Hexagonal and orthorhombic crystal structures of californium trichloride

J. inorg,nucl.Chem., 1973,Vol.35, pp. 117I- 1177. PergamonPress. Printedin Great Britain

HEXAGONAL STRUCTURES

AND ORTHORHOMBIC CRYSTAL OF CALIFORNIUM TRICHLORIDE*

J. H. BURNS, J. R. P E T E R S O N t and R. D. B A Y B A R Z Transuranium Research Laboratory, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 (Received 22 June 1972) Abstract-Californium trichioride has been prepared in two crystalline modifications and singlecrystal specimens of each have been studied by X-ray diffraction. The hexagonal UCl3-type crystal has unit-cell dimensions of a = 7.379 ( 1 ) A and c = 4.0900 (5) .~ and the orthorhombic PuBra-type, a = 3.869(2) ,~, b = 11.748(7) A and c = 8.561(4) A. Structural parameters including anisotropic thermal motion have been refined by the method of least squares. In the hexagonal form the Cfatom is 9-coordinated by six CI atoms at 2.815(3) A and three at 2-924(4) A, while in the orthorhombic form the coordination is 8-fold with two Cf-CI distances of 2.690(7) A; four of 2.806(4) A and two of 2-940(6) ,~. An ionic radius of 0.932(3) A was derived for the 6-coordinated Cf 3+ ion. INTRODUCTION

AMONG the lanthanide trichlorides are found three crystal modifications [1]. The stability range of each form is readily correlated to the size of the trivalent lanthanide ion involved. It is expected that this should also be the case with the actinide trichlorides provided compounds of actinide elements of comparable radii can be prepared. Until now, all the trichlorides from uranium to einsteinium have been shown to exhibit only the UC13 structure type[2-5] although the ionic size of Cf 3+ is estimated to be about the same as Gd 3+ whose trichloride crystallizes both in the UCI3 and in the PuBr3 forms [6]. The radius of Es 3+ is presumably smaller yet, but EsCI3 has been studied only at temperatures above 400°C because of self radiolysis. Our initial objective in studying CfCI3 was to refine the structural parameters of the hexagonal UCI3 form (h-CfCl3) from single-crystal data in order to derive accurate interatomic distances as we had done for AmCI3 [7] and CmCI3 [8] and as had been reported by Morosin [9] for LaCI3, NdCI3, EuCl3 and GdCl3. However, in the process we discovered the existence of the orthorhombic PuBr3 form (o-CfC13) and subsequently produced single crystals of each form and refined both structures. *Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation. t Department of Chemistry, University of Tennessee, Knoxville. 1. 2. 3. 4. 5. 6. 7. 8. 9.

D. Brown, Halides o f the Lanthanides andActinides, p. 154. Wiley-Interscience, London (1968). J. C. Wallmann, J. Fuger, J. R. Peterson and J. L. Green, J. inorg, nucl. Chem. 29, 2745 (I 967). J. R. Peterson and B. B. Cunningham, J. inorg, nucl. Chem. 30, 823 (1968). J. L. Green and B. B. Cunningharn, lnorg, nucl. Chem. Lett. 3, 343 (1967). D. K. Fujita, B. B. Cunningham and T. C. Parsons, Inorg. nucl. Chem. Lett. 5, 307 (1969). A. L. Harris and C. R. Veale, J. inorg, nucl. Chem. 27, 1437 (1965). J. H. Burns and J. R. Peterson, Acta crystallogr. B26, 1885 (1970). J. H. Burns andJ. R. Peterson, J. inorg, nucl. Chem. To be published (1973). B. Morosin, J. chem. Phys. 49, 3007 (1968). 1171

1172

J . H . BURNS, J. R. PETERSON and R. D. BAYBARZ

EXPERIMENTAL Preparation of crystals The synthesis of CfCls and growth of the two crystalline modifications were done similarly in both cases. A single cation-exchange resin bead, in each instance, was loaded with 24aCfand then calcined to an oxide in the usual way [ 10]. The 24aCfwhich yielded the orthorhombic crystal was from a batch purified for other work [ I 1], and the source of 24gCffor the hexagonal crystal was an oxide bead used in a study of the Cf-O system [12]. The oxide bead was placed in a quartz capillary drawn on the end of a standard taper joint, attached to a high vacuum system, and evacuated. Purification from possible Pt contamination[13] was achieved by heating to 600°C in a dynamic vacuum. After this treatment, admission of anhydrous HCl gas (99.5% purity, Air Products, Inc.) caused transformation of the oxide sample to emerald-green CfCla in less than 1 rain; or, by cooling to room temperature, admitting the HC1 gas and slowly raising the temperature, a noticeable color change was observed around 425°C. In either case, thermal cycling while observing the sample through a microscope established the melting point as 545 - 5°C. Careful cooling below this temperature produced the crystals used in this study. The conditions required to obtain each crystal modification have not been established. Indeed, essentially the same conditions have yielded one and then the other form; in one instance both forms appeared at different sites in the same capillary. Possible contributing factors are the stabilization of one phase by impurities or the ability to supercool past the transition temperature (known to be greater than 400°C) in some cases. Because of certain difficulties encountered on this microgram scale of experimentation, we have not yet been able to resolve this question.

Single-crystal data collection The intensity data and the observations for unit-cell determination were obtained with a computercontrolled Picker X-ray diffractometer equipped with a scintillation-counter detector. For unit-cell determination twelve reflections (in the r~mge 43-48 ° 20 for orthorhombic, 61-67 ° 20 for hexagonal) were carefully centered and the angles obtained were used as observations in a least-squares refinement. Intensities were measured by the 0-20 scan technique with background counts at the ends of each scan. A 6 mm receiving slit, a take-off angle of 2.5 °, unfiltered MoK~ X-rays, and a scan range varying from 1.2 ° to 2 ° for the h-CfCla sample and constant at 2.5 ° for o-CfCla were used. A reference reflection was measured hourly and indicated stability of the instrument and crystals. The effect of X-ray absorption by the crystal on the intensities is quite large and is difficult to correct for because of two factors: first, the linear absorption coefficient of this compound is quite large and not accurately known, and second, the shapes of the specimens are difficult to describe accurately because of their growth from a droplet of liquid on the inner surface of a capillary tube. The h-CfCla sample was estimated to have a mass of 3/~g and was described best by an ellipsoid, while the o-CfCla crystal, 13/~g, was approximated by nine bounding planes. With these descriptions of the crystals, absorption corrections were calculated[14] and applied to the observations. These corrections were not adequate to account for discrepancies among equivalent reflections at low angles in the orthorhombic case, so the final refinement was done with reflections having 40 ° < 20 < 70°. For the hexagonal case the intensities of equivalent reflections were in better agreement and were averaged; these included all reflections out to 20 = 65 °. CALCULATIONS Since both modifications of CfCla were recognized as known structural types, it w a s p o s s i b l e t o p r o c e e d d i r e c t l y t o t h e l e a s t - s q u a r e s r e f i n e m e n t . F o r t h i s p u r p o s e t h e a t o m i c c o o r d i n a t e s o f o r t h o r h o m b i c T b C l a [ 15] a n d h e x a g o n a l A m C l 3 [7] J. R. Peterson, J. A. Fahey and R. D. Baybarz, J. inorg, nucl. Chem. 33, 3345 (1971). P. G. Laubereau andJ. H. Bums, lnorg. Chem. 9, 1091 (1970). R. D. Baybarz, R. G. Haire and J. A. Fahey, J. inorg, nucl. Chem. 34, 557 (1972). D. K. Fujita, Ph.D. Thesis, Univ. of Calif. Lawrence Radiation Laboratory, Report UCRL19507 (1969). 14. D. J. Wehe, W. R. Busing and H. A. Levy, ORABS, A Fortran Prngram for Calculating SingleCrystal Absorption Corrections, Report No. ORNL-TM-229, Oak Ridge National Laboratory, Oak Ridge, Tenn. (1962). 15. J. D. Forrester, A. Zalkin, D. H. Templeton and J. C. Wallmann, Inorg. Chem. 3, 185 (1964). l 0, 11. 12. 13,

Hexagonal and orthorhombic CfCl3

1173

were used as starting values. Individual anisotropic thermal parameters were applied to each atom. Atomic scattering factors for neutral Cf and CI were taken from Cromer and Waber [16] with the values for Cf corrected for real and imaginary components of anomalous dispersion[17]. Other quantities related to the refinement are listed in Table 1 along with the refined unit-cell dimensions, which agree with previous measurements [4] within their error limits. (In all cases we have given the standard errors in parentheses following the corresponding number.) The full-matrix least-squares calculations were done with a recent version of program ORFLS[18]. The function minimized w a s ~(1/o'2)(Fo2-s2Fo2), in which Fo and F~ are observed and calculated structure factors, respectively, and Table 1 Orthorhombic

Crystal data a b c space group Z dealt Refinement details observations, n variable parameters, p R = EllFol-lFAl/Y.Fol (for reflections with Fo > 1o-) o'1 = ~w(Fo~--s2Fc2)2/(n-p), in which w = weight of an observation, and s = a scale factor

Hexagonal

3.869(2) A 11.748(7) 8.561(4) Cmcm 4 6.07 gcm -3

7.379(1)/~ 4.0900(5) P63/m 2 6.12 gcm -3

393 15 0.074

265 9 0.041

1.56

2-08

Table 2. Structural parameters for californium trichloride

x

y

Site

Cf CI

2(c) ~ 6(h) 0.3902(6)

Atom

Site

x

y

z

Cf CI(1) C!(2)

4(c) 4(c) 8(f)

0 0 0

0.2434(1) 0.5843(8) 0.1466(5)

¼ ¼ 0.5667(6)

0.3019(6)

z

Hexagonal flll

Atom

¼ 0.0054(1) ¼ 0-0060(7) Orthorhombic

/3,, 0.0134(7) 0.023(4) 0.022(3)

fl~2 0.0054 0.0052(6)

t~2~ 0.0028(1) 0-0025(5) 0.0028(3)

fl33 0.0113(4) 0.019(2)

~33 0.0031(1) 0.0060(9) 0.0028(5)

fl~2 0.0027 0.0030(5)

¢~3 0 0 -0.0004(3)

The thermal parameters are for the expression exp [ - (/3~h 2 +/3~2k2 +/333/2 + 213~2hk+ 2~3hl + 2flzsk/) ]. 16. D.T. Cromer and J. T. Waber, Acta Crystallogr. 18, 104 (1965). 17. D.T. Cromer, Acta Crystallogr. 18, 17 (1965). 18. W. R. Busing, K. O. Martin and H. A. Levy, ORFLS, A Fortran Crystallographic Least-Squares Program, Report ORNL-TM-305, Oak Ridge National Laboratory, Oak Ridge, Tenn. (1962).

1174

J . H . BURNS, J. R. PETERSON and R. D. BAYBARZ

the variance, o-2, is based on counting statistics plus an allowance of 5 per cent of the intensity for unknown, systematic errors. The refined parameters are given in Table 2, and values of Fo and Fc are given in Tables 3 and 4. Interatomic distances and their standard errors were calculated from the final parameters with program ORFFE [19]. The illustrations were made with program ORTEP [20]. RESULTS AND DISCUSSION

The two structures of CfCI3 are illustrated in Figs. 1 and 2. Bond lengths are Table 3. Observed and calculated structure factors for orthorhombic CfCI3 on an absolute scale L

00S

CRL

==m 0 O L u 10 137 -176 12 110 128 u

0 9 tO L1 12 t3 0 8 g 10 11 12 13

ml

=m 0 0 g 10 11 12 13 0 6 7 8 9 10 11 12

•m

mm 0 q 5 6 7 8 g LO 11 =m 0 0 1 2 3 ti 5 6 "1 0 9 10 == 0 0 I 2 3 q 5 6 7 0 g

L

081

CRL

m. 0 16 L am O ~9 ti6 t =43 50 2 "/8 -68 2 L == 3 0 -23 59 -5=4 tt 63 $5 125 13"/ 5 tt2 52 0 -tit 6 36 -=41 69 -103 7 ti8 -31 24 -3q =a 0 18 L m= q L nm 0 65 -69 217 19q t 35 -tie 38 23 2 33 tL9 111 -107 3 29 28 7 9 tt 66 -50 89 8"/ 0 19 J-, t 1 L a m 8 =41 -39 6 L mum 9 161 -167 227-196 t0 0 -5 0 -16 11 51 87 135 116 12 1=4 2 55 =47 13 113 -103 113 -9=4 33 -8 nu= I 3 L am 8 2q 23 8 L mm 9 161 lb-~ 102 - t S t t0 3g 3=4 60 -56 11 t3q -139 117 102 12 13 -21 35 3=4 13 88 89 113 -102 71 -6q am I 5 L me 8g 80 8 63 52 9 106 -105 10 L am 10 32 -36 130 -127 11 131 132 26 -36 12 35 25 15L 1=45 13 6q -60 30 3") 68 -66 mum t 7 L mm ")1 -37 7 1=4¼-13q 100 92 8 5"1 -6q 18 21 9 118 110 10 0 7 12 L ,=m 11 103 -93 1q6 l=46 12 O -10 t~7 50 1¼1 -130 u 1 9 L mum 101 -97 5 139 -131 1~7 130 6 t~0 -39 37 2"/ 7 153 1=45 11=4- 1 t 3 8 6~ 5-/ q5 -Sq 9 1110-130 92 92 10 32 -20 52 55 11 70 66 90 -"/8 u I 11 L u t=4 L mm 0 126-118 62 -56 1 118 116 63 -'71 2 32 63 105 103 3 1q9 -1=4¼ 76 80 =4 6'7 -73 76 -61 $ 86 07 66 -56 6 88 88 63 50 7 100 -g9 65 58 8 32 -3q 77 - 8 0 9 90 07 ¼9 -q8 10 65 52 11 ~ -=49

L

0BS CRL

L

=m I 13 L == 7 0 76 76 8 1 87 -102 9 2 29 -29 10 3 82 72 11 =4 =45 =42 5 92 -95 am 2 6 59 -57 0 7 59 69 1 8 0 15 2 9 q5 -52 3 ti =m t 15 L =" 5 0 37 -=46 6 t 99 100 7 2 62 58 8 3 61 -63 9 =4 39 -51 10 5 93 97 6 =4=4 38 = i 2 7 56 -63 0 8 =40 -=43 1 2 =u= 1 17 L mum 3 0 39 37 ti t 50 -50 5 2 65 -69 6 3 29 50 7 =4 62 94 8 5 53 --q=4 9 mm 2 0 L =J' m. 2 8 13~ 13=4 0 10 156 -156 I 12 110 115 2 3 =m 2 2 L N ti 7 tL5 =41 5 8 129 -136 6 9 =42 .-q7 7 10 tL~l 122 8 11 0 -12 12 92 -9~ == 2 13 0 -30 0 1 am 2 =4 L == 2 7 2q -16 3 8 169 171 =4 9 10 20 5 10 95 -96 11 0 7 ==, 3 12 8=4 79 5 6 =a, 2 6 L =u= 7 0 205-195 8 1 75 -68 9 2 267 256 10 3 23 tq 1! =4 211 -210 12 5 70 -70 6 156 151 =.= 3 7 =40 31 5 fl 175 -172 6 9 12 -15 7 10 106 tot/ 8 11 S0 =42 9 12 88 -85 10 11 == 2 0 L =u= =4 1¼=4 l q l m m 3 5 109 99 =4 6 1=46-1¼2 5

0B$

CRL

L

5q -49 96 92 22 30 911 -92 6=4 -57

6 7 8 9 10 11

3~ -t¢t lit 117 0 tiO 88 -86 3~ -28 122 106

3 0 t 2 3 q 5 6 7 8 g 10

7 L == 28 lti IS0 150 31 56 133 -133 q6 -31 128 131 0 -6 102 -106 =49 -50 87 89 0 7

10 L m. 173 - t 6 8 ¼7 -¼0 101 99 52 50 110 -112 30 -33 129 128 =41 3~ 58 -60 36 -33 83 83

u

12 L m= 118 129 n 3 38 ti=4 0 119 -121 1 8q -85 2 115 115 3 11 25 q 103 -101 5 =48 "-q8 S 8q 83 7 59 =49 8 9 It& L .m 10 50 -52 63 -63 =" 3 87 91 0 65 71 1 6~ -73 2 =41 -50 3 33 =46 q =47 5"2 5 75 -72 6 '7 16 L == 8 22 =43 9 39 ti5 63 -61 '='= 3 19 -21 0 ti6 50 1 38 t17 2 3 1 L mum =4 127 -131i 5 31 -12 6 I=4"/ 1=48 7 1=4 -28 131 -131 as 3 23 -=4 0 70 72 1 7 2 2 3 3 L m= q 198 202 5 tit 50 153 -157 u ¼ 31 16 0 12"/ 122 2 O 26 =4 IL:~ -112 6 8 5 L H 10 q6 53 17=4-180

085

CRL

9 L mm 33 36 1141 -137 60 -60 167 1"/3 =48 =47 100 -103 37 -31 11¼ 115 =¢9 =46 91t -1014 0 -23 11 L =m 9¢I -90 83 91 =49 50 111 -113 56 -57 75 71 62 69 70 -80 0 -28 72 71 13 L mmm =47 60 80 -81 28 -25 6=4 59 2l 3=4 "N -76 q6 -=46 53 5'7 15 50 80 52 63 50 '76

L mm -38 80 =47 -52 -=42 77

L

081

CRL

m,, q O I 2 3 ti 5 6 7 8 9 10

L

2 L m m am 5 160 -165 0 0 -15 I 11i5 Lq=4 2 62 63 3 139 -139 =4 0 ¼ 5 133 130 6 =42 27 7 102 -97 8 15 -31 99 90 =" 5 0 =m =4 =4 L == t 0 109 112 2 t 19 3 3 2 155 -161 q 3 33 -27 5 ti 13q 133 6 5 0 -6" 7 B 92 -9~ 7 26 -12 mum5 8 121 120 0 9 0 15 1 10 73 -72 2 3 =" ti 6 L =m =4 0 126 -130 5 1 20 -q=4 6 2 16=4 167 7 3 t=4 11 =4 135 -1=41 IN 5 5 =47 -q7 0 6 106 106 1 7 26 22 2 8 122 -121 3 g It& -12 ti 5 m= =4 8 L == 6 0 131 126 1 61 65 m= $ 2 101 -100 0 3 19 -27 I =4 100 100 2 5 73 6"/ 3 6 96 -100 =4 7 =41 -35 5 8 '72 69 me 5 m m ti 10 L m m 0 0 1L0 -117 1 1 28 -30 2 2 6=4 72 3 ¼'7 36 u 6 =4 '77 -81 0 5 21 -25 2 6 89 92 7 5 26 u 6 0 mm =4 12 L u 1 0 83 91 2 1 =42 33 3 2 8q -fl6 3 62 -60 a= 6 =4 81 82 0 5 2"/ 20 1 6 75 -73 2

0 L sm 226 228 IS=4 -150 169 1611 171t - I ' N 9"/ 96 m= =4 t=4 L a 120 "112 0 39 "q0 1 =45 -q5 2 63 66

08S

CRL

1 L n 0 It 112 -111 0 18 11i7 136 15 -8 92 -88 29 -7 102 96 16 -16 3 L =m 21 -tit 137 lti0 0 -3 129 -120 60 - i t 126 125 9 30 10=4 -101 5 L u 0 32 119 -117 38 -36 75 78 =4"/ 32 110 -113 27 -26 82 78 7 L ma 0 5 gl 95 q8 35 86 -86 ti2 -21 87 85 0 =4 9

0 85 5=4 100 29 76

L m= 2=4 -88 -38 108 31 -69

11 L m m 52 -56 63 62 10 33 0 L mm 133 126 90 -91 2 L m= 101 -95 0 -8 89 85 23 32 =4 L ms 69 69 37 ti 90 -93

19. W. R. Busing, K. O. Martin and H. A. Levy, ORFFE, A Fortran Crystallographic Function and Error Program, Report ORNL-TM-306, Oak Ridge National Laboratory, Oak Ridge, Tenn. (1964). 20. C. K. Johnson, ORTEP, A Fortran Thermal-Ellipsoid Plot Program for Crystal Structure Illustrations, Report ORNL-3794, Oak Ridge National Laboratory, Oak Ridge, Tenn. (1965).

Hexagonal and orthorhombic CfC13

1175

Table 4. Observed and calculated structure factors for hexagonal CfCl 3 o n an absolute scale L aeS CI~ ,,,, 0 0 L ,,= 2 183 -200 tl 12t 123 6 69 -72 =m I 0 1 2 3 4 5 6 == 2 0 1 2 3 q 5 6 == 3 0 1 2 3 14 5 u

0 l 2 3 tl 5

tt

,,a S 0 1 2 3 LI S ass 6 0 t 2 3 4

L ~

m= 7 0 t 2 3 0 L m= q l o t -96 t i t -).17 u 8 73 73 0 8S 84 l 46 -47 2 50 -53 3 28 28 -,, 9 0 L ss= 0 91 -~lq I t56 159 2 67 67 105 -109 ross I 45 -44 0 "/0 68 l 28 27 2 3 0 L == 4 170 166 5 1.3 -9 6 137 -137 12 6 =ss 2 92 92 0 B -3 1 2 0 L ss= 3 8B -83 tt 70 -67 5 73 70 53 53--3 49-49 0 35 -37 1 2 0 L == 3 72 -66 q 97 97 5 65 59 76 -"/6 =ss 4 43 -42 0 SO 51 1 2 0 L ms 3 78 76 q 26 -24 S 67 -68 16 18 == 5 49 51 0 l 2

CIr.

O L == 40 -40 54 -55 38 36 49 46 26 -2"/

L 08S Cl:i.

L 5BS CRL

L 08S ~

L ~$

3 q 5

tl 5

14 -15 46 46

3 q

3

mm 5 0 1 2 3 q

m, B 2 L u n 5 3 L m= 0 72 72 0 39 ~ 0 1 26 26 I 7tt "/'/ 2 63 -65 2 35 36 21 -20 3 62 -62 mm 7 48 t19 4 28 -2"/ 0

l L =. 96 99 0 0 85 -89 5 0

mm 6 0 1 2 3 q

2 L == 39 -39 55 -58 35 35 46 48 25 -26

ross 6 0 l 2 3

i L == 32 -34 39 -42 29 31

mm "/ 0 1 2 3

I= 6 0 l 0 L m= :~ 3"! -3"/ 3 qq qS q 35 33 qO -38 ~,, -/ 0 0 L..ss ! 4"/ t18 2 15 - t i t 3 tt3 -qq m=l IB 1 L m= 0 128 133 1 59 50 2 107 -110 31 -29 "= 9 71 74 0 19 16 1 46 -LI6 "= 1 1 L -" 0 100 -98 1 131 -142 2 79 76 3 9"/ 101 q 52 -52 5 64 -64 I= 2 1 L.. tO 89-86 -/6 73 2 "/4 -/2 3 60 -58 q 51 -49 5 37 39 ,m 3 I L =m 0 13~ 133 1 9 7 2 [ I q -116 3 O -5 q 92 82 5 6 3 == q 1 L == 0 54 -54 l 99 -99 2 46 48 3

78 35 52

78 -35 -53

t L m, 41 -qO 60 62 36 36 48 -50 2"/ -2"/

1 L It= L:m. -24 45 49 2 19 114 24 67 20 59

L ,t= -19 121 21 -88 -17 57

mm 8 0 1 =m 1 O t 2 3 4 5

2 L ss= 118 1199 " ' 0 2 '10 96 -102 1 11 6 2 68 71 3 =9 -q 4 5 2 L == tiP. -42 == 3 93 -93 0 39 3"/ 1 60 72 2 2"/ -2"/ 3 48 -48 q 5 2 L == 22 -20 == 4 85 86 0 19 ).9 1 70 -68 2

=" "/ 2 L =m 0 33 -35 1 46 48 2 29 31 38 -410 =lit I 0 2 L =m I 42 45 2 13 15 3 4 3 L == 5 57 -53 103 -103 H 2 52 46 0 -/6 78 l 38 -32 2 56 -St 3 4 L-ss'883 3 S 5 132 137 m, 3 69 71 0 92 -104 I 48 -q9 2 71 67 3 4 3 L == 93 93 == 4 13 -12 0 83 -82 1 12 9 2 56 60 3 6 -5 q 3 L == m 5 58 -57 0 83 -e~ 1 52 Sl 2

63 38

67 -3"/

3 L ss= == I 53 $q 0 20 -20 l 49 -49 2 14 16 3 tl 3 L =m 5 25 -26 54 -55 === 2 24 24 0 1 q L m= ;~ 110 105 3 IL -fl 4 92 -92 12 6 == 3 66 66 0 9 -tt 1 ;2 4 L == 3 41 -38 tl 86 -83 33 34 mm 4 614 66 0 2g -25 1 48-45 2 3 ¼ L am 61 -60 == 5 53 53 0 54 53 1 q-/ -43 2 43 -39 =. 6 q L == 0 93 95 1 6 -q ~ -~5 am t 9 3 O 66 63 1 2 q L m,, 3 14 - I q 4 55 -56 11 13

q'l

~ tl?

4 L m. 36 -3"/ 56 57 33 3tl

L 08S C~ =,. 2 6 L m= 0 46 -46 l "/3 "/2 2 45 42 3 64 -59 tl 35 "31

q L mm ss= 3 55 58 0 | 5 L == 2 0 -16 3 8-/ 82 16 15 ross q "/5 -65 0 15 -13 1 52 45 2

6 L m= 62 61 6 2 5"/ -56 e -2

5 L "= ,,m 5 101 99 0 10 "/ I 85 -88 6 -5 ss= I "/3 65 0 t S L ssss 2 53 -52 3 43 -43 tl"/ 46 ass 2 39 36 0 36 -31t I 2 5 L u 3 31 -2~ 53 52 == 3 26 25 0 45-43 1 2 5 L ss= SO 52 ss= tt 9 6 0 ¼6 -4-/ ss= t S L == 0 32 -32 1 31 -31 2

6 L m= 16 -16 qO tll

6 L ll II 2 50 -416 0 92 -87 t 42 42 71 "/0 a l l 30 -31 0 |

6 L ross 41 -40 6"/ -68 3"/ 3"/

"7 L ssss 64 60 24 -1"/ 53 -55 17 13 7 L ss= lq -9 55 -53 11 9 49 tlq "/ L == 25-24 48 tO 22 22 "/ L == 44 45 1] L == 33 -32 63 60 29 29 O L II 60 59 6 -5 S L == 32 ~32 Sl -SO

%

ORTHORHOMBIC

HEXAGONAL

Fig. l. Coordination polyhedra about Cf in the two forms of C f C I 3 . Atoms are represented by their 75%-probability thermal ellipsoids [20].

listed in Table 5 along with the corresponding distances in the closest available lanthanide analogues for comparison. T h e main point o f structural interest in these results is that in h-CfC13 the C f

1176

J . H . BURNS, J. R. PETERSON and R. D. BAYBARZ

(o)

)

-----0

(b)

-----0

Fig. 2. Stereoscopic drawings showing the relationship between the two structures of

CfCla. (a) orthorhombic; (b) hexagonal. Table 5. Bond lengths o-CfCI3

TbCla (Ref. [ 15])

Atoms

Distance

Number

Cf-Cl(1) -Cl(2) -C1(2) -CI(1)

2-690(7) fit 2-806(4) 2.940(6) 4.005(9)

2 4 2 1

h-CfC6

2.70(2) fit 2.79(2) 2.95(2) 3.79(2) GdCI3 (Ref. [9])

Atoms

Distance

Number

Cf-CI -CI

2.815(3) fit 2.924(4)

6 3

2.822(2) h 2.918(2)

ion has nine C1 neighbors while in o-CfCl3 the number is eight. Indeed, the Cf ion is of such a size that it represents the point in the actinide series at which 8- and 9-coordination for C1 ions are about equally stable; the corresponding lanthanide element is Gd. In Fig. 1 are shown the two coordination polyhedra in a similar orientation for comparison. In each case there is a trigonal prism of six CI ions, the base being equilateral in h-CfC6 and isosceles in o-CfCl3; and in h-CfC6 there are three equatorial CI ions but in o-CfCl3 only two. More detail on how these prisms are shared with others to make up the structure is shown in the stereoscopic drawings of Figs. 2(a) and 2(b). It is seen that the trigonal prisms share bases to form endless columns along the vertical direction (in the figures). It is primarily in the cross-linking of these columns that the structures differ. In h-CfC6 each column is cross-linked in three horizontal directions, while in o-CfCl3 this occurs in only two directions, resulting in sheets of cross-linked columns (Fig. 2a). The mechan-

Hexagonal and orthorhombic CfCI3

1177

ism of the hexagonal to orthorhombic phase transformation is not known, but it can be visualized as resulting from the breaking of one third of the equatorial linkages with the remainder of the structure remaining intact and undergoing small atomic shifts. Since there are now seven members of the isomorphous series of UC13 structure type which have been completely refined by single-crystal methods, it is of interest to examine changes in the structure with cation size. As a basis for cation size we will use the trivalent ionic radii derived by Templeton and Dauben[21] from the cubic sesquioxides (CN = 6). When the equatorial and apical bond lengths of the lanthanide trichlorides are plotted vs. the cationic radius (Fig. 3) 2.96

O R N L - DWG. 7 2 - 5 6 3 6

. , ~

'

I

'

I

'

I

'

I

'

I

'

_

2.94 2.92

bJ (J Z

2.90

03

2.88

i

2.86

APICAL~dl

Am Xi

2.84

~ Cm

Gd -

2.82 2.80

I

J

I

1.06 L04

J

I

,

I

,

I

1 . 0 2 1.00 0.98 r (M3÷),

,

I 0.96

,

l

,

0.94 0.92

Fig. 3. Interatomic distances vs. metal radii for seven hexagonal lanthanide and actinide trichlorides.

they are seen to vary in a smooth way. The shorter apical bond distances decrease monotonically while the equatorial ones go through a minimum, then rise slightly. The reason for this behavior is seen in the previously discussed tendency, near the transition point between 8- and 9-coordination, for the prismatic columns, which are held together by apical bonds, to remain intact while the cross-linking through equatorial bonds gives way to some extent. Thus it is more reasonable to define the ionic radius in terms of the apical distances which follow smoothly the cation's decreasing size than to include all nine distances, three of which actually reverse their trend as the cation size decreases. Accordingly we have derived trivalent radii from the actinide trichloride structures by use of their apical bond lengths and the graph in Fig. 3. The corresponding equatorial lengths are also plotted and show the same general trend as observed in the lanthanide trichlorides. The derived ionic radii consistent with the 6-coordinate radii of Templeton and Dauben [21] are: Am 8+ - 0 . 9 8 4 A, Cma+-0-971 ,~, and Cf3+-0.932 A~. 21. D. H. Templeton and C. H. Dauben, J. A m. chem. Soc. 76, 5237 (1954).