Indian Journal of Chemistry Vol. 42A, July 2003, pp. 1597-1603

Hydrides of TbM2 (M = Fe and Mn)

S K Kulshreshtha*, R Sasikala & 0 D Jayakumar

Novel Materials and Structural Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India

Received 28 January 2002; revised 25 March 2003

Intermetallics TbFe2, TbFeMn and TbMn2 having C-15 cubic Laves phase structure, have been prepared and hydrided to varying values of HIM using gas phase method up to a saturation composition of HIM == 1.6. TbFe2H. samples show the formation of a metastable rhombohedral phase for higher values of x, which on equilibration with hydrogen transforms to cubic form. Unlike this, TbFeMn shows the existence of both hydrided and unhydrided phases for x ::; 3.0 and fo r higher values of x, uniform hydriding takes place and a saturation composition of TbFeMnH4.8 which is characterized by cubic structure with 'a' = 8.137 A, is obtained. For TbMn2H., for x 2: 1.2, the formation of a tetragonal phase is observed. Mossbauer study of TbFe2H. samples reveals that for TbFe2, the Fe spins are aligned along ( Ill ) direction, which on hydriding become non-collinear and shift towards ( 100) direction for TbFe2H3.15 composition. Mossbauer spectra of TbFeMn are complex and suggest a non-collinear spi n alignment due to the existence of both ferromagnetic and anti ferromagnetic interactions between transition metal elements and show a systematic decrease in magnetic field and magnetic transition temperature due to hydrogen incorporation in the matrix. Temperature programmed hydrogen desorption studies of TbFe2H. and TbFeMnH. samples exhibit broad peaks in the region 225-350°C, due to overlap effects and peak maxima shift slightly towards higher temperature side with Mn substitution. Unlike this, TbMn2H. samples exhibit much better resolved TPR curves consisting of mainly three peaks placed over the region 275-400°C, possibly because of the tetragonal structure of these samples.

Many intermetallics of rare earth and transition metals are known to absorb large quantities of hydrogen under moderate conditions of temperature and pressure. In the past few years a number of investigations l

-5 have been carried out to understand

the hydriding characteristics of Laves phase cubic compounds because of the simplicity of their crystallographic and magnetic structures. The aim of these studies has been to establish the conditions and mechanism of hydride fo rmation, changes in crystallographic and magnetic characteristics due to hydrogen incorporation and the degradation of AB2 intermetallics during hydriding and dehydriding cycles. In this structure there are eight AB2 formula units that constitute the unit cell and hydrogen atoms occupy the tetrahedral voids formed in this structure. There are 12 equivalent A2B2, y-sites (96 g), 4 equivalent AB), ~-sites (32e) and one B4, a-site (8b) per formula unit available for hydrogen occupation. (The numbers in the brackets represent the Wyckoff notations of these sites.) However the relative occupancy of these sites and the sequence of filling of these sites are controlled by the relative size of different types of voids, mlmmum hydrogen­hydrogen separation and the electronic structure of rare earth and transition metal atoms. The occupancy of a -sites has not been well-established6

,7 as for most

of the systems the observed value of hydrogen content can be accounted by complete filling of only y and ~ sites. A number of neutron diffraction studies8

-lo

,

carried out for different systems suggest that predominantly the y sites, are occupied with some amount of hydrogen in ~ sites. This happens because of the higher affinity of hydrogen towards rare earth atoms as compared to the transition metal atoms, which is also reflected in the formation of their bulk hydrides. Temperature programmed hydrogen desorption studies can be used to get information about the occupation of hydrogen in different tetrahedral sites and their stability during desorption process as has been demonstrated by Park and Lee ll

and Kim et ai. 12 for ErFe2 and CeNi2 systems, respectively . Based on the observance of three peaks in the TPR curves, Park and Lee ll reported that there are two types of A2B2 sites and one type of AB3 sites for hydrogen occupation in ErFe2Hx system.

Another important aspect of the study of such hydrides, is to see the effect of hydrogen incorporation in the tetrahedral interstitial sites on the magnetic characteristics of these intermetallics, which have been extensively investigated by Mossbauer spectroscopyI3-16. In general it has been observed that on hydriding, the isomeric shift increases and both internal hyperfine field and magnetic transition

1598 INDIAN J CHEM. SEC A. JULY 2003

temperature decrease due to the de-localization of charges and increased volume effects. It has also been reported that irrespective of the direction of spin orientation in RFe2 compounds, Mossbauer spectra of ternary hydrides are a broad single sextet. The possible reasons for observing such a broad pattern are (a) the random occupation of interstitial sites by hydrogen atoms thereby locally affecting the direction of spin alignment which leads to a distribution of hyperfine interaction parameters, (b) the existence of more than one type of hydride phases of slightly different hydrogen stoichiometry and (c) the existence of very fine magnetic particles produced by the violent reaction of hydrogen with intermetallics, leading to the amorphisation of such material.

In the present communication we report the results of hydriding characteristics of isostructural TbM2 intermetallics with M = Fe and Mn to see the effect of replacement of Fe by Mn on the crystallographic and magnetic characteristics of these hydrided samples using Mossbauer effect and powder X-ray diffraction techniques. Temperature programmed hydrogen desorption studies of TbM2Hx samples have been calTied out to understand the filling of tetrahedral voids by hydrogen atoms .

Materials and Methods Intermetallics were prepared by repeated arc

melting of the constituent metals in ultra pure argon flow. The ingots so prepared were annealed at '" 1125K in evacuated and sealed quartz tubes for 200 h. The formation of single phase intermetallics, with cubic C-15 structure, was confirmed by powder X-ray diffraction technique. The hydrides of the intermetallics with varying values of 'x' were prepared by standard gas phase method using Sievert type instrument made of stainless steel. Freshly crushed samples weighing approximately 0.5 to 1.0 gm were placed in the hydriding reactor evacuated to a pressure of '" I 0-5 torr at room temperature and exposed to varying pressures of hydrogen to get the desired composition of hydrides. For TbMn2, one cycle of activation treatment is required before it starts absorbing hydrogen . For X-ray diffraction studies of these samples, monochromatic CuKa radiation' was used. 57Fe Mossbauer spectra were recorded at room temperature and 78K using a constant acceleration electromechanical drive coupled with a 1024 channel analyzer. The source used was 57CO in Rh matrix and absorbers contained approximately 0.25 to 0.30 mg 57Fe/cm2. All isomeric

shift values are reported with respect to iron metal at room temperature.

Temperature programmed hydrogen desorption spectra were recorded with a beating rate of 10 Klmin. Samples weighing approximately 50 mg, were used for these experiments. The instrument consists of a tubular stainless steel reactor surrounded by a furnace. The evolved hydrogen gas was detected by gas chromatographic method using a molecular sieve column and helium as carrier gas. A thermal conductivity detector was used for the analysis of evolved hydrogen.

Results and Discussion The intermetallics TbFe2 and TbFeMn start

absorbing hydrogen without any activation process. Unlike this, TbMn2 needs one activation cycle before it can absorb significant amount of hydrogen. This activation process comprises of heating the sample in vacuum at 575K for about two hours followed by heating at the same temperature for about half an hour under hydrogen pressure of about five atmospheres. The sample when cooled to room temperature under hydrogen atmosphere exhibited hydrogen absorption. Before starting the actual measurements for hydrogen absorption for this sample, the hydrogen absorbed during activation process was completely desorbed by heating it at about 725K for two hours under vacuum. The pressure composition isotherms, recorded at room temperature for these three intermetallics, are shown

16

14 f-

12 f-~ .. E -:; 10 I-

w a: :::> Sf-en en w a: D- 6 I-

4 f-

2 I-

0 0-4~ ~J

0 1 2 3 4 5 6

• X' VALUES

Fig. l - Pressure composition isotherms recorded at room temperature for TbFe2(- ). TbFeMn (0) and TbMn2 (1'.).

KULSHRESHTHA et al. : HYDRIDES OF TbM2 1599

in Fig.1. From this figure it is clear that these intermetallics absorb hydrogen quantitatively on exposure to hydrogen and the saturation composition could be achieved at few atmospheres of hydrogen pressure. The observed saturation composition for these intermetallics are TbFe2~.8. TbFeMn~.8 and TbMn2~.5 with saturation value of HIM ~ 1.6 and by adjusting the hydrogen pressure in the manifold, it was possible to prepare the hydrides with varying value of HIM.

An unusual metastable behaviour has been observed for the hydrides of TbFe2. The hydride samples with x:s 3.15, are found to be cubic in nature. But the samples with higher concentration of hydrogen show a rhombohedral distortion on initial exposure to hydrogen, as can be seen from Fig.2(c) where representative region of the X-ray diffraction pattern is shown for TbFe2H4.8. However when these samples were equilibrated under hydrogen pressure for extended time, the hydrided samples showed a cubic structure with systematic increase in unit cell volume with increase in hydrogen absorption as can be seen from Fig. 2(d). The observance of a metastable rhombohedral phase for TbFe2Hx is in conformity with the results reported by Berthier et ai.17 Based on the volume analysis of the saturated hydrogen composition, the average increase in the unit cell volume corresponds to ;:::: 3 A3 per hydrogen atom.

The intermetallic TbFeMn, which has a cubic structure with a = 7.440 A, when hydrided to different values of bulk HIM, shows the existence of both hydrided and unhydrided phases up to a bulk composition of HIM ~ 1.0 and for higher values of HIM, the observed structure was cubic, with a saturation composition of TbFeMn~.8 ' The relative fraction of unhydrided phase decreased systematically with the increasing values of HIM and the unit cell volume of hydrided phase show a continuous increase. Essentially similar behaviour has been observed for ErFe2Hx and Zr l -yHor;Co2Hx systems by Park and Lee ll and Ramesh et al. 8 respectively. The saturation composition TbFeMn~.8 showed a volume increase of approximately 31 % which is comparable to the value of 29% observed for TbFe2H4.8 and the average volume occupied by each hydrogen atom works out to be ;:::: 3.4 A3.

The X-ray diffraction pattern of TbMn2 shows the formation of cubic Laves phase structure with a = 7.633A. On hydriding, this sample retained its cubic structure for smaller values of 'x', however, the

~

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>-II: <l II:

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Z \oJ t-Z ...

45

N N N

40 35

29 (DEGREES)

o N N

30

Fig. 2-Representative region of the powder X-ray diffraction patterns of (a) cubic TbFe2 (b) cubic TbFe2H3.15 (c) rhombohedral TbFe2H4.8and (d) cubic TbFe2~ 8'

." t­i: :::>

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(0)

45 40

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Fig. 3-Representative region of the powder X-ray diffraction patterns of (a) cubic TbMN2, (b) tetragonal TbMn2H2 .. 6, (c) tetragonal TbMn2H, I' d (d) tetragonal TbMn2H4.5.

1600 INDIAN J CHEM. SEC A, JULY 2003

samples with x ~ 1.2 show a tetragonal distortion as most of the peaks are found to be split into two peaks (viz. Fig. 3) where a representative region of the X-ray diffraction patterns is shown for some of these samples. It may be mentioned that similar splitting of different Brag reflections has been reported for the hydrides of YFe219, YMn220 and some other systems2. These authors assigned it to the simultaneous formation of two metastable cubic hydrided phases with varying hydrogen composition. However in the present study, the relative intensity ratio of the splitted peaks corresponding to the different Bragg reflections of TbMn2Hx is found to be significantly different as can be seen from Fig. 3, thereby ruling out the possibility of two different phases with varying values of hydrogen composition . All the Bragg reflections for different hydrogen compositions could be indexed in terms of tetragonal unit cell with cia ratio"" 0 .512. For example for TbMn2H4.5 the values 'a' and 'c' parameters are 10.435 and 5.339A, respectively . This gives rise to a volume change of about 35% due to hydrogen incorporation and the average volume occupied by each hydrogen atom works out to be 3.8A3. The variation of unit cell parameters and unit cell volume as a function of hydrogen contents, is shown in Fig. 4. For TbMn2Hx samples, relatively larger increase in unit cell volume was observed when the system changed from cubic to tetragonal form. Further increase in hydrogen concentration did not give rise to any significant volume change.

All these intermetallic hydrides were found to be stable at room temperature and retained their hydrogen concentration on exposure to atmosphere over a period of almost two years. These hydrides are not expected to loose hydrogen as their desorption plateau pressure lies below atmospheric pressure. Further it was possible to get back the cubic crystalline form for TbFe2 and TbFeMn hydrides with decreasing values of x by slowly heating these samples to different temperatures. However, sudden heating of TbFe2Hx samples at 775K lead to the formation of amorphous Fe and terbium hydride. TbMn2Hx samples when heated up to different temperatures for the partial removal of hydrogen, transformed to cubic form on heating them up to different temperatures. On heating at 775K the hydrogen is completely lost and cubic crystalline TbMn2 could be recovered.

57Fe Mossbauer spectra of TbFe2Hx samples, recorded at room temperature and 78K, are shown in Fig. 5. The observed spectra for TbFe2 are complex

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Fig. 4--Variation of the lattice parameters and unit cell volume as a function of hydrogen composition for TbFe2Hx. (0 ), TbFeMnHx (x) and TbMn2Hx (Ll ) •

and consist of two overlapping magnetic sextets of unequal intensity and characterized by different values of internal hyperfine field. For C-15 cubic Laves phase structure, the Fe atoms in a unit cell are placed at the corner sharing network of regular tetrahedrons with trigonal symmetry. Thus there are four sets of local principal e.f.g. axes with V zz lying along the equivalent < III > directions for the four crystallographically equivalent Fe sites. These Fe sites are characterized by single values of isomeric shift and quadrupole splitting. However, depending on the spin alignment, the angle (8), between the directions of the principal electric field gradient and spin orientation, varies and the observed values of internal hyperfine field for these crystallogrphically equivalent Fe sites are found to be different because of the existence of anisotropic hyperfine fields. Thus for

KULSHRESHTHA ef al.: HYDRIDES OF TbM2 1601

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w IBIl.B ' .. "';.'~ ..... q : I-< (g) It: IBIl.B ~W

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Fig. 5- ~7Fe Mossbauer spectra of (a) TbFe2 at RT, (b) TbFe2 at 78K, (c) TbFe2HI.87 at RT, (d) TbFe2HI.87 at 78K, (e) TbFe2H2.97 at RT, (f) TbFe2H197 at 78K , (g) TbFe2H3.15 at RT and (h) TbFe2H3.15 at 78K

specific directions of spin orientation like (100), (110) and (III), the shape of 57Fe Mossbauer spectrum gi ves a clear idea of spin orientation (21-24). Fig. 5(a) and (b) show the 57Fe Mossbauer spectra of TbFe2 recorded at room temperature and 78K, respectively . The shape of these spectra suggests that the Fe spins are aligned along (III) direction which is in conformity with the observations of Atzmony et al. (21). From the analysis of room temperature spectrum of TbFe2 the values of isomeric shift 8 = -0.07 mm S-I, and quadrupole splitting ilEq = -0.49 mm S-I, have been obtained. The internal hyperfine field values for the two types of Fe atoms characterized by e = 700 3i and 00

, are 215 and 193 Koe, respectively. The analysis of liquid nitrogen temperature spectrum gave 8 = +0.03 mm S-I, ilEq = -0.52 mm S-1 and the corresponding values of internal hyperfine field were found to be 228 and 200 Koe, respectively. Fig. 5(c) to (f) show the room temperature and 78K, Mossbauer spectra of TbFe2H1.87 and TbFe2H2.37 compositions, respecti vely. The observed spectra are quite complex and exhibit significant line broadening. This is arising due to non-coll inear spin alignment at Fe sites due to varying number of hydrogen atoms in its vicinity .

Room temperature spectrum of TbFe2H3. 15 shown in Fig. 5(g) is more like a single symmetric sextet which is characteristic of spin orientation along (100) direction as has been reported for a number of RFe2 intermetallics22. Slight excess line width observed for this spectrum is arising due to the random occupation of hydrogen atoms in the tetrahedral sites thereby locally affecting the collinearity of spin alignment. The analysis of this room temperature spectrum gave 8 = +0.25 mm S-1 and an internal hyperfine fie ld - 176 KOe. This change in 8 values is arising both due to the increase in unit cell volume and changed electron density in the 3d band of the Fe atoms. The observed value of internal hyperfine field at room temperature for TbFe2H3.15 is much less thall the average value of the internal hyperfine field observed for TbFe2 at

room temperature. The 78K spectrum of thi s composition is again complex . This suggests a composition and temperature dependent spin orientation for TbFe2Hx system. The room temperature Mossbauer spectrum of TbFe2H4.8 is a paramagnetic doublet characterized by 8 = 0.57 mill S-1 and ilEq = 0.45 mm S-I. These observations suggest that the magnetic transition temperature for TbFe2H, decreases with increase in hydrogen contents as has been reported for a number of RFe2Hx systems25-27

.

The room temperature and 78K Mossbauer spectra of TbFeMnHx samples which exhibited uniform hydriding, are found to be complex and do not show any specific spin orientation down to 78K. Thi s is arising due to the magnetic frustration created in the lattice due to antiferromagnetic ordering between the Mn atoms28 and ferromagnetic ordering between Fe atoms29 as the magnetic ordering in these AB:! type compounds is mainly decided by the most prominent B-B interaction . The presence of varying number of hydrogen atoms in the '.'icinity of Fe atoms, further affects the nature of magnetic interactions and the direction of spin alignment. The hydrogen incorporation leads to a qualitative decrease in the values of internal ' hyperfine field and magneti c transition temperature and an increase in isomeric shift values. These features are consistent with the behaviour observed for TbFe2Hx systems.

Temperature programmed hydrogen desorption measurements, carried out for varying values of ·x· . for TbFe2HX! TbFeMnHx and TbMn2Hx, are shown in Fig. 6. From this figure it is clear that for a given intermetallic the overall line shape of the TPD curve is not affected by the extent of hydrogen loading and in general same number of c10sely overlapping peaks

1602 INDIAN J CHEM. SEC A. JULY 2003

( i )

( h)

~ <1 ~ (9) ~ <1 W 11.

C\J J: ( f ) l1. 0 (e) >-t: C/l (d) z w I-Z ....

( c)

( b)

(0)

Fig. 6-Temperature programmed hydrogen desorption curves for (a) TbFe2H L87. (b) TbFe2H2.97. (c) TbFe2H3.15. (d) TbFeMnH22• (e) TbFeMnH3.47. (f) TbFeMnHu (g) TbMn2H I.2. (h) Tl:Mn2H2.6 and (i) TbMn2~.5'

have been observed. For all the three intermetallics although some difference in the relative intensity of these peaks has been observed depending on the value of 'x' . However for TbMn2Hx the temperature region over which hydrogen desorption occurs is much broader than that observed for TbFe2 and TbFeMn. This may be because of the better stability of TbMn2 hydrides and also to some extent due to the change in environment of hydrogen site brought about by the structural transformation of TbMn2 on hydrogen incorporation. Further, the temperature at which hydrogen desorption Initiates is systematically increased with Mn substitution. The temperature region, over which hydrogen desorption occurs for these intermetallics, is much smaller than that observed for the hydrides of isostructural TbCo2 and TbNi2, both of which form amorphous hydrides3o, and ErFe2Hx reported by Park and Lee ll

. Based on the shape of these TPR curves, which is not appreciably

affected for different hydride compositIOns, it is inferred that for these compositions, most of the hydrogen atoms are mainly occupying the y-sites. If hydrogen atoms had also been placed at the other two a and ~ sites, there should have been significant difference in their hydrogen desorption temperatures because of the difference in the constitution of these sites.

The two factors, which mainly decide the stability of hydride phases of these terbium intermetallics, are the electron exchange between hydrogen and transition metal atoms and the relative volume of the tetrahedral voids, which are to be occupied by hydrogen . For TbMn2 the volume of the tetrahedral voids is slightly larger than that of TbFe2 as the ' a' values, for these two intermetaUics, are 7.633 and 7.355A respectively. Further the large positive shift of :::; 0.64 mms·1 observed in the room temperature values of the chemical shift of TbFe2 and TbFe2i4.8 cannot be completely accounted in terms of the increased volume of the unit cell as suggested by Weisenger31 who had invoked the transfer of 4s electrons of Fe to the hydrogen atoms as the Fe 3d states are not appreciably affected by hydrogen incorporation in the lattice. Similar suggestions regarding the 4s electron transfer from Fe to hydrogen site have been made by Gupta32 as well as Switendick33. This 4s electron transfer has also been substantiated from the XPS studies of Schlapbach34

for ZrMn2H3 system. The 4s electron transfer is expected to be easier for Mn as compared to that of Fe because of the difference in theirelectronegativity values, which are 1.55 and 1.83, respectively. Both these factors suggest a better stabili ty for TbMn2Hx samples as compared to TbFe2Hx samples, which is in agreement with the results reported here.

It can be concluded that in the present communication we have reported the formation of various hydride compositions for isostructural TbFe2, TbFeMn and TbMn2 with a saturation value of HIM equal to :::; 1.6 and their crystallographic characteristics have been reported. Based on Mossbauer effect studies, it is observed that the hydrogen incorporation in TbFe2 leads to non-linear spin alignment and in general the strength of magnetic interactions is decreased due to increased unit cell volume and partial charge transfer. Based on temperature programmed hydrogen desorption studies it is inferred that primarily the y-sites are being occupied by hydrogen atoms in these intermetallic hydrides.

KULSHRESHTHA et al. : HYDRIDES OF TbM2 1603

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