International
Tables for
Crystallography
Volume B
Reciprocal space
Edited by U. Shmueli

International Tables for Crystallography (2006). Vol. B. ch. 2.1, pp. 200-203   | 1 | 2 |

Section 2.1.7.3. Application to centric and acentric distributions

U. Shmuelia* and A. J. C. Wilsonb

aSchool of Chemistry, Tel Aviv University, Tel Aviv 69 978, Israel, and bSt John's College, Cambridge, England
Correspondence e-mail:  ushmueli@post.tau.ac.il

2.1.7.3. Application to centric and acentric distributions

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We shall summarize here the non-ideal centric and acentric distributions of the magnitude of the normalized structure factor E (e.g. Shmueli & Wilson, 1981[link]; Shmueli, 1982[link]). We assume that (i) all the atoms are located in general positions and have rationally independent coordinates, (ii) all the scatterers are dispersionless, and (iii) there is no noncrystallographic symmetry. Arbitrary atomic composition and space-group symmetry are admitted. The appropriate weight functions and the corresonding orthogonal polynomials are [\matrix {& & \hbox{Non-ideal} \cr p^{(0)}(|E|) & f_k(x) & \hbox{distribution} \cr\noalign{\hrule}\cr & & \cr (2/\pi)^{1/2}\exp(-|E|^2/2) & He_{2k}(|E|)/[(2k)!]^{1/2} & \hbox{Centric} \cr & & \cr 2|E|\exp(-|E|^2) & L_k(|E|^2) & \hbox{Acentric} \cr\noalign{\hrule} \cr} \eqno(2.1.7.4)] where [He_{k}] and [L_{k}] are Hermite and Laguerre polynomials, respectively, as defined, for example, by Abramowitz & Stegun (1972[link]). Equations (2.1.7.2)[link], (2.1.7.3)[link] and (2.1.7.4)[link] suffice for the general formulation of the above non-ideal p.d.f.'s of [|E|]. Their full derivation entails (i) the expression of a sufficient number of moments of [|E|] in terms of absolute moments of the trigonometric structure factor (e.g. Shmueli & Wilson, 1981[link]; Shmueli, 1982[link]) and (ii) calculation of the latter moments for the various symmetries (Wilson, 1978b[link]; Shmueli & Kaldor, 1981[link], 1983[link]). The notation below is similar to that employed by Shmueli (1982[link]).

These non-ideal p.d.f.'s of [|E|], for which the first five expansion terms are available, are given by [p_{c}(|E|) = p_{c}^{(0)}(|E|)\left[1+\sum_{k = 2}^{\infty}{{A_{2k}}\over{(2k)!}} He_{2k}(|E|)\right] \eqno(2.1.7.5)] and [p_{a}(|E|) = p_{a}^{(0)}(|E|)\left[1+\sum_{k = 2}^{\infty}{{(-1)^{k}B_{2k}}\over{k!}} L_{k}(|E|^{2})\right] \eqno(2.1.7.6)] for centrosymmetric and noncentrosymmetric space groups, respectively, where [p_{c}^{(0)}(|E|)] and [p_{a}^{(0)}(|E|)] are the ideal centric and acentric p.d.f.'s [see (2.1.7.4)[link]] and the unified form of the coefficients [A_{2k}] and [B_{2k}], for [k =] 2, 3, 4 and 5, is [\eqalign{&A_{4}\cr &A_{6}\cr &A_{8}\cr &A_{10}\cr &\cr}\eqalign{&\hbox{ or } \cr &\hbox{ or } \cr &\hbox{ or } \cr &\hbox{ or }\cr &\cr}\eqalign{B_{4} & = a_{4}Q_{4} \cr B_{6} & = a_{6}Q_{6} \cr B_{8} & = a_{8}Q_{8} + U(a_{4}^{2}Q_{4}^{2}-\gamma_{4}^{2})\cr B_{10} & = a_{10}Q_{10} + V(a_{4}a_{6}Q_{4}Q_{6}-\gamma_{4}\gamma_{6}Q_{10})\cr &\quad{}+ W\gamma_{4}^{2}Q_{10} }\eqno(2.1.7.7)] (Shmueli, 1982[link]), where U = 35 or 18, V = 210 or 100 and W = 3150 or 900 according as [A_{2k}] or [B_{2k}] is required, respectively, and the other quantities in equation (2.1.7.7)[link] are given below. The composition-dependent terms in equations (2.1.7.7)[link] are [Q_{2k} = {{\sum_{j = 1}^{m}f_{j}^{2k}}\over{\left(\sum_{n = 1}^{m}f_{n}^{2}\right)^{k}}}, \eqno(2.1.7.8)] where m is the number of atoms in the asymmetric unit, [f_{j},\; j = 1,\ldots,m] are their scattering factors, and the symmetry dependence is expressed by the coefficients [a_{2k}] in equation (2.1.7.7)[link], as follows: [{a_{2k} = (-1)^{k-1}(k-1)!\alpha_{k0} +\textstyle\sum\limits_{p = 2}^{k}(-1)^{k-p}(k-p)!\alpha_{kp} \gamma_{2p},} \eqno(2.1.7.9)] where [\alpha_{kp} = \left(\matrix{ k \cr p } \right) {{(2k-1)!!}\over{(2p-1)!!}} \quad{\rm or}\quad\left(\matrix{ k \cr p } \right) {{k!}\over{p!}} \eqno(2.1.7.10)] according as the space group is centrosymmetric or noncentrosymmetric, respectively, and [\gamma_{2p}] in equation (2.1.7.9)[link] is given by [\gamma_{2p} = {{\langle |T|^{2p} \rangle}\over{\langle |T|^{2} \rangle^{p}}}, \eqno(2.1.7.11)] where [\langle |T|^{k} \rangle] is the kth absolute moment of the trigonometric structure factor [T({\bf h}) = \textstyle\sum\limits_{s = 1}^{g}\exp[2\pi i{\bf h}^{T}({\bf P}_{s}{\bf r}+{\bf t}_{s})] \equiv \xi({\bf h})+i\eta({\bf h}). \eqno(2.1.7.12)] In equation (2.1.7.12)[link], g is the number of general equivalent positions listed in IT A (2005)[link] for the space group in question, times the multiplicity of the Bravais lattice, [({\bf P}_{s},{\bf t}_{s})] is the sth space-group operator and [{\bf r}] is an atomic position vector.

The cumulative distribution functions, obtained by integrating equations (2.1.7.5)[link] and (2.1.7.6)[link], are given by [\eqalignno{N_{c}(|E|)& = {\rm erf}\left({{|E|}\over{\sqrt{2}}}\right)-{{2}\over{\sqrt{\pi}}} \exp\left(-{{|E|^{2}}\over{2}}\right) \cr &\quad\times \left[\sum_{k = 2}^{\infty}{{A_{2k}}\over{(2k)!}} He_{2k-1}(|E|)\right] &(2.1.7.13)}] and [\eqalignno{N_{a}(|E|) & = 1 - \exp(-|E|^{2}) + \exp(-|E|^{2}) \cr &\quad\times \left\{\sum_{k = 2}^{\infty}{{(-1)^{k}B_{2k}}\over{k!}}[L_{k-1}(|E|^{2}) - L_{k}(|E|^2)]\right\}\cr &&(2.1.7.14)}] for centrosymmetric and noncentrosymmetric space groups, respectively, where the coefficients are defined in equations (2.1.7.7)[link]–(2.1.7.12)[link] [link] [link] [link] [link]. Note that the first term on the right-hand side of equation (2.1.7.13)[link] and the first two terms on the right-hand side of equation (2.1.7.14)[link] are just the cumulative distributions derived from the ideal centric and acentric p.d.f.'s in Section 2.1.5.6[link].

The moments [\langle |T|^{2k} \rangle] were compiled for all the space groups by Wilson (1978b[link]) for [k =] 1 and 2, and by Shmueli & Kaldor (1981[link], 1983[link]) for [k =] 1, 2, 3 and 4. These results are presented in Table 2.1.7.1[link]. Closed expressions for the normalized moments [\gamma_{2p}] were obtained by Shmueli (1982[link]) for the triclinic, monoclinic and orthorhombic space groups except [Fdd2] and [Fddd] (see Table 2.1.7.2[link]). The composition-dependent terms, [Q_{2k}], are most conveniently computed as weighted averages over the ranges of [(\sin\theta)/\lambda] which were used in the construction of the Wilson plot for the computation of the [|E|] values.

Table 2.1.7.1| top | pdf |
Some even absolute moments of the trigonometric structure factor

The symbols p, q, r and s denote the second, fourth, sixth and eighth absolute moments of the trigonometric structure factor T [equation (2.1.7.12)[link]], respectively, and the columns of the table contain (for some conciseness) [p, q, r/p] and [s/p^{2}]. The numbers in parentheses, appearing beside some space-group entries, refer to hkl subsets which are defined in the note at the end of the table. These subset references are identical with those given by Shmueli & Kaldor (1981[link], 1983[link]). The symbols q, r and s are also equivalent to [\gamma_{4}P^{2}], [\gamma_{6}P^{3}] and [\gamma_{8}P^{4}], respectively, where [\gamma_{2n}] are the normalized absolute moments given by equation (2.1.7.11)[link].

Space groups(s)pq[r/p][s/p^{2}]
Point group: 1        
P1 1 1 1 1
Point group: [\bar{\bf 1}]        
[P\bar{1}] 2 6 10 17½
Point groups: 2, m        
All P 2 6 10 17½
All C 4 48 160 560
Point group: [{\bf 2/}{\bi m}]        
All P 4 36 100 306¼
All C 8 288 1600 9800
Point group: 222        
All P 4 28 64 169¾
All C and I 8 224 1024 5432
F222 16 1792 16384 173824
Point group: mm2        
All P 4 36 100 306¼
All A, C and I 8 288 1600 9800
Fmm2 16 2304 25600 313600
Fdd2 (1) 16 2304 25600 313600
Fdd2 (2) 16 1280 7168 43264
Point group: mmm        
All P 8 216 1000 5359[3\over8]
All C and I 16 1728 16000 171500
Fmmm 32 13824 256000 5488000
Fddd (1) 32 13824 256000 5488000
Fddd (2) 32 7680 71680 757120
Point group: 4        
[P4, P4_{2}] 4 36 100 306¼
[P4_{1}] (3) 4 36 100 306¼
[P4_{1}] (4) 4 20 28 42¼
[I4] 8 288 1600 9800
[I4_{1}] (5) 8 288 1600 9800
[I4_{1}] (6) 8 160 448 1352
Point group: [\bar{\bf 4}]        
[P\bar{4}] 4 28 64 169¾
[I\bar{4}] 8 224 1024 5432
Point group: [{\bf 4}/{\bi m}]        
All P 8 216 1000 5359[3\over8]
[I4/m] 16 1728 16000 171500
[I4_{1}/a] (7) 16 1728 16000 171500
[I4_{1}/a] (8) 16 960 4480 23660
Point group: 422        
[P422], [P42_{1}2], [P4_{2}22], [P4_{2}2_{1}2] 8 136 424 1682[1\over8];
[P4_{1}22], [P4_{1}2_{1}2] (3) 8 136 424 1682[1\over8]
[P4_{1}22], [P4_{1}2_{1}2] (4) 8 104 208 470[1\over8]
I422 16 1088 6784 53828
[I4_{1}22] (7) 16 1088 6784 53828
[I4_{1}22] (8) 16 832 3328 15044
Point group: 4mm        
All P 8 168 640 2970[5\over8]
I4mm, I4cm 16 1344 10240 95060
[I4_{1}md, I4_{1}cd] (7) 16 1344 10240 95060
[I4_{1}md, I4_{1}cd] (8) 16 832 3328 15188
Point groups: [\bar{\bf 4}{\bf 2}{\bi m},\bar{\bf 4}{\bi m}{\bf 2}]        
All P 8 136 424 1682[1\over8]
[I\bar{4}m2, I\bar{4}2m, I\bar{4}c2] 16 1088 6784 53828
[I\bar{4}2d] (5) 16 1088 6784 53828
[I\bar{4}2d] (6) 16 832 3328 15044
Point group: 4/mmm        
All P 16 1008 6400 51985[15\over16]
[I4/mmm], [I4/mcm] 32 8064 102400 1663550
[I4_{1}/amd, I4_{1}/acd] (5) 32 8064 102400 1663550
[I4_{1}/amd, I4_{1}/acd] (6) 32 4992 33280 265790
Point group: 3        
All P and R 3 15 31 71
Point group: [\bar{\bf 3}]        
All P and R 6 90 310 1242½
Point group: 32        
All P and R 6 66 166 508½
Point group: 3m        
P3m1, P31m, R3m 6 66 178 604½
P3c1, P31c, (3); R3c (1) 6 66 178 604½
P3c1, P31c, (4); R3c (2) 6 66 154 412½
Point group: [\bar{\bf 3}{\bi m}]        
[P\bar{3}1m, P\bar{3}m1, R\bar{3}m] 12 396 1780 10578¾
[P\bar{3}1c, P\bar{3}c1] (3); 12 396 1780 10578¾
 [R\bar{3}c] (1)        
[P\bar{3}1c, P\bar{3}c1] (4); 12 396 1540 7218¾
 [R\bar{3}c] (2)        
Point group: 6        
P6 6 90 340 1522½
[P6_{1}] (9) 6 90 340 1522½
[P6_{1}] (10) 6 54 91 161½
[P6_{1}] (11) 6 54 97 193½
[P6_{1}] (12) 6 90 280 962½
[P6_{2}] (13) 6 90 340 1522½
[P6_{2}] (14) 6 54 97 193½
[P6_{3}] (3) 6 90 340 1522½
[P6_{3}] (4) 6 90 280 962½
Point group: [\bar{\bf 6}]        
[P\bar{6}] 6 90 310 1242½
Point group: [{\bf 6/{\bi m}}]        
[P6/m] 12 540 3400 26643¾
[P6_{3}/m] (3) 12 540 3400 26643¾
[P6_{3}/m] (4) 12 540 2800 16843¾
Point group: 622        
P622 12 324 1150 5506¼
[P6_{1}22] (9) 12 324 1150 5506¼
[P6_{1}22] (10) 12 252 577 1537¾
[P6_{1}22] (11) 12 252 583 1601¾
[P6_{1}22] (12) 12 324 1090 4746¼
[P6_{2}22] (13) 12 324 1150 5506¼
[P6_{2}22] (14) 12 252 583 1601¾
[P6_{3}22] (3) 12 324 1150 5506¼
[P6_{3}22] (4) 12 324 1090 4746¼
Point group: 6mm        
P6mm 12 396 1930 12818¾
P6cc (3) 12 396 1930 12818¾
P6cc (4) 12 396 1450 6098¾
[P6_{3}cm, P6_{3}mc] (3) 12 396 1930 12818¾
[P6_{3}cm, P6_{3}mc] (4) 12 396 1630 8338¾
Point groups: [\bar{\bf 6}{\bi m}{\bf 2}, \bar{\bf 6}{\bf 2}{\bi m}]        
[P\bar{6}m2, P\bar{6}2m] 12 396 1780 10578¾
[P\bar{6}c2, P\bar{6}2c] (3) 12 396 1780 10578¾
[P\bar{6}c2, P\bar{6}2c] (4) 12 396 1540 7218¾
Point group: 6/mmm        
P6/mmm 24 2376 19300 224328[1\over8]
P6/mcc (3) 24 2376 19300 224328[1\over8]
P6/mcc (4) 24 2376 14500 106728[1\over8]
P6/mcm, P6/mmc (3) 24 2376 19300 224328[1\over8]
[P6/mcm], [P6/mmc] (4) 24 2376 16300 145928[1\over8]
Point group: 23        
P23, [P2_{1}3] 12 276 760 2695¼
I23, [I2_{1}3] 24 2208 12160 86248
F23 48 17664 194560 2759936
Point group: [{\bi m}\bar{\bf 3}]        
[Pm\bar{3}, Pn\bar{3}, Pa3] 24 1800 9400 67703[1\over8]
[Im\bar{3}, Ia\bar{3}] 48 14400 150400 2166500
[Fm\bar{3}] 96 115200 2406400 69328000
[Fd\bar{3}] (1) 96 115200 2406400 69328000
[Fd\bar{3}] (2) 96 96768 1484800 28183680
Point group: 432        
[P432, P4_{2}32] 24 1272 4648 25216[7\over8]
[P4_{1}32] (15) 24 1272 4648 25216[7\over8]
[P4_{1}32] (16) 24 1176 3568 13916[7\over8]
[P4_{1}32] (17) 24 1080 2776 8664[7\over8]
[P4_{1}32] (18) 24 984 2272 6580[7\over8]
I432 48 10176 74368 806940
[I4_{1}32] (15) 48 10176 74368 806940
[I4_{1}32] (17) 48 8640 44416 277276
F432 96 81408 1189888 25822080
[F4_{1}32] (15) 96 81408 1189888 25822080
[F4_{1}32] (18) 96 62976 581632 6738816
Point group: [\bar{\bf 4}{\bf 3}{\bi m}]        
[P\bar{4}3m] 24 1272 5128 32896[7\over8]
[P\bar{4}3n] (1) 24 1272 5128 32896[7\over8]
[P\bar{4}3n] (2) 24 1272 4168 17536[7\over8]
[I\bar{4}3m] 48 10176 82048 1052700
[I\bar{4}3d] (15); (20) 48 10176 82048 1052700
[I\bar{4}3d] (15); (21) 48 10176 66688 561180
[I\bar{4}3d] (17) 48 8640 44416 277276
[F\bar{4}3m] 96 81408 1312768 33686400
[F\bar{4}3c] (15) 96 81408 1312768 33686400
[F\bar{4}3c] (18) 96 81408 1067008 17957760
Point group: [{\bi m}\bar{\bf 3}{\bi m}]        
[Pm\bar{3}m, Pn\bar{3}m] 48 8784 72160 972717[13\over16]
[Pn\bar{3}n, Pm\bar{3}n] (1) 48 8784 72160 972717[13\over16]
[Pn\bar{3}n, Pm\bar{3}n] (2) 48 8784 56800 488877[13\over16]
[Im\bar{3}m] 96 70272 1154560 31126970
[Ia\bar{3}d] (15); (20) 96 70272 1154560 31126970
[Ia\bar{3}d] (15); (21) 96 51840 432640 4497850
[Ia\bar{3}d] (17) 96 70272 908800 15644090
[Fm\bar{3}m] 192 562176 18472960 996063040
[Fm\bar{3}c] (1) 192 562176 18472960 996063040
[Fm\bar{3}c] (2) 192 562176 14540800 500610880
[Fd\bar{3}m] (1) 192 562176 18472960 996063040
[Fd\bar{3}m] (2) 192 414720 7782400 205432640
[Fd\bar{3}c] (1) 192 562176 18472960 996063040
[Fd\bar{3}c] (2) 192 414720 6799360 136619840

Note. hkl subsets: (1) [h + k + l = 2n]; (2) [h + k + l = 2n + 1]; (3) [l = 2n]; (4) [l = 2n + 1]; (5) [2h + l = 2n]; (6) [2h + l = 2n + 1]; (7) [2k + l = 2n]; (8) [2k + l = 2n + 1]; (9) [l = 6n]; (10) [l = 6n + 1, 6n + 5]; (11) [l = 6n + 2, 6n + 4]; (12) [l = 6n + 3]; (13) [l = 3n]; (14) [l = 3n + 1, 3n + 2]; (15) hkl all even; (16) only one index odd; (17) only one index even; (18) hkl all odd; (19) two indices odd; (20) [h + k + l = 4n]; (21) [h + k + l = 4n + 2].
And the enantiomorphous space group.

Table 2.1.7.2| top | pdf |
Closed expressions for [\gamma_{2k}] [equation (2.1.7.11)[link]] for space groups of low symmetry

The normalized moments [\gamma_{2k}] are expressed in terms of [M_{k}], where [M_{k} = {(2k)! \over 2^{k}(k!)^{2}} = {(2k - 1)!! \over k!},] and [l'], which takes on the values 1, 2 or 4 according as the Bravais lattice is of type P, one of the types A, B, C or I, or type F, respectively. The expressions for [\gamma_{2k}] are identical for all the space groups based on a given point group, except Fdd2 and Fddd. The expressions are valid for general reflections and under the restrictions given in the text.

Point group(s)Expression for [\gamma_{2k}]
1 1
[\bar{1}, 2, m] [l'^{k - 1} M_{k}]
[2/m, mm2] [l'^{k - 1} M_{k}^{2}]
mmm [l'^{k - 1} M_{k}^{3}]
222 [{l'^{k - 1} \over 2^{k} (k!)^{2}} \sum\limits_{p=0}^{k} (M_{p}M_{k - p})^{3}[p! (k - p)!]^{2}]

References

First citationInternational Tables for Crystallography (2005). Vol. A. Space-group symmetry, edited by Th. Hahn. Heidelberg: Springer.Google Scholar
First citationAbramowitz, M. & Stegun, I. A. (1972). Handbook of mathematical functions. New York: Dover.Google Scholar
First citationShmueli, U. (1982). A study of generalized intensity statistics: extension of the theory and practical examples. Acta Cryst. A38, 362–371.Google Scholar
First citationShmueli, U. & Kaldor, U. (1981). Calculation of even moments of the trigonometric structure factor. Methods and results. Acta Cryst. A37, 76–80.Google Scholar
First citationShmueli, U. & Kaldor, U. (1983). Moments of the trigonometric structure factor. Acta Cryst. A39, 615–621.Google Scholar
First citationShmueli, U. & Wilson, A. J. C. (1981). Effects of space-group symmetry and atomic heterogeneity on intensity statistics. Acta Cryst. A37, 342–353.Google Scholar
First citationWilson, A. J. C. (1978b). Variance of X-ray intensities: effect of dispersion and higher symmetries. Acta Cryst. A34, 986–994.Google Scholar








































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