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Structural data from X-ray powder diffraction for new high-temperature phases (Y1–xLnx)2Si2O7 with Ln=Ce, Pr, Nd
Journal of the European Ceramic Society 22 (2002) 721–730 www.elsevier.com/locate/jeurceramsoc
Structural data from X-ray powder diffraction for new high-temperature phases (Y1 xLnx)2Si2O7 with Ln=Ce, Pr, Nd F. Monteverde*, G. Celotti Research Institute for Ceramics Technology, Via Granarolo, 64, 48018 Faenza (RA), Italy Received 17 January 2001; received in revised form 14 May 2001; accepted 20 May 2001
Abstract Extending the investigation on possible mixed phases with composition (Y1 xLnx)2Si2O7 formed at high temperature under oxidizing conditions in silicon nitride samples doped with Y2O3 and Ln2O3 as sintering aids, the compounds with Ln= Ce, Pr, Nd were successfully synthesized and structurally characterized by Rietveld whole powder pattern refinement. Some fluctuations in the x values were found as well, but all the compounds are monoclinic (space group P21/c), with lattice parameters close to those of the previously studied (Y2/3La1/3)2Si2O7. Further efforts with Sm (and Dy) failed in producing the expected G-form phase, indicating that the family does not extend beyond Nd: in these cases Ln ions partially substitute Y in its already known a- and d-disilicates. Under the adopted experimental conditions (1 atm, ambient air), an explanation of the impossibility to obtain the phase in question containing lanthanides with Z562 was attempted on the basis of lattice strains introduced by foreign ions larger than Y. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: (Y,Ln)2 Si2O7; X-ray methods; Phase composition; Grain boundary phase
1. Introduction
2. Experimental
During previous studies of the oxidation resistance at high temperature in air of fully dense silicon nitride based materials densified with the addition of sintering aids (i.e Y2O3 and La2O3), a new mixed disilicate was found and completely characterized.1 A synthesis of this new phase, out of a silicon nitride matrix, was successfully carried out as well in excess of SiO2. To improve the knowledge of such (Y1 xLnx)2Si2O7 possible class of compounds with high temperature Gform structure,2,3 a systematic study with following light rare earths (Ln=Ce, Pr, Nd, Sm) was undertaken. Experiments were performed with Ln=Dy too, even if this element does not belong properly to the light group, due to the fact that its ionic radius is almost equal to that of Y3+. This paper describes the synthesis of these new mixed disilicates, the assessment of their stoichiometry and the relative structure determination. When the expected phase failed to form, the reasons for this finding were analyzed and tentatively explained.
Several powder mixtures were prepared varying the molar content of the rare earth oxides involved (Y2O3, H.C. Starck - Germany; CeO2, Merck- Germany; Pr6O11, Nd2O3, Sm2O3, Dy2O3, Techsnabexport - Russia), the test temperature and the dwell time in view to obtain the highest amount and/or the best defined X-ray diffraction pattern of the expected new mixed disilicate. The prerequisite of the amorphous silica (SiO2, Carlo Erba - Italy) in excess was kept constant for each prepared mixture. Once the initial composition was designed, the oxide precursors were ultrasonically mixed in pure ethanol and dried. The blend was then softly pestled in agate mortar, pellettized (100 kg/cm2), mounted on a platinum support, and finally placed in a laboratory kiln. The as-treated pellet was finely ground in agate mortar and, on this powdered sample, the XRD analyses were conducted. Accurate XRD patterns were recorded over an angular range 10 < 2 < 60 on a Siemens mod. D500 (Ni filtered CuKa, 25 mA 33 KV, step width 0.02 , sampling time 6 s). Table 1 summarizes the most significant attempts with starting composition, thermal cycle and crystalline phases identified.
* Corresponding author. E-mail address: [email protected] (F. Monteverde).
0955-2219/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(01)00338-7
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Parallel inspections were performed via SEM (Cambridge mod. S360) equipped with an EDX analyzer (eXL II Link Analytical Pentafet) in order to assess the composition associated to the new formed phases. For this need it was necessary to coat the selected powdered samples with a thin carbon film. The experimental diffraction patterns were processed using the following software packages: DBWS-9411 for Rietveld refinement,4 DMPLOT5 and PowderCell 2.36 for the necessary graphic illustrations.
3. Results and discussion Being the expected structure type of the possible (Y1 xLnx)2Si2O7 phases known from the early determined (Y2/3La1/3)2Si2O7,1 a systematic search was carried out on
mixtures thermally treated at different temperatures from 1400 to 1650 C. By varying the starting compositions, as shown in Table 1, the G-form structure was clearly identified in some Ce-, Pr- and Nd-containing samples. In the first three Nd-containing samples, this form is not detectable due to the fact that, even if small quantities would have been present, the overlapping peaks of a- or d-Y2Si2O7 made the identification largely uncertain. Quantitative EDX analyses yielded for Ce a stoichiometry very similar to the La compound,1 i.e X1/3, while for Pr X1/2 was found and for Nd X2/5, with no traces of other phases, excluding the unavoidable SiO2 (a-cristobalite). On the contrary the Sm-containing samples resisted to any effort: at temperatures 1400–1500 C only a-Y2Si2O7, and d-Y2Si2O7 at temperatures 1550–1650 C were detected. A careful inspection of the XRD patterns revealed
Fig. 1. Outputs of the whole-powder-X ray pattern fitting by Rietveld refinement [observed profile intensities (+) and calculated intensities (-)] for the selected compositions YCe3 (A), YPr5 (B), and YNd4 (C). The difference plot (d) between the observed and calculated intensities for each processed XRD patterns is shown. Short vertical bars represent the Bragg reflection positions of the expected new phase (upper) and a-cristobalite (lower)
F. Monteverde, G. Celotti / Journal of the European Ceramic Society 22 (2002) 721–730
that lattice parameters of the two Y-disilicates increased on the average by about 1% (a-form: a=6.62, b=6.71, c=12.34 A˚, =94, =89, =93 ; d-form: a=13.81, b=5.02, c=8.30 A˚), indicating that Sm substituted partially for Y: instead of forming the expected phase, Sm prefers to occupy Y sites in both the well known disilicate polymorphs.7 While La, Ce, Pr and Nd elements have ionic radius r exceeding that of Y by more than 10% (corresponding to a volume increase 533% in large cations fraction,
Fig. 2. Projection of the monoclinic structure (space group P21/c) seen along [100] at (101) of (Y2/3Ce1/3)2Si2O7 (a), (Y1/2Pr1/2)2Si2O7 (b), and (Y3/5Nd2/5)2Si2O7 (c).
723
representing 8–12% of the total cell volume), Sm is the first with r going below this limit: it seems that, when the linear strain induced by the substitution of Y by Ln element is lower that 10%, the lanthanide enters the Ydisilicate lattice (corresponding to the thermal treatment temperature) and does not give rise to G-form structure anymore. A further experiment was performed with Dy, which exhibits a linear lattice strain, if compared to Y, of merely 1%: such a element is the last showing a d-type disilicate among the lanthanides: mainly d-(Y,Dy)2Si2O7 and traces of g-(Y,Dy)2Si2O7 were found with lattice parameters increased on the average by 0.1% (a=13.70, b=5.01 c=8.18 A˚). Strong evidence can be deduced for the fact that (Y1 xLnx)2Si2O7 compounds with high temperature G-type structure are formed only with rare earth types characterized by a ionic radius sufficiently large. The structural data for the isomorphous new compounds (Y2/3Ce1/3)2Si2O7, (Y1/2Pr1/2)2Si2O7 and (Y3/5 Nd2/5)2Si2O7 are reported in Tables 2–5. The results of the whole-powder-pattern fitting are presented in Fig. 1a–c for the new formed high temperature Ln-Y disilicates, Ln=Ce(a), Pr(b), Nd(c). The comparison between observed and calculated intensities are shown in Tables 6–8 (excluded the reflections belonging to acristobalite). These outputs were obtained after Rietveld refinement by means of the DBWS-9411 program,4 starting from structural parameters predetermined for (Y2/3La1/3)2Si2O7 (Table 5). The drawings in Fig. 2a–c, built with PowderCell 2.36 represent the projection of the monoclinic (Y1 xLnx)2Si2O7 structure with Ln=Ce (a), Pr (b), and Nd (c). The reliability index RWP is quite good (Table 5), even if not as satisfactory as in the case of La: from Tables 2–4 it can be noted again the tendency of Y to occupy preferentially positions (2) leaving the major part of substituting Ln in position (1). In the case of Pr, the sharing is complete, with Y and Pr occupying in equal proportions position (2) and (1), respectively. SEM micrographs from some as-treated powder mixtures and corresponding EDX spectra are presented in Fig. 3. The apparent deviation of Pr-based compound cell volume from the expected monotonic decrease, coming from the well known ‘‘lanthanide contraction’’, is easily understood taking into account the fact that larger Pr ions substitute Y in an anomalously high proportion. The reason why more Ln enters in the compound (Y1 xLnx)2Si2O7 when Ln=Pr, Nd is not easy to understand. It seems that a sort of volume compensating mechanism is activated which prevents the cell volume from becoming too small (allow for the fact that all the cell volumes of the new mixed phases lie within a poor 1.5%). Theoretical calculations based on the volumes of the different ions involved in the formula units treated as cubes of edge 2r yielding a complete space filling, properly
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Fig. 3. BSE–SEM micrographs (left column) from the as treated mixtures YCe2 (a1), YPr6 (b1), YNd5 (c1), YSm4 (d1), and YDy1 (e1). The EDX spectra (right column) were collected on the bright features (placed upon the grey a-cristobalite particles) of the corresponding micrograph.
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F. Monteverde, G. Celotti / Journal of the European Ceramic Society 22 (2002) 721–730 Table 1 Summary of the most significant attempts: the complement to 100% (wt. or mol) for each composition consists of amorphous silica Mixture
Composition
Thermal treatment
Y
YPr1 YPr2 YPr3 YPr4 YPr5 YPr6 YPr7 YCe1 YCe2 YCe3 YCe4 YCe5 YNd1 YNd2 YNd3 YNd4 YNd5 YNd6 YSm1 YSm2 YSm3 YSm4 YSm5 YSm6 YSm7 YDy1 a b c d
Ln
wt.%
mol%
wt.%
mol%
6 15 6 10 6 10 15 14.1 15 15 14.1 15 15 15 15 14 18 14 10 15 15 15 14 10 10 18.7
2 5.3 2 1.6 2 1.6 5.3 5 6.6 6.6 5 6.6 5.2 5.2 5.2 5.1 4.2 5.1 3.5 5.2 5.2 5.2 5.1 3.5 3.5 5
24 15 24 20 24 20 15 20.3 15 15 20.3 15 15 15 15 21 12 21 20 15 15 15 21 20 20 11.3
2 1.2 2 3.6 2 3.6 1.2 9.2 5.0 5.0 9.2 5.0 3.5 3.5 3.5 5.2 4.2 5.2 4.5 3.4 3.4 3.4 5.2 4.5 4.5 5
Temperature ( C)
dwell (h)
NSPc
a-cristobalite
1450 1500 1500 1500 1550 1550 1600 1450 1500 1550 1550 1600 1450 1500 1550 1550 1600 1600 1400 1450 1500 1550 1550 1600 1650 1600
2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
Yd Y Y Y Y Y – Y Y Y Y – – – – Y Y Y – – – – – – – –
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
Notes
Y2SiO5*a d-Y2Si2O7 – – – – d-Y2Si2O7 Y2SiO5*, CeO2 – – – d-Y2Si2O7 a-Y2Si2O7 a-Y2Si2O7 d-Y2Si2O7 – d-Y2Si2O7 – a-Y2Si2O7 a-Y2Si2O7 a-(Y,Sm)2Si2O7 d-(Y,Sm)2Si2O7, Sm2O3/Sm2Si2O7* d-(Y,Sm)2Si2O7, Sm2O3/Sm2Si2O7* d-(Y,Sm)2Si2O7, Sm2O3/Sm2Si2O7* d-(Y,Sm)2Si2O7, Sm2O3/Sm2Si2O7* d-(Y,Dy)2Si2O7, g-(Y,Dy)2Si2O7*
**b Fused
** Fused
**
* Traces. ** Pattern processed for Rietveld refinement. NSP: new synthesized phase. Y: yes.
Table 2 Structural data of the new phase (Y2/3Ce1/3)2 Si2O7a Atom
X
Y
Z
Occupancy
Y(1) Ce(1) Y(2) Si(1) Si(2) O(1) O(2) O(3) O(4) O(5) O(6) O(7)
0.5172
0.8055
0.7689
0.8295 0.730 0.910 0.806 0.999 0.574 0.560 0.798 0.231 0.034
0.6008 0.253 0.510 0.421 0.164 0.168 0.268 0.493 0.451 0.655
0.5890 0.014 0.171 0.059 0.045 0.078 0.910 0.253 0.221 0.150
0.335 0.665 1 1 1 1 1 1 1 1 1 1
a
Crystalline phases
BOV= 0.4.
Table 3 Structural data of the new phase (Y1/2Pr1/2)2Si2O7a Atom
X
Y
Z
Occupancy
Pr(1) Y(2) Si(1) Si(2) O(1) O(2) O(3) O(4) O(5) O(6) O(7)
0.5161 0.8371 0.753 0.928 0.801 0.009 0.597 0.540 0.790 0.288 0.019
0.8044 0.5988 0.289 0.505 0.438 0.174 0.161 0.258 0.484 0.439 0.650
0.7688 0.5876 0.028 0.186 0.059 0.025 0.072 0.903 0.249 0.215 0.159
1 1 1 1 1 1 1 1 1 1 1
a
BOV=1.5.
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F. Monteverde, G. Celotti / Journal of the European Ceramic Society 22 (2002) 721–730
distance, gave 11% for La, 8% for Ce, 15% for Pr and 6% for Nd compounds, respectively. Once more Pr, besides the peculiar Ln-rich stoichiometry, involves the most perturbed structure. While Y-La compound is white, Y-Ce appears very pale yellow, Y-Pr very pale green and Y-Nd very pale purple coloured: of course the colours are faded by the contemporary presence of large fraction of white SiO2 (a-cristobalite).
Table 4 Structural data of the new phase (Y3/5Nd2/5)2Si2O7a Atom
X
Y
Z
Occupancy
Y(1) Nd(1) Y(2) Nd(2) Si(1) Si(2) O(1) O(2) O(3) O(4) O(5) O(6) O(7)
0.5234
0.8056
0.7662
0.8346
0.5984
0.5889
0.745 0.920 0.811 0.018 0.577 0.542 0.854 0.297 0.043
0.258 0.530 0.422 0.194 0.176 0.282 0.507 0.453 0.678
0.021 0.164 0.066 0.046 0.090 0.908 0.260 0.216 0.142
0.5 0.5 0.7 0.3 1 1 1 1 1 1 1 1 1
a
4. Conclusions Extending the study of the class of compounds (Y1 XLnX)2Si2O7 with high temperature G-form structure, whose first member was that with Ln= La and X=1/3, three new phases were synthesized (with Ln=Ce, Pr, Nd and X=1/3, 1/2, 2/5 respectively) in excess of silica, and structurally characterized by whole powder X-ray pattern Rietveld refinement. All the new phases are monoclinic, space group P21/c(14), with cell parameters as follows (Z=4 for everyone): (Y2/3Ce1/3)2Si2O7, a=5.365(1)A˚, b=8.509(1)A˚, c=13.804(1)A˚, =111.45 (1) , XRD= 4.305 g/cm3; (Y1/2Pr1/2)2Si2O7, a=5.370(1) A˚, b=8.542(2)A˚, c=13.838(2)A˚, =111.76(2) , XRD= 4.485 g/cm3; (Y3/5Nd2/5)2Si2O7, a=5.362(1)A˚, b=8.496 (1)A˚, c=13.782(1)A˚, =111.46(1) ; XRD=4.435 g/cm3. In these cases too the atomic coordinates showed that substitution of Ln for Y cations preferentially occurs in half of the possible lattice positions: only Nd presents replacement in both of them. Si-O tetrahedra are slightly distorted in Ce and Nd compounds, while Pr compound is characterized by a deformation even larger than that previously found for La. Attempts to obtain the high temperature G-form phase with Ln= Sm, Dy failed, probably because the lanthanides more easily substituted Y in a- and/or ddisilicates lattice when the produced strain is sufficiently small. If the ratio r(Ln)/r(Y), at least for lanthanides with cation radius larger than that of Y, deviates from unit by less than 10%, it looks energetically more favoured for Ln to replace Y in Y-disilicates instead of a general rearrangement to form the new phase. Clear
BOV= 0.7.
account for unit cell volumes found in the given stoichiometry: the monotonic reduction in O2- radius with decreasing trivalent cations size observed in lanthanides pyrosilicates has to be held in due consideration, being oxygen responsible of about 90% of cell volume occupation. The overall thermal factor BOV significantly higher for Pr compound (Table 3) has likely to be attributed to the lower stability of Pr oxides. The best fit implies not neglibible displacements of some oxygen atoms from the geometrically regular tetrahedral positions, leading to a typical distorted arrangement linked to the non-equilibrium conditions of nucleation and growth. On the other hand the spatial positions of the heavy ions in the real lattice remain remarkably constant, and those of Si undergo only minor changes (Tables 2–4). In general, the partial substitution of Y to Ln ions, doesn’t introduce substantial modification in the G-form structure as described in Refs [2] and [3] (accounting for the choice in Ref. [3] of a description by P21/n space group with b very close to 90 ), apart from the enhanced tetrahedra distortion. An estimate of SiO4 tetrahedra distortion, based both on the discrepancy of Si–O average bond distances as compared to the theoretical value (1.65 A˚) and on the half maximum spread of the same
Table 5 Structural parameters (a, b, c, , V, Z), density (XRD), and reliability index (RWP) for the Rietveld refinement of the high temperature new formed phases Compound
(Y2/3La1/3)2Si2O7 (Y2/3Ce1/3)2Si2O7 (Y1/2Pr1/2)2Si2O7 (Y3/5Nd2/5)2Si2O7 a b
Lattice parameter
Cell volume
a(A˚)
b(A˚)
c(A˚)
( )
(A˚3)
5.375(1) 5.365(1) 5.370(1) 5.362(1)
8.569(1) 8.509(1) 8.542(2) 8.496(1)
13.863(1) 13.804(1) 13.838(2) 13.782(1)
111.79(1) 111.45(1) 111.76(2) 111.46(1)
592.9 586.5 589.6 584.3
Z: formula units per cell. TW: this work.
Za
XRD
RWP 3
4 4 4 4
(g/cm )
(%)
4.250 4.305 4.485 4.435
6.6 8.6 8.6 7.3
Ref.
[1] TWb TW TW
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F. Monteverde, G. Celotti / Journal of the European Ceramic Society 22 (2002) 721–730 Table 6 X-ray diffraction data of the new phase (Y2/3Ce1/3)2Si2O7 (IOBS 55) h
k
0 0 0 1 1 0 1 0 0 1 1 0 1 1 1 1 1 0 1 1 0 0 1 1 0 2 1 1 1 1 0 0 2 2 1 2 2 1 0 1 2 0 2 1 1 1 1 2 2 2 2 1 0 0 2 0 2 0 1 2 2 2 1 0 2 1 2 1
1 0 1 0 0 2 1 2 1 1 1 2 0 2 0 2 2 0 1 1 2 1 2 2 3 0 2 1 2 1 3 2 1 1 3 1 0 3 1 3 1 3 1 3 0 2 0 2 2 1 2 3 0 3 2 4 0 1 2 0 1 2 2 4 2 3 1 4
l 1 2 2 0 2 0 2 1 3 1 3 2 2 1 4 0 2 4 2 4 3 4 1 3 1 2 2 3 4 5 2 4 2 1 1 3 0 0 5 2 0 3 4 1 4 3 6 2 1 5 0 4 6 4 4 1 2 6 4 6 2 1 6 2 5 3 6 1
d (A˚)
I calc
I obs
h
k
7.094 6.424 5.127 4.993 4.907 4.255 4.251 4.039 3.825 3.743 3.670 3.547 3.388 3.333 3.307 3.238 3.214 3.212 3.147 3.082 3.018 3.005 2.977 2.940 2.770 2.682 2.650 2.648 2.611 2.596 2.595 2.563 2.558 2.520 2.507 2.497 2.497 2.466 2.460 2.456 2.396 2.365 2.357 2.345 2.340 2.331 2.295 2.269 2.242 2.173 2.153 2.153 2.141 2.126 2.125 2.099 2.084 2.077 2.050 2.034 2.024 2.022 2.020 2.019 1.9876 1.9876 1.9783 1.9774
454 265 54 56 17 129 16 106 36 24 14 9 738 1000 941 783 523 82 240 238 439 77 5 16 114 494 55 254 46 108 103 146 13 10 8 16 31 176 97 121 31 25 16 25 6 7 72 57 38 13 171 13 175 7 52 337 6 182 44 6 74 404 37 40 309 43 97 22
396 224 43 52 11 128
130 28 49 32 29 774 1000 943 800 625
1 1 1 1 2 2 1 0 0 0 1 1 2 2 2 2 2 1 1 1 0 2 1 1 2 1 3 0 2 1 3 3 1 1 2 1 1 0 3 1 3 2 2 2 2 3 3 0 1 3 3 2 1 1 1 3 1 1 1 2 1 2 1 3 2 2 0
3 4 1 4 3 3 1 2 4 3 4 4 3 2 3 2 1 2 3 4 1 1 2 4 3 3 0 4 3 0 1 1 0 1 0 4 1 5 1 4 0 1 4 0 4 2 2 4 3 0 2 1 3 2 5 1 2 5 4 2 5 4 4 1 4 2 3
251 251 488 80 5 19 114 482 306 43 181
136 13 8 8 48
179 96 115 22 16 10 21 6 5 75 61 43 6 176
210 59
354 6 200 45 7 526
85
353
122
l 5 0 5 2 2 1 7 6 3 5 1 3 0 2 4 6 3 5 4 2 7 7 7 4 1 6 2 4 5 6 2 4 8 6 4 3 8 1 5 5 0 4 1 8 3 3 2 5 5 6 4 8 7 6 2 6 8 1 4 4 3 1 6 1 5 8 7
d (A˚)
I calc
1.9655 1.9571 1.9523 1.9517 1.9486 1.9317 1.9211 1.9127 1.9052 1.9043 1.8947 1.8851 1.8741 1.8715 1.8555 1.8351 1.8345 1.8143 1.8050 1.8015 1.7942 1.7918 1.7891 1.7891 1.7858 1.7843 1.7767 1.7736 1.7618 1.7498 1.7392 1.7280 1.7236 1.7140 1.6938 1.6908 1.6893 1.6871 1.6779 1.6771 1.6645 1.6612 1.6560 1.6535 1.6495 1.6482 1.6395 1.6386 1.6377 1.6355 1.6301 1.6232 1.6191 1.6183 1.6078 1.6061 1.5975 1.5755 1.5740 1.5737 1.5700 1.5612 1.5603 1.5523 1.5451 1.5412 1.5409
16 28 22 6 7 18 20 10 5 15 151 155 29 153 94 164 27 180 40 39 45 6 152 31 35 62 26 58 31 21 8 6 13 76 23 63 72 21 6 113 101 23 77 30 48 164 86 105 10 117 50 44 21 43 23 30 14 64 13 9 51 11 8 28 6 8 31
I obs
19 31 34
9 19 21 12 6 18 149 149 30 157 92 196
194 44 43 47 6 189
99
27 57 25 20 6 6 13 89 25 66 75 23 108
9 =
95 21 71 28 46 155 204
;
121 49 44 66
22 29 12 54 19
43 12
20 5 38
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F. Monteverde, G. Celotti / Journal of the European Ceramic Society 22 (2002) 721–730
Table 7 Diffraction data of the new phase (Y1/2Pr1/2)2Si2O7 (IOBS 55) h
k
0 0 0 1 1 1 0 1 0 0 1 1 0 1 1 1 1 1 0 1 1 0 0 0 2 1 1 1 1 0 0 2 1 2 2 1 1 0 2 2 0 2 1 1 1 2 2 2 1 2 0 2 0 0 2 0 1 0 1 2 2 2 1 2 1 1 1
1 0 1 0 0 1 2 1 2 1 1 1 2 0 2 0 2 2 0 1 1 2 1 3 0 2 1 2 1 3 2 1 3 1 0 3 3 1 0 1 3 1 3 2 0 2 2 1 3 2 0 2 3 4 0 1 2 4 2 2 1 2 3 1 4 3 4
l 1 2 2 0 2 0 0 2 1 3 1 3 2 2 1 4 0 2 4 2 4 3 4 1 2 2 3 4 5 2 4 2 1 3 0 0 2 5 4 0 3 4 1 5 6 2 1 5 4 0 6 4 4 1 2 6 4 2 6 1 2 5 3 6 1 5 0
d (A˚)
I calc
I obs
h
k
7.114 6.426 5.135 4.987 4.922 4.307 4.271 4.264 4.053 3.829 3.739 3.684 3.557 3.380 3.342 3.319 3.244 3.226 3.213 3.143 3.093 3.025 3.007 2.780 2.685 2.650 2.644 2.621 2.604 2.603 2.568 2.561 2.515 2.503 2.494 2.473 2.465 2.461 2.461 2.394 2.371 2.365 2.349 2.303 2.302 2.273 2.244 2.181 2.161 2.153 2.142 2.132 2.131 2.107 2.079 2.078 2.049 2.027 2.026 2.021 2.020 1.9948 1.9891 1.9855 1.9843 1.9723 1.9631
455 244 62 37 19 20 88 39 43 34 40 27 18 811 1000 899 659 484 100 198 180 514 70 72 505 68 233 31 90 103 109 31 6 27 33 133 131 131 9 23 27 11 19 9 43 27 16 15 9 141 154 52 17 285 5 168 28 47 49 365 40 257 25 77 26 9 23
391 217 63 48 14 11 94
53 20 67 41 39 843 1000 892 683 496 109 212 188 525 69 100 488 67 243 30 178
110 30 7 30 35 134 125 133
23 25 10 16 49
41 24 5 6 135 173 51 17 283 189
25 95
443
269 24 106
1 1 2 1 0 0 0 1 1 2 2 2 2 2 1 1 1 2 1 0 1 1 2 0 3 2 1 3 1 1 3 0 1 1 2 1 2 3 2 2 2 2 3 0 3 3 1 3 3 2 1 2 1 1 2 3 1 1 1 1 2 2 3 3 2 2 0
4 1 3 1 2 4 3 4 4 3 2 3 2 1 2 3 4 1 4 1 2 3 3 4 0 3 0 1 0 1 1 5 4 1 0 4 3 0 4 0 1 4 2 4 2 0 3 2 1 1 3 4 2 5 4 1 2 5 5 4 2 4 1 2 2 3 3
12 25
l 2 5 1 7 6 3 5 1 3 0 2 4 6 3 5 4 2 7 4 7 7 6 1 4 2 5 6 4 8 6 1 1 3 8 4 5 2 0 1 8 4 3 3 5 2 6 5 4 0 8 7 0 6 2 4 6 8 1 3 4 4 5 1 0 8 7 7
d (A˚)
I calc
I obs
1.9590 1.9496 1.9350 1.9259 1.9147 1.9112 1.9080 1.8997 1.8923 1.8759 1.8695 1.8618 1.8418 1.8306 1.8131 1.8057 1.8053 1.7983 1.7958 1.7950 1.7940 1.7900 1.7864 1.7785 1.7771 1.7682 1.7473 1.7314 1.7274 1.7118 1.6979 1.6935 1.6935 1.6931 1.6898 1.6831 1.6792 1.6625 1.6597 1.6593 1.6577 1.6548 1.6506 1.6426 1.6408 1.6405 1.6379 1.6336 1.6318 1.6288 1.6238 1.6220 1.6172 1.6139 1.6129 1.6111 1.6014 1.5802 1.5760 1.5760 1.5713 1.5509 1.5499 1.5492 1.5467 1.5451 1.5430
8 33 6 24 5 9 12 96 109 33 105 90 72 47 136 52 24 13 34 22 114 57 40 36 6 33 27 10 10 60 6 34 78 51 23 114 11 69 64 51 30 45 154 62 76 100 4 42 6 27 24 6 40 8 5 16 13 61 54 6 7 7 21 6 7 5 33
9 39 7 26 7 11 15 110 126 43 118 99 115 70 191 99
9 =
15 184
;
9 =
65 48 48 8 33 18 8 7 72 6 166
;
22 105 10 73 115
29 43 150 66 191
5 46 7 29 26 6 45 9 5 16 13 47 59
9 =
6 31
; 7 5 36
729
F. Monteverde, G. Celotti / Journal of the European Ceramic Society 22 (2002) 721–730 Table 8 Diffraction data of the new phase (Y3/5Nd2/5)Si2O7 (IOBS 55) h
k
0 0 0 1 1 1 1 0 1 0 1 1 0 1 1 1 1 1 0 1 1 0 0 0 2 1 1 1 1 0 0 2 2 2 2 1 0 1 2 2 0 2 1 1 2 1 2 2 1 2 1 0 2 0 0 2 0 1 2 2 2 1 0 2 1
1 0 1 0 0 1 1 2 1 2 1 1 2 0 2 0 2 2 0 1 1 2 1 3 0 2 1 2 1 3 2 1 1 1 0 3 1 3 0 1 3 1 3 0 2 1 2 1 1 2 3 0 2 3 4 0 1 2 0 1 2 2 4 2 3
l 1 2 2 0 2 1 0 0 2 1 1 3 2 2 1 4 0 2 4 2 4 3 4 1 2 2 3 4 5 2 4 2 1 3 0 0 5 2 4 0 3 4 1 6 2 4 1 1 6 0 4 6 4 4 1 2 6 4 6 2 1 6 2 5 3
d (A˚)
I calc
I obs
h
k
7.083 6.413 5.119 4.990 4.903 4.533 4.303 4.248 4.246 4.033 3.739 3.666 3.542 3.384 3.329 3.303 3.235 3.210 3.207 3.144 3.078 3.013 3.000 2.765 2.680 2.647 2.644 2.607 2.592 2.591 2.559 2.556 2.518 2.495 2.495 2.463 2.456 2.452 2.451 2.394 2.361 2.355 2.342 2.292 2.267 2.253 2.240 2.217 2.213 2.151 2.150 2.138 2.123 2.123 2.095 2.082 2.073 2.047 2.032 2.022 2.020 2.017 2.016 1.9854 1.9848
396 218 76 29 27 4 34 145 22 114 51 40 13 794 947 1000 614 487 35 234 155 482 49 64 514 59 191 26 68 141 141 33 6 39 42 193 81 138 5 17 20 31 13 71 16 5 21 6 15 137 6 191 42 23 280 5 153 35 13 57 365 37 25 268 44
352 199 86 41 21 7 36 140
130 57 58 29 802 956 1000 632 555
252 156 511 47 76 547 273
29 195
141 32 9 86
194 79 133 5 21 21 27 15 75 17 6 23 6 15 143
220 68
318 6 170 21 14 472
66
328
2 1 1 1 1 1 2 2 2 1 0 0 1 1 2 2 2 2 2 1 1 1 0 2 1 1 2 1 3 0 2 1 3 1 1 2 1 1 0 3 1 3 2 2 2 2 3 3 0 1 3 3 3 2 2 1 1 1 2 3 1 1 1 3 2
1 4 3 4 1 4 3 3 3 1 2 3 4 4 3 2 3 2 1 2 3 4 1 1 4 2 3 3 0 4 3 0 1 0 1 0 4 1 5 1 4 0 1 4 0 4 2 2 4 3 0 1 2 1 4 3 2 5 4 1 2 5 5 1 4
d (A˚)
l 6 1 5 0 5 2 2 1 3 7 6 5 1 3 0 2 4 6 3 5 4 2 7 7 4 7 1 6 2 4 5 6 2 8 6 4 3 8 1 5 5 0 4 1 8 3 3 2 5 5 6 0 4 8 0 7 6 2 4 6 8 1 3 1 5
1.9760 1.9746 1.9625 1.9543 1.9495 1.9489 1.9466 1.9298 1.9194 1.9180 1.9096 1.9013 1.8921 1.8823 1.8722 1.8696 1.8534 1.8328 1.8326 1.8117 1.8024 1.7990 1.7912 1.7895 1.7864 1.7863 1.7839 1.7815 1.7758 1.7708 1.7597 1.7474 1.7382 1.7208 1.7114 1.6919 1.6883 1.6865 1.6845 1.6766 1.6746 1.6634 1.6594 1.6541 1.6513 1.6475 1.6470 1.6384 1.6360 1.6353 1.6342 1.6324 1.6289 1.6210 1.6174 1.6166 1.6159 1.6055 1.6052 1.6048 1.5949 1.5733 1.5676 1.5511 1.5432
I calc 59 35 9 14 12 11 4 28 4 4 11 7 149 161 49 129 86 149 36 174 42 28 46 5 19 171 41 36 9 49 13 13 14 12 48 24 67 54 23 5 109 84 30 67 55 36 141 86 95 14 136 12 43 26 9 6= 20 59 ; 9 10 = 2 9; 24 47 45 35 8
I obs 102 12 20 37 6 32 5 5 12 7 138 149 51 134 85 205 207 43 30 49 5 192 41 36 9 44 6 5 8 6 55 25 69 56 24 5 96 84 30 64 51 164 88 115 144 12 43 25 85
22
17 32 30 10 5
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evidence of this phenomenon was supplied by SEM investigations and XRD determination of the modified lattice parameters.
3.
4.
References 1. Monteverde, F. and Celotti, G., Structural data from X-ray powder diffraction of a new phase formed in the Si3N4-La2O3Y2O3 system after oxidation in air. J. Eur. Ceram. Soc., 1999, 19, 2021–2026. 2. Greis, O., Bossemeyer, H. G., Greil, P., Breidenstein, B. and Haase, A., Structural data of the monoclinic high-temperature G-
5. 6. 7.
form of La2Si2O7 from X-ray powder diffraction. Materials Science Forum, 1991, 79-82, 803–808. Cuneyt Tas, A. and Akinc, M., Crystal structures of the hightemperature forms of Ln2Si2O7 (Ln=La, Ce, Pr, Nd, Sm) revisited. J. Am. Cer. Soc., 1994, 77(11), 2968–2970. Young, R. A., Sakthivel, A., Moss, T. S. and Paiva-Santos, C. O., DBWS-9411, Rietveld analysis of X-ray and neutron powder diffraction patterns. Georgia Institute of Technology, Atlanta, 1995. Marciniak, H., DMPLOT, Plot View Program for Rietveld Refinement Method. High Pressure Research Centre, Warsaw, 1995. Kraus, W. and Nolze, G., PowderCell for Windows 2.3. Federal Institute for Materials Research and Testing, Berlin, 1999. Liddell, K. and Thompson, D. P., X-ray diffraction data for Yttrium silicates. Br. Ceram. Trans. J., 1986, 85, 17–22.
Structural data from X-ray powder diffraction for new high-temperature phases (Y1 xLnx)2Si2O7 with Ln=Ce, Pr, Nd F. Monteverde*, G. Celotti Research Institute for Ceramics Technology, Via Granarolo, 64, 48018 Faenza (RA), Italy Received 17 January 2001; received in revised form 14 May 2001; accepted 20 May 2001
Abstract Extending the investigation on possible mixed phases with composition (Y1 xLnx)2Si2O7 formed at high temperature under oxidizing conditions in silicon nitride samples doped with Y2O3 and Ln2O3 as sintering aids, the compounds with Ln= Ce, Pr, Nd were successfully synthesized and structurally characterized by Rietveld whole powder pattern refinement. Some fluctuations in the x values were found as well, but all the compounds are monoclinic (space group P21/c), with lattice parameters close to those of the previously studied (Y2/3La1/3)2Si2O7. Further efforts with Sm (and Dy) failed in producing the expected G-form phase, indicating that the family does not extend beyond Nd: in these cases Ln ions partially substitute Y in its already known a- and d-disilicates. Under the adopted experimental conditions (1 atm, ambient air), an explanation of the impossibility to obtain the phase in question containing lanthanides with Z562 was attempted on the basis of lattice strains introduced by foreign ions larger than Y. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: (Y,Ln)2 Si2O7; X-ray methods; Phase composition; Grain boundary phase
1. Introduction
2. Experimental
During previous studies of the oxidation resistance at high temperature in air of fully dense silicon nitride based materials densified with the addition of sintering aids (i.e Y2O3 and La2O3), a new mixed disilicate was found and completely characterized.1 A synthesis of this new phase, out of a silicon nitride matrix, was successfully carried out as well in excess of SiO2. To improve the knowledge of such (Y1 xLnx)2Si2O7 possible class of compounds with high temperature Gform structure,2,3 a systematic study with following light rare earths (Ln=Ce, Pr, Nd, Sm) was undertaken. Experiments were performed with Ln=Dy too, even if this element does not belong properly to the light group, due to the fact that its ionic radius is almost equal to that of Y3+. This paper describes the synthesis of these new mixed disilicates, the assessment of their stoichiometry and the relative structure determination. When the expected phase failed to form, the reasons for this finding were analyzed and tentatively explained.
Several powder mixtures were prepared varying the molar content of the rare earth oxides involved (Y2O3, H.C. Starck - Germany; CeO2, Merck- Germany; Pr6O11, Nd2O3, Sm2O3, Dy2O3, Techsnabexport - Russia), the test temperature and the dwell time in view to obtain the highest amount and/or the best defined X-ray diffraction pattern of the expected new mixed disilicate. The prerequisite of the amorphous silica (SiO2, Carlo Erba - Italy) in excess was kept constant for each prepared mixture. Once the initial composition was designed, the oxide precursors were ultrasonically mixed in pure ethanol and dried. The blend was then softly pestled in agate mortar, pellettized (100 kg/cm2), mounted on a platinum support, and finally placed in a laboratory kiln. The as-treated pellet was finely ground in agate mortar and, on this powdered sample, the XRD analyses were conducted. Accurate XRD patterns were recorded over an angular range 10 < 2 < 60 on a Siemens mod. D500 (Ni filtered CuKa, 25 mA 33 KV, step width 0.02 , sampling time 6 s). Table 1 summarizes the most significant attempts with starting composition, thermal cycle and crystalline phases identified.
* Corresponding author. E-mail address: [email protected] (F. Monteverde).
0955-2219/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(01)00338-7
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Parallel inspections were performed via SEM (Cambridge mod. S360) equipped with an EDX analyzer (eXL II Link Analytical Pentafet) in order to assess the composition associated to the new formed phases. For this need it was necessary to coat the selected powdered samples with a thin carbon film. The experimental diffraction patterns were processed using the following software packages: DBWS-9411 for Rietveld refinement,4 DMPLOT5 and PowderCell 2.36 for the necessary graphic illustrations.
3. Results and discussion Being the expected structure type of the possible (Y1 xLnx)2Si2O7 phases known from the early determined (Y2/3La1/3)2Si2O7,1 a systematic search was carried out on
mixtures thermally treated at different temperatures from 1400 to 1650 C. By varying the starting compositions, as shown in Table 1, the G-form structure was clearly identified in some Ce-, Pr- and Nd-containing samples. In the first three Nd-containing samples, this form is not detectable due to the fact that, even if small quantities would have been present, the overlapping peaks of a- or d-Y2Si2O7 made the identification largely uncertain. Quantitative EDX analyses yielded for Ce a stoichiometry very similar to the La compound,1 i.e X1/3, while for Pr X1/2 was found and for Nd X2/5, with no traces of other phases, excluding the unavoidable SiO2 (a-cristobalite). On the contrary the Sm-containing samples resisted to any effort: at temperatures 1400–1500 C only a-Y2Si2O7, and d-Y2Si2O7 at temperatures 1550–1650 C were detected. A careful inspection of the XRD patterns revealed
Fig. 1. Outputs of the whole-powder-X ray pattern fitting by Rietveld refinement [observed profile intensities (+) and calculated intensities (-)] for the selected compositions YCe3 (A), YPr5 (B), and YNd4 (C). The difference plot (d) between the observed and calculated intensities for each processed XRD patterns is shown. Short vertical bars represent the Bragg reflection positions of the expected new phase (upper) and a-cristobalite (lower)
F. Monteverde, G. Celotti / Journal of the European Ceramic Society 22 (2002) 721–730
that lattice parameters of the two Y-disilicates increased on the average by about 1% (a-form: a=6.62, b=6.71, c=12.34 A˚, =94, =89, =93 ; d-form: a=13.81, b=5.02, c=8.30 A˚), indicating that Sm substituted partially for Y: instead of forming the expected phase, Sm prefers to occupy Y sites in both the well known disilicate polymorphs.7 While La, Ce, Pr and Nd elements have ionic radius r exceeding that of Y by more than 10% (corresponding to a volume increase 533% in large cations fraction,
Fig. 2. Projection of the monoclinic structure (space group P21/c) seen along [100] at (101) of (Y2/3Ce1/3)2Si2O7 (a), (Y1/2Pr1/2)2Si2O7 (b), and (Y3/5Nd2/5)2Si2O7 (c).
723
representing 8–12% of the total cell volume), Sm is the first with r going below this limit: it seems that, when the linear strain induced by the substitution of Y by Ln element is lower that 10%, the lanthanide enters the Ydisilicate lattice (corresponding to the thermal treatment temperature) and does not give rise to G-form structure anymore. A further experiment was performed with Dy, which exhibits a linear lattice strain, if compared to Y, of merely 1%: such a element is the last showing a d-type disilicate among the lanthanides: mainly d-(Y,Dy)2Si2O7 and traces of g-(Y,Dy)2Si2O7 were found with lattice parameters increased on the average by 0.1% (a=13.70, b=5.01 c=8.18 A˚). Strong evidence can be deduced for the fact that (Y1 xLnx)2Si2O7 compounds with high temperature G-type structure are formed only with rare earth types characterized by a ionic radius sufficiently large. The structural data for the isomorphous new compounds (Y2/3Ce1/3)2Si2O7, (Y1/2Pr1/2)2Si2O7 and (Y3/5 Nd2/5)2Si2O7 are reported in Tables 2–5. The results of the whole-powder-pattern fitting are presented in Fig. 1a–c for the new formed high temperature Ln-Y disilicates, Ln=Ce(a), Pr(b), Nd(c). The comparison between observed and calculated intensities are shown in Tables 6–8 (excluded the reflections belonging to acristobalite). These outputs were obtained after Rietveld refinement by means of the DBWS-9411 program,4 starting from structural parameters predetermined for (Y2/3La1/3)2Si2O7 (Table 5). The drawings in Fig. 2a–c, built with PowderCell 2.36 represent the projection of the monoclinic (Y1 xLnx)2Si2O7 structure with Ln=Ce (a), Pr (b), and Nd (c). The reliability index RWP is quite good (Table 5), even if not as satisfactory as in the case of La: from Tables 2–4 it can be noted again the tendency of Y to occupy preferentially positions (2) leaving the major part of substituting Ln in position (1). In the case of Pr, the sharing is complete, with Y and Pr occupying in equal proportions position (2) and (1), respectively. SEM micrographs from some as-treated powder mixtures and corresponding EDX spectra are presented in Fig. 3. The apparent deviation of Pr-based compound cell volume from the expected monotonic decrease, coming from the well known ‘‘lanthanide contraction’’, is easily understood taking into account the fact that larger Pr ions substitute Y in an anomalously high proportion. The reason why more Ln enters in the compound (Y1 xLnx)2Si2O7 when Ln=Pr, Nd is not easy to understand. It seems that a sort of volume compensating mechanism is activated which prevents the cell volume from becoming too small (allow for the fact that all the cell volumes of the new mixed phases lie within a poor 1.5%). Theoretical calculations based on the volumes of the different ions involved in the formula units treated as cubes of edge 2r yielding a complete space filling, properly
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F. Monteverde, G. Celotti / Journal of the European Ceramic Society 22 (2002) 721–730
Fig. 3. BSE–SEM micrographs (left column) from the as treated mixtures YCe2 (a1), YPr6 (b1), YNd5 (c1), YSm4 (d1), and YDy1 (e1). The EDX spectra (right column) were collected on the bright features (placed upon the grey a-cristobalite particles) of the corresponding micrograph.
725
F. Monteverde, G. Celotti / Journal of the European Ceramic Society 22 (2002) 721–730 Table 1 Summary of the most significant attempts: the complement to 100% (wt. or mol) for each composition consists of amorphous silica Mixture
Composition
Thermal treatment
Y
YPr1 YPr2 YPr3 YPr4 YPr5 YPr6 YPr7 YCe1 YCe2 YCe3 YCe4 YCe5 YNd1 YNd2 YNd3 YNd4 YNd5 YNd6 YSm1 YSm2 YSm3 YSm4 YSm5 YSm6 YSm7 YDy1 a b c d
Ln
wt.%
mol%
wt.%
mol%
6 15 6 10 6 10 15 14.1 15 15 14.1 15 15 15 15 14 18 14 10 15 15 15 14 10 10 18.7
2 5.3 2 1.6 2 1.6 5.3 5 6.6 6.6 5 6.6 5.2 5.2 5.2 5.1 4.2 5.1 3.5 5.2 5.2 5.2 5.1 3.5 3.5 5
24 15 24 20 24 20 15 20.3 15 15 20.3 15 15 15 15 21 12 21 20 15 15 15 21 20 20 11.3
2 1.2 2 3.6 2 3.6 1.2 9.2 5.0 5.0 9.2 5.0 3.5 3.5 3.5 5.2 4.2 5.2 4.5 3.4 3.4 3.4 5.2 4.5 4.5 5
Temperature ( C)
dwell (h)
NSPc
a-cristobalite
1450 1500 1500 1500 1550 1550 1600 1450 1500 1550 1550 1600 1450 1500 1550 1550 1600 1600 1400 1450 1500 1550 1550 1600 1650 1600
2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
Yd Y Y Y Y Y – Y Y Y Y – – – – Y Y Y – – – – – – – –
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
Notes
Y2SiO5*a d-Y2Si2O7 – – – – d-Y2Si2O7 Y2SiO5*, CeO2 – – – d-Y2Si2O7 a-Y2Si2O7 a-Y2Si2O7 d-Y2Si2O7 – d-Y2Si2O7 – a-Y2Si2O7 a-Y2Si2O7 a-(Y,Sm)2Si2O7 d-(Y,Sm)2Si2O7, Sm2O3/Sm2Si2O7* d-(Y,Sm)2Si2O7, Sm2O3/Sm2Si2O7* d-(Y,Sm)2Si2O7, Sm2O3/Sm2Si2O7* d-(Y,Sm)2Si2O7, Sm2O3/Sm2Si2O7* d-(Y,Dy)2Si2O7, g-(Y,Dy)2Si2O7*
**b Fused
** Fused
**
* Traces. ** Pattern processed for Rietveld refinement. NSP: new synthesized phase. Y: yes.
Table 2 Structural data of the new phase (Y2/3Ce1/3)2 Si2O7a Atom
X
Y
Z
Occupancy
Y(1) Ce(1) Y(2) Si(1) Si(2) O(1) O(2) O(3) O(4) O(5) O(6) O(7)
0.5172
0.8055
0.7689
0.8295 0.730 0.910 0.806 0.999 0.574 0.560 0.798 0.231 0.034
0.6008 0.253 0.510 0.421 0.164 0.168 0.268 0.493 0.451 0.655
0.5890 0.014 0.171 0.059 0.045 0.078 0.910 0.253 0.221 0.150
0.335 0.665 1 1 1 1 1 1 1 1 1 1
a
Crystalline phases
BOV= 0.4.
Table 3 Structural data of the new phase (Y1/2Pr1/2)2Si2O7a Atom
X
Y
Z
Occupancy
Pr(1) Y(2) Si(1) Si(2) O(1) O(2) O(3) O(4) O(5) O(6) O(7)
0.5161 0.8371 0.753 0.928 0.801 0.009 0.597 0.540 0.790 0.288 0.019
0.8044 0.5988 0.289 0.505 0.438 0.174 0.161 0.258 0.484 0.439 0.650
0.7688 0.5876 0.028 0.186 0.059 0.025 0.072 0.903 0.249 0.215 0.159
1 1 1 1 1 1 1 1 1 1 1
a
BOV=1.5.
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F. Monteverde, G. Celotti / Journal of the European Ceramic Society 22 (2002) 721–730
distance, gave 11% for La, 8% for Ce, 15% for Pr and 6% for Nd compounds, respectively. Once more Pr, besides the peculiar Ln-rich stoichiometry, involves the most perturbed structure. While Y-La compound is white, Y-Ce appears very pale yellow, Y-Pr very pale green and Y-Nd very pale purple coloured: of course the colours are faded by the contemporary presence of large fraction of white SiO2 (a-cristobalite).
Table 4 Structural data of the new phase (Y3/5Nd2/5)2Si2O7a Atom
X
Y
Z
Occupancy
Y(1) Nd(1) Y(2) Nd(2) Si(1) Si(2) O(1) O(2) O(3) O(4) O(5) O(6) O(7)
0.5234
0.8056
0.7662
0.8346
0.5984
0.5889
0.745 0.920 0.811 0.018 0.577 0.542 0.854 0.297 0.043
0.258 0.530 0.422 0.194 0.176 0.282 0.507 0.453 0.678
0.021 0.164 0.066 0.046 0.090 0.908 0.260 0.216 0.142
0.5 0.5 0.7 0.3 1 1 1 1 1 1 1 1 1
a
4. Conclusions Extending the study of the class of compounds (Y1 XLnX)2Si2O7 with high temperature G-form structure, whose first member was that with Ln= La and X=1/3, three new phases were synthesized (with Ln=Ce, Pr, Nd and X=1/3, 1/2, 2/5 respectively) in excess of silica, and structurally characterized by whole powder X-ray pattern Rietveld refinement. All the new phases are monoclinic, space group P21/c(14), with cell parameters as follows (Z=4 for everyone): (Y2/3Ce1/3)2Si2O7, a=5.365(1)A˚, b=8.509(1)A˚, c=13.804(1)A˚, =111.45 (1) , XRD= 4.305 g/cm3; (Y1/2Pr1/2)2Si2O7, a=5.370(1) A˚, b=8.542(2)A˚, c=13.838(2)A˚, =111.76(2) , XRD= 4.485 g/cm3; (Y3/5Nd2/5)2Si2O7, a=5.362(1)A˚, b=8.496 (1)A˚, c=13.782(1)A˚, =111.46(1) ; XRD=4.435 g/cm3. In these cases too the atomic coordinates showed that substitution of Ln for Y cations preferentially occurs in half of the possible lattice positions: only Nd presents replacement in both of them. Si-O tetrahedra are slightly distorted in Ce and Nd compounds, while Pr compound is characterized by a deformation even larger than that previously found for La. Attempts to obtain the high temperature G-form phase with Ln= Sm, Dy failed, probably because the lanthanides more easily substituted Y in a- and/or ddisilicates lattice when the produced strain is sufficiently small. If the ratio r(Ln)/r(Y), at least for lanthanides with cation radius larger than that of Y, deviates from unit by less than 10%, it looks energetically more favoured for Ln to replace Y in Y-disilicates instead of a general rearrangement to form the new phase. Clear
BOV= 0.7.
account for unit cell volumes found in the given stoichiometry: the monotonic reduction in O2- radius with decreasing trivalent cations size observed in lanthanides pyrosilicates has to be held in due consideration, being oxygen responsible of about 90% of cell volume occupation. The overall thermal factor BOV significantly higher for Pr compound (Table 3) has likely to be attributed to the lower stability of Pr oxides. The best fit implies not neglibible displacements of some oxygen atoms from the geometrically regular tetrahedral positions, leading to a typical distorted arrangement linked to the non-equilibrium conditions of nucleation and growth. On the other hand the spatial positions of the heavy ions in the real lattice remain remarkably constant, and those of Si undergo only minor changes (Tables 2–4). In general, the partial substitution of Y to Ln ions, doesn’t introduce substantial modification in the G-form structure as described in Refs [2] and [3] (accounting for the choice in Ref. [3] of a description by P21/n space group with b very close to 90 ), apart from the enhanced tetrahedra distortion. An estimate of SiO4 tetrahedra distortion, based both on the discrepancy of Si–O average bond distances as compared to the theoretical value (1.65 A˚) and on the half maximum spread of the same
Table 5 Structural parameters (a, b, c, , V, Z), density (XRD), and reliability index (RWP) for the Rietveld refinement of the high temperature new formed phases Compound
(Y2/3La1/3)2Si2O7 (Y2/3Ce1/3)2Si2O7 (Y1/2Pr1/2)2Si2O7 (Y3/5Nd2/5)2Si2O7 a b
Lattice parameter
Cell volume
a(A˚)
b(A˚)
c(A˚)
( )
(A˚3)
5.375(1) 5.365(1) 5.370(1) 5.362(1)
8.569(1) 8.509(1) 8.542(2) 8.496(1)
13.863(1) 13.804(1) 13.838(2) 13.782(1)
111.79(1) 111.45(1) 111.76(2) 111.46(1)
592.9 586.5 589.6 584.3
Z: formula units per cell. TW: this work.
Za
XRD
RWP 3
4 4 4 4
(g/cm )
(%)
4.250 4.305 4.485 4.435
6.6 8.6 8.6 7.3
Ref.
[1] TWb TW TW
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F. Monteverde, G. Celotti / Journal of the European Ceramic Society 22 (2002) 721–730 Table 6 X-ray diffraction data of the new phase (Y2/3Ce1/3)2Si2O7 (IOBS 55) h
k
0 0 0 1 1 0 1 0 0 1 1 0 1 1 1 1 1 0 1 1 0 0 1 1 0 2 1 1 1 1 0 0 2 2 1 2 2 1 0 1 2 0 2 1 1 1 1 2 2 2 2 1 0 0 2 0 2 0 1 2 2 2 1 0 2 1 2 1
1 0 1 0 0 2 1 2 1 1 1 2 0 2 0 2 2 0 1 1 2 1 2 2 3 0 2 1 2 1 3 2 1 1 3 1 0 3 1 3 1 3 1 3 0 2 0 2 2 1 2 3 0 3 2 4 0 1 2 0 1 2 2 4 2 3 1 4
l 1 2 2 0 2 0 2 1 3 1 3 2 2 1 4 0 2 4 2 4 3 4 1 3 1 2 2 3 4 5 2 4 2 1 1 3 0 0 5 2 0 3 4 1 4 3 6 2 1 5 0 4 6 4 4 1 2 6 4 6 2 1 6 2 5 3 6 1
d (A˚)
I calc
I obs
h
k
7.094 6.424 5.127 4.993 4.907 4.255 4.251 4.039 3.825 3.743 3.670 3.547 3.388 3.333 3.307 3.238 3.214 3.212 3.147 3.082 3.018 3.005 2.977 2.940 2.770 2.682 2.650 2.648 2.611 2.596 2.595 2.563 2.558 2.520 2.507 2.497 2.497 2.466 2.460 2.456 2.396 2.365 2.357 2.345 2.340 2.331 2.295 2.269 2.242 2.173 2.153 2.153 2.141 2.126 2.125 2.099 2.084 2.077 2.050 2.034 2.024 2.022 2.020 2.019 1.9876 1.9876 1.9783 1.9774
454 265 54 56 17 129 16 106 36 24 14 9 738 1000 941 783 523 82 240 238 439 77 5 16 114 494 55 254 46 108 103 146 13 10 8 16 31 176 97 121 31 25 16 25 6 7 72 57 38 13 171 13 175 7 52 337 6 182 44 6 74 404 37 40 309 43 97 22
396 224 43 52 11 128
130 28 49 32 29 774 1000 943 800 625
1 1 1 1 2 2 1 0 0 0 1 1 2 2 2 2 2 1 1 1 0 2 1 1 2 1 3 0 2 1 3 3 1 1 2 1 1 0 3 1 3 2 2 2 2 3 3 0 1 3 3 2 1 1 1 3 1 1 1 2 1 2 1 3 2 2 0
3 4 1 4 3 3 1 2 4 3 4 4 3 2 3 2 1 2 3 4 1 1 2 4 3 3 0 4 3 0 1 1 0 1 0 4 1 5 1 4 0 1 4 0 4 2 2 4 3 0 2 1 3 2 5 1 2 5 4 2 5 4 4 1 4 2 3
251 251 488 80 5 19 114 482 306 43 181
136 13 8 8 48
179 96 115 22 16 10 21 6 5 75 61 43 6 176
210 59
354 6 200 45 7 526
85
353
122
l 5 0 5 2 2 1 7 6 3 5 1 3 0 2 4 6 3 5 4 2 7 7 7 4 1 6 2 4 5 6 2 4 8 6 4 3 8 1 5 5 0 4 1 8 3 3 2 5 5 6 4 8 7 6 2 6 8 1 4 4 3 1 6 1 5 8 7
d (A˚)
I calc
1.9655 1.9571 1.9523 1.9517 1.9486 1.9317 1.9211 1.9127 1.9052 1.9043 1.8947 1.8851 1.8741 1.8715 1.8555 1.8351 1.8345 1.8143 1.8050 1.8015 1.7942 1.7918 1.7891 1.7891 1.7858 1.7843 1.7767 1.7736 1.7618 1.7498 1.7392 1.7280 1.7236 1.7140 1.6938 1.6908 1.6893 1.6871 1.6779 1.6771 1.6645 1.6612 1.6560 1.6535 1.6495 1.6482 1.6395 1.6386 1.6377 1.6355 1.6301 1.6232 1.6191 1.6183 1.6078 1.6061 1.5975 1.5755 1.5740 1.5737 1.5700 1.5612 1.5603 1.5523 1.5451 1.5412 1.5409
16 28 22 6 7 18 20 10 5 15 151 155 29 153 94 164 27 180 40 39 45 6 152 31 35 62 26 58 31 21 8 6 13 76 23 63 72 21 6 113 101 23 77 30 48 164 86 105 10 117 50 44 21 43 23 30 14 64 13 9 51 11 8 28 6 8 31
I obs
19 31 34
9 19 21 12 6 18 149 149 30 157 92 196
194 44 43 47 6 189
99
27 57 25 20 6 6 13 89 25 66 75 23 108
9 =
95 21 71 28 46 155 204
;
121 49 44 66
22 29 12 54 19
43 12
20 5 38
728
F. Monteverde, G. Celotti / Journal of the European Ceramic Society 22 (2002) 721–730
Table 7 Diffraction data of the new phase (Y1/2Pr1/2)2Si2O7 (IOBS 55) h
k
0 0 0 1 1 1 0 1 0 0 1 1 0 1 1 1 1 1 0 1 1 0 0 0 2 1 1 1 1 0 0 2 1 2 2 1 1 0 2 2 0 2 1 1 1 2 2 2 1 2 0 2 0 0 2 0 1 0 1 2 2 2 1 2 1 1 1
1 0 1 0 0 1 2 1 2 1 1 1 2 0 2 0 2 2 0 1 1 2 1 3 0 2 1 2 1 3 2 1 3 1 0 3 3 1 0 1 3 1 3 2 0 2 2 1 3 2 0 2 3 4 0 1 2 4 2 2 1 2 3 1 4 3 4
l 1 2 2 0 2 0 0 2 1 3 1 3 2 2 1 4 0 2 4 2 4 3 4 1 2 2 3 4 5 2 4 2 1 3 0 0 2 5 4 0 3 4 1 5 6 2 1 5 4 0 6 4 4 1 2 6 4 2 6 1 2 5 3 6 1 5 0
d (A˚)
I calc
I obs
h
k
7.114 6.426 5.135 4.987 4.922 4.307 4.271 4.264 4.053 3.829 3.739 3.684 3.557 3.380 3.342 3.319 3.244 3.226 3.213 3.143 3.093 3.025 3.007 2.780 2.685 2.650 2.644 2.621 2.604 2.603 2.568 2.561 2.515 2.503 2.494 2.473 2.465 2.461 2.461 2.394 2.371 2.365 2.349 2.303 2.302 2.273 2.244 2.181 2.161 2.153 2.142 2.132 2.131 2.107 2.079 2.078 2.049 2.027 2.026 2.021 2.020 1.9948 1.9891 1.9855 1.9843 1.9723 1.9631
455 244 62 37 19 20 88 39 43 34 40 27 18 811 1000 899 659 484 100 198 180 514 70 72 505 68 233 31 90 103 109 31 6 27 33 133 131 131 9 23 27 11 19 9 43 27 16 15 9 141 154 52 17 285 5 168 28 47 49 365 40 257 25 77 26 9 23
391 217 63 48 14 11 94
53 20 67 41 39 843 1000 892 683 496 109 212 188 525 69 100 488 67 243 30 178
110 30 7 30 35 134 125 133
23 25 10 16 49
41 24 5 6 135 173 51 17 283 189
25 95
443
269 24 106
1 1 2 1 0 0 0 1 1 2 2 2 2 2 1 1 1 2 1 0 1 1 2 0 3 2 1 3 1 1 3 0 1 1 2 1 2 3 2 2 2 2 3 0 3 3 1 3 3 2 1 2 1 1 2 3 1 1 1 1 2 2 3 3 2 2 0
4 1 3 1 2 4 3 4 4 3 2 3 2 1 2 3 4 1 4 1 2 3 3 4 0 3 0 1 0 1 1 5 4 1 0 4 3 0 4 0 1 4 2 4 2 0 3 2 1 1 3 4 2 5 4 1 2 5 5 4 2 4 1 2 2 3 3
12 25
l 2 5 1 7 6 3 5 1 3 0 2 4 6 3 5 4 2 7 4 7 7 6 1 4 2 5 6 4 8 6 1 1 3 8 4 5 2 0 1 8 4 3 3 5 2 6 5 4 0 8 7 0 6 2 4 6 8 1 3 4 4 5 1 0 8 7 7
d (A˚)
I calc
I obs
1.9590 1.9496 1.9350 1.9259 1.9147 1.9112 1.9080 1.8997 1.8923 1.8759 1.8695 1.8618 1.8418 1.8306 1.8131 1.8057 1.8053 1.7983 1.7958 1.7950 1.7940 1.7900 1.7864 1.7785 1.7771 1.7682 1.7473 1.7314 1.7274 1.7118 1.6979 1.6935 1.6935 1.6931 1.6898 1.6831 1.6792 1.6625 1.6597 1.6593 1.6577 1.6548 1.6506 1.6426 1.6408 1.6405 1.6379 1.6336 1.6318 1.6288 1.6238 1.6220 1.6172 1.6139 1.6129 1.6111 1.6014 1.5802 1.5760 1.5760 1.5713 1.5509 1.5499 1.5492 1.5467 1.5451 1.5430
8 33 6 24 5 9 12 96 109 33 105 90 72 47 136 52 24 13 34 22 114 57 40 36 6 33 27 10 10 60 6 34 78 51 23 114 11 69 64 51 30 45 154 62 76 100 4 42 6 27 24 6 40 8 5 16 13 61 54 6 7 7 21 6 7 5 33
9 39 7 26 7 11 15 110 126 43 118 99 115 70 191 99
9 =
15 184
;
9 =
65 48 48 8 33 18 8 7 72 6 166
;
22 105 10 73 115
29 43 150 66 191
5 46 7 29 26 6 45 9 5 16 13 47 59
9 =
6 31
; 7 5 36
729
F. Monteverde, G. Celotti / Journal of the European Ceramic Society 22 (2002) 721–730 Table 8 Diffraction data of the new phase (Y3/5Nd2/5)Si2O7 (IOBS 55) h
k
0 0 0 1 1 1 1 0 1 0 1 1 0 1 1 1 1 1 0 1 1 0 0 0 2 1 1 1 1 0 0 2 2 2 2 1 0 1 2 2 0 2 1 1 2 1 2 2 1 2 1 0 2 0 0 2 0 1 2 2 2 1 0 2 1
1 0 1 0 0 1 1 2 1 2 1 1 2 0 2 0 2 2 0 1 1 2 1 3 0 2 1 2 1 3 2 1 1 1 0 3 1 3 0 1 3 1 3 0 2 1 2 1 1 2 3 0 2 3 4 0 1 2 0 1 2 2 4 2 3
l 1 2 2 0 2 1 0 0 2 1 1 3 2 2 1 4 0 2 4 2 4 3 4 1 2 2 3 4 5 2 4 2 1 3 0 0 5 2 4 0 3 4 1 6 2 4 1 1 6 0 4 6 4 4 1 2 6 4 6 2 1 6 2 5 3
d (A˚)
I calc
I obs
h
k
7.083 6.413 5.119 4.990 4.903 4.533 4.303 4.248 4.246 4.033 3.739 3.666 3.542 3.384 3.329 3.303 3.235 3.210 3.207 3.144 3.078 3.013 3.000 2.765 2.680 2.647 2.644 2.607 2.592 2.591 2.559 2.556 2.518 2.495 2.495 2.463 2.456 2.452 2.451 2.394 2.361 2.355 2.342 2.292 2.267 2.253 2.240 2.217 2.213 2.151 2.150 2.138 2.123 2.123 2.095 2.082 2.073 2.047 2.032 2.022 2.020 2.017 2.016 1.9854 1.9848
396 218 76 29 27 4 34 145 22 114 51 40 13 794 947 1000 614 487 35 234 155 482 49 64 514 59 191 26 68 141 141 33 6 39 42 193 81 138 5 17 20 31 13 71 16 5 21 6 15 137 6 191 42 23 280 5 153 35 13 57 365 37 25 268 44
352 199 86 41 21 7 36 140
130 57 58 29 802 956 1000 632 555
252 156 511 47 76 547 273
29 195
141 32 9 86
194 79 133 5 21 21 27 15 75 17 6 23 6 15 143
220 68
318 6 170 21 14 472
66
328
2 1 1 1 1 1 2 2 2 1 0 0 1 1 2 2 2 2 2 1 1 1 0 2 1 1 2 1 3 0 2 1 3 1 1 2 1 1 0 3 1 3 2 2 2 2 3 3 0 1 3 3 3 2 2 1 1 1 2 3 1 1 1 3 2
1 4 3 4 1 4 3 3 3 1 2 3 4 4 3 2 3 2 1 2 3 4 1 1 4 2 3 3 0 4 3 0 1 0 1 0 4 1 5 1 4 0 1 4 0 4 2 2 4 3 0 1 2 1 4 3 2 5 4 1 2 5 5 1 4
d (A˚)
l 6 1 5 0 5 2 2 1 3 7 6 5 1 3 0 2 4 6 3 5 4 2 7 7 4 7 1 6 2 4 5 6 2 8 6 4 3 8 1 5 5 0 4 1 8 3 3 2 5 5 6 0 4 8 0 7 6 2 4 6 8 1 3 1 5
1.9760 1.9746 1.9625 1.9543 1.9495 1.9489 1.9466 1.9298 1.9194 1.9180 1.9096 1.9013 1.8921 1.8823 1.8722 1.8696 1.8534 1.8328 1.8326 1.8117 1.8024 1.7990 1.7912 1.7895 1.7864 1.7863 1.7839 1.7815 1.7758 1.7708 1.7597 1.7474 1.7382 1.7208 1.7114 1.6919 1.6883 1.6865 1.6845 1.6766 1.6746 1.6634 1.6594 1.6541 1.6513 1.6475 1.6470 1.6384 1.6360 1.6353 1.6342 1.6324 1.6289 1.6210 1.6174 1.6166 1.6159 1.6055 1.6052 1.6048 1.5949 1.5733 1.5676 1.5511 1.5432
I calc 59 35 9 14 12 11 4 28 4 4 11 7 149 161 49 129 86 149 36 174 42 28 46 5 19 171 41 36 9 49 13 13 14 12 48 24 67 54 23 5 109 84 30 67 55 36 141 86 95 14 136 12 43 26 9 6= 20 59 ; 9 10 = 2 9; 24 47 45 35 8
I obs 102 12 20 37 6 32 5 5 12 7 138 149 51 134 85 205 207 43 30 49 5 192 41 36 9 44 6 5 8 6 55 25 69 56 24 5 96 84 30 64 51 164 88 115 144 12 43 25 85
22
17 32 30 10 5
730
F. Monteverde, G. Celotti / Journal of the European Ceramic Society 22 (2002) 721–730
evidence of this phenomenon was supplied by SEM investigations and XRD determination of the modified lattice parameters.
3.
4.
References 1. Monteverde, F. and Celotti, G., Structural data from X-ray powder diffraction of a new phase formed in the Si3N4-La2O3Y2O3 system after oxidation in air. J. Eur. Ceram. Soc., 1999, 19, 2021–2026. 2. Greis, O., Bossemeyer, H. G., Greil, P., Breidenstein, B. and Haase, A., Structural data of the monoclinic high-temperature G-
5. 6. 7.
form of La2Si2O7 from X-ray powder diffraction. Materials Science Forum, 1991, 79-82, 803–808. Cuneyt Tas, A. and Akinc, M., Crystal structures of the hightemperature forms of Ln2Si2O7 (Ln=La, Ce, Pr, Nd, Sm) revisited. J. Am. Cer. Soc., 1994, 77(11), 2968–2970. Young, R. A., Sakthivel, A., Moss, T. S. and Paiva-Santos, C. O., DBWS-9411, Rietveld analysis of X-ray and neutron powder diffraction patterns. Georgia Institute of Technology, Atlanta, 1995. Marciniak, H., DMPLOT, Plot View Program for Rietveld Refinement Method. High Pressure Research Centre, Warsaw, 1995. Kraus, W. and Nolze, G., PowderCell for Windows 2.3. Federal Institute for Materials Research and Testing, Berlin, 1999. Liddell, K. and Thompson, D. P., X-ray diffraction data for Yttrium silicates. Br. Ceram. Trans. J., 1986, 85, 17–22.