Structural and magnetic phase transitions in ZrMn2D3

Journal of the Less-Common

STRUCTURAL

Metals, 103 (1984)

AND MAGNETIC

267

261 - 275

PHASE TRANSITIONS

IN ZrMn2D3*

J. J. DIDISHEIM Department of Materials Science and Engineering, Cambridge, MA 02139 (U.S.A.)

Massachusetts Institute of Technology,

P. FISCHER Institut fiir Reaktortechnik, Wiirenlingen (Switzerland)

Eidgenijssische

Technische

Hochschule

Ziirich,

CH 5303

(Received June 19,1984)

Summary

The intermetallic compound ZrMn, and its deuteride ZrMn,D, were investigated using neutron powder diffraction at room temperature and below. The neutron diffraction patterns of ZrMn, at 293 and 4.2 K are almost identical, which confirms the absence of any structural or magnetic transition. Two transitions are observed in ZrMn,Ds: an order-disorder phase transformation of the deuterium atoms at 230 + 10 K, and a magnetic transition at 175 + 5 K. The metal atoms appear to retain an undistorted Cl4 hexagonal structure at low temperatures, but further experimental work is needed to determine the ordered ~s~bution of the deuterium atoms. The magnetic scattering gives rise to weak peaks at low Bragg angles, which can be indexed on the basis of the orthohexagonal cell.

1. Introduction

At present there is a great deal of interest in the hydrides of ZrMn,based intermetallics as potential hydrogen storage systems [l - 41. The binary compound has a homogeneity field ranging from ZrMn,.s to ZrMnsa4 f2], which is unusually wide for a Friauf-Laves phase. The hydrogen content x of the hydride phase ZrMn, +y HX lies between 2.8 and 3.8. Much of the current interest in this compound is also due to its magnetic properties. Whereas ZrMn, +Y is a Pauli paramagnet, magnetic measurements [ 2, 5,6] on the hydrrde phase ZrMn,,, HX show an increase in magnetization with decreasing temperature below 145 K, indicating weak ferromagnetism or ferrimagnetism with a small moment of 0.04 pg (formula unit)-’ at 4.2 K in a field of 21 kOe. Different behaviour is observed, however, when the samples are cooled in the absence of an applied magnetic field; in this case *Paper presented at the International Symposium on the Properties and Applications of Metal Hydrides IV, Eilat, Israel, April 9 - 13, 1984. oozz-5068/64/$3.00

0 Elsevier ~quoia/~inted

in The Netherlan~

268

the magnetization has a broad maximum at about 70 - 100 K. The origin of this “spin glass” behaviour and the nature of the magnetic interactions are not completely understood at present. It should also be noted that the related hydride Ti,.,Mn,.sHs shows “well-behaved” ferromagnetism with T, = 213 K [7,8]. In this paper we present the results of a neutron diffraction investigation of ZrMn, and its de&ride ZrMn,D3 at low temperatures. The structure at room temperature has been studied previously [9]. It was shown that the C14-type structure of ZrMn, is not modified by the absorption of hydrogen (or deuterium). The deuterium atoms are located on four sites which all correspond to tetrahedral interstices surrounded by two zirconium and two manganese atoms. The interstices are only partially occupied, and it has been suggested [9] that an order-disorder transformation may take place at a lower temperature. The neutron diffraction data presented below will show the occurrence of such a transition at 230 K and of a magnetic transition at 175 K. 2. Experimental

details

Samples of ZrMnz were prepared in an arc furnace. An X-ray powder photograph showed that the alloy was a single-phase hexagonal Cl4 structure with lattice constants a = 5.035(5) A and c = 8.277(3) A. These values are in close agreement with those obtained previously [9] and also with those given in ref. 2 for stoichiometric ZrMn,. The deuteride was prepared by heating the powder to 373 K in deuterium at 10 bar, followed by slow cooling. A gravimetric measurement gave the formula ZrMnzDs.orl,,,7. The X-ray lines, which were somewhat broader than those of the starting material but still measurable, gave the lattice constants a = 5.426(5) A and c = 8.804(6) A. Neutron powder spectra were recorded at the Saphir reactor, Wiirenlingen, on a two-axis diffractometer equipped with a CT1 closedcycle helium refrigerator. The sample was enclosed in a sealed cylindrical vanadium container. The Rietveld program [lo] was used for crystallographic refinements.

3. Results and discussion 3.1. ZrMn2 The neutron diffraction pattern of ZrMn? was recorded at 293 and 4.2 K before deuleration of the sample. The two spectra were essentially identical, thus confirming the absence of any structural or magnetic transition. The following lattice constants were obtained by the Rietveld profilefitting method (the numerals in parentheses are the estimated standard deviations calculated using the Rietveld program; they do not include possible errors due to wavelength calibration): u = 5.0307(2) A and c = 8.2679(6) A at 293 K; a = 5.0191(4) A and c = 8.2596(8) A at 4.2 K.

269

1

Drl

I

2-THETA n *

(DEGREES>

,* 10 ’

IOBS - 'ICALC ' neutron diffraction patterns Fig. 1. Comparison of the observed (0) and calculated ( -) of ZrMnz at room temperature (the diffraction pattern at T = 4.2 K is almost identical) for a neutron wavelength x of 2.335 ,&. The difference between the observed and refined profiles is shown at the bottom of the figure. The background and an unidentified peak at 28 = 19.7” (see also Fig. 2) were subtracted. The positions of the Bragg reflections are shown as vertical bars together with their Miller indices (space group, PGs/mmc).

No other crystallographic details are reported here; at both temperatures the two refined positional parameters were identical, within the standard deviation, with those given in ref. 9. We also investigated the possibility of a deviation from stoichiometry by refining the manganese occupancy of the zirconium site. From the values refined at both temperatures we obtained Mnz, which may represent the true composithe formula Zro.9s5(5)Mno.~s(5) tion of our sample. Agreement factors [lo] for the refinements with one temperature factor were R, = 0.030 (0.065) and RprofWeig = 0.065 (0.075) at T = 293 K (4.2 K). The diffraction pattern at room temperature is shown in Fig. 1. 3.2. ZrMn,D, The neutron diffraction patterns are shown in Fig. 2. The diffraction pattern at 293 previously [9]. The structure at room was refined and structural parameters

of ZrMn,D, at 293, 230,180

and 7 K

K is very similar to that measured temperature (space group, PGs/nmc) identical with those reported in ref. 9

270

1.50

(b)

3. 80

1.50

2-THETA

6.06

7.58

(.OEGREES>

9.I

10.50

12.00

.50

*10 '

Fig. 2. Neutron powder diffraction pattern of ZrMnzIfs at (a) 293 K, (b) 230 K, (c) 180 K and (d) 7 K for a neutron wavelength x of 2.341 a The intensity scale is fogarithmic and hence weak peaks are emphasized, An unidentified peak at 28 = 19.0” is denoted by a full diamond. The peaks indicated by arrows in (b) (230 K) and (c) (180 K) are due to the ordering of the deuterium atoms. The bars in (c) (180 K) show the position

271

IIn

Cd)

1.311

3. II)

a. 50

2-THETA

6.00

7.50

(DEGREES)

10.50

9.00

*18

12.m

50

l

of the Bragg reflections in the hexagonal cell; the lower row corresponds to the reflections of the Cl4 structure (see also Fig. I), and the upper row shows the reflections which are forbidden in space group P6a/mmc. The arrows in (d) (7 K) show the magnetic peaks together with their indexing in the orthohexagonal cell (see text).

272

2-

l-

I 50

I 100

I 150

I 200

t(K)

Fig. 3. Temperature dependence of the integrated intensity of one peak in ZrMnzD3 owing to the ordering of the deuterium atoms (0, 28 = 56.2” in Fig. 2(c) and hkl = 103 in the hexagonal cell (measured on cooling)) and of one magnetic peak (m, 28 = 21.0” in Fig. 2(d) and hkl = 101 in the orthohexagonal cell (measured on heating)).

were obtained, except for slightly higher occupancy factors of the deuterium atom sites owing to a larger deuterium content of the sample. The refined occupancy factors give the composition ZrMn,Dz.9 k o. 1, which is in good agreement with the value obtained by gravimetry. Figure 2 shows the presence of additional reflections at 230 and 180 K and of yet other new reflections at 7 K, indicating that two phase transitions take place. As can be seen in Fig. 3, they appear to be second order and occur at 230 f 10 K and 175 f 5 K. It will be shown below that the transition at 230 K is an order-disorder transformation involving the deuterium atoms, whereas the transition at 175 K is of magnetic origin. 3.2.1. The order-disorder phase transformation The strong diffuse background peak in the diffraction pattern of ZrMn,D3 at 293 K (Fig. 2(a)) is due to the short-range order between the deuterium atoms which is such that two adjacent tetrahedral interstices are never occupied simultaneously [9]. The disappearance of this diffuse peak between 293 and 180 K shows that the transition at 230 + 10 K is associated with a redistribution of the deuterium atoms.* This inference is *The relatively large error in the temperature of the phase transition is due to a discrepancy between Fig. 3, which shows zero intensity for the 103 peak at 230 K, and Fig. 2(b), in which superstructure peaks are clearly present at 230 K. We do not know whether this discrepancy is due to an error in the temperature calibration or to a slight hysteresis effect.

273

also supported by pulsed proton magnetic resonance measurements on ZrMn,H, [ll] in which an unexplained minimum in the Knight shift was observed at about 220 K. However, it was concluded from the temperature dependence of the relaxation times that “this minimum occurs at temperatures where the fast hydrogen diffusion starts” [ 111. An identical disappearance of the diffuse short-range order peak with decreasing temperature has recently been observed in order-disorder phase transitions of the cubic Friauf-Laves phase deuterides ZrVZD3.6 [ 121, HfV2D4 [ 131 and ZrCr,D, [14]. Their low temperature structures are characterized by an ordered arrangement of the deuterium atoms in tetrahedral interstices which do not share faces and are about 2 A apart. The transitions in these compounds are accompanied by marked changes in the neutron diffraction intensities and by the splitting of the Bragg peaks as a result of reduction in the Bravais lattice symmetry. In contrast, a comparison of Figs. 2(a) and 2(c) shows no line splitting and far less dramatic changes in the intensities for the 230 K transition in ZrMn,D3, which indicates that the ordering of the deuterium atoms might be of a different type from that in the cubic compounds. Table 1 indicates that the additional peaks may be indexed in the same hexagonal cell as used for the room temperature structure. The symmetry TABLE 1 Miller indices and observed and calculated Bragg angles for the peaks due to ordering of the deuterium atoms (nuclear peaks indicated by arrows in Figs. 2(b) and 2(c)) and for the magnetic peaks (indicated by arrows in Fig. 2(d)) Nuclear peaks hkl, hexagonal cell 102 110 111* 103 201 104 I 113a 114 205 106 221a

Magnetic peaks 28 obs

43.1 50.6 54.1 56.3 62.7

f f f f +

0.3 0.4 0.3 0.3 0.5

+ 12.3 0.3 36.7 + 0.3 112.5 + 1.0 123.8 f 1.0

28 Cl3lC

hkl, orthohexagonal cell

28 obs

42.9 51.3 53.9 56.3 62.3 72.2 72.3 86.9 113.0 114.1 122.4

100 101 010 012

14.3 21.0 23.1 40.1

f + f f

28 CdC

0.3 0.3 0.6 0.3

14.4 21.1 25.0 40.2

The measured angles are all at T = 7 K and h = 2.341 A. Indexing is based on the hexagonal cell (an, ch) for th e nuclear peaks and on the orthohexagonal cell, with a, = 3cn1’2, b, = ah and Co = Ch, for the magnetic peaks. The cell constants at 7 K used for the computation of the Bragg angles are an = 5.405 and ch = 6.170 and were obtained from the position of strong non-overlapping peaks of the Cl4 structure. *Bragg reflections of the type hhl with 1 odd are forbidden in space group P63/mmc.

274

of the low temperature phase is reduced, however, as shown by the presence of hhl reflections with 1 odd, which implies the suppression of the c glide plane, whereas the absence of 001 reflections with 1 odd (see Fig. 2(c)) indicates that the 6a axis is conserved. Accordingly, we have tested various superstructure models, with ordering schemes similar to those found in the cubic compounds, in space groups P6,22, P6,/m and P6s which are subgroups of the high temperature space group PG,/mmc. None of these models was found to give satisfactory agreement with the experimental data. Although we probably did not test all possible superstructure models, we feel that, for the reasons mentioned above, a more complicated ordering scheme may operate; more experimental work is now in progress to determine the ordered structure. In particular, some uncertainty remains concerning the true symmetry at low temperatures, as the agreement between observed and calculated angles for some of the peaks listed in Table 1 is not quite satisfactory. 3.2.2. The magnetic phase transition The diffraction pattern of ZrMn,D, at 7 K (Fig. 2(d)) contains weak additional lines at small angles compared with the patterns measured at higher temperatures. As shown in Table 1, they can be indexed on the basis of an orthorhombic unit cell with lattice constants a, = 31’2ah, b, = ah and c, = ch. The temperature dependence of the corresponding reflection (101) is shown in Fig. 3 and indicates a second-order transition at 175 ? 5 K. Table 1 shows that only reflections with h + k = 2n + 1 are observed, which corresponds to the extinction rule for antiferromagnetic coupling between moments separated by the translation (a, + b,)/2. Consequently, we consider these peaks and the transition at 175 K to be of magnetic origin. If it is assumed that the distortion of the metal configuration is negligible compared with the Cl4 structure, the simplest model suggested by the observed extinctions is an antiferromagnetic arrangement of Mn 3d moments parallel to c, with antitranslations along (a, + b,)/2. This model, however, gives only an approximate qualitative agreement with the experimental intensities. Further work will be needed in order to improve these preliminary results. The magnetic transition temperature (T = 175 K) is markedly higher than that obtained from magnetic measurements (T = 145 K) [ 5,6]. The most probable causes of this discrepancy are different manganese concentrations and/or different hydrogen (deuterium) concentrations in the samples, as the magnetic properties appear to depend critically on the exact composition (see the discussion in ref. 6). As mentioned in Section 1, magnetic measurements on ZrMn*+,H, indicate that the compound is a weak ferromagnet or ferrimagnet which exhibits spin glass behaviour when cooled in the absence of an applied field. Our results show that ZrMn2Ds is not a simple ferromagnet, but the exact nature of the magnetic structure has yet to be determined. Finally, we might ask whether the structural transition at 230 K and the magnetic transition at 175 K are related. The ordering of the deuterium

275

atoms is accompanied by a reduction in crystal symmetry; this could, for instance, produce sufficient change in the environment of the manganese atoms to affect the magnetic interactions. If there exists such a direct relation, the determination of the ordered arrangement of the deuterium atoms would prove to be essential for a complete understanding of the magnetic properties.

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