Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 166

a b c

Fig. 20 (a) The structure of KLiBeF 4 projected on (110). Both the Li and Be atoms ( red and blue ,
respectively) are forming a wurtzite-like array stuffed with the K atoms ( grey ). The F atoms ( small
green spheres ) are located close to the hypothetical Li-Be bonds. (b) A perspective view of the
KLiSO 4 exhibiting the same wurtzite-like array of the [LiS] subarray ( red and yellow spheres).
The K atoms are also filling the four-connected network. The O atoms have been omitted for
clarity. (c) The cation array of the hexagonal HT phase of Ca 2 GeO 4 . The Ca (1) atoms located at
(0, 0, 0) and (0, 0, 1/2) form 3 6 ( hcp ) layers. Both the Ca (2) and Ge atoms produce a four-
connected, distorted wurtzite-like network. The O atoms have been omitted

transfer from Na (Cs) to the most electronegative Li atoms. The same occurs with
the HT phase of CsLi(CrO 4 )[ 81 ] which is converted into (

C -Xe)[ C -BeCrO 4 ], with

a blende-type -BeCr subarray.

It is important to remark that KLiBeF 4 [ 58 ] , with a cation array of the Fe 2 P type,
transforms into filled wurtzite-like structures at HT, as it can be seen in Fig. 20a, b ,
respectively. The related LiKSO 4 also adopts this wurtzite-type arrangement [ 59 ] .
In terms of the

C

, this transition would imply that the most electropositive K
atoms would transfer one electron to the Li atoms converting them, respectively,
into K + [

EZKC

-BeBeF 4 ] and K + [ C C -BeSO 4 ], whose Be and BeS skeletons are, in turn,

wurtzite type.
In the case of the anti-fluorite-type compounds Na 2 S and Li 2 SO 4 , the electron
transfer between atoms of the same kind could be seen as weakly founded, because
each Na(Li) atom is forming, with the S atom, two indistinguishable structures,
which have been interpreted as resonance structures [ 33 ]. However, the formation,
at HT, of a stuffed wurtzite-type structure by Ca 2 GeO 4 (Fig. 20c ) gives support to
our approach. In this special case, the two Ca atoms are located in unequivalent
positions. Because in ZnS the blende wurtzite transition occurs at HT, it is also

possible that our walk , depicted in Scheme 1 , might well commence with this new
polymorph. The phase transitions undergone by Ca 2 GeO 4 will be analysed in
greater extent below.

!

8 The Post-Spinel Structures Under Pressure

The structural sequence initiated with the anti-fluorite structure does not stop at the
spinel structure. At higher pressures, the

AB 2 binary alloys and the spinel-type

oxides ( AB 2 O 4 ) undergo additional phase transitions.