Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 175

parameter in HP-GeO 2 (2.87 ˚ )[ 102 ] . When all these magnitudes are compared,
one sees that the structure of HP-Mn 2 GeO 4 [ 18 , 19 ] is a compromise to satisfy all
these structural and geometrical requirements.
As an example, we can outline that the shortest Mn-Mn distances, i.e. 3.35 ˚ in
bixbyite (Mn 2 O 3 )[ 98 ], 2.88 ˚ in pyrolusite (MnO 2 )[ 104 ], 2.87 ˚ in the rutile-like,
HP-GeO 2 [ 102 ] and 3.14 ˚ in manganosite (MnO) [ 103 ] average to a value of
3.06 ˚ , which is quite close to that of 2.95 ˚ observed in Mn 2 GeO 4 . This is the

value of the projection axis in Fig. 20 [ 18 , 19 ].

8.3 Ca 2 SnO 4 and the FeB Structure

We have mentioned in this section that we could not find any alloy isostruc-

tural to the cation subarray of Ca 2 SnO 4 . However, the subarrays Ca 2 Sn and Mn 2 Ge,
drawn in Figs. 21a and 23a , admit an alternative interpretation. When Fig. 21a is
rotated around the projection axis, Fig. 24a is obtained. On the right side in Fig. 24b ,
we have drawn the FeB structure (

A 2 B

P nma), viewed along b. In spite of their different

stoichiometry ( ), both structures exhibit a surprising similarity in

projection. The FeB-type structure is adopted by the cation arrays of many oxides,
such as BaS in barite (BaSO 4 ), BaMnO 4 , BaFeO 4 , BaSeO 4 , SnSO 4 and PbSeO 4 [ 5 ] ,

A 2 B vs. AB

Fig. 24 (a) The structure of Ca 2 SnO 4 (as in Fig. 16a ) , but rotated around the projection axis to
facilitate its comparison with the TiSi structure (FeB type, nma), represented in (b). In (c) and

(d), we have drawn the isolated Sn-filled prisms which centre the respective unit cells. [ 57 ]

P