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): 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.
back
Armel Le Bail
Think to
learn how to determine a crystal structure by powder diffractometry...