NOTES
2862
Vol. 67 THE CRYSTAL STRUCTURE OF STRONTIUM BROMIDE BY RONALD L. SASS,THOMAS BRACKETT, AND ELIZABETH BRACKETT Department of Chemistry, Rice University, Houston 1, Texas Received July 18,1963
0.19
1
,
1
13
15
14
I
lo3 Fig. 1.-Standard potential of the silver-silver chloride electrode at 25" vs. reciprocal of the dielectric constant of the solvent: 0 , water; 0 , water-mannitol; 0, water-glucose; 0, waterglycerol; x, water-ethylene glycol.
TABLEI ELECTROMOTIVE FORCES ASD STANDARD POTENTIALS OF CELL H2/HC1(0.01&f),KCl ( n z z ) ,~IANNITOL, ljrATER/AgC1, Ag AT 25" E, y
e-'.'
___-_ __-_
7
I n 1939, DO11 and Klemml published a powder X-ray diffraction diagram for strontium bromide. These authors were unable to give any interpretation of the diagram except that it appeared to be qualitatively similar to those of europium bromide and samarium bromide. A single crystal X-ray diffraction investigation of the structure of strontium bromide was reported in 1939 by Kamerrnam2 The assigned structure is a distorted version of the lead chloride structure. The space group is D2hl6-Pbnm (Pnma) with a = 9.20, b = 11.42, and c = 4.3 8. The structure is quite an open one with most of the nearest strontium-bromine distances considerably longer than the sum of the ionic radii. Also it was observed that the structure proposed by Kamermans did not fit the powder pattern presented by Doll and Klemm. I n an attempt to clarify the structural properties of strontium bromide, this paper describes a reinvestigation of the crystalline state by the method of powder X-ray diffraction techniques. A sample of hydrated strontium bromide was heated under vacuum a t 200' for several hours. A flat powder sample of the resulting anhydrous material was prepared in a drybox and transferred in a desiccator to the X-ray diffractometer. The diffractometer radiation shield was covered by a polyethylene envelope enclosing both sample and a silica gel desiccant. All diffraction data were recorded on an automatic recording powder diff ractometer equipped with a Geiger counter detector. The resulting diffraction pattern mas indexed on the basis of a tetragonal cell with a = 11.633 i 0.009, c = 7.155 f 0.008 A. (Cu Kal = 1.5405 A.). The observed systematic absences
_ _ _ ~ I _ _
5 1111
0 0.01 .015 .02 .03 .04 ,05 .06 .065 .07 .08 .09 Em0
,-----
0.46142 ,44545 ,44034 .43632 .42984 .42489 .42089 ,41753 .41602 .41466 ,41213 ,40978 .21948 ,01546
10
Wt.
13
mannitol
0.45844 .44245 .43741 I43334 .42686 ,42194 .41801 .41454 ,41309 ,41168 .40914 ,40685 ,21639 ,01494
0.45527 .43927 .43422 ,43010 .42366 .41874
,41471 .41124 ,40974 ,40831 ,40585 ,40349 ,21310' ,01419
EN' the change in free energy when a mole of hydrochloric acid is transferred from its hypothetical standard state in water to the corresponding state in mannitol solution. It is a measure of the "medium effect" when the transfer occurs without the interference of interionic forces. We now find 47 j . mole-l for 5% mannitol solution, 97 j. mole-' for 10% mannitol, and 170 j. mole-I for 15% mannitol. The medium effect is, therefore, approximately proportional to the mannitol concentration. A short extrapolation gives 173 j. mole-I or 41.3 cal. mole-1 in 15.41% mannitol (1 144) compared with 13.6 cal. mole-1 for sodium chloride and 9.3 cal. mole-I for potassium ~ h l o r i d e . ~ Figure 1 is B plot of EO us. the reciprocal of the dielectric constant of the solvent for the following systems a t 25': water, 5 , 10, and 15% mannitol, 5, 10, 20, and 3001, glucose, 3.06, 10, 21.2, and 30% glycerol, and 5, 10, 15, 20, 30, and 40% ethylene glycol.' This plot suggests, admittedly from a study of only a few solvent mixtures, that the standard potential of the electrode depends not so much on the chemical nature (hexose or polyhydric alcohol) of the solvent component, but more on the length of the carbon chain of the organic component of the mixed solwnt. (7) See H. S. E a r n e d a n d E. B. Owen "The Physical Chemistry of Electrolytic Solutions," 3rd Ed., Reinhold Publishing Co., New York, N. Y., 1958. p. 462.
hkO; h
+ k = 1 (mod 2 )
are consistent with space group D4h7 - P;lmm or C4ha Pi4. Density measurements are consistent with 10 molecular units per cell. This structure is in good agreement with the pattern pictured by Doll and Klemm. It thus appears that SrBrz, EuBrs, and SmBrz are all isostructural tetragonal structures. A sample of hydrated strontium bromide was kept under vacuum a t room temperahre for several hours. A powder X-ray diffraction pattern of the resulting TABLE I SrBrz ATOMIC PAR.4METERS Xumber a n d position notation
5
21
2
8g
0.088
0.588
2c 2c
'/P
1/4
l/4
'/4
8g
0.155 0.345
0.460 0.460
0.245 ,360 ,860 .604 ,115 0
8g
2a 2b
'14
'14
f/4
"4
(1) W. DQ1 a n d W. Klemm, Z . anorg. allgem. Chem., 241, 239 (2) M. A. Kamermans, Z. Riist., 101, 406 (1939).
'12
(1939).
NOTES
Dee., 1963 TABLE I1 OBSERVED AND CALCULATED INTESSITIES O F
hkl
100 110 001 101 200 111 210 201 211 220 300 310
{E j 102
Iulmd.
II
1
0
