A. Kolb – K. O. Klepp · K4Ti3Te9
637
irregular. K(1) and K(3) are both in distorted mono-
Partial Te-Te bonds are well established in com-
capped trigonal prismatic coordinations (Fig. 3). The pounds containing hypervalent tellurium [24] in
trigonal prisms are connected via common edges to square planar (pseudo octahedral) coordination by its
build infinite corrugated chains running along [010] homonuclear neighbour leading to the formation of
which in turn are connected via the ditelluride pairs discrete Te65− groups as for instance in Ga2Te5 [25],
yielding a layered arrangement. No proper rationaliza- K2SnTe5 [26] or Re2Te5 [27], or polymerisation
tion is possible describing the coordinations of K(2) products thereof as in the infinite chains, 1 (Te5)2−
,
∞
or K(4) (seven and nine Te-neighbours, respectively). of Rb2Te5 [28] or Cs2Te5 [29]. The Te-Te distances
˚
Average K-Te distances are calculated as 3.644(6) A.
originating from the hypervalent centre are typically
˚
The most striking feature of the present compoundis in the range of 2.90 – 3.03 A. As has been stated in the
certainly the presences of Te-Te pairs with exception- introduction, similar Te-Te distances (but no square
ally long bond distances. The application of Pauling’s Te5-groups) are found in many alkali polytelluromet-
bond order – bond length relationship [23]
allates of the group IVa-metals but no attempt has
so far been made to reach a rational interpretation of
their occurrence. Since they show a wider spread of
homonuclear bond distances, such an interpretation
appears less obvious and probably would require
extensive theoretical calculations, in particular for
tellurium-rich compounds like K4Hf3Te17 [18] where
the system of partial Te-Te bonds extends over several
tellurium atoms. For K4Ti3Te9 it should be easier
since only tellurium pairs need to be considered.
Conductivity measurements and the determination of
the magnetic susceptibility should, in addition, help
dn = d1 −0.71log n
reveals fractional bond orders of 0.55 and 0.45 for
˚
d1 = 2.78 A which comes close to 0.5. The reduc-
tion of the bond order with respect to that of a “nor-
mal” ditelluride (Te22−) group could be explained by
the presence of one further electron occupying an anti-
bonding level of the ditelluride unit yielding Te32−. Ne-
glecting weak bonding interactions between the ditel-
luride groups and their tellurium neighbours (the near-
est homonuclear neighbours of the two pairs are at
to clarify the bonding situation within the [Ti3Te9]4−
anions.
-
˚
3.215(2) and 3.333(2) A apart) the Te2-units might
be considered as isolated groups. Under this assump-
tion the crystal chemical formula can be rationalized
as (K+)41∞[(Ti4+)3(Te2)3−2(Te2−)5]. In the normal va-
lence compound Tl2TiTe3 [22] which – for easier com-
parison – may be formulated as Tl6Ti3Te9 a sufficient
number of electrons can be supplied by the cations and
homonuclear Te-Te bonding is no longer necessary.
Acknowkedgement
Financial support of this work through the Jubila¨ums-
¨
fonds der Osterreichischen Nationalbank (project no. P6427)
is gratefully acknowledged.
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Kolb, Andreas
