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Journal of Alloys and Compounds 330 ­ 332 ( 2002 ) 110 ­ 116

www.elsevier.com / locate / jallcom

L

Phase transformations, crystal and magnetic structures of high-pressure hydrides of d -metals
V.E. Antonov*
Institute of Solid State Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow District, Russia

Abstract This paper will briefly discuss the high-pressure-hydrogen techniques used at the Institute of Solid State Physics, Russian Academy of Sciences, the T ­ P diagrams of the studied binary metal ­ hydrogen systems and the crystal and magnetic structures of high-pressure hydrides formed in those systems. A compilation of the available experimental data and a list of relevant publications are provided for reference purposes. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Metal hydrides; Crystal structure; Magnetic properties

1. Introduction Most transition metals form hydrides at sufficiently high pressures of hydrogen. The technique for compressing gaseous hydrogen to pressures of up to 9 GPa at temperatures up to 5008C [1], and recently to 10008C [2], developed at the Institute of Solid State Physics, Russian Academy of Sciences ( ISSP RAS ), made it possible to synthesise hydrides of all 3d -metals and of all 4d -metals except Ru. In particular, hydrides of Fe, Co, Mo, Tc, Rh and Re have been synthesised at the ISSP for the first time. The technique also allows the rapid cooling of the hydrides under high pressure to liquid nitrogen temperature. At such a low temperature all hydrides are metastable at ambient pressure and can be removed from the highpressure cell for studying their composition, crystal structure and physical properties. At the ISSP RAS, hydrogen is compressed in the capsules shown in Fig. 1, which are placed in a quasihydrostatic Toroid-type high-pressure chamber. Hydrogen gas inside the capsule is produced by thermal decomposition of AlH 3 after a preliminary compression. If a Teflon capsule is used, the dead space in it is initially filled with heptane or silicon oil to prevent leaks of hydrogen, which forms a bubble around the sample in the upper part of the capsule. To seal a copper capsule, the gap between the capsule and the plug is filled with gallium, which is liquid at ambient temperature, diffuses into the

*E-mail address: antonov@issp.ac.ru ( V.E. Antonov).

copper when the temperature is raised and forms Cu ­ Ga alloys of increasing thermal stability as its concentration in the copper matrix decreases. Teflon, Cu and Ga are largely impermeable to hydrogen. The dead space in the copper capsule is filled with kaolin wool which does not react with H 2 , Cu and d -metals in the T ­ P range of interest. Due to the hardness of its fibres, the wool remains porous even at high pressures, leaving enough room for the hydrogen gas inside the capsule and ensuring good access of the hydrogen to the sample. The maximum amount of hydride produced in one experiment varies from 30 to 500 mm 3 depending on the pressure of synthesis. The high-pressure technique has been most effective in hydrogenation of the group VI ­VIII transition metals, neither of which except Pd forms hydrides at low hydrogen pressures. The hydrides formed at high pressures were shown to have close-packed metal sublattices with fcc ( g ), hcp ( e) or double hcp ( e9 ) structures, in which hydrogen occupies octahedral interstitial positions. The hydrides are metals and most of them exist in wide composition ranges and can be considered as solid solutions of hydrogen distributed over interstices either randomly or in a superstructure order. According to the phase rule, only singlephase fields are possible in the equilibrium T ­ P diagrams of binary Me ­ H systems, but the composition of every phase can vary with T and P within the corresponding field. The T ­ P diagrams constructed at the ISSP RAS and also the diagram of the Pd ­ H system from literature [3] are presented in Fig. 2. The solid lines show the transitions accompanied by a decrease in the hydrogen content of the sample ( hydride decomposition), as these lines are much

0925-8388 / 02 / $ ­ see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01532-8


V.E. Antonov / Journal of Alloys and Compounds 330 ­ 332 ( 2002 ) 110 ­ 116

111

Fig. 1. Schematic diagrams of the high-pressure cells used for electrical measurements [1] ( left ) and for production of hydrides [2] (right ).

closer to the equilibrium phase boundaries than the lines of hydride formation [4]. Table 1 compiles the neutron diffraction data of our group on the crystal and magnetic structure of Me ­ H phases prepared under a high hydrogen pressure and studied in a metastable state at ambient pressure and also literature data on e-CrH and g-PdH.

2. Formation conditions and structures of highpressure hydrides Cr ­ H system : The hydrogen solubility in bcc (a) chromium is small. Chromium hydride with a hcp ( e) metal lattice can be produced electrolytically and under high hydrogen pressure [5]. Its composition is always close to CrH. The NiAs-type ( Fig. 3a) crystal structure of e-CrH was determined by neutron diffraction using the sample loaded with hydrogen electrolytically [6] while the values of lattice parameters indicated in Table 1 were more accurately determined by X-ray diffraction on the sample synthesised under a high hydrogen pressure [5]. Mn ­ H system : At atmospheric pressure, there are four allotropic modifications of manganese: a-Mn and b-Mn with complex cubic lattices, fcc g-Mn and bcc d-Mn. Under high pressures of hydrogen, manganese forms e hydrides [7,8] with the H-to-metal atomic ratio x ranging from 0.65 to 0.96 [9], g hydrides with 0,x ,0.5 [10] and also primary solid solutions of hydrogen in a-Mn and b-Mn with x #0.1 [2]. In the e manganese hydrides with x $0.83, hydrogen atoms are randomly distributed over octahedral interstices in the hcp metal lattice (a deficient NiAs type structure, Fig. 3a) [11]. At x 50.65, hydrogen forms a superstructure, presumably, of the anti-CdI 2 type [12] shown in Fig. 3b. ´ The hydrides are antiferromagnets with the Neel temperature of T N ¯360 K and have a layered collinear spin structure formed of ferromagnetic ( 110 ) hex planes coupled antiferromagnetically [11,12] as shown in Fig. 4a. The g hydride MnH 0.41 has a fcc metal lattice with ° a 0 53.776 A at 300 K, in which hydrogen randomly occupies octahedral interstices [13]. The hydride is an

antiferromagnet with T N ..300 K and a layered collinear structure formed of ferromagnetic ( 001 ) planes coupled antiferromagnetically [13] as shown in Fig. 4b. Its mag] netic Bravais cell is tetragonal with a 5a 0 /OE2 and c 5 a 0 ( Table 1 ). In a-MnH 0.073 , hydrogen randomly occupies interstitial positions of the 12e type inside distorted octahedra of manganese atoms [14] as shown in Fig. 5. The 12e sites ° form dumb-bells 0.68 A long positioned at the centres of the edges and faces of the cubic unit cell of a-Mn. Because of the small distance between the sites in a dumb-bell, these positions cannot be occupied by hydrogen at the same time due to the `blocking effect' [15] requiring that the distance between hydrogen atoms in a metal should not ° be less than 2 A. There are 58 manganese atoms in the unit cell of a-Mn, and occupancy of half of 12e sites corresponds to a H / Mn atomic ratio of x 5 6/58 ¯ 0.103. Thus, about 70% of the accessible interstices were filled with hydrogen in the a-MnH 0.073 solid solution studied. ° The small distance of 0.68 A between the sites in a dumb-bell also results in a giant effect of hydrogen tunnelling which is clearly visible in the inelastic neutron scattering spectra of a-MnH 0.073 even at temperatures as ¨ high as 100 K [14,16]. Using 119 Sn Mossbauer spectroscopy it was shown that T N of a-Mn doped with 0.2 at.% Sn increases from 97 to 128 K on dissolution of x 5 0.05 of hydrogen [17]. Fe ­ H system : The hydrogen solubility in a-Fe is small. Iron forms dhcp e9 hydride with x ¯ 1 [18,19], fcc g hydride of unknown composition [20] and also a metastable intermediate hcp e phase [20] with x ¯ 0.4 [21]. The e9 iron hydride is a ferromagnet with the Curie ¨ temperature much exceeding 300 K [22]. 57 Fe Mossbauer spectroscopy revealed a large concentration of stacking faults in its dhcp metal lattice [23]. In the neutron diffraction investigation [21], the stacking faults manifested themselves by the presence of Fe and H atoms on `defect' 2d and 4f sites as indicated in the last two lines of description of e9-FeH in Table 1. The schematic plot of the e9-FeH crystal structure in Fig. 6 shows displacements of hydrogen layers that are not indicated in Table 1 because the effect was largely within the error limits. Nevertheless, ° these displacements of the order of dZ 5 0.007c ¯ 0.03 A


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V.E. Antonov / Journal of Alloys and Compounds 330 ­ 332 ( 2002 ) 110 ­ 116

Fig. 2. T ­ P diagrams of the systems Cr ­ H [42], Mn ­ H [2], Fe ­ H [43], Co ­ H [24,44], Ni ­ H [28], Mo ­ H [45], Tc ­ H [32,33], Rh ­ H [37] and Pd ­ H [3]. Metal lattices of the phases are: a5 bcc, a9 5 a-Mn, b9 5 b-Mn, g5 fcc, e5 hcp, e9 5 dhcp. The subscripts `0', `1' and `2' mark isomorphous phases in the order of increasing hydrogen content. The superscripts `f' and `p' mark ferro- and paramagnetic phases, respectively. The dashed portion of the e g boundary in the Co ­ H diagram is tentative. The vertical dashed line in this diagram is a schematic plot of the line of supercritical anomalies of the transformation between the metastable g1 and g2 phases. The dashed curves in the Tc ­ H diagram represent the lines of supercritical anomalies of the e 0 e 1 and e 1 e 2 transformations. The circles in the Ni ­ H and Pd ­ H diagrams show the position of the critical point of the g1 g2 transformation ( the g1 and g2 phase in the Pd ­ H system are usually called the a and b phase, respectively ).

are very likely to occur on physical grounds [21] as they increase the distance between hydrogen atoms in equally stacked octahedral layers that is the shortest distance between any hydrogen atoms in the hydride. This is the very effect one can expect in view of the strong long-range repulsive interaction between hydrogen atoms, which is one of the main factors governing formation of hydrides of transition metals [15]. The e-FeH 0.42 hydride is paramagnetic down to 4.2 K

[21,23]. Hydrogen is randomly distributed over octahedral interstices of the hcp metal lattice [21]. Co ­ H system : At temperatures of 250 ­ 3508C, the equilibrium solubility of hydrogen in the low-temperature hcp ( e) modification of cobalt monotonically increases with pressure and reaches x ¯ 0.6 at 7 GPa [24]. At higher pressures, hydride with x ¯ 1 is formed on the base of the high-temperature fcc ( g ) cobalt modification [24]. A neutron diffraction investigation at ambient pressure and


V.E. Antonov / Journal of Alloys and Compounds 330 ­ 332 ( 2002 ) 110 ­ 116 Table 1 Positional parameters (X, Y, Z ) and site occupancies for high-pressure hydrides of the group VI ­VIII transition metals Phase e-CrH x ; x 5 1.00, T 5 300 K; P63 / mmc, M 5 2 ° ° a 5 2.717 A, c 5 4.436 A, c / a 5 1.633 e-MnH x ; x 5 0.83, T 5 120 K; P63 / mmc, M 5 2 ° ° a 5 2.692 A, c 5 4.355 A, c / a 5 1.618 AFM, s 5 0.73 mB / Mn atom g-MnH x ; x 5 0.41, T 5 300 K; PI 4 / mmm, M 5 2 ] ° ° a 5 2.670 A, c 5 3.776 A, c / a 5OE2 AFM, s 5 1.88 mB / Mn atom Atom Cr H Mn H Mn, s Mn, s H H Mn Mn Mn Mn H Fe Fe H Fe H Fe D Co H Site 2c 2a 2c 2a X 1/3 0 1/3 0 Y 2/3 0 2/3 0 Z 1/4 0 1/4 0 Occupancy 1.00 1.00 1.00 0.83

113

Ref. [5,6]

[11]

1a 1d 1b 1c 2a 8c 24g 24g 12e 2a 2c 4f 2d 4f 2c 2a 2c 2a

0 1/2 0 1/2 0 0.319 0.355 0.091 0 0 1 1 1 1

0 1/2 0 1/2 0 0.319 0.355 0.091 0 0 2 2 2 2

0 1/2 1/2 0 0 0.319 0.034 0.281 0.538 0 1 7 3 1

1.00 1.00 0.41 0.41 1.00 1.00 1.00 1.00 0.34 1.000 0.935 0.935 0.065 0.065 1.00 0.42 1.00 0.26

[13]

a-MnH x ; x 5 0.073, T 5 300 K; ¯ I43 / mmc, M 5 58 ° a 5 8.9403 A

[14]

1 2

e9-FeH x ; x 5 1.00, T 5 90 K; P63 / mmc, M 5 4 ° ° a 5 2.679 A, c 5 8.77 A, c / a 5 2 ? 1.637 FM, s 5 2.2 mB / Fe atom

[21]

/ / / /

3 3 3 3

/ / / /

3 3 3 3

/ / / /

4 8 4 8

e-FeD x ; x 5 0.42, T 5 90 K; P63 / mmc, M 5 2 ° ° a 5 2.583 A, c 5 4.176 A, c / a 5 1.617 e-CoH x ; x 5 0.26, T 5 120 K; P63 / mmc, M 5 2 ° ° a 5 2.544 A, c 5 4.116 A, c / a 5 1.618 FM, s 5 1.6 mB / Co atom e-CoH x ; x 5 0.34, T 5 120 K; P63 , M 5 6 ° ° a 5 2.555 A, c 5 12.406 A, c / a 5 3 ? 1.619 FM, s 5 1.6 mB / Co atom

1/3 0 1/3 0

2/3 0 2/3 0

1/4 0 1/4 0

[21]

[25]

Co Co Co H H H Co D D Ni H Mo H Tc H H Tc H Rh H Pd H Re H

2b 2b 2b 2a 2a 2a 2d 1a 1b 4a 4b 2c 2a 2d 1a 1b 2c 2a 4a 4b 4a 4b 2c 2a

1/3 2/3 1/3 0 0 0 1/3 0 0 0 1/2 1/3 0 1/3 0 0 1/3 0 0 1/2 0 1/2 1/3 0

2/3 1/3 2/3 0 0 0 2/3 0 0 0 1/2 2/3 0 2/3 0 0 2/3 0 0 1/2 0 1/2 2/3 0

1 / 12 0.245 0.421 1/6 0 1/3 0.262 0 1/2 0 1/2 1/4 0 0.26 0 1/2 1/4 0 0 1/2 0 1/2 1/4 0

1.00 1.00 1.00 0.02 0.02 0.97 1.00 0.97 0.04 1.00 1.00 1.00 0.95 1.00 0.90 ¯0 1.00 0.69 1.00 0.95 1.00 1.00 1.00 0.20

[25]

¯ e-CoD x ; x 5 0.50, T 5 120 K; P3m1, M 5 2 ° ° a 5 2.579 A, c 5 4.162 A, c / a 5 1.614 FM, s 5 1.5 mB / Co atom g-NiH x ; x 5 1.05, T 5 120 K; Fm3 m, M 5 4 ° a 5 3.740 A e-MoH x ; x 5 1.05, T 5 120 K; P63 / mmc, M 5 2 ° ° a 5 2.937 A, c 5 4.758 A, c / a 5 1.620 ¯ e-TcH x ; x 5 0.45, T 5 300 K; P3m1, M 5 2 ° ° a 5 2.801 A, c 5 4.454 A, c / a 5 1.590

[25]

[31]

[31]

[34]

e-TcH x ; x 5 0.69, T 5 300 K; P63 / mmc, M 5 2 ° ° a 5 2.838 A, c 5 4.465 A, c / a 5 1.573 g-RhH x ; x 5 1.00, T 5 120 K; Fm3 m, M 5 4 ° a 5 4.010 A g-PdH x ; x 5 1.00, T 5 77 K; Fm3 m, M 5 4 ° a 5 4.090 A e-ReH x ; x 5 0.20, T 5 90 K; P63 / mmc, M 5 2 ° ° a 5 2.801 A, c 5 4.465 A, c / a 5 1.594

[34]

[31]

[38,39]

[12]

T is the temperature of the neutron diffraction measurement; M is the number of formula units per unit cell, s is the magnetic moment.


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Fig. 3. Crystal structures of e hydrides: (a) the NiAs type structure, space ¯ group P63 / mmc ; ( b) the anti-CdI 2 type structure, P3m1 ; (c) the Co 3 H structure, P63 . The solid circles show positions of metal atoms. The crosses and open circles mark octahedral interstitial positions, respectively, empty and occupied by hydrogen atoms. The arrows indicate the directions of displacements of metal atoms from the hcp positions due to hydrogen ordering.

Fig. 4. Crystal (solid lines) and magnetic (dashed lines) Bravais cells of manganese hydrides. (a) e-MnH 0.83 : a deficient NiAs type crystal structure, space group P63 / mmc ; an orthorhombic magnetic structure, Shubnikov space group PC cmm [11]. ( b) g-MnH 0.41 : a deficient NaCl type crystal structure, Fm3 m ; a tetragonal magnetic structure, PI 4 mmm [13]. Solid circles represent manganese atoms, open circles show the octahedral positions partly occupied by hydrogen, arrows indicate the positions and directions of the magnetic moments.

120 K showed [25] that hydrogen atoms in the e solutions with x # 0.26 are randomly distributed over octahedral interstices ( Fig. 3a). In the solutions with x $ 0.34, hydrogen atoms form layered superstructures, occupying every third octahedral base layer at x 5 0.34 ( Fig. 3c) and every second layer at x $ 0.38 ( Fig. 3b). In the ordered structures, as seen from Table 1 and Fig. 3b and c, the metal layers separated by hydrogen atoms move apart while the layers containing nearly no hydrogen between them move closer together. Cobalt and its hydrides are ferromagnets. The magnetic moments in the e hydrides are directed along the c -axis and decrease with increasing hydrogen concentration at a rate of about 0.36 Bohr magneton per H atom [26]. Ni ­ H system : Nickel hydride is formed via the isomorphous g1 g2 transition and can be produced electrolytically and under high hydrogen pressure [27]. At room temperature, the g1 g2 transition is accompanied by an abrupt increase in the hydrogen solubility from x 5 0.01 to x ¯ 1 [24]. The compositions of the coexisting g1 and g2 phases get closer to each other with increasing temperature, and the line of the g1 g2 equilibrium terminates at a

critical point [28] as shown in Fig. 2. The Curie temperature of the ferromagnetic g1 solutions decreases with hydrogen pressure due to the increase in the equilibrium

Fig. 5. A dumb-bell of two adjacent positions 12e partly occupied one at a time by hydrogen (open circles 9 and 10 ) and (a) its nearest neighbourhood of two tetrahedra formed by manganese atoms at positions 24g1 (shaded circles 1 ­ 4 ) and at the positions 24g2 (solid circles 5 ­ 8 ) and ( b) the resulting distorted octahedral interstice around the 12e position (open circle 9 ) in the crystal structure of a-MnH 0.073 [14].


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pressures of the order of 1 GPa are necessary to approach x 5 1 [38] ( 1 atm 5 101 325 Pa). The octahedral hydrogen coordination in palladium hydrides was established by neutron diffraction for the sample with x 5 0.709 [39]. The lattice parameter of g-PdH indicated in Table 1 is from the X-ray measurements on the hydrides synthesised at high pressures [38]. Re ­ H system : At 170 ­ 3008C, the hydrogen solubility in hcp rhenium monotonically increases with pressure and reaches x ¯ 0.2 at 9 GPa [40,41]. Concluding, it seems worth mentioning that the above variety of the magnetic properties and, to a great extent, of the crystal structures of high-pressure hydrides can be explained in the framework of a simple phenomenological `rigid d -band' model [46].

Acknowledgements The author thanks the Organising Committee of MH2000 for financial support to attend the Symposium. The support from the Russian Foundation for Basic Research under the grant No. 99-02-17299 also is gratefully acknowledged.

Fig. 6. Crystal structure of e9-FeH [21]. The letters a, c and f mark the layers of equivalent positions in the P63 / mmc space group originated, respectively, from the positions 2a, 2c and 4f with Z 5 7 / 8 listed in Table 1. The solid circles show regular positions of Fe atoms, open circles of H atoms. The arrows indicate the directions of displacements of H atoms from the centres of octahedral interstices. The letters A, B and C represent the standard notation for close-packed layers in the dhcp structure.

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hydrogen solubility [28]. The g2 phase is paramagnetic down to 4.2 K [29]. Mo ­ H system : The hydrogen solubility in bcc (a) molybdenum is small. The composition of the e hydride is close to MoH [30,31]. Tc ­ H system : At high pressures, the system undergoes two isomorphous phase transitions, e 0 e 1 and e 1 e 2 . The lines of both transitions terminate in the critical points at T , 3008C [32,33]. At 3008C, the hydrogen solubility in Tc reaches x ¯ 0.5 at 0.7 GPa and x ¯ 0.8 at 2 GPa [33]. Under ambient conditions, hydrogen is randomly distributed over octahedral interstices in the hydride with x 5 0.69 (a deficient NiAs type structure, Fig. 3a) and forms a layered superstructure of the anti-CdI 2 type ( Fig. 3b) at x 5 0.45 [34]. Ru ­ H system : The hydrogen solubility in hcp ruthenium at 2508C monotonically increases with pressure and reaches x ¯ 0.03 at 9 GPa [35]. Rh ­ H system : The hydrogen content of the primary g1 solutions is small. The composition of the g2 hydride is close to RhH [36,37]. Pd ­ H system : A hydrogen pressure of about 0.007 atm is sufficient to form g2 hydride of palladium (usually called the b-phase) with x ¯ 0.61 at room temperature [3], but


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