Tetragonal rare earth hydrides REH(D)≈2.33 (RE  La, Ce, Pr, Nd, Sm) and a neutron diffraction study of NdD2.36

Journal

of the Less-Common

Metals,

TETRAGONAL RARE EARTH Pr, Nd, Sm) AND A NEUTRON

PETER KNAPPE

323

95 (1983) 323-333

HYDRIDES REH(D)22,33 (RE = La, Ce, DIFFRACTION STUDY OF NdD2.36

and HORST MfSLLER

Abteilung fiir Radiochemie, Znstitut fiir Anorganische und Analytische Freiburg, Albertstrasse 21, D-7800 Freiburg im Breisgau (F.R.G.)

Chemie,

Universitat

HUGO W. MAYER Fachbereich Chemie, Sonderforschungsbereich 127 Kristallstruktur Universitiit Marburg, Lahnberge, D-3550 Marburg an der Lahn (F.R.G.)

und Chemische

Bindung,

(Received March 5,1983)

Summary The rare earth (RE) metals lanthanum, cerium, praseodymium, neodymium and samarium form hydrides REHz22,33 and deuterides RED,,,3, with a tetragonally distorted fluorite structure. X-ray investigations show a facecentred tetragonal arrangement of the rare earth cations and a lattice parameter ratio c/a 2 1. Neutron diffraction studies of NdDZ,36 yield a tetragonal superstructure with cfa k 2 and space group I4,md. The structure is related to the REH, (CaF,) cell; some ofthe tetrahedral sites are vacant, but half or almost half of the octahedral sites are occupied by deuterium atoms in an ordered manner. These results are compared with those of other workers.

1. Introduction In previous papers on the hydrides and deuterides of cerium Cl] and samarium [2] we have reported the existence of the tetragonal phases REH, and RED, (RE = rare earth; x x 2.33). Other workers have observed tetragonal deuterides of lanthanum, cerium and praseodymium [3,4]. Because the data on the structure, the composition and the temperature range of these compounds show some contradictions, we have prepared tetragonal hydrides and deuterides of lanthanum, praseodymium and neodymium for X-ray studies and in one case (NdD,,,,) for neutron diffraction studies. 2. Experimental

methods

2.1. Preparation and X-ray investigations The hydrides and deuterides of lanthanum, praseodymium and neodymium were prepared by direct reaction of the metals (purity, 99.9%; Auer-Remy, 0 Elsevier Sequoia/Printed

in The Netherlands

324

Hamburg) with hydrogen (purity, 99.999%; Messer-Griesheim, Dusseldorf) and deuterium (purity, 99.7%; Messer-Griesheim, Dusseldorf). The metals were heated to 400-500 “C and were then cooled under hydrogen or deuterium at a rate of 4 “C h- I. Further details / of the preparation/ and the I Guinier X-ray diffraction measurements have been given elsewhere [1,5,6]. The experimental intensities 1/l, were measured using a photometer (G III; Carl Zeiss, Jena).

2.2. neutron ~i~ructio~ ~eusure~e~ts The neutron diffraction measurements were performed using the P14 powder diffractometer at the Forschungsreaktor 2, Kernforschungszentrum Karlsruhe. A sample of NdD2,36 p owder was irradiated with thermal neutrons (Cu(220) monochromator; 2& x 48”; 2 = 1.034 A). The neutrons scattered at the sample were registered on 14 3He counter tubes located on a horizontal circle around the sample position (Fig. 1). The counters, which were fixed on a bank at a mutual angular separation of 3.5”, moved together. Therefore the reflected intensity was measured at 14 angles simultaneously. Step scans were performed with a fixed step width of typically 0.05”. Scanning over 3.5” resulted in measurement over a total angular range of 49”. In this way the region from 0” to 120” could be measured with constant statistical accuracy by three scans of 3.5 each. The counting rate at a specific detector position was determined using a fission chamber (monitor) to ensure a constant neutron flux for each step. A more detailed description of this type of diffractometer is given by Hewat and Bailey [7]. To calibrate the efficiency of the individual counters and to determine the neutron wavelengths and zero-point deviations, difl?raction measurement over the whole angular range were performed periodically using a standard Al,O, sample. The sample was prepared for the diffraction experiments as follows. An aluminium cylinder 5 mm in diameter with a wall thickness of 1 mm was filled with 1 cm3 (approximately 5 g) of NdD,.,, using a dry-box. The height (5 cm) of the powder in the tube was sufficient to cover the counter tubes. The effect of L/2 contamination was found to be less than 0.4% and was neglected. Since there was a very small amount of aluminium in the beam, corresponding aluminium reflections were not observed in the diffraction pattern.

3. Results

and discussion

3.1. X-ray investigation The tetragonal phases of the hydrides and deuterides prepared in this work are related to the fluorite structure of the dihydrides REH2. X-ray patterns of the REH, and RED, samples (2.30 < x < 2.38) showed a broadening or splitting of most of the diffraction peaks. Only the (111) and (222) peaks remained sharp. This behaviour is characteristic of a cubic-to-tetragonal phase transition. The intensities of the split peaks indicate that the tetragonal phase has a c/a ratio greater than unity. In our previous papers [l, 23 we transformed the tetragonal face-centred cell F into a tetragonal body-centred cell I (a, = +ar2”2; cI = cr).

325

This procedure is correct according to X-ray measurements alone, but our neutron diffraction studies have shown that the face-centred cell should be doubled in the c direction. Therefore we now prefer not to transform the facecentred cell. The lattice parameters of the tetragonal hydrides and deuterides of lanthanum, praseodymium and neodymium prepared in this work are listed in Table 1.For-comparison Table 1 also contains the results of previous work on the cerium [l]and samarium [2] compounds, low temperature X-ray measurements of CeD 2.75141 and neutron diffraction data for LaD2.30, CeD,.,, and PrD,.,, E33. The X-ray powder diffraction data for NdD,.,, are given in Table 2. The hydrides REH, and the deuterides RED, (x < 2.30) can be characterized as purely cubic; the lattice parameters decrease linearly with increasing x owing to the uptake of hydrogen Cl, 2,8]. Samples with x > 2.38 revealed only a cubic phase without any change in the lattice parameters corresponding to the results obtained with higher cooling rates [l, 81. Figure 2 shows the volumes per formula unit RED, of the tetragonal rare earth deuterides from Table 1. It can be seen that the data of Titcomb et al. [S] are TABLE

1

Lattice parameters deuterides

and volumes

Sample 5.619(l) 5.611(l) 5.636(Z) 5.528(l) 5.529(l) 5.520(l) 5.521(l) 5.538( 1) 5.507(3) 5.475(l) 5.472(l) 5.466(l) 5.463(l) 5.494(Z) 5.424(l) 5.422(l) 5.415(l) 5.414(l) 5.413(l) 5.343(l) 5.334(l)

per formula

unit of the tetragonal

rare earth hydrides

and

e (A)

v’ (A”,

Reference a

5654(l) 5.638(l) 11.325(7) 5.562(3) 5.558(l) 5553(Z) 5.550(l) 11.046(2) 5.547(3) 5.506(l) 5.510(3) 5.499(l) 5*496(l) 11.078(6) 5.455( 1) 5.454(l) 5.445(l) 5446(l) 10.87(l) 5366(l) 5.350(l)

44.63(l) 44.38(l) 44.97 42.49(Z) 42.48(2) 42.30(l) 42.29(2) 42.35 42.06 41.26(l) 41.25(Z) 41.08(2) 41.00(l) 41.89 40.12(l) 40.09(l) 39.92(l) 39.90(l) 39.81(l) 38.28(l) 38.05(l)

This work, X This work, X

C31, N Cl19 x PI? x PI, x IIll>x II319 N c439 x

This This This This

work, work, work, work,

E3I N

This This This This This

work, work, work, work, work,

X X X X X X X X N

ca x c219 x

*X, X-ray diffraction; N, neutron diffraction. The lattice parameters for tetragonal body-centred cells [l, 2 J have keen transformed to those for tetragonal face-centred cells (see text). b O&y phase with c/u < 2. =At - 170 “C.

TABLE 2 X-ray data for NdD,.,, hkl

111 002 206 202 226 113 311 222 004 400 313 331 204 402 420 224 422

II&

3.1318 2.7228 2.7070 1.9197 1.9141 1.6402 1.6332 1.5659 1.3614 1.3535 1.2455 1.2424 1.2163 1.2120 1.2106 1.1094 1.1062

3.1319 2.1220 2.7056 1.9190 1.9135 1.6400 1.6333 1.5666 1.3616 1.3532 1.2455 1.2427 1.2161 1.2122 1.2103 1.1093 1.1063

100 35 50 46 30 15 28 20 5 6 15 11 10 10 8 6 10

a = 5.414(l) A; c = 5.44611) A.

La Fig. 1. Principal configuration of the P14 powder diffractometer. the monochromator and the 3He counter are in millimetres.

Pr

Nd

(Pm)

Sm

The distances from the specimen to

Fig. 2. Volumes V’ per formula unit RED, of the tetragonal rare earth deuterides: l , this work: CD, ref.l;#@,ref.2;(),ref.3;@,ref.4.

rather higher than the results of other workers except for CeD,.,, which was the only deuteride for which they reported a c/a ratio of less than 2. The results of Libowitz et ab [4], which were obtained by X-ray investigations, are in good

327

agreement considering the lower temperature at which their measurements were performed. However, their neutron diffraction study of CeD,.,, revealed no evidence of the tetragonal phase. Moreover the deuterium content of their sample CeD,,,, is much higher than those of the other RE deuterides in Table 1. Tetragonal cells related to a distorted fluorite structure have also been reported for the binary rare earth fluorides Sm,F, [9], Eu,F, [lo] and YbJF, [ll], the ternary fluorides M,REF, [lo-141 andM,YF, (M = Ca, Sr, Ba) [14,15], and the actinide oxides lXJ,O-, [16], Ui603, [1’7] andPa,O, [lS]. In the fluorides and oxides these intermediate phases are produced by the ordering of interstitial anions and cations of different oxidation states (RE” and REI”; M” and REI**;WV and II’*; PaIV and Pa’). There must be an alternative explanation for the ordering in the hydrides and deuterides. Since the valence state of the rare earth cations is 3 + even in the dihydrides, the tetragonal distortion of the fluorite structure is primarily due to the anions. However, in contrast with the behaviour of the fluorides, no superstructure reflections were observed on the Guinier patterns of the hydrides and deuterides. Therefore a neutron diffraction study was performed to obtain more information about the assumed ordering of the rare earth cations and/or the hydrogen (deuterium) anions. 3.2. Neutron d~~ruct~on study of NdD,, The diffraction diagram profiles of NdD z.36 were fitted using the PERNOD program [19] with 40 reflection groups up to a 28 value of 119”. Approximate atomic parameters were derived from the structure of CeD,,,, [3]. In doing this one deuterium position was treated differently. NdD,,,, crystallizes in tetragonal symmetry with eight formula units per cell and was refined in the acentric space group 14,md (Cib). In the structural model the neodymium atoms were located on two special positions (O,O,z). As one z parameter of the structure was to be chosen freely in this space group, one of the neodymium atoms was fixed at (O,O,O), the coordinate of the other then being (0, 0,0.51). The deuterium atoms were found in two positions. One must be in a general position (D(l)), whereas the other is close to (0, 0, z) (D(2)). A refinement of the population parameters showed that the D(2) atoms tend to occupy approximately four positions even if their x,y,z parameters are assumed to be unrestricted. If a total deuterium content of 2.36 atoms per formula unit is assumed, an occupancy of 0.93 is obtained for D(1). This distribution was fixed and D(2) was tested in (x,y, z), (0,y,.a) and (O,O,z) using various refinements. Comparison of the results shows that (O,O,z) is a reasonable choice (Table 3). To test the occupation of the deuterium positions a series of refinements was made using various values of the population parameters included in the refinements (Table 4).In this approach one free thermal parameter was allowed for the neodymium atoms and one for the deuterium atoms. The best reliability factors were found either for fully occupied D(2) positions or for an equal occupation of D(1) and D(2). The deviations of the position parameters of D(1) and D(2) were within 1% of the lattice constants. The reliability value R represents the fit of the structural amplitudes Fcalc

Nd(l)(a) Ml)(a) Ml)(a) Nd(l)(a)

D(1) (c) D(1) (c) D(l) (c) D(l)(c)

1 2 3 4

1 2 3 4

0.247(3) 0.246(4) 0.246(3) 0.247(3)

0 0 0 0

xb

0.233(l) 0.234(l) 0.234(l) 0.234(l)

0 ‘0 0 0

Yb

0.137(l) 0.137(l) 0.136(l) 0*138(l)

0 0 0 0

Zb

0.930 0.930 0.930 0.944

1 1 1 1

PP’

0.7(l) 0.8(l) 0.8(l) 0.8(l)

1.0(2) 0.9(3) 0.9(3) 0.9(3)

Bd (AZ)

D(g) (c) D(2)(b) D(2)(a) D(2)(a)

Nd(2) (a) Nd(2) (a) Nd(2) (a) Nd(2) (a)

Atom”

0.01(5) 0 0 0

0 0 0 0

xb

0.028(6) O.OO(3) 0 0

0 0 0 0

Yb

0.736(l) 0.736(2) 0.735(l) 0.738(Z)

0.513(l) 0.512(l) 0.512(l) 0.614(l)

Zb

0.25 0.5 1 0.944

1 1 1 1

PP’

1.3(4) 1.6(4) 1.x2) U(2)

0.9(2) O&2) 0.8(Z) 0.7(Z)

Bd(A2)

0.061 0.060 0.060 0.060

R

0.211 0.209 0.213 0.210

R,

a = 5.413(l) A; c = 10.87(l) A; c/a = 2.01; V = 318.5(4)A” (from refinement 4); 2 = 8; space group, 14,md (C::); scattering lengths, bNd= 7.30 x lo-l3 cm and b, = 6.674 x 10eL3 cm [ZO]. “The Wyckoff notation is given in parentheses. bThe position parametersin fractions of the cell. cPopulation parameter. dIsotropic temperaturefactor.

Atom”

NO.

TABLE 3 Structure refinementstested for NdD,.,,

329

TABLE 4 Comparison ofthe various deuterium distributions

at the positions D(1) (0.246,0.234,0.136)

and D(2)

(0, 0,0.735)

Pf’ (D(O)

PP (00)

R

RP

0.930

1.000 0.944 0.860 0.800 0.720

0.059 0.059 0.061 0.064 0.068

0.214 0.214 0.214 0.216 0.223

0.944 0.965 0.980 1.000 PP, population

parameter.

given by the refined structure intensity measurements :

model

to the amplitudes

Fobs derived

from

w=l

The value R, represents the accuracy of reproduction the model. It is summed over all steps of the scan :

of the intensity profile by

R, = 1 lIZBobs - 12eca1CI c 12eobs 28

i 28

The integral diffraction pattern and its fit are shown in Fig. 3. The symbols Y represent the measured intensities divided by 20. The full curves show the background and the calculated diffraction patterns composed of gaussian profiles. The agreement is good. The chosen ranges of the various intensity groups are assumed to be independent [19]. The resulting unit cell of NdD,.,, is shown in Figs. 4 and 5 where the deuterium atoms D(1) are located near the fourfold screw axis 1 49 L 49 z * The structure of NdD,,,, can be considered as a sequence of identical neodymium layers which undulate slightly in the c direction. If the undulation is neglected, the various sheets at z = $, 3 and $ can be constructed from the plane at z = 0 by a counterclockwise rotation of x/2 at (&i, 0) and a shift of (0,0,:) which is performed by the operation of the screw axis 4, at (& $, 0). Thus the layer sequence can be denoted ABCD. The layers contain all the neodymium atoms and are connected by D(2) atoms in the a and b directions. The D(2) atoms are located in half the octahedral sites. The D(1) atoms are embedded between the different layers in tetrahedral sites at z = 4, z = & z = 2 and z = g at a mean distance of 2.35A (Table 5). Therefore both neodymium positions have an eightfold coordination of D(1). The distances between the neodymium atoms and the D(2) atoms in the a-b plane are 2.71 and 2.72 A. In the c direction the D(2) atoms are found at a distance of 2.85 A from the Nd(1) atoms and 2.44 A from the Nd(2) atoms; the octahedral site on the other side of the neodymium atom is empty. Therefore it can be concluded that deuterium prefers a small separation

330

Fig. 4. Stereoscopic

plot of NdD,.3, in the a-c plane.

L

Fig. 5. Stereoscopic

plot of NdD,,,,

in the u-b plane.

TABLE 5 Atomic distances in the unit cell of NdD,,,, A tams

Distances (A)

Nd(l)-Nd(1) -Nd(2)

3.84(l) 3.73(l), 2.31(l); 2.71(l), 3.84(l) 2.31(2), 2.44(2),

-DO) -D(2) Nd(2)-Nd(2) -D(l) -D(2)

3.83(l), 3.94(l) 2.38(l) 2.85(2) 2.40(l) 2.72(l)

332

of 2.44~% in the large octahedral space. Refinements have shown that the D(2) position is occupied fully or at least as much as the D(1) position (Table 4).

4. Conclusion The tetragonal phases could only be prepared for the lanthanum, cerium, praseodymi~, neodymium and samarium compounds for which the solid solution of the dihydrides Cl, 2, 8J exists over a wide range. The upper phase boundary of the homogeneity range of the dihydrides REH, of the rare earth metals dysprosium, holmium, erbium [5] and thulium [21] is below x = 2.33 (e.g. TmH,&. No tetragonal phase was observed in these systems at hydrogen contents of about x = 2.33, but instead a two-phase region of cubic dihydride and hexagonal trihydride was present. It appears that the tetragonaf phases are revealed only when the cubic dihydrides REH, exhibit a solid solution up to x x 2.30. Neutron diffraction studies of NdD,,,, resulted in the same tetragonal superstructure (2 = 8) as that reported by Titcomb et al. [3f for LaD2.30,CeD,,,, and PrD,.,,. However, no satisfactory explanation could be found for the discrepancy between the deuterium contents of the compounds RED, (x = 2.292.37) and the ideal stoichiometry of x = 2.50 for this ordered tetragonal phase. Therefore further neutron diffraction investigations of samples with lower and higher deuterium contents, where no tetragonal distortion has been observed by X-ray diffraction, should be performed.

Acknowledgments Th. Goldschmidt AC., the Freiburger Wissenschaftliche Gesellschaft and the Fond der Chemie supported the X-ray investigation. We are grateful to Mrs. B. Niefer and Mr. T. Dengler for their assistance with some experiments. The numerical evaluation of the X-ray data was performed on the UNIVAC 1100/81 computer at the Computer Centre of the University of Freiburg im Breisgau. Thanks are due to the Kernforschungszentrum Karlsruhe for providing the facilities for the neutron diffraction measurements and the numerical evaluations, to Mr. W. Abel for his assistance with the computer plots and Dr. G. Heger for useful discussions. References P. Knappeand H. Miiller, 2. Anorg. AlEg. Chem., 487(1982) 63. 0. Greis, P. Knappe and H. Miiller, J. Solid State Chem., 39 (1981) 49. 3 C. G. Titcomb, A. K. Cheetham and B. E. F. Fender, J. Phys. C, 7(1974) 2409. 4 G. G. Libowitz, J. G. Pack and W. P. Binnie, Phys. Rev. B, 6(1972) 4540. 5 P. Knappe, Dissertation, Freiburg im Breisgau, 1981. 6 0. Greis and T. Petzel, 2. Anorg. Allg. Chem., 403 (1974) 1. 1

2

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7 8 9 10 11 12

13 14 15 16 17 18 19 20 21

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