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 firstname.lastname@example.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
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
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...!
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...