Page 68 - Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005)

IR-11.7 S O L I D S

Example:

1. (Bi,Pb) n S n 4
The parent structure is PbS which has the cF8 ( NaCl type) structure. The twin planes
are (311) with respect to the PbS unit cell. Two members of the homologous s eries are
known, Bi 8 Pb 24 S 36 and Bi 8 Pb 12 S 24 , b ut other members are found in the quaternary
Ag-Bi-Pb-S system. The difference between compounds lies in the separation of
the twin planes; each structure is built from slabs of PbS o f v arying t hickness,
alternate s labs being twinned across (311) with respect to the parent structure.

IR-11.6.5 Infinitely adaptive structures

In some s ystems it would appear that any composition can yield a f ully ordered crystal
structure over certain t emperature and composition ranges. As the composition changes, so
the structure changes to meet this need. The term infinitely adaptive structures has been
applied to this g roup of substances. 8
Examples:

1. Compounds in the Cr 2 O 3 -TiO 2 system between the composition ranges
(Cr,Ti)O 2.93 and (Cr,Ti)O 2.90 .
2. Compounds in the Nb 2 O 5 -WO 3 system with block-type structure between t he
:
composition limits N b 2 O 5 and 8WO 3 9Nb 2 O 5 (Nb 18 W 8 O 69 ).
IR-11.6.6 Intercalation compounds

There are several materials in which a g uest species is inserted into a h ost matrix. The
process is called intercalation, and the product i s c alled an intercalation c ompound.
Common examples of intercalated materials are found in the clay s ilicates, layered
dichalcogenides and electrode materials for lithium batteries; graphite intercalation is
considered in detail in Ref. 9. Intercalated m aterials can be designated by conventional
chemical formulae such as Li x TaS 2 (05x51) or by host-guest designations, such as
TaS 2 :xLi (05x51). If the stoichiometry is definite, ordinary c ompound designations may
:
:
be used, e.g. 3TaS 2 4N 2 H 4 , C 5 H 5 N 2TiSe 2 and KC 8 .
Many intercalation compounds are layered structures a nd intercalation is a t wo-
dimensional reaction. The term insertion is sometimes used for three-dimensional e xamples,
as in the tungsten bronzes, e.g. Na x WO 3 , a nd the spinels, e.g. Li x Mn 2 O 4 , a nd also as a
general term for a r eaction involving the transfer of a g uest atom, ion or molecule into a h ost
4
crystal lattice instead o f i ntercalation. More specifically, intercalation is used for an
4
insertion reaction that does not cause a m ajor structural m odification of the host. If the
structure of the host is modified significantly, for example b y b reaking of bonds, then the
insertion can be referred to as topochemical or topotactic. 4

IR-11.7 P O L Y M O R P H I S M

IR-11.7.1 Introduction

A n umber of chemical compounds a nd elements change their crystal structure with external
conditions such as temperature and pressure. T hese various structures a re termed p olymorphic

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