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Rietveld refinements are generally used to confirm crystal structures solved from powder diffraction data. If the Rietveld refinement converges with low R values and with a smooth difference curve, and the structure looks chemically sensible, the resulting structure is generally considered to be close to the correct crystal structure. Here we present a counter example: The Rietveld refinement of the X-ray powder pattern of γ-quinacridone with the crystal structure of β-quinacridone gives quite a smooth difference curve; the resulting crystal structure looks reasonable in terms of molecular conformation, molecular packing and intermolecular hydrogen bonds. However, neither the lattice parameters, the molecular packing nor the conformation of the molecules show any similarity with the actual structure, which was determined from single-crystal data. This example shows that a successful Rietveld refinement is not always final proof of the correctness of a crystal structure; in special cases the resulting crystal structure may still be wrong.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108768107050823/av5092sup1.cif
Contains datablock BETA_MIT_GAMMA_publ

rtv

Rietveld powder data file (CIF format) https://doi.org/10.1107/S0108768107050823/av5092sup2.rtv
Contains datablock BETA_MIT_GAMMA_publ

CCDC reference: 671627

Computing details top

Program(s) used to refine structure: GSAS.

Figures top
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(BETA_MIT_GAMMA_publ) top
Crystal data top
C20H12N2O2β = 100.58 (2)°
Mr = 314.34V = 694.14 (14) Å3
Monoclinic, P21/cZ = 2
a = 6.6661 (10) Å? radiation, λ = 1.5406 Å
b = 3.8680 (5) Å?, ? × ? × ? mm
c = 27.386 (5) Å
Data collection top
Refinement top
Least-squares matrix: fullProfile function: CW Profile function number 4 with 21 terms Pseudovoigt profile coefficients as parameterized in P. Thompson, D.E. Cox & J.B. Hastings (1987). J. Appl. Cryst.,20,79-83. Asymmetry correction of L.W. Finger, D.E. Cox & A. P. Jephcoat (1994). J. Appl. Cryst.,27,892-900. Microstrain broadening by P.W. Stephens, (1999). J. Appl. Cryst.,32,281-289. #1(GU) = 351.449 #2(GV) = 271.092 #3(GW) = -166.729 #4(GP) = 177.162 #5(LX) = 1.062 #6(ptec) = 5.16 #7(trns) = 10.93 #8(shft) = -2.5040 #9(sfec) = 0.00 #10(S/L) = 0.0248 #11(H/L) = 0.0227 #12(eta) = 1.0000 #13(S400 ) = 1.3E+00 #14(S040 ) = 9.4E+01 #15(S004 ) = 0.0E+00 #16(S220 ) = 3.4E+02 #17(S202 ) = 3.2E-01 #18(S022 ) = -3.4E-02 #19(S301 ) = 1.1E+00 #20(S103 ) = -5.7E-02 #21(S121 ) = 3.4E+01 Peak tails are ignored where the intensity is below 0.0100 times the peak Aniso. broadening axis 0.0 0.0 1.0
Rp = 0.05210 parameters
Rwp = 0.08583 restraints
Rexp = 0.010(Δ/σ)max = 3.68
χ2 = 98.804Background function: GSAS Background function number 1 with 14 terms. Shifted Chebyshev function of 1st kind 1: 7866.19 2: -6734.34 3: 1960.36 4: -1043.41 5: 393.031 6: -405.838 7: 350.344 8: -133.807 9: 172.298 10: -167.459 11: 155.814 12: -153.533 13: 69.2807 14: -31.5552
1551 data points
Crystal data top
C20H12N2O2β = 100.58 (2)°
Mr = 314.34V = 694.14 (14) Å3
Monoclinic, P21/cZ = 2
a = 6.6661 (10) Å? radiation, λ = 1.5406 Å
b = 3.8680 (5) Å?, ? × ? × ? mm
c = 27.386 (5) Å
Data collection top
Refinement top
Rp = 0.0521551 data points
Rwp = 0.08510 parameters
Rexp = 0.01083 restraints
χ2 = 98.804(Δ/σ)max = 3.68
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
C010.514010.206570.667420.025*
C020.660980.227390.705080.025*
C030.863230.241610.697670.025*
C040.898230.21360.657340.025*
C050.7320.075030.615910.025*
C060.555280.091670.620440.025*
C070.655480.33410.542290.025*
C080.421410.301070.542880.025*
C090.369550.164180.576060.025*
C100.729870.598250.506520.025*
Dummy0.50.50.50.025*
H10.934530.266750.583210.025*
H010.356280.232640.673490.025*
H020.633770.197830.738430.025*
H030.964310.244690.723490.025*
H041.053410.241630.65130.025*
H100.875210.657240.511460.025*
N10.793040.187430.578310.025*
O10.207460.249350.587840.025*

Experimental details

Crystal data
Chemical formulaC20H12N2O2
Mr314.34
Crystal system, space groupMonoclinic, P21/c
Temperature (K)?
a, b, c (Å)6.6661 (10), 3.8680 (5), 27.386 (5)
β (°) 100.58 (2)
V3)694.14 (14)
Z2
Radiation type?, λ = 1.5406 Å
Specimen shape, size (mm)?, ? × ? × ?
Data collection
Diffractometer?
Specimen mounting?
Data collection mode?
Scan method?
2θ values (°)2θmin = ? 2θmax = ? 2θstep = ?
Refinement
R factors and goodness of fitRp = 0.052, Rwp = 0.085, Rexp = 0.010, χ2 = 98.804
No. of data points1551
No. of parameters10
No. of restraints83
(Δ/σ)max3.68

Computer programs: GSAS.

 

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