EAR'I[~I AND PLANETARY SCIENCE LETTERS 5 (1969) 245-250. NORTIt-IIOLLAND PUBLISIIING COMP., AMSTERDAM

H I G H P R E S S U R E T R A N S F O R M A T I O N S O F SPI N E L S O)

A. E. RINGWOOD Department of Geophysics and Geochemistry.

Australian National University, Canberra. Australia

and

A. F. REID Divi~lion of Mineral Chemistry. C.S.LR. 0..

Melbourne. Australia

Received 18 November 1968

The effects of high pressures (up to 200 kb) and high temperatures (~ 1000°C) upon 36 sl,inels haY.; been inves- tigated in an attempt to define the principal patterns of spinel transformations. Fifteen new transformations were found. Among A2÷B3÷O, l spinels, the most common transformation mode is complete dispro~,ortionation into oxide m xtures A +B In eontrasl a 2 a* • i . ' :, O 205. ', :, mon~, A2"B 04 spmels, the most common transformatiors are either to denser single A~*Ba*Oa phases or to mixtures of A2*O (rocksalt) + A2"Ba*O a (ilmenite) type phases, l'wo ease~ of complese disproportionation into AO + B(3,2 (rutile) were ob';erved. The bearing of these obsel'vations u~on the probable trans- Ibrmation mode of Mg2SiO4 (spinel-like phase) in the Earth's mantle is discussed.

I. lnt~lroduction

Th,e mineral olivine is an imporrtant constituent o f the Upper Mantle. It is known that common olivines transform to denser spinel or spind-ralated structures at high pressures and it is probable that this transfor- mation has an important effect on the properties of the mantle at depths of about 400, km. e.~. [1,2], Accordingly, it is of interest to stttdy the effects of hig~ pressure nn spinels, since further hight pressure transformations of spinels deeper in the mantle would be of ,:onsiderable geophysical importance [I ,3]. This paper presents the first results of a series of investiga- tions aimed at elucidating some of the systematic crystal chemical and thermodynamic factors gover- ning spinel transformations. In the present investiga- tion, emphasis is laid upon a particular class of trans- formations i~a which spinels disproportionate cora- pletely into their constituent oxides. We also present preliminary results upon olther types of spinel trans- forma lions which we have observed. These will he discussed in greater detail in a subsequent paper [4].

Finally. we present the results of sotne experiments upon AI2GeO 5 . Although t]~is compound does not possess a spinel structure, the result,~; are relevant to the more general topic eonsldered herein,

It has been suggested by ~everal workers (e.g. refs. [5 -10] ) that under the higlr~ pressures existing in the Lower Mantle, silicate m;,nerals may actually dispro- portionate into physical mi;~,tures of' their constituent oxides - principally SiO 2 (;s~ stishovite), MgO, FeO, CaO and AI20 3. Such reactions are :heoretically possible if the density of thc~ oxide mixture is greater than that of the mineral or c, ompound. The free energy change AG in these reactions is equal to /`G 0 + .fP/`u dP where AG O is the fr~e energy of for- mation of the compound farom its constituent oxides at atraospheri~ pressure, arid AVp is the difference i:t molar volume at pressure F i)etweet~ the compound and its isocherfnieal oxide n'~ixture. 'l.he condition for disproportionalion is for ~3U to be zero, hence .f~ AodP= --/,G 0. If we eun,~ider Av to be constant (a good approximation in rll ~ny cases)PA~ = -AG n and hence P= -AGo/AV. I~'I my calculations; of the

246 A,E.RINGWOOD and A.F.REID

pressures at which magnesium silicates might be ex- pected to disproportionate into oxide components have been carried out [3,6,7,9,10-13]. The calculated pressures are in the vicinity of 200-300 kb at about 1000-2000°C.

?dthough such reactions are formally possible, and the relevant equilibria are readiliy calculated, it is de- bated whether they actually oc~:ur in the mantle. Ringwood [1] has argued that silicates are more likely to transform to new dense binary and ternary polynlorphs possessing densities similar to, or greater than the oxide mixtures. Such transformations would bypass the oxide-disproportionation step, rendering ~he latter equilibria metastable. One of the objectives of the present investigation was to obtain a more gen- er~il understanding of the conditions under which oxide disproportionation reactions might, or migl~t not OCCUr.

2. Experimental

Single phase starting materials were prepared by intimately mixing the oxide components in their correct proportion!;, pelletizing, and beating at 10(]0-1500°C for several hotlrs. In most cases, heating, was carried out in air. llowe.ver, the prep- ar~tions of FeAI~O4. MnAlzOa. Mn~.SnO4, FezGeOa and FezSiOa were carried oat in a non-oxidizing atmosphesu. Tile Ibllowing phases were prepared utilizing where possible reac- tion conditions already published:

MnAI204, FeAI204, NiAI204, CoAI2Oa, ZnAI204, MgA12Oa, C.oGa2Oa, NiGa~Oa, ZnGaaO4. MgCr2On, MgFea04, ZnFeaOa FezOn, CdFe204, CdCr2Oa, Mg2SiO4. Co2SiO4, FeaSiO4, Ni2SiOa, Mg~GeO4, CozGeO~ Fe~GeO4, Ni2GeO~,, Zn2GeOa, Mg2TiOa, MgZnTiO4, Z:~2Ti<)a. Mn2TiO4, Mn2SnO4, MgaSnO4, Co25nOa. Zn2SnO4, MIIsOa. LiFeTiO4, LiAIGeO4, LiAISiOa.

All thl2se phasL~ erystMlize in spinel or spinel-like structures at atmospheric pressures, oz can be converted into the!;e structures at hT~h pressures. In addition, we prepared tkl2GeO s and At2SiOs pha~s. Al~er preparation, each of the samples was checked for homogeneity by X-ray diffraction arm opti- cal exaralnation in immersion liquids, and reground antd re- hea~.ed, if necessary, until homogeneity had been obtained.

Samples so prepared were subjected to controlled high pressure at about 1000°C for 3-5 rain in a Bridgman-anvil apparatus equipped with an in~.emai heater [ 14]. Most runs were carried out at about 120 kb. However, in a few cases, runs were c:~rb:d out at higher pressures, up to 200 kb. In the experiments on AI2GeOs, ~he samples were also !;ubjec end to 15 and 35 kb, 120Oc~C in a piston-cylinder appaxae~s [ 151. Alter completion of ea,.'h run, the sample was quenched Ull- der pressure, removed h'om the apparatus, and examined by microscope and X-ray diffraction m~.'thods.

3. Remits

3.1. A2+B~3*O4 type spinals The spinels MnAI204, FeAI204 and NiAI204 ~,vere

observed to di:;proportionate completely into their oxide components in runs at 120 kh, whilst COA120¢ was about 70% disproportionated. For the reacti!on

AB204 (spine[) -~ An (rocksalt) + 13203 (corundum)

the inere:as¢ in density is about 8-1G%. This arb~es largely because of the increase in averagL. • metal-.oxygen co-ordination from 4, 6, 6 in the spinels to 6, 6, 6 in the oxide mixtures.

The spinel!~ ZnAI204, MgAI204, MgCr204, CoGa204, NiGa20 , , ZnGa204 failed to transform. Althoagh the free energies of formation of all the above spirr, els are not known, Navrotsky and Kleppa [16] have ,~ecently provided a considerable amount of data on the enthal- pies of formation of spinels from their eompe.nent oxides. The enthalpies of ZnAI204, MgAI20 ~ , MgCr204, ZnGa204, NiGa204 and CoGa204 are with- in the range of the enthalpies of formation of MnAl204, FeAI2G a, CoAI20 a and NiAI204 from the e:~mponen~t oxides, and it appears probable that this is ~lso the case for theix retative free energies of formation. Accord- ingly, we would expect the entire group to dispru- portion,ate under the pressures which sufficed to dis- proportionate some of their number. Theit failure to do so is probably to be attributed to kinetic dift3=ul- ties. This is iin accord wJith the observation th~.t CoAl204 displayed only par',ial dispropor tionation when subjected to conditions ueder which MnAI~O4, FeAI204 and NiAI204 disproportionated completely.

The results on Fe30, l, MgFe20 a and ZnFe204 were inconclusive because elf partial reductions of Fe 3. to Fe 2+ during the runs. Tiffs occurred de.cpite the fact that Pt - 4 0 % Rh heating strips were u,!.ed. The partial reduction was indicated by the presence of substantial amounts of Fee , (Fe, blg)O and (Zn, F;e)O solid solu- tions in the: run products without any of the comple- mentary Fe203. The samples of CdC/204 and CdFe204, however, appeared to transform to flew single phases

The disl?roportionx.qon reaetion.,J observed provide the first examples of this class of tr;msformations

t which had previously been predict,,M on thermo- dynamic grounds. Looking at the results from a slightly different viewpoint, one might also regard them as examples of another general class of reaction: the disproportionation of a spinel into a rock-salt

HIGH PRESSURE TRANSFORMATIONS OF SPINELS (1) 247

type oxide plus an ilmenite or corundum type oxide, as di~:;eussed later in this paper.

3.2. Silicate and g,~rmanate spinels "[he silicate o]li~ines Fe2SiO4, Ni2SiO 4 and Co2SiO 4

are lmown to transform to spinel struetun~s at 20 to 70 kb, 700°C [1 ], whilst pure Mg2SiO, 1 t~ansforms to a structure probably related to a spinel at 145 kb, 10000C I2]. All of these olivines have bean subjected to pressures up to 150 kb (200 kb for Mg2SiO4) but no further transformations to phases other than spi~ads have been observed.

The germanate spinels Ni2GeO 4, Co2GeO4, Fe2GeO 4 and Zn2GeO 4 have been subjected to 150 l~;b at 1000°C, whilst Mg2GeO 4 Was subjected to 200 kb. No further transformations were observed. The related compound Mn2GeO 4 which possesses the olivine structure at low pl:essures, was found by Wadsley, Reid and R~ngwood ['7] to transform directly to the strontium plumbate strueture. A spinel modification of Mn2GeO 4 has not been found.

3.3. Titanate spinels Akimoto and Syono [18] observed that the spinels

Mg2TiO 4, Fe2TiO 4 and Co2TiO 4 disproportionated at pressure.'; up to 40 kb, 800-1500°C into mixtures of MO (rocksalt) plus MTiO 3 (ilmenite str~cture). We have found that MgZnTiO 4 spinel also disproportio- nares under pressure in this manner to a mixture of ZnO (rocksalt structure, 33) plus MgTiO 3. The com- position Ni2TiO 4 is not stable as a spinel and crystal- lizes to a mixture of NiO plus NiTiO 3 (ilmanite) at atmospheric pressure.

On the other hand, the spinels Mn2TiO 4 and Zn2TiO~ ' did not disproportionate m'der pr~.'ssure, but instead transformed into new denser sirtgle phases. The X-ray diffractS~on patterns of these phases display a marked resemblance to patterns of high pressure Mn2GeO 4 and Ca2SnO 4 wb.[eh possess the strontium p]umbate structure and it appears likely that these phases possess a similar or related structure. A detailed intvestigation of this subject is in progress [4]. The tetragonal spinel Mn304 0aausmanite) transformed to what appeared to be a single new phase in a run at 120 kb. This phase may possibly be related to high pressure Mn~TiO~, and possess a compl~sition Mn2+lvln4+O4. A11ternatively, if the corrlposigon of the'high pressure phase is Mn2+Mn~+O~l, it may be

related to the high pressure ~,hase of magnetite Fe2~'Fe~+O4, which is form~,~l under shock wave con- ditions [23] . Further detailed studies are it: pro- gross [41.

3.4. Stannate spinels The spinels Mg2SnO 4 aad Co2SnO 4 dispropor-

tionated comp!etely into m~J.;tures of their oxide components, MO (roekssll:) plus SnO 2 (futile), in runs at 120 kb, 1000°C. Ni2Sn,04 is unstable at zero pres- sure and 1200°C, this composition crystallizing as a mi::ture of NiO and SnO:!. On the other hand, Zn2SnO 4 and Mn2SnO 4 spinels transformed to single new phases possessing powd~r patterns very similar to those of Zn2TiO 4 and ~/I n 2TIO4 . It is probable that all these phases are isostruc:tural [,I 1 .

3.5. LiFe Ti0 4, LiA IG eO 4 ar~'d LiA ? SiO 4 LiFeTiO 4 crystallizes a!~ a spineI at atmospheric

pressure [31]. At 120 kb thrs spim.q was found to transform completely into a single new phase. Studies on the structure of this ph;~s', are in progress [4].

LiAIGeO 4 possesses a phenacite structure at atmospheric pressure [30]. Gaines et ai. [30] and Rooymans [ 19] observed that Li.~.lGeO 4 phenacite transformed to the spinel ~!alucture (a 0 = 7.996 A) in the pressure range 4 -12 k:b ',tt 3O0-1000*C. In our experiments at 120 kb, spitnel was the most abtr:ndant phase present, although the presence of a set of non- spinel lines on the X-ray diftraction photograph may be indicative of partial tr~:n'fformation into a denser phase.

LiAISiO 4 (eucryptite) al!~,) possesses a phenacite structure at atmospheric pressure below about 900°C [20]. At 20 kb, 1000°C, Rcoymans I19] observed that l.iAISiO 4 disproportianated t~) ~', mixture of LiA1Si206 (spodumene) ÷ I.~AIO2 . In a run at 170 kb, 1000°C, we have observed 'flint LiAISiO 4 glass crys- tallized mainly to a spinel phase p~ssessing a latlice parameter of 7.86 +- 0.01 ~. ;md a :t:efractive index of about 1.71. Transformatio~a was ir complete and a substantial amount of the Io ~¢er pressure (LiAISi206 + LiAlO2) assemblage rer~,ained. In runs at lower pressures, down to 120 kb, smaller amounts of the spinel phase were present

It appears probable th~.t the sy~thesized phase is LiA1SiO 4 spinel, isotypie ~vith LiA.1GeC- 4 spinel. This furnishes another exampl~ o F the z elationship whereby

248 A.E.RINGWOOD and A.F.REI I~

germanates display similar types of transformations to their silicate isotypas but at lower pressures [ 1 ] . The principal difference between phase relationships in the two systems is the intervention of the (LiA1Si206 + LiAIO2) field between the phenacite and spinel forms ff LiAlSiO 4. The analogous assemblage is not oh- m;erred to in~tervene between the LiAlGeO 4 phenacite and spinel fields.

3.6. Aluminogermanates and aluminosfficates The polymorphisln of AI2SiO 5 has been extensively

discussed in the literature. In runs up to 150 kb, ]000°C, kyanite appears stable, and no new transfor- mations have been discovered. Although germanium- mullite (2GeO2.3A1203) is the only stable binary com- pound in the zero pressure GeO2-Al203 system [32], the composition AI2GeO 5, presinteted at 1400aC, was observed to transform to the kyanite (A12$iO5) struc- ture at 15 k5, 1200°C. The kyanite form of Al2GeO5 thus occurs within the field of stability of AI2SiO 5 sillimanite [21 ] , a further example of the model rela- tivnship [t] wh~ereby germanates display .~dgh pres- sure forms similar to those of correspondi~'lg silicates, but at lower pressures. In runs above about 70 kb, Ai2GeO 5 (kyaaite) was found to be unstable, and dis- proportionated completely into the cansti;.uent oxides Al203 plus GeO 2 (futile).

4. Discussion

Spine[s may 'transform under high pressures either by direct conversion to a single denser phase, or by disproportionation. For A2+B32+O 4 spinels, dispropor- tionation will produce a mixture of AO (rocksalt) + B20 3 (corundum), while for A2+B4+O4 spinels, either 2AO (rocksalt) + BO 2 (rutile) or AO (rocksalt) + ABO 3 (ilmenito or perovskite) can be the products. Which of" the transf¢rmation modes will be displayed by a given spinel is stron#y influenced by its free energy of formation from constituent c~xides. Where this is relatively sm',dl, a correspondin~Jy small pros- sore may be sufficient to cause dissociation according to the relationship P = --/AGo/A,. On the other hand, where -AG O is large (i.e. a strong compound forming tendency exists between the oxides) a correspondingly higher pressure will be required to cause dissociation into oxides, and transformation if it occurs, is more

likely to result in the formation of a new single phase, providing this is stereoebemically possible.

Fern' of the A2+B3+O4 spinels which we investigat- ed were fi)und to dissociate into constituent simple odixas under high pressure. This behaviour is proba- bly a consequence of the rather small free energies of formation of these spinels, which, based mainly on enthalpy ,data [ 16], are probably in the range l - 1 2 kcal/mol. Under shock wave conditions, at pressures of several hundred kilobass, MgAI204 and Fe304 transform to phases which are substz.ntially denser than the isochemical mixed oxides [22,23]. It ap- pears that the oxide-disproportionation reactions which mi,!,,ht have been expected under equilibrium conditions have been bypassed, perhaps for kinetic reasons, and that transformation into single A2BO 4 phases possessing relatively close packed structures may have occurred. The observed transformations of CdCr204, CdFe204 and Mn304 spinels into new structures may be relevant to these shock transforma- tions.

We turn now to the A2+B4+O4 spinels. Ramberg [24,25] has discu~ssed the general systematic relation- ships which exist between the free energies of forma- tion of binary oxide compounds from their oxide components and the positions of the oxide forming elements in the periodic table. These relationships indicate that the free energies of formation of spinals from a given oxide AO (rocksalt structure) and a futile-type BO 2 oxide, where B is a group IV element, should become smaller in the order A2SiO2, A2GeO 4, A2TiO4, A2SnO 4. The behaviour of spinels at high pressure generally reflec~ts this trend. All of the silicate and germ~nate st)tools failed to transform, whereas the stanmlte spinels Mg2SnO 4 and Co2SnO 4 dispro- portionated completely into oxides. It was not possi- ble to synl:hesize the sp:inel Ni2SnO4, which crystal- lized to a mixture of NiO and SnO 2 at atmospheric ptessure, 12009C.

The behaviour of the titanatas was generally inter- mediate between those of the silicates and germanates on the one hand, and tllle stannates on the other. While Zn2'HO4, Mn2TiO 4 a:ad LiFeTiO 4 transformed to new single phases, M~2TiO4, Co2TiO 4 and Fe2TiG 4 1II8] and MgZnTiO 4 behave in a truly intermediate raanner by exhibiting pe~tial disproportionation into mixturas of AO (rocksal0 + ATiO 3 (ilmenite) where A = Mg, Fe, Co, Zn. The densities of these assemblages

are generally similar to those of the isochemical mixed oxide:s, however the energy required for partial dis- proportional[on is much smaller than for complete disproportionafion "~nto oxides. Thus, i.n cases where the formation of a sufficiently dense binary com- pound ABe 3 is possible on stereochemical grounds, A2BO 4 compounds, unless they transfi3rm to a denser single phase, are more likely to disproportionate initially into an (An + ABe3) assembhtgc than into the oxides. Further transfoEmations at higher pres- sures will depend on the relative densil~ies of the binary compounds as compared with the cor~;tituent oxides. The hehaviours of Ca2GaO 4 and Cd2GeO 4 [26] illus- trate two of the above alternatives. These compounds possess olivine sl!ructures at atmospheric pressure, and the free energy of formation of Ca2GeO 4 is probab,ly much higher than that of Cd2GeO 4 [24,25]. Under pressure, Ca2GeO 4 transforms to a single phase pos- sessing the K2NiF 4 structure, whereas Cd2GeO 4 dis- plays disproportional[on into a mixture of CdO + CdGeO 3 perovskite [26].

The above considerations may be applied 1o the probable behaviour of the spinal-like form of Mg2SiO 4 at very high pressures. It is known [23] that Mg2SiO 4 transforms under shock pressures to a phase or phase assemblage which possesses very n¢,:ly the density of an isochemical mixture of 2MgO + sin 2 (stishovitO. An assemblage of MgO + MgSiO 3 (ilmenite) or of a form of Mg2SiO 4 possessing the strontium plurnbalte structure would possess similar densities [I ,26]. Per- haps other single phases with similar densities have yet to be discovered. Studies of the system MgGeO 3 - MgSiO 2 at high pressures [27, 28] have shown that an ilraenite form of MgSiO 3 will probably become stable between 200 and 300 kb. In the Earth's man- tie, such a phase would be further stabilized by solid solutiton of AI203 and Cr203. From the previous dis- cuss[on, in view of the relatively high free energy of formation of.Mg2SiO4, and the probable existence of MgSiO 3 ilmenite as a stable phase at high pressure, we woul,a expect that deep in the Ealrth's mantle, the spinel-like phase lVlg2SiO 4 is much more likely to transform ultimately either to a mixture of MgSiO 3 (ilmenite) 4 MgO or into a single dense phase such as the strontium plumbate type, than to dissociate com- pletelty into an oxide mixture of 2MgO + sin 2 (st[she- vile), "l'bis inference is strongly supported by the experimental evidence. Of ten high pressure transfer-

HIGH PKESSURE TRAN::;FORMATIONS OF SPINELS (1) 249

malleus now known to occur in A~+B4+O4 spine|s, only two have been to the rr ixed oxides An + BO 2, and these two spinals we~'e probably characterized by the lowest free energies of fixmation of those studied. Of the remaining eigllt (runs ['e'mations four have been to a single phase and fern" to ilmenite-rocksalt type mixtures.

The free ener~ of formatinn of AI2SiO 5 (kyanite) is on'~y about 2 kcal/mol at 29g°K [22] and it appears probable that the free energ:,.t of fo;rmation of Al2GeO:i will be even smaller [24,25]. A,:;colldingly, the com- plete disproportionation und~:r pressure of AI2GeO 5 (kyanite) into the much denser ( ~ 11%) assemblage A[203 + Gee 2 (rutile) was not ,mexpected.

Acknowledgements

The assistance of Mr. A~an Majolr in the experi- mental investigations is gratt:ft~lly acknowledged.

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