Crystal Structures of Inorganic Oxoacid Salts Perceived as Cation Arrays: A Periodic-Graph Approach - Inorganic 3D Structures - page 56

Table 11 Underlying nets in sulfates and selenates
Underlying net Sulfate/selenate Correspondence XO 4 coordination

type

K 42 ,K 51 ,K 06 pcu-b (NaCl) MSO 4 (M ¼ Ca, Sr, Ba, Cd, Hg);

HgSeO 4

Normal

K 6

MSO 4 (M ¼ Mg, Mn, Fe, Co, Ni,
Cu, Cd, Zn); MSeO 4 (M ¼ Mg,
Mn, Co, Ni, Cu)

nia (NiAs) Normal

K 8 flu (CaF 2 ) Li 2 SO 4 Anti

K 43 ,K 61 FeB MSO 4 (M ¼ Ca, Sr, Ba, Pb, Eu);

BaSeO 4

Normal

K 52 ,K 61 bcu-7- P 2 1 / c

(RhSi)

CaSO 4 -HP, MSeO 4 (M ¼ Sr, Pb) Normal

K 42 sma g -CaSO 4

K 4 dia-b (ZnS,

sphalerite)

MSO 4 (M ¼ Be, Zn) Normal

K 4 pts (PtS) PdXO 4 (X ¼ S, Se) Normal

K 461 ,K 47 ,K 65 ,K 56 ,
K 911 ,K (10)1

Ni 2 In (BaF 2 -HP) M 2 SO 4 (M Li, Na, K, Rb, Cs,

Tl); M 2 SeO 4 (M

¼ Anti

¼ K, Cs, Tl)

K 82 TiSi 2 M 2 XO 4 (M ¼ Na, Ag; X ¼ S, Se) Anti

K 4 alw (Rh 2 O 3 -HP) M 2 (SO 4 ) 3 (M Fe, In, Y, Er);

M 2 (SeO 4 ) 3 (M

¼ Normal

¼ Fe, Sc, Yb)

K 4 cor ( a -Al 2 O 3 ) 2 (SO 4 ) 3 (M ¼ Al, Ga, Cr, Fe, Normal

In, Sc)

T 3 ,K 4 PbCl 2

(cotunnite)

a -Zr(SO 4 ) 2 Normal

[ 7 ] (Fig. 4 ) , while 8-coordinated Ca and Hg provide a pcu-b underlying net (Fig. 3 ) .
At ambient conditions, sulfates and selenates with large alkaline-earth cations adopt
a 7,7-coordinated FeB-like motif (Fig. 10 ) and transform to pcu-b only at high
temperature. CaSO 4 has 7,7-coordinated underlying nets FeB and RhSi (monazite-
like) under high pressure [ 76 ]. The high-temperature

-CaSO 4 is an example of
reorganization of the anhydrite cation array (pcu-b) to another 6,6-coordinated net
with the sma topology (Fig. 17 ) . Note that

g

g -CaSO 4 is obtained by a “soft chemis-

try” method (heating CaSO 4 0.5H 2 O, cf. the method of obtaining orthophosphates

with the sma cation array; Table 10 ) . 4-coordinated Be 2+ ,Zn 2+ , and Pd 2+ cations
together with 4-coordinated S or Se cations form sphalerite- or PtS-like arrays. The
topology is predetermined by the coordination figures of the M and X cations:
tetrahedral coordination figures give rise to the sphalerite motif while the rect-
angle tetrahedron combination (PdXO 4 ) results in the similar PtS (cooperite)

array (Fig. 18 ).
g ¼

þ

2:1. Most sulfates and selenates of this type have the 5,6,11-coordinated
Ni 2 In-like cation array [ 16 ] (Fig. 6 ) . In [ 7 ] , caesium sulfate and selenate were
related to the cotunnite (PbCl 2 ) structure type. This is an example of discrepancies
between geometrical and topological descriptions obviously caused by the close
topologies of the PbCl 2 and Ni 2 In crystal structures. At the same time, the cation
array in the high-temperature Na 2 SO 4 phase was assigned to the Ni 2 In type [ 6 ] that
confirms the conclusion of the periodic-graph approach. At ambient conditions,