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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: mardan@kge.msu.ru

Chemistry Department, Moscow State University

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ExperimentalIn 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.

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The synthesized products were investigated by physical-chemical 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.

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Introduction

The preceding investigations of kinetics and mechanism of

formation of fine crystalline oxides in sub- and supercritical

water (P = 2-26 MPa, T = 200-400C) 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.

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The process of corundum formation from hydrargillite in water vapor passes through the following stages:

O22H3O2Alα400C

O2HAlOOHAl(OH)3O2H

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.

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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 structure.

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.

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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.

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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+.

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.

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Fig. 4. Corundum Mn-doped

Dopant MnCl24H2O. Temperature of synthesis 400°С, PH2O = 26 MPa.

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 510-4%.

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 2.510-2 DS20

46.6 129.6 510-2 DS50

38.4 137.7 510-3 DS32

54.7 161.1 510-4 DS65

The crushing strength of these crystals equal to strength synthetic of diamond DS65.

Table 1

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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.

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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.

Boehmite and corundum doped by chrome

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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 NO3

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ions during boehmite synthesis. In the table 2 the content of chrome in doped 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.

*Stainless steel

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

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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.

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

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.

From Fig.8 follows that the intensive transformation Cr6+→Cr3+ in doped boehmite begins under pressure water vapor higher 26 MPa.

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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.

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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

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,.

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.

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Fig. 12. EPR-spectra of boehmite and corundum. a) Boehmite synthesized out the chromic dopant; b) corundum synthesized out the chromic dopant; c) corundum doped by (NH4)2Cr2O7;

d) boehmite doped by Cr(NO3)3.

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. Only samples containing less of 0.1 % chromes completely correspond to a true solid solution of Cr3+ in corundum with statistically homogeneous allocation of Cr3+ 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.

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Corundum obtained in presence B-containing additives

Corundum obtained in presence Cr – containing additives

Fig. 13

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+.

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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.

Doped Yttrium–aluminium garnet (Y3Al5O12).

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.

Fig.14.Y3Al5O12 structure:

dodecahedrons, denoted with points-joint location of YO8

with [OH]4 substituting [AlO4].

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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.

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.

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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 luminescenceThe 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.

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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.

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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.

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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.