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A Raman spectroscopy study of the formation of Ln2−xLnx∗TiyO2y + 3 solid solutions
Journal of the Less-Common
Metals, 109 (1985)
147
147 - 153
A RAMAN SPECTROSCOPY STUDY OF THE FORMATION OF Lnz__xLn~Ti,O,, + 3 SOLID SOLUTIONS C. E. BAMBERGER
and G. M. BEGUN
Transuranium Research 37831 (U.S.A.) (Received
October
Laboratory,
Oak Ridge National Laboratory,
Oak Ridge,
TN
19, 1984)
Summary The existence of lanthanide titanates LnzO,-yTi0, in the range 3.0 < y < 4.5 has been determined by Raman spectroscopy. It has been established that such compounds form in the pure state only with Ln = La - Nd and y = 4.33 - 4.50. However, solid solutions of the type (Lnz_,Ln,*0&4.33Ti02, where Ln* = Sm - Lu, were prepared. The solubility of the elements from lutetium to samarium in (La, Pr, or Nd),Os-4.33Ti0, increases as a function of the ionic radius of the “solute” element.
1. Introduction Considerable information is available on the formation of substitutional solid solutions of dititanates of the type Ln,_ xM,Ti,O,, where M is a lanthanide element [l] or bismuth [2], and of the trititanates LnxBi4-xTi3012 [3,4]. However, we found no information on the occurrence of solid solutions of the lanthanide titanates Lnz_ xM,Ti,Ozv+ s with values of y larger than 3. Actually very little information is available on such pure lanthanide titanates. McChesney and Sauer [ 51 have reported the X-ray powder diffraction (XRD) pattern of the compound La,0s.4.5Ti02. Kolar et al. [6] have reported the XRD pattern of the compound Ndz0s*4Ti02 and have indexed it on the basis of an orthorhombic cell. In addition they indexed the pattern of La,03*4.5Ti0, [ 51 for an orthorhombic cell with lattice parameter values very close to those of Nd,0s*4Ti02. From these data Kolar et al. [6] conclude that Laz0s.4.5Ti02 exists as a homogeneous solid solution of 0.5Ti02 in La,Os-4Ti0,. It can be surmised from the literature that lanthanide elements heavier than neodymium do not form titanates with y > 3. Collongues et al. [7] in their study of the phase diagrams of Gd,O,-TiO, and Dy,03TiO? observed only TiOz and pyrochlore phases (Ln,Ti,O,) in the region 0 30 mol.% Ln,O,. Similar results have been reported for the systems Tbz03TiOz [8], Er,03-TiOz [9], YzOs-Ti02 [9 - 111, LuzO,--TiO, [12], Yb,O,TiOz [13] and Gd,O,--TiO, [ 141. In the present work we have sought to 0022-5088/95/$3.30
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determine the range of homogeneity of the titanates Ln,O,*yTiOz (y > 3) and furthermore the existence of solid solutions of the type Ln? _xLn,*Ti,Oay + s. Raman spectroscopy was used throughout this study because we have previously established [ 1 - 31 that it is a useful technique for rapidly identifying various titanate phases.
2. Experimental
details
The synthesis of solid species with the compositions LnzOs.yTiOz and by heating intimate mixtures of the Lnz-xLn,*Ti,OZy +3 were accomplished powdered oxides, contained in platinum boats, in air at 1350 - 1400 “C for various periods of up to 40 h followed by rapid cooling in air. This temperature range was chosen to avoid the formation of liquid phases since it had been reported that La,03. 4.5Ti02 and Nd,03* 4Ti01 melt incongruently at 1455 f 5 “C [5] and 1505 f 15 “C [6] respectively. Lanthanide oxides with a minimum purity of 99% were obtained from several commercial sources. The oxides Pr,03 +% and Tbz03+, were used as such after determining the value of x from the oxygen evolution during the formation of their corresponding orthophosphates [ 151. We have established previously that these lanthanide elements reduce spontaneously to the trivalent state on reaction with TiOz. Samples were ground and reheated repeatedly until their Raman spectra remained unchanged. The Raman spectra were excited using the 514.4 or 488.0 nm line from a continuouswave argon ion laser. Spectra were observed using a Ramanor HG-2S spectrophotometer (Instruments SA) as previously described [ 161.
3. Results and discussion Preparations containing La203, Pr,03 and Ndz03, each with 4.7 TiOz, exhibited Raman spectra that had not been observed before in lanthanide titanate systems. Because their XRD patterns matched those reported for La,03*4.5Ti02 [ 51 and for Nd,03* 4Ti02 [6] we assigned most of the peaks of these spectra, which are shown in Figs. 1 and 2, to Ln,03*yTiOz (Ln = La, Pr and Nd; y w 4.7). Similar preparations containing EuZ03, Ho203, Er,03 and Luz03 produced the Raman spectra of the respective LnzTizO, pyrochlores and TiOz (rutile) in agreement with information from phase diagrams [7 - 141. The spectra of Ln,03.yTiOz included, in addition to the new spectra, the two strongest peaks of TiOz @utile) at 446.8 and 608.5 cm-’ (Fig. 1, curve c) indicating that the compounds required a stoichiometry with y < 4.7. Additional preparations of Laz03, Prz03 and Nd,03 with yTiO,, where y was varied between 4 and 6, were made, and the Raman spectra (see Figs. 1 and 2) were examined for the presence of monoclinic Ln,Ti,O.l and TiO? (rutile). The presence of rutile, which provided the higher limit to the value of y, was ascertained from the ratio of the intensity of the
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Fig. 1. Curve a, Raman spectrum of a mixture of LazOs-4.5TiO2 and X.+Ti@7 obtained by heating La*03 and 4.03Ti02 at 1400 “C for a total of 38 h (excitation, 514.5 nm; *, monoclinic LazTi,O, peaks); curve b, Raman spectrum of LazOs-4.38TiO.r obtained by heating La203 and 4.38TiOs at 1400 “C for a total of 19 h (excitation, 514.5 nm); curve c, Raman spectrum of a mixture of LazOs.4.5Ti02 and about 13.8 wt.% TiOs obtained by heating La203 and 5.87 TiOz at 1350 “C for a total of 41 h (excitation, 488.0 nm; 0, TiOz (rutile) peaks}. Fig. 2. Curve a, Raman spectrum of a mixture of PrzOa*4,5TiOa and Pr2Ti207 obtained from PrsOs + 4.04Ti02; (*, monoclinic PrzTis07 peaks); curve b, Raman spectrum of Prz03*4.33Ti0s; curve c, Raman spectrum of a mixture of NdsOs-4.5Ti02 and NdaTizO, obtained from Nd20s + 4.04Ti02 ; (*, monoclinic NdzTi207 peaks); curve d, Raman spectrum of NdzOa-4.31TiOs. All preparations were heated at 1370 “C for a total of 19 h and excited at 514.5 nm.
Raman peak at 446.8 cm-’ (low intensity for LnzOS+yTiOz and strong intensity for rutile) to the intensity of the peak at 663.5 cm-’ (strong intensity for Ln,O, l yTiO2). The values of this ratio measured for about 30 preparations, including the solid solutions Lnz_xLn:Ti,,0zY+3, was 0.27 ?s 0.13 for values of y between 4 and 4.5 and increased linearly to about 1.51 * 0.4 for y = 6. These results indicate that the value of y lies between 4.0 and 4.5. The presence of monoclinic Ln,TizO, phases in some Raman spectra of preparations with y = 4 - 4.5 yielded a value of about 4.3 for the lower limit of y. This value was confirmed by examining the Raman spectra of mixtures
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Fig. 3. Curve a, Raman spectrum of a mechanical mixture of Laz03.4.7Ti02 with 7.3 wt.% monoclinic LaNdTizOT (excitation, 514.5 nm; *, monoclinic LaNdTiz07 peak); curve b, Raman spectrum of a mechanical mixture of Prz03-4.33Ti02 with 10.2 wt.% LuzTizO, showing the presence of the latter (excitation, 514.5 nm; *, cubic LuzTiz07 peaks).
prepared by grinding together monoclinic LnzTizO, and the product of the reaction of Laz03 with 4.7Ti02 (Fig. 3(a)). The monoclinic phases were observed with some certainty at a minimum concentration of 7.3 wt.%. Since we have observed LazTizO, in experiments in which y = 4.03 (Fig. 1, curve a), we conclude that in the compound LazOs*yTiOz y has a value between 4.25 and 4.33 which corresponds to mixtures containing 7.0 - 9.4 wt.% La,Ti,O,. We have thus arbitrarily selected y = 4.33 which gives the stoichiometry 3Ln,03-13Ti02. The Raman spectra of various mixtures of the pyrochlore Lu,TizO, with La*O,-4.33Ti0, (or the analogous neodymium compound) (Fig. 3, curve b) were recorded in order to set the limit of detectability for the pyrochlore phase and thus assign uncertainties to the solubility of the heavy lanthanide elements (Ln*) in the Lnz_,Ln:Os-4.33Ti02 solid solutions reported below. The minimum concentration of LuzTizO, that could be detected using the Raman spectra was about 10 wt.%. Solid solutions of Ln,_,Ln,*0,*4.33Ti02 (Ln 5 La, Pr and Nd; Ln* = Lu, Yb, Tm, Er, Ho, Tb, Eu, Nd and Pr; y = 4.35 - 4.7) were prepared with various ratios [Ln*]/ [Ln + Ln*], and some of their selected Raman spectra are shown in Fig. 4. These spectra were interpreted in terms of the presence of either a single phase or a polyphase, ,and the results were plotted in a graph of S = 100 {(IR8L, - IR8L,B)/IR8L,*} uersus IRsmean = IR8n,X,, + IR8nn*XLn* where IR’ refers to the ionic radii with coordination number 8 obtained from ref. 17 and Xr,, is the cationic fraction of the lanthanide host (X,, = nLn/(nLn + nLn*)). The resulting graph (Fig. 5) shows the boundary between single phases of
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Fig. 4. Curve a, Raman spectrum of a mixture of 0.79PrzOs + O.ZlHozO3 + 4.2TiOz heated to 1370 “C for a total of 18 h (excitation, 514.5 nm) (from material balance calculations and using the solubjlity data in Fig. 6 we estimated that the product consists of a mixture of 0.92(Pr1,,2 H0cS2s03*4.5Ti02) and 0.08HozTiz07 (6.4 wt.%) with 0.07TiOz; both minor phases are below the limit of detection); curve b, Raman spectrum of a mixture of 0.4SLaz03 + 0.55Ho203 + 4.3TiOz heated at 1370 “C for a total of 39 h (excitation, 514.5 nm;*, HozTizO, peaks; 0, TiOz (rutile) peaks) (from material balance calculations and the solubility data of Fig. 6 we estimated that the product consists of a mixture of about 0.7(La~.~Ho~,~O3*4.5TiO~), 0.31HozTi20, (about 27.7 wt.%) and 0.58Ti02); curve c, Raman spectrum of a solid solution La ~,3~Euc.~sO~.4.5TiO~ obtained from a mixture of 0.66La203 + 0.34Euz0s + 4.5TiOz heated at 1400 “C for a total of 23 h (excitation, 488.0 nm). Fig. 5. Boundary of the saturated solid solutions Lnz_,Ln$0s.4.33Ti02 as a function of the percentage difference ?i between the ionic radii with coordination number 8 (IRS) of Ln and Ln*, and the mean ionic radius IR8-: 0, single phase; A, polyphase.
solid solutions and saturated solid solutions. This boundary was used to calculate the solubility limits of the lanthanide elements from samarium to lutetium in 3Ln,03*13Ti0,, where Ln 3 La, Pr and Nd. Values for the .. . maximum solubrhtres XLn+,axf were calculated from the interpolated values on the boundary in Fig. 5. A plot of maximum solubility versus 6 of IR*,,,, is shown in Fig. 6 where the portion above samarium is based on experimental data which indicated total miscibility among the lanthanum, praseodymium and neodymium titanates. In addition, in Fig. 6 we have drawn tie lines between the “solute” elements samarium and lutetium, and we have
152
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Fig. 6. Maximum solubility of Ln* in Lnz_,Ln,*03.4.33%02 as a function of 6. The curves for An2 _XLn,Os ‘4.33Ti02, where An = Pu3+ and Am3+, are drawn lighter than those for the lanthanides to reflect the larger uncertainty,
extrapolated/interpolated on those lines the values of 6 calculated for Am3+ and Pu3’. These elements were considered to form the solvent 3An,0313Ti02, while the elements s~a~urn and lutetium were considered to be the solutes. This generated two new solubility curves which are also shown in Fig. 6 with a lighter trace to reflect a larger uncertainty. The ions Am’+ and Pu3* were selected here to provide a clue to their predicted behavior in lanthanide titanate systems, which are of interest in nuclear waste management. 4. Conclusions We have established by Raman spectroscopy that the minimum value of y in compounds Ln,03*yTi0,, where Ln = La, Pr and Nd, is about 4.33 and the maximum value is about 4.5, While similar compounds containing Ln = Sm - Lu do not occur, solid solutions of the type Ln~_~Ln~03*4,33TiO~ were prepared in which Ln* = Sm - Lu. It has also been established that the value of x increases as a function of the ionic radius of Ln*. We can conclude, based on their Raman spectra, that although the crystal symmetry of the compounds Ln,03*4.33Ti02 has not been established, the solid solutions have the same symmetry as the pure compounds with Ln = La, Pr and Nd. Based on extrapolation and interpolation of the solubility data obtained, we predict that Pu3+ will form solid solutions with all the lantbanide elements and that Am3+ will behave similarly, except with thulium, ytterbium and lutetium.
153
Acknowledgments This research was sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems Inc.
References 1 C. E. Bamberger, H. Dunn, G. M. Begun and S. Landry, Substitutional solid solutions from heterotypic rare earth dititanates, Lnz _,Ln,Tiz07, J. Less-Common Met., 109 (1985) 209. 2 C. E. Bamberger, H. Dunn, G. M. Begun and S. Landry, Substitutional solid solutions of bismuth-containing lanthanide dititanates, Ln2_-*BixTi207, J. Solid State Chem., 58 (1985) in the press. 3 R. W. Wolfe and R. E. Newnham, Rare earth bismuth titanates, J. Electrochem. Sot., 116 (6) (1969) 832. 4 C. E. Bamberger, G. M. Begun, J. Brynestad and H. W. Dunn, Lanthanide titanates and LnxBi4_-xTis012 solid solutions: their Raman spectra, Znorg. Chim. Acta, 109 (1985) 141. 5 J. B. MacChesney and H. A. Sauer, The system LazOs-Ti02; phase equilibria and electrical properties, J. Am. Ceram. Sot., 45 (9) (1962) 416. 6 D. Kolar, S. Gaberscek and A. Barbulescu, The NdzOs.4TiOz phase in the Nd@TiOz system, J. Less-Common Met., 60 (1978) 137. 7 R. Collongues, F. Queyroux, M. Perez y Jorba and J.-C. Gilles, Structures et propribt6s des composes form& par les oxydes des terres rares avec les oxydes des eI6ment.s du groupe IVA, Bull. Sot. Chim. Fr., (1965) 1141. 8 L. G. Shcherbakova, V. B. Glushkova, K. N. Guseva, L. G. Mamsurova, L. V. Sazonova and G. E. Sukhanova, The TiO?-TbzOa system at high temperatures, Znorg. Mater. (U.S.S.R.), 16 (8) (1980) 996. 9 L. G. Shcherbakova, V. B. Glushkova, L. P. Lyashenko, A. P. Zaitseva and L. V. Sazonova, High temperature interactions and diffusion in the Er@-Ti02 and YzOaTiOz systems, Dokl. Akod. Nauk S.S.S.R., 236 (6) (1978) 619. 10 N. Mizutani, Y. Tajima and M. Kato, Phase relationship in the system Yz0s-Ti02, J. Am. Ceram. Sot., 59 (4) (1976) 168. 11 J. D. Ault and A. J. E. Welch, The yttrium oxide-titanium dioxide systems, Acta Crystallogr., 20 (1966) 410. 12 M. A. Petrova, A. S. Novikova and R. G. Gerbenshchikov, Phase equilibria in the LuzOa-Ti02 system, Znorg. Mater. (U.S.S.R.), 18 (4) (1982) 599. 13 F. Queyroux, Sur l’existence d’un compose nouveau YbsTiOir et sur le diagramme d’equilibre YbzOs-Ti02, Bull. Sot. Fr. Mineral. Cristallogr., 88 (3) (1965) 519. 14 J. L. Waring and S. J. Schneider, Phase equilibrium relationships in the system GdzOs-TiOz, J. Res. Natl. Bur. Stand., Sect. A, 69 (3) (1965) 255. 15 G. M. Begun and C. E. Bamberger, Raman spectra of the rare earth trimetaphosphates, J. Raman Spectrosc., 13 (3) (1982) 284. 16 C. E. Bamberger, Preparation of metal phosphates by metathesis reaction with BP04, J. Am. Ceram. Sot., 65 (7) (1982) C-107. 17 R. D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr., Sect. A, 32 (1976) 751.
Metals, 109 (1985)
147
147 - 153
A RAMAN SPECTROSCOPY STUDY OF THE FORMATION OF Lnz__xLn~Ti,O,, + 3 SOLID SOLUTIONS C. E. BAMBERGER
and G. M. BEGUN
Transuranium Research 37831 (U.S.A.) (Received
October
Laboratory,
Oak Ridge National Laboratory,
Oak Ridge,
TN
19, 1984)
Summary The existence of lanthanide titanates LnzO,-yTi0, in the range 3.0 < y < 4.5 has been determined by Raman spectroscopy. It has been established that such compounds form in the pure state only with Ln = La - Nd and y = 4.33 - 4.50. However, solid solutions of the type (Lnz_,Ln,*0&4.33Ti02, where Ln* = Sm - Lu, were prepared. The solubility of the elements from lutetium to samarium in (La, Pr, or Nd),Os-4.33Ti0, increases as a function of the ionic radius of the “solute” element.
1. Introduction Considerable information is available on the formation of substitutional solid solutions of dititanates of the type Ln,_ xM,Ti,O,, where M is a lanthanide element [l] or bismuth [2], and of the trititanates LnxBi4-xTi3012 [3,4]. However, we found no information on the occurrence of solid solutions of the lanthanide titanates Lnz_ xM,Ti,Ozv+ s with values of y larger than 3. Actually very little information is available on such pure lanthanide titanates. McChesney and Sauer [ 51 have reported the X-ray powder diffraction (XRD) pattern of the compound La,0s.4.5Ti02. Kolar et al. [6] have reported the XRD pattern of the compound Ndz0s*4Ti02 and have indexed it on the basis of an orthorhombic cell. In addition they indexed the pattern of La,03*4.5Ti0, [ 51 for an orthorhombic cell with lattice parameter values very close to those of Nd,0s*4Ti02. From these data Kolar et al. [6] conclude that Laz0s.4.5Ti02 exists as a homogeneous solid solution of 0.5Ti02 in La,Os-4Ti0,. It can be surmised from the literature that lanthanide elements heavier than neodymium do not form titanates with y > 3. Collongues et al. [7] in their study of the phase diagrams of Gd,O,-TiO, and Dy,03TiO? observed only TiOz and pyrochlore phases (Ln,Ti,O,) in the region 0 30 mol.% Ln,O,. Similar results have been reported for the systems Tbz03TiOz [8], Er,03-TiOz [9], YzOs-Ti02 [9 - 111, LuzO,--TiO, [12], Yb,O,TiOz [13] and Gd,O,--TiO, [ 141. In the present work we have sought to 0022-5088/95/$3.30
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determine the range of homogeneity of the titanates Ln,O,*yTiOz (y > 3) and furthermore the existence of solid solutions of the type Ln? _xLn,*Ti,Oay + s. Raman spectroscopy was used throughout this study because we have previously established [ 1 - 31 that it is a useful technique for rapidly identifying various titanate phases.
2. Experimental
details
The synthesis of solid species with the compositions LnzOs.yTiOz and by heating intimate mixtures of the Lnz-xLn,*Ti,OZy +3 were accomplished powdered oxides, contained in platinum boats, in air at 1350 - 1400 “C for various periods of up to 40 h followed by rapid cooling in air. This temperature range was chosen to avoid the formation of liquid phases since it had been reported that La,03. 4.5Ti02 and Nd,03* 4Ti01 melt incongruently at 1455 f 5 “C [5] and 1505 f 15 “C [6] respectively. Lanthanide oxides with a minimum purity of 99% were obtained from several commercial sources. The oxides Pr,03 +% and Tbz03+, were used as such after determining the value of x from the oxygen evolution during the formation of their corresponding orthophosphates [ 151. We have established previously that these lanthanide elements reduce spontaneously to the trivalent state on reaction with TiOz. Samples were ground and reheated repeatedly until their Raman spectra remained unchanged. The Raman spectra were excited using the 514.4 or 488.0 nm line from a continuouswave argon ion laser. Spectra were observed using a Ramanor HG-2S spectrophotometer (Instruments SA) as previously described [ 161.
3. Results and discussion Preparations containing La203, Pr,03 and Ndz03, each with 4.7 TiOz, exhibited Raman spectra that had not been observed before in lanthanide titanate systems. Because their XRD patterns matched those reported for La,03*4.5Ti02 [ 51 and for Nd,03* 4Ti02 [6] we assigned most of the peaks of these spectra, which are shown in Figs. 1 and 2, to Ln,03*yTiOz (Ln = La, Pr and Nd; y w 4.7). Similar preparations containing EuZ03, Ho203, Er,03 and Luz03 produced the Raman spectra of the respective LnzTizO, pyrochlores and TiOz (rutile) in agreement with information from phase diagrams [7 - 141. The spectra of Ln,03.yTiOz included, in addition to the new spectra, the two strongest peaks of TiOz @utile) at 446.8 and 608.5 cm-’ (Fig. 1, curve c) indicating that the compounds required a stoichiometry with y < 4.7. Additional preparations of Laz03, Prz03 and Nd,03 with yTiO,, where y was varied between 4 and 6, were made, and the Raman spectra (see Figs. 1 and 2) were examined for the presence of monoclinic Ln,Ti,O.l and TiO? (rutile). The presence of rutile, which provided the higher limit to the value of y, was ascertained from the ratio of the intensity of the
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Fig. 1. Curve a, Raman spectrum of a mixture of LazOs-4.5TiO2 and X.+Ti@7 obtained by heating La*03 and 4.03Ti02 at 1400 “C for a total of 38 h (excitation, 514.5 nm; *, monoclinic LazTi,O, peaks); curve b, Raman spectrum of LazOs-4.38TiO.r obtained by heating La203 and 4.38TiOs at 1400 “C for a total of 19 h (excitation, 514.5 nm); curve c, Raman spectrum of a mixture of LazOs.4.5Ti02 and about 13.8 wt.% TiOs obtained by heating La203 and 5.87 TiOz at 1350 “C for a total of 41 h (excitation, 488.0 nm; 0, TiOz (rutile) peaks}. Fig. 2. Curve a, Raman spectrum of a mixture of PrzOa*4,5TiOa and Pr2Ti207 obtained from PrsOs + 4.04Ti02; (*, monoclinic PrzTis07 peaks); curve b, Raman spectrum of Prz03*4.33Ti0s; curve c, Raman spectrum of a mixture of NdsOs-4.5Ti02 and NdaTizO, obtained from Nd20s + 4.04Ti02 ; (*, monoclinic NdzTi207 peaks); curve d, Raman spectrum of NdzOa-4.31TiOs. All preparations were heated at 1370 “C for a total of 19 h and excited at 514.5 nm.
Raman peak at 446.8 cm-’ (low intensity for LnzOS+yTiOz and strong intensity for rutile) to the intensity of the peak at 663.5 cm-’ (strong intensity for Ln,O, l yTiO2). The values of this ratio measured for about 30 preparations, including the solid solutions Lnz_xLn:Ti,,0zY+3, was 0.27 ?s 0.13 for values of y between 4 and 4.5 and increased linearly to about 1.51 * 0.4 for y = 6. These results indicate that the value of y lies between 4.0 and 4.5. The presence of monoclinic Ln,TizO, phases in some Raman spectra of preparations with y = 4 - 4.5 yielded a value of about 4.3 for the lower limit of y. This value was confirmed by examining the Raman spectra of mixtures
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Fig. 3. Curve a, Raman spectrum of a mechanical mixture of Laz03.4.7Ti02 with 7.3 wt.% monoclinic LaNdTizOT (excitation, 514.5 nm; *, monoclinic LaNdTiz07 peak); curve b, Raman spectrum of a mechanical mixture of Prz03-4.33Ti02 with 10.2 wt.% LuzTizO, showing the presence of the latter (excitation, 514.5 nm; *, cubic LuzTiz07 peaks).
prepared by grinding together monoclinic LnzTizO, and the product of the reaction of Laz03 with 4.7Ti02 (Fig. 3(a)). The monoclinic phases were observed with some certainty at a minimum concentration of 7.3 wt.%. Since we have observed LazTizO, in experiments in which y = 4.03 (Fig. 1, curve a), we conclude that in the compound LazOs*yTiOz y has a value between 4.25 and 4.33 which corresponds to mixtures containing 7.0 - 9.4 wt.% La,Ti,O,. We have thus arbitrarily selected y = 4.33 which gives the stoichiometry 3Ln,03-13Ti02. The Raman spectra of various mixtures of the pyrochlore Lu,TizO, with La*O,-4.33Ti0, (or the analogous neodymium compound) (Fig. 3, curve b) were recorded in order to set the limit of detectability for the pyrochlore phase and thus assign uncertainties to the solubility of the heavy lanthanide elements (Ln*) in the Lnz_,Ln:Os-4.33Ti02 solid solutions reported below. The minimum concentration of LuzTizO, that could be detected using the Raman spectra was about 10 wt.%. Solid solutions of Ln,_,Ln,*0,*4.33Ti02 (Ln 5 La, Pr and Nd; Ln* = Lu, Yb, Tm, Er, Ho, Tb, Eu, Nd and Pr; y = 4.35 - 4.7) were prepared with various ratios [Ln*]/ [Ln + Ln*], and some of their selected Raman spectra are shown in Fig. 4. These spectra were interpreted in terms of the presence of either a single phase or a polyphase, ,and the results were plotted in a graph of S = 100 {(IR8L, - IR8L,B)/IR8L,*} uersus IRsmean = IR8n,X,, + IR8nn*XLn* where IR’ refers to the ionic radii with coordination number 8 obtained from ref. 17 and Xr,, is the cationic fraction of the lanthanide host (X,, = nLn/(nLn + nLn*)). The resulting graph (Fig. 5) shows the boundary between single phases of
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Fig. 4. Curve a, Raman spectrum of a mixture of 0.79PrzOs + O.ZlHozO3 + 4.2TiOz heated to 1370 “C for a total of 18 h (excitation, 514.5 nm) (from material balance calculations and using the solubjlity data in Fig. 6 we estimated that the product consists of a mixture of 0.92(Pr1,,2 H0cS2s03*4.5Ti02) and 0.08HozTiz07 (6.4 wt.%) with 0.07TiOz; both minor phases are below the limit of detection); curve b, Raman spectrum of a mixture of 0.4SLaz03 + 0.55Ho203 + 4.3TiOz heated at 1370 “C for a total of 39 h (excitation, 514.5 nm;*, HozTizO, peaks; 0, TiOz (rutile) peaks) (from material balance calculations and the solubility data of Fig. 6 we estimated that the product consists of a mixture of about 0.7(La~.~Ho~,~O3*4.5TiO~), 0.31HozTi20, (about 27.7 wt.%) and 0.58Ti02); curve c, Raman spectrum of a solid solution La ~,3~Euc.~sO~.4.5TiO~ obtained from a mixture of 0.66La203 + 0.34Euz0s + 4.5TiOz heated at 1400 “C for a total of 23 h (excitation, 488.0 nm). Fig. 5. Boundary of the saturated solid solutions Lnz_,Ln$0s.4.33Ti02 as a function of the percentage difference ?i between the ionic radii with coordination number 8 (IRS) of Ln and Ln*, and the mean ionic radius IR8-: 0, single phase; A, polyphase.
solid solutions and saturated solid solutions. This boundary was used to calculate the solubility limits of the lanthanide elements from samarium to lutetium in 3Ln,03*13Ti0,, where Ln 3 La, Pr and Nd. Values for the .. . maximum solubrhtres XLn+,axf were calculated from the interpolated values on the boundary in Fig. 5. A plot of maximum solubility versus 6 of IR*,,,, is shown in Fig. 6 where the portion above samarium is based on experimental data which indicated total miscibility among the lanthanum, praseodymium and neodymium titanates. In addition, in Fig. 6 we have drawn tie lines between the “solute” elements samarium and lutetium, and we have
152
0.8
20.6 E .Y x” 0.4
0.2
0
0
5
10
15
20
Fig. 6. Maximum solubility of Ln* in Lnz_,Ln,*03.4.33%02 as a function of 6. The curves for An2 _XLn,Os ‘4.33Ti02, where An = Pu3+ and Am3+, are drawn lighter than those for the lanthanides to reflect the larger uncertainty,
extrapolated/interpolated on those lines the values of 6 calculated for Am3+ and Pu3’. These elements were considered to form the solvent 3An,0313Ti02, while the elements s~a~urn and lutetium were considered to be the solutes. This generated two new solubility curves which are also shown in Fig. 6 with a lighter trace to reflect a larger uncertainty. The ions Am’+ and Pu3* were selected here to provide a clue to their predicted behavior in lanthanide titanate systems, which are of interest in nuclear waste management. 4. Conclusions We have established by Raman spectroscopy that the minimum value of y in compounds Ln,03*yTi0,, where Ln = La, Pr and Nd, is about 4.33 and the maximum value is about 4.5, While similar compounds containing Ln = Sm - Lu do not occur, solid solutions of the type Ln~_~Ln~03*4,33TiO~ were prepared in which Ln* = Sm - Lu. It has also been established that the value of x increases as a function of the ionic radius of Ln*. We can conclude, based on their Raman spectra, that although the crystal symmetry of the compounds Ln,03*4.33Ti02 has not been established, the solid solutions have the same symmetry as the pure compounds with Ln = La, Pr and Nd. Based on extrapolation and interpolation of the solubility data obtained, we predict that Pu3+ will form solid solutions with all the lantbanide elements and that Am3+ will behave similarly, except with thulium, ytterbium and lutetium.
153
Acknowledgments This research was sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems Inc.
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