Chemistry Reference
In-Depth Information
Table 3.2 Summary of hyperfine parameters for goethite
Crystallinity
T (K)
Spectrum
B
av
(T)
B
p
(T)
2e or D (mm/s)
d
Fe
(mm/s)
Very high
RT
S
38.1
38.1
-0.28
0.37
(e.g. Harz)
80
S
50.0
50.0
-0.26
0.48
4
S
50.7
50.7
-0.26
0.49
Related high
RT
S
30-35
31-38
-0.26
0.37
80
S
47-49
49-50
-0.26
0.47
Moderate
RT
coll S
20-25
25-33
-0.25
0.37
+D
-
-
0.55
0.36
80
S
43-47
48-49
-0.25
0.47
Poor
RT
D
-
-
0.55-0.6
0.36
+ coll S
\20
-
-0.25(f)
0.36
80
S
40-43
47-49
-0.25
0.47
Very poor
RT
D
-
-
0.6
0.36
80
S
35-40
47-48
-0.24
0.46
+D (Fh?)
0.6
0.46
S sextet, Coll S collapsing sextet, D doublet, f value fixed in the fit
could roughly serve as a measure for the average particle size or crystallinity of the
goethite phase in a sample. A summary of the hyperfine parameters for goethite
obtained from own research is given in Table
3.2
. The remaining doublet at 80 K is
in many cases most probably ferrihydrite. Examples of different spectra will be
shown in
Sect. 3.7
.
Unfortunately, similar field-reducing and distributive effects are also caused by
isomorphous substitutions which are frequently encountered in natural samples.
Because of the high abundance of Al in nature the substitution of Al for Fe in
goethite has attracted most attention and has been intensively studied [
22
-
25
].
Substitution by diamagnetic Al first of all lowers T
N
so that a doublet is obtained at
RT for an Al substitution of 12 at % and larger [
26
]. Secondly, this substitution
reduces the supertransferred contribution to B, leading to a reduction and a dis-
tribution of B. From systematic studies on synthetic aluminous goethites an
average field reduction of about 0.05 T at 4 K and of about 0.14 T at 80 K per
at % Al has been derived. Because the degree of crystallinity plays a similar role in
the dependence of the hyperfine field, both Al concentration and a crystallinity
parameter have been introduced in various linear equations of the type,
or b
=
MCD
111
Þ
b S m
2
=
g
B
¼
B
0
a C %Al
ð
ð
3
:
4
Þ
where a and b are given coefficients. Either the specific surface area S from the
BET method or the mean crystallite diameter MCD
111
obtained from XRD
broadening of the [
27
] reflection has been taken as a measure for the crystallinity
[
22
,
24
,
25
].
These relationships were initially considered as promising complementary
means to characterize goethite in natural samples. From the most probable
hyperfine
field
B
p
or
better,
the
average
hyperfine
field
B
av
,
either
the
Al
