- Email: [email protected]
REDUCED ALKALINE EARTH TANTALATES
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3 N 7 pp. 8 Q1 E S L i t USA. All rights reserved 0025-5408/97
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$17.00 + .00
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BellLaboratories,LucentTechnologies 700MountainAvenue,MurrayHill,NJ 07974USA (Refereed) (ReceivedFebruary10, 1997;AcceptedFebruary18, 1997)
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Among reduced ternary transition metal oxides, tantalates are rare, even though interestingelectronicpropertiesmight be expected.Here, we report on new reduced alkaline earth tantalates Ca2Ta206F,Sr+,Ta5015,and Ba2Ta,503zgrown under vacuum conditionsand high temperatures.These reduced ternary tantalates are isostructural with known reduced ternary niobates,indicatinga strong similarityof the crystal chemistrybetween the tWOSyStemS.0 1 E L l
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KEYWORDS: A. fluorides, A. oxides, C. X-ray diffraction, D. crystal structures I
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Reduced oxides of titanium,vanadium,niobium,molybdenum,and tungsten are common amonginterestingelectronicmaterials,displayingpropertiesthat range frommetal insulator transitions through superconductivity.It is surprising,therefore, that reduced oxides of tantalumare only rarely found.Recently,a smallnumberof reducedternaryzirconiumand tantalum oxides have been reported [1,2], synthesizedvia a high temperaturesealed tube method. Here, we report some of our initial expirations of the synthesis and crystal structures of reduced ternary tantalum oxides synthesizedin a similar manner. We have foundthat they formstructuralanalogsto someknownreducedternaryniobiumoxides. S
Starting materials for the synthesis were Ta metal powder (<200 mesh, 99.99Y0purity), Ta*05powder (99.999%purity), and dry SrCl,, BaCl~,and CaF, powders(99.9% purity). 881
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Vol.32,No.7
TABLE 1 CrystallographicData ca2Ta206F group Crystalsize(mm) a (A)(at 23”C) c (A) V(A3) z D. (g/cm3) ~ (MCIKct,cm-’) Minimumtransmission Maximumtmnsmission Radiation Scantype Reelections measured Independentreelections Observedr ( 2.5cr(l)) Refinedparameters S
R R ( E
Sr3Ta5015
F 0.10
P
x 0.20x 0.15 0 10.421(2)
1131.69(3)
8 6.539 4.0 0.019 0.080 Mo Kc( 42;1 147 142
l
Q
R
0.24 12.282(6) 3.864(15) 582.9(4) 2 7.936 102.2 0.041 0.22 Cu Ka 2e/w 1972 372 363 0
X
2
0.084 0.079
0
0.10x 0.10x 0.03 7.773(2) 35.414(9) 1853(1) 3 9.41 6.86 0.033 0.20 Mo Ka 5;9 1828 e 1409 50 0.064 / 0.044
2
0.048 0.051
l
13azTal@32
x0
1
Powders were mixed in a variety of ratios in a nitrogen-filledglove bag. Crystals of the compoundsdescribedin this report,BazTa@Jz,SrJ-.Ta@ls,ad CazTa@bF,were obtained fi-ommixturesof 0.2 g of BaCIZ,SrClz,or CaFzwith 0 g of Ta@s + Ta metal, the latter two componentsmixed in the appropriateratio to yield the formulaTa@zg. The mixtures were heated in sealed tantalumtubes. Samplesin the Ta tubes were heated in a vacuum furnace to 1450”C,held for 40 h, and cooled to 1200°Cat 50°C/h before being cooled quicklyto ambienttemperature.The resultingmaterialswere very well crystallized,yielding singlecrystalssuitablefor structuralstudy. C
I
R
All crystals were measured at 23”C, using an Enraf-Nonius CAD-4 single crystal diffractometerwith graphite-monochromatizedMo Kct or Cu K(x radiation, controlled by the NRCCAD program [3]. All subsequentcalculationsused the NRCVAX program suite [4].A Gaussianintegrationabsorptioncorrectionwas appliedto all the measuredintensities. Latticeparameterswere determinedfromthe absolute20 valuesof at least24 reflectionsat high angles.Crystallographicresultsare givenin Tables1,2, and 3. Ca2Ta~OdF. This phase is isostructuralto the known CazTazOTpyrochlore-typephase where Ta is fully oxidized(Ta+s)(see Fig. 1). Here, the additionof fluorineprovidesfor a reduced tantalate with a dark bluish color, compared to white for the oxidized phase.
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REDUCED TANTALATES
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TABLE2 AtomicPositionalParametersand IsotropicTemperatureFactors x
z
Y
O
B (
C T C o F
o 1 1
o
0
1
0
5/8
1/8
Srl
o
0
1/8 sl”3Ta5015 0
S T
0 o
0
0
Ta2 01 02 03 04 05
1/2 1/2 0.07659(20) 0.21119(20) 1/2 l/~ o 0 0.276(4) 0.776 1/2 0.208(4) O 0.078(4) 0.011(3) 0.338(3) 1/2 0.140(4) 0.066(4) 1/2 Ba*Tal@Jz
B T T T T 0 0 0 0 0 0
o o 0 o 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 -
B i t
m
o t
0
p
a
0
8
o t
t
0 2 1
2.2(3) 3.3(4)
0.83(4)
5
1 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0
,
1.56(15) 1.54(11) lo(5) 2.7(11) 4.2(11) 2.5(7) 3.5(9)
.
.
.
.
.
. . .
8
. . .
e
. . . 0 r
Oxyfluoridesof this type are well representedby the mineral microliteof the pyrochlore. microlite series CazTazOT[5] and several similar phases reported of the general formula A1+B2+Ta@,F [6]. In these phases, however, the tantalum is always fullYoxidized. The fluorineintroducedinto the structureis not part of the Ta-O tlamework,but it is found in the large holes produced by the framework and is tetrahedrally coordinated by four Ca atomsat 2.256~. S X-ray powder diffractionindicatedthat SrJ-.Ta0s r possessesthe tetragonal tungstenbronze-type(TTB-type)structure(Fig.2). This is similarto the TTB type observed for the reduced niobate BaJNbsOls [ T C W ! P w chosen,correspondingto the reducedniobate.A refinementof the Sr occupancyparameters
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TABLE3 SelectedInteratomicDistances Ca2Ta206F Ta
–O
1.971(6)
Ca
(jx
–O -
2 2
6 2
S S
Sr2
Ba
T
r
–
2.75(5)
-05 -01 -02 -03
2.74(3) 3.033(9) 2.64(3) 3.03(4)
-
2.82(3)
–05
3.24(3)
4X 8x lx 2x 2x 4X 4x
–01
3.04(2)
Ba2Ta15032 3x Tal
– – – –
2 2 3 2 2 1 2 2 2
3 3 3 l l l l l
Tal
-01 -04 Ta2 -02 -03
T T
1.932(1) 1.96(4) 1,96(2) 1.933(2)
2x 4x 1X 2x
-
1.90(4)
lx
–05 -05
1.93(4) 1.97(4)
lx lx
-06
1.97(3)
6x
– -
1 2 2 2 1 1 2 1
3x 3x lx lx lx lx lx lx
showed a slightlyreduced Sr content,with a stoichiometryof Srz,~TasOIS(within 40’s of full occupancy).This vacancy on the Sr site shiftsthe overall oxidationstate of Ta from Ta+4’8towards Ta+4’88.Identical to the isostructural BaJNbsO 1s, there are two crystallographicallydifferentTaOboctahedrapresent,with one of them (T@ distorted in the sameway as in the niobate:the distancesrangingfrom 1.89to 1.98~, comparedto 1.93 to 1.99~ for Tal. B This structure is isostructuralwith the reducedaphase BaZNb@Jz [7]. It containsclustersof [Ta6012]06and groups of [ a i l a t ca The clusters show short intraclusterTa4-Ta4 distancesof 2.756 and 2.798 & indicating strongTa–Ta interactions.The clustersthemselvesare linkedvia anothersingleTa atom, at a distanceof Ta4–Ta3of 3.113 & forminglayer A. The adjacentlayers containgroups of edge-sharingTaOb-octahedra,forming the Ta@lJ SOUPS(layer B). Them the TcQ-Ta2 distancesare 3.181~, indicatingthe absenceof Ta–Tainteractionsand a valencestate of +5 for the Ta atom. As a consequence,the Ta–O distance is largest to the central oxygen common to all three octahedra.The third layer is formed by TaOboctahedra and barium, which is located in dodecahedralcavities (Ba-O distancesfrom 2.891 to 3.138 ~). The
Vol.3 N 7
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FIG. 1 View of CaZTa@sFpyrochlore-type st~cture< Large spheres indicate calcium; small spheres,fluorine. 4
FIG.2 View of tie Sr3-XTa@15 tetragonaltungstenbronze-typestructure.Large spheres indicate strontium.
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Vol. 32, No. 7
0 0 o
0 (
(a) 0
0
0
0
0
0
( FIG.3 (a) View of the C layer along the c axis of BaZTalsOJz. Large spheresindicatebarium. (b) View of the B layer along the c axis showingthe ITajOlj] cluster.(c) View of the A layer alongthe c axis,showingthe [T%O12]06 cluster.Smallspheresindicatethe linking tr@lum. overallstackingsequencesis then [CBAB].. The variouslayersare depictedin Figure3. In this phase,the Ta atomsbehaveidenticallyto Nb atomsin the [email protected] lattice constants as well all the positional parameters are virtually identical. This close relationshipalso suggeststhe likelyexistenceof a solidsolutionseriesof Baz(TaxNbl-.)@Jz with O< x <1. C
The phases described here have been presented previously [1,2]; however, only the tetragonaltungstenbronze-typephasesare accessiblein the literature[1].Here we reportthe strong similarity in the structural chemistry between reduced tantalates and reduced niobates. Most striking is the correspondencebetween BaZNblQz and BazTa@Jz, with nearly identicallatticeand positionalparametersand,therefore,virtuallyidenticaldistances. This is also the case for the other two types of structures,the tetragonaltungsten bronze (TTB) type and the pyrochloretype. Again,only small variationsin the lattice parameters are observed,and usuallyonly smallvariationsin the positionalparametersoccur. Sincethe TTB-type structure has channels along the c axis, it is not uncommonto find vacancy formation in the cation substructure occupying those channels. In the case of the
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pyrochlore/microlite-typefamily, that structure is known to accommodatequite a large number of differentelementsin an oxideor oxyfluorideanion lattice.The overallunit-cell dimensionsvary only slightly with the transition metal-oxygen distance and are usually found in the range of 10to 10.5~ [8]. Researchinto the electricaland magneticproperties of these and relatedphaseswillbe of considerableinterest. R 1. 2. 3. 4. 5. 6. 7.
C F a R Z C Mater. 7,373 (1995). C F a R Z Abstracts of Papers, V21O, 32-INOR, A N M N W D O 1 A C S Proc. Amer. Ctystallogr. Assoc. Ann. Meeting 1986, Y L P W a E G Hamilton, Canada, A N Y ( ( P E G Y L J C F L a P W J Appl. Cryst. 22,384 (1989). A B Ark. Kemi Mineral. Geol. 18A(21), 1 (1945) (1944 i p i 1 y EA a R R J Amer. Ceram. Soc. 45, 18-25 (1962). B H S S T S A F a J W Chem. Mater. 3, 528
E i i
a
r
u
(1991).
8
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Chemie a’erEra’e8, 186 (1933).
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3 N 7 pp. 8 Q1 E S L i t USA. All rights reserved 0025-5408/97
P
R
S
0
A
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e
$17.00 + .00
E
R
C
a
0
T
J
K
i
BellLaboratories,LucentTechnologies 700MountainAvenue,MurrayHill,NJ 07974USA (Refereed) (ReceivedFebruary10, 1997;AcceptedFebruary18, 1997)
A
B
Among reduced ternary transition metal oxides, tantalates are rare, even though interestingelectronicpropertiesmight be expected.Here, we report on new reduced alkaline earth tantalates Ca2Ta206F,Sr+,Ta5015,and Ba2Ta,503zgrown under vacuum conditionsand high temperatures.These reduced ternary tantalates are isostructural with known reduced ternary niobates,indicatinga strong similarityof the crystal chemistrybetween the tWOSyStemS.0 1 E L l
s
KEYWORDS: A. fluorides, A. oxides, C. X-ray diffraction, D. crystal structures I
N
Reduced oxides of titanium,vanadium,niobium,molybdenum,and tungsten are common amonginterestingelectronicmaterials,displayingpropertiesthat range frommetal insulator transitions through superconductivity.It is surprising,therefore, that reduced oxides of tantalumare only rarely found.Recently,a smallnumberof reducedternaryzirconiumand tantalum oxides have been reported [1,2], synthesizedvia a high temperaturesealed tube method. Here, we report some of our initial expirations of the synthesis and crystal structures of reduced ternary tantalum oxides synthesizedin a similar manner. We have foundthat they formstructuralanalogsto someknownreducedternaryniobiumoxides. S
Starting materials for the synthesis were Ta metal powder (<200 mesh, 99.99Y0purity), Ta*05powder (99.999%purity), and dry SrCl,, BaCl~,and CaF, powders(99.9% purity). 881
882
e a T. SIEGRIST
Vol.32,No.7
TABLE 1 CrystallographicData ca2Ta206F group Crystalsize(mm) a (A)(at 23”C) c (A) V(A3) z D. (g/cm3) ~ (MCIKct,cm-’) Minimumtransmission Maximumtmnsmission Radiation Scantype Reelections measured Independentreelections Observedr ( 2.5cr(l)) Refinedparameters S
R R ( E
Sr3Ta5015
F 0.10
P
x 0.20x 0.15 0 10.421(2)
1131.69(3)
8 6.539 4.0 0.019 0.080 Mo Kc( 42;1 147 142
l
Q
R
0.24 12.282(6) 3.864(15) 582.9(4) 2 7.936 102.2 0.041 0.22 Cu Ka 2e/w 1972 372 363 0
X
2
0.084 0.079
0
0.10x 0.10x 0.03 7.773(2) 35.414(9) 1853(1) 3 9.41 6.86 0.033 0.20 Mo Ka 5;9 1828 e 1409 50 0.064 / 0.044
2
0.048 0.051
l
13azTal@32
x0
1
Powders were mixed in a variety of ratios in a nitrogen-filledglove bag. Crystals of the compoundsdescribedin this report,BazTa@Jz,SrJ-.Ta@ls,ad CazTa@bF,were obtained fi-ommixturesof 0.2 g of BaCIZ,SrClz,or CaFzwith 0 g of Ta@s + Ta metal, the latter two componentsmixed in the appropriateratio to yield the formulaTa@zg. The mixtures were heated in sealed tantalumtubes. Samplesin the Ta tubes were heated in a vacuum furnace to 1450”C,held for 40 h, and cooled to 1200°Cat 50°C/h before being cooled quicklyto ambienttemperature.The resultingmaterialswere very well crystallized,yielding singlecrystalssuitablefor structuralstudy. C
I
R
All crystals were measured at 23”C, using an Enraf-Nonius CAD-4 single crystal diffractometerwith graphite-monochromatizedMo Kct or Cu K(x radiation, controlled by the NRCCAD program [3]. All subsequentcalculationsused the NRCVAX program suite [4].A Gaussianintegrationabsorptioncorrectionwas appliedto all the measuredintensities. Latticeparameterswere determinedfromthe absolute20 valuesof at least24 reflectionsat high angles.Crystallographicresultsare givenin Tables1,2, and 3. Ca2Ta~OdF. This phase is isostructuralto the known CazTazOTpyrochlore-typephase where Ta is fully oxidized(Ta+s)(see Fig. 1). Here, the additionof fluorineprovidesfor a reduced tantalate with a dark bluish color, compared to white for the oxidized phase.
N
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REDUCED TANTALATES
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TABLE2 AtomicPositionalParametersand IsotropicTemperatureFactors x
z
Y
O
B (
C T C o F
o 1 1
o
0
1
0
5/8
1/8
Srl
o
0
1/8 sl”3Ta5015 0
S T
0 o
0
0
Ta2 01 02 03 04 05
1/2 1/2 0.07659(20) 0.21119(20) 1/2 l/~ o 0 0.276(4) 0.776 1/2 0.208(4) O 0.078(4) 0.011(3) 0.338(3) 1/2 0.140(4) 0.066(4) 1/2 Ba*Tal@Jz
B T T T T 0 0 0 0 0 0
o o 0 o 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 -
B i t
m
o t
0
p
a
0
8
o t
t
0 2 1
2.2(3) 3.3(4)
0.83(4)
5
1 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0
,
1.56(15) 1.54(11) lo(5) 2.7(11) 4.2(11) 2.5(7) 3.5(9)
.
.
.
.
.
. . .
8
. . .
e
. . . 0 r
Oxyfluoridesof this type are well representedby the mineral microliteof the pyrochlore. microlite series CazTazOT[5] and several similar phases reported of the general formula A1+B2+Ta@,F [6]. In these phases, however, the tantalum is always fullYoxidized. The fluorineintroducedinto the structureis not part of the Ta-O tlamework,but it is found in the large holes produced by the framework and is tetrahedrally coordinated by four Ca atomsat 2.256~. S X-ray powder diffractionindicatedthat SrJ-.Ta0s r possessesthe tetragonal tungstenbronze-type(TTB-type)structure(Fig.2). This is similarto the TTB type observed for the reduced niobate BaJNbsOls [ T C W ! P w chosen,correspondingto the reducedniobate.A refinementof the Sr occupancyparameters
e
T SIEGRIST et al.
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TABLE3 SelectedInteratomicDistances Ca2Ta206F Ta
–O
1.971(6)
Ca
(jx
–O -
2 2
6 2
S S
Sr2
Ba
T
r
–
2.75(5)
-05 -01 -02 -03
2.74(3) 3.033(9) 2.64(3) 3.03(4)
-
2.82(3)
–05
3.24(3)
4X 8x lx 2x 2x 4X 4x
–01
3.04(2)
Ba2Ta15032 3x Tal
– – – –
2 2 3 2 2 1 2 2 2
3 3 3 l l l l l
Tal
-01 -04 Ta2 -02 -03
T T
1.932(1) 1.96(4) 1,96(2) 1.933(2)
2x 4x 1X 2x
-
1.90(4)
lx
–05 -05
1.93(4) 1.97(4)
lx lx
-06
1.97(3)
6x
– -
1 2 2 2 1 1 2 1
3x 3x lx lx lx lx lx lx
showed a slightlyreduced Sr content,with a stoichiometryof Srz,~TasOIS(within 40’s of full occupancy).This vacancy on the Sr site shiftsthe overall oxidationstate of Ta from Ta+4’8towards Ta+4’88.Identical to the isostructural BaJNbsO 1s, there are two crystallographicallydifferentTaOboctahedrapresent,with one of them (T@ distorted in the sameway as in the niobate:the distancesrangingfrom 1.89to 1.98~, comparedto 1.93 to 1.99~ for Tal. B This structure is isostructuralwith the reducedaphase BaZNb@Jz [7]. It containsclustersof [Ta6012]06and groups of [ a i l a t ca The clusters show short intraclusterTa4-Ta4 distancesof 2.756 and 2.798 & indicating strongTa–Ta interactions.The clustersthemselvesare linkedvia anothersingleTa atom, at a distanceof Ta4–Ta3of 3.113 & forminglayer A. The adjacentlayers containgroups of edge-sharingTaOb-octahedra,forming the Ta@lJ SOUPS(layer B). Them the TcQ-Ta2 distancesare 3.181~, indicatingthe absenceof Ta–Tainteractionsand a valencestate of +5 for the Ta atom. As a consequence,the Ta–O distance is largest to the central oxygen common to all three octahedra.The third layer is formed by TaOboctahedra and barium, which is located in dodecahedralcavities (Ba-O distancesfrom 2.891 to 3.138 ~). The
Vol.3 N 7
R
885
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FIG. 1 View of CaZTa@sFpyrochlore-type st~cture< Large spheres indicate calcium; small spheres,fluorine. 4
FIG.2 View of tie Sr3-XTa@15 tetragonaltungstenbronze-typestructure.Large spheres indicate strontium.
8
T SIEGRIST et af.
Vol. 32, No. 7
0 0 o
0 (
(a) 0
0
0
0
0
0
( FIG.3 (a) View of the C layer along the c axis of BaZTalsOJz. Large spheresindicatebarium. (b) View of the B layer along the c axis showingthe ITajOlj] cluster.(c) View of the A layer alongthe c axis,showingthe [T%O12]06 cluster.Smallspheresindicatethe linking tr@lum. overallstackingsequencesis then [CBAB].. The variouslayersare depictedin Figure3. In this phase,the Ta atomsbehaveidenticallyto Nb atomsin the [email protected] lattice constants as well all the positional parameters are virtually identical. This close relationshipalso suggeststhe likelyexistenceof a solidsolutionseriesof Baz(TaxNbl-.)@Jz with O< x <1. C
The phases described here have been presented previously [1,2]; however, only the tetragonaltungstenbronze-typephasesare accessiblein the literature[1].Here we reportthe strong similarity in the structural chemistry between reduced tantalates and reduced niobates. Most striking is the correspondencebetween BaZNblQz and BazTa@Jz, with nearly identicallatticeand positionalparametersand,therefore,virtuallyidenticaldistances. This is also the case for the other two types of structures,the tetragonaltungsten bronze (TTB) type and the pyrochloretype. Again,only small variationsin the lattice parameters are observed,and usuallyonly smallvariationsin the positionalparametersoccur. Sincethe TTB-type structure has channels along the c axis, it is not uncommonto find vacancy formation in the cation substructure occupying those channels. In the case of the
O
v
3 N 7
R
T
8
pyrochlore/microlite-typefamily, that structure is known to accommodatequite a large number of differentelementsin an oxideor oxyfluorideanion lattice.The overallunit-cell dimensionsvary only slightly with the transition metal-oxygen distance and are usually found in the range of 10to 10.5~ [8]. Researchinto the electricaland magneticproperties of these and relatedphaseswillbe of considerableinterest. R 1. 2. 3. 4. 5. 6. 7.
C F a R Z C Mater. 7,373 (1995). C F a R Z Abstracts of Papers, V21O, 32-INOR, A N M N W D O 1 A C S Proc. Amer. Ctystallogr. Assoc. Ann. Meeting 1986, Y L P W a E G Hamilton, Canada, A N Y ( ( P E G Y L J C F L a P W J Appl. Cryst. 22,384 (1989). A B Ark. Kemi Mineral. Geol. 18A(21), 1 (1945) (1944 i p i 1 y EA a R R J Amer. Ceram. Soc. 45, 18-25 (1962). B H S S T S A F a J W Chem. Mater. 3, 528
E i i
a
r
u
(1991).
8
E R
Chemie a’erEra’e8, 186 (1933).
e