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The nitrides and oxide-nitrides of tungsten
THE NITRIDES AND OXIDE-NITRIDES OF TUNGSTEN* R. KIESSLING and L. PETEWONt A tungsten oxide-nitride with a composition close to the formula W,*~~.~.~s has been investigated by analyticat and X-ray methods. It was obtained by reducing ammonium paratungstate or tungsten trioxide with ammonia, probably has metallic properties, and belongs to the interstitial compounds. It has a defective MeX-lattice with the sodium chloride structure. Only 62 per cent of the Mepositions, however, are statistically occupied by tungsten atoms, whereas probably all the X-positions are occupied, 62 per cent by nitrogen atoms and 38 per cent by oxygen. The -r-nitride in the tungsten-nitrogen system has also been shown to be an oxide-nitride. This phase has also a defective tun sten lattice, but the vacancies are ordered. Its ideal formula is W. Z, (N, O)r.o~,where the ratio N: 6. IS unknown but certainly greater than for W.s2N.stO.s~+ In conclusion there is a short discussion regarding the electronic structure of the oxide-nitrides. LES NITRURES ET LES OXYDES-NITRURES
DE TUNGSTRNE
On a investigud par des methodes analytiques et aux rayons x, un oxyde-nitrure de tungstene, N 0,s~ 0 0,~s.Ce compose fut obtenu par la reduction dont la composition se rapproche de la formule W 0.62 d’un paratuugstate d’ammoniaque ou un trioxyde de tungstene evec I’amm~~aque. 11a probablement des proprietds metalli ues et appartient B la classe des composes interstitiels. Ce compose a un t-t&au defectueux du type- ckl eX avec la structure du chlorure de sodium. Toutefois. seulement 62 pour cent des positions Me sont statistiquement occupees par des atomes de tung&me, alors qu;? les positions X sent probablement toutes oceup&s; ies atomes d’azote en occupant 62 pour cent et les atomes d’oxyg&ne 38 pour cent. On a montr4 que les nitrures y du systeme tungstene-azote sent au& des oxydes-nitrues. Cette phase a aussi un r&&au dtsfectueux du tungst&ne, mais les facunes y sont ardonn&s. Sa formuleideateest W O.&N, O)Z,OO, oh le rapport N: 0 est inconnu, mais rrrtainement plus grand que dans le cas de W~,E~NO,~~O~,~~. Pour conclure, la structure electronique des oxydesnitrures est bri&vement disc&e. DIE NITRIDE UND OXYDNITRIDE DES WOLFRAMS Ein Wolframoxydnitrid mit einer etwa der Formel W 0,~aN~r~~0~,8s entsprechenden Zusammensetzung wurde analytisch und r&rtgenographischuntersucht. Es wurde durch Reduktion von Ammoniumparawolframat oder Wolframtrioxyd mit Ammoniak erhalten; es hat ~~~einlich metallisehe Eigenschaften und ehort zu den Einlagerungsverbindungen, Es hat ein fehlerhaftes MeX- Gitter mit einer Natrium4 lorid-Struktur, in dem jedoch nur 627s der Me-Gitterplltze statistisch von Wolframatomen besetzt sind. Dagegen sind wahrscheinlich alle X-Gitterplltze besetzt, 62% mit Stickstoffatomen und 38yo mit Sauerstoff, Es wurde ausserdem gezeigt, dass das y-Nitrid des Wolfmm~Stj~stoffsystems ein Oxydnitrid ist. Diese Phase ist ebenfails ein fehierhaftes Wolframgitter, in dem die Leerstellen jedoch geordnet sind. Die theoretische Formel dieser Verbindung ist WO,,~(N,O)r,so; das Verh<nis N: 0 ist nicht bekannt, jedoch bestimmt grosser als fiir W~~N~,~O~~* Absfhliessend wird die ~iektro~enst~ktur der Oxydnitride kurz diskutiert.
Two nitrides of tungsten are known from earlier work. Hagg [l] reported that a nitride with a face-centred cubic metal lattice was formed if tungsten was nitrided with dry ammonia at ?QO-8OO“C. The formation of this nitride was very slow, however, and it was not possible to obtain it in a pure form; it always appeared mixed with tungsten. The phase was called the p-phase in the tungsten-nitrogen system, and assumed to have a composition of W2N, but the formula coufd not be verified. Later, Kiesshng and Liu [2] observed a new nitride, the,r-phase, formed at about 800~900° if tungsten was nitrided with ammonia. The y-phase was assumed to be closely related to the o-phase. Its lattice was of the simple cubic type. The structure could not be determined but was probably closely related to that of the B-phase. The nitrogen content was assumed to be about 33 atomic per cent, as for the &phase, but *Received January 15,1954. $%derfors Bruk, Siiderfors, Sweden. ACTA ~ET‘~LLURGI~A,
VOL. 2, SEPT. 1954
it was also impossible to obtain this phase in a pure state. Tt wiI1 be shown befow that the y-phase has an ordered defective lattice containing both nitrogen and oxygen.
The Existence of a Tungsten ode-Ni~de During experiments with the reduction of ammonium paratungstate to tungsten with ammonia, the present authors observed that -a phase with a face-centred cubic metal lattice could be obtained in a pure state as the final reduction product if the reduction was carried out at a temperature of about 700% The phase had a grey, metallic lustre and was usually difficult to distinguish from tungsten powder. The lattice constant, about 4,138 A was slightly greater than that of the /3- and y-phases in the tungstennitrogen system (4.126 and 4.122 - 4,130 A resp.). The phase could also be obtained by reducing tungsten trioxide, a process which has been described in a patent journal f3]. However,
676
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the chemical analysis showed a tungsten content much too low and a nitrogen content too high to correspond to the formula W2N. Large amounts of oxygen were also found to be present in the lattice. On structural grounds the formula may be written W,N,O+, where the values of x and y both are about 0.62. Before discussing the results, which are given in Table I, the methods of analysis will be described. The chemical analyses were carried out in the following way. Tungsten was determined by oxidizing the specimens in dry oxygen to tungsten trioxide and weighing the amount, of trioxide formed. Some control analyses were carried out by dissolving the samples in a mixture of nitric and hydrofluoric acids, precipitating the tungsten as tungstic oxide, and igniting the precipitate. Nitrogen was usually determined by heating the compound to about llOO°C and pumping the gases formed. It had been found that the phase was completely converted to a mixture of tungsten and tungsten dioxide if heated in vacuum at this temperature for 2-3 hours. During this process the nitrogen was given off, which could be checked by comparing the results with some analyses by the Kjeldahl method. The method was usually avoided, however, because of the vigorous reaction which often occurred. The oxygen content was directly determined by reduction of the oxide-nitride with hydrogen at about 1000°C and weighing the amount of water
VOL.
2, 1954
absorbed by phosphorous pentoxide when the gas had passed the reaction zone. The residue after reduction was pure tungsten, and the completeness of the reduction could be tested by comparing the tungsten content after reduction and oxidation of the specimens. Because of the presence of oxygen in the lattice the specimens always formed a mixture of tungsten and tungsten dioxide if they were heated in vacuum at lOOO-11OO’C. (No trace of tungsten dioxide was observed if tungsten powder was heated in the same apparatus under the same conditions.) A probable process for this formation of tungsten dioxide is illus~at~ by the following reaction formula Wo.ar No.62 Oo.z, + 0.31 Nz + 0.19 WOz + 0.43W. The tungsten dioxide is thus formed by the oxygen which is present interstitially in the oxide-nitride and which will form the dioxide when the lattice of the oxide-nitride is broken down by the evolution of nitrogen in vacuum. A mixture of 63 w.t. per cent tungsten and 37 w.t. per cent tungsten dioxide will thus be formed if the oxide-nitride is heated in vacua. Such a mixture was prepared and the X-ray photograph compared with the residue after vacuum extraction. The photographs were very similar. As is shown by the results in Table I, the composition of the oxide-nitride is independent of the period of nitriding, within the limits of error, if this period has been long enough. It is also
TABLE I Density (a = 4,14A) observed CdC. talc. 4 defective metal atoms lattice
Method of preparation
W weights OI /cJ
N weights %
0 weights %
W,sNr&
89.5
5.1
4.3
12.2
19.0
14.0
paratungstate 3h, NHs, 700” (trace of WOZ)
WaoN&o
88.0
6.8
5.0
11.3
19.4
11.7
paratungstate 5h, NHz, 700”
W~N~~O~~
89.1
6.5
4.8
11.7
19.3
12.2
paratungstate li”, NHz, 700”
WsaN&r
89.1
6.9
4.7
11.8
19.4
12.0
paratungstate, 4@, NHI, 700”
WwNro&
86.8
7.3
5.8
11.4
20.3
10.5
WOJ, 3h, NH,, 700”
Wr~NssOas
88.5
6.9
4.3
12.0
19.4
12.2
WO,, 7b, NHP, 700*
w0&0rs
88.4
6.4
4.7
11.5
19.4
12.4
WO,, lIh, NHa, 700“
Formula* (according to the analyses)
.
*The formulas have been written with the sum of the N and 0 atoms in each case = 100. They thus itiustrate a MeX Iattice with NaCI structure where alI the X-positions are occupied by the N and 0 atoms together and only part of the Me-positions by W atoms (73, 60, 63, 62, 53, 63 and 64 per cent resp.).
KIESSLING
AND
PETERSON:
independent of whatever compound, tungsten trioxide or paratungstate, has been used for the preparation. The formula obtained if the mean value of all the analyses are used is W.SZ N.62 0.38 (the values after 3h nitriding of WOO or paratungstate have been excluded because of incomplete reaction time). On structural grounds, which will be discussed below, the sum of the oxygen and nitrogen atoms has been put equal to 1.00. The homogeneity range seems to be very narrow. The X-ray reflexions are those of a facecentred metal lattice with a cube-edge of 4.138 f 0.002 A. The intensities, but not the densities, are compatible with the assumption that 4 tungsten atoms are present in the unit cell, occupying the positions 000, $30, $03, O$$. The densities, experimentally obtained by the pycnometric method with about 5g of the sample each time, have also been given in Table I. These densities are far too low, however, compared with the densities calculated from X-ray data with the assumption that 4. metal atoms are present in the unit cell, i.e., occupy the corners of the cube and the centres of the faces of the cubic unit cell. These calculated densities are also given, and it is evident that, even with a large experimental error in the density determination, the densities obtained are much too low to agree with the assumption of 4 metal atoms in the unit cell. The densities obtained indicate that between 2 and 3 metal atoms in the unit cell is much more probable. Furthermore, space considerations strongly support the assumption of a face-centred tungsten lattice with tungsten atoms missing. The number of nonmetal atoms nitrogen and oxygen together is about 1, 5 to 2 times that of the number of the tungsten atoms. In a fully occupied face-centred metal lattice with a parameter of 4.14 A there will be two types of interstices, “octahedral” and “tetrahedral” interstices: In the “octahedral” interstices a nonmetal atom will be octahedrally surrounded by six metal neighbours and the greatest radius possible for the atom will be 0.630.69 A (depending on whether the radius of the tungsten atom is assumed to be 1.46 A as obtained from this structure or 1.40 A as obtained from other similar compounds). In the “tetrahedral” interstices a nonmetal atom will be tetrahedrally surrounded by four tungsten atoms and the greatest possible radius for an interstitial atom will be 0.33-0.39 A. The radius of the nitrogen atom has been found for several structures to be about 0.71 A. The radius of the oxygen atom seems to be about 0.60 A, and if some ionisation
NITRIDES
OF
TUNGSTEN
677
to a negative oxygen ion occurs the value will be greater. It is this evident that only the octahedral interstices will have space for the nonmetal atoms, and the total number of such interstices is equal to four for the face-centred cubic cell. Thus there cannot be an oxide-nitride with a face-centred lattice with a greater number of nonmetal atoms than metal atoms unless some of the metal atoms in the face-centred lattice are missing. There are several possibilities for the structure of such a defective lattice (see below). The most probable, in the authors’ opinion, is that the nonmetal atoms oxygen and nitrogen together occupy all the “octahedral” positions of the cubic unit cell (with coordinates 003, 040, 300 and +$$), whereas statistically about 2/3 of the metal positions in the face-centred lattice (033, 803, O++ and 000) are occupied by tungsten atoms and about l/3 empty. In Table I the densities for the different specimens have been calculated on this assumption, using the formulas obtained from chemical analysis. The agreement between observed and calculated densities is very good. It has been assumed above that the vacant sites of the metal lattice are empty. The possibility that these “holes” may be filled with nitrogen or oxygen atoms must also be considered, but may be rejected because the resulting densities would not be in accordance with the observed ones. If, for instance, all the “holes” of the lattice of W.62 N.6, 0.38 are assumed to be occupied by oxygen and nitrogen atoms, the formula could be written about W. Z, N. I6 0.10 (N. 00O.JO).The density calculated for this compound would be 14,6, far too great compared with the observed density. of 11.3-12.0. There is also the possibility of vacancies both in the metal and in the nonmetal lattices. This has been suggested by Brauer for the NbO-lattice [4]. The experimental evidence makes such an assumption rather unfeasible. (A lattice with 10 per cent of the sites of the nonmetal atoms in W.az N.62 0.38 unoccupied would give a calculated density of 10.5 compared with the observed density of 11.3-12.0.) Finally, several attempts were made to produce an ordered defective lattice of this phase. They were not successful (see below).
The Tungsten-Nitrogen System Because of the high nitrogen content of this oxide-nitride, the presence of oxygen, and the existence of a defective lattice, a reinvestigation
ACTA
678
METALLURGICA,
of the tungsten-nitrogen system was undertaken. Starting materials were tungsten powder and ammonia, and the nitriding was carried out in a tube furnace at various temperatures and for various periods. In some experiments the ammonia was carefully dried, and all oxygen was finally removed from it by passing over zirconium hydride at about 800 degrees: in other experiments the ammonia contained small amounts of oxygen and water vapor. The following are the essential results: The P-phase [l], assumed by H@g to have the composition W2N, was formed between 600°C and 85O”C, and was always mixed with a much greater amount of tungsten. It had a face-centred cubic metal lattice and was formed by both pure and impure ammonia. The y-phase [Z] was only formed if the ammonia contained small amounts of oxygen or water. It had a simple cubic metal lattice closely related to the &phase. All the reflexions with unmixed indexes present in the &phase were also present and strong in the y-phase, but in addition there were weak reflexions with mixed indexes (Table II). The r-phase too was always mixed with a much greater amount of tungsten. It was possible to determine the crystal structure of the y-phase, previously unknown. The reflexions are those of a simple cubic lattice (Table II), TABLE II r-phase. Simple cubic, 0% Pm3m. Calculated for 3 W-atoms in 3: (d) 3 + 0, 4 0 4, 0 3 4. The observed intensities have been obtained from a film taken with monochromatic Cu radiation in a Guinier camera. hkl
100 110 111 200 210 211 220 221 300 310 311 222 320 321
PI FI’ bW
1
27 47 255 176 72 67 268 58 14 1 53 456 147 47 91
NT
(ohs)
29 50 255 169 63 75 264 54 33 370 124 39 78
except that the reflexions with unmixed indexes are much stronger than the others. The earlier investigation by Kiessling and Liu was based on
VOL.
2, 1954
the assumption that all metal positions were occupied by tungsten atoms, and it was never possible to obtain acceptable agreement between observed and calculated intensities with this assumption. A reinvestigation by the present authors showed, however, that close agreement between observed and calculated intensities was obtained if the face-centred metal lattice of the y-phase was assumed to be an ordered defective lattice, with the metal atoms in the positions with coordinates 440, aO&, and 033, but with the atom at the corner of the cube, 000, missing (Table II). The formula for the y-phase should thus be W.r5 N, O1_-r, where z is unknown, as compared with W. 62 N. 62 0.38 for the oxide-nitride described above. Attempts to determine z by nitrogen and oxygen analyses gave no reproducible results because of the small amounts present. The oxygen content of W.72 N, Or_, is certainly much smaller than for W.SZ N.62 0.88, since the former is formed from pure tungsten and impure ammonia whereas the latter is formed from oxygen-rich tungsten compounds. The results seem to show that at least two different oxide-nitrides exist. One, W. $2 N.62 0.38, has a defective disordered face-centred tungsten lattice, and the length of the axis of the cubic cell is 4.138 A. The other, W.U, N, 01+ formerly called the y-phase in the tungsten-nitrogen system, has an ordered, defective, simple cubic tungsten lattice with the cube-edge 4.122-4.130 A. Because of the structural relationship between the two defective lattices of the ordered W.rs N, O1_, and the disordered W,sz N.aa 0. as, the effect of various heat treatment was tried. All attempts to order the defective tungsten lattice of the W. 62 N. $2O_ss-phase were unsuccessful. The ordered W.Q N, Ol_,-phase (the y-phase) changed to a face-centred cubic lattice on annealing at 890 degrees for some hours and quenching. This new face-centred lattice could again be transformed to the y-phase if annealed for some hours in ammonia or a vacuum at temperatures below about 850°C. These reactions indicate an order-disorder transformation for the y-phase, but no definite conclusions can be obtained, as the phases always appeared mixed with a much greater amount of tungsten. The B-phase formed by pure, dry ammonia could not be transformed to the y-phase, whereas the @-phase containing oxygen could be transformed.
KIESSLING
AND
PETERSON:
This also indicates that the y-phase is an oxidenitride and not a nitride of tungsten.
Discussion The oxide-nitrides described are of interest because they have oxygen atoms interstitially in the metal lattice. They thus belong to the same group of compounds as the carbides, borides, and nitrides of the transition metals, and are related to the oxide-nitrides of titanium [5], as well as to the oxides with the structure of the high-speedsteel carbide [S]. It is of special interest to compare the compounds WZN, W.75 N, CL., and W.SZ IV.62 0.~8 where the amount of oxygen in the lattice increases. The metal lattice is the same face-centred cubic lattice for all the phases. With increasing amount of oxygen, however, an increasing number of vacancies in the tungsten lattice appear. This seems to support the theory advanced by one of the present authors (FL K.) for the borides of the transition metals 171, that the interstitial boron atoms (and similarly for carbon and nitrogen [S; 91) give part of their electrons to the metal lattice. When oxygen is introduced into the tungsten nitride, some kind of N-O bonds are pro-
NITRIDES
OF
TUNGSTEN
679
bably formed. Part of the electrons of the nitrogen atoms are thus engaged in bond formation with the strongly positive oxygen atoms, which tend to form negative oxygen ions. The number of electrons which can be transferred from the interstitial nitrogen atoms to the tungsten lattice thus decreases with increasing oxygen content. The electron condensation of the metal lattice thus decreases, though defects appearing in the tungsten lattice tend to maintain it.
Acknowledgements The authors wish to thank Mr. N. Schonberg for his kind interest in this investigation and Miss B. &honing for valuable help with various preparations and calculations.
References 1. 2. 3. 4‘ 5. 6. 7. 8. 9.
H&G,G.
2. phys. Chem. B7 (1930) 339. KIESSLING,R. and LIU, Y, H. J. Metals 3 (1951) 639. Brit. Pat. 635.221, August 29, 1946. BRAUEB, G. 2. anorg. u. allg. Chem. 248 (1941) 1. EHRLICH,P. Z. anorg. Chem. 259 (1949) 1. KAXUSSON, N. Nature 168 (1951) 558. KIESSLING,R. Acta Chem. Stand. 4 (1950) 269. JACK, K. H. Proc. Roy. Sot. London A195 (1948) 63. JACK, K. H. Chem. and Ind. (1951) 1994.
DE TUNGSTRNE
On a investigud par des methodes analytiques et aux rayons x, un oxyde-nitrure de tungstene, N 0,s~ 0 0,~s.Ce compose fut obtenu par la reduction dont la composition se rapproche de la formule W 0.62 d’un paratuugstate d’ammoniaque ou un trioxyde de tungstene evec I’amm~~aque. 11a probablement des proprietds metalli ues et appartient B la classe des composes interstitiels. Ce compose a un t-t&au defectueux du type- ckl eX avec la structure du chlorure de sodium. Toutefois. seulement 62 pour cent des positions Me sont statistiquement occupees par des atomes de tung&me, alors qu;? les positions X sent probablement toutes oceup&s; ies atomes d’azote en occupant 62 pour cent et les atomes d’oxyg&ne 38 pour cent. On a montr4 que les nitrures y du systeme tungstene-azote sent au& des oxydes-nitrues. Cette phase a aussi un r&&au dtsfectueux du tungst&ne, mais les facunes y sont ardonn&s. Sa formuleideateest W O.&N, O)Z,OO, oh le rapport N: 0 est inconnu, mais rrrtainement plus grand que dans le cas de W~,E~NO,~~O~,~~. Pour conclure, la structure electronique des oxydesnitrures est bri&vement disc&e. DIE NITRIDE UND OXYDNITRIDE DES WOLFRAMS Ein Wolframoxydnitrid mit einer etwa der Formel W 0,~aN~r~~0~,8s entsprechenden Zusammensetzung wurde analytisch und r&rtgenographischuntersucht. Es wurde durch Reduktion von Ammoniumparawolframat oder Wolframtrioxyd mit Ammoniak erhalten; es hat ~~~einlich metallisehe Eigenschaften und ehort zu den Einlagerungsverbindungen, Es hat ein fehlerhaftes MeX- Gitter mit einer Natrium4 lorid-Struktur, in dem jedoch nur 627s der Me-Gitterplltze statistisch von Wolframatomen besetzt sind. Dagegen sind wahrscheinlich alle X-Gitterplltze besetzt, 62% mit Stickstoffatomen und 38yo mit Sauerstoff, Es wurde ausserdem gezeigt, dass das y-Nitrid des Wolfmm~Stj~stoffsystems ein Oxydnitrid ist. Diese Phase ist ebenfails ein fehierhaftes Wolframgitter, in dem die Leerstellen jedoch geordnet sind. Die theoretische Formel dieser Verbindung ist WO,,~(N,O)r,so; das Verh<nis N: 0 ist nicht bekannt, jedoch bestimmt grosser als fiir W~~N~,~O~~* Absfhliessend wird die ~iektro~enst~ktur der Oxydnitride kurz diskutiert.
Two nitrides of tungsten are known from earlier work. Hagg [l] reported that a nitride with a face-centred cubic metal lattice was formed if tungsten was nitrided with dry ammonia at ?QO-8OO“C. The formation of this nitride was very slow, however, and it was not possible to obtain it in a pure form; it always appeared mixed with tungsten. The phase was called the p-phase in the tungsten-nitrogen system, and assumed to have a composition of W2N, but the formula coufd not be verified. Later, Kiesshng and Liu [2] observed a new nitride, the,r-phase, formed at about 800~900° if tungsten was nitrided with ammonia. The y-phase was assumed to be closely related to the o-phase. Its lattice was of the simple cubic type. The structure could not be determined but was probably closely related to that of the B-phase. The nitrogen content was assumed to be about 33 atomic per cent, as for the &phase, but *Received January 15,1954. $%derfors Bruk, Siiderfors, Sweden. ACTA ~ET‘~LLURGI~A,
VOL. 2, SEPT. 1954
it was also impossible to obtain this phase in a pure state. Tt wiI1 be shown befow that the y-phase has an ordered defective lattice containing both nitrogen and oxygen.
The Existence of a Tungsten ode-Ni~de During experiments with the reduction of ammonium paratungstate to tungsten with ammonia, the present authors observed that -a phase with a face-centred cubic metal lattice could be obtained in a pure state as the final reduction product if the reduction was carried out at a temperature of about 700% The phase had a grey, metallic lustre and was usually difficult to distinguish from tungsten powder. The lattice constant, about 4,138 A was slightly greater than that of the /3- and y-phases in the tungstennitrogen system (4.126 and 4.122 - 4,130 A resp.). The phase could also be obtained by reducing tungsten trioxide, a process which has been described in a patent journal f3]. However,
676
ACTA
METALLURGICA,
the chemical analysis showed a tungsten content much too low and a nitrogen content too high to correspond to the formula W2N. Large amounts of oxygen were also found to be present in the lattice. On structural grounds the formula may be written W,N,O+, where the values of x and y both are about 0.62. Before discussing the results, which are given in Table I, the methods of analysis will be described. The chemical analyses were carried out in the following way. Tungsten was determined by oxidizing the specimens in dry oxygen to tungsten trioxide and weighing the amount, of trioxide formed. Some control analyses were carried out by dissolving the samples in a mixture of nitric and hydrofluoric acids, precipitating the tungsten as tungstic oxide, and igniting the precipitate. Nitrogen was usually determined by heating the compound to about llOO°C and pumping the gases formed. It had been found that the phase was completely converted to a mixture of tungsten and tungsten dioxide if heated in vacuum at this temperature for 2-3 hours. During this process the nitrogen was given off, which could be checked by comparing the results with some analyses by the Kjeldahl method. The method was usually avoided, however, because of the vigorous reaction which often occurred. The oxygen content was directly determined by reduction of the oxide-nitride with hydrogen at about 1000°C and weighing the amount of water
VOL.
2, 1954
absorbed by phosphorous pentoxide when the gas had passed the reaction zone. The residue after reduction was pure tungsten, and the completeness of the reduction could be tested by comparing the tungsten content after reduction and oxidation of the specimens. Because of the presence of oxygen in the lattice the specimens always formed a mixture of tungsten and tungsten dioxide if they were heated in vacuum at lOOO-11OO’C. (No trace of tungsten dioxide was observed if tungsten powder was heated in the same apparatus under the same conditions.) A probable process for this formation of tungsten dioxide is illus~at~ by the following reaction formula Wo.ar No.62 Oo.z, + 0.31 Nz + 0.19 WOz + 0.43W. The tungsten dioxide is thus formed by the oxygen which is present interstitially in the oxide-nitride and which will form the dioxide when the lattice of the oxide-nitride is broken down by the evolution of nitrogen in vacuum. A mixture of 63 w.t. per cent tungsten and 37 w.t. per cent tungsten dioxide will thus be formed if the oxide-nitride is heated in vacua. Such a mixture was prepared and the X-ray photograph compared with the residue after vacuum extraction. The photographs were very similar. As is shown by the results in Table I, the composition of the oxide-nitride is independent of the period of nitriding, within the limits of error, if this period has been long enough. It is also
TABLE I Density (a = 4,14A) observed CdC. talc. 4 defective metal atoms lattice
Method of preparation
W weights OI /cJ
N weights %
0 weights %
W,sNr&
89.5
5.1
4.3
12.2
19.0
14.0
paratungstate 3h, NHs, 700” (trace of WOZ)
WaoN&o
88.0
6.8
5.0
11.3
19.4
11.7
paratungstate 5h, NHz, 700”
W~N~~O~~
89.1
6.5
4.8
11.7
19.3
12.2
paratungstate li”, NHz, 700”
WsaN&r
89.1
6.9
4.7
11.8
19.4
12.0
paratungstate, 4@, NHI, 700”
WwNro&
86.8
7.3
5.8
11.4
20.3
10.5
WOJ, 3h, NH,, 700”
Wr~NssOas
88.5
6.9
4.3
12.0
19.4
12.2
WO,, 7b, NHP, 700*
w0&0rs
88.4
6.4
4.7
11.5
19.4
12.4
WO,, lIh, NHa, 700“
Formula* (according to the analyses)
.
*The formulas have been written with the sum of the N and 0 atoms in each case = 100. They thus itiustrate a MeX Iattice with NaCI structure where alI the X-positions are occupied by the N and 0 atoms together and only part of the Me-positions by W atoms (73, 60, 63, 62, 53, 63 and 64 per cent resp.).
KIESSLING
AND
PETERSON:
independent of whatever compound, tungsten trioxide or paratungstate, has been used for the preparation. The formula obtained if the mean value of all the analyses are used is W.SZ N.62 0.38 (the values after 3h nitriding of WOO or paratungstate have been excluded because of incomplete reaction time). On structural grounds, which will be discussed below, the sum of the oxygen and nitrogen atoms has been put equal to 1.00. The homogeneity range seems to be very narrow. The X-ray reflexions are those of a facecentred metal lattice with a cube-edge of 4.138 f 0.002 A. The intensities, but not the densities, are compatible with the assumption that 4 tungsten atoms are present in the unit cell, occupying the positions 000, $30, $03, O$$. The densities, experimentally obtained by the pycnometric method with about 5g of the sample each time, have also been given in Table I. These densities are far too low, however, compared with the densities calculated from X-ray data with the assumption that 4. metal atoms are present in the unit cell, i.e., occupy the corners of the cube and the centres of the faces of the cubic unit cell. These calculated densities are also given, and it is evident that, even with a large experimental error in the density determination, the densities obtained are much too low to agree with the assumption of 4 metal atoms in the unit cell. The densities obtained indicate that between 2 and 3 metal atoms in the unit cell is much more probable. Furthermore, space considerations strongly support the assumption of a face-centred tungsten lattice with tungsten atoms missing. The number of nonmetal atoms nitrogen and oxygen together is about 1, 5 to 2 times that of the number of the tungsten atoms. In a fully occupied face-centred metal lattice with a parameter of 4.14 A there will be two types of interstices, “octahedral” and “tetrahedral” interstices: In the “octahedral” interstices a nonmetal atom will be octahedrally surrounded by six metal neighbours and the greatest radius possible for the atom will be 0.630.69 A (depending on whether the radius of the tungsten atom is assumed to be 1.46 A as obtained from this structure or 1.40 A as obtained from other similar compounds). In the “tetrahedral” interstices a nonmetal atom will be tetrahedrally surrounded by four tungsten atoms and the greatest possible radius for an interstitial atom will be 0.33-0.39 A. The radius of the nitrogen atom has been found for several structures to be about 0.71 A. The radius of the oxygen atom seems to be about 0.60 A, and if some ionisation
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to a negative oxygen ion occurs the value will be greater. It is this evident that only the octahedral interstices will have space for the nonmetal atoms, and the total number of such interstices is equal to four for the face-centred cubic cell. Thus there cannot be an oxide-nitride with a face-centred lattice with a greater number of nonmetal atoms than metal atoms unless some of the metal atoms in the face-centred lattice are missing. There are several possibilities for the structure of such a defective lattice (see below). The most probable, in the authors’ opinion, is that the nonmetal atoms oxygen and nitrogen together occupy all the “octahedral” positions of the cubic unit cell (with coordinates 003, 040, 300 and +$$), whereas statistically about 2/3 of the metal positions in the face-centred lattice (033, 803, O++ and 000) are occupied by tungsten atoms and about l/3 empty. In Table I the densities for the different specimens have been calculated on this assumption, using the formulas obtained from chemical analysis. The agreement between observed and calculated densities is very good. It has been assumed above that the vacant sites of the metal lattice are empty. The possibility that these “holes” may be filled with nitrogen or oxygen atoms must also be considered, but may be rejected because the resulting densities would not be in accordance with the observed ones. If, for instance, all the “holes” of the lattice of W.62 N.6, 0.38 are assumed to be occupied by oxygen and nitrogen atoms, the formula could be written about W. Z, N. I6 0.10 (N. 00O.JO).The density calculated for this compound would be 14,6, far too great compared with the observed density. of 11.3-12.0. There is also the possibility of vacancies both in the metal and in the nonmetal lattices. This has been suggested by Brauer for the NbO-lattice [4]. The experimental evidence makes such an assumption rather unfeasible. (A lattice with 10 per cent of the sites of the nonmetal atoms in W.az N.62 0.38 unoccupied would give a calculated density of 10.5 compared with the observed density of 11.3-12.0.) Finally, several attempts were made to produce an ordered defective lattice of this phase. They were not successful (see below).
The Tungsten-Nitrogen System Because of the high nitrogen content of this oxide-nitride, the presence of oxygen, and the existence of a defective lattice, a reinvestigation
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of the tungsten-nitrogen system was undertaken. Starting materials were tungsten powder and ammonia, and the nitriding was carried out in a tube furnace at various temperatures and for various periods. In some experiments the ammonia was carefully dried, and all oxygen was finally removed from it by passing over zirconium hydride at about 800 degrees: in other experiments the ammonia contained small amounts of oxygen and water vapor. The following are the essential results: The P-phase [l], assumed by H@g to have the composition W2N, was formed between 600°C and 85O”C, and was always mixed with a much greater amount of tungsten. It had a face-centred cubic metal lattice and was formed by both pure and impure ammonia. The y-phase [Z] was only formed if the ammonia contained small amounts of oxygen or water. It had a simple cubic metal lattice closely related to the &phase. All the reflexions with unmixed indexes present in the &phase were also present and strong in the y-phase, but in addition there were weak reflexions with mixed indexes (Table II). The r-phase too was always mixed with a much greater amount of tungsten. It was possible to determine the crystal structure of the y-phase, previously unknown. The reflexions are those of a simple cubic lattice (Table II), TABLE II r-phase. Simple cubic, 0% Pm3m. Calculated for 3 W-atoms in 3: (d) 3 + 0, 4 0 4, 0 3 4. The observed intensities have been obtained from a film taken with monochromatic Cu radiation in a Guinier camera. hkl
100 110 111 200 210 211 220 221 300 310 311 222 320 321
PI FI’ bW
1
27 47 255 176 72 67 268 58 14 1 53 456 147 47 91
NT
(ohs)
29 50 255 169 63 75 264 54 33 370 124 39 78
except that the reflexions with unmixed indexes are much stronger than the others. The earlier investigation by Kiessling and Liu was based on
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the assumption that all metal positions were occupied by tungsten atoms, and it was never possible to obtain acceptable agreement between observed and calculated intensities with this assumption. A reinvestigation by the present authors showed, however, that close agreement between observed and calculated intensities was obtained if the face-centred metal lattice of the y-phase was assumed to be an ordered defective lattice, with the metal atoms in the positions with coordinates 440, aO&, and 033, but with the atom at the corner of the cube, 000, missing (Table II). The formula for the y-phase should thus be W.r5 N, O1_-r, where z is unknown, as compared with W. 62 N. 62 0.38 for the oxide-nitride described above. Attempts to determine z by nitrogen and oxygen analyses gave no reproducible results because of the small amounts present. The oxygen content of W.72 N, Or_, is certainly much smaller than for W.SZ N.62 0.88, since the former is formed from pure tungsten and impure ammonia whereas the latter is formed from oxygen-rich tungsten compounds. The results seem to show that at least two different oxide-nitrides exist. One, W. $2 N.62 0.38, has a defective disordered face-centred tungsten lattice, and the length of the axis of the cubic cell is 4.138 A. The other, W.U, N, 01+ formerly called the y-phase in the tungsten-nitrogen system, has an ordered, defective, simple cubic tungsten lattice with the cube-edge 4.122-4.130 A. Because of the structural relationship between the two defective lattices of the ordered W.rs N, O1_, and the disordered W,sz N.aa 0. as, the effect of various heat treatment was tried. All attempts to order the defective tungsten lattice of the W. 62 N. $2O_ss-phase were unsuccessful. The ordered W.Q N, Ol_,-phase (the y-phase) changed to a face-centred cubic lattice on annealing at 890 degrees for some hours and quenching. This new face-centred lattice could again be transformed to the y-phase if annealed for some hours in ammonia or a vacuum at temperatures below about 850°C. These reactions indicate an order-disorder transformation for the y-phase, but no definite conclusions can be obtained, as the phases always appeared mixed with a much greater amount of tungsten. The B-phase formed by pure, dry ammonia could not be transformed to the y-phase, whereas the @-phase containing oxygen could be transformed.
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This also indicates that the y-phase is an oxidenitride and not a nitride of tungsten.
Discussion The oxide-nitrides described are of interest because they have oxygen atoms interstitially in the metal lattice. They thus belong to the same group of compounds as the carbides, borides, and nitrides of the transition metals, and are related to the oxide-nitrides of titanium [5], as well as to the oxides with the structure of the high-speedsteel carbide [S]. It is of special interest to compare the compounds WZN, W.75 N, CL., and W.SZ IV.62 0.~8 where the amount of oxygen in the lattice increases. The metal lattice is the same face-centred cubic lattice for all the phases. With increasing amount of oxygen, however, an increasing number of vacancies in the tungsten lattice appear. This seems to support the theory advanced by one of the present authors (FL K.) for the borides of the transition metals 171, that the interstitial boron atoms (and similarly for carbon and nitrogen [S; 91) give part of their electrons to the metal lattice. When oxygen is introduced into the tungsten nitride, some kind of N-O bonds are pro-
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bably formed. Part of the electrons of the nitrogen atoms are thus engaged in bond formation with the strongly positive oxygen atoms, which tend to form negative oxygen ions. The number of electrons which can be transferred from the interstitial nitrogen atoms to the tungsten lattice thus decreases with increasing oxygen content. The electron condensation of the metal lattice thus decreases, though defects appearing in the tungsten lattice tend to maintain it.
Acknowledgements The authors wish to thank Mr. N. Schonberg for his kind interest in this investigation and Miss B. &honing for valuable help with various preparations and calculations.
References 1. 2. 3. 4‘ 5. 6. 7. 8. 9.
H&G,G.
2. phys. Chem. B7 (1930) 339. KIESSLING,R. and LIU, Y, H. J. Metals 3 (1951) 639. Brit. Pat. 635.221, August 29, 1946. BRAUEB, G. 2. anorg. u. allg. Chem. 248 (1941) 1. EHRLICH,P. Z. anorg. Chem. 259 (1949) 1. KAXUSSON, N. Nature 168 (1951) 558. KIESSLING,R. Acta Chem. Stand. 4 (1950) 269. JACK, K. H. Proc. Roy. Sot. London A195 (1948) 63. JACK, K. H. Chem. and Ind. (1951) 1994.