Crystal Structures of Inorganic Oxoacid Salts Perceived as Cation Arrays: A Periodic-Graph Approach - Inorganic 3D Structures - page 55

Fig. 17 Topological
resemblance between the sma
net ( P 6 2 22) and the LaPO 4
crystal structure

zeolite-like [ 69 ] and can hardly be considered within the cation array model. Three
“dense” structure types are “grey-isomorphic” to quartz (qtz), cristobalite, or
tridymite [ 70 ] ; in the quasi-binary representation they correspond to

a

-HgS, sphal-

erite (dia-b), and wurtzite (lon), respectively.
The underlying nets of other orthophosphates correspond to the phases metasta-
ble at ambient conditions. Hexagonal modifications of lanthanum, cerium, and
neodymium phosphates are based on the 6,6-coordinated sma topology (Fig. 17 ) .
FePO 4 obtained by the “soft” chemistry method [ 71 ] has the nia-5,5- P 2 1 / c cation
array that was also revealed in sulfites and selenites (Table 7 ). Alternative descrip-
tions of both FeLiPO 4 and FePO 4 , in terms of the extended Zintl-Klemm concept,
can be found in this volume [ 72 ]
g ¼

2:3. Two orthophosphates with this g ratio have similar, but not equivalent
topologies: Nb 2 (PO 4 ) 3 has the topology of the Nasicon framework with the corundum-
like underlying net (Fig. 7 ), while Sb 2 (PO 4 ) 3 adopts the motif of a high-pressure
modification of Rh 2 O 3 [ 73 ] to be topologically close to corundum (Table 5 ) . This is
one more example of resemblance between the oxygen-stuffed alloys and high-
pressure forms of the corresponding binary compounds (cf. [ 7 ]).
g ¼ 3:2. Most of the orthophosphates and orthoarsenates of this g ratio have

complex polynodal underlying nets with a unique topology; the relations to known
topologies of nets are occasional. Thus, the structure type of farringtonite,
Mg 3 (PO 4 ) 2 , has the trinodal 4,6,7-coordinated underlying net that was found in
Mn 3 ( m 7 -phosphonopropionato) 2 (CSD Reference code XEBHOB) [ 74 ].

g ¼ 3:1. According to [ 4 ], the cation array in Ag 3 PO 4 is similar to the 14,12-

coordinated Cr 3 Si ( -W) crystal structure that reflects the spatial arrangement of Ag

and P atoms. If one neglects Ag-Ag contacts, a 4,12-coordinated ith underlying net
(UH 3 structure type [ 16 ]) emerges that is a subnet of Cr 3 Si. Lithium orthophosphate
and orthoarsenate contain a Cu 3 Sb-like cation array [ 4 ] that represents a distorted
hexagonal close packing where the underlying net with only Cu-Sb (Li-P) contacts
is trinodal 4,4,12-coordinated. The recently investigated crystal structure of
K 3 AsO 4 [ 75 ] is the first example of the complex trinodal 4,6,14-coordinated wzz
underlying net.

b

3.3.3 Sulfates and Selenates

g ¼

1:1. The pcu-b and nia underlying nets are quite common for sulfates and
selenates (Table 11 ). Small 6-coordinated M 2+ cations promote a nia cation array