Unknown crystal structures

Why to keep secret your favourite unknown in you desk ? Let a chance to others to fail also in determining their structure, or to succeed in ! Public-domain identified problems are listed below : hitherto unknown structures (due to absence of suitable single crystal) having a chance to be solved from powder diffraction data one day or another. Send your propositions to alb@cristal.org with a form similar to the following :

Estimated Formula ; Reference ; Short comment

X-KAlF4 (X=?) ; J. Solid State Chem. 98 (1992) 151-158 ; A new variety obtained from the thermal dehydration of K2(H5O2)Al2F9. A cell proposition was given. The attempt to determine the structure from powder diffraction data failed. The X-ray pattern is complex, reflections are slightly broadened. The pattern may be obtained but not the sample. After H. Duroy, the starting compound is extremely hard to prepare : forget K(H3O)2AlF6 in a drawer for a year under specific (but still remaining to define) atmospheric conditions, then observe that crystals have lost their transluscence, make a powder pattern (could be a good candidate for the "irreproducible results" review). Try to determine the structure ! DOWNLOAD KAlF4-X.zip containing the X-ray Cu-K-alpha pattern in various standard formats.

SOLVED - HAlF4. Participate to The DuPont Powder Challenge and win US$ 1000 (I suggest that if you solve their problem, you should ask for more money because they have probably yet spend 50 or 100.000$ if not more for syntheses, synchrotron and neutron patterns, and still have not the solution !-). Moreover, I have rarely seen so bad data : lot of noise and background, plus preferred orientation : the challenge would be easier if good patterns were provided (even a conventional X-ray powder pattern would have chances to be better than the low-quality synchrotron patterns - which may be due to low sample quality and waek quantity, and low conting time, and unstability under the beam and so on...).
ANYWAY, The DuPont Powder Challenge has been won, see the discussion at the SDPD forum under the title "Chemical analysis of the year".

One hundred phosphates at least (!) ; in 'Crystal Chemistry of Condensed Phosphates', A. Durif, Plenum Press (1995) ; A book giving a huge list of (in principle) well identified compounds with hitherto unknown structure. Most of them are candidates for a SDPD because no sufficiently large single crystal was obtain due to unfavourable way of synthesis (dehydration, precipitation...). Here is a partial list with the page where the compound is cited in the book (see the book at these pages for more precise references) :
Na6P4O13 (p.8); trömelite Ca4P6O19 (p.15)[too late, see: Z. Anorg. Allg. Chem. 631 (2005) 1272-1276]; A4P2O7, A=Li, Na, K, Tl (p.16); NaH3P2O7 (p.19); Be2P2O7 (p.21); CaH2(P2O7)2.H2O, Ca3H2(P2O7)2.4H2O (p.23); Ca2P2O7.2H2O and 4H2O (p.28); AB2P2O7, A=Zn, Pb, B=K, Na, Rb (p.30); Pb3K2(P2O7)2, CdK2P2O7.4H2O, CuNa6(P2O7)2.16H2O (p.31); Ca3K2(P2O7)2.2H2O (p.32); CuA2P2O7, A=Li, Na (p.34); etc, an unknown each 3 pages as a mean...

> 300 minerals, and probably more - see the list... amazing...!

Bi1-xLnxO1.5Ln ; J. Solid State Chem. 124 (1996) 287-291 ; The author states : 'Although single crystal X-ray diffraction analysis is strictly necessary for structure determination, single crystal preparation is very difficult...'. You may try to demonstrate that single crystal is not strictly necessary.

Homologous series C13H28 to C60H122 ; J. Mater Chem. 4 (1994) 977-981 ; Beautiful synchrotron patterns, indexing realized, but no try for structure determination ?

Metal-free phthalocyanine ; J. Crystal Growth 128 (1993) 1257-1262 ; The problem is to prepare metal-free phthalocyanine of sufficient perfection, in (micro)crystal terms...

CaO2 ; J. Solid State Chem. 140 (1998) 103-115; "the crystal structure of the compound is uncertain"...

Just see some chemical catalogs, they contain compounds (many organics) whose the crystal structures are unknown.


Armel Le Bail

Think to learn how to determine a crystal structure by powder diffractometry...