Rietveld refinement of a wrong crystal structure

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.

Computing details

Program(s) used to refine structure: GSAS.

Figures

(BETA_MIT_GAMMA_publ)

Crystal data

C20H12N2O2	β = 100.58 (2)°
Mr = 314.34	V = 694.14 (14) Å^3
Monoclinic, P21/c	Z = 2
a = 6.6661 (10) Å	? radiation, λ = 1.5406 Å
b = 3.8680 (5) Å	?, ? × ? × ? mm
c = 27.386 (5) Å

Data collection

Refinement

Least-squares matrix: full	Profile 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 = 0.052	10 parameters
Rwp = 0.085	83 restraints
Rexp = 0.010	(Δ/σ)max = 3.68
χ^2 = 98.804	Background 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

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
C01	0.51401	−0.20657	0.66742	0.025*
C02	0.66098	−0.22739	0.70508	0.025*
C03	0.86323	−0.24161	0.69767	0.025*
C04	0.89823	−0.2136	0.65734	0.025*
C05	0.732	−0.07503	0.61591	0.025*
C06	0.55528	−0.09167	0.62044	0.025*
C07	0.65548	0.3341	0.54229	0.025*
C08	0.42141	0.30107	0.54288	0.025*
C09	0.36955	0.16418	0.57606	0.025*
C10	0.72987	0.59825	0.50652	0.025*
Dummy	0.5	0.5	0.5	0.025*
H1	0.93453	0.26675	0.58321	0.025*
H01	0.35628	−0.23264	0.67349	0.025*
H02	0.63377	−0.19783	0.73843	0.025*
H03	0.96431	−0.24469	0.72349	0.025*
H04	1.05341	−0.24163	0.6513	0.025*
H10	0.87521	0.65724	0.51146	0.025*
N1	0.79304	0.18743	0.57831	0.025*
O1	0.20746	0.24935	0.58784	0.025*