JOURNAL OF RESEARCH of the Nationa l Bureau of Standards - A. Physics and Chemistry

Vol. 74A, No. 3, May- June 1970

A Refinement of the Crystal Structure of Na2 C0 3 • H2 0

B. Dickens, F. A. Maue r, and W. E. Brown*

Insti tute for Materials Research , National Bureau of Standards Washington, D.C. 20234

(January 28, 1970)

The c rysta l s tru cture of synthe ti c Na2 Co.,,· H2O. has been re fin ed us ing 1231 un ique x· ray dif· fr~ctlOn da ta c~l ec led by the peak he ight m~thod on a diffractome te r. R = 0.034. The unit ce ll is a - 6.474(2), b- 1O.724(3) and c=5.259(2) A with z = 4 a nd s pace group P2,ab. The ca lc ulat ed de nsit y IS the sa me as the obse rve d dens it y, 2.26 g. c m-3 . Th e s tru c tu re cont a in s shee ts of C0.§- ions bonded to Na+ IOn s a nd water mo lec ules rough ly ha lfway be tween the s hee ts. Each C0 2- bonds ed rre· wise to both Na+ ions. The Na+ ions have irregul a r but simila r coordi nations of se ve n "' ~ i ghbo rs . Ea~h wa ter molec u le is bonded to both Na+ ions and form s hydrogen bond s to bot h ne ig hboring Co. ~- layers.

Key words : C rys tal s truc ture; h yd ra ted ca rbona tes; hydroge n bond ing ; sodium carbonat e; the rmonatrit e; x·ray diffrac ti on.

1. Introduction

The crystal structure of Na 2C0 3 . H 20 was deter· mined except for the hydrogen positions, by Harper [1] I in 1936 usin g qualitative es timates of the x·ray intensiti es. In our program of studies on coordination in hydrated carbonates [2] and phosphates [3], we have re fin ed Harper 's structure for Na2CO:! · H 20 using new x·ray data. Na2CO:1·H 20 is the mineral thermona· trite and often occurs with Na 2CO:) . 1OH 20 (natron) and Na2C0 3 • NaHC03 • 2H 20 (trona) [4,5,6].

2. Determination of the Structure

Formula: Na2 COOl' H20. Unit ceLL: Orthorhombic with ( a = 6.472(2) A, b = 1O.724(3) A, c=5.259(2) A at 24 °C

as calculated from three pairs of 28 values of axial re fl ec tions from a single crys tal and observed on a diffractometer. The s tandard deviations of the cell parameters are in parentheses and are esti mates based gn experience with the techniqu e.o Cell vo lume: 365.1 A3. Wavelength used: 0.710688 A (Mo Ka). Filter:

r 0.025 mm Nb. Space Croup: P2 l ab. CeLL contents in formula Wts: 4. EqLtivalent positions: x, y, z; 1/2 + x , -y, -z; 1/2 +x, 1/2-y, z; x, 1/2 +y, -z. Reciprocal lattice extinctions: hOl, h 7'= 2n; hkO , k 7'= 2n. Observed

[density: 2.255 g' c m- 3 [7]. Calculated density: 2.256 g . cm - 3 . Habit: Fragment from plate. Size of crystal: - 0.35 mm max. - 0.05 mm min. Origin: Evaporation

*Di rf'C IOr. Research Associat e Progra m of the American Dent al Associat ion at the National Bureau of S tandar<]s.

1 Fi ~ lI res in bracke ts ind ir' atc the lit era ture refe rences at the end (If thi s pape r.

of aqueous solution a t 60 °C. Linear absorption coeffi­cient: 4.37 c m- I . Absorption corrections: None applied. Maximum error in any intensity from absorption is - 10 percent. Number of reflections: 2189 were col­lected from 2 octants and merged into a unique se t of 1231 of which 1132 are "observed" re fl ections and 99 are less th a n 20- above bac kground and are " un ­observed". Max imum sin e/i--. for data.: 0.904 A - I.

Method used to estimate data: peak height meas ure­ment [8] with a s in gle crys ta l diffractometer [9] and so me peak heights s tandardized again s t 8/28 scans. Scattering factors: Na , C, 0 , for neutral atoms give n in reference rIO]; H from. reference [Ill Least·squares refin~me"ts: Full·mat6x, with !'(wllFol-IFcll)2 mini­mized. Refine ments include unobserved reflections for which the calculared intensities are more than 20- above background. Least·squares weights: 1/0-2

normalized so that maximum weight is 1. Definitions: counts in peak = 1= P - (T/2TB ) (BI. + BH ) , 0-(1) = (P + (BI. + BH ) (T/ (2TB ) )2)1 /2, F = (AF)(LP)(I))I /2, o-(F) = (0-(1)/2) (LP/I) 1/ 2 where P= co unts at the peak position , BI. and Bli = background counts at lower and higher 28 respectively , T= time spent counting peak, TIJ = time spent counting each background , AF = attenuator· factor , LP = Lorentz polarization correction.

319

Final R w: 0.029. Final R: 0.032. A verage shift/error for last cycle: 0.015. Thermal parameters: anisotropic with form

exp (-I/4(a*2B ll h2+ b*2B22k2+ c*2B33l 2+ 2a*b*Blzhk

+ 2a*c*B 13hl + 2b*c* B23kL).

sheets at x - 0.25 and x - 0.75. Because the C atoms lie close to the planes of th e a glides, they form columns along a at y= 0.25 , z - 0.60. Two adjacent C03 groups in a column are held together by both being ionically bonded to four Na ions and hydrogen bonded by one water molecule. The four Na ions and the water mole­c ule form a pentagon roughly halfway between the C03 s heets. Adjacent C03 groups in a given sheet are linked by the Na ion s that lie above and below the sheet.

3.1. The Carbonate Anion and Invironment

The dimensions in the C03 anion are given in table 4 and the e nvironment is d etailed in table 4 and fi gure

The structure was refined isotropically from Harper's parameters using the x-ray 67 sys tem [12] of computer programs to Rw=0.065 ; the x parameter of Na( I ) was fixed at x = O. The structure was refined ani so­tropically to Rw = 0.044 and the hydroge ns were found unambiguously as th e two highest peaks in the dif­ference synthes is in which the coeffi cients were weighted by the leas t squares weights. The two next highest peaks were less th an 3/4 as high as the peaks assigned to hydrogen s and were (a) halfw \}y, between C and 0(3) in the C03 group and (b) 0.7 A from 0(1) of the C03 group. The hydrogens were included with variable pOOiitional parameters and fixed thermal parameters (BH=I A2) in the final refinement to Rw= 0.029. The largest correlation coefficients are - 0.25 between the scale factor and the B II thermal parameters of the two N a ions and - 0.15 between these B II thermal parameters. Most correlation coeffi­cients are less than 0.05.

2. The C03 group is nearly trigo nal. As can be seen in ) fi gures 1 and 2, 0 (2) is the only oxygen which is not hydrogen bonded , being instead ionically bonded to four Na ions. The absence of hydrogen bonding to 0(2) may account for the observation that the C-0(2) bond is apparently the s hortes t of the three. 0 (1) is coordinated to three Na ions and is the acceptor in the hydrogen bond 0 (1) ... H(I )-0(4) from the water molecule. 0 (3) is bonq,ed strongly to two Na ions and more weakly (2. 822 A) to a third Na(2). 0(3)

The atomic parameters are given in table 1. The observed and calculated structure factors are given in table 2. The hydrogen posi tions obtained from the weighted difference synthoesis and from the refin e­ments differ by - 0.17 A. "Calculated" hydrogen pos itions were derived by applying the geometry of free water (0-H=0.958 A, LH-O-H= 104.5°) with the constraint that the o-H . . . 0 angles be as near to linear as possible. These hydrogen pq,sitions differ from the other positions by about 0.3 A. The three sets of hydroge n positions are compared in table 3. The distances and angles which involve hydrogen were obtained using the "calculated" hydrogen positions.

3. Description of the Structure

The structure (fig. 1) con tains C03 anions whose planes are almost perpendicular to a and which form

is the acceptor in the hydrogen bond 0(3) ... H(2)-0(4) from the water molecule.

Since the planes of the C03 groups are all essen­tially perpendicular to a, thi s is expected to be the direction of lowest r efractive index, as was found by Harper [1] . Later workers [13, 14] appare ntly did not permute the refractive indexes when they permuted the unit cell axes to fit cr ys tallographic convention.

3.2. The Sodium Environments

The two crystallographically distinct Na ions in the structure are in general positions. Their environments are shown in fi gure 3 and are give n in table 5.

Na( I) is bonded ionic ally to five oxygens from C0 3

groups, and strongly to one water oxygen. Four of the C03 oxygens, 0 (1), 0 (2'), 0(3), 0 (3'), figure 3, define an approximate square about Na(1), and water oxygen 0(4) forms the apex of a square pyramid. The fifth carbonate oxygen, 0(2), is in the same C0 3 group as 0 (1); the C03 group is therefore coordinated edgewise to Na(I). The position of 0(2) is s uch that the coordina-

TABLE 1. Atomic parameters of NazCO"' H20

Atom x y Z Bil B22 B33 B'2 BI3 B23

Na(1) 0.0000 0.1938(1) 0.1398(1) 1.40(3) 1.54(3) 1.36(3) 0.17(3) 0.17(3) 0.08(2) Na(2) .0651 (2) .0020(1) -.3786(2) 1.44(3) 1.19(3) 1.76(3) .31(2) .25(3) .09(3)

C - .2151(3) .2556(2) .6025(3) .45(6) 1.11(6) 1.17(5) -.06(5) -.05(5) -.03(5) 0(1) -.2010(3) .1381(1) .5457(3) 1.56(6) 0.72(5) 2.72(7) .08(5) - .15(6) - .47(4) 0(2) - .2027(3) .3369(1) .4267(2) 1.73(6) 1.27(5) 1.24(5) - .05(5) -.04(5) .47(4) 0(3) - .2398(3) .2876(1) -.1639(2) 1.42(6) 2.14(6) 0.90(4) - .28(5) .25(4) -.32(4) 0(4) .0325(3) - .0296(1 ) .0683(3) 1.64(7) 1.43(6) 1.92(6) - .21(5) - .25(5) .39(4)

Figures in parentheses are standard errors in last significant figure quoted, and were computed in the final cycle of full· matrix least· squares refin ement.

*Thermal parame ters are in ,.\z.

320

J

~

l

>

z no 211 .. 42 1 ,~z

~ 55t. H, I 8 IH I II

I :) II ~ II q

lZ 4 8 4' \4 50 !to l~ !t o H 18 4'

l SI. lSI 52:) 519 ZH ZS 4 I n U6

1 0 5d 811 12 ' 5 41 \4 1)9 t18 I II U 75 18 t ~ 15

''f n II ' } ",2

.. SH H9 to 281 28110 S 125 I llI

10 HI Jl8 12 )0 H 111 H n B ,.. "" 18 U H

tHo 111 1028 415 ZH lS I

II 127 1 21 t o lH

12 " " "" u u U 1/:0 55 5'1 18 I q 1'J

2 )\' nq .. 2h 22) b z~s Z 14 8 2H 22 8

10 52 52 12 H 14 52 H U ) 25 18 52

2 t H "to .. 111 11>1 10 Z9 30 8 ILS III

10 h n 12 H l'll H I II 11 9 III ~o H

U H )ll 21ft. 1'1" l eU 1) U

II n o 129 I) 12& us 12 50 52 II, H U

" ,

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lB ISS lU 1"8

.. 40 "2 /0 U 1t1 8 IH IH

11:1 50 79 lZ 1) 72 I .. H H

2 l5 2 8 .. ) 1 311 !t )) H II SO 53

1 0 II> 1<J 12 0, 6 51

1:1 .... 0

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10 " 0

z~~ l~: 21 1'J

, . J 211 19 2

'" 200 1" 2 159 1St 100 lbS

9 i&b us 10 118 III

" , 12 18 8 191 13 q . q

\ 4 85 " U 12 )6 16 57 55

11 Il " HI 28 )l 19 n_ I

, . 0 .... 39 66 bt.

)07 H " 350 no 206 10 0, 89 IU

2l.9 27) II 1't4 \4 ' q IllS 16b

10 14 1 l SI 1 1 )) )1 12 " 0 4 ) 1) 1 20 121 \ '" 8) 8 5 I S 2 et 26 1 6 61 &,9 \7 "' ...... 18 1 7 7

17 )'

2.11.. 1

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32' JZIJ 275 no

28 21 10) 10 1 80 80 ~, " I

9 I U 12S 10 205 20/1. 11 b l &4.

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" " .. n ., 11 ' 8

III " 2 .. " " ,. .

151 18" 12' 125 2Q2 29l 182 117 12'1 127

6 2 bL 22) 12"

• " <n' 9 1 1 70

10 93 960

11 ~ I " " 12 11 1~ !) 88 !!I8 14 90 q]

IS 15 38 16 75 711 11 43

" . ) 68 n~

61 62 21 27 l5 23 In 121 12 8 12b

6 l in 1112 1 105 103 8 19' I'n 9 III 110

10 I U 114 11 28 HI 12 n o 12 7 !) U 11

TABLE 2. Observed and cal cui ated structure factors for Na2co3• H20 a

14 'I U 15 U 20 lb 55 53 11 )) 32 18 II 14

h 0 121 126

4 5 U 29 28

15) 151 219 221 101 102 ~7 115 H 14

9 lOS 111

10 qo '" 11 62 '57 12 48 13 49 50 U 2! J2 15 1115 41 17 25 2)

In lq5 16b 111

98 9! In til

4 125 :24 5 ~8 H , H 14 1 U 90 9 29 25 <J 1 0 U

10 1'1 11 .. 1 ' 6 12 21 Z7 lJ 19 20 1lt )9 H 1 5 ll - 8 1.. )8 U

5' • 108 III

H 53 .. .. " 40

11 0 10" .. " " " 2l II

9 1l II 10 2) 21 1 1 " 10 12 H ) 0 l) 12 10

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'" 111 n~

U) 1'1 152 166 12<J 125

51 56 132 112

75 12 I 111 1t5 9 B 5"

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" lJ ,. 2) 2lt ,. , I" l O_ I S H 11 16 19 19 11 10' U6

2t. 28

" 0 5 5 1:0 IIll 11) 113 427 UO

27 20 49 5 504 121 I II \13 IU

II 41 ,q 9 IH 12)

10 77 12 II 11119 11119 12 1111 ,III II 22 Ib I' U 4 ) 15 U III I' ,7 5 0 17 19 36 " ,

'" '" IH In 222 22 7 221 2)) I ~1 Ibb

12 7Z 206 205

16 39 9 2U U'5

10 H H 11 H 72 12 )J )9

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, 0 U9 , o2 II ' 0

119 122 57 '5'5

317 )8 1 41 41 I~' U5

59 60 9 110 129

10 115 U 11 ~9 98 12 25 26 13 18 8l 14 .. I to)

15 58 58 U U 49 " , " .

2 1 0 20'f 115 111 "I 90 91 87 55 55 51 '55 all 84

120 111 72 1I

9 12 85 10 Q2 9)

11 25 II 1 2 19 19 I} III 16 U 14 15 15 69 '6 U 13 Il 17 11 12

5.K.2

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11 19 135 154

b "2 "2 7 U) HI 8 I) 14 9 "0 19

10 U 4 5 11 50 52 12 )I H 13 11 7Z 14 25 27 l'5 12_ U lb 21 )0

11 ' 5 46

III III 139 14) l' 111 15 h III 8 1 ao 81 2lt 22 115 115 21 2<J

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b H )5

1 "I 98 8 24 2 4

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10_ 0 19 11 10- 5

10 4 101 16 14 2" 11 '2 43 51 51

> 9 ' 9 53

10 11 Il I I 1 ~

30 It 66 12 30 H )1 41 22 22 40 U 12- 10 12_ 1 11 1'5 38 41

11,11..2

12_ 0 24 25 12_ , JI 34 te 11 Il- 1 I

21" 2 U )5 U

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U8 110 20. 21'

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1 121 I 20. 21n ~ 205 205

to 3) 21 11 26 30

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12 2 120 10_ 0 21 30 11_ 5 21 23 53 53 ..

I.K.3

6,0 '0 !!I8 n

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41 ' 8 161 113 181 U5

17 80 9 125 121

10 22 20 II 59 '51

12 41 " 13 104 l 'n H 50 51 15 43 It)

U U 18 11 '54 53 11 22 22

l ,K , 3

211 2 15 120 122 119 Ub 14' 148

1 0 11 " , 1 20 120 151 U2 1 5) 1'59

9 )5 35 10 l' )8

11 17 Il 12 20 19 13 5) ' 0 14 9 0 91 15 50 46 lb l' )8

11 6 1 6' II 30 26

, . 0 196 198

94 9)

11 2 175 144 141 104 10) In U8 14. 148 6) b5

9 12 I) 10 ) 1 12 11 67 68 12 611 6' Il 11 11 1111 2. 29 15 5' 53 Ib 12_ • 11 45 4 2

,.It .)

231 231 5111 '54

220 220 2b 22

149 1'53 12 7 II I

I ' 86 11 11

10) 9'f 9 131 Ub

10 1 06 10et 11 35 38 12 15 12 13 15 12 14 36 H 1'5 12- 8 Ib II H 11 54 5 2

, . 4 ) 43

102 10 ' 111 III II 89

114 lIet 1. 79

14 " H H 9 n q2

10 25 2b \I bt 51 12 )0 11 1) 57 51 14 29 12 15 3) II 16 21 11

IU UI 64 66 41 41 66 10 21 21 21 2 1

10 1 10' tU 115 2l 25

9 40 39 10 )9 39 11 25 20 12 2 9 29 13 21 )1 14 )1 J1 15 26 23

.. . 101 111

It) " 29 21 78 80 14 lS n 14 78 11

I 3b 38 9 2 5 2l

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9 5 1 52 10 13 16 11 19 2 1 12 t5 12

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53 5S 24 H 16 14 12 - 5

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15 :1 H7 1 ~ 1 IH 2) 20

~ tl 14 10 148 " , 1 2 ~ 7 11 4 1) 7, H

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101 10 " lH 165 )) 3Z

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11 ):l 31 12 . 0 U 13 )l 28 14 U 47 15 29 H U III 11 11 15

2:)5 201 1~3 U2 117 182 ' 9 53 12 80

107 III 214 220 26 25 H 15

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12 0 121 115 1) 3

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I ' ) ' ) 9 U 51

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"II 91 " 112 bI

10 " 11 ' 2 12 )to H I ) 16 III 14 59 6 0

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51 '51 102 10 ' 80 8' II 10

6 51 52 1 41 4) I 25 29 9 29 II

10 U 11 11 12_ 10 t2 U q

" iii) 4. 18 19 92 9' 32 311

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26 25 19 19

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III 111 42 4)

~ 11 \1 10 20 15 11 1) H 12 25 29 1) 41 49 I.. )8 39 15 25 2'

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9 5 97 98 1 00 65 lIS ' II 41 511 51 12 12

6 4 11 U 7 55 56 8 11 I b 9 H ))

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21 30 3 0 H U 17 ..1 42 31 n 2b 1)

3 " )4

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12 22

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2 1 2 5 bO '2 1) 10

9 20 2 1 I () 81 12 11 11_ " 12 b8 lit U U 4 2 14 '6 44 15 211 20

l.k., 9) 90 55 53 1) 11 29 II

124 119 H 11 U U 5' 61

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~ H H 10 H )9 11 II) lit 12 51 51 13 U zq l' 32 31

112 III III 12

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, " " " "

" "

" " " " " JJ

" " " " " H

II .. " H

" " " " " " " " "

.. " 41 51 '5'5 79 lq 28 32

9 ) ' )5 10 3) H II 40 "2 1 2 U 16

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O.K.'

90 ee 17 zq

UI 15b 11 It

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9 H )'5

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9 25 2 1 10 h 12 I I 25 24 12 39 )" II 10 21

9) ee 10_ 10

'1 '4 20 14 13 2" . 2 )9

4' 4 2 H 18

I U ,. ~ H III

10 19 J1 II 3~ n 1 2 25 24

4' U U H 2 ' 2' 15 H 6 ' U 6' lIS 2' l b

I 2 1 21 9 )2 31

10 18 1'5 11 H 34 12 )) 2 ~

81 78 2 1 2 1

101 101 30 21 4 9 U )1 32

, 22 22 7 ' 0 U 8 35 l ' 9 31 21

10 41 ' 0 II 32 ) 0

,. 20 11 4 2 41 II_ ,

II_ " 4 ) 42 III) 43 12- 10

q U - 12 10 )0 25

6,1t,1

82 10 " , II_ 5 U 21 U 12 39 lb 48 U 2. n 19 11

' .1t ,7

" 36 11 2l 24 49 41 )II )7

" O.K.'

" 11- 0 /» U 20 \6 20 U 41 41

I. K.8

11- " l8 U 4 0 3q l8 l8 21 2' " U )1 U 18 I ~

~ ) 9 )8

10 2 ' 22

1 11 11 1 2 65 3 ~ 32 ~II U 1 5 I I 58 51 7 2 61 II \6 H 14 I)_ t2

l, .. ,a

11_ 2

25 2l 4 2 19 12 - II 25 23 39 )7 3 8 )b

15 18 ]I H

, I b " H )0

" "I 44 1118 4 5

" ' 9 ' 3 H 21

U 13 11 21 211 2b 31 31 n H

b . ... 1

ll_ 11 '4 5 1 4 2 ' 0 4 2 4 0

70 61 22 I ' II II 21 11 1 4 U 28 24

" 2 0 16 13_ ~

51 50 24 22 25 21 26 21

" 4' 42 68 U 11_ 6 21 24 25 2l

20 12 15 II )5 )l

a Th e column s are k.l0F o . l0Fc • "\jnobserved" refl ections are marked by Fo and Fc are on an absolute scale.

TABLE 3. The hydrogen positions in Na,C03 'H,O

Weighted Least Alam diffe re nce squares Calculated*

synthes is refinements

x y z x y z x y

H(l ) 0.12 - 0.07 0.14 0.11 -0.06 0.15 0.134 -0.056 H(2) -.08 -.08 .14 -.10 -.08 .12 - .077 -.089

* Assumin g the geomel ry of free water with O-H= 0.958 A and L H-o--H = 104.5· , a nd mak ing the o--H . as linear as possible .

321

z

0.190 .084

. 0 hyd roge n bond s

01

FIGURE 1. A stereoscopic iLlustration of Na2C0 3 • H2 0 viewed aLong a. The urigin of the uni t cell is marked by the aste ri sk.

TABLE 4. The CO" group

Atoms

C, O(l ) .................. . C, 0(2). ................ . C,0(3) .................. . 0(1 ), 0 (2) . .... ... . .. .... . 0(1), 0 (3) ....... ..... .. . . 0 (2), 0(3) . .. . O(l), C, 0 (2) .......... .. 0(1), C, 0(3) .......... .. 0(2), C, 0(3) .......... .. 0 (1), Na(l) .. ... ........ .

Oil), Na(2) .. .. .. 0 (1), Na(2 ') .. .... ...... .. 0(1 ), 0 (4) ...... .. ...... .. 0(1 ), H(l). .............. . 0 (2), Na(l) ............ ..

0 (2), Na(l') ............ . 0(2), Na(2') ............ . 0(2), Na(2) .......... .. .. 0(3). Na(l) ...... .. 0(3), Na(l') .. . 0(3), Na(2) ............. . 0 (3),0(4) .............. .. 0 (3), H(2) .. ............ ..

Di sta nces, A, or angle , deg.

1.299(3) 1.274(2) 1.285(2) 2.223(2) 2.229(2) 2.230(2)

119.6(2) 119.2(2) 121.2(2)

2.570(2) 2.293(2) 2.307(2) 2.907(2) 1.96 2.521(2) 2.468(2) 2.508(2) 2.491(2) 2.444(2) 2.330(2) 2.822(2) 2.685(2) 1.74

In all tables of inte ratomic di stances and angles, the quantiti es in parentheses are s tandard e rrors in the last s ignificant fi gure and were computed from the standard e rrors in the atomic positional parameters and in the cell parameters. The primes refer to atoms in fi gure 2.

FIGURE 2. The carbonate group environment in Na2 CO,, ' H2 0. The primes refer to atoms in tab le 4.

ti on about Na(l) cannot be considered octahedral. The nex t oxygen ill the direction of the octahedral apex is the water oxygen 0(4'), which is relatively far (3.669 A) from Na (1).

The coordination of Na(2) is similar to that of Na(I). Na(2) is slightly displaced from the center of an ap­proximate square of carbonate oxygens, 0 (1') , 0 (2") , 0 (1"), 0 (2'"). The displacement is towards the strongly bonded water molecule 0 (4), which is the

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::<

l t

03

FI GU RE 3. The water and sodiu m environments in Na,CO, · H 2 0. The pri rn e~ refer 10 atoms in Tables 5 and 6.

TAB LE 5. The sodium environments

Atoms Distance, A

Na(l) , 0(1) ........... 2.570(2) A N a(l) , 0(2) . . . . . . . . . . . . . 2.521(2) Na(l), 0 (3) .............. 2.444(2) Na(l). 0(4) .............. 2.434(2) Na(1), 0(2') .......... 2.468(2) Na(l),0(3') ... . .. .. .. .. 2.330(2 ) Na(l),O(4') 3.669(2)

Na(2), 0(4). 2.384(2) A Na(2), 0(1' ) ....... . 2.293(2) Na(2), 0(1") .. . 2.307(2) Na(2),0(2") .... . 2.508(2) Na(2), 0 (2'") . ... 2.491(2) Na(2), 0(4") ....... . .... 2.936(2) Na(2),0(3') ............ 2.822(2)

Th e prim es refer to th e aIoms in fi gure 3.

apex of a square based pyramid (base down in figure 3). Theo coordination of Na(2) is completed by 0(4") (2 .936 A) which is th e re maining apex of an approxi­mate octahedro n, and by 0(3') which is in the same CO:1 group as 0(2"). Thus the CO~ group is coordi­nated edgewise to Na(2) also , this time using 0(2) and 0(3) in stead of 0(1 ) and 0(2) , whi ch are used to coordinate to Na(l). This edgewise coordination is shown in figure 2. The coordination to Na(2) comprises five carbonate oxygens a nd two water molecules ins tead of four carbonate oxyge ns and two water molecules as sugges ted by Harper [lJ and noted by Wells [15].

and Na(2) with distances of 2.434 and 2.384 A respec­tively , and form s hydrogen bonds to oxygens 0(1) and 0(3) of neighboring CO:! groups. Na(l) , Na(2), 0(1) , and 0(3) are arranged ap proximately tetra­hedrally about the water oxygen. The distortion of this tetrahedron is considerable as can be see n from the angles listed in table 6. The closes t H .. . Na distance is H(I) ... Na(2) = 2.42 A, which is in the normal range.

TAB LE 6. The water environment

Atom Distan ce A or a ngle, deg.

0 (4),Na(1 ) 2.434(2) A 0 (4),Na(2) 2.384(2)

0(4),Na(2') 2.936(2) 0 (4),0(1) 2.907(2) 0(4).0(3) 2.684(2) H(1),O(l) 1.96 H(2) ,0(3) 1.74

Na(l ),0(4),Na(2) 91. 15(6)0 0 (1),0(4),Na(1) 109. 70(7) 0 (1),0 (4),N a(2) 133.83(9) 0 (3),O(4),Na(l ) 129.28(9) 0 (3),0(4),Na(2) 110.29(7)

0(1) ,0 (4),0(3) 88. 12(6) 0 (4) ,H(l ),O(l) 168. 0 (4), H(2),0(3) ]67.

Th e prim e refe rs to an a tom in fi gure 3.

The calcul ated hyd roge n pos itions in table 3 were obtain ed usin g the geometry offree water and imposing the co ndition that th e O-H . .. 0 a ngles both be as linear as possible. Because H(2) ' 0 ' • 0(3) is shorter (1.74 A) than H(l) ... 0(1) (1.96 A) it is possible th at the hydrogen bond 0(4)-H(2) ... 0(3) is s tri ctly lin ear. Assum ing th e same 'Yater geo metry, the hydro­ge ns would then be = 0.14 A away from the positions given in table 3 at 0.145 , -0.046 , 0.182 for H(I) and - 0.065,-0.095 , O.102for H(2). The 0(4)-H(l) ... 0(1) angle would then be 156°, and the H(I) <I _ • 0(1) an,d H(2) . _ . 0(3) distances would be 2.01 A and 1.73 A, respectively. The closes! H ... Na distan ce would be H(l) ... Na(2)=2.39A.

Collection of th e diffractometer data was made poss ible through the cooperation of E. C. Prince. The x·ray 67 syste m of computing programs (J . M. Stewart , University of Maryland, Editor) was used for most calc ulations. We thank Joy S. Bowen and Pamela B. Kingsbury for technical help.

3.3. The Water Environment This investigation was supported in part by researc h

grant DE- 00572- 09 to the American De ntal Associa· tion from the National In s titute of Dental Research and

The water en vironment is given in table 6 and shown is part of the de ntal researc h program condu cted by in figure 3. The water molecule is bonded to Na(l) the National Bureau of S tandards, in cooperation with

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th e Council on Dental Research of the American Dental Association; the United States Army Medical Research and Development Command; the Dental Sciences Division of the School of Aerospace Medicine, USAF; the National Institute of Dental Research; and the Veterans Administration.

References

[1] Harper. J. P. , Crystal structure of sodium carbonate mono· hydrate, Na2 CO,,' H2 0 , Z. Krist. 95,266-273 (1936).

[2] Dickens, B., and Brown , W. E., The crystal structures of CaNa2(CO,,), · 5H 2 0 , synthet ic gaylussite, and CaNa2(CO,,)2 '2H20 , syntheti c pirssonite, Inorg. Chem. 8 , 2093-2103 (1969).

[3] Brown , W. E., Crys ta l structure of octacalcium phosphate; Nature 196, 1048- 1050 (1962).

[4] Brown , C. J" P eiser , H. S. , and Turner·Jones, A. , T he crystal of sodium sesquicarbonate , Acta Cryst. 2, 167- 174 (1949).

[5] Bacon , G. E., and Curry, N. A. , A neutron·diffracti on study of sodium sesqui carbonate, Acta Crys t. 2,82- 85 (1956).

[6] Candlin , R. , Thermal changes in the structure of sodium sesquicarbonate, Acta Cryst. 9,545-554 (1956).

[7] Pabst , A., On the hydrates of sodium carbonate , Am er. Min. 15, 69-73 (1930).

[81 Reim,"", C. W. , Migh,lI , A. D. , "dM"",, F. A., The 0<",,1 l and molecular structure of te tra isopyrazole·nickel chloride, Ni(C 3 H4N2 )4Cl" Acta Cryst. 23, 135-141 (1967).

[9] Mauer, F. A. , and Koenig, A. L., An automatic diffractometer for off·lin e operati on. American Crystallograph ic Association , Summer Meeting, Abstract E1O, University of Minnesota, Minneapoli s, Minn. Aug. 20- 25 , (1967).

[101

[12]

[13]

[14]

[1 5]

International Tables for X·ray C rys tallography 3, p. 202 (The Kynoch Press, Birmingham , England , 1962).

McWeeney, R , x·ray scattering by aggregates of bonded atoms. I. Analytical approximations in s ingle·a tom scattering, Acta Cryst. 4,513-519 (1951).

C hastain. R. Y., An algorithm for findin g a set of phases directly from sigma two relation ships, In , x·ray 67·Progra m System for x·ray Crystallography, T echni cal Report 67-58, Ed. J. M. Stewart, pp 71-75, Univers ity of Maryland , College Park, Md. (1967).

Palache, c., Berman , H. , and Frondel , c., Thermonatrite [Na2C03 • H 20], The System of Mineralogy of J. D. Dana and E. S. Dana , 7th ed., 2, p. 224 (J. Wiley and Sons, New York , N.Y., 1951 ).

Winchell , A. N. , and Winc hell , H., The Microscopi cal Char· act ers of Artifical Inorganic Solid Substances: Optical Prop· erties of Artifical Mine ral s (Academic Press, Ne w York, 1964).

Wells, A. F. , S tru ctural Inorganic Chemi stry , 3rd ed. p. 587 (Oxford Unive rs ity Press, London , 1962).

(Paper 74A3-603)

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