Download Cr 6+ ions

Mechanism of the incorporating of the dopant ions
into the structure of oxides under water vapor fluid.
Danchevskaya M. N., Ivakin Yu. D, Torbin S. N.,
Ovchinnikova O. G., Muravieva G.P.
Chemistry Department, Moscow State University, Leninskie Gory, Moscow
119992, Russia
E-mail: [email protected]
Chemistry Department, Moscow State University
1
Experimental
In the present report are given the results of the
mechanism study of incorporation of doping ions in
structure of fine-crystalline corundum (α-Al2O3) and
yttrium-aluminium garnet (Y3Al5O12) formed in
water vapor in sub- and supercritical conditions.
Synthesis of oxides was carried out in autoclaves
under temperature up to 417°C and under pressure
of water fluid up to 31MPa. The pressure of water
vapor was created by water filled between walls of
autoclave and container with starting material,
during heating of autoclave.
2
The synthesized products were investigated by physicalchemical methods.
The electron microscopic photographies were carried out on
the device «Cam Scan Series 2».
For the X-ray analysis of the synthesis products was using a
diffractometer DRON-3M (CuKa radiation).
A size and form of crystals were determined using optical and
electronic microscopes.
EPR measurements of samples were performed with a Varian
E-109 RS X-band radiospectrometer at room temperature.
The photoluminescence spectra were measured with the
device SDL-2M. The diffuse reflection spectra were
determined with spectrometer M-40. These spectra were using
for study of absorption bands of doped oxides.
The doping ions impurities in boehmite and corundum were
defined with usage of PLAZMA – spectrometer.
3
4
Introduction
The preceding investigations of kinetics and mechanism of
formation of fine crystalline oxides in sub- and supercritical
water (P = 2-26 MPa, T = 200-400C) and in soft hydrothermal
conditions shown that this process is multistage. It proceeds
through the formation of solid phases intermediate - the
hydrated forms of precursors.
It proceeds through the formation of solid-phase intermediate
representing the hydrated forms of precursor. It was
established that molecules of water fluid actively participate in
the reorganization of a solid phase both at the first and second
stages of transformation of precursors.
5
The process of corundum formation from hydrargillite in
water vapor passes through the following stages:
H2O
400C
 AlOOH  H O 
 α  Al O  2H O
Al(OH) 3 
2
2 3
2
Hydrargillite
Boehmite
Corundum
The rate of the corundum formation and the properties of
corundum, such as a habitus, degree of perfection and sizes of
crystals depend on feature of structure of doped intermediate
(boehmite) .
The dopants were added either at primary stage into
hydrargillite, or into intermediate – boehmite. In both cases the
doped corundum was obtained, but with different
characteristics.
6
Boehmite (AlOOH) and corundum (-Al2O3) doped by
manganese.
The researches by optical method and by method EPR have revealed that
the including of doping Mn2+ ions in structure of boehmite is
accompanied by change of a charge of ions.
Boehmite has laminar structure (Fig. 1). From the beginning the
associative entrance of doping ions of manganese in boehmite structure
occurs. At rise of temperature of boehmite synthesis up to 360ºС the
formation of [MnO3]2- ions in boehmite structure takes place.
Fig. 1. Boehmite
At increasing of temperature and time of
thermovaporous treatment (TVT) of boehmite the
manganese ions occupy an octahedral position in of
an oxygen sublattice of boehmite. During entrance of
manganese ions into boehmite structure and then
into corundum a degree of an oxidizing of ions from
Mn2+ up to Mn3+ and Mn4+ changes.
structure.
7
Fig. 2. The reflection spectrum of
boehmite samples.
Temperature of synthesis 277°С,
duration TVT 2 h, PH2O = 5 MPa.
The numerals are shown
the content of manganese mass
% in boehmite. Dopant MnCl2.
In a Fig. 2 the spectruma of
reflection of boehmite with the
different content of manganese in
reaction medium represented. At
small concentration of manganese in
boehmite (0.0072 %) the absorption
bands about 380nm, which can be
referred to Mn2+ in orthorhombic
lattice of boehmite and Mn3+ (250
and 500 nm) clearly are visible. At
increasing of сoncentration only the
absorption bands of Mn3+ (250 and
500 nm) are discovered.
8
At transformation of boehmite into corundum (Fig. 3) wide peak in area
500 - 530 nm resolve into two peaks at 499.7 and 528.9 nm. Dichroism
characteristic for Mn3+ in a trigonal field of corundum is observed.
Fig. 3. Dependence of reflection
spectrum of a boehmite on
duration of synthesis in interval
0 - 4.5 hours. Temperature
417°С, PH2O = 29.6 MPa. The
content of manganese in
reaction medium 0.04% relative
to
aluminous
constituent.
Dopant MnCl2.
It corresponds to change of state of
manganese ions at introduction them
into structure of corundum. After
formation of corundum the intensity
of peak at 256 nm increases, the
absorption peak at 370 nm
disappears (Fig. 3, 4.5 h), and the
peaks in area 495 - 530 nm appear.
The occurrence of absorption peak at
495 nm corresponds to presence of
Mn4+ ions in structure of corundum.
Peak of absorption about 260 nm is
conditioned by processes of charge
transport from O2- to Mnn+.
9
Synthesized corundum doped by manganese has
mainly bipyramidal habitus. The crystals sizes
are in the range from 100 – 270 μm.
Fig. 5. Distribution on the sizes of
corundum crystals doped by
manganese. Concentration of
manganese is 510-4%.
Table 1
Fig. 4. Corundum Mn-doped
Dopant MnCl24H2O.
Temperature of synthesis 400°С,
PH2O = 26 MPa.
The crushing strength of
these crystals equal to
strength synthetic of
diamond DS65.
Average
value
of crushing
strength, N
Maximal
value of
crushing
strength, N
The content
(%) of
manganese.
Class of
diamond of
similar crushing
strength.
26.1
85.2
2.510-2
DS20
46.6
129.6
510-2
DS50
38.4
137.7
510-3
DS32
54.7
161.1
510-4
DS65
10
Fig. 5. EPR-spectrum of boehmite,
doped by manganese at T= 360°C,
PH2O=18 MPa. Dopant MnCl2 (0.01%).
Fig. 6. EPR-spectrum of corundum doped
by Mn. Dopant MnCl2 (0.01%),
TTVT=410°C, PH2O=28.5 MPa.
EPR spectra of boehmites doped by manganese reveals that between Mn2+
ions exists exchange interactions (The signal with g=2.32). Besides, the signal
at g = 3.14 testifies that Mn4+ (d3) has a high degree of a covalent bond of ions
with ligandes, as [MnO3]2-. In EPR spectrum of Mn-corundum the signal at
g~3.8 is assigned to Mn2+ (d5), in a structural position with the expressed
orthorhombic anisotropy. That is caused by a difference of charges of Аl3+
and Mn2+ occupying nodal position D3 symmetry. The form of EPR-signal at
g=2.00 testifies to axial distortion of cubic symmetry of tetrahedral position
of Mn2+ ion in spinel type structure, possibly, as of aluminate fragments
MnAl2O4:Al2O3. Besides, the exchange interaction between unlike charges
Mn2+ and Mn4+ ions forming fragment of a type Mn2+-Mn4+O3 arises.
11
Boehmite and corundum doped by chrome
The investigations of influence of a charge of a doping ion on incorporation
it into boehmite and corundum structure during their synthesis from
hydrargillite were carried out with usage of chemical compounds of
chrome with different valences: (NH4)2Cr2O7; Cr(NO3)3; CrCl3.
Fig. 7. Reflection Spectra of Boehmite
doped by Cr under TVT 234°С, P=2.5
MPa, 20 h, dopants:
a – (NH4)2Cr2O7; b - CrCl3 (container
from teflon); c - Cr(NO3)3. The content
of chromium in reaction medium 0.4%
relative to aluminous constituent.
The content of Cr3+ and Cr6+ in boehmite
was determined from reflection spectra
according to equation of Kubelka - Munk
F (R) = (1-R) 2/2R = k/s, where k absorption of a sample, s - dispersion of a
not immersing matrix. k = 2.303 ac;
a - coefficient extinction, c - concentration
of an immersing component. The values R
were calculated from reflection spectrum
for bands with minima of reflection at 372
nm (Cr6+) and 560 nm (Cr3+) . From Fig. 7
is shown that maximal amount of Cr3+
ions is in boehmite doped by dopants
containing trivalent chrome.
12
The absorption band with the minimum of reflection near 560 nm is attributed to
Cr3+ ions, which are in octahedral oxygen environment in aluminous matrix, as in
corundum phase. The absorption band with the reflection minimum near 370 nm is
due to charge transfer transition of Cr6+ ions in tetrahedral oxygen environment.
The Cr3+ ions could be formed as consequence of reduction of Cr6+ by NH4+-ions in
the course of boehmite synthesis with addition of ammonium dichromate. Detection
of absorption band at 370 nm (Cr6+ ions), in case the using as dopant Cr3+-nitrate,
can be explained by partial oxidation of Cr3+ ions by water fluid containing NO3ions during boehmite synthesis.
In the table 2 the content of chrome in doped
The chrome content in samples Cr-doped
boehmite.
Table 2
Dopant
Container
material
Cr %
Fe %
(NH4)2Cr2O7
St. steel*
0.07
0.0012
K2Cr2O7
St. steel
0.035
0.0018
CrCl3
St. steel
0.15
0.053
CrCl3
Teflon
0.235
0.0007
Cr(NO3)3
St. steel
0.176
0.0029
Cr(NO3)3
Teflon
0.235
0.00047
*Stainless steel
boehmite synthesized from hydrargillite
(235°C, P=2.5 MPa, 20 h) is given.
The content of chrome in reactionary medium
in all cases was 0.4 % concerning aluminous
component (Al2O3), but the chrome was
inserted into reactionary medium with
various valence of chrome. It is shown that
the maximal amount of chrome incorporates
into boehmite structure during its synthesis in
the container from teflon and using as
dopants Cr3+ compounds: CrCl3 and
Cr(NO3)3. This synthesized boehmite is
minimally polluted by iron.
13
Influence of pressure of water vapor on redox process during
thermovaporous treatment of boehmite beforehand synthesized and doped
by chrome (K2Cr2O7 T=270oC) becomes apparent in change of the relation
Cr3+/Cr6+ in structure of boehmite. In Fig. 8 are given the relations Cr3+/Cr6+
in samples doped (0.076% Cr) boehmite and processed under T=410o C,
24 h, and under different pressure of water vapor.
The relation Cr3+/Cr6+ in boehmite
was defined from relation of values
parameter R for bands with minima
of reflection at 370 nm (Cr6+) and at
560 nm (Cr3+) . The values R were
calculated according to equation of
Kubelka – Munk.
Fig.8. The change of the relation
Cr3+/Cr6+ in boehmite versus the
increasing of water vapor pressure
under thermovaporous treatment.
T=410oС. Dopant (NH4)2Cr2O7
From Fig.8 follows that the intensive
transformation Cr6+→Cr3+ in doped
boehmite begins under pressure water
vapor higher 26 MPa.
14
Fig. 9. Boehmite doped by Cr under
TVT 234°С, 20h, dopant (NH4)2Cr2O7.
Fig. 10. Boehmite doped by Cr under
TVT 234°С, 20h, dopant Cr(NO3)3.
At successful doping of corundum the ions Cr3+ isomorphously substitute
ions of aluminium. They are in trigonal distorted octahedrons.
The absorption spectrum of corundum containing Cr3+ is characterized
by three wide bands with maximuma at 550nm, 410nm and 260nm.
15
Fig. 11. Reflection spectra of
corundum synthesized and doped
by Cr under TVT 410°С, 120 h.
Dopants K2Cr2O7 .
a - P=21 MPa, b – 30 MPa,.
Fig. 12. Dependence of the content of
Cr3+ in corundum on water vapor
pressure during synthesis from
hydrargillite under 410°C, 120 h. The
chromium concentration in reaction
medium 0.4%. Dopant K2Cr2O7
The concentration of Cr3+ in synthesized corundum was rise with the
increase of water vapor pressure. It is noticeably especially under
pressure above 28 MPa (Fig. 12). The concentration of Cr3+ in corundum
was defined from parameter R for bands with minima of reflection at
560 nm.
16
The investigations by EPR-method have
shown that the chromium ions, initially
chaotically
distributed
in
disordered
boehmite structure, interact with hydroxyl
groups, and partially in aluminium-oxygen
octahedrons
are
built.
During
transformation boehmite into corundum the
signals on g=3.4, g=1.48 and g=1.25
correspond to spectrum of Сr3+ ions in a field
of trigonal symmetry of corundum appear.
Fig. 12. EPR-spectra of boehmite and
corundum. a) Boehmite synthesized Only samples containing less of 0.1 %
out the chromic dopant; b) corundum chromes completely correspond to a true
solid solution of Cr3+ in corundum with
synthesized out the chromic dopant;
c) corundum doped by (NH4)2Cr2O7;
statistically homogeneous allocation of Cr3+
d) boehmite doped by Cr(NO3)3.
in nodal positions of a crystal lattice. At
concentration of chrome in synthesized
corundum more than 0.1 % the
homogeneous distribution of the included
chrome in structure of corundum is broken.
The
exchange-bounded
paramagnetic
chromium ions appear.
17
During the doping by Cr6+ of hydrargillite, the Cr6+ mainly place in defects of
boehmite structure. The Cr6+ ions posed in defects of boehmite structure, at small
concentrations they also are isomorphously incorporated into crystal during the
formation structure of corundum changing valence from Cr6+ to Cr3+.
Fig. 13
Corundum obtained in presence
B-containing additives
Corundum obtained in presence
Cr – containing additives
18
Doped Yttrium–aluminium garnet (Y3Al5O12).
The doping of yttrium-aluminum garnet (YAG) was carried out during its
formation from the stoichiometric mixture of oxide yttrium and oxide
aluminium under hydrothermal and thermovaporous treatment in the
temperature range 200 – 400°C and under pressures of water vapor 4.0 - 26
MPa. It was found that the synthesis of YAG proceeds with formation of
intermediate substance with Y(OH)3 structure and amorphous aluminous
component. The diffusion of this aluminous component into the Y(OH)3
matrix resulted in the reorganization of oxygen sublattice accompanied with
dehydroxylation and formation the hydroxylated YAG.
Fig.14.Y3Al5O12 structure:
dodecahedrons, denoted with
points-joint location of YO8 with
[OH]4 substituting [AlO4].
The ions of dopant with aluminum ions occupy
octahedral positions and partly of tetrahedral
positions with structure formation of hydroxylated
doped YAG. By EPR-investigations and the study
of luminescence properties of YAG doped by Nd or
Cr ions has shown participation of hydroxyl groups
and oxygen vacancies in the formation defects of
doped YAG structure.
19
Luminescent properties of neodymium and chromium ions in garnet
allow to conclude that hydroxyl groups are located in the tops of
tetrahedral group (Fig. 14), having joint side with dodecahedron.
Fig. 15. The spectra of luminescence YAG:Cr (0.1%).
1 – monocrystal; 2 - powder of monocrystal; 3 - synthesized
fine-crystalline YAG:Cr.
Fig. 16. The spectra of
luminescence YAG: 1%Nd: 1 –
synthesized YAG; 2 - the same
sample, treated 4 h at 1100°C;
3 - powder of water-freeYAG
monocrystal.
20
The spectrum of a luminescence of garnet doped by chrome testifies to
presence in its structure of vacancies (430 nm) and Cr3+-ions substituting
isomorphously Al3+ ions (688 nm) (Fig. 15) .
In Fig. 16 are exhibited the spectra of luminescence of synthesized of
hydroxylated YAG doped by Nd (1 at.%) and of crushed water-free
monocrystal of YAG:Nd. The broadening of spectral bands of a
luminescence of a neodymium in garnet (Fig. 17) is stipulated by influence
of hydroxyl groups on centres luminescence
The entrance of Cr3+ occurs only into octahedral a - positions of a crystal
lattice of yttrium-aluminum garnet, that had found by EPR-method.
Fig. 17. The yttrium-aluminum garnet
doped by chrome.
21
Fig. 18. EPR- spectra of Cr3 +
in YAG (Х-gamut, ambient
temperature of measuring):
a - Powder of single crystal
YAG:Cr3+ ;
b - synthesized YAG:Cr3+,
(T=400°C;P=25MPa);
c - synthesized YAG:Cr3+ after
annealing at 465°С (2h).
d - YAG-:Cr3+ after heating at
550°С (2h).
EPR spectra of YAG, synthesized and doped by
chrome in thermovaporous conditions, differ from a
spectrum of a high-temperature YAG:Cr3+(Fig. 18).
The first signal at g = 1.99 corresponds with ions of
chrome taking place in nonuniform field of hydrated
ions ligandes. The second signal at g ~ 3.56 – 3.50
corresponds to the most intensive line of thin
structure of a spectrum of Cr3+, which occupies an
octahedral a - positions in a lattice YAG. . The line
broadening of thin structure in a spectrum of
synthesized garnet to ΔH ~ 800 Gs is a consequence of
a wide scattering of crystalline fields owing to
hydration of garnet structure.
The annealing of synthesized garnet at 465oC and
550°C does not influence on structural position of
doping ions of chrome, but promotes diminution of a
scattering degree of the crystalline fields. It may by
conclude that chrome ions included in garnet
structure during its synthesis under TVT are disposed
in oxygen octahedrons containing hydroxyl group in
vertexes, and in hydrated clustered vacancies of
structure of garnet.
22
Conclusion
The process of corundum formation from hydrargillite in water
fluid passes through the formation of intermediate (boehmite).
At the doping by Cr3+- or Cr6+-compounds originally chaotically
distributed in disordered structure of boehmite, chromium ions
partially place in defects of boehmite structure and interact with
hydroxyl groups, are partially built in aluminium-oxygen
octahedrons, isomorphously substituting aluminium and
changing valence from Cr6+ to Cr3+. At transforming of boehmite
into corundum during it dehydroxylation in quasi-equilibrium
with water fluid and the formation of structure of corundum,
Cr3+-ions homogenous are distributed in lattice nodal of
corundum.
23
The Mn4+ and Mn2+ ions occupy positions in a lattice of
corundum, which symmetry is differing from trigonal. As a
result of rise of temperature, water pressure and time of
synthesis the ions of manganese interact with oxygen vacancies
and hydroxyl-groups in structure of corundum to form
composite centre. Besides, the ions of Mn4+ in a trigonal lattice
of corundum can place together with a compensator-ion (for
example Mg2+, Sr2+, Mn2+ or charged vacancy) in octahedral
environment of anions.
The chrome ions included in garnet structure during its
synthesis under thermovaporous treatment (TVT) are disposed
in two basic positions: in oxygen octahedrons containing
hydroxyl group in vertexes and in hydrated clustered vacancies
of garnet structure. The formation of associates of hydrated ions
of chrome in garnet also was revealed.
24