X-Ray and Density Study of Nonstoichiometry in Uranium Oxides

5 X-Ray and Density Study of Nonstoichiometry in Uranium Oxides LAHMER LYNDS, W. A. YOUNG, J. S. MOHL, and G. G. LIBOWITZ 1

1

Research Division, Atomics Canoga Park, Calif.

1

International,

2

Division

of North

American

Aviation,

Downloaded by EMORY UNIV on March 10, 2016 | http://pubs.acs.org Publication Date: January 1, 1963 | doi: 10.1021/ba-1964-0039.ch005

The lattice parameters of nonstoichiometric uranium oxides, quenched from 1100° C., were determined within the composition range UO. 4

9

UO

to

2

Two separate linear relations for the lat-

tice parameter as a function of oxygen content were obtained: the other of

one characteristic of

UO . 4

9-y

5.4705 - 0.094 x (0 5.4423 + 0.029 y (0

UO

≤ ≤

x

≤ 0.125) and

y ≤ 0.31).

and

2+x

The two functions are:

a

=

a

=

0

0

Helium dis-

placement densities were determined for some samples; the values obtained are consistent with an oxygen interstitial model for oxygen vacancy model for

UO

and an

2+x

UO . 4

9-y

The nature of the defects causing nonstoichiometry i n uranium dioxide has not been unequivocally established, although m u c h previous work is consistent w i t h an oxygen interstitial model. Possibly the most straightforward approach to the problem is the study of density variations as a function of composition. Such a study requires a determination of accurate a n d precise lattice constants and densities, and a knowledge of the phase relationships. Phase relationships i n the UO -U O system have been studied b y many authors using a variety of techniques. T h e results obtained b y several investigators are given i n F i g u r e 1 (14). A c c o r d i n g to the results of Grønvold (12) a n d Schaner (22) there is essentially no deviation from stoichiometry i n UO at room temperature. Therefore, previous density determinations (2, 8, 11, 12) w i t h i n this system, w h i c h were conducted at room temperature, are not meaningful i n terms of nonstoichiometric uranium dioxide, since the observed increase i n density w i t h increasing oxygen content is due to the increased amount of the UO phase. Schaner has demonstrated that the single phase at elevated temperatures can be frozen i n b y quenching. I n the present investigation, limited to compositions between UO a n d U O , samples were prepared b y direct oxidation of stoichio2

4

9

2

4

2

4

9-y

9

Present address, North American Aviation Science Center, Division of North American Aviation. Present address, Materials Sciences Laboratory, Aerospace Corp., El Segundo, Calif. 1

2

58

Ward; Nonstoichiometric Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1963.


S. LYNDS ET AL.

1200

Γ

Ι 100

h

1000

h

2.00

Uranium Oxides

2j05

2.10

59

2.15

2.20

2.25

0 / U ATOM RATIO

Figure 1.

υθ —υ^0 2

phase diagram

9

metric U 0 , equilibrated i n the h i g h temperature monophasic region, and quenched. Schaner's phase diagram was used as a guide. Precise lattice con stants and densities were determined for the single phase thus frozen i n . 2

Experimental Sample Preparation. T h e samples were prepared b y the direct oxidation of stoichiometric uranium dioxide obtained b y the thermal decomposition of uranyl iodide ( 1 7 ) . E a c h sample was prepared i n the same manner. A degassed quartz tube fitted w i t h a Vycor standard taper joint was weighed on an analytical balance before and after loading w i t h stoichiometric U 0 (1 to 5 grams) and evacuated at 700° to 800° C . to 10~ m m . Purified oxygen (99.5 mole %) was added to a pressure sufficient to attain the desired composition (50 to 600 m m . ) ; pressures were meas2

6



60

ADVANCES IN CHEMISTRY SERIES

ured to ± 0.5 m m . using a mercury manometer. T h e tube was then almost en tirely immersed i n a room temperature water bath and sealed off at a narrowed region just above the water level. F r o m four to six tubes were arranged sym metrically i n a resistance furnace so that each sample w o u l d receive equivalent thermal treatment, heated to 1100° to 1150° C , and maintained at temperature for 60 to 100 hours. E a c h tube and its contents were then quenched i n a matter of seconds by a quick transfer into water. Application of a spark coil to each tube indicated that only a negligible amount of gas remained i n the tube after this treatment. A s an additional check a manometric measurement after oxidation of one sample yielded zero pressure. E a c h tube was carefully cut open, the sample removed, and the tube volume determined to ± 0.01 rc. b y filling w i t h distilled water and weighing. A l l samples were handled i n an argon-filled glove box after quenching and during preparation for x-ray diffraction studies. X-Ray Measurements. A Norelco T y p e 52058 symmetrical focusing backreflection camera was used to obtain precise lattice constants. This camera has a radius of 60 m m . a n d provides excellent resolution between θ = 59° a n d θ = 88.74°. A small portion of each sample was ground to a fine powder, mixed with a small quantity of D o w - C o r n i n g silicone grease, and applied to the camera target as a thin film. T h e diffraction patterns were obtained using 2- to 4-hour expo sures w i t h Ni-filtered C u radiation (Ka = 1.54051 A . and Ka — 1.54433 A . ) (16) at 34 k v . and 3 0 ma. Sample temperatures were monitored during expo sure and the lattice constants adjusted to 25° C , using a correction factor of 0.00006 A . per degree (12). T h e films were developed on one side only, to minimize parallax errors. T h e positions of the lines were measured to ± 0.1 m m . ; film shrinkage or expansion was considered i n all calculations. The lattice constants were calculated b y the method of Mueller, Heaton, and M i l l e r (19), adapted for the I B M - 7 0 9 b y Korst, Tannenbaum, and M i l l e r (15). This program assumes that systematic errors i n Aa /a are directly proportional to φ tan φ, where φ = π/2 — θ, and also corrects for random errors b y a least squares method. T h e maximum error i n the lattice constants is estimated to be ±: 0.0005 A . Density Measurements. Densities were obtained using the helium displace ment apparatus described b y Schumb a n d Rittner (23) w i t h minor modifications. Compressed nitrogen was used to raise the mercury level; a steel metric scale and cathetometer were used to measure the heights of the mercury levels i n the manometer and telescopes were used to observe the levels defining the constant volume. Sample Analysis. T h e composition of each sample was determined after oxidation b y one or more of three methods: polarographic analysis, volumetric analysis, and gas l a w calculation. T h e polarographic method (9) provides a quantitative determination of U ( V I ) formed as a result of oxidation. It was assumed that only U ( I V ) , U ( V I ) , and O were present a n d a value for the ratio of oxygen to uranium was then calculated. T h e error i n the O / U ratio determined b y this method is a function of the ratio and varies from ± 0.0005 i n the vicinity of U 0 to ± 0.005 near U 0 5 . Occasional difficulties w i t h the polarograph necessi tated the use of a eerie ammonium sulfate-ferric ammonium sulfate volumetric method (21) w h i c h is quantitative for U ( I V ) ; O / U ratios were then calculated to ± 0.005 using the assumption given above. T h e O / U ratios were also calculated b y use of the ideal gas l a w from the volumes of the samples and tubes and the pressures a n d temperatures of the oxygen added. Suitable choice of sample a n d tube volumes yielded an uncertainty of ± 0 . 0 0 0 5 . x

0

2

0

2

2

2 i 2

Results X-Ray Data. T h e x-ray data and the O / U ratios obtained b y the different analytical methods are shown i n Table I. F o r several samples, as a check for homogeneity, two random portions were chosen for x-ray examination. T h e maxi m u m difference found was only 0.0007 A . W h e r e differences occurred, the average lattice parameter was used. T h e high angle a — a doublets were fully resolved and very sharp lines were obtained for a l l compositions reported i n Table I, the x

2


5. LYNDS ET AL.

Uranium Oxides

61

patterns obtained for O / U > 2.17 being somewhat sharper than those for O / U < 2.13. However, attempts to prepare usable samples in the O / U range 2.13 to 2.17 were unsuccessful; the diffraction lines were always extremely diffuse. Possible reasons for the diffuse nature of the patterns are discussed below. Table I. From U(VI)

Lattice Constants in U 0 - U 0 2

Oxygen/Uranium

Ratio

From U(IV)

Cas law

Âv.

2.001 2.005

Obsd. 5 .4706

5.4706

5. 4701

5.4701

2.012

5..4694 5..4693

5.4694

2 .034

2.034

5 .4672 5 .4671

5.4672

2 .071

2.072

5 .4636 5 .4635

5.4636

2 .090

2.088

5 .4622 5 .4617

5.4620

2.116

2 .102

2.109

5 .4606

5.4606

2.124

2 .115

2.120

5 .4591

5.4591

2.123

5 .4589 5 .4589

5.4589 5.4512

2. 020

2.074 2.086

2.130

2 .114

2.167

2. 180

2.174

5 .4515 5 .4510

2 .198

2.198

5 .4485

5.4485

2.193

2. .205

2.199

5 .4486 5 .4479

5.4482

2.210

2. 218

2.214

5..4462 5..4462

5.4462

2.217

2.217 2.227

2. 220

2.224

5 .4466 5 .4464 5 .4453

A.

Âv.

2.001

2.034


9

2.004

2.004

2.005

2.126

4

Lattice Constant at 25° C,

5.4465 5.4453

Density Data. The density data are given in Table II and in Figure 2. Here the error is estimated to be 0.2%, based on determinations of the density of mercury which yielded results differing from the accepted value by no more than 0.16%. The standard deviations for seven to ten measurements on each sample varied from 0.02 to 0.10%. The first sample listed in Table II is "iodide" U 0 ; the others were prepared from the same batch of this material with S6a and S9a being prepared by oxidation of S6 and S9, respectively. 2

Table II. Sample S S6 S6a S9 S9a

Helium Displacement Densities O/U

Ratio

2.001 2.109 2.120 2.198 2.224

Density at 25° C , ±0.02 G./Cc. 10.23 10.62 10.70 11.06 11.10

Discussion

Lattice Parameters. Previous investigators (I, 10, 13) of lattice parameters as a function of O / U ratio have shown a continuous linear change in lattice Ward; Nonstoichiometric Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1963.

ADVANCES IN CHEMISTRY SERIES

62

1.50 r -

11.25

r-

S90

0s9o


0S6a

10.25

k

10.00

2.00

2.05

2.20

2.15

2.10

2.25

0 / U RATIO

Figure 2.

Densities in

O0 —O O> 2

k

parameter from U 0 to U 0 . Therefore, it was generally assumed that the U 0 structure was merely that of the fluorite U 0 lattice w i t h a n additional oxygen i n the ( 1 / 2 , 1/2, 1/2) position. T h e fact that the U 0 structure is more complex has recently been demonstrated b y Belbeoch, Piekarski, a n d Perio ( 7 ) . A s seen i n Figure 3, the present data can best be represented b y two straight lines, one characteristic of ϋ Ό and the other probably characteristic of U 0 _ Least squares treatment of the data yields the following relations, i n Angstrom units: 2

4

4

9

9

2

4

9

2 + Λ ?

4

9

r

a

0

=

5.4705 ±

0.001 -

(0.094 ±

0.001)*, (0 < χ

X-Ray and Density Study of Nonstoichiometry in Uranium Oxides

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