Structure determination of ζ-Fe2N by neutron and synchrotron powder diffraction

Journal of

ALLOY5 £~D COB@OU~D5 ELSEVIER

Journal of Alloys and Compounds 235 (1996) 15-22

Structure determination of (-Fe2N by neutron and synchrotron powder diffraction 1 D. Rechenbach, H. Jacobs* Fachbereich Chemie der Universitiit Dortmund, 44221 Dortmund, Germany Received 15 August 1995

Abstract Microcrystalline samples of (-Fe2N were prepared from iron powder in a glass apparatus under flowing ammonia. The crystal structure was studied by neutron and high resolution synchrotron ?owder diffraction at room temperature. ~'-FezN crystallizes orthorhombic: space group Pbcn (No. 60), Z = 4, a = 4.4373(2) A, b = 5.5413(1) .A and c = 4.8429(1) A; N(Fo)=29/46 and seven structural parameters with both sets of data commonly refined; RBragg = 4,2% (neutron), RBragg = 5.6% (synchrotron). Keywords. (-Fe2N; Structure determination; Neutron diffraction

1. Introduction Iron nitrides are of fundamental importance for steel production and steel hardening. Up to now, details of reactions are not known, e.g. structural information is required in order to obtain reproducible qualities of the hardened steel surface (for example, see Ref. [1]). The preparation of single crystals and single phase samples is difficult on account of the low thermal stability and high decomposition pressures of nitrogen at moderate temperatures [2]. Our unpublished investigations on a mixture of a-Fe/y'-Fe4N show N 2 pressures higher than 3200 bar (so far limited by the apparatus used) at 600°C. This value is in agreement with data for the equilibrium pressure of nitrogen for these phases found by Krichevskij and Khazancva [3] at 500°C: p(N2) = 2700 atm. Decomposition ot (-FeeN into the elements begins at about 300°C. In previous studies the crystal structures of phases of the iron-nitrogen system were often investigated with partially different results (for example, see Refs. [4-7]). Very detailed investigations were carried out by Jack in the 1940s and 1950s (for example, see Refs. [8-10]). We found in previous studies [11] that in * Corresponding author. I Dedicated to Professor Dr. Hk. Miiller-Buschbaum on the occasion of his 65th birthday. 0925-8388/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI 0925 -8388 ( 95 )02097-7

general he described the structures correctly. In the case of (-Fe2N Jack suggested an orthorhombic distorted hexagonal close packed arrangement of iron atoms with nitrogen atoms in octahedra [8]. He gave no space group and for all atoms in the unit cell 12 atomic positions for an ideal arrangement. In our investigations we want to resolve the crystal structures of the compounds of the iron-nitrogen system in detail and to correlate them with physical properties. Recently we published [11] the structures of y'-Fe4 N based on X-ray single crystal data and of e-Fe3N based on neutron powder diffraction data.

2. Synthesis of ~'-FezN Powder samples of pure e-Fe3N were prepared with great success in technical nitridation furnaces [2,11]. For (-Fe2N this is not possible. The synthesis of (Fe2N needs a high nitriding potential [8]. It would be too expensive to charge a commercial furnace with a sufficient flowing rate of ammonia. Samples of about 3 g of Fe powder (Alfa, Karlsruhe, Germany, purity 99.9 wt.%) placed in a corundum container were treated with flowing ammonia (Messer Griesheim, purity 99.999 vol.%) in the temperature range of 420 to 450°C and 1 bar total pressure in a glass tube. Glass wool was inserted at the inlet of the tube to heat the NH 3 gas quickly. The reaction time

16

D. Rechenbach, H. Jacobs / Journal of Alloys and Compounds 235 (1996) 15-22

took about 24 h. The furnace was cooled down by 50°C h -1 under flowing ammonia. In agreement with the results obtained by Jack [8], single phase products resulted with a nitrogen content of 11.1(1) wt.% (automatic C, N, H, S analyser CHNS-932, Leco, St. Joseph (USA)).

3. Structure determination

Guinier diagrams were indexed on the basis of an orthorhombic unit cell. The refined cell parameters are a -- 4.430(4) ~,, b = 5.526(5) .A and c = 4.833(5) ,A (Cu Kc~1 radiation). The observed reflection conditions Okl k = 2n, hOl 1 = 2n and hkO h + k = 2n led to the space group Pbcn (No. 60 [12]). All 12 atomic positions in the unit cell (8 Fe and 4 N) proposed by Jack [8] could be transferred to 8 Fe in site 8d x,y,z (ideal parameters: x = 1/4, y = 1/8, z = 1/12) and 4 N in site 4c 0,y,1/4 (ideal parameter y = 3/8). For a detailed structure analysis, neutron powder diffraction data were collected at the DR3 reactor at the Rise National Laboratory (Denmark) with instrument TAS1. The sample (11.2 g of (-Fe2N) was placed in a vanadium can of 8 mm diameter. Diffraction data were recorded at room temperature. The scan covered the angular range of 20.0-99.8 ° in 20 in steps of 0.1 °. The wavelength of neutrons was 2.016 A. The structure refinements were carried out with the program FULLPROV[13] based on the Rietveld profile method [14] applying a Gaussian function. An initial refinement was made starting with the data given above. A purely nuclear calculation is justified by the paramagnetism of (-Fe2N. A satisfactory fit between observed and calculated neutron diffraction data of (-Fe2N for the refined model led to a final reliability factor RBragg of about 2.5% [15]. (-FezN shows a very small deviation from hexagonal symmetry. The profiles of the reflections in the neutron diffraction diagram are too broad and n o splitting was detected. To prove the correctness of the unit cell parameters used so far, synchrotron diffraction data were taken at the high resolution powder diffractometer at HASYLAB (Hamburg; synchrotron source DORIS III). A double monochromated X-ray beam was used. The intensity of Bragg-reflections was registered by a single detector with an analyser. The wavelength was 1.90746 ,~. Both sets of data were commonly refined with the program PROFIL [16] based on the Rietveld profile method [14] applying a Gaussian function for the neutron data and a pseudo-Voigt function for the synchrotron data. The adaptions of parameters were done in a weighting scheme which depends on the scattering power of the atoms and the information

about the different data sets. The wavelength of the synchrotron radiation is very well known. On this basis the wavelength of neutrons was also refined. The final results of technical and crystallographical data are given in Table 1. The low value of B ( N ) = 0.0(3) A may be caused by the small number of reflections in the neutron experiment and for the synchrotron data N is of low scattering power. Fits between observed and calculated data of both experiments are illustrated in Figs. 1 and 2. The reliability factors obtained for the profile agreement Rwp are high. The reason is that the number of parameters for profile correction in the program PROVm is restricted (e.g. there are no asymmetry corrections). Furthermore, the synchrotron data includes about 8000 points with bad counting statistics (background) and the neutron data contains additional reflections caused by the wavelength A/2. In contrast, the structure relevant reliability factor RBragg shows, with an average of 5%, the correctness of the solution of the proposed structure. In Table 2 a comparison is made between observed and calculated intensities.

Table 1 Results of refinement and structural parameters of (-Fe2N (neutron diffraction data from Ris0 National Laboratory (TAS1), synchrotron diffraction data from HASYLAB (high resolution powder diffractometer)) Neutron 20-range (deg) Step rate (deg) Wavelength (A) Number of reflections

Rwp (%) Rexp (%) Rm..g (%) Synchrotron 20-range (deg) Step rate (deg) Wavelength (A) Number of reflections Rwo (%) Re~p (%)

RBragg (%) Space group Cell parameters: a (,~) b (A)

c (A) Volume (~3) Fe on site 8d B(Fe) (,h,2) N on site 4c B(N) (•2) Occupation factor of N

20.0-99.9 0.1 2.0206(6) 28 11.85 4.24 4.29

31.50-111.51 0.01 1.90746 40 24.87 4.76 5.64 Pbcn (No. 60 [12])

4.4373(2) 5.5413(1) 4.8429(1) 119.081(8) 0.249(1), 0.128(1), 0.0827(6) 0.64(8) 0, 0.364(2), 1/4

0.0(3) 0.94(3)

D. Rechenbach, H. lacobs / Journal of Alloys and Compounds 235 (1996) 15-22 Intensity (count) 1000C

800C 600C 400£

A

200(

I

20

II

3'0

II ~

4'o

I

I

do

~'o

I

~""

Ill

I

7'0

I

8'0

90

28 (degrees)

Fig. 1. Neutron diffraction diagram of (-FezN measured at room temperature at TAS1 (Ris~). Dotted curves give measured, full lines calculated values. A difference curve is shown below. The positions of the reflections (vertical signs) are given. Excluded regions are marked. "['hey show weak reflections caused by M2 radiation.

Intensity (count) 1200C 1000C 800C 600( 4000 [

2000 0

, L II

40

II

I

,.

[

50

II

i

I

60

III

1

A III

I IIIII

1

f~

7'0

80

I

9'0

100 110 20(degrees)

Fig. 2. Synchrotron diffraction diagram of (-Fe2N measured at room temperature at the high resolution powder diffractometer (HASYLAB). Dotted curves give measured, full lines calculated values. A difference curve is shown below. The positions of reflection,; (vertical signs) are given.

4. Results and discussion

(-Fe2N crystallizes in the a-PbO 2 structure type. Iron atoms show the motif of a slightly distorted hexagonal close packing (h.c.p.) as also found for e-Fe3N [11]. In e-Fe3N, only corner sharing octahedra of Fe are occupied in an ordered way by nitrogen. In (-FezN, nitrogen occupies corner and edge sharing octahedra. The arrangement of edge sharing octahedra shows one-dimensional infinite cis-linked chains parallel to the direction [001] (see Fig. 3). All chains are linked with one another with neighbouring layers by corner sharing (see Fig. 4). The structure of (-FezN is illustrated in Figs. 3 and 4 by shaded octahedra of iron containing nitrogen. Each iron atom has three nitrogen neighbours in a

17

nearly trigonal-planar geometry. Nitrogen is well ordered in (-Fe2N and occupies only one crystallographic site with full occupancy (see Table 1). To verify the full occupancy of the Fe position the pycnometric density of (-FezN was measured (gas displacement pycnometer, AccuPyc 1330, micromeritics, Neuss, Germany). The value found is P~xp = 7.03 + 0.01 g cm -3 and is in good agreement with the calculated density (data from Table 1) of Pca~c= 7.012 g cm 3. Both atomic sites are fully occupied. The h.c.p, of iron in e-F%N is nearly retained in (-F%N. The hexagonal unit cell parameters ahc× of e-Fe3N split into the parameters borth and Corth in (-FezN. In Table 3 the unit cell parameters of e-Fe3N are transformed with respect to the orthorhombic setting in (-FezN. According to the data, the greatest increase is in the orthorhombic c-axis. This is the direction of edge sharing chains of octahedra Fe6N. The elongation is due to repulsion effects between closest neighboured nitrogen atoms. In Table 4 some distances and angles of (-Fe2N are given. Fig. 5 shows an anti-cuboctahedron of iron atoms with sharing octahedra Fe6N. The distances d ( F e - N ) in the octahedron Fe6N are split into three values. This is caused by the repulsion of the nitrogen atoms in edge sharing octahedra with each other and is indicated by a shift of about 0.03 of nitrogen atoms from the centres of the iron octahedra parallel to the orthorhombic b direction. This is schematically shown in Fig. 6. In principle the arrangement of iron atoms in (Fe2N and ~-Fe3N is marginally changed. A first look at Fig. 7 gives the impression that it may be possible to construct the structure of ~'-Fe2N by filling up empty iron octahedra in e-Fe3N by additional nitrogen. A closer inspection of the phases shows that this is impossible. One half of all nitrogen atoms of E-Fe3N have to be moved into the next face sharing octahedra above or below. This formalism has already been described in general by Jack [10]. The similarity of the structures of (-Fe2N and eFe3N is quite obvious. B/irnighausen [17] developed a useful way for the comparison of similar structures by symmetry relationships. Between the space groups P6322 and Pbcn there is no direct group/subgroup relation [12]. P6 3/mcm is a supergroup of Pbcn and a minimal supergroup of P6322. In the space group P6 3/mcm a hypothetical 'tri-NiAs' can be constructed. 'tri-NiAs' is derived from the NiAs structure type by enlarging the unit cell parameter a with the factor ,/3. In 'tri-NiAs', arsenic forms an ideal h.c.p, with nickel occupying all octahedral sites. From this point of view (-F%N and e-Fe3N can be interpreted as ordered defect structures of the aristotype 'tri-NiAs'. The

18

D. Rechenbach, H. Jacobs / Journal of Alloys and Compounds 235 (1996) 15-22

Table 2 Positions (20, deg), observed and calculated intensities of reflections of evaluated neutron diffraction data (k = 2.0206 ,~) and synchrotron diffraction data (,~ = 1.90746 A) of (-Fe2N hkl

110 111 020 002 021 200 102 121 112 211 022 220 130 122 202 221 131 212 113 310 222 023 132 040 311 231 041 123 141 213 302 321 312 232 004 042 223 240 133 104

Neutron

Synchrotron

20

lob~

1c~1¢

20

l,h~

lc,L~

33.81 41.92 42.67 49.21 49.58 54.07 56.65 56.98 61,10 64.32 67.19 71.27 72.56 73.50 76.18 76.46 77.72 80.07 87.23 89.87 91.55 92.72 92.77 93.55 94.86 95.64 98.57 98.69

147 290 13 0 0 99 535 1000 121 0 0 9 75 4 429 790 52 0 88 52 7 714 110 395 86 0 5 0

146 274 5 1 2 106 500 1002 112 0 1 0 69 1 408 787 57 0 91 38 5 737 112 358 85 0 3 0

31.99 39.60 40.29 46.41 46.75 50.94 53.34 53.65 57.47 60.44 63,10 66.85 68.04 68.90 71.35 71.61 72.76 74.90 81.38 83.75 85.25 86.30 86.35 87.04 88.21 88.90 91.49 91,60 96.79 97.31 98.18 98.42 101.62 102.32 103.97 104.99 107.76 108.54 108.91 109.50

12 25 0 58 132 280 457 1000 5 0 0 0 1 0 109 230 1 0 1 1 0 232 2 101 1 0 0 0 1 0 108 247 1 0 11 23 216 100 0 87

ll 19 2 73 136 286 469 937 5 0 0 0 2 1 108 206 2 0 2 1 0 226 2 110 1 0 0 0 1 0 108 218 1 0 11 23 207 103 0 93

d e r i v e d s y m m e t r y r e l a t i o n s w i t h all t r a n s f o r m a t i o n s o f sites a n d cell p a r a m e t e r s a r e s h o w n in Fig. 8. A c o m p a r i s o n o f sites o c c u p i e d b y n i t r o g e n in ( F e 2 N a n d e - F e 3 N r e v e a l s n o p o s s i b i l i t y for a d e r i v a tion of group/subgroup relations between these two c o m p o u n d s . T h e t w o f o l d site 2c in e - F % N w i t h fract i o n a l c o o r d i n a t e s 1 / 3 2 / 3 1 / 4 a n d 2 / 3 1 / 3 3 / 4 (only c o m e r s h a r i n g o c t a h e d r a ) c h a n g e s f o r ( - F e 2 N in a f o u r f o l d site w i t h t h e i d e a l a t o m i c p o s i t i o n s 0 3 / 8 1/4, 0 5 / 8 3 / 4 , 1 / 2 7 / 8 1 / 4 a n d 1 / 2 1 / 8 3/4. I n t h e s p a c e g r o u p P b c n (with l o w e r s y m m e t r y w i t h r e s p e c t to t h e s p a c e g r o u p P6322 ) t h e r e is n o p o s s i b i l i t y o f o b t a i n i n g an a r r a n g e m e n t o f o n l y c o r n e r s h a r i n g o c t a h e d r a Fe6N. T h e r e f o r e , n o s y m m e t r y r e l a t i o n s h i p b e t w e e n ( - F e 2 N a n d ~ - F e 3 N exists.

D u r i n g t h e d i s c u s s i o n o f t h e s t r u c t u r e s of T ' - F e 4 N a n d e - F e 3 N t h e r e g u l a r a r r a n g e m e n t o f all a t o m s was e x p l a i n e d m a i n l y f r o m an e l e c t r o s t a t i c p o i n t o f view [11]. B o t h s t r u c t u r e s r e p r e s e n t s i m p l e s o l u t i o n s for a c.c.p, a n d h.c.p, o f i r o n with n i t r o g e n in o n l y c o m e r s h a r i n g o c t a h e d r a l interstice. W i t h t h e c o m p o s i t i o n F e 2 N an o n l y c o r n e r s h a r i n g c o m b i n a t i o n o f o c c u p i e d o c t a h e d r a o f i r o n is n o t p o s s i b l e in e i t h e r close p a c k i n g . A n o r t h o r h o m b i c d i s t o r t e d h.c.p, of i r o n o c c u p i e d b y n i t r o g e n in o c t a h e d r a l i n t e r s t i c e s results as e x p l a i n e d a b o v e . T h e c i s - l i n k e d chains o f e d g e s h a r i n g o c t a h e d r a F e 6 N give t h e p o s s i b i l i t y o f d i l a t a t i o n p a r a l l e l to this c h a i n - d i r e c t i o n a n d for a l t e r n a t i n g shifts o f n i t r o g e n a t o m s (see Fig. 6). T h e a r r a n g e m e n t o f n e x t n e i g h b o u r i n g N - a t o m s to o n e n i t r o g e n will

D. Rechenbach, H. Jacobs I Journal of Alloys and Compounds 235 (1996) 15-22

19

ot

,~c

a

b Fig. 3. Part of the structure of (-FezN: octahedra Fe6N in [100].

13

"b Fig. 4. Part of the structure of (-Fe2N: octahedra Fe6N in [001].

D. Rechenbach, H. Jacobs / Journal of Alloys and Compounds 235 (1996) 15-22

20

Table 3 Comparison of the cell parameters of e-Fe3N in orthorhombic setting with the cell parameters of (-Fe2N; the elongation of (-Fe2N and the "/°'-ratios in (-Fe2N are calculated with respect to the ideal h.c.p. (1.633)

e-Fe3N (-Fe2N

a = 4.6982 A c = 4.8429 A

2a/x/-3 = 5.4250 ,~ b = 5.5413 ,~

c = 4.3789 A a = 4.4373 A.

A (A) [%]

0.1447 [+3.1]

0.1163 [+2.1]

0.0584 [+1.3]

-f3 '/ a = 1.614 ~t ,3 /, = 1.587 2"/h = 1.602

Table 4 Distances (,~) and angles (deg) in (-Fe2N d(Fe-Fe)

d(N-Fe)

d(N-N)

2 x 2.719(7) 1 x 2.739(6) 1 × 2.743(7) 1 × 2.756(6) 1 x 2.761(7) 2 x 2.771(9) 2 × 2.774(6) 2 x 2.806(6) 2 x 1.894(8) 2 x 1.960(4) 2 x 2.009(9) 2 × 2.852(7) 4 × 3.519(5) 4 x 3.550(9)

Fe-N-Fe

1x 2x 2x 2x 2x 2x 1x 3x 1x 1x 1x

N-Fe-N

N - N - N (b,c-plane)

86.6(4) 88.1(4) 89.7(4) 90.0(4) 90.4(5) 92.1(4) 92.6(4) 177(9) 91.4(4) 130.8(9) 131.9(7) 116.2(3)

Table 5 Distances (.~) and angles (deg) for a-Fe, 7'-Fe4N, e-Fe3N (see Ref. [11]) and (-Fe2N a-Fe

Distances ( A ) d(Fe-Fe) 8 x 2.48 6 × 2.87 (12 × 2.57) a

y'-Fe4N

e-Fe3N

12 × 2.68

6 x 2.68 6 × 2.74

d(N-Fe)

6 x 1.90

6 x 1.93

d(N-N)

6 x 3.79

6 x 3.49

Angles (deg) Fe-N-Fe N-Fe-N

12 x 90 3 x 180 1 × 180

12 x 90 3 x 180 1 × 129.5

(-Fe2N 2x 2x 6× 2x 2x 2x 2x 2x 8×

12 × 3× 1x 2×

Fig. 5. Anti-cuboctahedron of Fe in (-Fe2N with sharing nitride occupied octahedra Fe6N; distances d(Fe-Fe) are given in angstroms.

2.72 2.74 2.77 2.81 1.89 1.96 2.01 2.85 3.53

/~

1,894X /

1,960

,)<2,009

(87-93) 177 91.4 131

"Distance for hypothetical face centred cubic y'-Fe.

n o w b e d i s c u s s e d . F o r d a t a , s e e T a b l e 4. I n y ' - F e 4 N six N a t o m s f o r m a r e g u l a r o c t a h e d r o n . F o r e - F e 3 N this r e s u l t s i n a t r i g o n a l p r i s m . I n ( - F e 2 N t h e r e is a t r i g o n a l p r i s m a t i c s u r r o u n d i n g ( d ( N - N ) = 3.53 ,~) a n d an additional distorted tetrahedron: d(N-N)=2 x 3.53 ,~ a n d 2 x 2.85 ,~. A n a l t e r n a t i v e d e s c r i p t i o n is a t r i g o n a l p r i s m o f n i t r o g e n w h i c h is c a p p e d b y f o u r n i t r o g e n a t o m s at t h e r e c t a n g u l a r p l a n e s . T w o o f t h e s e planes are capped by one and the third by two nitrogen atoms. T a b l e 5 gives a c o m p a r i s o n o f d i s t a n c e s a n d a n g l e s

tc

~

~~A\

Fe6N

Fig. 6. Part of a one-dimensional infinite chain of edge sharing octahedra in (-Fe2N; the shift of the nitrogen atoms from the centre of the octahedra Fe6N is indicated; distances d(Fe-N) are given in angstroms. for a - i r o n a n d i r o n n i t r i d e s . T h e a v e r a g e d i s t a n c e s d(Fe-Fe) increase with increasing nitrogen content f r o m 2.68 ,~ ( y ' - F e 4 N ) to 2.70 A ( ~ - F e a N ) t h e n to 2.76 ,~ i n ( - F e z N . T h e s t r u c t u r e s w i t h o n l y c o r n e r s h a r i n g o c c u p i e d o c t a h e d r a o f i r o n s h o w a slight i n c r e a s e , b u t

D. Rechenbach, H. Jacobs I Journal of Alloys and Compounds 235 (1996) 15-22

a

21

b

Fig. 7. Comparison of the structures of e-Fe3N and (-F%N perpendicular to the hexagonal close packed iron layers.

in (-Fe2N it is more distinct. As pointed out before [11], nitrogen is inserted in the arrangement of iron atoms in such a way that they are the maximum possible distance from one another. This view is clearly driven by electrostatic aspects. 7'-Fe4N and e-Fe3N are metallic ferromagnets and show only small distortions in the iron substructure. (-F%N is paramagnetic. The calculation of volume increments [18] for these nitrides leads to very small values for nitrogen that cannot be compared to N 3 ions, as in Li3N [19]. Besides metallic bonding, strong covalent interactions between iron and nitrogen seem to be favoured. Up to now we have no indication for this assumption, e.g. by infrared or Raman spectroscopy, but first qualitative results of X-ray absorbtion near edge spectroscopy (XANES) experiments (unpublished results) support this point of view.

tri-NiAs P 6Jm 2/c 2/m 4 Ni 1/3, 2/3, 0 2 Ni0,0,0 k2 16as x, 0, I/4 x=l/3 i 2a+b, b-a, c t3 i 2a+b, b, c

tri-NiAs

NiAs P 6 j m 2/m 2/c 2 Ni0, 0,0 2 AS 1/3, 2/3, 1/4

P6322 2 Ni 1/3,2/3,1/4 2 Ni I/3, 2/3, 3/4 2 Ni 0, 0, 1/4 6 As x, 0,0 x~l/3

]i

SNix, 0,0 i 4 Ni0,0,0 8Asx, y, 1/4 ~4 x=l/3 y=2/3 As 0, y, 1/4 y=2/3 ]

I

k2 ] 2/3a, b, c

* tetra-NiAs p 2/b 2,!n2!{a

J

[ c,a,b

i [ tetra-NiAs P 2/b2/c 2Jn i 4 Ni 0, y, 1/4 5,=7/8 ! 4 Ni 0 y 1/4 y-3/8 i 8Asx, y,z x~l/4 y=l/8 z-l/12:

Acknowledgements

[

~-FezN P 2,/b 2/c 2,/.

i

4 N 0, y, I/4 y~) 364 8 Fe x, y, z l x-0.249 y=0.128 z-~083 ]

' ]

c-F%N

]

P6322 ] 2 N 1/3, 2/3, 1/4 i i6 Fe x, 0, 0 x=0.328J

Fig. 8. Symmetry relationship between (-Fe2N and ~-Fe3N; the common aristotype is a hypothetical 'tri-NiAs' (see text); t~translationengleich and k~klassengleich; transformations of cell parameters are given.

Financial support by the 'Bundesministerium ffir Forschung und Technologie' (BMFT; Contract No. 03-JA2DOR) and the 'Fonds der Chemischen Industrie' is gratefully acknowledged. For recording neutron diffraction data we thank Dr. P. MOiler (Inorganic Chemistry, RWTH Aachen, Germany) and for assistance during the stay of one of us (D.R.) at HASYLAB we thank Dr. T. Wroblewski and Dr. S. Doyle.

22

D. Rechenbach, H. Jacobs / Journal of Alloys and Compounds 235 (1996) 15-22

References [1] B. Prenosil, Hiirterei-Tech. Mitt., 28 (1973) 157. [2] H. Jacobs, D. Rechenbach and U. Zachwieja, Hiirterei-Tech. Mitt., 50 (1995) 205. [3] I.R. Krichevskij and N.E. Khazanova, Z. Fiz. Chim., 21 (1947) 719. [4] G. H~igg, Nature, 121 (1928) 826. [5] S.B. Hendricks and ER. Kosting, Z. Kristallogr., 74 (1930) 511. [6] R.J. Bouchard, C.G. Frederick and V. Johnson, J. Appl. Phys., 45 (1974) 4067. [7] A. Burdese, Metall. Ital., 3 (1957) 195. [8] K.H. Jack, Proe. R. Soc. London Ser. A:, 195 (1948) 34. [9] K.H. Jack, Acta Crystallogr., 3 (1950) 392. [10] K.H. Jack~ Acta Crystallogr., 5 (1952) 404. [11] H. Jacobs, D. Reehenbach and U. Zaehwieja, J. Alloys Comp., 50 (1995) 205.

[12] T. Hahn (ed.), International Tables For Crystallography, Vol. A, Space Group Symmetry, Kluwer Academic Publishers, Dordrecht, 1992. [13] J. Rodriguez-Carvajal, Program FULLPROF,Version 2.4.2, December 1993 (Institut Laue-Langevin). [14] H.M. Rietveld, Aeta Crystallogr., 22 (1967) 151; J. Appl. Crystallogr., 2 (1969) 65. [15] D. Rechenbach, Thesis, University of Dortmund, 1995. [16] J.K. Cockcroft, Program PROFm,Version 5.02 (Birkbeck College, London). [17] H. B~irnighausen, MATCH, 9 (1980) 139. [18] W. Biltz, Raumchemie fester Stoffe, Leopold Voss, Leipzig, 1934. [19] H.J. Beister, S. Haag, R. Kniep, K. Str6Bner and K. Syassen, Angew. Chem., 100 (1988) 1116.