IIOBERT
s. S C H W A R T Z ,
[CONTRIBUTIOX FROM
I. FANKUCHEN A N D ROLAND iir.ARD
THE
vo1. 74
POLYTECHNIC INSTITUTE OF BROOKLYX 1
The Products of Thermal Decomposition of Chromium Trioxide'm2 BY
ROBERT s. S C H W A R T Z , I. FANKUCHEN A N D ROLAND 1fr.4RD3
The products of the thermal decomposition of chromium trioxide have been studied by means of X-ray diffraction and chemical analysis. Three compounds intermediate between CrOa and ere03 have been found. These are Cr308, Cr20jand CrOn. By using oxygen pressures up t o 900 p.s.i. in the temperature range 322 t o 563 O , the preparation of each of these oxides in relatively pure form was accomplished. No evidence was obtained for the existence of oxide phases of variable composition. It appears likely that some of the formulas previously ascribed to oxides of chromium are derived from the analysis of heterogeneous mixtures
The black products which are obtained as intermediates in the thermal decomposition of chromium trioxide to chromic oxide have been studied by many investigators but there is considerable disparity among their conclusions. From weight changes observed in heating chromium trioxide in vacuo Simon and Schmidt4 found that Cr6OI3 was formed between 260 and 280" and Cr5012between 360 and 366' and these were the only intermediate oxides. From pressure measurements, on the other hand, Ryss and Selyanskayaj came to the conclusion that the oxides Cr6013, Cr8021,Cr7O18 and CrsOlt were formed. A different idea was presented by Cameron, Harbard and King6 who studied the volume of oxygen liberated from chromium trioxide a t constant pressure as a function of temperature. The volume-temperature curves showed gradual changes a t approximately 250 and 360" extending over composition ranges CrO2.6-2 2 and Cr01.9-1.7which they interpret as an indication of non-stoichiometric compound formation. More recently, Vasenin7 from differential thermal analysis reported that only Cr205 is formed between 267 and 348'. He gave the X-ray diffraction powder pattern of the product. Changes in magnetic susceptibility of chromium trioxide and its products during thermal decomposition suggested to Honda* that two compounds Cr6015 and CrcOlgwere formed successively. The latter is ferromagnetic and was obtained a t 450'. A subsequent publication by Michel and Benard$ establishes the ferromagnetic oxide as CrOz which has the rutile structure. This paper gives the results of the examination of decomposition products of chromium trioxide by X-ray diffraction and chemical analysis. The thermal decomposition was carried out first in an evacuated apparatus and later under relatively high oxygen pressures. Experimental A sample of about 5 t o 10 g. of dried, reagent grade chro(1) Abstracted from a thesis by Robert S. Schwartz submitted t o t h e Graduate School of t h e Polytechnic Institute of Brooklyn, 1960, in partial fulfillment of the requirements for the degree of Doctor cf Philosophy. ( 2 ) P a r t of the work described in this paper was performed under Contract W 36-039 Sc 33719 with t h e L.' S. Army Signal Corps Engineering Laboratcries, Belmar, New Jersey. ('3) University of Connecticut, Storrs, Connecticut. (4) Simon and Schmidt, Z. unoyg. Chem., 163, 191 (1926). ( 5 ) I. G. Ryss and A . I. Selyanskaya, Acla Physiochem. (L'. S. S R.)> 8 , I323 (1938). (0) A. Camerou, E. H. Harbard and A. King. J . CAem. .Soc., 53 (1!J36).
(7) F. I. Vasenin, J . Gen. C h i n . (27.S. S. R . ) , 17, 459 (1'1-171 ( 8 ) Honda, Sci. Re$. T o h o k i ~U n i v . , 3, 223 (1913). i!i) A . hIiche1 a u d J. Renard, ( ' o m p l . Y C J I ~ . , 200, 1310 (1933).
rnium trioxide in a silica glass boat was placed in an assembly of two Pyrex test-tubes, the mouth of the smaller tube being toward the closed end of the larger. This served t o prevent sublimation of the chromium trioxide t o the cooler parts of the reaction tube. The reaction tube was placed in the cold furnace, the system evacuated and the furnace allowed t o reach the desired temperature which was held to 3 ~ 2 ~ . The heating was continued until constant pressure was reached. The temperature range was 210 to 460" and the heating periods varied from 20 t o 200 hr. );-Ray diffraction patterns were obtained for all the products. By visual comparison of these patterns, the presence of several phases could be detected by the appearance and disappearance of certain sets of diffraction lines from one pattern t o another. I n this way it was possible t o arrive a t the powder patterns for three different phases which are listed in Table I. These are numbered in order of their formation with rising temperature. Sone of the products consisted of a single phase.
TABLE I X-RAYPOWDER PATTERXS Phase I (CraOs) d
:A
11 8 ti 19 5 84 4 95 4 49 3 88 3 72 3 34 3 21 :i08 2 95 2 81 2 tii 2 60 2 13 2 01.1 1 938 1 856 1 718 1 659 1 511 1 351
Phase I1 (CrZOs) Intensity (-4
d
8 45 6 24 .5 95 5.13 3 8i 3 48 3 26 3 08 3 81 2 i,5
Phase 111 (CrOd d (A,)
3 2 2 2 1 1 1 1 1 1 1
13 435 213 135 638 566 459 382 318 066 016 0 850 0 812 0 793
Below 360" the products were principally mixtures of I and I1 with small amounts of 111. Extraction of this mixture with water gave a residue which was essentially phase I1 contaminated with phase 111. Between 360 and 460' the main ingredient of the product was chromic oxide with small amounts of phase 111. Since it appeared likely that a stepwise decomposition was occurring, the reaction was carried out under high oxygen pressure in the hope that conditions could be adjusted t o yield each of the phases in relatively pure form. This was accomplished by means of a steel bomb consisting of a thickwalled steel cylinder t o the open end of which a machined cover was bolted. An annealed copper gasket was used between the cylinder and cover. The cylinder was attached to :I commercial oxygen cylinder by means of S.A.E. flare
April 5, 1932
PRODUCTS OF THERMAL DECOMPOSITION OF CHROMIUM TRIOXIDE
1677
amine as indicator. Another aliquot was treated with hot 70% perchloric acid to oxidize the Cr+3 to CrO4-2 and the total chromium determined by titration. The average oxidation number of chromium was thus found t o be 5.34 and the percentage chromium 55.2. This corresponds to the formula Cr308(54.93% Cr). A weighed sample of phase I1 was dissolved in boiling 70% perchloric acid and the total chromium determined to be 56.8 which corresponds reasonably well t o Cr205 (56.5% Cr). Perhaps it is of interest t o note that the X-ray diffraction pattern used by Vasenin t o substantiate the formation of CrnOs as an intermediate in the decomposition of chromium trioxide corresponds t o that of a mixture of Cr308, Crz06 TABLE I1 and Cr203. PRODUCTS OF THERMAL DECOMPOSITION OF C r 0 3 UNDER Phase I11 corresponds t o the rutile type oxide and presumably has the ideal formula CrOz. OXYCEX PRESSURE The ranges of variable composition in the oxides CrPressure, 0 2 . 2 - Z , B and CrOl.7- 1.9 reported by Cameron, Harbard and Temp., O C . p.s.1. Time, min. Phases King6 should lead to some detectable variation in lattice IU 50 120 322 spacings. This could not be checked very well for Cr308 and I" 300 120 322 CrnObsince the diffraction patterns of the compounds we obtrace I11 I1 tained did not have any back-reflection lines. N6 variation 25 60 441 I1 I11 Cr20a in the spacings was detectable with the forward reflection 50 210 441 lines. I n the case of chromium dioxide, on the other hand, trace I11 I1 300 90 441 the lines in the back reflection region were quite adequate. I1 trace I11 500 120 44 1 Two samples were carefully examined; one which was conI1 trace I11 taminated with CrtOs and another which contained crd&. 300 120 49 5 These samples represent the oxide intermixed with phases I1 I11 513 5 300 of higher and lower .oxygen contents. The cell dimensions 10 11" 515 200 in these cases should represent the extremes in composition I11 60 Trace I1 900 545 of the oxide. Examination of the lines that correspond to Bragg angles close to 78' show that the lattice constants I11 I1 75 545 900 cannot have differed over the extremes of composition by 90 Trace 11 I11 54 5 900 more than 2 parts in a thousand. This indicates that marked I11 trace CrZO3 nonlstoichiometry is not possible and that the range of non54 5 120 900 CrzOa 111 180 300 563 stoichiometry is rather limited. This interpretation is consistent with the fact that the relative intensities of the lines in 0 Samples used for analysis. the diffraction pattern did not change in any case. These data show that the decomposition of phase I can be Formulas such as Cr8021, Cr5013, Cr7018, Cr5Ol2' arrested a t 322" with moderate oxygen pressures so that none of the strongest lines of phase I1 or I11 appear in the diffrac- and Cr5098were arrived a t by interpretation of tion patterns. Phase I1 is the main product in the tem- pressure measurements and magnetic susceptibility perature range 441 to 495 a t oxygen pressures above 300 data neither of which is unequivocal. It is most p.s.i. and with heating periods up to 2 hours. It is usually likely that the products analyzed were all heterogecontaminated with phase I11 under these conditions. The purest samples of phase I1 were obtained a t 545' under 200 neous. While it must be admitted that our analytip.s.i. for 10 minutes. Longer heating periods a t 545' with cal data are not sufficiently precise to distinguish the oxygen pressure a t 900 p.s.i. usually gave phase I11 between Cr205and Cr7018or between CrsOs and as the principal product but none of these samples was Cr8021, we are inclined to favor the simpler formulas sufficiently pure to warrant the establishment of the formula until the necessity for a more complex one can be by chemical analysis. demonstrated. The complete determination of the The samples indicated in Table I1 were used for analysis of Dhases I and 11. Phase I was readilv soluble in water crystal structure of these two phases could settle giving a solution containing both CrOaLzand Cr+3. The this point. chromate was determined in an aliquot by titration with standard ferrous sulfate solution using barium diphenyl- STORRS,CONN. RECEIVED AUGUST29, 1951
fittings and copper tubing. A metal Bourdon gage was used to determine the pressure. A weighed quantity of dried chromium trioxide in a Pyrex tube with a loosely fitting cover was placed in the bomb which was then flushed with oxygen several times and charged with oxygen to the desired pressure. The bomb was then placed in a vertical tube furnace and brought to temperature, the pressure being maintained constant by a manually operated valve. The reaction was stopped by quenching the bomb in ice-water. The conditions and results of a number of experiments are given in Table 11.
+ + + + + +
+
+ + + + +
