FRANCIS GALASSO, LEWISKATZAND ROLAXD !,YARD
5898
[CONTRIDUTION FROM
THE
Vol. 81
DEPARTMENT O F CHEMISTRY, UNIVERSITY OF CONNECTICUr]
Tantalum Analogs of the Tetragonal Tungsten Bronzes' BY FRANCIS GALASSO, LEWISKATZAND ROLAND WARD RECEIVEDhfAY 28, 1939 Phases of composition Ao.5TaOa and Ao.5TaOz.awhere A = S r f 2 or B n f 2 were found to have the tetragonal tungsten bronze structure the ideal formula for which is Ko.aW01. This structure apparently may exist with both anion and cation deficiencies. Analogous phases were found in the niobium system with barium but not with strontium. Partial substitution of the alkali metals in the tetragonal and hexagonal tungsten bronzes was effected by valence compensation using tantalum(V). Measurement of t h e electrical conductivity of pressed pellet samples showed t h a t the substitution of Nb(IV) for T a ( I V ) leads to much higher conductivity.
Introduction Several ternary oxide systems have been described which are related to the cubic sodium tungsten bronze. SrMoOs and BaAIoOa2have the same structure but appear to be stoichiometric. CahloOS3s4is also stoichiometric, but has an orthorhombic structure. La1-.Ti03~ and SrO.b+xNb03,6 on the other hand, are both non-stoichiometric cubic phases, the latter showing the variation in color from blue to red with increasing strontium content. None of the other phases exhibits this property of color change with changing composition so characteristic of the sodium-tungsten bronzes. I t is strange that tantalum has not been found to form similar phases in view of the negligible difference in size of the tantalum and niobium ions. Attempts to prepare such phases led to the isolation of a hexagonal barium-tantalum oxide.' This phase was produced when a particular batch of tantalum pentoxide was heated with metallic tantalum and barium oxide. Tantalum pentoxide from all other sources yielded a tetragonal phase under the same conditions. The hexagonal phase may be formed from any sample of tantalum pentoxide by the use of various fluxes and the study of its composition and structure is being continued. This paper is concerned with the characterization of the tetragonal phase and related compounds. Experimental Reactants.-The preparations of BaO and Sr08 and of iTb026 have been described in earlier papers. T h e other reactants were reagent grade chemicals. In general, the preparations were effected by heating appropriate mixtures of the alkaline earth carb:nates and the transition metal oxide in Leco boats at 1100 . Phases containing tantalum in a lower oxidation state were obtained by using metallic tantalum as reducing agent, those containing niobium(1V) by using NbOz as reactant. I n these cases the mixtures were heated in evacuated sealed silica capsules. Chemical analyses were obtained only for the compound Bao.sTaO3. The compositions of other phases were assumed t o be given approximately by the composition of the mixtures used to prepare them. The phases were identified from X-ray powder diagrams.
Results The compound B a o ~ , T a 0formed ~ by the reaction BaCC,
+ Tar06 +2Bao.aTa03CO~
(1) This research was sponsored by the Office of Kava1 Research; reproduction in whole or in p a r t is permitted for a n y purpose of t h e United States Government. (2) R . Scholder and W. Klemm, Angew. Chcm., 66, 467 (19.54). (3) W. H. RIcCarro!!, L. Katz and R . Ward, THISJ O U R N A L78, , 2909 (1956). (4) R . Scholder and L. Brixner, Z . Nalurfovsch., lob, 178 (1955). ( 5 ) h i . Kestigian and K . Ward, THISJOURNAL, 71, Gl99 (19563. (6) D.Ridgley and R . Ward. i h i d . , 7 1 , GI32 11955). (7) F. S. Galasso, L. Katz and R. Ward, i b i d . , SO, 12G2 (1968). (8) F. Galasso, L. Katz and R. Ward, i b i d . . 81, 820 (1959).
at 1100" was obtained as a single crystal by heating the product with lead(I1) oxide.8 Precession photographs served to determine the tetragonal symmetry. The complex X-ray powder diagram could be ind$xed satisfactorily with the parameters a = 12.60 A., and c = 3.95 A. The dimensions suggested an analogy with the tetragonal potassium tungsten bronze structure.1° The relative intensities of the reflections given in Table I were calcuTABLE I COMPARISON OF OBSERVED A N D CALCULATED INTENSITIES OF REFLECTIOSS FOR Bao jTaOl Plane
110 220
{
-
W ' R1
320 201 211 400 410
1;: {
lobad
IV
M
W ' M
+
Ti7
M++
\v
+
Iciiled
x
13.8 4.8 250.0 246.0 7.0 358 0 33.6 532.0 142.0
S
968.0
321
M
w-
289.0 23.7
411 002 630 60 1
TV' MW LV
2.4 141.1 46.9 57.0
lated upon the assumption that the parameters for barium and tantalum were the same as those of potassium and tungsten. The agreement between the calculated and observed values is shown to be satisfactory. Analysis of the compound gave Ra = 23.15%, Ta = G0.32%. The values calculated from Bao.bTaOs are Ba = 23.05%, Ta = 60.78%. Further evidence that the two structures are similar was obtained by the preparation of similar phases of intermediate composition exemplified by the substances listed in Table 11. In contrast to the highly colored tungsten bronzes, all of these preparations are colorless, The replacement of tungsten by tantalum apparently permits all of the elements to assume the highest oxidation state. A series of oxygen-deficient phases with the same structure was prepared corresponding to the gen(9) J. P. Remeika, ibid., 78, 4259 (1956). (10) A , XIagneli and B. Blomberg, A c f a C k e m . Scand.. S, 372 (19.51).
Nov. 20, 1959
TANTALUM
ANALOGSOF
TETRAGONAL
TUNGSTEN BRONZES
5899
Discussion
TABLE I1 TETRAGONAL BRONZE STRUCTURE
Not all of the phases were obtained as homogencPHASESWITH THE ous products. Bao.sNb02.sfor example was always Unit cell parameters (A,) Formula a C contaminated with other phases. The entire X-ray powder pattern of these preparations, however, &s(Wo.aTao.s)Oa 12.36 3.90 could be accounted for by reflections due to the K0.4Baa.I( WO. cTa0.61 0 3 12.41 3.92 phases Ba6Nb4013L a cubic phase with lattice con12.60 3.95 Bao.sTaO3 stant of about 4 A. and the phase with the tetrag~ao.rSro.~(Wo.nTao.e)Os 12.33 3.87 12.41 3.90 onal bronze type of structure. The identification Sro.6Ta08 of the phase was thus possible although the sample era1 formula Ao.5Ta02.6 where A = Ba, Sr. Par- was not pure enough to,use for resistance measuretial and complete substitution of niobium for tan- ments. It should be observed that the barium-tantalum talum could be made without change in structure when the A cation was barium but this substitution oxide BaobTaOa is cation-deficient only with recould not be effected in the presence of strontium spect to the number of equivalent A-cation sites available in the structure. The full complement of without change in structure. All of these phases have a deep blue color which six A cations to ten B cations has not been obbecomes darker with increasing niobium content. tained in this system. Magneli's preparations of The resistivities of some of the preparations were the potassium-tungsten bronze also fell short of compared by use of cylindrical pellets 0.45 cm. the maximum amount of potassium. Some variadiam., 0.1 cm. long prepared by sintering com- tion in potassium content was reported, but in the pressed samples a t 1200'. The contacts were made barium-tantalum system no variation in lattice through platinum discs held against the ends by a constant such as would be expected with changes in spring. By heating these compounds a t l l O O o in barium content was ever observed. It is interestair, oxidation to the colorless compounds occurred ing to note that in this system the phases containwithout change in structure. The gain in weight ing tantalum in an oxidation state lower than +5 mas close to that expected for oxidation from h f f 4 were arrived a t by oxygen deficiencies, whereas in to M+5. The lattice constants and resistivities of the strontium-niobium system the lower oxidation states of niobium were secured by the introduction the oxygen deficient phases are given in Table 111. of additional strontium ions. The evidence for oxygen deficiencies in the strucTABLE 111 ture is not yet complete. Direct chemical deterOXYGEN-DEFICIENT PHASESOF TANTALUM ASD NIOBIUM mination of the oxidation state of the tantalum i n WITH THE TETRAGONAL BRONZE STRUCTURE the reduced phases is difficult because of their unreUnit cell parameters active nature. The deep blue color assumed by all Resistivity 4.) C of those phases, their relatively high conductivity, Formula a (ohm cm.) their oxidation to colorless phases a t high temperaBa0.5NbOz.5 12.60 3.95 tures by air to give the expected gain in weight, and Bao.s(Tao.sNbo.s)Oz.6 12.59 3.95 3.5 the correspondence of the X-ray diagrams of the Bao.sTaOa.6 12.60 3.95 21.2 oxidized phases to that of the analyzed phase Sro.zsBao.zsTaOz.6 12.52 3.93 2500 (Bao.5Ta0J all suggest that we are dealing with oxyS r o ~ T a 0 ,2K 12.41 3.93 200,000 gen deficient phases. The lattice constants are the same as the oxidized phases within the limits of our Some preliminary experiments on substitution of measurements. This is a common phenomenon in tungsten by tantalum in other structures indicate systems of this type. that the phenomenon is fairly general. Mixtures The mixed systems such as Ko.4Bao.l(Wo 4Tao00, of potassium, rubidium and cesium carbonates with all contain tungsten and tantalum in their highest tantalum(V) oxide and tungsten(V1) oxide gave oxidation state only. It would seem reasonable to colorless phases such as Ko.s(Wo.,Tao3 ) 0 3 , which suppose that such phases could be formed containseemed to be similar in structure to the hexagonal ing tungsten in a lower oxidation state, but none rubidium-tungsten bronze. l 1 has been obtained so far. (11) A. Magneli, Acta Chem. Scand., 7, 315 (1953).
STORRS, CONN.
