Results of a systematic search for crystal structures being of interest as potential superconductors
or superconductor parent compounds according to a crystal chemical approach.
On this web page we publish results of a search for inorganic crystal structures which
fulfill the condition claimed in [LKW11] and [KL12] as necessary for unconventional
superconductors. Structure data have been retrieved from the Inorganic Crystal Structure
Database (ICSD) as well as from the scientific literature.
We start with tables which
may be considered as a motivation to pursue the way we propose for the detection of new
groups of unconventional superconductors. In the first set of tables we are showing results
for structure data which are contained in the ICSD with publication date before 1986 and after
1975 containing Cu and O. We might consider this as
a test whether our approach could have been helpful to predict cuprates
as unconventional superconductors before they have been detected.
It must be observed that only those structure data are qualifying for the analysis which have
been determined under ambient conditions (temperature less than 313 K and higher than 273 K,
pressure between 0.095 and 0.11 MPa) and with sufficient precision (R-value less than 0.15) or
if no R-value is given.
Furthermore, the occupation of positions has to be 90% or higher and it should be possible
to assign integer oxidation numbers to all elements of a formula according to the proceeding
described in [LKW11]. In case of multiple occupation of positions the possible alternatives are
analysed separately.
In the ICSD as of July 2012 there are 366 entries for compounds containing Cu and O (including
data for some compounds several times) whose data have been published between 1975 and 1985. We are
left with 79 structures qualifying according to a minimal strength value of 1.2, i.e. having
Infinite Units of Strongly Overlapping Orbitals (IUSOOs)
with a strength at least 20% higher than expected from the stoichiometric valences and Paulings rule
of equal distribution of the valences among the bonds. In the table below we show data for those
22 structures among these structures which have layers as IUSOOs, i.e. dimensionality 2. If no bond in a IUSOO
is formed by Cu and O the entry is marked by a star.
The CC value from the ICSD is given in bold faces if the corresponding structure is a known parent
compound of an unconventional superconductor.
It is remarkable that this property holds for 7 compounds out of the 22.
The CC numbers of the structure data in the ICSD database are given together with the formula
of the structure, the maximal strength value of a IUSOO, the year of publication, and the
dimensionality of the IUSOOs. In case of multiple publications of data for the same compound the newest
one is included if there are no substantial differences in the result of the analysis. The formula notation is taken from
the ICSD. In case the compound has a single IUSOO the strength value denotes the minimum of all lowest bond
valences of bond paths in the IUSOO of a structure; if there are several IUSOOs the maximum of these
strength values is shown.
Cuprates with strong two-dimensional IUSOOs; crystal structure data published before 1986
| CC |
Formula |
Year |
Max. strength |
| |
|
|
of a IUSOO |
| |
| 4087 | Cl2 Cu1 O2 Sr2 | 1975 | 1.21 |
| 4327 | Cu1 I2 O6 | 1975 | 1.27 |
| 4202 | Cu2 Mg1 O3 | 1975 | 1.23 |
| *4203 | Cu1 Nd2 O4 | 1975 | 1.47 |
| 64956 | H3 Cl1 Cu2 O3 | 1975 | 1.33 |
| *115 | Ba3 Cl2 Cu2 O4 | 1976 | 1.22 |
| *33569 | Ba2 Cu1 O6 W1 | 1976 | 1.56 |
| *36128 | Ba2 Br2 Cu3 O4 | 1976 | 1.53 |
| 1027 | Ca2 Cl2 Cu1 O2 | 1977 | 1.51 |
| 1028 | Br2 Ca2 Cu1 O2 | 1977 | 1.49 |
| *1038 | Ba2 Cl1 Cu1 O2 | 1977 | 1.40 |
| 1076 | Cu1 La2 O4 | 1977 | 1.62 |
| 1178 | Br2 Cu1 O2 Sr2 | 1977 | 2.13 |
| *41844 | Cu1 Gd2 O4 | 1977 | 1.51 |
| 100072 | H2 Cu5 O14 Si4 | 1977 | 1.20 |
| 8268 | H2 As2 Cu3 O10 Pb1 | 1979 | 1.55 |
| 100702 | Al1 Cu1 In1 O4 | 1980 | 1.29 |
| 100761 | Cu2 La2 O6 Sr1 | 1980 | 1.24 |
| 201837 | H10 Cu4 O12 S1 | 1984 | 1.21 |
| 30683 | H4 As2 Cu5 O12 | 1985 | 1.23 |
| 61389 | Ca1 Cu1 Fe2 O5 | 1985 | 1.20 |
The complete list (80 structures) can be found here.
In a path table we show for all qualifying structures
information on the paths forming
the IUSOOs. IUSOOs of a structure are numbered and additionally to their strength (Str) the
periodicity (Per, length of repeating unit), the elements involved (cation A and anion X)
as well as the weakest bond valence (Min_path_str) are given for each path.
There are several entries for a single path of a IUSOO if edges in the path differ in the
cation or anion element or in the strength value or both.
In the same manner as for cuprates we checked the existence of structure data for iron pnictides in the
ICSD fulfilling our necessary condition for superconductivity. The following table shows the results for
compounds with layered IUSOOs with all ICSD data published before 2008 and after 1975.
Iron pnictides with strong two-dimensional IUSOOs; crystal structure data published before 2008
| CC |
Formula |
Year |
Max. strength |
| |
|
|
of a IUSOO |
| |
| 610431 | As2 Eu1 Fe2 | 1978 | 1.58 |
| 610527 | As2 Fe2 Sr1 | 1980 | 1.48 |
| 31473 | As2 Fe2 K1 | 1981 | 1.71 |
| 42327 | Fe1 Li1 P1 | 1985 | 1.29 |
| 42331 | As1 Fe1 Li1 | 1985 | 1.63 |
| 633102 | Fe2 P2 Sr1 | 1990 | 2.11 |
| 602466 | As1 Fe1 Nd1 O1 | 1993 | 1.81 |
| 602489 | As1 Fe1 La1 O1 | 1993 | 1.62 |
| 602490 | As1 Ce1 Fe1 O1 | 1993 | 1.71 |
| 80200 | Fe1 O1 P1 Pr1 | 1995 | 1.97 |
| 80202 | Fe1 O1 P1 Sm1 | 1995 | 2.04 |
| *83394 | C6 Fe1 N6 Na1 Rb2 | 1996 | 1.24 |
| 54408 | Ba1 Fe2 P2 | 1997 | 2.06 |
| 90316 | As1 Fe1 O1 Pr1 | 2000 | 1.73 |
| *280850 | C6 Fe1 K1 N3 O3 | 2002 | 1.33 |
In the ICSD as of July 2012 there are 2102 entries for compounds containing Fe and a pnictide (N,P,As,Sb,Bi)
whose data have been published between 1975 and 2007. We are
left with 43 structures qualifying according to a minimal strength value of 1.2. From 15 structures with layered
IUSOOs 11 (!) are known as superconductors or parent compounds of superconductors. Furthermore, the non-superconducting
Ba1Fe2P2 is end member of the BaFe2(As1-xPx)2 series.
You may miss superconducting BaFe2As2 in this table; this compound, however, has a three-dimensional IUSOO with strength 1.25
since Ba-As bonds are strong as well. If only Fe-As bonds are considered the IUSOO of BaFe2As2 is a layer.
Two compounds in the table (CC with a star) have IUSOOs without Fe bonds: In C6 Fe1 N6 Na1 Rb2 IUSOOs are built
by Rb-N and Na-N bonds; in C6 Fe1 K1 N3 O3 there are only K-N bonds in IUSOOs.
The complete list with 43 entries comprising also structures with one- and three-dimensional IUSOOs can be found
here
For the path table with the same format as given for the cuprates see here.
References
[LKW11] Liebau, F., Klein, H.-J., Wang, X.: A crystal chemical approach to superconductivity. I. A bond-valence sum analysis
of inorganic compounds, Zeitschrift für Kristallographie 226, pp. 309-318, 2011
[KL12] Klein, H.-J., Liebau, F.: A crystal chemical approach to superconductivity. II. An analysis of bond-valence
distribution in inorganic compounds, Zeitschrift für Kristallographie 227, pp. 179-189, 2012