedra. The symmetry about the vanadium is therefore probably tetragonal rather than octahedral. A molecule of tetragonal (D4h) symmetry has three infrared active fundamentals (doubly degenerate E, species) rather than the two triply degenerate infrared active fundamentals of an octahedral molecule. Assuming that small changes in symmetry such as this will not cause large shifts in the vibrational frequencies, the 780-837 cm.-l band in vanadium tetrafluoride may be considered one of these three fundamentals. The strong band a t 540 cm.-l probably corresponds to the 583 cm.-l band observed in K2VF6,l6and the remaining fundamental may be below 400 crn.-'. The reason for the doublet structure of the 780-837 cm.-l peak is not clear, but it may be due to crystal field effects or to differences in bonding.
A New Polymorph of CrOOH BY AT. C. TOMBS, W. J. CROFT,J. K. CARTER, J. F. FITZGERALD
ASD
Received June 18, 1964
Compounds in the system chromium-oxygen-hydrogen have been of interest in regard to magnetic properties, catalytic action,2 and pigment applicat i o n ~ . The ~ chemistry of the system is complex, in view of the multivalence of chromium, and experimental work is hindered by the occurrence of poorly crystallized products. The present work involved reactions under hydrothermal conditions, using sealed platinum tubes inside Stellite pressure vessel^,^ and has yielded additional information concerning the solid phases and their interconversion. The use of hydrothermal reaction conditions is favorable in this type of system, both in order to promote crystallization and to permit retention of the higher valence states of (1) (2) (3) (4)
A. hlichel and J . BQnard, Bull roc. chiin. P ~ a n c a ,10, 315 (1943) M , P. Matuszak, IJ. S. Fatents 2,403,671; 2,403,672 (1946). M . Uarrin, U. S. Patent 2,350,960 (1944). G. W. Morey, J . A m . Chenz. Soc., 36, 215 (1914).
Experimental The compound CrOz was prepared b y decomposition of CrOo, b y heating for 72 hr. a t 450" and 40,000 p.s.i., using welded thinwalled platinum tubing as a reaction container. T h e pressure and temperature were provided b y means of a Stellite pressure vessel heated in a tube furnace, according t o conventional techniques. T h e CrOE was removed from the reaction tube, washed in water, and -0.2 g. was resealed in a new platinum tube, together with -2 ml. of water. The CrOl-water mixture was then heated for 72 hr. a t 450' and 40,000 p.s.i. T h e reaction product was an olive-green powder (111) which gave the X-ray diffraction powder data shown in Table I. X-Ray diffraction powder patterns were obtained using a 114.59-mm. Straumanis-loaded camera, with vanadium-filtered chromium radiation. Intensity measurements were made b y visual estimation. The powder pattern was indexed onoan orthorhombic cell with a = 4.861, b = 4.292, and c = 2.960 A. T h e indexing was done (5) A. W. Lauhengayer a n d H. W. McCune, ibid., 74, 2362 (1952). (6) M . W. Shafer a n d R . Roy, Z . anorg. allgem. Cizem., 276, 275 (1964). (7) €3. J. Thamer, R. M. Douglass, and E. Staritzky, J . Am. Chem. SOL, 79, 547 (1957). (8) T. J . Swoboda, P. Arthur, J r . , N. L. Cox, J. N. Ingraham, A. L. Oppegard, and M. S . Sadler, J . A p p l . P h y s . , 32, 3745 (1961). (0) A. L. Oppegard, U. S.Patent 2,888,365 (1959). (10) J. N . Ingraham and T. J. Swohoda, U. S. Patent 2,923,682 (1!160). (11) P. Arthur, Jr., U. S. Patent 2,956,955 (1960). (12) Y. Goto and T. Kitamura, Funlai Oyobi Fu?t~natsziynkiit,9 , 109 (1962).
1792 NOTES
Inorganic Chemistry TABLE
1
CrOOH. This appears to be a new phase modification of CrOOH, of orthorhombic structure, in contrast to the previously reported red, rhombohedral form. Direct interconversion of the two forms has not been observed. Heating of the green, orthorhombic form in air a t about 350" causes complete and ready conversion to Cr02,which is stable under these conditions. Similar heating of the red, rhombohedral form causes a slower conversion to Cr02, mixed with some Crz03. The CrzOacontent does not increase once all the CrOOH has been decomposed. This difference in behavior may be explained by the physical form of the green, orthorhombic CrOOH, which was much more finely divided and would be expected to oxidize more readily during decomposition, thereby avoiding the formation of unreactive aggregates of Cr203. Somewhat similar behavior is also found in the oxides and oxyhydroxides of iron. The following reversible reaction has thus been demonstrated for the green, orthorhombic CrOOH
X-RAYDATAFOR CrOOH (111) Orthoihombic indices hkl
110 101 011 200 111 020 210 211 121 220 310 002 301 130 112 03 1 202 131 230 212 400 \ 122 321
I 1
d obsd.,
I obsd. S
m-
+
b 3.214 2,522
in
2,429
mw-
2 1i4 2.145 2.112 1 720 1.636 1.608 1.515 1.478 1.421 1.373 1.343 1.288 1.263 1.245 1,233
111-
m ni
+
mw
w m-
w in w TV
1%'
TV vy
1.212
w
1 185
d calcd.,
A. 3.217 2,528 (2.137 (2.431 2.178 2.146 2.115 1.721 1.636 1,609 1.516 1.480 1.421 1.372 1.345 1.288 1.264 1.245 1.233 (1.213 (1.215
450' under
2Cr02
by examining differences of sin2 B values. T h e unit cell was corroborated by electron diffraction data. An approximate mean refractive index of 1.9 was determined microscopically by the immersion method. T h e material was light yellow-green in transmission, hut was too finely divided for a n y individual crystal to be distinguished. Individual crystals as observed in the electron microscope had no recognizable outline, although they were shown to he single by selected area electron diffraction. Bn identical product 111 was obtained by similar treatment of C r 0 2in 3 iM NaOH or 0.25 AMK2Cr207 solutions, instead of water. Heating of I11 a t 110' in air caused no loss in weight. Ignition a t 1000" yielded Cr203, with a weight loss of 11.05%. No foreign lines were detected in the X-ray powder diffraction pattern of the Cr203,using Cr K a radiation. The weight loss was shown to represent H 2 0 by direct collection and weighing. h weighed sample of I11 was sealed in an evacuated fused silica tube fitted with a small-bore side arm. T h e side arm mas cooled with Dry Ice, and the sample was heated to red heat with a hydrogen torch. Water was seen t o condense in the side arm and was ultimately caused to accumulate a t the end of the side arm, which was then sealed off and separated from the main tube. T h e tube containing the collected water was allowed to reach room temperature, and a small opening was made by breaking off the pointed end of the glass. T h e tube was immediately weighed. It was then heated a t 110' and reweighed. The weight loss correpondiiig t o the collected water was l l , l l ~ofc the sample weight. The weight loss calculated for the reaction 2CrOOH -+ Cr2Oa H20 is 10,597,. T h e empirical formula of I11 is therefore assumed t o be CrOOH. Samples of I and I11 were placed in unsealed envelopes of platinum foil and heated simultaneously a t 335" in air in a tube furnace. After 6 hr., X-ray diffraction showed t h a t I11 had converted completely to Cr02, whereas I showed only partial conversion. After a further 72 hr., I11 still yielded only CrOz, hut I gave a mixture of C r 0 2and Cr203. More prolonged heating, for a total of 225 hr. a t 335", produced no further change in either sample.
+
Discussion Treatment of CrOn in water a t temperatures near 450" yields a green compound of empirical formula
Dressure + H2O ---2CrOOH
-3500inair
+ (1/2)02
(1)
This reaction appears to be the first reported route to synthesis of pure CrOz under atmospheric conditions,' although it is necessary to use hydrothermal condition5 to prepare the intermediate green, orthorhombic CrOOH. Formation of the red, rhombohedral CrOOH has been considered7in terms of a reaction between protons and polymeric CrV1ions (HCrO4).-Pz nH+
+ (HCr04),-"
=
nCrOOH
+ (3n/4)02 + ( n / 2 ) H 2 0
(2)
In contrast, the green, orthorhombic CrOOH now reported appears to result from direct hydrolysis of Cr02,according to eq. 1. Acknowledgments.-This work arose out of a study of CrOl suggested by Dr. D. Chapin, N.I.T., Lincoln Laboratory, whose interest and encouragement are appreciated. Thanks are due t o J. J. Comer and I
