Geoscience Reference
In-Depth Information
[242]
. The study of ionic conductivity in condensed phosphates needs a thorough
knowledge of crystalline structure, which insists on single crystals. Similarly, rare
earth phosphates, particularly orthophosphates have very high melting points
(2300
2500
C) and they are known as high-temperature ceramics in addition to
their common names like color luminophors and scintillators. More recently, mag-
netic properties have also been reported in some orthophosphates. All these proper-
ties have not been understood precisely. Therefore, the growth of these materials in
the form of single crystals and their various physical characteristics need to be
understood. There may be over 1000 reports on the synthesis of phosphates bearing
rare earth elements, alkali, alkaline earth, and transitional elements. One-third of
them might deal with hydrothermal technique.
7.13.1 Structural Chemistry of Rare Earth Phosphates
The formation of a wide variety of phosphates, particularly rare earth phosphates is
connected with the reaction susceptibility of phosphoro-oxygen anions to give vari-
ous degrees of condensation. As a result of such reactions among the phosphoro-
oxygen anions, there develops bridging oxygen bonds
forming pyr-
ophosphates—[P
2
O
7
]—initially followed by more complex linear anions like
[P
n
O
3n
1
1
], rings like [P
n
O
3n
](n
a
P
a
O
a
O
a
P
a
3), and so on. It is extremely difficult to classify
phosphates unlike silicates, because of high complexity in the phosphates' internal
structures. For example, a recently reported structure of (NH
4
)
2
Ce(PO
3
)
5
[243]
has
two infinite crystallographically nonequivalent (PO
3
)
2
chains, one running parallel
to the a-axis, the other along c-axis, both with a period of five tetrahedra. This
compound is the first example of a long chain polyphosphate with crystallographi-
cally independent chains and such a peculiarity has not been reported in silicates or
germanates so far.
The phosphorus atoms participate in the form of phosphoro-oxygen anions
[H
2
PO
4
]
2
1
, [HPO
4
]
2
2
, and [PO
4
]
2
1
in the reactions forming respective phosphates.
The [PO
4
]
2
3
anions are less reaction susceptible among these three, and that makes
the solid-state synthesis difficult. Therefore, ammonium phosphates are used in the
solid phase synthesis. The acidic phosphoro-oxygen anions easily take part in the
reactions during heating and in fact, high reaction susceptibility of acidic
phosphoro-oxygen anions serve as a basis in working out various methods of obtain-
ing rare earth phosphates with a particular reference to the highly condensed ultra-
phosphates
[244]
. With a slow dehydration of the acid orthophosphate anion, the
removal of the (OH)
2
1
ions and the condensation of phosphoro-oxygen anions
occurs. This was a popular method, even in the nineteenth century, but for a long
time the mechanism of the process remained unclear. Growth conditions for the
required phosphate anions were selected empirically and compelled to participate in
the reactions. However, the higher the degree of condensation of phosphate anion,
the more difficult it is to ascertain the growth conditions. The process is quite com-
plicated and very often results in the formation of a mixture of various degrees of
condensed anions. McGilvery and Scott
[245]
concluded that water molecules play a
distinct role in the dehydration process, particularly its concentration influences, not
,
