Zeitschrift fUr Kristallographie, Bd. 142, S. 129-141 (1975)

A neutron diffraction study of mercury(II)chromate

hemihydrate, HgCr04' t H20

By KARIN AURIVILLIUS and CLAES STALHANDSKE

Chemical Center, Divisions of Inorganic Chemistry, University of Lund*

(Received 12June 1975)

Auszug

Die Kristallstruktur des Quecksilber(II)chromathemihydrates HgCr04.,H20 wurde mit Hilfe von Neutronenbeugungsaufnahmen eines Einkristalles(aus 870 unabhangigen Reflexen) zu R = 0,030 und Rw = 0,023 verfeinert.Die Kristalle sind monoklin, die Raumgruppe ist 02/c, mit den Zellparameterna = 11,832(1), b = 5,2616(6), c = 14,637(2) A, fJ = 121,01(1)° und Z = 8.

Das Quecksilberatom ist nul' an zwei Sauerstoffatome gebunden, jedeseinem separaten Chromattetraeder zugehorig, mit den Abstanden 2,055(2) und2,064(2) Aund dem (O-Hg-O)-Winkel von 179,95(5)°. Da jedeChromatgruppe

an zwei Quecksilberatome gebunden ist, sind die fundamentalen Bauelemente

del' Struktur, endlose Ketten del' Zusammensetzung (HgCr04)n, annaherndparallel del' Diagonale del' ab-Ebene laufend. Die Ketten werden durch Wasser-stoffbriicken zu BlOcken von del' Zusammensetzung (HgCr04 . !H20)n und del'Dicke! c verkniipft. Zwischen den BlOcken besteht cine schwache Wechsel-wirkung, die fUnf Quecksilber-Sauerstoff-Abstande variieren zwischen 2,655und 2,743 A.

1m WassermolekiiI sind die Abstande O(W)-H(2 x) und H-H 0,945(4)bzw. 1,487(6) A. Del' Winkel H-O(W)-H ist 103,7(4)°. In den Wasserstoff-briicken ist del' Abstand O(W)-H..'O zwischen Wassersauerstoff undChromatsauerstoff 2,961(2) A, del' fiir die Wasserstoffbriicke H...O 2,133(4) Aund del' Winkel O(W)-H"'O 145,6(3)°.

Abstract

The crystal structure ofmercury(II)chromate hemihydrate, HgCr04 . !H20,has been refined from neutron single-crystal data (870 independent reflections)to R = 0.030 and Rw = 0.023. The crystals are monoclinic, space group 02/cwith a = 11.832(1), b = 5.2616(6), c = 14.637(2) A, fJ = 121.01(1)° and Z = 8.

* S-220 07 Lund 7, Sweden.

Z. Kristallogr. Ed. 142, 3/4 9

130 KARIN AURIVILLIUS and CLAES STALHANDSKE

The mercury atom is bonded only to two oxygen atoms, each belonging toa separate chromate tetrahedron, at the distances 2.055(2) and 2.064(2) A, theangle O-Hg-O being 179.95(5)°. As each chromate group is bonded to twomercury atoms, fundamental building elements of the structure are endlesschains of the composition (HgCr04)n running approximately parallel to thediagonals of the ab plane. The chains are fused by hydrogen bonds to blocksof composition (HgCr04 . tH20)n with thickness t c. There are weak interac-tions between the blocks, five mercury to oxygen distances varying between2.655-2.743 A.

In the water molecule the distances O(W)-H (2 x) and H-H are 0.945(4)and 1.487(6) A respectively. The angle H-O(W)-H is 103.7(4)°. In the hydro-gen bonds the distance O(W)-H'" 0 between water and chromate oxygen

atoms is 2.961(2) A with a distance H... 0 of 2.133(4) A and an angle

O(W)-H...O of 145.6(3)°.

Introduction

The crystal structure of HgCr04' lH20 was determined earlierfrom x-ray diffraction data (AURIVILLIUS,1972). The structure iscomposed of endless chains of the formula (HgCr04)n with watermolecules in cavities.

It was not possible to locate the positions of the hydrogen atomsfrom the x-ray intensity data available. Some possibilities for thehydrogen bonds were, however, discussed but could not be confirmedat that time. The present neutron-diffraction study was undertakento refine the structure and to elucidate the hydrogen bonding system.

Experimental

An aqueous solution of CrOs (0.50 M) was boiled with a deficit ofyellow HgO (molar ratio HgO : CrOs = 1: 2). By slow evaporation the

crystal used for the neutron-diffraction study was formed. Themorphology of the crystal is given in Fig. 1. The crystal dimensionsare presented in Table 1 and some crystal data in Table 2.

The intensity measurements were carried out at room temperatureon a Hilger & Watts computer-controlled four-circle diffractometerlocated at the R2 reactor at the Swedish Atomic Energy Company,Studsvik. The neutron flux at the crystal was about 106 cm-2 S-1 andthe wavelength of the monochromated neutron beam was 1.210 A.A quadrant in the reciprocal space up to (sin 0)/ A = 0.693 A-1 was

examined using the w-20 step-scan technique. Each reflection wasmeasured for approximately 20 minutes. The most intense reflectiongave a net intensity of 57500 counts. Three standard reflections weremeasured at regular intervals as a check of crystal and electronic

Plane d Plane

(100) 0.90 mm (lI0)(lOO) 0.90 (rIO)(110) 0.90 (101)(HO) 0.90 (lOl)

A neutron diffraction study of mercury(II)chromate hemihydrate 131

(11O)

\

(JOO)

Fig. 1. The morphology of the crystal used in the data collection. The planes(101) and (10l) are parallel to the plane of the paper, with (101) in front

and (10l) behind

stabilities. No significant fluctuations in their intensities were observed.Out of the collected 1081 independent reflections 211 were consideredunobserved as they gave I values less than 2 Gc(I), where I was thenet intensity and Gc(I) the standard deviation based on counting sta-tistics. The intensities and their variances were corrected for Lorentz

Table 1. Orystal dimensionsBoundary planes and their distances from an internal origin

d

0.90 mm0.900.740.74

Crystal volume = 5.5 mm3.

Table 2. Orystal data

HgCr04 . tHaO, F.W. 325.6Monoclinic 02jc; a = 11.832(1), b = 5.2616(6), c = 14.637(2) AfJ = 121.01(1)°; V = 780.99 A3*Z=8Dm = 5.51, Dx = 5.53 g . cm-3

Neutrons, A = 1.210 A; fI = 2.84 cm-1

*Misprint in previous publication (1972).

9*

Table 3. Final fractional atomic positional parameters from the neutron refinementwith estimated standard deviations in parentheses

In brackets are given values according to AURIVILLIUS (1972)

Atom x y z

Hg - 0.00110(5) 0.02619(9) 0.12474(4)[- 0.00126(10)] [0.02676(12)] [0.12468(8)]

Cr 0.19953(14) 0.4391 (3) 0.09833( 11)[0.1999(3)] [0.4385(4)] [0.0989(2)]

0(1) 0.36707(10) 0.3913(2) 0.16566(8)[0.3687(17)] [0.393(3)] [0.1646(14)]

0(2) 0.16291(13) 0.6174(2) 0.16827(10)[0.161(2)] [0.619(3)] [0.1660(16)]

0(3) 0.12129(10) 0.1514(2) 0.07324(9)[0.1195(17)] [0.156(3)] [0.0728(14)]

0(4} 0.15138(11) 0.5733(2} - 0.01409(9)[0.149(2)] [0.576( 4}] [- 0.0161(16)]

O(W) 0 0.3967(3} t[0.399( 4)]

H 0.0725(3) 0.5076(6 } 0.2878(2}

132 KARIN AURIVILLIUS and CLAES STALHANDSKE

and absorption effects. The linear absorption coefficient 2.84 cm-1 wascalculated assuming a value of 40 barns for the incoherent scatteringcross-section of hydrogen. The mass absorption coefficients for theother elements were taken from International tables for x-ray crystal-lography (1962). The resulting transmission factors, evaluated bynumerical integration, varied from 0.54 to 0.73.

Structure refinement

The position of the hydrogen atom was found from a three-dimen-sional difference Fourier synthesis using the coordinates of mercury,chromium and oxygen given in the x-ray study (AURIVILLIUS, 1972).Full-matrix least-squares refinements were then performed minimizingthe function L;Wi(iFo 1- IFe 1)2with the weights calculated from theexpression Wi-1 = a2(IFol) = ae2(IFo12)/4IFo12 + alFol2. With a =0.00005 the average value of L;Wi(IFo I-We 1)2was nearly constant

AtomI

fJll I fJ22 I fJ33I

fJ12 I fJ13 fJ23

Hg 276(3) 0.0137(2) 209(2) -130(6) 153(2) - 14(5)Or 203(12) 0.0097(5) 168(8) - 51(19) 97(8) 1(15)0(1) 224(8) 0.0194(4) 248(6) - 77(13) 117(6) 139(12)0(2) 640(12) 0.0161 (4) 445(7) 27(16) 366(8) -195(14)0(3) 361(9) 0.0144(4) 308(6) -247(14) 223(6) -126(12)0(4) 460(10) 0.0216(4) 262(6) 55(17) 126(6) 322(13)O(W) 621(16) 0.0134(5) 288(9) 0 219(10) 0H 0.0099(3) 0.0329(10) 0.0077(2) -0.0069(5) 0.0031(2) -0.0027(4)

Atom Rl R2 R3

Hg 0.100(1) .A 0.129(1) .A 0.145(1) .AOr 098(3) 116(3) 120(3)0(1) 102(2) 133(2) 177(2)0(2) 122(2) 169(2) 199(2)0(3) 106(2) 139(2) 169(2)0(4) 118(2) 161(2) 200(2)O(W) 137(3) 149(2) 182(2)H 172(4) 253(3) 279(3)

A neutron diffraction study ofmercury(II)chromate hemihydrate 133

in the different intervals of IFo Iand sinO. The Fo values were correctedfor extinction using the expression

I I I I [2'P'jFcI2.g'A,3

]

114

F0 corr = F0 1 + V2. sin 20'

where g is the isotropic extinction parameter (ZACHARIASEN, 1967;COPPENS and HAMILTON, 1970), A. is the wavelength in A and V thevolume in A3. The mean path-length T through the crystal is definedby T = -In Alfl, where A is the transmission factor and f-t the linearabsorption coefficient. The final refinement including anisotropicthermal parameters for all atoms and the isotropic extinction coef-ficient, converged with the agreement factors

Table 4a. Final anisotropic thermal parameters fJi} with estimated standarddeviations in parentheses

The expression used is exp [-(fJllh2 + fJ22k2 + fJ33l2 + 2fJ12hk + 2fJ13hl + 2fJ23kl)].

The fJi} (except fJ22 and fJH) values are multiplied by 105

Table 4 b. The root-mean-square component,y, Ri, of thermal displacement alongprincipal axes of the ellipsoids of vibration

134 KARIN AURIVILLIUS and CLAES STALHANDSKE

Table 5. Observedand calculatedneutron structure-factoramplitudes (x 100)

1 IF.I IF.I IF.I IF.! IF.I [F.I IF.I IF.I 1 IF.I IF.I IF.I IF.I IF.II F.I 1 IF.I IF.I

° °1

8 325 320 _12')9 272- '°5 '02 6 5'7 560 -, ''0 "2 -2 16' 16, _1 656 652

9 315 '28 0 590 586 ,,6'01 -8 7," "0

_12'7 225 12

'98 395

"502 ,,6 1 '02

,68 :H5 359 5 1 1 -6 '90 175°

'89 '089 1,

1259 1263 12 80, 790 2"7

)'5 "272 -19 182 168 -5 851 862 1 28} 282 _20 978 1006

6 '78 '8, 13 153 122 , 751 745 20' 176 _18 285 270 -,"3 '"

230 2'9_18

'9'230

8 796 787"

237 255 , 506 its:; 2/.0'"'

_16 208 162 -, 12, 55 , 298 '0' -17 165"910 16' 152 1] }07 333 5 267 256 178 169 -15 142 171 -2 "0 }',O

,80' 8°9

_16 1060 106)12 532 533 1 5 1

6''0

341, , 7 1 -"257 261 -1 101,2 1030 ,. 176 18' -15 162

'""

278 259 8 2" 280 -n 290 283°

1387 138) 7 "9 '96 -"228 "516 520 507 -12 283 296 , 233 2U4 -5

'"1')1 -12 11134 1447 1 685 678 8 700 701 -1J 24/) 211

°2 1 -" '07 '°9

1O 155 163 -, 162 180-"

157 "3 2 )1,1, 336 9 602 602-" "0

,8B->0 '33 346

"511 SliU -2 "3

332 _106" 669 3 '22 "9 -10 288 283

0 1144 11/,7 -9 4~4 463 12"8

1,66 -1 169 B1 -9 '99 '95, 691 6"7

7 5 1-9 )0') ,'o

2 335 322 -8 831 8}O2 6 ,

°1

_8 lI,la9 1452 5 '08 "7 -1' 462 "8_6 721 730, 570 562 -, 237 273 1 -7 621 6jo 8 7" 76J -12 207 203 -', 41.4 '52, 898 89' -b 227 2J2 -8 J53 350 -16 1229 1219 -5 55' 55!' 9 198 173

-"167 169 -, 5" ",5 255 2'5 -5 ''" '" "3 407

-" "3 '"-, 1I.1t91459 10

2" 17') -9 1002 999 -2 357 '596 241 228 -, 444 ", -r, 322 330 -12 1679 1674 -3 33' 319"

251 26;1 -8 717 717 _133'

,,,8 1091 1107 -, 558 573 -:J '06 399 -10 525 529 -- "0 '21 6 ,

1 -7 2872"

°1152 1169

9'0'

,B6 -2 '"8 275 -, ,B6 ,81 -8 1578 '586_1 221 221 -5 30] 299 1 375 ,66

10 165 159 -I 585 611 -, 11.8 6J -6 372 3"0

"711') -16 635 653 -', 222 221 253 208

12 %13 39'°

867 859 -, 461 %65 -', 6}0 638 I 18" "8 -15 765 761 -, 21,22" 1192 1209

13'"

707 1 '06 281. _I935 943 -2 81, 811. , 127 1/.5 -13 727 732 0

'"%0') 249 211

"352 337 2 '96 50'

0 42; %26°

1.)5

'" J '"59 -12 1212 1209 1 508 suo 230 22J

IS 167 ,", 172 159 1 225 2J9 2 18' 187 503 501 -11 '16 1i10 637 6J6 725 7"16 1.28 412 5 700 688 , 891 869 , 601 60' 8 523 511 -10 '99 222 8

°1 9 , 17 '90 171

,581 580 6 1lit 5 1146 9 ,,} 464 -9 26, 2970 ,

1 8 407 398 5 2lt5 208 8 1109 1112 10 397 '03_6 21<) 223 -20 1024 1029 -18 390 J70

739 725 9 5910 60' 7 ", "8 10 20' 218 11 160'"

-5 '06 1015 -18 153 ," -16 '21'"116 108 10 1.05

39' , 1 12 1035 1038 12 76'} 77'} -, 1')7 136 -16 633 6}0 -15 435 ,"3610 378 11 571 }61", 182 22' 5 , 1 -, 223 20' -12 620 608 -1} 21' 226

571 5')') -17 295'"

-2 136 1'}4 -10 81,0'"

_12 809 8105 701 692 1 ,

1 _16 6310 6JO, 2 1 -11, 89' ,07 -1 '46 458 -8 791 789 _10 181 181

6 271 2BJ -5'" '"

-15 ,,6'26 -16 8" 8" -15 ,,6

'81 0 159 173 -6 205 186 -9 7"7)6

7 237 242 -, 506 533 -12 717 707 -13 '97 155 -13 167 127}

J70 367 -, 10)!. 1028 -8 1239 12%28 BJO BJ2 -2 290 305 _11 ,60 373 -12 827 8J2 -12 '02 '09

,896 886 -2 382 385 -7 ,28 ,J1

9 257 267 _I'"

,,, -105"

~)105 _11 780 773 -10'" "0 5 '89 392 u 166%1648 -, ,8' ,,,

10 263 2)0

°539 535 -9 705 697 -10 14' 182 -, 166 150 6 3%5 ,46 , 287 287 -,

7'3 72911 256 262 1

6" 592 -8 91.69'° -9 207 205 -7 908 910 , 534 522 6 178 155 -3 5"

53012 1163 1166 3 '90 506 -7 J7, ,81 -8 "0 862 -5 555 578 8 679 672 8 371 39' -2 1'.0 12313 MH 660 4

'"',,, -6 }18 '27 7" 772 -, 270 278 , 161

"9 8 2 1_I 688 6BJ

°6 1 2 0 1

-} 9'9 559 -6 132'"

-, 127'0' 6 6 1

0 185 171-,

"0171 -5 196 20' -.

'"112 -19 166 165 1 228 21'241. 208 -18 JO}

'08 -, V3 2h5 -, 1214 1242 _1'" '08

_11 245 ", -1' 242 26} 2 176 178226

'99 -16 989 980 412 ", -, 117 '35 0 1050 1044 -10 269"9 -16 5'JO 6"

, 1510 153"5 535

-" 25' 260 -I 323 330 -2 208 219 I 789 786 -9 730 ,,6 -15 521 509 9 5"7 '0' -12 11<)2 1192

"1327 1292 _I 169 189 "8 285 -8 268

'"' -"253 225,,, 332 -10 979 973 1

'"731

°879 872 , 666 663 -, 547 550 -I' 892 889 -13 "9 235

'"J372 -8 290

'"0 '39 44.~, I 267 263 , 1207 11'}8 -6 '90 15') -12 1075 1()76 -12 671 678}22 517 _6 288 286 476 '71

, 191 '91 7 20' 221 -5 310 '12 -10 34<)'"

_11 721 715-, 4')2 1056 11.70 1449 , 707 702 8 68' 678 -, 665 671 -, 216 216 -10 318 28J

331 ,,,~673 661 5 11}') 1157 9 '92 '"

-, 332 335 -8 866 868 -9 516 5'7-17 "5

472°

"0 "91,88 4<)3

(,735 7"

10 173

'"-2 1.40

"9 -7 1034 1032 -8 825 831_16 1045 1()39 2 607 606 8 1163 1169 8'" '"

11 278 273 _12'.6 186 -r, 179 206 -7 529

5"-"

146 102 ,9'° 9" 10 554 5103 9 66,

6" 5 5 11 26, 279 -5 120 152 -6 523

'"-', 251 2102 6 ,"8 "0 11 221 200 10 187 205 2 22(, 220 -, 55', 555 -5 ,16 723_12 956 961, 8 1738 1767 12 806 815 11 336 338-" ''" '"

, 1 1-, 1015 1015 -, ,25 716_11 115 7') 12 465 '152

"275

2(;_) 12 564 521) -1J 299 312 -2 '374}0 -2 221 lB5_10

'97 396

"'9'

232'5

:!o(, ::!1t! , , -10 271 260 -20 "9508 -1 561 558 -1 ,81 ;J79

-9 218 205 2 2 1, , _8 278 298 -18 H;9 161

°786 780 11 208 216

-8 367 371 -16 172 196 -, '25 '18 -1' 809 '85 1 361'58

1 ,11 ,,,-6 176 '50 -17 22J 205 -17 175 121 -15 203 201 -5 "7

178 -1(, 527 523 3 52' 516 10°

1-5 97' 989 -16 552 5"5_16 277 293

-" 28' 280 -, '56 460 -12 676 668 ,'05 ,"8-, 555 555 -15 lJB 1}8 -IS 618 632 -11 266 2102 _2

5'7 5'6 -11'" "0 5 534 559 _20

'" ,"-, 367 368-"

177 136-"

182 18' -IU 110 112 _I JOI '12 -10 319 312(,

233 25' -16 1429 1429-2 763 771 -1' B36 850 -13 17 ~I 235 -9 151

'" °79' 793 -9 2JO 232 7 162 206

-"}91 ,"2

°999 990 -12 712 727 -12 2')0 293 -8 175 '58 1 ,,6

82' -8 700 723 8 621 61J -12 1255 12%51

'"233 -10 254 239 _11

"0 '75 -7 262 26) 2 1510 162 -,'" '57 9 298 255 _10 151 136

2 '52,,6

-9 688 686 -9 732 72'} -6 5'6 560 3 101,,0 1030 -6 212 190 8 , -8 }98'°5,

'91 18' -8 "8 "2 -8 1002 lOOy -5 158 "2, 881 892 -5 621 6}}

-6 1034 1031, 1879 1879 -7'"

'68 -6 '0'7HJ -, 1310 13100 5 165

"7-, 1921 1875 -16 531 531 -2 '55

'"5 15' "'-5 751 761 -5 B38 855 -, 80' 816 7 505 ", -2 278 257 -15

'"105 0 562 580

6 317 }" -, 1104 1121 -, 8B1 891 -2 '55 3" 8 616 609 -1,,,

33' -1'42} '12 2

'" '"7 821 824 -3 992 1013 _I 911 913 _I 453 '66 6 0 10 71' 702 -12 1038 1040 ,

1480 110908 '00 301 _I 10110 1024

°11MB 1186 0 1336 1330 1 460 '78 -11 J22 }O1 6 797 819

10'01 392

°510 517 I 743 72<) 1 273 282 -20 33'

31t3 2 37'),6,

-10 383'80 10 2 111 16, 13') I 928 920 2 317 ,2O 3 '56 457 -18

,,8 157 3 251 275 -9 19} 18112 761 768 2 103

"

,"9 ,61 670 649 -16 261 261 '.1302 1'290 -8 581 586 -19 516 5'9

"36J 3/.4 3 2B8 275

, 522 520 5 561 5109 _I' '97,,0 5 34} 33' -, 572 586 -17

'" "31 , 1, 588 562 5 (,13 617 6 26' 293 _12 352 JI.9 6 266 295 -5 622 626 _16 }40 5J25 261 2Sl, 8

"3,01 ,

"7 186 _10"7 "3

, 257 23':1 -', 277 265-"

162 ,"_16 J25 ,,,8 l1}O 11210 10 136 72 8 '56 90 _8 1435 11036 9 365 359 -3 224 212

-1'770 75'

-I''"

10' 9 599 60' 11 '00 399 9 J75 '71_6

'55 '" 7 ,1 -2 lJ7 171 -12 7'5 716

-12 669 670 10"7 "7 12 5BJ 555 11

2"252 -, 1179 1183 _I 22, 2Hi _11

'9'237_11

'07 '08 11 835'"

3 5 1,

6 o 2210 211.3 -18 273 277 0

'"9' -10 179 181

-9 521 531 12 609 607 2 515 516 -1' 1093 1091 1 565 569 -9 5" 5"-8 11061 141)0

"'02 '16 -13 528 542 -9 '35 464 , 166 88 -16 758 765 2 1(,6 62 -8 9" 929

-7 '99 '97 ", 186 128 _12'78 '79 -8 182

"96 1,.67 '65 -1)

"6 1'5 3 '"9 '29 -7 48}'79

-5 932 9105 2 ,1

_11 278 267 -7 '72 '76 8 609 610-"

208 22\ ,'81 '93 -6 '6J ,"-, 976 997 _10

5°7 521 -6 581 586 12 569 57' -I' 2J5 '98 177 18' -, 905 897-, 61' 615 -12 "5 3" -8 7" 730 -, 293 262 6 ,1 -12 811 812 8 6 ~-, '0' 2"-2 506 510 -11 "0 '30 -7 697 682 -, 681 697 -11 606 631 -2 '77 '"-1 J20 325 -10 330

'"-6 ,56 ,82 -2 "8 173 -19 250 26' -10 J16 315 -10 363 36'

_12" '"°

7O, 698 -9 180 171 -5,,, ,,, -1 278 262 _16 675 665 -9 J60 354 -9 "6

'" °62J 62J

1 216 221 -8 500 508 -, 512 516 0 31',OB -15 '10 "9 -8 945 972 -8 '99 170 1 297 292

2'" '"

-, 8" 856 -, 33' "5 1 618 593-"

188 210 -,'9' 292 -, '59 150 , 821 8J}, 131 56 -6 '5' 35' -2 2J6 252 2 218 216 -1J 177 159 -6 '92 219 -6 '18 4'12 5 211 22'

5 178 189 -5 1059 1079 _1597 5"

,18' 127 -12 '99 "9 -5 797 795 -5 "0 '66 6 206 201

6 223 212 -4 1223 1245 0 '22 '02,

21' 228 -11'"

526 -, 589 609 -, '9} '"9 10 ,17 1052 10102 -, 59' 588 1 275 241 5 157 85 _10 167 98 -,

'"103 -2 261 266

Table 5. (Continued)

1I

F,II

F,I

1I

F,I

[F,I

1I

F,II

F,I 1

I

F,IIF,

I

1I

F,IIF,

I1

I

F,II

F,I

1I F,II',I 1 IF,

11', I

10 ,1 -12 1718 1707 -9

'8' '69 -8 1375 1374- 0 561 56' -10 202 180

"0 1 '5

1 !-11 '02 '06 -8 506 507 -6 '72 '58,

'95 209 -8 79' 78'-"

201 165 -10'8'

155 -7 '60 355 0 252 285 12 ,! -6 31' "8 -" IS' 133 -16 6'6 656

-"25' 26' -9 509 526 -5 609 611 2 351 355 -4 1137 1133 -12 508 506

-"266 2"

-I'259 26' -8 1037 1034 -, 969 967

,266 ,M

-" 20' '90 -, 53' S'8 -10 356 328-I'

222 235-12 186

'67 -6 285 270 -, 179 '57 12 2 !-12 '02 398 -1 18' 225 -8 1637 16'-1

_1119' 171

-11 '72'"

-5 298 '02 -2'0' '82 -11 ,'6 338 0 "0

'"-6 '0, 268 -9 19' 129

...626 625 -, 727 727 0 895 900 -1. 1'3,,8

-8 257 216 1 262 26, -4 1397 lltOl -8 '69 "7-. 966 957 -2 226 2" 1 86' 851 -17 559 575 -7 278 257 2 220

'"0 871 895 -7 ,'6 "7

-6 289 296_1

219 201 2 2562"

-16 856 8'9 -6 255 185I'

, !"

2 ! -6 576 582-, '95 515 0 292 292 ,

'7'9.

-'5522 525 -5 337 '22 -, 669 6.7

-'"1230 1225 1

6'8653 11 5 ! -" 39' 358 -, 618 62' -16 6'6 6'6 -16 605 620 -, 153 201

-2 157 122 2 291 297-I' "5

,M -, '90 510 -15 52' 50' -15 16, 125-, 326

'27,

'7'200 -9 "9 '25 -12 '25 '80

_1 850 867-"

187 166-I'

335 301 16 0 !0 70' 697 5 2" '"

-7'9' '92 -11 3'1 352

I'1 ! -12 295 272 -12 698 691 -12 21' 21'

2 16' 82 -5 301 319 -10'"

92 _1160' 587 _11 222 "011 ,

! -, 337 336 -9 251 215 -1.'"

"0 -10 '39 159 -10 18' 25711 1 ! -16 "8 "8 -, 271 29' -8 801 803-'7

215 25' -9'0'

296 -9 2'3 20'-17 237 227 -15 153

8' 12 0 ! -7 592 601-'6

257 206 -8 16, 177 -8 752 75.-16 725 70.

-"202 151 -5 733 728 -15 ",

'"-6 "9 170 -7 '76

511-15 139

"9 -I'229 212 -16 1024 1029 -,

'71 '7' -I'196 206 -,

5'9 558 -6 160 "7-"

181 82 -12 311 '08 -12 67' 6S' -2 1'3 ," -12 1082 1068 -, '35 ''3 -5 297 278

-'3290 27'

_11 552 560 -10 288 ,00 -1 7'7 136 -11 555 557_1

'50 '36 -, 688 706

Table 6. Selected interatomic distances and anglesEstimated standard deviations are given in parentheses. Notations of the atoms,

cf. Fig. 2, Table 3

A neutron diffraction study of mercury(II)chromate hemihydrate 135

Distances mercury to oxygen

within the chains between the chainsHg-0(3) 2.055(2) A Hg-0(1) 2.724(2) AHg-0(1) 2.064(2) Hg-0(2) 2.743(2)

Hg-0(3) 2.655(2)Angle 0(1)-Hg-0(3) 175.95(5)0 Hg-0(4) 2.694(2)

Hg-O(W) 2.672(2)

Distances and angles in the Cr04 tetrahedron

Cr-0(1) 1.718(2) A 0(1)-0(2) 2.711(2) A 0(2)-0(3)Cr-0(2) 1.604(2) 0(1)-0(3) 2.798(2) 0(2)-0(4)Cr-0(3) 1.713(2) 0(1)-0(4) 2.727(2) 0(3)-0(4)Cr-0(4) 1.602(2)

0(1)-Cr-0(2)

0(1)-Cr-0(3)0(1)-Cr-0(4)

2.736(2) A2.612(2)2.676(2)

109.3(1) 0

109.3(1)110.4(1)

0(2)-Cr-0(3)0(2)-Cr-0(4)0(3)-Cr-0(4)

111.1(1) 0

109.1(1)107.6(1)

Distances oxygen-oxygen ::;: 3.06 A outside the tetrahedron

0(1)-0(1) 2.826(2) A 0(2)-0(3) 3.060(2) A 0(W)-0(1) 2.961(2) A (2 X)0(1)-0(2) 3.000(2) 0(2)-0(W) 2.979(2) 0(W)-0(4) 2.964(2) (2X)0(1)-0(W) 2.962(2) 0(3)-0(3) 3.005(3) 0(W)-0(2) 2.979(2) (2 X)0(1)-0(W) 3.011(2) 0(4)-0(4) 2.843(3) 0(W)-0(1) 3.011(2) (2x)

Distances and angles involving the hydrogen atoms

O(W)-H 0.945(4) A (2x) H-O(W)-H 103.7(4)0

H-H 1.487(6)0(1)-H 2.133(4)0(2)-H 2.543(4)0(4)-H 2.583(3)0(1)-H 2.950(4)0(3)-H 3.201(4)

0(W)-H-0(1) .

0(W)-H-0(2)0(W)-H-0(4)0(W)-H-0(1)0(W)-H-0(3)

145.6(3)108.3(2)104.4(2)

81.5(2)155.1(2)

136 KARIN AURIVILLIUS and CLAES STALHANDSKE

and

andg = 0.129(6) X 104.

The value of S, defined by

was 1.30, where m and n are the number of observations and parametersvaried. Attempts with anisotropic extinction corrections gave nofurther improvement. The coherent scattering amplitudes used werebo = 0.580 and bH = - 0.374 . 10-12 em (BACON, 1972). The scattering

amplitudes for mercury and chromium were included as parametersin the refinements. The resulting values were bHg = 1.263(5) andber = 0.360(4) . 10-12 em compared to the values bHg = 1.27 and

ber = 0.352 . 10-12 em (BACON, 1972).

The final atomic and thermal parameters are given in Tables 3 and4. Observed and calculated neutron structure-factor amplitudes arelisted in Table 5. Interatomic distances and angles are given in Table 6.The computer programs used were briefly described by STALHANDSKE(1974).

Description and discussion of the structure

By comparing the results of the present neutron-diffraction studyof HgCr04 . tH20 with those obtained from the earlier x-ray diffrac-tion work (integrated Weissenberg data, 1702 independent reflectionsvisually estimated) there are, as expected, no essential differences foundin the fractional coordinates or in the bond distances as far as theinteractions of the nonhydrogen atoms are concerned. The standarddeviations of the positional parameters of the oxygen atoms are,however, now ten or twenty times smaller than those found in thex-ray work. Concerning the mercury and chromium atoms the standarddeviations are about half the values found before (ct. Table 3). Con-sistently, the interatomic distances are now determined with a muchhigher degree of accuracy, e. g. in the mercury to oxygen distances(J = 0.002 A.

The aim of the present investigation was to locate the hydrogenbonds of the structure. As the positional parameters of the hydrogen

A neutron diffraction study of mercury (II) chromate hemihydrate 137

x

Fig. 2. Projection of the structure along [010]. The infinite chains of composition(HgCr04)n are running approximately parallel to the diagonals of the ab plane.The water molecules join the chains into infinite blocks (HgCr04' tH20)n of

thickness t c

Hg

~iI Hg

\ ~\ ,:'

lad'r!

0(1) 0(1)

Fig. 3. The tetrahedral environment of the water molecule. Covalent bonds aredrawn with filled lines and the hydrogen bonds with open lines. Dashed lines

indicate possible interactions with the mercury atoms

z

138 KARIN AURIVILLIUS and CLAES STALHANDSKE

atoms are now determined with high accuracy, the hydrogen bondingsystem is reliably elucidated.

The building principle of HgCr04'! H20 was thoroughly de-scribed earlier (AURIVILLIUS, 1972). A projection of the structure onthe ac plane is given in Fig.2. The atom labelling is given in Table 3and Fig. 2. The structure is composed of endless chains

V-Hg-O-Cr-O-Hg-, running approximately parallel to thediagonals of the ab plane, with water molecules in cavities.

The mercury atoms are linearly two-coordinated, the mercury tooxygen distances are 2.055(2) and 2.064(2) A and the angle O-Hg-Ois 179.95(5) ° (ct. Table 6). These values are in good agreement withthose previously found for the coordination number two of mer-cury(II) in e.g. the infinite zig-zag chains -O-Hg-O- of or-thorhombic mercury(II)oxide, the distances Hg-O being 2.04(3),2.07(3) A and the angle O-Hg-O 179.5(1.1)° (AURIVILLIUS, 1956,1964).

The oxygen atoms 0(1)-0(4) belong to the Cr04 tetrahedron.The tetrahedron is somewhat distorted due to a significant elongationof the bridging Cr-O distances, 1.713(2), 1.718(2) A, compared to thenon-bridging distances, 1.602(2), 1.604(2) A. The same effect con-cerning the Cr-O distances is found in e. g. the structures of Rb2Cr207,Rb2CrSOlO and Rb2Cr401s (LOFGREN, 1974). A similar elongation isalso found for the briding S-O distances in the finite zig-zag chain ofHg(OHh' 2 HgS04 . H20 (BJORNLUND, 1974) and to a smaller extentfor the P-O bridging distances in the endless network of HgS(P04)2(AURIVILLIUSand NILSSON, 1975).

The remaining oxygen atom O(W), belongs to the water molecule,situated between the endless chains of formula (HgCr04)n, which thusmay be connected by hydrogen bonds. The distance O(W)-H of0.945(4) A is in a good agreement with values reported for othersalts containing water of crystallization (BAUR, 1972). The angleH-O(W)-H of 103.7(4)° is a rather small water angle but is in therange 102 °-116 ° given in the summary by BAUR (1972). The H-Hdistance is 1.487(6) A. The water molecule has a tetrahedral environ-ment including two mercury and two oxygen atoms (Fig. 3). Accordingto the classification of hydrates described by FERRARIS and FRANCHINI-ANGELA (1972) the compound belongs to class 2B, also includinge.g. Fes(P04)2 . 4H20 (ABRAHAMS,1966) and BaCh' 2H20 (PADMAN-ABHANet al., 1963).

A neutron diffraction study ofmercury(II)chromate hemihydrate 139

A hydrogen bond will be defined by a H. . . 0 distance shorter thanthe sum of the van der Waals radii 2.4 A (HAMIL'l'ONand hERS, 1968;HAMILTON, 1968; BAuH, 1972). The only possibility for a hydrogenbonding in the structure is between the atoms 0(W)-H"'0(1), thedistance 0(W)-0(1) being 2.961(2) A and H'''0(1) 2.133(4) A. Theangle 0(W)-H"'0(1) is 145.6(3r.

As the H... O( 1) distance is rather long and the deviation of theangle O(W)- H. .. O( 1) from linearity is considerable, the hydrogen

Fig.4. A stereoscopic view along [001] showing the connection of endless chainsby water molecules to an infinite block of thickness t c. The hydrogen bonds are

drawn with thin lines

bond may be considered as being rather weak. It is also noteworthythat the angle 0(1)-0(W)-0(1) of 55° is smaller than in most otherhydrates.

The water molecules provide the hydrogen bonds between adjacentV

endless chains -Hg-O-Cr-O-Hg-. These are fused togetherforming infinite blocks of thickness i c, shown in a stereoscopicview (Fig. 4). Between the blocks there are weak mercury-oxygeninteractions (see below).

In the chains the chromate oxygen atom 0(1) is bonded at shortdistances to three atoms (Cr, Hg and H at 1.718, 2.064 and 2.133 A),but the atom 0(3) only to two atoms (Cr and Hg at 1.713 and 2.055 A).The atoms 0(2) and 0(4) are each bonded only to one chromium atomat 1.604 and 1.602 A respectively.

Geometrically the polyhedron around mercury can be described asa pentagonal bipyramid (Fig. 5). Five oxygen atoms 0(W),0(1) to 0(4)

140 KARIN AURIVILLIUS and CLAES STALHANDSKE

0(3)

2694

0(2)

0(4)

Fig.5. Distances within the pentagonal bipyramid of mercury. The covalentbonds between mercury and oxygen are drawn by heavy lines. Thin lines indicateweak mercury-oxygen interactions. These more distant oxygen atoms form the

pentagonal plane

at Hg-O distances 2.655(2)-2.743(2) A form approximately a penta-gonal plane. The covalently bonded atoms 0(1) and 0(3) at the dis-tances 2.065(2) and 2.055(2) A form the apices of the polyhedron.

These studies form part of a research program financially sup-ported by the Swedish Natural Science Research Council.

References

S. C. ABRAHAMS (1966), Ferromagnetic and crystal structure of ludlamite,Fe3(P04)2 . 4H20 at 4.2°K. J. Chern. Physics 44,2230-2237.

K. AURIVILLIUS (1956), The crystal structure of mercury(II)oxide studied byx-ray and neutron diffraction methods. Acta Chern. Scand. 10, 852-866.

K. AURIVILLIUS (1964), Least-squares refinement of the crystal structures oforthorhombic HgO and of Hg202NaI. Acta Chern. Scand. 18, 1305-1306.

K. AURIVILLIUS (1972), The crystal structure of mercury(II)chromate hemi.hydrate, HgCr04 . !H20. Acta Chern. Scand. 26, 2113-2124.

K. AURIVILLIUS and B. A. NILSSON (1975), The crystal structure of mercury(II)phosphate, Hg3(P04)2. Z. Kristallogr. 141, 1-10.

G. E. BACON (Chairman Neutron Diffraction Commission) (1972), Coherentneutron-scattering amplitudes. Acta Crystallogr. A 28, 357-358.

A neutron diffraction study of mercury(II)chromate hemihydrate 141

W. H. BAUR (1972), Prediction of hydrogen bonds and hydrogen atom positionsin crystalline solids. Acta Crystallogr. B 28, 1456-1465.

G. BJORNLUND (1974), The crystal structure of Hg(OH)2 . 2HgS04' H20. ActaChern. Scand. A 28,169-174.

P. COPPENS and W. C. HAMILTON (1970), Anisotropic extinction corrections inthe Zachariasen approximation. Acta Crystallogr. A 26, 71-83.

G. F. FERRARIS and M. FRANCHINI-ANGELA (1972), Survey of the geometry andenvironment of water molecules in crystalline hydrates studied by neutrondiffraction. Acta Crystallogr. B 28, 3572-3583.

W. C. HAMILTON (1968). On hydrogen bonding in inorganic crystals in A. RICHand N. DAVIDSON (Eds.), Structural chemistry and molecular biology. SanFrancisco and London: W. H. Freeman, p. 466.

W. C. HAlIHLTON and J. A. IBERS (1968), Hydrogen bonding in solids. New York:W. A. Benjamin, p. 15.

International tables for x-ray crystallography, Vol. III (1962). Birmingham: TheKynoch Press, p. 197.

P. LOFGREN (1974), On the crystal chemistry of alkalichromates(VI). Diss.Dept. of Inorg. Chern., University of Stockholm, Sweden.

V. M. PADMANABHAN, W. R. BUSING and H. A. LEVY (1963), A single-crystalneutron-diffraction study of BaCh . 2H20. Acta Crystallogr.16, Supp!. A 26.

C. STALHANDSKE (1974), The crystal structure of o-phenylenediamine dihydro-chloride. Acta Crystallogr. B 30, 1586-1589.

W. H. ZACHARIASEN (1967), A general theory of x-ray diffraction in crystals.Acta Crystallogr. 23, 558-564.