Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 185

the high-temperature phase of Li 2 SO 4 [ 32 ] , this hypothesis could be seen as weakly
founded, because the two resonance structures are identical. However, in the HT
phase of Ca 2 GeO 4 , one Ca atom provokes the formation of a unique, four-
connected [

-TiGe] skeleton of the wurtzite type, which does not need the addi-
tional “ resonance concept ” to account for its structure.
The same occurs with the HT phase of CsLi(CrO 4 )[ 81 ], which is converted into

C

( -BeCr subarray is forming a blende-type struc-

ture. In the compound under discussion, the wurtzite-like network is formed,
instead of the more common fluorite structure. Because in ZnS, the blende

C -Xe)[ C -BeCrO 4 ], where the C

!
wurtzite transition occurs at HT, the walk might commence from any of these two
polymorphs.
Looking at Fig. 29a, b , it is noticeable that, in both phases, the Ca atoms (green)
form 3 6 layers ( hcp ) which remain unaltered during the transition. On the contrary,
the atom pair [CaGe] (grey and red, respectively) forms irregular 6 3 graphene layers
in the olivine structure (Fig. 5 ), becoming puckered in the HT-Ca 2 GeO 4 , when the
wurtzite-like structure is formed (Fig. 29a ).
The distorted wurtzite-like structure of Ca 2 GeO 4 is chemically meaningful when
we see that in compounds such as KLiBeF 4 [ 58 ] and KLiSO 4 [ 59 ] , the most
electropositive atoms (K) transfer one electron to the Li atoms, converting them
into K + [ -BeBeF 4 ] and K + [ -BeSO 4 ], respectively. In reality, they become

K-stuffed tridymite-like structures, whose respective Be and BeS skeletons form,
in turn, the londsdaleite and wurtzite structures.
This interpretation agrees with the fact that BeF 2 , quartz type at ambient condi-
tions [ 126 ], could transform into either cristobalite or tridymite, as it does SiO 2
itself. Moreover, real BeS is blende type and real BeSO 4 is cristobalite-like!
Remember in this regard that the cristobalite-type structure is also implicit in the
cubic Cs-filled blendes Cs[LiCrO 4 ][ 81 ] and Cs[LiMoO 4 ][ 78 , 80 ] (see Scheme 1 ) .
By examining the drawings of Fig. 20 , one has the impression that when the two
cations are of the same species, as in Ca 2 GeO 4 and Ca 2 SiO 4 , the conversion of one
Ca atom into a

C C

-Ge) seems to be more difficult than in compounds with
unlike cations. In this latter case, it seems that the difference in electronegativity
favours the electron transfer. This could be a reason why in Ca 2 GeO 4 (Fig. 20c ), the
wurtzite-like structure is more distorted (it is taking shape!), whereas in KLiBeF 4
and KLiSO 4 (Fig. 20a ) the arrangement is closer to the ideal tridymite structure.

C -Ti ( C

11 Conclusions

The results discussed in this chapter indicate that the structural transformations
undergone by different compounds may be correlated in a rational manner.
This correlation is reinforced by the observation that the same transitions occur in
the oxides as well as in the corresponding alloys. These new relationships give
additional support to the statement that the “cations subarrays govern the structures
of oxides”, an intuition first expressed by Wondratschek et al. [ 127 , 128 ] , later