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Int. J. Hydrogen Energy, Vol. 14, No. 6, pp. 371-377, 1989. Printed in Great Britain.

0360-3199/89 $3.00 + 0.00 Pergamon Press plc. © 1989 International Association for Hydrogen Energy.

CRYSTAL STRUCTURE AND MAGNETIC PROPERTIES OF HIGH-PRESSURE PHASES IN THE Fe-H AND Fe-Cr-H SYSTEMS
V. E. ANTONOV, I. T. BELASH, V. F. DEGTYAREVA, D. N. MOGILYANSKY, B. K. PONOMAREVand V. SH. SHEKHTMAN Institute of Solid State Physics, U.S.S.R. Academy of Sciences, 142432, Chernogolovka, Moscow district, U.S.S.R.

(Received for publication 6 December 1988)
Abstract--Isotherms of hydrogen solubility at T = 325°C and PH2 ~< 7 GPa were plotted for the Fe-Cr alloys with 50, 25 and 5.3 at %Cr. At T = 350°C and PH2 = 9 GPa, nearly single-phase samples of iron hydride with H/Me = 0.9 were synthesized. At atmospheric pressure, the hydrides obtained were X-rayed (T = 90 K) and their magnetization measured (80 ~< T ~< 180 K, H ~< 5 T). The hydrides of the alloys with 50 and 25 at %Cr were shown to have an hcp (2H) metal lattice. The hexagonal metal lattice of hydrides of the alloy with 5.3 at %Cr contains stacking faults partly ordered to the Sm-type (9R) structure, and the iron hydride has a dhcp (4H) lattice. The hydrides of iron and the alloys with 5.3 and 25 at %Cr are ferromagnetics, the values of their spontaneous magnetization being in accordance with the predictions of the rigid d-band model. composition and crystal structure of hydrides forming under high hydrogen pressure on the base of the iron-chromium alloys and by further extrapolation of the data obtained to the zero chromium content. Under high hydrogen pressure chromium forms an e-hydride [5] which is much more stable at atmospheric pressure than the iron hydride (Td.... p = -30°C [5, 6]); thermal treatment permits production of sufficiently homogeneous bcc (a) Fe-Cr alloys in the whole range of concentrations from Fe to Cr [7]. So, one could hope that hydrides of the Fe-Cr alloys will also be more stable at atmospheric pressure than the iron hydride, and that changes in their crystal structure, if any, with increasing the iron content of the alloys will be restricted to modifications of the e-lattice. According to the rigid d-band model [3], the e-hydrides of Fe-Cr alloys containing up to ~ 30 at % Cr, must be ferromagnetically ordered, and along with the structural studies we have examined the magnetic properties of the synthesized hydrides. Besides, an analysis of the data obtained upon investigating phase transformations in the Fe-Cr-H systems allowed us to clear up the reasons of the presence of a-Fe in the earlier synthesized Fe-H samples. As a result, nearly single-phase samples of iron hydride were synthesized and their X-ray examination performed.

INTRODUCTION The iron hydride was first synthesized under hydrogen pressure of 6.7 GPa at T = 250°C [1]. The X-ray investigation of the Fe-H samples at atmospheric pressure and T ~ 80 K has shown them to contain a considerable amount of the starting a-Fe. Besides the a-Fe lines, there were other 8 lines in the diffraction patterns referred to the iron hydride and indexed in the hcp (E) cell. At an atmospheric pressure the iron hydride is extremely thermally unstable and decomposes rapidly to H2 and a-Fe at T i>-120°C, and it was not clear whether the presence of a-Fe in the samples was due to a partial decomposition of the hydride while transferring the samples to the X-ray cryostat at atmospheric pressure or due to the incompleteness of the process of the hydride formation at a high hydrogen pressure. It has'been shown in [2] that the iron hydride is a ferromagnetic with spontaneous magnetization ao at T = 0 K close to that of a-iron and the Curie temperature Te >> 80 K, that does not contradict the rigid d-band model for the e-hydrides [3]. The Mrssbauer investigation [4] of the Fe-H samples, synthesized under the same conditions as in [1], has given, however, unexpected results. Along with the magnetic sextet, corresponding to a-Fe, the spectra displayed two additional sextets with approximately equal intensities instead of one sextet which could have been expected for a ferromagnetic E-hydride. As a most probable explanation of the observed spectra, it has been assumed in [4] that the crystal structure of iron hydride has two non-equivalent positions of iron atoms with equal multiplicity factors. Therefore, the data available on the hydrogen content of the iron hydride as well as on its crystal structure were not reliable. The present work was aimed at elucidating these characteristics of the iron hydride by studying the

EXPERIMENTAL The ingots of the Fe-Cr alloys containing 50, 25 and 5.3 %Cr were melted from Armco-iron and electrolytical chromium in an induction furnace in vacuum. After a 24 h homogenization in vacuum at 1100°C and water-quenching these ingots and also an ingot of Armco-iron were cold-rolled into 0.2 mm thick strips, then subjected to stress-relief annealing in vacuum at

371


372

V.E. ANTONOV et al.

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HIGH PRESSURE PHASES IN THE Fe-H AND Fe-Cr-H SYSTEMS
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ll00°C for 15 min and again quenched in water. The 1.2 2~o-o--oobtained strips had a bcc lattice, the values of their 1.0 o unit-cell parameters and ty0 (see the lines for n = 0 in Table 1, n is H/Me atomic ratio) well agreed with the a8 c literature data [8, 9]. The samples intended for hyd0.6 rogenation weighed about 50 mg and were made up of qB 3 x 3 mm 2 plates cut from the strips. O.& ·Hydrogenation of the alloys was carried out by a 24 h 0.2 exposure in a hydrogen atmosphere at 325°C and pressures up to 7 GPa with subsequent quenching down c~O.--o-o--o to -~ -170°C (the method is given in [3]). The error in 1.6 estimating the pressure grew from _+0.05 GPa at Prh ~<1 GPa to _+0.3 GPa at PH2 ~ 7 GPa, the temperature being measured accurate to _ 10°C. Thermal stability of the synthesized hydrides at atmospheric pressure decreased with decreasing chromium concentration in the alloys, from Tdecomp ~ q20°C for the alloy with 50 at %Cr to Toccomp ~ (60-80)°C for the alloy with 5.3 at %Cr. The hydrogen Fig. 1. Hydrogen content, n, (a) and an increase in the volume content of the samples was determined with a relative per metal atom, AVa = V, (n) - V, (0) for the FesoCrso-H solid solutions produced by a 24 h exposure at 325°C and under the accuracy of 3%, the method is given in [10]. An X-ray study was performed by a phototechnique hydrogen pressures indicated on the abscissa, e--data for using a DRON-2.0 diffractometer with FeK, radiation. a-solutions (bcc metal lattice), o--for e-solutions (hcp metal The magnetization of the samples was measured with lattice), half-blackened symbols refer to two-phase (a + e) samples. 5% error in a pulsed magnetic field up to 6 T by the induction method [11] in the temperature range 80-200 , -~ 0.03, the parameter of the bcc alloy lattice somewhat K, the pulse duration being ~0.01 s. When not in use, the Fe-H and Fe-Cr-H samples increasing, and at higher pressures solid e-solutions were stored in liquid nitrogen to prevent hydrogen (hydrides) with n ~ 0.75 and an hcp metal lattice are losses. The installation permitted loading the samples formed. Dependences n(PH2 ) and AVa (PH2) are charwithout their heating much above liquid nitrogen acterized by the presence of two plateaux in the pressure ranges =2 to 3 GPa and =4 to 7 GPa, separated by the temperature. region of a rather steep increase in the both parameters. STRUCTURAL STUDIES
n J

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For each of the three Fe-Cr-H systems, isotherms of hydrogen solubility in the metal at T= 325°C and PH2~<7 GPa were plotted. The phase composition of the synthesized samples and the crystal structure of metal sublattice of the forming phases were studied at atmospheric pressure and T = 90 K. The basic results are presented in Figs 1-3 and Tables 1, 2. The X-ray data in Figs l(b) and 3(b) are given in the form of dependences AVa = Va(n) -- Va(0) upon the pressure of synthesis and the hydrogen content of the samples, respectively, where Va is tl/e volume per metal atom in the phase under study. For the Me-H phases on the base of a great number of different transition metals and their alloys, the dependences AVa(n) are approximately linear and close to each other within wide concentration ranges [3, 12], and determination of the AVa-values is useful, in particular, for estimating the hydrogen concentration of the phases in multi-phase samples. The FesoCrs0-H system As is seen from Fig. 1, an increase in the hydrogen pressure to ~0.8 GPa at 325°C induces growth of the hydrogen concentration in the a-alloy FesoCrs0 up to n

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Fig. 2. Hydrogen content, n, vs pressure, Pn2, of the samples synthesis (24 h at 325°C) for the Fe75Crzs--H(a) and Fe94.7Crs.3H (b) solid solutions, e--a-solutions, o--hydrides (based on the hcp metal lattice (a) and on the lattice with stackings partly ordered to the 9R structure (b)), half-blackened symbols stand for the two-phase samples.


374

V.E. ANTONOV et al.

Table 2. Experimental (T = 90 K) and calculated values of lattice spacings, d, and integral intensities of the X-ray diffraction maxima, 1, for the hydrides of iron and Fe-Cr alloy with 5.3 at %Cr. The values of the unit cell parameters from Table 1 have been used in the calculation
Fe94.7Cr5.3-n ,

n=0.92+0.03 Calculation

Fe-H, n=0.88+0.03 Experiment 9R (Sin-type) Calculation 4H (double hcp)

Experiment 2H (hcp) No. 1 2 3 4 5

d(,~)
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I
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26 28 100

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lOt 2.313 012 009 104 015 107 018 10.10 01.11 110 1.270 2.267 2.189 2.106 2.005 1.795 1.692 1.504 1.420 1.345 22

I/lmax(%) NO. d (A)
42 16 54 100 86 7 13 9 3 35 12" 22 30 55 45 1 2 3 4 5* 6 7 8* 9 10 11 1.171 13 14 15 16 2.328 2.250 2.201 2.058 2.029 1.823 1.600 1.436 1.404 1.341 1.238 <1 1.152 1.145 1.124 1.080

I/lmax(%)
10 60 66 100 2 30 10 <1 12 58 30 12 40 25 10

hkl
100 101 004 102 103 104 105 110 106 201 114 202 203

d (A)
2.324 2.246 2.194 2.053 1.819 1.595 1.400 1.342 1.238 1.152 1.144 1.123 1.080

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* Lines of a-iron. Designation: m--medium, w--weak, vw--very weak~ b---broadened.

In the vicinity of PH2 = 3.5 GPa, dependences n(PH2 ) and AVa(PH2) are very similar to supercritical ones for the isomorphous el ¸'-e2 transformation terminated at a critical point at T < 325°C. At the present stage of investigation the FesoCrs0-H system one cannot neglect other possible explanations of the observed anomaly either, for instance, it may be due to the ordering of the solutions with n = 0.75 to the Me4H3 type.

The Fe75Cr25-H system
Figure 2 shows that at T -- 325°C the hydrogen solubility in the a-alloy Fe75Cr25 at Pm -,51.7 GPa is very low (n .,.<0.005), and at higher pressures e-solutions are formed with the hydrogen concentrations growing monotonically from n ~ 0.6 at PH2 = 2.8 (JFa to n = 1 at PH2 = 7 GPa. As is seen from Fig. 3, the AVe-values of the single-phase e-samples increase linearly with the hydrogen concentration. In the two-phase (a + e) samples,

the a-phase has AV a -~ 0 (that points to low hydrogen solubility in this phase) and the AVa-value of the e-phase (and, consequently, the hydrogen concentration of the e-phase) is independent of the relative content of the phase in the samples within the experimental error and constitutes ~0.6 /~3 atom-1. The intersection of the linear prolongation of dependence AVe(n) for the single-phase e-samples with the line AV~ - 0.6 A3 atom-'(see Fig. 3b) gives n ~ 0.55 for the composition of the e-phase in the two-phase samples and, accordingly, for the minimum hydrogen concentration in the e-solutions forming at T = 325°C and high hydrogen pressures, Fig. 2a. The extrapolation of the linear dependence AVa (n) for the e-solutions to even less values of the hydrogen concentrations (dotted line in Fig. 3b) produces an unexpected result. The extrapolation suggests that formation of e-solutions (virtual) with n ~ 0.3 should cause a decrease in Va instead of its increase. Such an effect has not been observed so far in any of the


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lines were observed to which fractional indices of the (10//m) type can be ascribed in the hexagonal cell. The broadening of the (hOl) type diffraction lines with h ,/: 3n is typical for the hcp metals with chaotic stacking faults. An estimation of the density of stacking, ast, by the experimental value A(20)lm ~ 1° of the (101) line width, according to the formula [13]
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Fig. 3. Spontaneous magnetization, Oo, at T = 0 K (in Bohr magnetons per metal atom) (a) and AVa (b) as functions of the hydrogen content, n, of the Fe75Crzs-H samples. *---data for a-solutions, o--for e-solutions, half-blackened symbols refer to two-phase samples. The Oo-values were obtained by extrapolation from T/> 80 K.

gave a~t ~- 0.3 for the Fe94.7Crs.3-H hydride (0 is the Debye angle, c is the hcp cell parameter, X is the wavelength of the radiation used). The presence of lines with the (10 l/m) type indices in the hcp cell suggests formation of a multilayer closepacked structure (polytype). The shortest repetition units along the c axis are characteristic of the polytype 4H (double hcp structure consisting of alternating cubic (c) and hexagonal (h) layers, chchch .... and polytype 9R (Sm-type structure) with the sequence of layers chhchhchh... Taking into consideration the value ast 1/3 of the density of stacking faults in the Fe94.7Crs.3-H hydride, it seems quite probable that the presence of the (10//m) type lines in the X-ray patterns of this hydride is due to the partial ordering of the stackings just to the 9R type, see Table 2.

The Fe-H system
Figures 1 and 2 show that as the chromium content of the Fe-Cr alloys decreases, the pressure for a hydride to start forming at 325°C increases monotonically from PH2 1 GPa for the alloy with 50 at %Cr to ~ 3.5 GPa for the alloy with 5.3 at %Cr, and the interval of the transition (where the forming Fe-Cr-H samples are two-phase mixtures) comprises ~ 0.5 GPa for the alloys with 50 and 25 at %Cr and extends to ~ 2 GPa for the alloy with 5.3 at %Cr. An examination of the T-PH~ phase diagram of the Fe-H system showed [14] that at T 250 to 350°C the iron hydride starts forming at PH2 ~ 5 'to 5.5 GPa. If one assumes the interval of the transition in the Fe-H system, as well as in the Fe94.7Crs.3-H system, to be ~ 2 GPa, then the pressures of ~ 7-7.5 GPa, exceeding those used in [1, 4], are required to synthesize single-phase samples of the iron hydride. In this work we succeeded in preparing nearly singlephase samples of the iron hydride (see Table 2) by a 24 h exposure to a hydrogen pressure of 9 GPa at T = 350°C. Therefore, the main reason lying behind the presence of a-Fe in the Fe-H samples studied in [1, 4] was incompleteness of the process of hydride formation under the conditions of synthesis. As is seen from Table 2, all the diffraction lines of the iron hydride obtained are well indexed in the double hcp cell (structure 4H). A higher intensity of the (hkl) type lines, as compared to the calculated one, is likely to be related to the sample texture (we could not get rid of the texture, since grinding the Fe-H samples even in liquid nitrogen causes considerable losses in hydrogen).

d-metal-hydrogen systems investigated. Moreover, in the most systems (for example, Pd-H, Ni-H and on the base of their alloys [3, 12]) the derivative OVa~On > 0 and decreases with the hydrogen concentration (i.e., 0 2Va/o2n < 0); as one can see from the A V,-values listed in Table 1, this is valid for FesoCrso-H e-hydrides too.

The Fe94.7Cr 5.3-H system
At T = 325°C, the equilibrium hydrogen concentration in a-solutions Fe94.7Crs.3-H did not exceed n 0.005, and starting with PH, = 3.5 GPa, the appearance of a hydride with an hcp metal lattice was observed in the samples, see Fig. 2(b). At 3.5 ~ PH2 ~ 4.5 GPa (that is, in the pressure range where the produced samples consisted of a mixture of the a-phase and the hydride), the parameters of the hcp metal lattice of the hydrides somewhat increased with the pressure of synthesis, and for the single-phase hydride samples obtained at 6 ~

376

V. E. ANTONOV et al.

It is apparently the particular texture of the samples that the values AVa and c/a for the iron hydride well (bits of the 1.1 mm dia. rod) that was responsible for the agree with the extrapolated ones. This suggests that the absence of "superstructural" lines of the 4H polytype in hydrogen content of the iron hydride does not much X-ray photographs of the iron hydride in [1]. The differ from the extrapolated value, n ~ 0.9, either, and two-phase Fe-H sample with n = 0.70 + 0.03, synthe- the Fe-H samples with n ~ 0.88 obtained in the sized in the present experiment under the same condi- experiment are practically single-phase indeed, i.e. the tions as in [1] (10 h at PH_~= 6.7 GPa and T = 250°C), texture does not distort the X-ray data in this respect. exhibited these lines. An additional argument in favour of the choice of the MAGNETIC MEASUREMENTS 4H cell to describe the metal lattice of the iron hydride is The values of spontaneous magnetization, at,. did not the existence of two non-equivalent positions of atoms change within the experimental error in the temperature with equal multiplicity factors in the 4H structure, since it agrees with the existence of the two magnetic sextets in range 80 to 180 K for all the investigated samples, the Mrssbauer spectra of the iron hydride, observed in FesoCrss-H and Fe77Cr25-H hydrides with very low as [4]. It is also worth noting that in the sequence of the 0.01 to 0.03 /~B (metal atom) ~ among them. The polytype structures 2H (hcp) --* 9R ~ 4H, proposed for observed behaviour of as is indicative of high (much the Fe-Cr-H hydrides, an increase in the fraction of above 180 K) temperatures of magnetic disordering of cubic (c) layers with decreasing the chromium content of the samples that allows us to consider their values as (80 the alloys is monotonous: zero--for the 2H structure K) as ao at T = 0 K, without introducing any essential (ccccc...) 1/3~for 9R (chhchhchh...) 1/2--for 4H error. The Oo-values for the a-solutions of hydrogen in the (chchch...). The sequence of structures 2H ~ 9R --* 4H is characteristic of the systems with the decreasing Fe-Cr alloys coincided with those for the starting alloys. The results of magnetic measurements for the samples energy of stackings. Extrapolation of properties of the Fe-Cr-H hydrides with a higher hydrogen content can be illustrated on an is also useful for elucidating the hydrogen concentration example of the Fe75Cr25-H system (Fig. 3a). It is seen that the Oo-values for the single-phase in the iron hydride which is somewhat uncertain due to Fe75Cr2s-H e-solutions increase from oo ~< 0.03 /uB the presence of the a-phase in the investigated Fe-H samples with n = 0.88, see Table 2. As follows from Figs atom -~ at n ~ 0.6 to 0.8 to Oo ~ 0.5 ~B atom ~at n ~ 1. 1 and 2 and Table 1, the characteristics of the Fe-Cr-H Magnetization of the two-phase (a + e) samples is hydrides practically stop changing at PH, ~ 6 GPa. proportional to the content of the ferromagnetic aFigure 4 depicts the data for the Fe-Cr-H hydrides phase. The approximation by a straight line (dotted line synthesized at PH_~ = 7 GPa and the iron hydride (to in Fig. 3a) provides for ao = 0 the value n ~ 0.57, which make it easier to compare the X-ray data, a doubled is close to the minimum hydrogen concentration n distance between the close-packed planes, as in the hcp 0.55, in the single-phase e-samples, determined from lattice, is taken as a c parameter for the Fe94.7Crs.3-H the X-ray measurements. An analogous dependence ao(n) for the two-phase (a (+) and Fe-H (â) hydrides where it constitutes 2/9 and 1/2 of the repetition period, respectively). One can see + e) samples was observed in the FesoCrs0-H system. All the single-phase FesoCrs0-H e-samples with 0.75 ~ n ~< 1.15 had ao~ 0.01 ~B ato m-l. < For the case of the Fe94.7Crs.3-H system the hydride 3, i i iI was ferromagnetic, and the Oo-values for the two-phase samples displayed an approximately linear decrease from 2.10 /~B atom -~ for the starting Fe94.7Crs.3 alloy down to 1.45 /~B atom -1 for the hydride with n ~ 0.92. As for the interpretation of the data obtained, the Ur ~ -i4.u following should be noted. The magnetic properties of -~ 2.0- o-o -o -]0.8 the fcc (7) and hcp (e) alloys of 3d-metals, which are the ! ~o --~~. closest neighbours in the Periodic Table, are well described by the rigid band model [9]. The concentra>g1.2~- ~ -~1.62 tion dependences of oo for the ferromagnetic alloys and i the N6el points, TN, for the antiferromagnetic ones are F~ , , .60 plotted in Fig. 5, the composition of the alloys being 0 25 50 Cr content (at.%) given in the N ~ units of the average number of external (3d + 4 s) electrons per atom of the alloy (the so-called Pauling-Slater curves). Fig. 4. Values of ao (A), AVa (o), n (A) and the ratio c/a for At present a large scope of the data on magnetic the hcp cell (e) and hcp pseudocell (+, â--see the text) as functions of chromium concentration in the Fe-Cr alloys, for properties of y- and e-hydrides of 3d-metals and their the hydrides synthesized at maximum hydrogen pressures (7 alloys is available, see [3]. An analysis of these data GPa, 325°C for Fe-Cr-H hydrides and 9 GPa, 350°C for iron permitted formulating the phenomenological rigid dhydride; the Oo-value for iron hydride is taken from [2]). band model [3] to describe concentration dependences

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HIGH PRESSURE PHASES IN THE Fe-H AND Fe-Cr-H SYSTEMS
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electrons supplied by hydrogens, the value of r/being of the order of 0.5 electrons per H atom. In case this is valid for the FesoCrs0 alloy (N ~ = 7 electrons per atom = ~n) too, its e-hydrides, as is seen from Fig. 5, should be antiferrogmagnetics with high Nrel points. Note, that the presence of stacking faults (and the hydrides of Fe-Cr alloys are liable to it) in an e-antiferromagnetic must lead to a decompensation of its magnetic sublattices that gives rise to a relatively small spontaneous moment only slightly depending on temperature at T << TN. Maybe the small spontaneous moment of the FesoCrs0-H e-hydrides (Table 1) is just of this origin.
Acknowledgements--The authors are grateful to V. G. Glebovsky in whose laboratory the Fe-Cr alloys were melted and 1. M. Romanenko for the chemical analysis of these alloys.

Fig. 5. Concentration dependences of cr o and Nrel temperature, TN, for the fcc 0') (thin lines, experimental data [9]) and hcp (e) (sections of solid lines, experimental data [9], and dotted lines, an estimation [3]) alloys of 3d-metals that are nearest neighbours in the Periodic Table, and also the experimental values of TN for the manganese e-hydride [16] (*) and ofo o for thee -solutions Co-H [17] (â) and Fe77.rMn224-H [3] (+), iron hydride [2] (&), e-hydrides of Fe75Cr2s alloy (o) and Fe94.TCrs.3-Hhydride (A), data of the present paper, at r/ = 0.5 electron per H atom, see the text.

of the magnetic properties of such hydrides. The essence of the model is that the change in these properties upon hydrogenation is mainly caused by the increase in the degree of filling up the d-band of the host metal with electrons, hydrogen being considered as the donor of a fractional number of electrons r/~ 0.3 to 0.7 electrons per H atom. Thus, according to the model, in the case of formation of y- or e-hydrides on the basis of the alloys of 3d-metals which are close neighbours in the Periodic Table, their magnetic properties should be described by the curves given in Fig. 5, provided me quantity N~eff = N ~ + on is taken as the electron concentration. The r/-values may differ considerably for the hydrides of different metals and alloys. The values of ~/for some of the y-hydrides (in particular, for the Pd and Ni hydrides [15]) were obtained directly from the quantummechanical calculation of their band structure, but those for the e-hydrides we are interested in, have not been estimated yet. Having chosen r/ = 0.5 electrons per H atom, we plotted in Fig. 5 the values of TN and cro for all the e-hydrides studied earlier and also for the hydrides of iron and Fe75Cr25 and Fe94.7Crs. 3 alloys. One can see that these values well fit the dependences TN(N~) and oo(N ~) for the hcp alloys. If the observed agreement is not assumed to be of a random character, then we have: (i) the magnetic properties of the virtual Fe-Cr alloys with an hcp lattice and polytypes on its basis are described satisfactorily by the rigid band model for the chromium concentrations up to 25 at% at least; (ii) the properties of their hydrides are mainly determined by the degree of filling up the d-band of these alloys with the

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