Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 153

a b

c d e

Fig. 12 The structures of the two phases of Cs 2 SO 4 .(a) The cotunnite-like structure stable at
ambient conditions. (b) The HT phase in which the Cs 2 S subarray forms the Ni 2 Al-type structure,
as in Na 2 SO 3 and K 2 SO 3 .(c) The trigonal structure of K 2 SO 3 , isostructural with Na 2 SO 3 .Itis
projected on (110) to show the K 2 S subarray of the Ni 2 Al type. (d) The HT structure of K 2 SO 4 ,
isostructural to the HT phase I-Na 2 SO 4 (compare with Fig. 7 ). (e) The structure of the low
temperature phase (Pnma) of b -K 2 SO 4

the d phase is stable at ambient conditions whereas the y -phase is obtained at HT

(lower pressures). On the contrary, in Na 2 S, the PbCl 2 ! Ni 2 In transition occurs by

increasing pressure. The same occurs in oxides, such as -K 2 SO 4 , cotunnite-like at

RT, which transforms into Ni 2 In type above 859 K. On the contrary, KLiSO 4
undergoes the inverse transition Ni 2 In

b

!

PbCl 2 at 941 K.

5 Oxidation, Temperature and Pressure

At this point, it is convenient to delve in depth in the phase transitions of Cs 2 SO 4 .
Although advanced in the above Sect. 4 , its structural changes deserve an additional
discussion, specially as regards its comparison with its analogues K 2 SO 3 and
K 2 SO 4 . Here, beyond what was advanced in the previous section, our aim is to
discuss the structural changes of this family of compounds, by considering