1 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 , Russia Chemistry Department, Moscow State University

2 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-Al 2 O 3 ) and yttrium-aluminium garnet (Y 3 Al 5 O 12 ) 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.

3 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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5 Introduction The preceding investigations of kinetics and mechanism of formation of fine crystalline oxides in sub- and supercritical water (P = 2-26 MPa, T =  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.

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

7 Boehmite (AlOOH) and corundum (  -Al 2 O 3 ) doped by manganese. The researches by optical method and by method EPR have revealed that the including of doping Mn 2+ 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 [MnO 3 ] 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 Mn 2+ up to Mn 3+ and Mn 4+ changes.

8 Fig. 2. The reflection spectrum of boehmite samples. Temperature of synthesis 277°С, duration TVT 2 h, P H2O = 5 MPa. The numerals are shown the content of manganese mass % in boehmite. Dopant MnCl 2. 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 ( %) the absorption bands about 380nm, which can be referred to Mn 2+ in orthorhombic lattice of boehmite and Mn 3+ (250 and 500 nm) clearly are visible. At increasing of сoncentration only the absorption bands of Mn 3+ (250 and 500 nm) are discovered.

9 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 nm appear. The occurrence of absorption peak at 495 nm corresponds to presence of Mn 4+ ions in structure of corundum. Peak of absorption about 260 nm is conditioned by processes of charge transport from O 2- to Mn n+. At transformation of boehmite into corundum (Fig. 3) wide peak in area nm resolve into two peaks at and nm. Dichroism characteristic for Mn 3+ in a trigonal field of corundum is observed. Fig. 3. Dependence of reflection spectrum of a boehmite on duration of synthesis in interval hours. Temperature 417°С, P H2O = 29.6 MPa. The content of manganese in reaction medium 0.04% relative to aluminous constituent. Dopant MnCl 2.

10 Fig. 4. Corundum Mn-doped Dopant MnCl 2  4H2O. Temperature of synthesis 400°С, P H2O = 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 5  %. Average value of crushing strength, N Maximal value of crushing strength, N The content (%) of manganese. Class of diamond of similar crushing strength  DS  DS  DS  DS65 The crushing strength of these crystals equal to strength synthetic of diamond DS65. Table 1

11 Fig. 5. EPR-spectrum of boehmite, doped by manganese at T= 360°C, P H2O =18 MPa. Dopant MnCl 2 (0.01%). Fig. 6. EPR-spectrum of corundum doped by Mn. Dopant MnCl 2 (0.01%), T TVT =410°C, P H2O =28.5 MPa. EPR spectra of boehmites doped by manganese reveals that between Mn 2+ ions exists exchange interactions (The signal with g=2.32). Besides, the signal at g = 3.14 testifies that Mn 4+ (d 3 ) has a high degree of a covalent bond of ions with ligandes, as [MnO 3 ] 2-. In EPR spectrum of Mn-corundum the signal at g~3.8 is assigned to Mn 2+ (d 5 ), in a structural position with the expressed orthorhombic anisotropy. That is caused by a difference of charges of Аl 3+ and Mn 2+ occupying nodal position D 3 symmetry. The form of EPR-signal at g=2.00 testifies to axial distortion of cubic symmetry of tetrahedral position of Mn 2+ ion in spinel type structure, possibly, as of aluminate fragments MnAl2O4:Al2O3. Besides, the exchange interaction between unlike charges Mn 2+ and Mn 4+ ions forming fragment of a type Mn 2+ -Mn 4+ O 3 arises.

12 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: (NH 4 ) 2 Cr 2 O 7 ; Cr(NO 3 ) 3 ; CrCl 3. Fig. 7. Reflection Spectra of Boehmite doped by Cr under TVT 234°С, P=2.5 MPa, 20 h, dopants: a – (NH 4 ) 2 Cr 2 O 7 ; b - CrCl 3 (container from teflon); c - Cr(NO 3 ) 3. The content of chromium in reaction medium 0.4% relative to aluminous constituent. The content of Cr 3+ and Cr 6+ 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 = 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 (Cr 6+ ) and 560 nm (Cr 3+ ). From Fig. 7 is shown that maximal amount of Cr 3+ ions is in boehmite doped by dopants containing trivalent chrome. Boehmite and corundum doped by chrome

13 The absorption band with the minimum of reflection near 560 nm is attributed to Cr 3+ 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 Cr 6+ ions in tetrahedral oxygen environment. The Cr 3+ ions could be formed as consequence of reduction of Cr 6+ by NH 4 + -ions in the course of boehmite synthesis with addition of ammonium dichromate. Detection of absorption band at 370 nm (Cr 6+ ions), in case the using as dopant Cr 3+ -nitrate, can be explained by partial oxidation of Cr 3+ ions by water fluid containing NO 3 - 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 (Al 2 O 3 ), 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 Cr 3+ compounds: CrCl 3 and Cr(NO 3 ) 3. This synthesized boehmite is minimally polluted by iron. *Stainless steel The chrome content in samples Cr-doped boehmite. Table 2 DopantContainer material Cr %Fe % (NH 4 ) 2 Cr 2 O 7 St. steel* K 2 Cr 2 O 7 St. steel CrCl 3 St. steel CrCl 3 Teflon Cr(NO 3 ) 3 St. steel Cr(NO 3 ) 3 Teflon

14 Influence of pressure of water vapor on redox process during thermovaporous treatment of boehmite beforehand synthesized and doped by chrome (K 2 Cr 2 O 7 T=270 o C) becomes apparent in change of the relation Cr 3+ /Cr 6+ in structure of boehmite. In Fig. 8 are given the relations Cr 3+ /Cr 6+ in samples doped (0.076% Cr) boehmite and processed under T=410 o C, 24 h, and under different pressure of water vapor. Fig.8. The change of the relation Cr 3+ /Cr 6+ in boehmite versus the increasing of water vapor pressure under thermovaporous treatment. T=410 o С. Dopant (NH 4 ) 2 Cr 2 O 7 The relation Cr 3+/ Cr 6+ in boehmite was defined from relation of values parameter R for bands with minima of reflection at 370 nm (Cr 6+ ) and at 560 nm (Cr 3+ ). The values R were calculated according to equation of Kubelka – Munk. From Fig.8 follows that the intensive transformation Cr 6+ →Cr 3+ in doped boehmite begins under pressure water vapor higher 26 MPa.

15 Fig. 9. Boehmite doped by Cr under TVT 234°С, 20h, dopant (NH 4 ) 2 Cr 2 O 7. Fig. 10. Boehmite doped by Cr under TVT 234°С, 20h, dopant Cr(NO3) 3. At successful doping of corundum the ions Cr 3+ isomorphously substitute ions of aluminium. They are in trigonal distorted octahedrons. The absorption spectrum of corundum containing Cr 3+ is characterized by three wide bands with maximuma at 550nm, 410nm and 260nm.

16 Fig. 12. Dependence of the content of Cr 3+ in corundum on water vapor pressure during synthesis from hydrargillite under 410°C, 120 h. The chromium concentration in reaction medium 0.4%. Dopant K 2 Cr 2 O 7 Fig. 11. Reflection spectra of corundum synthesized and doped by Cr under TVT 410°С, 120 h. Dopants K 2 Cr 2 O 7. a - P=21 MPa, b – 30 MPa,. The concentration of Cr 3+ 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 Cr 3+ in corundum was defined from parameter R for bands with minima of reflection at 560 nm.

17 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 (NH 4 ) 2 Cr 2 O 7 ; d) boehmite doped by Cr(NO 3 ) 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 Сr 3+ 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 Cr 3+ in corundum with statistically homogeneous allocation of Cr 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.

18 Corundum obtained in presence B-containing additives Corundum obtained in presence Cr – containing additives Fig. 13 During the doping by Cr 6+ of hydrargillite, the Cr 6+ mainly place in defects of boehmite structure. The Cr 6+ 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 Cr 6+ to Cr 3+.

19 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 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 (Y 3 Al 5 O 12 ). Fig.14.Y 3 Al 5 O 12 structure: dodecahedrons, denoted with points-joint location of YO 8 with [OH] 4 substituting [AlO 4 ]. 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.Y 3 Al 5 O 12 structure: dodecahedrons, denoted with points-joint location of YO 8 with [OH] 4 substituting [AlO 4 ].

20 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.

21 The spectrum of a luminescence of garnet doped by chrome testifies to presence in its structure of vacancies (430 nm) and Cr 3+ -ions substituting isomorphously Al 3+ 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 Cr 3+ 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.

22 Fig. 18. EPR- spectra of Cr 3 + in YAG (Х-gamut, ambient temperature of measuring): a - Powder of single crystal YAG:Cr 3+ ; b - synthesized YAG:Cr 3+, (T=400°C;P=25MPa); c - synthesized YAG:Cr 3+ after annealing at 465°С (2h). d - YAG-:Cr 3+ 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:Cr 3+ (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 Cr 3+, 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 465 o C 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.

23 Conclusion The process of corundum formation from hydrargillite in water fluid passes through the formation of intermediate (boehmite). At the doping by Cr 3+ - or Cr 6+ -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 Cr 6+ to Cr 3+. At transforming of boehmite into corundum during it dehydroxylation in quasi-equilibrium with water fluid and the formation of structure of corundum, Cr 3+ -ions homogenous are distributed in lattice nodal of corundum.

24 The Mn 4+ and Mn 2+ 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 Mn 4+ in a trigonal lattice of corundum can place together with a compensator-ion (for example Mg 2+, Sr 2+, Mn 2+ 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.