Geoscience Reference
In-Depth Information
Table 7.25 Cations in Superionic Pyrophosphates
[297]
At. Radii
a
(A
)
Entropy at 298
K (e.u.)
Element
At. No.
At. Wt.
Al
13
65.38
1.82
6.769
Mn
25
54.93
1.79
7.590
Co
27
58.93
1.67
6.800
Ni
28
58.70
1.62
7.137
Zn
30
65.38
1.53
9.950
Cd
48
112.41
1.71
12.300
Pb
82
207.20
1.81
15.490
a
Table of periodic properties of the elements, Sargent
Welch Scientific Company.
HNaMP
2
O
7
(M
Mn, Co, Al, Ni) show good crystallinity and well-developed
crystal morphology, whereas the crystals HNaMP
2
O
7
(M
5
Zn, Cd, Pb) with higher
entropy values have a lower degree of crystallinity. These crystals show poor mor-
phology and are often polycrystalline
[297]
.
The authors have obtained good crystals up to 6 mm size. Twinning was absent
in general, except in crystals doped with additives. In the case of (Na
2/3
Zr
1/3
)
2
P
2
O
7
,
Bi and Ce were added as additives even in higher concentration, but they did not
enter the composition. The cations play a major role in the crystallization of pyro-
phosphates. Also, divalent metals enter the composition more easily than trivalent
metals, particularly in condensed phosphates. A large number of experiments have
been carried out using Al, Cr, Fe, Bi, Co, and rare earths even in surplus amount.
Only Al enters the composition easily and others need higher temperature of syn-
thesis. The volatiles and type of cations play a major role in the crystallization of
condensed phosphates. The morphology of these crystals depends upon the concen-
tration of P
2
O
5
and H
2
O in the system
[263,298]
. For example, in NaHCo(P
2
O
7
)
crystals, the prism faces dominate over the pyramidal faces with an increase in
P
2
O
5
and H
2
O concentration. The morphology of these superionic pyrophosphates
is very interesting and it
5
is shown together with their schematic diagrams in
Figure 7.64a
e
. The crystallization was carried out by spontaneous nucleation,
which was controlled by a systematic rate of heating. Hence, the crystals were well
developed and of excellent quality.
The X-ray diffraction studies (both powder and single crystal) show that these
crystals belong to the lower symmetry and show very interesting morphology.
A general morphology of these pyrophosphates is given in
Table 7.25
.
In order to grow these crystals, it is very important to know their solubility,
internal atomic structure, macro- and micromorphology. The solubility of these
pyrophosphates was found to be positive. Similarly, the crystals obtained by the
hydrothermal technique show very interesting morphology. Hence, it becomes
rather easy to grow them as bulk single crystals of several centimeters in length,
despite the phosphoric acid media in which the growth occurs.
The most common morphological habits observed in pyrophosphates are perfect
rhombohedral, prismatic, rod shaped, amongst others. The crystal faces are more or
