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Silicon lanthanide oxynitrides of the M4Si2O7N2 type
Journal of the Less-Common 0 Elsevier
Sequoia
S.A.,
Metals, 68 (1979)
Lausanne
SILICON LANTHANIDE
M. MONTORSI
Istituto (Italy) (Received
-
Printed
193
193 - 197 in the Netherlands
OXYNITRIDES
OF THE M,&IiaO,Ns TYPE
and P. APPENDINO
di Chimica Generate e Applicata
e di Metallurgia,
Politecnico
di Torino,
Turin
April 4, 1979)
Summary Four silicon lanthanide oxynitrides of the M,Si,O,Ns type (M = Ce, Pr, Tm, Lu) with a monoclinic elementary cell similar to that of cuspidine were prepared and their lattice constants were determined.
The possibility that silicon nitride is a substance suitable for electronic and electrotechnical applications [l] and for the production of materials designed for use at high temperatures has been put forward in recent years. Its resistance to oxidation, creep, thermal shock and wear has enabled it to act as a substitute for metal alloys in various gas turbine components [ 2 - 61. Two crystalline modifications of silicon nitride, (Y (low temperature) [ 71 and fl (high temperature) [ 81, are known. SisN,, which is obtained by nitriding silicon powder with nitrogen at about 1400 “C, forms a noncohesive white powder which must be very energetically compacted to ensure good mechanical properties. This is done by adding small quantities of a substance which generates a liquid phase by reaction with Si3N4 and with other impurities which are present. In this condition the density, when the powder is hot pressed, almost reaches the theoretical value. Initially a few per cent of MgO appeared to be suitable [ 9, lo] , until it was discovered that the reaction between MgO, Si3N4 and the silica and impurities accompanying the nitride was forming a vitreous phase on the grain boundaries, and that this phase appreciably lowered the resistance to deformation at temperatures slightly above 1000 “C [ 11,121. A search was made for additives that would give a more refractory phase on the crystal boundaries with a thermal dilatation coefficient nearer to that of silicon nitride. Good results have been achieved with trivalent lanthanide oxides and yttrium oxide [ 13 - 151. The following mechanism during hot pressing has been postulated. Initially, at 1500 - 1600 “C, a reaction occurs at the grain boundaries of SiaN, between the grains, the small amounts of SiOa which are always present and the trivalent oxides. A trivalent oxide-rich liquid phase appears and greatly assists sintering, but has no more than a slight
194
effect on the final mechanical properties. As the reaction proceeds, this phase gradually disappears so that eventually the edges are substantially coated with a high melting M,0s*SisN4 oxynitride [ 161. If the reaction is incomplete, another high melting oxynitride, M,Sis07Nz, may also be formed during the solidification of the liquid phase that remains. Clearly, it is important to investigate the structure and behaviour of the phases that may be formed during the manufacture and employment of silicon nitride. This work concerns the preparation and crystallographic classification of some silicon lanthanide oxynitrides of the M&,O,Ns type. Wills et al. [17] reported five of these compounds with M = La, Sm, Dy, Er and Yb and showed that they are isomorphous with cuspidine Ca,SisO,Fs, a monoclinic calcium fluorosilicate the crystalline structure of which is well known [ 181. The existence of these oxynitrides was confirmed by Marchand et al. [ 191, although they assigned to them slightly different lattice constants (see Table 3). These authors also showed that phases with M = Nd, Gd and Ho display analogous features; our own work has served to bring Tb within this category
[201. This paper tries to complete the picture by examining the products of the reaction between Si3N4, SiOs and MsOs when M = Ce, Pr, Tm or Lu. Specimens were prepared using amorphous silica and an Si3N4 powder consisting of about 90% and 10% of the (Yand 0 modifications, respectively (Starck). Tm and Lu oxides (Fluka) of at least 99.9% purity were employed. Pr,Os was obtained by the reduction of Pr,O, in a hydrogen current at 650 “C [21, 221, while CesOs was prepared from CeO, by heating at 1250 “C in the presence of metallic niobium in vacuum-sealed containers. Owing to the relative oxygen tension values in the phases [ 231, Ce02 is reduced from Cen’ to Gem with partial simultaneous oxidation of the Nb. The mixtures were ground to give a grain size of less than 1 pm, compacted at 3000 kg cmp2, placed in graphite containers coated with BN to prevent reactions and heated at 1500 “C in a nitrogen atmosphere for 8 h. Preliminary tests were made to ensure that equilibrium conditions were reached in this way. The samples were then cooled in the oven, finely ground and subjected to X-ray analysis (powder method, Cu K, radiation with h = 1.5418 a). The X-ray results showed that the four oxides gave rise to M4Si207N2 oxynitrides with closely similar powder patterns, especially in the cases of Ce and Pr and of Tm and Lu, respectively. For this reason, the values of the inter-planar distances and the relative intensities of the main X-ray reflections are listed separately for the two pairs in Tables 1 and 2. Once again, the monoclinic cells obtained were comparable with that of cuspidine. The lattice constant values are as follows Ce4Si207N2
a = 7.949 A
b = 10.874 a
c = 11.066 A
fl = 110.57”
Pr,Si20,N2
a = 7.875 /$
b = 10.808 a
c = 11.016 a
fl = 110.65”
Tm,Si20,N2
a = 7.492 A
b = 10.351 A
c = 10.792 A
Ir = 111.24”
195 TABLE
1
Powder
X-ray diffraction
hkl
110
and Pr@2O,Nz Pr&&07N2
Ce.@20,N2 &bs
020 201 I.20 210 221 130 310 122 022 321 312 230 320 322 103 402 212 412 132 142 512 423 510 204
data for Ce4Si20,N2
(A)
7.48 5.433 5.187 4.813 4.667 3.748 3.421 3.286 3.206 3.066 3.033 3.019 2.966 2.912 2.717 2.627 2.597 2.540 2.522 2.383 2.243 2.119 2.056 2.034 1.987
Lu4Siz07N2
dmc(h
I/lo
dabs 6%
he
7.50 5.431 5.192 4.814 4.677 3.755 3.421 3.291 3.207 3.070 3.037 3.017 2.970 2.915 2.719 2.630 2.596 2.544 2.525 2.384 2.243 2.120 2.058 2.035 1.986
10 3 ‘2 2 30 8 5 35 100 20 25 25 50 60 10 10 20 5 5 5 10 15 5 20 25
7.443 5.400 5.163 4.766 4.652 3.742 3.398 3.271 3.186 3.043 3.017 3.003 2.953 2.900 2.699 2.605 2.586 2.520 2.514 2.364 2.221 2.109 2.044 2.026 1.972
7.459 5.404 5.165 4.786 4.660 3.134 3.401 3.275 3.182 3.044 3.022 2.999 2.953 2.897 2.703 2.606 2.582 2.518 2.512 2.366 2.228 2.110 2.045 2.025 1.969
a = 7.375 A
b = 10.315 A
c = 10.765 A
(A)
I& 10 3 3 4 20 10 5 30 100 15 25 25 40 70 8 15 12 5 5 8 7 20 15 20 25
,!I= 111.30”
Lastly, it should be mentioned that numerous attempts were made to prepare a phase with M = Eu by varying, within wide limits, the temperature range and the composition of the gas in contact with the solids. Variously coloured solids with poorly reproducible X-ray patterns were obtained but the presence, even partial, of a phase with a powder pattern similar to those given by the other oxides, was never observed. This divergent behaviour of EusOs may be due to its well-known inability to maintain the oxidation number three at high temperatures. With the exception of Eu, therefore, the modalities for the preparation and crystallographic classification of the entire series of silicon lanthanide oxynitrides M,Si20,N, have now been established. The full set of X-ray data is summarized in Table 3.
196 TABLE
2
Powder
X-ray
diffraction
hkl
data for TmqSi207N2
and Lu&i207N2 LqSi207N2
Tm&li207N2 d&s
110 020 201 120 210 221 130 310 122 312 022 112 230 320 401 322 312 140 331 232 410 420 222 113 312 500 501
(A)
dcalc (A)
z/lo
dabs (A)
dd,
(A)
z/zo
7.201 5.163 5,029 4.600 4.525 3.606 3.259 3.188 3.031
7.214 5.176 5.037 4.602 4.524 3.610 3.264 3.190 3.034
10 2 2 5 20 4 3 20 100
2.891 2.868 2.845 2.813 2.701 2.616 2.584 2.507 2.483
2.894 2.864 2.845 2.814 2.698 2.613 2.584 2.506 2.481
20 15 30 50 3 25 10 20 15
7.178 5.157 5.006 4.571 4.512 3.584 3.238 3.182 3.001 2.879 2.855 2.827
7.191 5.158 5.009 4.587 4.510 3.593 3.253 3.180 2.999 2.883 2.859 2.827
10 2 2 2 20 2 2 15 100 15 12 30
2.440 2.259 2.235 2.064 4.028 2.015 1.975
2.444 2.262 2.235 2.058 2.032 2.012 1.975
4 7 5 8 10 10 15
2.806 2.693 2.599 2.566 2.499 2.474 2.449 2.433 2.253 2.215 2.037 2.012 2.004 1.971
2.805 2.691 2.595 2.568 2.498 2.472 2.448 2.436 2.255 2.213 2.035 2.013 2.006 1.969
40 5 60 8 10 10 15 4 5 10 15 5 5 8
References 1 Handbook
of Electronics Materials, Vol. 3: Silicon Nitride for Microelectric J. T. Milek (ed.), Plenum Press, New York, 1971. J. F. Lynch (ed.), Engineering Properties of Selected Ceramic Materials, American Ceramic Society, Columbus, Ohio, 1966. D. J. Godfrey, Use of Ceramics in High Temperature Engineering, Met. Mater., 2 (1968) 305. N. L. Parr, Engineer, 222 (1966) 18. A. F. McLean, Am. Ceram. Sot. Bull., 52 (1973) 464,482. K. H. Jack, Trans. Br. Ceram. Sot., 72 (1973) 376. D. Hardie and K. H. Jack, Nature ~~ondo~~, 180 (1957) 332. D. S. Thompson and P. L. Platt, in G. H. Stewart (ed.), Science of Ceramics, Vol. 3, Academic Press, New York, 1967, p. 33. C. G. Deeley, J. M. Herbert and N. C. Moore, Powder Metall., 8 (1961) 145. P. Drew and M. H. Lewis, J. Mater. Sci., 9 (1974) 261. D. W. Richerson,Am. Cemm. Sot. Bull., 52 (1973) 560.
Applications, 2 3 4 5 6
7 8 9 10 11
197
TABLE Lattice
3 constants
for silicon ianthanide
oxynitrides
of composition
M&,&N~
Gxynitride
Ref.
e (8)
b (A)
c (8)
P
La4SizOTNz
17 19
15.538 8.03
10.437 10.99
23.948 11.05
113.53 110.1
7.949
10.874
11.066
110.57
Ce_&&&N2 Pr,Si,U,N,
7.875
10.808
11.016
110.65
Nd,Si$&Nz
19
7.839
10.737
11.022
110.64
Sm&lizG,N,
7.761 7.75
10.292 10.60 -
11.965 10.99
Eu4Siz0,Nz
17 19 -
113.69 111.1 -
Gd4Si&Nz
19
7.66
10.52
10.93
111.0
Tb$i&N2
20
7.640
10.156
11.033
108.53
Hy&$?&N2
17
7.527
10.052
11.053
108.59
Ho4Si207N2
19
7.57
10.46
10.81
111.0
Er&XzG~Nz
17 19
7.528 7.53
10.100 10.40
10.987 10.80
108.72 Ill.1
7.492
10.351
10.792
111.24
7,519 7.47
10.046 10.32
11.071 10.77
109.15 111.3
7.375
10.315
10.765
111.30
Tm4Si207N2 Yb4Si207NZ Lu4Si2G7N2 _
17 19
-
12 F. F. Lange and J. L. Iskae, Ceramics for High Performance Applicafions, 13 14 15 16 17 18 19 20 21 22 23
Brook Hill, Chestnut Hill, Mass., 1974, p. 223. G. E. Gazza,J. Am. Ceram. Sot., 56 (1973) 602. I. C. Huseby and G. Petzow, PotLlder Mefall. Inf., 6 (1974) 17. K. S. Mazdiyasni and C. M. Cooke, J. Am. Ceram. Stlc., 57 (1974) 536. R. R. Wills, S. Halmquist, J. M. Wimmer and J. A. Cunningham, J. &fafer. Sci., f f (1976) 1305. R. R. Wills, R. W. Stewart, J. A. Cunningham and J. M. Wimmer, J. Mater. Sci., 11 (1976) 749. R. W. G. Wickoff, Cryst. Strucf. Commun., 4 (1968) 239. R. Marchand, A. Jayaweera, P. Verdier and J. Lang, C. R. Acad. Sci., S&r. C, 283 (1916) 675. M. Montorsi and P. Appendino, to be published in Am. Cerum. Sot. BuE. R. E. Ferguson, E. Guth and L. Eyring, J. Am. Chem. Sot., 76 (1954) 3890, C. T. Stubblefield, E. Eick and L. Eyring, cl; Am. Chem. Sot., 78 (1956) 3877, J. F. Elliatt and H. Gleiser, Thermochemistry for Steelmaking, Addison-Wesley, Reading, Mass., 1963.
Sequoia
S.A.,
Metals, 68 (1979)
Lausanne
SILICON LANTHANIDE
M. MONTORSI
Istituto (Italy) (Received
-
Printed
193
193 - 197 in the Netherlands
OXYNITRIDES
OF THE M,&IiaO,Ns TYPE
and P. APPENDINO
di Chimica Generate e Applicata
e di Metallurgia,
Politecnico
di Torino,
Turin
April 4, 1979)
Summary Four silicon lanthanide oxynitrides of the M,Si,O,Ns type (M = Ce, Pr, Tm, Lu) with a monoclinic elementary cell similar to that of cuspidine were prepared and their lattice constants were determined.
The possibility that silicon nitride is a substance suitable for electronic and electrotechnical applications [l] and for the production of materials designed for use at high temperatures has been put forward in recent years. Its resistance to oxidation, creep, thermal shock and wear has enabled it to act as a substitute for metal alloys in various gas turbine components [ 2 - 61. Two crystalline modifications of silicon nitride, (Y (low temperature) [ 71 and fl (high temperature) [ 81, are known. SisN,, which is obtained by nitriding silicon powder with nitrogen at about 1400 “C, forms a noncohesive white powder which must be very energetically compacted to ensure good mechanical properties. This is done by adding small quantities of a substance which generates a liquid phase by reaction with Si3N4 and with other impurities which are present. In this condition the density, when the powder is hot pressed, almost reaches the theoretical value. Initially a few per cent of MgO appeared to be suitable [ 9, lo] , until it was discovered that the reaction between MgO, Si3N4 and the silica and impurities accompanying the nitride was forming a vitreous phase on the grain boundaries, and that this phase appreciably lowered the resistance to deformation at temperatures slightly above 1000 “C [ 11,121. A search was made for additives that would give a more refractory phase on the crystal boundaries with a thermal dilatation coefficient nearer to that of silicon nitride. Good results have been achieved with trivalent lanthanide oxides and yttrium oxide [ 13 - 151. The following mechanism during hot pressing has been postulated. Initially, at 1500 - 1600 “C, a reaction occurs at the grain boundaries of SiaN, between the grains, the small amounts of SiOa which are always present and the trivalent oxides. A trivalent oxide-rich liquid phase appears and greatly assists sintering, but has no more than a slight
194
effect on the final mechanical properties. As the reaction proceeds, this phase gradually disappears so that eventually the edges are substantially coated with a high melting M,0s*SisN4 oxynitride [ 161. If the reaction is incomplete, another high melting oxynitride, M,Sis07Nz, may also be formed during the solidification of the liquid phase that remains. Clearly, it is important to investigate the structure and behaviour of the phases that may be formed during the manufacture and employment of silicon nitride. This work concerns the preparation and crystallographic classification of some silicon lanthanide oxynitrides of the M&,O,Ns type. Wills et al. [17] reported five of these compounds with M = La, Sm, Dy, Er and Yb and showed that they are isomorphous with cuspidine Ca,SisO,Fs, a monoclinic calcium fluorosilicate the crystalline structure of which is well known [ 181. The existence of these oxynitrides was confirmed by Marchand et al. [ 191, although they assigned to them slightly different lattice constants (see Table 3). These authors also showed that phases with M = Nd, Gd and Ho display analogous features; our own work has served to bring Tb within this category
[201. This paper tries to complete the picture by examining the products of the reaction between Si3N4, SiOs and MsOs when M = Ce, Pr, Tm or Lu. Specimens were prepared using amorphous silica and an Si3N4 powder consisting of about 90% and 10% of the (Yand 0 modifications, respectively (Starck). Tm and Lu oxides (Fluka) of at least 99.9% purity were employed. Pr,Os was obtained by the reduction of Pr,O, in a hydrogen current at 650 “C [21, 221, while CesOs was prepared from CeO, by heating at 1250 “C in the presence of metallic niobium in vacuum-sealed containers. Owing to the relative oxygen tension values in the phases [ 231, Ce02 is reduced from Cen’ to Gem with partial simultaneous oxidation of the Nb. The mixtures were ground to give a grain size of less than 1 pm, compacted at 3000 kg cmp2, placed in graphite containers coated with BN to prevent reactions and heated at 1500 “C in a nitrogen atmosphere for 8 h. Preliminary tests were made to ensure that equilibrium conditions were reached in this way. The samples were then cooled in the oven, finely ground and subjected to X-ray analysis (powder method, Cu K, radiation with h = 1.5418 a). The X-ray results showed that the four oxides gave rise to M4Si207N2 oxynitrides with closely similar powder patterns, especially in the cases of Ce and Pr and of Tm and Lu, respectively. For this reason, the values of the inter-planar distances and the relative intensities of the main X-ray reflections are listed separately for the two pairs in Tables 1 and 2. Once again, the monoclinic cells obtained were comparable with that of cuspidine. The lattice constant values are as follows Ce4Si207N2
a = 7.949 A
b = 10.874 a
c = 11.066 A
fl = 110.57”
Pr,Si20,N2
a = 7.875 /$
b = 10.808 a
c = 11.016 a
fl = 110.65”
Tm,Si20,N2
a = 7.492 A
b = 10.351 A
c = 10.792 A
Ir = 111.24”
195 TABLE
1
Powder
X-ray diffraction
hkl
110
and Pr@2O,Nz Pr&&07N2
Ce.@20,N2 &bs
020 201 I.20 210 221 130 310 122 022 321 312 230 320 322 103 402 212 412 132 142 512 423 510 204
data for Ce4Si20,N2
(A)
7.48 5.433 5.187 4.813 4.667 3.748 3.421 3.286 3.206 3.066 3.033 3.019 2.966 2.912 2.717 2.627 2.597 2.540 2.522 2.383 2.243 2.119 2.056 2.034 1.987
Lu4Siz07N2
dmc(h
I/lo
dabs 6%
he
7.50 5.431 5.192 4.814 4.677 3.755 3.421 3.291 3.207 3.070 3.037 3.017 2.970 2.915 2.719 2.630 2.596 2.544 2.525 2.384 2.243 2.120 2.058 2.035 1.986
10 3 ‘2 2 30 8 5 35 100 20 25 25 50 60 10 10 20 5 5 5 10 15 5 20 25
7.443 5.400 5.163 4.766 4.652 3.742 3.398 3.271 3.186 3.043 3.017 3.003 2.953 2.900 2.699 2.605 2.586 2.520 2.514 2.364 2.221 2.109 2.044 2.026 1.972
7.459 5.404 5.165 4.786 4.660 3.134 3.401 3.275 3.182 3.044 3.022 2.999 2.953 2.897 2.703 2.606 2.582 2.518 2.512 2.366 2.228 2.110 2.045 2.025 1.969
a = 7.375 A
b = 10.315 A
c = 10.765 A
(A)
I& 10 3 3 4 20 10 5 30 100 15 25 25 40 70 8 15 12 5 5 8 7 20 15 20 25
,!I= 111.30”
Lastly, it should be mentioned that numerous attempts were made to prepare a phase with M = Eu by varying, within wide limits, the temperature range and the composition of the gas in contact with the solids. Variously coloured solids with poorly reproducible X-ray patterns were obtained but the presence, even partial, of a phase with a powder pattern similar to those given by the other oxides, was never observed. This divergent behaviour of EusOs may be due to its well-known inability to maintain the oxidation number three at high temperatures. With the exception of Eu, therefore, the modalities for the preparation and crystallographic classification of the entire series of silicon lanthanide oxynitrides M,Si20,N, have now been established. The full set of X-ray data is summarized in Table 3.
196 TABLE
2
Powder
X-ray
diffraction
hkl
data for TmqSi207N2
and Lu&i207N2 LqSi207N2
Tm&li207N2 d&s
110 020 201 120 210 221 130 310 122 312 022 112 230 320 401 322 312 140 331 232 410 420 222 113 312 500 501
(A)
dcalc (A)
z/lo
dabs (A)
dd,
(A)
z/zo
7.201 5.163 5,029 4.600 4.525 3.606 3.259 3.188 3.031
7.214 5.176 5.037 4.602 4.524 3.610 3.264 3.190 3.034
10 2 2 5 20 4 3 20 100
2.891 2.868 2.845 2.813 2.701 2.616 2.584 2.507 2.483
2.894 2.864 2.845 2.814 2.698 2.613 2.584 2.506 2.481
20 15 30 50 3 25 10 20 15
7.178 5.157 5.006 4.571 4.512 3.584 3.238 3.182 3.001 2.879 2.855 2.827
7.191 5.158 5.009 4.587 4.510 3.593 3.253 3.180 2.999 2.883 2.859 2.827
10 2 2 2 20 2 2 15 100 15 12 30
2.440 2.259 2.235 2.064 4.028 2.015 1.975
2.444 2.262 2.235 2.058 2.032 2.012 1.975
4 7 5 8 10 10 15
2.806 2.693 2.599 2.566 2.499 2.474 2.449 2.433 2.253 2.215 2.037 2.012 2.004 1.971
2.805 2.691 2.595 2.568 2.498 2.472 2.448 2.436 2.255 2.213 2.035 2.013 2.006 1.969
40 5 60 8 10 10 15 4 5 10 15 5 5 8
References 1 Handbook
of Electronics Materials, Vol. 3: Silicon Nitride for Microelectric J. T. Milek (ed.), Plenum Press, New York, 1971. J. F. Lynch (ed.), Engineering Properties of Selected Ceramic Materials, American Ceramic Society, Columbus, Ohio, 1966. D. J. Godfrey, Use of Ceramics in High Temperature Engineering, Met. Mater., 2 (1968) 305. N. L. Parr, Engineer, 222 (1966) 18. A. F. McLean, Am. Ceram. Sot. Bull., 52 (1973) 464,482. K. H. Jack, Trans. Br. Ceram. Sot., 72 (1973) 376. D. Hardie and K. H. Jack, Nature ~~ondo~~, 180 (1957) 332. D. S. Thompson and P. L. Platt, in G. H. Stewart (ed.), Science of Ceramics, Vol. 3, Academic Press, New York, 1967, p. 33. C. G. Deeley, J. M. Herbert and N. C. Moore, Powder Metall., 8 (1961) 145. P. Drew and M. H. Lewis, J. Mater. Sci., 9 (1974) 261. D. W. Richerson,Am. Cemm. Sot. Bull., 52 (1973) 560.
Applications, 2 3 4 5 6
7 8 9 10 11
197
TABLE Lattice
3 constants
for silicon ianthanide
oxynitrides
of composition
M&,&N~
Gxynitride
Ref.
e (8)
b (A)
c (8)
P
La4SizOTNz
17 19
15.538 8.03
10.437 10.99
23.948 11.05
113.53 110.1
7.949
10.874
11.066
110.57
Ce_&&&N2 Pr,Si,U,N,
7.875
10.808
11.016
110.65
Nd,Si$&Nz
19
7.839
10.737
11.022
110.64
Sm&lizG,N,
7.761 7.75
10.292 10.60 -
11.965 10.99
Eu4Siz0,Nz
17 19 -
113.69 111.1 -
Gd4Si&Nz
19
7.66
10.52
10.93
111.0
Tb$i&N2
20
7.640
10.156
11.033
108.53
Hy&$?&N2
17
7.527
10.052
11.053
108.59
Ho4Si207N2
19
7.57
10.46
10.81
111.0
Er&XzG~Nz
17 19
7.528 7.53
10.100 10.40
10.987 10.80
108.72 Ill.1
7.492
10.351
10.792
111.24
7,519 7.47
10.046 10.32
11.071 10.77
109.15 111.3
7.375
10.315
10.765
111.30
Tm4Si207N2 Yb4Si207NZ Lu4Si2G7N2 _
17 19
-
12 F. F. Lange and J. L. Iskae, Ceramics for High Performance Applicafions, 13 14 15 16 17 18 19 20 21 22 23
Brook Hill, Chestnut Hill, Mass., 1974, p. 223. G. E. Gazza,J. Am. Ceram. Sot., 56 (1973) 602. I. C. Huseby and G. Petzow, PotLlder Mefall. Inf., 6 (1974) 17. K. S. Mazdiyasni and C. M. Cooke, J. Am. Ceram. Stlc., 57 (1974) 536. R. R. Wills, S. Halmquist, J. M. Wimmer and J. A. Cunningham, J. &fafer. Sci., f f (1976) 1305. R. R. Wills, R. W. Stewart, J. A. Cunningham and J. M. Wimmer, J. Mater. Sci., 11 (1976) 749. R. W. G. Wickoff, Cryst. Strucf. Commun., 4 (1968) 239. R. Marchand, A. Jayaweera, P. Verdier and J. Lang, C. R. Acad. Sci., S&r. C, 283 (1916) 675. M. Montorsi and P. Appendino, to be published in Am. Cerum. Sot. BuE. R. E. Ferguson, E. Guth and L. Eyring, J. Am. Chem. Sot., 76 (1954) 3890, C. T. Stubblefield, E. Eick and L. Eyring, cl; Am. Chem. Sot., 78 (1956) 3877, J. F. Elliatt and H. Gleiser, Thermochemistry for Steelmaking, Addison-Wesley, Reading, Mass., 1963.