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A study of phase stability in the Lu2O3–Al2O3 system
Journal of Crystal Growth 377 (2013) 178–183
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
A study of phase stability in the Lu2O3–Al2O3 system A.G. Petrosyan a,n, V.F. Popova b, V.L. Ugolkov b, D.P. Romanov b, K.L. Ovanesyan a a b
Institute for Physical Research, National Academy of Sciences, 0203 Ashtarak-2, Armenia Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, 2 Admiral Makarov Embankment, 199034 St. Petersburg, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 19 February 2013 Received in revised form 10 April 2013 Accepted 27 April 2013 Communicated by S. Uda Available online 27 May 2013
Differential thermal analysis (DTA) of LuAlO3 perovskite phase was performed in argon/hydrogen atmosphere in the temperature range up to 2100 1C. Single-phase LuAlO3 perovskite crystals grown by the vertical Bridgman method were used in measurements. Under thermal conditions excluding phase decomposition in the course of measurements, a single endothermal peak at 1847 1C (corresponding to the melting of pure perovskite phase) and a single exothermal peak at 1834 1C (corresponding to the solidification of the perovskite phase) were registered. DTA measurements were performed with single crystal garnet (Lu3Al5O12) and polycrystalline monoclinic (Lu4Al2O9) as well. Thermal instabilities of LuAlO3 and monoclinic Lu4Al2O9 phases were reinvestigated by x-ray diffraction and micro-probe spectral analysis. Indexing of the LuAlO3 perovskite, so far not reported, is done. & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Phase equilibria A2. Growth from melt B1. Rare-earth compounds B2. Scintillator materials
1. Introduction Three compounds have been reported in the Lu2O3–Al2O3 system with molar ratios Lu2O3:Al2O3 equal to 2:1 (Lu4Al2O9; monoclinic structure), 1:1 (LuAlO3; perovskite-type structure) and 3:5 (Lu3Al5O12; garnet-type structure) [1–3]. The Lu2O3– Al2O3 system was studied in [1] by means of annealing and quenching at the 1000–2300 1C temperature range. Two compounds are shown on the phase diagram: Lu3Al5O12 (3:5) and Lu4Al2O9 (2:1). The garnet phase (3:5) is the most stable in the system and melts congruently at 2060 1C without any sign of decomposition. According to the phase diagram, the monoclinic 2:1 phase melts incongruently at 2000 1C. A possible metastable version corresponding to the thermal maximum of the third phase, LuAlO3 (1:1), is also given in the diagram and valid for cooling from temperatures above the liquidus curve. The phase diagram of the Lu2O3–Al2O3 system defined in argon–hydrogen atmosphere [3] describes all the three compounds; in addition, the stability range of the 1:1 perovskite phase, the range of solid solutions of the garnet phase (with account of non-equivalent substitutions by Lu for octahedral Al sites) and the lower temperature stability limit of the monoclinic phase are shown. The calculated phase diagram of the Lu2O3–Al2O3 system reported in [2] states peritectic melting of LuAlO3 and congruent melting of Lu4Al2O9.
n
Corresponding author. Tel.: +374 1028 8150; fax: +374 2323 1172. E-mail address: [email protected] (A.G. Petrosyan).
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.04.054
The most conflicting results concern the phase equilibrium relations in the 1:1 range and associated with difficulties encountered in preparation of single phase LuAlO3 perovskite. Attempts to produce this phase by applying traditional methods were unsuccessful. Under normal pressures, solid state reactions with heating the stoichiometric mixtures (Lu2O3+Al2O3) up to the melting point or crystallization from low-temperature solutions did not yield this phase [1,4,5], the products always being the more stable garnet and monoclinic phases or garnet and lutetium oxide. LuAlO3 was produced as single crystals by Czochralski [6] and later by the vertical Bridgman [7]. A critical step in preparation of single phase LuAlO3 is melt overheating required for destruction of structural units of the garnet phase. Detailed analysis of processes occurring in melts and crystallization schemes developed for preparation of LuAlO3 in the form of polycrystals and single crystals were given in references [3,8]. Composition-stability relationships in this system are functions of crystalline quality (single crystals or polycrystals), temperature and atmosphere. According to [9], LuAlO3 taken in powdered form (as-crushed single crystals) decomposes upon heating in air at 1100–1650 1С, the products being Lu3Al5O12 and Lu4Al2O9; bulk single crystals decompose upon heat treatment in excess of 1300 1C under argon atmosphere, the products being Lu3Al5O12 and Lu2O3, and melt with formation of Lu3Al5O12 and Lu4Al2O9. According to [10], single crystals of LuAlO3 decompose upon heating (at a rate of 100 K/min) from room temperature to 1700 1C under argon–hydrogen atmosphere, the products being also Lu3Al5O12 and Lu2O3 (reaction starts from the surface and is typical for a diffusion favored process), and melt with formation of Lu3Al5O12 and Lu4Al2O9; it is possible to prevent the
A.G. Petrosyan et al. / Journal of Crystal Growth 377 (2013) 178–183
decomposition reaction from occurring if the samples are heated to 1700 1C at a high temperature raising rate (2000 K/min). The behavior of LuAlO3:Ce was studied in [11] by differential thermal analysis (DTA) up to 2000 1С in Ar atmosphere at heating/ cooling rates of 10 K/min; three quite wide endothermal peaks upon heating (at 559 1C, 1292 1C and 1905 1С) and two exothermal peaks upon cooling (at 1905 1C and 1854 1С) have been registered. The studied sample corresponded mainly to the perovskite phase but contained some small amounts of Lu3Al5O12 and Lu4Al2O9 phases. The melting behavior of LuAlO3 applying DTA up to 1970 1C in argon was studied in [12] in an attempt to explain the contradictions in results reported in [2] and [3]. The phase composition of samples used in measurements [12] was not however determined. The observed endothermal peak with onset at 1901 1С is attributed to melting of the eutectic between 2:1 and 1:1; peritectic melting of LuAlO3 at 1907 1C is proposed, as defined earlier in [2]. The different results reported in [2] and [3] are attributed to reduction of Al2O3 to suboxides or even Al metal under reducing conditions used in [3]; the latter is considered as the main reason for decomposition of LuAlO3 below its peritectic melting temperature. Lutetium aluminates have large application potential in many fields of optical technologies. Single crystals of based on LuAlO3 and Lu3Al5O12 hosts and activated with various rare-earth ions emit in a wide spectral range at wavelengths not offered by other laser crystals [13]. Due to high density and high Zeff, lutetium aluminates doped with Ce3+ or Pr3+ are under active studies as scintillators for applications in nuclear medical imaging and in calorimetry in high energy physics [14–18]. A small animal PET tomograph based on (Lu,Y)AlO3 (LuYAP)/Lu2SiO5 (LSO) has been developed by the Crystal Clear collaboration [15]. Single crystals of Lu3Al5O12:Pr have been used in medical imaging [16]. Lu3Al5O12: Ce, after demonstration of high light yield [17,18] comparable to LSO, is in the list of potential materials for application in high energy physics and nuclear medical imaging. High crystalline and optical quality parameters are the necessary requirements to materials for efficient use in various applications. Reliable production of single crystals with desired properties requires in-depth investigation of processes occurring in oxide melts under different conditions. In view of contradictory reports concerning the stability of compounds existing in the Lu2O3–Al2O3 system and their melting character, studies of lutetium aluminates have been undertaken in this work by means of x-ray diffraction, micro-probe electron microscopy and DTA up to 2100 1C.
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Tm were performed in air using a high-temperature microscope equipped with an iridium heater [19]. The phase composition of as-grown and heat-treated samples was determined by x-ray powder diffraction (XRD) using DRON-3 and DRF-2,0 diffractometers (CuKα-radiation, Ni-filter). The microstructure, elemental composition and composition of individual phases were studied by energy dispersive micro x-ray spectral analysis (MPSA) using CamScan MV2300 scanning electron microscope equipped with microprobe unit from Oxford Link Pentafet. The mean inaccuracy in measurements was 0.3 mass%.
3. Results and discussion 3.1. LuAlO3 The x-ray powder diffraction of as-grown LuAlO3 is welldefined and does not show any evidence of traces of other phases (Fig. 1, pattern 1). The pattern is indexed (so far not reported for this compound) to the orthorhombic perovskite structure (Table 1); its crystallographic parameters (a ¼5.106 Å, b¼ 5.335 Å, c¼ 7.304 Å; V¼198.9 Å3; space group Pbnm) are in a good agreement with JCPDS (No. 24-0690). According to electron microscopy and MPSA measurements, the composition of as-grown LuAlO3 single crystals corresponds to Lu2O3:Al2O3 ¼ 1:1, Fig. 2a and Table 2(a). Thermal decomposition of LuAlO3 has been reinvestigated in this work under various conditions (Figs. 1 and 2). It follows from Fig. 1 that LuAlO3 decomposes, the products being 3:5+Lu2O3 (or 3:5+2:1). Eutectic melting between these phases and final melting should follow upon further heating. Under annealing at 1500 1C and 1800 1C (heating rate 10 K/min), the perovskite decomposes in solid phase, the products being 3:5+Lu2O3. However, if a high rate heating up to 1800 1C is applied, the XRD shows decomposition of the perovskite phase to 3:5+2:1 already before melting. At lowrate heating (10 K/min) decomposition starts from low temperatures and the phase relations are described by the lowtemperature two-phase range on the diagram [3]; upon further heating up to 1800 1C the 2:1 phase is not seen, as it should be the case following [3]. It can therefore be concluded that at 10 K/min raising rate complete decomposition of the perovskite phase to 3:5 +Lu2O3 takes place, followed by eutectic melting. If however
2. Experimental procedures LuAlO3 and Lu3Al5O12 aluminates were grown as single crystals by the vertical Bridgman method [7,8]. The monoclinic Lu4Al2O9 was obtained in a polycrystalline form by melting and crystallization of the component oxides taken in stoichiometric quantities. Differential thermal analysis (DTA) was performed using STA 429 CD set-up (produced by NETZSCH 20–2100 1C) equipped with W heater, W/Re thermocouple and W crucible. Crystal pieces weighing 20–25 mg were used in DTA studies. Measurements were carried out in 99.999% pure Ar with 2% of H2 up to 2100 1C applying heating/cooling rates of 10–20 K/min. Peak temperature values recorded on the DTA curves were taken for the corresponding thermal effects. Heat treatments were performed in air up to 1500 1C and in Ar/ H2 atmosphere up to the melting point (Tm) in resistance furnaces, crystal growth chambers and DTA cameras. Quenching runs from
Fig. 1. X-ray diffraction patterns illustrating decomposition of the LuAlO3 perovskite phase: (1)—as-grown LuAlO3; (2)—LuAlO3 after annealing at 1500 1C for 24 h (heating/cooling rate 10 K/min, air); (3)—after annealing at 1800 1C for 0.5 h (DTA, 10 K/min, argon/hydrogen); (4)—LuAlO3 after melting (high-temperature microscope, 3000 K/min, air); (5)—as-grown Lu3Al5O12 and (6)—as-obtained Lu4Al2O9.
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high-rate heating is applied, the sample falls directly into the hightemperature region 3:5+2:1 and eutectic melting between these phases takes place, in accordance with the phase diagram [3]. Experiments performed in this work (in addition to [3]) confirm that at heat treatments in Ar–H2 the monoclinic phase undergoes decomposition, the products being 3:5+Lu2O3. It can therefore be concluded that, in the course of slow-rate heating, decomposition of LuAlO3 to 3:5+2:1 is accompanied by parallel decomposition of the 2:1 phase to 3:5+Lu2O3. Phase decomposition of LuAlO3 single crystals is illustrated in Fig. 2(b and c). The reaction starts from the surface showing features randomly distributed over the surface. Individual cluster composition is given in Table 2(b) and corresponds to either 3:5 or Lu2O3. It should be noted that the atmosphere used in the growth of single crystals places no detectable influence on the thermal
Table 1 Indexing of LuAlO3. I/I
2θexp
2θcal
d/nexp
hkl
2 81 21 34 27 100 28 27 8 22 28 13 3 2 40 14 29 2 3 4 6 9 9 24 9 2 15 9 9 2 3
21.20 24.09 24.35 27.06 33.57 34.52 35.12 35.81 41.06 41.91 43.20 44.53 45.65 46.57 49.38 50.72 51.03 54.86 55.77 56.75 58.24 60.66 61.48 62.63 63.19 64.93 67.73 71.91 72.82 73.90 74.25
21.21 24.11 24.35 27.06 33.57 34.53 35.13 35.81 41.07 41.90 43.20 44.55 45.67 46.69 49.37 50.72 51.03 54.83 55.79 56.73 58.24 60.64 61.49 62.64 63.20 64.94 67.72 71.90 72.83 73.91 74.24
4.19 3.690 3.653 2.293 2.6674 2.5962 2.5533 2.5057 2.1963 2.1540 2.0924 2.0328 1.9857 1.9484 1.8439 1.7983 1.7881 1.6721 1.6470 1.6209 1.5828 1.5254 1.5069 1.4819 1.4702 1.4350 1.3824 1.3119 1.2977 1.2813 1.2762
101 110 002 111 020 112 200 201 211,103 022 202 113 122 212 220 023 221 213 222 310 311 132 024, 204 312 223 320 133 041 224 025 400, 034
behavior of LuAlO3. Single crystals grown by Czochralski in pure argon atmosphere [6] and those grown by Bridgman in argon– hydrogen atmosphere [7] undergo similar phase transformations, when subjected to heat treatments [9,10]. Thermal instability of LuAlO3 should be taken into account in DTA measurements. The thermal rout used in present DTA measurements follows the crystallization scheme of LuAlO3 [8], which involves melt overheating stage necessary to destroy the structural units of the garnet phase; under such conditions the melts acquire an ability to undergo high supercoolings followed by nucleation and solidification of the phase with the most simple unit cell and structure in the system, the perovskite phase. The results are given on Fig. 3 including DTA, TG and temperature measurements. In the first DTA heating/cooling circle (Fig. 3, left section; 20 K/min) the perovskite single crystal is heated from 300 K up to 2097 1C and undergoes partial decomposition; three endothermal peaks (at 1847 1C, 1862 1C and 1903 1C) are registered reflecting the multi-phase composition of initially single-phase sample. The peaks at 1847 1C and 1862 1C correspond to eutectic melting of co-existing phases and the peak at 1903 1C corresponds to final melting of the sample. The exothermal peak upon subsequent cooling is registered at 1623 1C showing that the melt has acquired the ability to undergo a high supercooling ensuring solidification of the perovskite phase [3,8]. It should be noted that the extent of supercooling may exhibit some scattering but the Table 2 Composition measured by MPSA in different regions over the surface of as-grown and heat treated LuAlO3 (Fig. 2). Sample (Fig. 2)
Examined region
Components content (mol%) Al2O3
Lu2O3
Phases
a
1 2 3 4 5 6 7 SQ
47.78 47.63 48.37 47.03 48.00 47.79 48.49 48.08
52.22 52.37 51.63 52.97 52.00 52.21 51.51 51.92
LuAlO3
b
1 2 3 4 5 6 SQ1 SQ2
18.07 18.51 16.74 57.52 57.74 57.92 49.28 48.26
81.93 81.49 83.26 42.48 42.26 42.08 50.72 51.74
Lu2O3
Lu3Al5O12
LuAlO3
Fig. 2. Microphotographs of samples: (a) as-grown LuAlO3, (b) LuAlO3 after annealing at 1800 1С for 0.5 h in argon/hydrogen (heating/cooling rate 10 K/min) and (c) different scales of (b).
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Fig. 3. (a) DTA heating/cooling curves, TG and T data for heating of a single phase LuAlO3 to 2100 1C (20 K/min), cooling to 1500 1C and subsequent second DTA measurement (10 K/min).
tendency is that both the extent of overheating and the time of heating in the molten state favor higher supercoolings observed at the solidification point [20]. In the subsequent second DTA measurement (10 K/min) the sample is heated again just shortly after passing the exothermal peak registered in the first DTA run, not to escape the stability range of the perovskite phase. In contrast to the first DTA run, there is now one clear endothermal peak at 1847 1C which corresponds to melting of the pure LuAlO3 phase; the exothermal peak at 1834 1C corresponds to solidification of the LuAlO3 phase. The endothermal peak at 1847 1C is registered in both DTA measurements suggesting that in the first measurement this peak corresponds to melting of the perovskite phase remaining in some amounts in the decomposing sample. It should be noted that the accuracy of correspondence of thermal effects being registered on DTA to actual melting and solidification temperatures noticeably depends on the used method of temperature measurement, heating/cooling rates and the surrounding atmosphere. The upper curve (TG/%) gives no evidence for weight loss of the sample during the measurements, even if heated up to 2100 1C. Conclusions [12] on the role of hydrogen in favoring reduction of Al2O3 and modification of the behavior of LuAlO3 cannot be thus drawn from the present study. 3.2. Lu3Al5O12 The phase Lu3Al5O12 is cubic (Oh10-Ia3d) and has a garnet-type structure with unit cell dimension ao ¼11.905–11.920 Å. According to electron microscopy and MPCA measurements the composition of the sample under studies corresponds to Lu2O3:Al2O3 ¼3:5. The lattice constant is ao ¼11.910 Å evidencing of only a small octahedral occupancy by Lu ions [21]. The garnet phase is stable and melts without decomposition, as confirmed by DTA studies. One clear endothermal effect corresponding to congruent melting of Lu3Al5O12 is observed on the DTA curve at 1962 1C; the exothermal effect is at 1868 1C (Fig. 4, left curve; 20 K/min). The endothermal effect in the second DTA measurement (Fig. 4) is registered at the same temperature (1961 1C) as in the first DTA run. In both measurements the temperature is raised to the same point (1977 1C). The second measurement was done at 10 K/min; the time of heating in the molten state was therefore longer and even
the small overheating by some 16 1C was sufficient to destroy the garnet structural units and observed high extent of supercooling at the solidification point. The exothermal peak is registered at 1567 1C evidencing for solidification of a two-phase mixture (1:1 +Al2O3), which is a typical behavior of garnet melts including Y3Al5O12 [22] and Lu3Al5O12 [7]. XRD taken after DTA measurements confirm that the sample corresponds to LuAlO3/Al2O3 (Fig. 5). The convergence with earlier reported data [1] (where the melting temperature of Lu3Al5O12 was estimated as 2060 1C) should be attributed to different methods used for temperature measurements. 3.3. Lu4Al2O9 The third compound in the Lu2O3–Al2O3 system with molar ratio 2:1, Lu4Al2O9, has a monoclinic cell (space group P21/c) [23]. Only a few and contradictory reports on the melting behavior of this phase are available. As already mentioned, incongruent melting of this compound is suggested in [1,3], while according to [2], Lu4Al2O9 melts without decomposition. The polycrystalline Lu4Al2O9 sample prepared in this work corresponded to the monoclinic system; the crystallographic parameters were in a good agreement with JCPDS (No. 33–844). DTA measurements with Lu4Al2O9 are given in Fig. 6. The peritectic melting and final melting of the compound is at 1864 1C (the extended view of this range revealed two nearby peaks). Due to a negligible difference in the temperature, the two processes are given on the DTA curve as a summary clear endoeffect at 1864 1C. The two exothermal peaks correspond to crystallization of decomposition phases. Two effects upon heating (at 1855 1C and 1864 1C) and two upon cooling (at 1847 1C and 1833 1C) are registered in the second DTA measurement. Comparison of x-ray diffraction patterns (Fig. 7) of as-grown Lu4Al2O9 sample and that after quenching from 1800 1C (heated at this temperature for 30 min) gives evidence for peritectic melting of this compound. The phase composition of the quenched sample corresponds mainly to Lu2O3, the garnet phase Lu3Al5O12 and a small amount of the monoclinic Lu4Al2O9. The results of DTA and XRD (Figs. 6 and 7) reconfirm the incongruent melting character of the monoclinic phase, as suggested earlier in [3].
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Fig. 4. DTA of Lu3Al5O12 measured at 20 K/min (1st circle) and 10 K/min (2nd circle).
quenched sample which can be attributed to decomposition of the monoclinic phase. Analysis of the chemical composition in different parts over the surface of the as-grown sample showed relatively uniform component distribution corresponding approximately to Lu2O3/Al2O3 ¼66.6/33.3 mol%. The component distribution over the surface of the sample quenched from 1800 1C is highly non-uniform (from 80/20 to 50/50 mol%).
4. Conclusions
Fig. 5. X-ray diffraction pattern of solidification products of supercooled Lu3Al5O12 melt (a two-phase mixture LuAlO3/Al2O3).
The surfaces of as-grown sample and the sample quenched from 1800 1C (heated for 30 min) were studied by MPSA. Clusters differing in contrast appeared on the surface structure of the
The results of x-ray diffraction and micro-probe spectral analyses have confirmed decomposition of LuAlO3 already before melting followed by peritectical melting between Lu3Al5O12 and Lu2O3 or between Lu3Al5O12 and Lu4Al2O9 depending on experimental conditions. DTA measurements were performed with LuAlO3 under conditions preventing perovskite phase decomposition in the course of measurements. A single endothermal peak at 1847 1C upon heating of the single-phase perovskite at 1500 1C and a single exothermal peak at 1834 1C have been registered. The measurements were carried out in Ar/H2 atmosphere up to 2100 1C with no observed evidence for weight loss. The results substantially differ from DTA reports in [11,12] performed on samples of multi-phase or unknown phase composition and heated from 300 K. A single endothermal peak at 1962 1C and a single exothermal peak at 1868 1C were registered on DTA curves of Lu3Al5O12 garnet confirming high stability of this phase; if in the course of measurements the melt is overheated, the observed exothermal peak is by some 200–300 1C lower and, in accordance with the previous knowledge, corresponds to solidification of a two-phase mixture LuAlO3/Al2O3. DTA performed with the Lu4Al2O9 phase, as well as the XRD and MPSA studies, confirm phase separation upon heating and incongruent melting of this phase. Decomposition of the monoclinic phase (to Lu3Al5O12 and Lu2O3) is sluggish and the reaction can be prevented from occurring, if sufficiently high heating rates are applied. Indexing of LuAlO3 to orthorhombic perovskite structure, so far not reported, is done.
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Fig. 6. (a) DTA of monoclinic Lu4Al2O9.
References
Fig. 7. XRD patterns of samples with 2:1 composition: (1) as-obtained monoclinic Lu4Al2O9 and (2) quenched after heating for 30 min at 1800 1C.
Acknowledgements The authors (b) acknowledge the support of Project no. 11-0800801 granted by the Russian Foundation for Basic Research and Project no. 2012-1.2.2-12-000-2003-052 of Federal SpecialPurpose Program of Russia. Thanks are due to engineers of NETZSCH firm for construction of measurement devices used in the present studies. The authors (a) acknowledge the support of Project no. 11-1c322 of Science Committee Armenia, the International Associated laboratory IRMAS and the European Union Seventh Framework Program (FP-7/2007-2013) under Grant Agreement no. 295025.
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
A study of phase stability in the Lu2O3–Al2O3 system A.G. Petrosyan a,n, V.F. Popova b, V.L. Ugolkov b, D.P. Romanov b, K.L. Ovanesyan a a b
Institute for Physical Research, National Academy of Sciences, 0203 Ashtarak-2, Armenia Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, 2 Admiral Makarov Embankment, 199034 St. Petersburg, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 19 February 2013 Received in revised form 10 April 2013 Accepted 27 April 2013 Communicated by S. Uda Available online 27 May 2013
Differential thermal analysis (DTA) of LuAlO3 perovskite phase was performed in argon/hydrogen atmosphere in the temperature range up to 2100 1C. Single-phase LuAlO3 perovskite crystals grown by the vertical Bridgman method were used in measurements. Under thermal conditions excluding phase decomposition in the course of measurements, a single endothermal peak at 1847 1C (corresponding to the melting of pure perovskite phase) and a single exothermal peak at 1834 1C (corresponding to the solidification of the perovskite phase) were registered. DTA measurements were performed with single crystal garnet (Lu3Al5O12) and polycrystalline monoclinic (Lu4Al2O9) as well. Thermal instabilities of LuAlO3 and monoclinic Lu4Al2O9 phases were reinvestigated by x-ray diffraction and micro-probe spectral analysis. Indexing of the LuAlO3 perovskite, so far not reported, is done. & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Phase equilibria A2. Growth from melt B1. Rare-earth compounds B2. Scintillator materials
1. Introduction Three compounds have been reported in the Lu2O3–Al2O3 system with molar ratios Lu2O3:Al2O3 equal to 2:1 (Lu4Al2O9; monoclinic structure), 1:1 (LuAlO3; perovskite-type structure) and 3:5 (Lu3Al5O12; garnet-type structure) [1–3]. The Lu2O3– Al2O3 system was studied in [1] by means of annealing and quenching at the 1000–2300 1C temperature range. Two compounds are shown on the phase diagram: Lu3Al5O12 (3:5) and Lu4Al2O9 (2:1). The garnet phase (3:5) is the most stable in the system and melts congruently at 2060 1C without any sign of decomposition. According to the phase diagram, the monoclinic 2:1 phase melts incongruently at 2000 1C. A possible metastable version corresponding to the thermal maximum of the third phase, LuAlO3 (1:1), is also given in the diagram and valid for cooling from temperatures above the liquidus curve. The phase diagram of the Lu2O3–Al2O3 system defined in argon–hydrogen atmosphere [3] describes all the three compounds; in addition, the stability range of the 1:1 perovskite phase, the range of solid solutions of the garnet phase (with account of non-equivalent substitutions by Lu for octahedral Al sites) and the lower temperature stability limit of the monoclinic phase are shown. The calculated phase diagram of the Lu2O3–Al2O3 system reported in [2] states peritectic melting of LuAlO3 and congruent melting of Lu4Al2O9.
n
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0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.04.054
The most conflicting results concern the phase equilibrium relations in the 1:1 range and associated with difficulties encountered in preparation of single phase LuAlO3 perovskite. Attempts to produce this phase by applying traditional methods were unsuccessful. Under normal pressures, solid state reactions with heating the stoichiometric mixtures (Lu2O3+Al2O3) up to the melting point or crystallization from low-temperature solutions did not yield this phase [1,4,5], the products always being the more stable garnet and monoclinic phases or garnet and lutetium oxide. LuAlO3 was produced as single crystals by Czochralski [6] and later by the vertical Bridgman [7]. A critical step in preparation of single phase LuAlO3 is melt overheating required for destruction of structural units of the garnet phase. Detailed analysis of processes occurring in melts and crystallization schemes developed for preparation of LuAlO3 in the form of polycrystals and single crystals were given in references [3,8]. Composition-stability relationships in this system are functions of crystalline quality (single crystals or polycrystals), temperature and atmosphere. According to [9], LuAlO3 taken in powdered form (as-crushed single crystals) decomposes upon heating in air at 1100–1650 1С, the products being Lu3Al5O12 and Lu4Al2O9; bulk single crystals decompose upon heat treatment in excess of 1300 1C under argon atmosphere, the products being Lu3Al5O12 and Lu2O3, and melt with formation of Lu3Al5O12 and Lu4Al2O9. According to [10], single crystals of LuAlO3 decompose upon heating (at a rate of 100 K/min) from room temperature to 1700 1C under argon–hydrogen atmosphere, the products being also Lu3Al5O12 and Lu2O3 (reaction starts from the surface and is typical for a diffusion favored process), and melt with formation of Lu3Al5O12 and Lu4Al2O9; it is possible to prevent the
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decomposition reaction from occurring if the samples are heated to 1700 1C at a high temperature raising rate (2000 K/min). The behavior of LuAlO3:Ce was studied in [11] by differential thermal analysis (DTA) up to 2000 1С in Ar atmosphere at heating/ cooling rates of 10 K/min; three quite wide endothermal peaks upon heating (at 559 1C, 1292 1C and 1905 1С) and two exothermal peaks upon cooling (at 1905 1C and 1854 1С) have been registered. The studied sample corresponded mainly to the perovskite phase but contained some small amounts of Lu3Al5O12 and Lu4Al2O9 phases. The melting behavior of LuAlO3 applying DTA up to 1970 1C in argon was studied in [12] in an attempt to explain the contradictions in results reported in [2] and [3]. The phase composition of samples used in measurements [12] was not however determined. The observed endothermal peak with onset at 1901 1С is attributed to melting of the eutectic between 2:1 and 1:1; peritectic melting of LuAlO3 at 1907 1C is proposed, as defined earlier in [2]. The different results reported in [2] and [3] are attributed to reduction of Al2O3 to suboxides or even Al metal under reducing conditions used in [3]; the latter is considered as the main reason for decomposition of LuAlO3 below its peritectic melting temperature. Lutetium aluminates have large application potential in many fields of optical technologies. Single crystals of based on LuAlO3 and Lu3Al5O12 hosts and activated with various rare-earth ions emit in a wide spectral range at wavelengths not offered by other laser crystals [13]. Due to high density and high Zeff, lutetium aluminates doped with Ce3+ or Pr3+ are under active studies as scintillators for applications in nuclear medical imaging and in calorimetry in high energy physics [14–18]. A small animal PET tomograph based on (Lu,Y)AlO3 (LuYAP)/Lu2SiO5 (LSO) has been developed by the Crystal Clear collaboration [15]. Single crystals of Lu3Al5O12:Pr have been used in medical imaging [16]. Lu3Al5O12: Ce, after demonstration of high light yield [17,18] comparable to LSO, is in the list of potential materials for application in high energy physics and nuclear medical imaging. High crystalline and optical quality parameters are the necessary requirements to materials for efficient use in various applications. Reliable production of single crystals with desired properties requires in-depth investigation of processes occurring in oxide melts under different conditions. In view of contradictory reports concerning the stability of compounds existing in the Lu2O3–Al2O3 system and their melting character, studies of lutetium aluminates have been undertaken in this work by means of x-ray diffraction, micro-probe electron microscopy and DTA up to 2100 1C.
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Tm were performed in air using a high-temperature microscope equipped with an iridium heater [19]. The phase composition of as-grown and heat-treated samples was determined by x-ray powder diffraction (XRD) using DRON-3 and DRF-2,0 diffractometers (CuKα-radiation, Ni-filter). The microstructure, elemental composition and composition of individual phases were studied by energy dispersive micro x-ray spectral analysis (MPSA) using CamScan MV2300 scanning electron microscope equipped with microprobe unit from Oxford Link Pentafet. The mean inaccuracy in measurements was 0.3 mass%.
3. Results and discussion 3.1. LuAlO3 The x-ray powder diffraction of as-grown LuAlO3 is welldefined and does not show any evidence of traces of other phases (Fig. 1, pattern 1). The pattern is indexed (so far not reported for this compound) to the orthorhombic perovskite structure (Table 1); its crystallographic parameters (a ¼5.106 Å, b¼ 5.335 Å, c¼ 7.304 Å; V¼198.9 Å3; space group Pbnm) are in a good agreement with JCPDS (No. 24-0690). According to electron microscopy and MPSA measurements, the composition of as-grown LuAlO3 single crystals corresponds to Lu2O3:Al2O3 ¼ 1:1, Fig. 2a and Table 2(a). Thermal decomposition of LuAlO3 has been reinvestigated in this work under various conditions (Figs. 1 and 2). It follows from Fig. 1 that LuAlO3 decomposes, the products being 3:5+Lu2O3 (or 3:5+2:1). Eutectic melting between these phases and final melting should follow upon further heating. Under annealing at 1500 1C and 1800 1C (heating rate 10 K/min), the perovskite decomposes in solid phase, the products being 3:5+Lu2O3. However, if a high rate heating up to 1800 1C is applied, the XRD shows decomposition of the perovskite phase to 3:5+2:1 already before melting. At lowrate heating (10 K/min) decomposition starts from low temperatures and the phase relations are described by the lowtemperature two-phase range on the diagram [3]; upon further heating up to 1800 1C the 2:1 phase is not seen, as it should be the case following [3]. It can therefore be concluded that at 10 K/min raising rate complete decomposition of the perovskite phase to 3:5 +Lu2O3 takes place, followed by eutectic melting. If however
2. Experimental procedures LuAlO3 and Lu3Al5O12 aluminates were grown as single crystals by the vertical Bridgman method [7,8]. The monoclinic Lu4Al2O9 was obtained in a polycrystalline form by melting and crystallization of the component oxides taken in stoichiometric quantities. Differential thermal analysis (DTA) was performed using STA 429 CD set-up (produced by NETZSCH 20–2100 1C) equipped with W heater, W/Re thermocouple and W crucible. Crystal pieces weighing 20–25 mg were used in DTA studies. Measurements were carried out in 99.999% pure Ar with 2% of H2 up to 2100 1C applying heating/cooling rates of 10–20 K/min. Peak temperature values recorded on the DTA curves were taken for the corresponding thermal effects. Heat treatments were performed in air up to 1500 1C and in Ar/ H2 atmosphere up to the melting point (Tm) in resistance furnaces, crystal growth chambers and DTA cameras. Quenching runs from
Fig. 1. X-ray diffraction patterns illustrating decomposition of the LuAlO3 perovskite phase: (1)—as-grown LuAlO3; (2)—LuAlO3 after annealing at 1500 1C for 24 h (heating/cooling rate 10 K/min, air); (3)—after annealing at 1800 1C for 0.5 h (DTA, 10 K/min, argon/hydrogen); (4)—LuAlO3 after melting (high-temperature microscope, 3000 K/min, air); (5)—as-grown Lu3Al5O12 and (6)—as-obtained Lu4Al2O9.
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high-rate heating is applied, the sample falls directly into the hightemperature region 3:5+2:1 and eutectic melting between these phases takes place, in accordance with the phase diagram [3]. Experiments performed in this work (in addition to [3]) confirm that at heat treatments in Ar–H2 the monoclinic phase undergoes decomposition, the products being 3:5+Lu2O3. It can therefore be concluded that, in the course of slow-rate heating, decomposition of LuAlO3 to 3:5+2:1 is accompanied by parallel decomposition of the 2:1 phase to 3:5+Lu2O3. Phase decomposition of LuAlO3 single crystals is illustrated in Fig. 2(b and c). The reaction starts from the surface showing features randomly distributed over the surface. Individual cluster composition is given in Table 2(b) and corresponds to either 3:5 or Lu2O3. It should be noted that the atmosphere used in the growth of single crystals places no detectable influence on the thermal
Table 1 Indexing of LuAlO3. I/I
2θexp
2θcal
d/nexp
hkl
2 81 21 34 27 100 28 27 8 22 28 13 3 2 40 14 29 2 3 4 6 9 9 24 9 2 15 9 9 2 3
21.20 24.09 24.35 27.06 33.57 34.52 35.12 35.81 41.06 41.91 43.20 44.53 45.65 46.57 49.38 50.72 51.03 54.86 55.77 56.75 58.24 60.66 61.48 62.63 63.19 64.93 67.73 71.91 72.82 73.90 74.25
21.21 24.11 24.35 27.06 33.57 34.53 35.13 35.81 41.07 41.90 43.20 44.55 45.67 46.69 49.37 50.72 51.03 54.83 55.79 56.73 58.24 60.64 61.49 62.64 63.20 64.94 67.72 71.90 72.83 73.91 74.24
4.19 3.690 3.653 2.293 2.6674 2.5962 2.5533 2.5057 2.1963 2.1540 2.0924 2.0328 1.9857 1.9484 1.8439 1.7983 1.7881 1.6721 1.6470 1.6209 1.5828 1.5254 1.5069 1.4819 1.4702 1.4350 1.3824 1.3119 1.2977 1.2813 1.2762
101 110 002 111 020 112 200 201 211,103 022 202 113 122 212 220 023 221 213 222 310 311 132 024, 204 312 223 320 133 041 224 025 400, 034
behavior of LuAlO3. Single crystals grown by Czochralski in pure argon atmosphere [6] and those grown by Bridgman in argon– hydrogen atmosphere [7] undergo similar phase transformations, when subjected to heat treatments [9,10]. Thermal instability of LuAlO3 should be taken into account in DTA measurements. The thermal rout used in present DTA measurements follows the crystallization scheme of LuAlO3 [8], which involves melt overheating stage necessary to destroy the structural units of the garnet phase; under such conditions the melts acquire an ability to undergo high supercoolings followed by nucleation and solidification of the phase with the most simple unit cell and structure in the system, the perovskite phase. The results are given on Fig. 3 including DTA, TG and temperature measurements. In the first DTA heating/cooling circle (Fig. 3, left section; 20 K/min) the perovskite single crystal is heated from 300 K up to 2097 1C and undergoes partial decomposition; three endothermal peaks (at 1847 1C, 1862 1C and 1903 1C) are registered reflecting the multi-phase composition of initially single-phase sample. The peaks at 1847 1C and 1862 1C correspond to eutectic melting of co-existing phases and the peak at 1903 1C corresponds to final melting of the sample. The exothermal peak upon subsequent cooling is registered at 1623 1C showing that the melt has acquired the ability to undergo a high supercooling ensuring solidification of the perovskite phase [3,8]. It should be noted that the extent of supercooling may exhibit some scattering but the Table 2 Composition measured by MPSA in different regions over the surface of as-grown and heat treated LuAlO3 (Fig. 2). Sample (Fig. 2)
Examined region
Components content (mol%) Al2O3
Lu2O3
Phases
a
1 2 3 4 5 6 7 SQ
47.78 47.63 48.37 47.03 48.00 47.79 48.49 48.08
52.22 52.37 51.63 52.97 52.00 52.21 51.51 51.92
LuAlO3
b
1 2 3 4 5 6 SQ1 SQ2
18.07 18.51 16.74 57.52 57.74 57.92 49.28 48.26
81.93 81.49 83.26 42.48 42.26 42.08 50.72 51.74
Lu2O3
Lu3Al5O12
LuAlO3
Fig. 2. Microphotographs of samples: (a) as-grown LuAlO3, (b) LuAlO3 after annealing at 1800 1С for 0.5 h in argon/hydrogen (heating/cooling rate 10 K/min) and (c) different scales of (b).
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Fig. 3. (a) DTA heating/cooling curves, TG and T data for heating of a single phase LuAlO3 to 2100 1C (20 K/min), cooling to 1500 1C and subsequent second DTA measurement (10 K/min).
tendency is that both the extent of overheating and the time of heating in the molten state favor higher supercoolings observed at the solidification point [20]. In the subsequent second DTA measurement (10 K/min) the sample is heated again just shortly after passing the exothermal peak registered in the first DTA run, not to escape the stability range of the perovskite phase. In contrast to the first DTA run, there is now one clear endothermal peak at 1847 1C which corresponds to melting of the pure LuAlO3 phase; the exothermal peak at 1834 1C corresponds to solidification of the LuAlO3 phase. The endothermal peak at 1847 1C is registered in both DTA measurements suggesting that in the first measurement this peak corresponds to melting of the perovskite phase remaining in some amounts in the decomposing sample. It should be noted that the accuracy of correspondence of thermal effects being registered on DTA to actual melting and solidification temperatures noticeably depends on the used method of temperature measurement, heating/cooling rates and the surrounding atmosphere. The upper curve (TG/%) gives no evidence for weight loss of the sample during the measurements, even if heated up to 2100 1C. Conclusions [12] on the role of hydrogen in favoring reduction of Al2O3 and modification of the behavior of LuAlO3 cannot be thus drawn from the present study. 3.2. Lu3Al5O12 The phase Lu3Al5O12 is cubic (Oh10-Ia3d) and has a garnet-type structure with unit cell dimension ao ¼11.905–11.920 Å. According to electron microscopy and MPCA measurements the composition of the sample under studies corresponds to Lu2O3:Al2O3 ¼3:5. The lattice constant is ao ¼11.910 Å evidencing of only a small octahedral occupancy by Lu ions [21]. The garnet phase is stable and melts without decomposition, as confirmed by DTA studies. One clear endothermal effect corresponding to congruent melting of Lu3Al5O12 is observed on the DTA curve at 1962 1C; the exothermal effect is at 1868 1C (Fig. 4, left curve; 20 K/min). The endothermal effect in the second DTA measurement (Fig. 4) is registered at the same temperature (1961 1C) as in the first DTA run. In both measurements the temperature is raised to the same point (1977 1C). The second measurement was done at 10 K/min; the time of heating in the molten state was therefore longer and even
the small overheating by some 16 1C was sufficient to destroy the garnet structural units and observed high extent of supercooling at the solidification point. The exothermal peak is registered at 1567 1C evidencing for solidification of a two-phase mixture (1:1 +Al2O3), which is a typical behavior of garnet melts including Y3Al5O12 [22] and Lu3Al5O12 [7]. XRD taken after DTA measurements confirm that the sample corresponds to LuAlO3/Al2O3 (Fig. 5). The convergence with earlier reported data [1] (where the melting temperature of Lu3Al5O12 was estimated as 2060 1C) should be attributed to different methods used for temperature measurements. 3.3. Lu4Al2O9 The third compound in the Lu2O3–Al2O3 system with molar ratio 2:1, Lu4Al2O9, has a monoclinic cell (space group P21/c) [23]. Only a few and contradictory reports on the melting behavior of this phase are available. As already mentioned, incongruent melting of this compound is suggested in [1,3], while according to [2], Lu4Al2O9 melts without decomposition. The polycrystalline Lu4Al2O9 sample prepared in this work corresponded to the monoclinic system; the crystallographic parameters were in a good agreement with JCPDS (No. 33–844). DTA measurements with Lu4Al2O9 are given in Fig. 6. The peritectic melting and final melting of the compound is at 1864 1C (the extended view of this range revealed two nearby peaks). Due to a negligible difference in the temperature, the two processes are given on the DTA curve as a summary clear endoeffect at 1864 1C. The two exothermal peaks correspond to crystallization of decomposition phases. Two effects upon heating (at 1855 1C and 1864 1C) and two upon cooling (at 1847 1C and 1833 1C) are registered in the second DTA measurement. Comparison of x-ray diffraction patterns (Fig. 7) of as-grown Lu4Al2O9 sample and that after quenching from 1800 1C (heated at this temperature for 30 min) gives evidence for peritectic melting of this compound. The phase composition of the quenched sample corresponds mainly to Lu2O3, the garnet phase Lu3Al5O12 and a small amount of the monoclinic Lu4Al2O9. The results of DTA and XRD (Figs. 6 and 7) reconfirm the incongruent melting character of the monoclinic phase, as suggested earlier in [3].
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Fig. 4. DTA of Lu3Al5O12 measured at 20 K/min (1st circle) and 10 K/min (2nd circle).
quenched sample which can be attributed to decomposition of the monoclinic phase. Analysis of the chemical composition in different parts over the surface of the as-grown sample showed relatively uniform component distribution corresponding approximately to Lu2O3/Al2O3 ¼66.6/33.3 mol%. The component distribution over the surface of the sample quenched from 1800 1C is highly non-uniform (from 80/20 to 50/50 mol%).
4. Conclusions
Fig. 5. X-ray diffraction pattern of solidification products of supercooled Lu3Al5O12 melt (a two-phase mixture LuAlO3/Al2O3).
The surfaces of as-grown sample and the sample quenched from 1800 1C (heated for 30 min) were studied by MPSA. Clusters differing in contrast appeared on the surface structure of the
The results of x-ray diffraction and micro-probe spectral analyses have confirmed decomposition of LuAlO3 already before melting followed by peritectical melting between Lu3Al5O12 and Lu2O3 or between Lu3Al5O12 and Lu4Al2O9 depending on experimental conditions. DTA measurements were performed with LuAlO3 under conditions preventing perovskite phase decomposition in the course of measurements. A single endothermal peak at 1847 1C upon heating of the single-phase perovskite at 1500 1C and a single exothermal peak at 1834 1C have been registered. The measurements were carried out in Ar/H2 atmosphere up to 2100 1C with no observed evidence for weight loss. The results substantially differ from DTA reports in [11,12] performed on samples of multi-phase or unknown phase composition and heated from 300 K. A single endothermal peak at 1962 1C and a single exothermal peak at 1868 1C were registered on DTA curves of Lu3Al5O12 garnet confirming high stability of this phase; if in the course of measurements the melt is overheated, the observed exothermal peak is by some 200–300 1C lower and, in accordance with the previous knowledge, corresponds to solidification of a two-phase mixture LuAlO3/Al2O3. DTA performed with the Lu4Al2O9 phase, as well as the XRD and MPSA studies, confirm phase separation upon heating and incongruent melting of this phase. Decomposition of the monoclinic phase (to Lu3Al5O12 and Lu2O3) is sluggish and the reaction can be prevented from occurring, if sufficiently high heating rates are applied. Indexing of LuAlO3 to orthorhombic perovskite structure, so far not reported, is done.
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Fig. 6. (a) DTA of monoclinic Lu4Al2O9.
References
Fig. 7. XRD patterns of samples with 2:1 composition: (1) as-obtained monoclinic Lu4Al2O9 and (2) quenched after heating for 30 min at 1800 1C.
Acknowledgements The authors (b) acknowledge the support of Project no. 11-0800801 granted by the Russian Foundation for Basic Research and Project no. 2012-1.2.2-12-000-2003-052 of Federal SpecialPurpose Program of Russia. Thanks are due to engineers of NETZSCH firm for construction of measurement devices used in the present studies. The authors (a) acknowledge the support of Project no. 11-1c322 of Science Committee Armenia, the International Associated laboratory IRMAS and the European Union Seventh Framework Program (FP-7/2007-2013) under Grant Agreement no. 295025.
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