Availableonlineatwww.sciencedirect.com SCIENCE DIRECT非 JOURNAL OF PHYSIC EMISTRY OF SOLIDS ELSEVIER Journal of Physics and Chemistry of Solids 65(2004)1103-1112 www.elsevier.com/locate/jpcs Stress-induced cubic-tetragonal transformation in partiall stabilized zro2: Raman spectroscopy study A.A. Sobol, Yu.K. Voronko Laser Materials and Technology Research Center, General Physics Institute, Russian Academy of Sciences, Vavilov street 38, Building 'D, Moscow 119991, Russia Received 9 May 2003: revised 7 October 2003: accepted 10 November 2003; available online 7 February 2004 Abstract Regularities of the cubic-tetragonal transformation(C-t)in partially stabilized zirconia were studied with Raman spectroscopy and high-temperature Raman spectroscopy techniques. New ' low temperature mechanism of tetragonal nanoparticles formation in a volume of cubic solid solution was revealed in ZrO2-GdO3(Eu,O3)(6-8 mol%) single crystals. This mechanism includes nucleation of the tetragonal nanoparticles due to diffusionless C-t phase transformation at the first stage and gradual decrease of the stabilizer concentration insidet'. domains after subsequent low-temperature annealing. Predominant orientation of tetragonal domains due to the stress-induced C transformation was registered in ZrO2-Gd2O3(8 mol%)single crystals. C 2004 Elsevier Ltd. All rights reserved. Keywords: D. Phase transformations; C. Raman spectroscopy 1. Introduction ZrO2-based ceramics and single crystals [6-12. Raman spectroscopy was useful in studies in situ m+t transform Partially stabilized zirconia(PSZ) has great potentialities ations in the heating-cooling process [7-9, 13, 14]. Several as engineering and refractory materials [1]. The stress- experiments were carried out for studying transformations induced martensitic tetragonal to monoclinic(t-m)phase of ZrOz monoclinic phase into orthorhombic structures transition in PSZ ZrOz-Y2O3 solid solution was studied under high pr ressure [516] As to t→ C andt… C phase previously [2, 3]. However, there is another diffusionless transformations, there is a small experimental information cubic-tetragonal(C-t) transformation intrinsic to this on the nature of this phenomenon. Cubic-tetragonal phase system. This transformation almost was not studied due to transition at heating-cooling was studied only for Zro2 enormous experimental difficulties caused by the high Yb2O3(Eu2O3)(6-8 mol%) single crystals by high- temperature of the C-t phase transition temperature Raman spectroscopy technique [17]. Raman There exist different opinions on the nature of the C!' spectra of nanometric-size tetragonal zirconia under high transformation in PSZ(ZrO2-Y2O3) According to Ref [4], pressure up to 40 GPa were studied in Ref. [18] this transformation was displacive but nonmartensitie The goal of this paper is the application of the polarized Other authors described the transformation as martensitic Raman spectroscopy method for studying the stress-induced and similar to the t-m phase transition [5]. Both models C-t phase transformation of PSz single crystals suggest a possibility to induce the C-t transformation by deformation of PSz samples. However, up to now, the existence of such phenomenon in PSZ has not been proved 2 Experimental procedure Raman spectroscopy was shown to have advantage Single crystals of Zro2-Gd2O3(Eu2O3)(6-8 mol%) in studying the phase transformation and the structure under study were grown by cold-container technique [7 Plate-shaped 7X7X 3-mm' samples were cut and then Corresponding author. Tel. +7-95-135-03-01; fax: +7.95-135-02-70. polished. The samples were oriented along the three four E-mail address: sobol lst gpi. ru(AA. Sobol). fold axes of the cubic structure by means of the X-ray 0022-3697/S- see front matter o 2004 Elsevier Ltd. All rights reserved. doi:10.1016jpcs.2003.11.038
Stress-induced cubic–tetragonal transformation in partially stabilized ZrO2: Raman spectroscopy study A.A. Sobol*, Yu.K. Voronko Laser Materials and Technology Research Center, General Physics Institute, Russian Academy of Sciences, Vavilov street 38, Building ‘D’, Moscow 119991, Russia Received 9 May 2003; revised 7 October 2003; accepted 10 November 2003; available online 7 February 2004 Abstract Regularities of the cubic–tetragonal transformation (C ! t 0 ) in partially stabilized zirconia were studied with Raman spectroscopy and high-temperature Raman spectroscopy techniques. New ‘low temperature’ mechanism of tetragonal nanoparticles formation in a volume of cubic solid solution was revealed in ZrO2 –Gd2O3 (Eu2O3) (6–8 mol%) single crystals. This mechanism includes nucleation of the tetragonal nanoparticles due to diffusionless C ! t 0 phase transformation at the first stage and gradual decrease of the stabilizer concentration inside t0 - domains after subsequent low-temperature annealing. Predominant orientation of tetragonal domains due to the stress-induced C ! t 0 transformation was registered in ZrO2 –Gd2O3 (8 mol%) single crystals. q 2004 Elsevier Ltd. All rights reserved. Keywords: D. Phase transformations; C. Raman spectroscopy 1. Introduction Partially stabilized zirconia (PSZ) has great potentialities as engineering and refractory materials [1]. The stressinduced martensitic tetragonal to monoclinic (t ! m) phase transition in PSZ ZrO2 –Y2O3 solid solution was studied previously [2,3]. However, there is another diffusionless cubic–tetragonal (C ! t 0 ) transformation intrinsic to this system. This transformation almost was not studied due to enormous experimental difficulties caused by the high temperature of the C ! t 0 phase transition. There exist different opinions on the nature of the C ! t 0 transformation in PSZ (ZrO2 –Y2O3). According to Ref. [4], this transformation was displacive but nonmartensitic. Other authors described the transformation as martensitic and similar to the t ! m phase transition [5]. Both models suggest a possibility to induce the C ! t 0 transformation by deformation of PSZ samples. However, up to now, the existence of such phenomenon in PSZ has not been proved experimentally. Raman spectroscopy was shown to have advantages in studying the phase transformation and the structure of ZrO2-based ceramics and single crystals [6–12]. Raman spectroscopy was useful in studies in situ m $ t transformations in the heating–cooling process [7–9,13,14]. Several experiments were carried out for studying transformations of ZrO2 monoclinic phase into orthorhombic structures under high pressures [15,16]. As to t $ C and t0 $ C phase transformations, there is a small experimental information on the nature of this phenomenon. Cubic–tetragonal phase transition at heating–cooling was studied only for ZrO2 – Yb2O3 (Eu2O3) (6–8 mol%) single crystals by hightemperature Raman spectroscopy technique [17]. Raman spectra of nanometric-size tetragonal zirconia under high pressure up to 40 GPa were studied in Ref. [18]. The goal of this paper is the application of the polarized Raman spectroscopy method for studying the stress-induced C ! t 0 phase transformation of PSZ single crystals. 2. Experimental procedure Single crystals of ZrO2 –Gd2O3 (Eu2O3) (6–8 mol%) under study were grown by cold-container technique [7]. Plate-shaped 7 £ 7 £ 3-mm3 samples were cut and then polished. The samples were oriented along the three fourfold axes of the cubic structure by means of the X-ray 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2003.11.038 Journal of Physics and Chemistry of Solids 65 (2004) 1103–1112 www.elsevier.com/locate/jpcs * Corresponding author. Tel.: þ7-95-135-03-01; fax: þ7-95-135-02-70. E-mail address: sobol@lst.gpi.ru (A.A. Sobol)
l104 A.A. Sobol, Y.K. Voronko / Journal of Physics and Chemistry of Solids 65 (2004)1103-1112 Load ds of the as- received zro -Ln,o Number of Type of F the sample Undoped uuuu 246888 Sample 100] C, hydroxides coprecipitation; m, rapidly quenched melt; s.c., single ig. 1.(a) Scheme of the experiment on the sample deformation during th stal grown by cold-container technique C-I phase transformation. (b)Orientation of the sample under study; 1 and 2 denote the non-deformed and deformed zones, respectively technique. We also analyzed samples obtained by water quenching ZrO2-Eu2O3(2-8 mol%)melts, which had been previously investigated in Ref. [17]. Undoped powder 3. Potentialities of Raman spectroscopy in studying tetragonal ZrO2-sample was synthesized by hydroxides the c-t phase transformation co-precipitation technique [19]. A list of the sample under study is presented in Table 1. Raman spectra of the samples The Raman spectrum of tetrag were investigated at 300 K with Spex-Ramalog-1403 group) consists of six lines of the Aig 2B1g +3Es pectrometer using conventional back scattering geometry ymmetries [10]. This spectrum is characteristic of both under Ar(488 nm)excitation. High-temperature Raman pure ZrOz and ZrOz-Ln2O3 solid solutions (Ln implies the spectroscopy studies up to 1900 K were carried out with the lanthanide series and Y) in the region of 0-8 mol% of original method described in Ref. [20]. In contrast to Ln2 O3 [19]. The Raman spectrum of the tetragonal phase conventional Raman spectroscopy technique, we used the was shown to essentially differ from those for cubic and laser monoclinic zirconia [19]. Intensities of the Raman lines of operated in a pulse repetition regime at a frequency of different symmetries in the polarized Raman spectra 10 kHz and the power pulse duration of 10 ns. The laser depended on the orientation of tetragonal fragments. The average power was 1-3 W at the peak pulse power of about C-d phase transformation in the absence of deformation 10-30 kW.High peak pulse power of the laser resulted in a stimulated nucleation of three types of tetragonal domains high contrast between the Raman scattering signal and the (B, D and F)in the volume of primary cubic structure(O hermal-radiation background. We also used a signal gating space group)(Fig. 2). According to Ref [21 ], the tetragonal circuit with the signal duration time of 11 ns, which locked z-axes of the domains should be oriented along the three C4 out the registration system for the time of the absence of an axes, and vectors x, y are rotated through the angle of 45 exciting laser pulse, the thermal irradiation background with respect to the cubic axes of the primary fluorite being suppressed by a factor of 10 structure. The calculated intensities of the a,.B, and Fig. la displays the scheme of the stress-induced C-I modes for B, D and F tetragonal domains are displayed phase transformation experiments. A single crystal was separately in Table 2. The exciting beam and the laced into the furnace between two sapphire rods. The polarization vector Eex were directed along the [001] and upper rod had a sharp end of 1 mm in diameter. The sample [100] axes, respectively (Er-position in Fig. 2). Two was gradually heated for 3 h up to the temperature of scattering geometries were used in calculations. The 1800 K. A load(30 kg/mm) was applied to the sample analyzer was aligned in parallel and perpendicularly to the after holding at 1800 K for 30 min. The loading was realized Eex-vector in the cases of the ll and 1 geometries(Fig. 2) in the direction along one of the three cubic axes of the The Raman tensors corresponding to the Alg, BIg and e sample as shown in Fig. 1b. After keeping under loading modes are shown in the bottom of Table 2. Intensities of (15 min) at 1800 K, the sample was rapidly cooled to the plarized lines calculated for the sum of all three domains temperature below the C-t transformation (1350 K). are displayed in the >B, D, F column and for the sum of D Then, the sample was cooled for 2 h without loading to and F domains--in the 2D, F column. The data of the 300 K. Polarized Raman spectra were registered at 300K >B,D, F column correspond to equiprobable orientations of for both nonstressed (1)and stressed(2) zones of the the z axes of the tetragonal domains along the three cubic C4 crystals(Fig. 1b). Registration of the Raman spectra was axes. Only one line of the Ag -mode could be registered in carried out through the 40-um holes burned in the carbon the Raman spectrum for the geometry while the rest film by a focused excitation laser beam. This film was 2Blg+ 3Eg-modes appeared in the spectrum for the 1 preliminarily deposited on the surface of the single crystal geometry in this case
technique. We also analyzed samples obtained by water quenching ZrO2 –Eu2O3 (2–8 mol%) melts, which had been previously investigated in Ref. [17]. Undoped powder tetragonal ZrO2-sample was synthesized by hydroxides co-precipitation technique [19]. A list of the sample under study is presented in Table 1. Raman spectra of the samples were investigated at 300 K with Spex-Ramalog-1403 spectrometer using conventional back scattering geometry under Ar (488 nm) excitation. High-temperature Raman spectroscopy studies up to 1900 K were carried out with the original method described in Ref. [20]. In contrast to conventional Raman spectroscopy technique, we used the copper vapor laser as an excitation source. This laser operated in a pulse repetition regime at a frequency of 10 kHz and the power pulse duration of 10 ns. The laser average power was 1–3 W at the peak pulse power of about 10–30 kW. High peak pulse power of the laser resulted in a high contrast between the Raman scattering signal and the thermal-radiation background. We also used a signal gating circuit with the signal duration time of 11 ns, which locked out the registration system for the time of the absence of an exciting laser pulse, the thermal irradiation background being suppressed by a factor of 104 . Fig. 1a displays the scheme of the stress-induced C ! t 0 phase transformation experiments. A single crystal was placed into the furnace between two sapphire rods. The upper rod had a sharp end of 1 mm in diameter. The sample was gradually heated for 3 h up to the temperature of 1800 K. A load (<30 kg/mm2 ) was applied to the sample after holding at 1800 K for 30 min. The loading was realized in the direction along one of the three cubic axes of the sample as shown in Fig. 1b. After keeping under loading (15 min) at 1800 K, the sample was rapidly cooled to the temperature below the C ! t 0 transformation (1350 K). Then, the sample was cooled for 2 h without loading to 300 K. Polarized Raman spectra were registered at 300 K for both nonstressed (1) and stressed (2) zones of the crystals (Fig. 1b). Registration of the Raman spectra was carried out through the 40-mm holes burned in the carbon film by a focused excitation laser beam. This film was preliminarily deposited on the surface of the single crystal. 3. Potentialities of Raman spectroscopy in studying the C ! t 0 phase transformation The Raman spectrum of tetragonal zirconia (D15 4h space group) consists of six lines of the A1g þ 2B1g þ 3Eg symmetries [10]. This spectrum is characteristic of both pure ZrO2 and ZrO2 –Ln2O3 solid solutions (Ln implies the lanthanide series and Y) in the region of 0–8 mol% of Ln2O3 [19]. The Raman spectrum of the tetragonal phase was shown to essentially differ from those for cubic and monoclinic zirconia [19]. Intensities of the Raman lines of different symmetries in the polarized Raman spectra depended on the orientation of tetragonal fragments. The C ! t 0 phase transformation in the absence of deformation stimulated nucleation of three types of tetragonal domains ðB; D and FÞ in the volume of primary cubic structure (O5 h- space group) (Fig. 2). According to Ref. [21], the tetragonal z-axes of the domains should be oriented along the three C4 axes, and vectors x, y are rotated through the angle of 458 with respect to the cubic axes of the primary fluorite structure. The calculated intensities of the A1g; B1g and Eg modes for B; D and F tetragonal domains are displayed separately in Table 2. The exciting beam and the polarization vector Eex were directed along the [001] and [100] axes, respectively (E1-position in Fig. 2). Two scattering geometries were used in calculations. The analyzer was aligned in parallel and perpendicularly to the Eex-vector in the cases of the k and ’ geometries (Fig. 2). The Raman tensors corresponding to the A1g; B1g and Eg modes are shown in the bottom of Table 2. Intensities of polarized lines calculated for the sum of all three domains are displayed in the PB; D; F column and for the sum of D and F domains—in the PD; F column. The data of the PB; D; F column correspond to equiprobable orientations of the z axes of the tetragonal domains along the three cubic C4 axes. Only one line of the Ag-mode could be registered in the Raman spectrum for the k geometry while the rest 2B1g þ 3Eg-modes appeared in the spectrum for the ’ geometry in this case. Fig. 1. (a) Scheme of the experiment on the sample deformation during the C ! t 0 phase transformation. (b) Orientation of the sample under study; 1 and 2 denote the non-deformed and deformed zones, respectively. Table 1 The compositions and synthesis methods of the as-received ZrO2 –Ln2O3 solid solutions Number of the sample Type of Ln Ln2O3-concentration (mol%) Synthesizing method 1 – Undoped c 2 Eu 2.5 m 3 Eu 4 m 4 Eu 6 m 5 Eu 8 m 6 Eu 8 s.c. 7 Gd 8 s.c. c, hydroxides coprecipitation; m, rapidly quenched melt; s.c., single crystal grown by cold-container technique. 1104 A.A. Sobol, Y.K. Voronko / Journal of Physics and Chemistry of Solids 65 (2004) 1103–1112
A.A. Sobol, Y.K. Voronko Journal of Physics and Chemistry of Solids 65 (2004)1103-1112 1105 Load domains results in the appearance of only lines of the 3E modes in the Raman spectrum for the 1 geometry (D, F column in Table 2). Thus, registration of polarized Raman spectra is the reliable method of revealing results of the stress- [001] induced C-I transformation under loading However, at first sight, it is impossible to carry out above-mentioned experiments. Bulk tetragonal single crystals of own. as to crystals of Y-PSZ synthesized by cold container technique, they appear as milky color due to the presence of large dimension tetragonal precipitates and are inadequate for the [010] polarized Raman spectroscopy studies The application of the polarized Raman spectroscopy for studying the C-t transformation in PSZ single crystals is possible due to the specific mechanism of tetragonal phase formation in ZrO2-Gd2O3(Eu2O3)(6-8 mol%)[17, 22) solid solutions. which will be described belot [100] Ea 4. Formation of the tetragonal phase in ZrOz-Gd2O3 (Eu2O3)solid solutions Fig. 2. Orientation of the isolated tetragonal domains B, D and F in the volume of ZrO2-Gd2 O3(Eu,O3)( 8 mol%)cubic solid solution. En and E, are the directions of Eex-electric vector of the excitation A nature of t-domain formation can be qualitatively explained for ZrO2-rich region of the equilibrium ZrO2 Gd2O3(Eu2O3) phase diagram(Fig 3). The diagram of the Deformation along the definite CA axis in the process of description of the systems under study. The temperature T1 C-d transformation created the different conditions for separates the cubic and the cubic +tetragonal (C+t) nucleation of B, D and F types of t' domains(Fig. 2) regions, whereas To denotes the temperature of the C-t Loading can induce the predominant formation of either b phase transformation. These temperatures were determined or D+ Domain. The predominant formation of the B domain to be in the region of 1400-1700 K for ZrO2-Gd2O3 would result in decreasing the intensity of the Alg mode in the (Eu2O3)(6-8 mol%)samples [17]. There was an essential conditions of the gec omer difference in the phase formation at different annealin component of the Alg Raman tensor from the summarized temperatures(points 2 and 3 in Fig 3). The temperature of intensity equation of the 2B, D, F column (Table 2). More- the point 2 lies between T1 and To. The usual decomposition over,only lines of the 2B,g modes should be registered in the of the C-solid solution due to diffusion-controlled reaction spectrum of the 1 geometry at the predominant B domain must occur at this temperature [22]. Decomposition formation. In contrast, the predominant formation of D+F products are the low-Gd2O3(Eu2O3) t-phase(=2.5 mol% Table 2 Calculated intensities of the tetragonal vibrational modes in the parallel (ID)and the crossed( l )scattering geometries for three types of domains according to Fg.2(Eax‖E1) The domain SD.F E(1) 000 E
Deformation along the definite C4 axis in the process of C ! t 0 transformation created the different conditions for nucleation of B; D and F types of t0 domains (Fig. 2). Loading can induce the predominant formation of either B orD þ F domain. The predominant formation of theB domain would result in decreasing the intensity of the A1g mode in the conditions of the k geometry due to canceling the azz component of the A1g Raman tensor from the summarized intensity equation of the PB; D; F column (Table 2). Moreover, only lines of the 2B1g modes should be registered in the spectrum of the ’ geometry at the predominant B domain formation. In contrast, the predominant formation of D þ F domains results in the appearance of only lines of the 3Eg modes in the Raman spectrum for the ’ geometry (PD; F column in Table 2). Thus, registration of polarized Raman spectra is the reliable method of revealing results of the stressinduced C ! t 0 transformation under loading. However, at first sight, it is impossible to carry out the above-mentioned experiments. Bulk tetragonal single crystals of a pure ZrO2 cannot be grown. As to single crystals of Y–PSZ synthesized by cold container technique, they appear as milky color due to the presence of largedimension tetragonal precipitates and are inadequate for the polarized Raman spectroscopy studies. The application of the polarized Raman spectroscopy for studying the C ! t 0 transformation in PSZ single crystals is possible due to the specific mechanism of tetragonal phase formation in ZrO2 –Gd2O3 (Eu2O3) (6–8 mol%) [17,22] solid solutions, which will be described below. 4. Formation of the tetragonal phase in ZrO2 –Gd2O3 (Eu2O3) solid solutions A nature of t0 -domain formation can be qualitatively explained for ZrO2-rich region of the equilibrium ZrO2 – Gd2O3 (Eu2O3) phase diagram (Fig. 3). The diagram of the ZrO2 –Y2O3 system [23] was used as a prototype for the description of the systems under study. The temperature T1 separates the cubic and the cubic þ tetragonal (C þ t) regions, whereas T0 denotes the temperature of the C ! t 0 phase transformation. These temperatures were determined to be in the region of 1400–1700 K for ZrO2 –Gd2O3 (Eu2O3) (6–8 mol%) samples [17]. There was an essential difference in the phase formation at different annealing temperatures (points 2 and 3 in Fig. 3). The temperature of the point 2 lies between T1 and T0: The usual decomposition of the C-solid solution due to diffusion-controlled reaction must occur at this temperature [22]. Decomposition products are the low-Gd2O3 (Eu2O3) t-phase (<2.5 mol% Fig. 2. Orientation of the isolated tetragonal domains B; D and F in the volume of ZrO2 –Gd2O3 (Eu2O3) (8 mol%) cubic solid solution. E1 and E2 are the directions of Eex-electric vector of the excitation beam. Table 2 Calculated intensities of the tetragonal vibrational modes in the parallel (k) and the crossed ( ’ ) scattering geometries for three types of domains according to Fig. 2 (EexkE1) The mode The domain BDF PB; D; F PD; F k ’ k ’ k ’ k ’ k ’ A1g a 2 0 a 2 0 b 2 0 2a 2 þ b 2 0 a 2 þ b 2 0 B1g 0 c 2 00 00 0 c 2 0 0 Eg 00 0 e 2 0 e 2 0 2e 2 0 2e 2 A1g ¼ a 0 0 0 a 0 0 0 b ; B1g ¼ c 0 0 0 2c 0 000 ; Egð1Þ ¼ 000 0 0 e 0 e 0 ; Egð2Þ ¼ 0 0 2e 000 2e 0 0 A.A. Sobol, Y.K. Voronko / Journal of Physics and Chemistry of Solids 65 (2004) 1103–1112 1105
A.A. Sobol, Y.K. Voronko / Journal of Physics and Chemistry of Solids 65 (2004)1103-1112 (8 mol%)crystals can be repeated using the subsequent low- temperature annealing [17]. By contrast, the particles of e t-phase formed in the diffusion-controlled reaction are retained even after a long-time(20-30 h) annealing at T>T 5. Raman spectroscopy study of the formation of different tetragonal phases in ZrO2-Gd,O3(Eu2O3) solid solutions The above-mentioned two mechanisms of phase for- mation in the ZrO2-Gd2O3(Eu2O3) system are easily 1000 registered by the Raman spectroscopy technique. Fig. 4 displays the Raman spectra at 300 K for a series of rapidl Eu2O3(Gd2O3)-mol% quenched ZrO2-Eu2O3 melts under the variation of the Eu2O3 concentration from 2.5 up to 8 mol%. The Raman t trum of a is also shown in Fig 4. In this figure, all six Raman lines of tetragonal D4, phase are presented in the Raman spectra of the samples. A Fig. 3. Equilibrium ZrO2-Gd2O3(Eu 03)phase diagrams. Ti is the continuous change in the line frequencies is observed with o the cr bet ween turma nd t apd s e egte s nd e i h e temper atse the growth of Eu2O3 concentration, a shift of theRamanline in the experiments. Bottom-the boundaries of the t' and C metastable position within the range 240-270 cm being the most phases at 300K remarkable(Fig. 4). Redistribution of intensities and broadening of the Raman lines followed a growth in the stabilizer concentration. Thus. the most intensive narrow line in the 240-270 cm spectral range for a pure ZrO2 and 9mol% of Gd, 03(Eu,,). This process resulted in the ZrO2-EzO3(2.5 mol% )is registered as the weakest broad formation of large-size t-particles and the milky single band in the spectrum of the ZrO2-Eu2O3(8 mol%)sample crystals. In turn, there is a growth of relative intensities of the two Q The temperature at the point 3 is lower than To of the lines in the ptin o im me srd s ion of 600 cm with increasing Eu2O3 content. These tw merged practically into a unified is similar to the Gd2O3(Eu2O3) cubic phase should nucleate and dominating in the spectrum of the ZrO2-Eu2O3 in the crystal volume as a result of this transformation. (8 mol%)sample(Figs. 4 5). Previously, the band with Diffusion processes at T< To for ZrO2-Gd2O3(Eu2 03) O5 Fluorite structure on the basis of the polarized Raman (8 mol%) was shown to proceed very slowly [17]. They interfere with the nucleation of t-precipitates due to the diffusion-controlled reaction in the case of annealing at t< 242cm265cm To [17]. For the solid solution with Ln2O3 stabilizers of small Ln-ions (Y2O3, Yb2O3, Lu2O3), this phenomenon resulted in the freezing stabilizer concentration in 8-mol% domains. In contrast, the primary concentration of Ln2O oxides with large Ln cations(Gd2O3 and Eu2O3) did not 6-mol%o retain in supersaturated t'-domains even at low-temperature nnealing. gradual decrease in the concentration of Gd,O 4-mol% (Eu?O3) inside the t'-domains occurred, the volume around these domains being fertilized by Gd2O3(Eu2O3) oxides 5-mol% The Gd2O3(Eu2O3)concentration in t'-particles can be reduced to 2-3 mol%o due to long-time low-temperature 人 a pure Zr annealing [22]. The low-temperature phase formation mechanism was reversible for ZrO2-Gd,O3(Eu2O3) (8 mol%) system omposition of the material can be rapidly homogenized by annealing at T> T(point 1 in Raman shift (cm") Fig 3). Then, the process of formation of the low-Gd2O (Eu2O3)t'-domains in the volume of ZrO2-Gd2O3(Eu2O3) Fig 4. Raman spectra at 300 K of the rapidly quenched ZrO2-(Eu203 melts at variation of Eu,O3-conter
stabilizer) plus the cubic solid solution containing <9 mol% of Gd2O3 (Eu2O3). This process resulted in the formation of large-size t-particles and the milky single crystals. The temperature at the point 3 is lower than T0 of the C ! t 0 phase transition. The t0 -particles whose composition is similar to the Gd2O3 (Eu2O3) cubic phase should nucleate in the crystal volume as a result of this transformation. Diffusion processes at T , T0 for ZrO2 –Gd2O3 (Eu2O3) (8 mol%) was shown to proceed very slowly [17]. They interfere with the nucleation of t-precipitates due to the diffusion-controlled reaction in the case of annealing at T , T0 [17]. For the solid solution with Ln2O3 stabilizers of small Ln-ions (Y2O3, Yb2O3, Lu2O3), this phenomenon resulted in the freezing stabilizer concentration in t0 domains. In contrast, the primary concentration of Ln2O3 oxides with large Ln cations (Gd2O3 and Eu2O3) did not retain in supersaturated t0 -domains even at low-temperature annealing. Gradual decrease in the concentration of Gd2O3 (Eu2O3) inside the t0 -domains occurred, the volume around these domains being fertilized by Gd2O3 (Eu2O3) oxides. The Gd2O3 (Eu2O3) concentration in t0 -particles can be reduced to 2–3 mol% due to long-time low-temperature annealing [22]. The low-temperature phase formation mechanism was reversible for ZrO2 –Gd2O3 (Eu2O3) (8 mol%) system. The composition of the material can be rapidly homogenized by annealing at T . T1 (point 1 in Fig. 3). Then, the process of formation of the low-Gd2O3 (Eu2O3) t0 -domains in the volume of ZrO2 –Gd2O3 (Eu2O3) (8 mol%) crystals can be repeated using the subsequent lowtemperature annealing [17]. By contrast, the particles of the t-phase formed in the diffusion-controlled reaction are retained even after a long-time (20–30 h) annealing at T . T1: 5. Raman spectroscopy study of the formation of different tetragonal phases in ZrO2 –Gd2O3 (Eu2O3) solid solutions The above-mentioned two mechanisms of phase formation in the ZrO2 –Gd2O3 (Eu2O3) system are easily registered by the Raman spectroscopy technique. Fig. 4 displays the Raman spectra at 300 K for a series of rapidly quenched ZrO2 –Eu2O3 melts under the variation of the Eu2O3 concentration from 2.5 up to 8 mol%. The Raman spectrum of a pure ZrO2 ceramic is also shown in Fig. 4. In this figure, all six Raman lines of tetragonal D15 4h phase are presented in the Raman spectra of the samples. A continuous change in the line frequencies is observed with the growth of Eu2O3 concentration, a shift of the Raman line position within the range 240–270 cm21 being the most remarkable (Fig. 4). Redistribution of intensities and broadening of the Raman lines followed a growth in the stabilizer concentration. Thus, the most intensive narrow line in the 240–270 cm21 spectral range for a pure ZrO2 and ZrO2 –Eu2O3 (2.5 mol%) is registered as the weakest broad band in the spectrum of the ZrO2 –Eu2O3 (8 mol%) sample. In turn, there is a growth of relative intensities of the two lines in the region of 600 cm21 with increasing Eu2O3 content. These two lines merged practically into a unified band dominating in the spectrum of the ZrO2 –Eu2O3 (8 mol%) sample (Figs. 4 and 5). Previously, the band with the frequency of 600 cm21 was assigned to F2g mode of the O5 h fluorite structure on the basis of the polarized Raman Fig. 3. Equilibrium ZrO2 –Gd2O3 (Eu2O3) phase diagrams. T1 is the boundary between cubic and C þ t phases regions and T0 is the temperature of the C ! t 0 transformation. 1,2 and 3 denote the annealing regimes used in the experiments. Bottom—the boundaries of the t0 and C metastable phases at 300 K. Fig. 4. Raman spectra at 300 K of the rapidly quenched ZrO2 –(Eu2O3) melts at variation of Eu2O3-content. 1106 A.A. Sobol, Y.K. Voronko / Journal of Physics and Chemistry of Solids 65 (2004) 1103–1112
A.A. Sobol, Y.K. Voronko Journal of Physics and Chemistry of Solids 65 (2004)1103-1112 V-cm ZrO2-Eu2O3(& mol%) melt after several annealing cycles of with different time duration at To <T<T. This figure Eg displays the process of a nucleation and the volume growth 600Bg of the low-Eu2O3(2.5-3 mol%)t-phase with increasing the F2(C) annealing time. The Raman spectrum of the sample after the longest annealing(& h)revealed an intense broad line in he region of 600 cm-I and a narrow Raman line at 400 265 cm. This demonstrates the presence of the low-Eu2O3 t-phase in addition to a considerable quantity of the Eu2O3- B1 rich phase in the sample after long-time annealing at To A1 T<71(Fig.6) 200 The presence of two phases in the sample under study Eg resulted in the doublet form of lines in the tetragonal raman spectra. The positions of the Raman lines of a certain phase practically were not shifted with variation of the annealing time. This fact showed the absence of considerable change in the composition of these two phases during the thermal treatment as exemplified by the phase diagram. It should be Eu2O3- mol% noted that it takes a long annealing time(above 1 h)to initiate ig. 5. Variation of the tetragonal modes frequencies in the rapidly the decomposition process via the diffusion-controlled reac quenched ZrO2-Eu2O3 melts versus the Eu2O3 concentration. tion(Fig. 6) study of the cubic ZrO2-Eu2O3(12 mol%) single crystals Evolution of the Raman spectra at 300 K for the rapidl [11]. The frequency of the F2g mode for ZrO2-Eu2 0 quenched Z1O2-Eu2O3(8 mol%)melt after annealing at T (12 mol%)also points out in Fig. 5. Thus, Figs. 4 and 5 To(point 3 of the diagram in Fig 3)essentially differed from demonstrate the evolution of the ran those considered above. Changes in the Raman spectra tetragonal phase while moving from a pure ZrO, with the became noticeable already after 5-min annealing(Fig. 7) highest tetragonality(the cla ratio) to the practically owever, they are not associated with the appearance of stabilized cubic solid solution (cla- 1) in ZrO2-Eu2O3 narrow Raman lines of the low-Eu2O3 t-phase. Gradual (8 mol%) variations in positions and intensities of the Raman lines with The position and width of the Raman lines in the Raman growing annealing time were observed(Fig. 7). The spectra shown in Fig. 4 allow us to reliably indicate the regularities in transformations of the Raman spectra shown of a stabilizer. Thus. it was not difficult to register the changes of Raman spectra, which were caused by decreasing process of the decomposition of initially quenched ZrO Eu2O3(8 mol%) melt due to the diffusion-controlled 42cm1265cm1 reaction at To <T<Ti(point 2 in Fig. 3). Fig. 6 illustrates the Raman spectra at 300 K of the preliminary quenched as-quenched 5 min 20 55 min 0 200400600 0200400600800 Raman shift(cm) Fig. 6. Raman spectra at 300 K of the rapidly quenched ZrO2-(Eu2O3) Fig. 7. Raman spectra at 300 K of the rapidly quenched ZrO2-(Eu2O3 (8 mol%) melt after repeated annealing at 1700 K(above the C (8 mol%) melt after repeated annealing at 1350 K(below the C transformation, point 2 in Fig. 3)with different time expositions. transformation, point 3 in Fig. 3)with different time expositions
study of the cubic ZrO2 –Eu2O3 (12 mol%) single crystals [11]. The frequency of the F2g mode for ZrO2 –Eu2O3 (12 mol%) also points out in Fig. 5. Thus, Figs. 4 and 5 demonstrate the evolution of the Raman spectra for the tetragonal phase while moving from a pure ZrO2 with the highest tetragonality (the c=a ratio) to the practically stabilized cubic solid solution ðc=a ! 1Þ in ZrO2 –Eu2O3 (8 mol%). The position and width of the Raman lines in the Raman spectra shown in Fig. 4 allow us to reliably indicate the presence of the tetragonal phase with a certain concentration of a stabilizer. Thus, it was not difficult to register the process of the decomposition of initially quenched ZrO2 – Eu2O3 (8 mol%) melt due to the diffusion-controlled reaction at T0 , T , T1 (point 2 in Fig. 3). Fig. 6 illustrates the Raman spectra at 300 K of the preliminary quenched ZrO2 –Eu2O3 (8 mol%) melt after several annealing cycles of with different time duration at T0 , T , T1: This figure displays the process of a nucleation and the volume growth of the low-Eu2O3 (2.5–3 mol%) t-phase with increasing the annealing time. The Raman spectrum of the sample after the longest annealing (8 h) revealed an intense broad line in the region of 600 cm21 and a narrow Raman line at 265 cm21 . This demonstrates the presence of the low-Eu2O3 t-phase in addition to a considerable quantity of the Eu2O3- rich phase in the sample after long-time annealing at T0 , T , T1 (Fig. 6). The presence of two phases in the sample under study resulted in the doublet form of lines in the tetragonal Raman spectra. The positions of the Raman lines of a certain phase practically were not shifted with variation of the annealing time. This fact showed the absence of considerable change in the composition of these two phases during the thermal treatment as exemplified by the phase diagram. It should be noted that it takes a long annealing time (above 1 h) to initiate the decomposition process via the diffusion-controlled reaction (Fig. 6). Evolution of the Raman spectra at 300 K for the rapidly quenched ZrO2–Eu2O3 (8 mol%) melt after annealing at T , T0 (point 3 of the diagram in Fig. 3) essentially differed from those considered above. Changes in the Raman spectra became noticeable already after 5-min annealing (Fig. 7). However, they are not associated with the appearance of narrow Raman lines of the low-Eu2O3 t-phase. Gradual variations in positions and intensities of the Raman lines with growing annealing time were observed (Fig. 7). The regularities in transformations of the Raman spectra shown in Fig. 7 at low-temperature annealing are similar to changes of Raman spectra, which were caused by decreasing Fig. 5. Variation of the tetragonal modes frequencies in the rapidly quenched ZrO2 –Eu2O3 melts versus the Eu2O3 concentration. Fig. 6. Raman spectra at 300 K of the rapidly quenched ZrO2 –(Eu2O3) (8 mol%) melt after repeated annealing at 1700 K (above the C ! t 0 transformation, point 2 in Fig. 3) with different time expositions. Fig. 7. Raman spectra at 300 K of the rapidly quenched ZrO2 –(Eu2O3) (8 mol%) melt after repeated annealing at 1350 K (below the C ! t 0 transformation, point 3 in Fig. 3) with different time expositions. A.A. Sobol, Y.K. Voronko / Journal of Physics and Chemistry of Solids 65 (2004) 1103–1112 1107