Geoscience Reference
In-Depth Information
BaF 2 a
Ta or Nb) under hydrothermal
conditions have been identified [36,37] . Among them, Ba 3 Nb 2 O 2 F 12
TiO 2 a
FH aq and BaF 2 a
M 2 O 5 a
HF aq (M
5
2H 2 O,
Ba 4 Nb 2 O 3 F 12 , and Ba 3 Ta 2 O 2 F 12 have interesting structures. Ba 3 Ta 2 O 2 F 12 is a new
oxyfluoride with sevenfold coordinated Ta 5 1 . In the monocapped trigonal prism
TaOF 6 , the distances Ta
O 2 or F 2 ) are close to those found in an octahedral
X(X
5
environment, which led to X
X distances smaller than the calculated values. The
isolated TaOF 6 groups are connected together by two types of Ba 2 1 polyhedra. The
Ba 3 Nb 2 O 2 F 12
2H 2 O is a new acentric hydrated niobium oxyfluoride. Its structure is
built from isolated NbOF 6 pentagonal bipyramids. The Ba 4 Nb 2 O 3 F 12 is a 3d oxy-
fluoride, which exhibits two kinds of coordination polyhedra for Nb atoms:
The octahedral form with (NbO 2 F 4 ), (NbO 3 F 3 ) formulations, and the pentagonal
bipyramid (NbOF 6 ). Four (NbX 6 ) octahedra are cris-oxygen connected to form sepa-
rated [Nb 4 O 7 F 14 ] 8 2 condensed blocks, while (NbOF 6 ) pentagonal bipyramids are
isolated but linked together by Ba 2 1 cations.
In all these cases, single crystals have been prepared by the hydrothermal tech-
nique and not by other conventional methods.
The other materials of technological importance among fluorides are
-PbF 2
single crystals. Lead fluoride stands out as an important optical material because of
its chemical and mechanical stability, wide transparency range (from UV to
medium IR), and rich substitutional chemistry (permitting the variation of its com-
position and, consequently, its main operational parameters), and also because its
single crystals can be readily obtained from both melt and solution. Lead fluoride
is poorly soluble and can be easily prepared by the reaction of HF acid with lead
hydroxide or carbonate or by precipitation from aqueous solutions of these reagents
with NH 4 F. The
α
transformation, orthorhombic-to-cubic in PbF 2 , takes place at
460 C. Beta-PbF 2 is chemically more stable and can be readily obtained as large
single crystals from the melt [38] . However, the
αβ
-PbF 2 cannot be prepared by
melt or solid-state reacts or flux methods. Nikolskaya and Demianets [39] have car-
ried out the growth of
α
280 C using Teflon liners and at T
450 C
α
-PbF 2 at T
5
5
using copper ampoules. Extra-pure-grade PbF 2 ,6
20% KF
solutions have been used as the starting materials. The temperature gradient has
been maintained at 35
10% NH 4 F and 10
42 C. At a lower growth temperature of 280 C, the quan-
-PbF 2 is higher than the crystals obtained at 450 C. The crystals are highly
transparent, well faceted, elongated, and up to 8 mm in length. In some cases, the
crystal
tity of
α
length was up to 10 mm, but
in this case the crystals were fragile.
Figure 8.8 shows the micrographs of
-PbF 2 crystals obtained under hydrothermal
conditions by Nikolskaya and Demianets [39] .
α
8.6 Physical Properties of Transition Metal Fluorides and
Fluorocarbonates/Fluorophosphates/Oxyfluorides
These compounds exhibit essentially magnetic properties, ionic conducting, and
nonlinear optical properties. The oxyfluorides exhibit microporous structures, which
Search WWH ::




Custom Search