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Solid hydrogen in high pressure
ELSEVIER
Solid State Ionics 101-103
(1997)
1003-1005
Solid hydrogen in high pressure B. Baranowski” Institute of Physical Chemistry, Polish Academy of Sciences, 01-224 Warsaw, Poland
Abstract The vibron discontinuity in solid hydrogen around 150 GPa is explained as caused by a charge transfer, whereby the intra process is preferred. The ionic form of hydrogen is presented as an intermediate phase between homopolar and metallic hydrogen. As extremal case the formation of hydrogen hydride is foreseen, whereby an ionic lattice with opposite charged hydrogen ions should be realized. Keywords: Hydrogen:
High pressure;
Charge transfer:
Hydrogen
hydride
Materials: H; H,
Metallization
of
hydrogen
is
an
old
dream
of
laboratory static pressures reached the megabar region, due to the development of the diamond anvil technique [3], the investigations of solid hydrogen could be carried out in pressures where its metallization was expected from theoretical estimations. The most spectacular results presented a vibron discontinuity in solid hydrogen at low temperatures around 150 GPa [46]. The most simple explanation seemed the metallization of hydrogen, but this concept was rejected as soon as no dielectric anomaly was found [7,8] and a strong absorption in the IR region was stated, starting simultaneously with the onset of the vibron discontinuity [9]. The last property contradicted the symmetric, homonuclear structure of the hydrogen molecule, as well as the assumption of its metallization. Even earlier a hypothesis was formulated, supposing an intra charge transfer inside each hydrogen molecule, leading in the extremal case to the physics
and chemistry
[ 1,2]. When
*Fax: +48 22 325276. 0167-2738/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOl67-2738(97)00298-l
formation of hydrogen hydride, in full analogy to the hydrides of alkaline metals [lo]. Of course, this extremal case of the intra charge transfer would require the disappearance of hydrogen vibrons as an ionic lattice of hydrogen hydride would remove the vibron structure of hydrogen molecules. In other words, it is suggested that the valence bond of homonuclear hydrogen molecules is replaced by an ionic bond in which one hydrogen particle is a proton and the other is anionic. This means a radical symmetry breaking of the previous structure. Do there exist indications for such a behavior of hydrogen in high pressure? First of all, let us remark that the position of hydrogen in the periodic table is far from being unique and well defined. Usually, one is placing hydrogen in the group of alkaline metals, and in fact some properties are supporting such a classification: Here one can mention the large affinity of hydrogen and alkaline metals to the halogens, forming the corresponding salts (LiCl, LiBr...) and acids (HF, HCI...). The outer electronic shell involves in all
1004
B. Baranowski
I Solid State tonics
cases one electron, and the energies for the adoption of an additional electron (electron affinities) are similar, ranging from 0.75 eV for hydrogen to 0.47 for Cs. But on the other hand, there exist considerable similarities between hydrogen and the halogens. First of all the ionization energies are similar, ranging from 13.6 eV for the atomic hydrogen to 10.45 eV for iodine. Hydrogen as well as the halogens form diatomic molecules at ambient conditions with low melting temperatures as molecular liquids. In many organic compounds the replacement of hydrogen by a halogen does sometimes not change considerably the properties of corresponding compounds. The above arguments illustrate the bivalent role of hydrogen in chemistry. In a certain sense hydrogen behaves opportunistic. Its position in the periodic table should be somewhere between the alkaline metals and halogens. Due to these properties it is not surprising that hydrogen, not carbon participates in the maximal number of chemical compounds. In metallic environments, that is in metallic hydrides, hydrogen behaves also different: In alkaline metal hydrides hydrogen exhibits anionic character, comparable to the halides of the same metals. The alkaline hydrides present an evident proof of the difference between hydrogen and alkaline metals; thus the inconsequence of the classification of hydrogen as an alkaline metal in the periodic table of elements is here evident. On the other hand, in hydrides of the transition metals hydrogen exhibits a clear protonic character. The physical properties of the transition metal hydrides are typically by metallic, including for instance the superconductivity of palladium hydride. Thus, in a metallic matrix hydrogen can be either cationic or anionic. In hydrogen hydride both behaviors should be presented. A strong support of the ionic form of hydrogen, as a precursor of its metallization, can be found on a pure energetic ground [ 11,121: Namely the sequence of transitions H,@2HwH+
+ H-e2HC
(11 PI with the corresponding (1) -4.5 eV, (2) -12.8 eV,
energies:
+ 2e
101-103
(1997) 1003-1005
{3} - 14.4 eV proves that the ionic phase of hydrogen (2) requires only about half of the energy necessary for the full metallization (3). Thus as the increase of pressure increases the chemical potential of hydrogen in a continuous way, the transition to the ionic phase should precede its metallization, which is energetically nearly twice as expensive as the formation of hydrogen hydride. As the transition from the homopolar to the ionic bond can be performed in a continuous way, the existence of the vibron structure of hydrogen with an electric dipole contribution, as visualized by the strong absorption in the IR region, may not contradict the above reasoning. The collapse of the vibron and the formation of an ionic lattice can require an even higher pressure, but still a lower one than that characteristic for the metallization of hydrogen, where the analogy with the alkaline metals is created. An alternative for the intra charge transfer presented in Eq. (1) would be the simplest inter charge transfer [13-151 2H,++H;
+ H,
(2)
whereby the ionization energy of the H, molecule equals - 15.3 eV (I could not find the energy for electron affinity of the H, molecule). The model for this charge transfer was taken over from the known numerous examples of charge transfers in organic molecules. But one has to remark that the soft, multiatomic organic molecules may be not an adequate model for the homonuclear hydrogen molecules in the high pressure region. On the other hand, the energies for the intra charge transfer Eq. (1) and the inter charge transfer Eq. (2) are quite comparable, with a slight preference of Eq. (2), but on the other hand the charge transfer in Eq. (1) occurs on a closer distance than in Eq. (2), as the interatomic distance in the hydrogen molecule is smaller than the intermolecular distance in the pressure range considered. Furthermore, there exists no optical evidence for the change of the interaction between the hydrogen molecules after passing through the vibron discontinuity [9,11], which could be expected from Eq. (2). A remarkable
feature of the vibron discontinuity
is
B. Baranowski
I Solid State Ionics 101-103
its phase character: The discontinuity mentioned disappears above a critical temperature [16]. Thus, above a critical point the transition between both modifications of hydrogen can occur in a continuous way, which excludes a solid-solid phase transition of first order in terms of Landau’s criteria. This behavior is similar to the known phase diagrams of alkaline metals and transition metal hydrides, where the transition from the cr (low hydrogen content) to the hydride (high hydrogen content) phases exhibits a critical point too. Being optimistic one could expect that the critical point in the solid hydrogen around 150 GPa may characterize the transition from the covalent to at least partial ionic bond in hydrogen molecules. At ambient conditions the ionic configuration in hydrogen molecule requires an excited state, laying much above the dissociation state (see Eq. (1)). The existence of hydrogen hydride is therefore impossible. The volume conditions make such an existence also unprobable, as the negative hydrogen ion is in such conditions very voluminous, because of the imprecise, soft structure of its electron distribution. A large volume exhibits the anionic hydrogen in the alkaline metal hydrides too. But at high pressures this volume is reduced radically [17], thus making the formation of hydrogen hydride probable, also from the point of view of volume changes. Besides the above energetic arguments, recently some theoretical estimations seem to support the formation of hydrogen hydride in high pressures [18-201.
(1997) 1003-1005
1005
References Dl E. Wigner, H.B. Huntington, J. Chem. Phys. 3 (1935) 764. VI B. Baranowski, Z. Chemie 13 (1973) 281. 131 141 [51 161 171 PI 191 [lOI r111 (121
[I31 [I41 [I51 [I61 1171
1181 u91
ml
A. Jayaraman, Rev. Mod. Phys. 55 (1983) 65. R.J. Hemley, H. K Mao, Phys. Rev. Lett. 61 (1988) 857. R.J. Hemley, H.K. Mao, Phys. Rev. Lett. 63 (1989) 1393. H.E. Lorenzana, IF. Silvera, K.A. Goettel, Phys. Rev. Lett. 63 (1989) 2080. R.J. Hemley, M. Hanfland, H.K. Mao, Nature 350 (1991) 448. J.H. Eggert, F. Moshary, W.J. Evans, H.E. Lorenzana, K.A. Goettel, IF. Silvera, Phys. Rev. Len. 66 (1991) 193. M. Hanfland, R.J. Hemley, H.K. Mao, Phys. Rev. Lett. 70 (1993) 3760. B. Baranowski, Polish J. Chem. 66 (1992) 1737. B. Baranowski, Polish J. Chem. 69 (1995) 981. B. Baranowski, in: Chemical Approach to the Possibility of Hydrogen Hydride Formation, Proceedings of the 15th AIRAPT Conference, Warsaw, 1995. R.J. Hemley, Z.G. Soos, M. Hanfland, H.K. Mao, Nature 369 (1994) 384. Z.G. Soos, J.H. Eggert, R.J. Hemley, M. Hanfland, H.K. Mao, Chem. Phys. 245 (1995) 194. Z.G. Soos, D. Mukhopadhyay, Chem. Phys. Lett. 245 (1995) 194. R.J. Hemley, H.K. Mao, Science 249 (1990) 391. B. Baranowski, M. Tkacz, S. Majchrzak, Pressure dependence of hydrogen volume in some metallic hydrides, in: R. Pucci. G. Piccito (Eds.), Molecular Systems under High Pressure, Elsevier Science, 1991, pp. 139-156. A.L. Ruoff, in: Hydrogen at Multimegabar Pressures, Proceedings of the 15th AIRAPT Conference, Warsaw, 1995. N.W. Ashcroft, in: S. Ichimam, S. Ogata (Eds.), Elementary Processes in Dense Plasmas. Addison-Wesley, New York. 1995, p. 251. F. Siringo, R. Pucci, G.G.N. Angilella (preprint).
Solid State Ionics 101-103
(1997)
1003-1005
Solid hydrogen in high pressure B. Baranowski” Institute of Physical Chemistry, Polish Academy of Sciences, 01-224 Warsaw, Poland
Abstract The vibron discontinuity in solid hydrogen around 150 GPa is explained as caused by a charge transfer, whereby the intra process is preferred. The ionic form of hydrogen is presented as an intermediate phase between homopolar and metallic hydrogen. As extremal case the formation of hydrogen hydride is foreseen, whereby an ionic lattice with opposite charged hydrogen ions should be realized. Keywords: Hydrogen:
High pressure;
Charge transfer:
Hydrogen
hydride
Materials: H; H,
Metallization
of
hydrogen
is
an
old
dream
of
laboratory static pressures reached the megabar region, due to the development of the diamond anvil technique [3], the investigations of solid hydrogen could be carried out in pressures where its metallization was expected from theoretical estimations. The most spectacular results presented a vibron discontinuity in solid hydrogen at low temperatures around 150 GPa [46]. The most simple explanation seemed the metallization of hydrogen, but this concept was rejected as soon as no dielectric anomaly was found [7,8] and a strong absorption in the IR region was stated, starting simultaneously with the onset of the vibron discontinuity [9]. The last property contradicted the symmetric, homonuclear structure of the hydrogen molecule, as well as the assumption of its metallization. Even earlier a hypothesis was formulated, supposing an intra charge transfer inside each hydrogen molecule, leading in the extremal case to the physics
and chemistry
[ 1,2]. When
*Fax: +48 22 325276. 0167-2738/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOl67-2738(97)00298-l
formation of hydrogen hydride, in full analogy to the hydrides of alkaline metals [lo]. Of course, this extremal case of the intra charge transfer would require the disappearance of hydrogen vibrons as an ionic lattice of hydrogen hydride would remove the vibron structure of hydrogen molecules. In other words, it is suggested that the valence bond of homonuclear hydrogen molecules is replaced by an ionic bond in which one hydrogen particle is a proton and the other is anionic. This means a radical symmetry breaking of the previous structure. Do there exist indications for such a behavior of hydrogen in high pressure? First of all, let us remark that the position of hydrogen in the periodic table is far from being unique and well defined. Usually, one is placing hydrogen in the group of alkaline metals, and in fact some properties are supporting such a classification: Here one can mention the large affinity of hydrogen and alkaline metals to the halogens, forming the corresponding salts (LiCl, LiBr...) and acids (HF, HCI...). The outer electronic shell involves in all
1004
B. Baranowski
I Solid State tonics
cases one electron, and the energies for the adoption of an additional electron (electron affinities) are similar, ranging from 0.75 eV for hydrogen to 0.47 for Cs. But on the other hand, there exist considerable similarities between hydrogen and the halogens. First of all the ionization energies are similar, ranging from 13.6 eV for the atomic hydrogen to 10.45 eV for iodine. Hydrogen as well as the halogens form diatomic molecules at ambient conditions with low melting temperatures as molecular liquids. In many organic compounds the replacement of hydrogen by a halogen does sometimes not change considerably the properties of corresponding compounds. The above arguments illustrate the bivalent role of hydrogen in chemistry. In a certain sense hydrogen behaves opportunistic. Its position in the periodic table should be somewhere between the alkaline metals and halogens. Due to these properties it is not surprising that hydrogen, not carbon participates in the maximal number of chemical compounds. In metallic environments, that is in metallic hydrides, hydrogen behaves also different: In alkaline metal hydrides hydrogen exhibits anionic character, comparable to the halides of the same metals. The alkaline hydrides present an evident proof of the difference between hydrogen and alkaline metals; thus the inconsequence of the classification of hydrogen as an alkaline metal in the periodic table of elements is here evident. On the other hand, in hydrides of the transition metals hydrogen exhibits a clear protonic character. The physical properties of the transition metal hydrides are typically by metallic, including for instance the superconductivity of palladium hydride. Thus, in a metallic matrix hydrogen can be either cationic or anionic. In hydrogen hydride both behaviors should be presented. A strong support of the ionic form of hydrogen, as a precursor of its metallization, can be found on a pure energetic ground [ 11,121: Namely the sequence of transitions H,@2HwH+
+ H-e2HC
(11 PI with the corresponding (1) -4.5 eV, (2) -12.8 eV,
energies:
+ 2e
101-103
(1997) 1003-1005
{3} - 14.4 eV proves that the ionic phase of hydrogen (2) requires only about half of the energy necessary for the full metallization (3). Thus as the increase of pressure increases the chemical potential of hydrogen in a continuous way, the transition to the ionic phase should precede its metallization, which is energetically nearly twice as expensive as the formation of hydrogen hydride. As the transition from the homopolar to the ionic bond can be performed in a continuous way, the existence of the vibron structure of hydrogen with an electric dipole contribution, as visualized by the strong absorption in the IR region, may not contradict the above reasoning. The collapse of the vibron and the formation of an ionic lattice can require an even higher pressure, but still a lower one than that characteristic for the metallization of hydrogen, where the analogy with the alkaline metals is created. An alternative for the intra charge transfer presented in Eq. (1) would be the simplest inter charge transfer [13-151 2H,++H;
+ H,
(2)
whereby the ionization energy of the H, molecule equals - 15.3 eV (I could not find the energy for electron affinity of the H, molecule). The model for this charge transfer was taken over from the known numerous examples of charge transfers in organic molecules. But one has to remark that the soft, multiatomic organic molecules may be not an adequate model for the homonuclear hydrogen molecules in the high pressure region. On the other hand, the energies for the intra charge transfer Eq. (1) and the inter charge transfer Eq. (2) are quite comparable, with a slight preference of Eq. (2), but on the other hand the charge transfer in Eq. (1) occurs on a closer distance than in Eq. (2), as the interatomic distance in the hydrogen molecule is smaller than the intermolecular distance in the pressure range considered. Furthermore, there exists no optical evidence for the change of the interaction between the hydrogen molecules after passing through the vibron discontinuity [9,11], which could be expected from Eq. (2). A remarkable
feature of the vibron discontinuity
is
B. Baranowski
I Solid State Ionics 101-103
its phase character: The discontinuity mentioned disappears above a critical temperature [16]. Thus, above a critical point the transition between both modifications of hydrogen can occur in a continuous way, which excludes a solid-solid phase transition of first order in terms of Landau’s criteria. This behavior is similar to the known phase diagrams of alkaline metals and transition metal hydrides, where the transition from the cr (low hydrogen content) to the hydride (high hydrogen content) phases exhibits a critical point too. Being optimistic one could expect that the critical point in the solid hydrogen around 150 GPa may characterize the transition from the covalent to at least partial ionic bond in hydrogen molecules. At ambient conditions the ionic configuration in hydrogen molecule requires an excited state, laying much above the dissociation state (see Eq. (1)). The existence of hydrogen hydride is therefore impossible. The volume conditions make such an existence also unprobable, as the negative hydrogen ion is in such conditions very voluminous, because of the imprecise, soft structure of its electron distribution. A large volume exhibits the anionic hydrogen in the alkaline metal hydrides too. But at high pressures this volume is reduced radically [17], thus making the formation of hydrogen hydride probable, also from the point of view of volume changes. Besides the above energetic arguments, recently some theoretical estimations seem to support the formation of hydrogen hydride in high pressures [18-201.
(1997) 1003-1005
1005
References Dl E. Wigner, H.B. Huntington, J. Chem. Phys. 3 (1935) 764. VI B. Baranowski, Z. Chemie 13 (1973) 281. 131 141 [51 161 171 PI 191 [lOI r111 (121
[I31 [I41 [I51 [I61 1171
1181 u91
ml
A. Jayaraman, Rev. Mod. Phys. 55 (1983) 65. R.J. Hemley, H. K Mao, Phys. Rev. Lett. 61 (1988) 857. R.J. Hemley, H.K. Mao, Phys. Rev. Lett. 63 (1989) 1393. H.E. Lorenzana, IF. Silvera, K.A. Goettel, Phys. Rev. Lett. 63 (1989) 2080. R.J. Hemley, M. Hanfland, H.K. Mao, Nature 350 (1991) 448. J.H. Eggert, F. Moshary, W.J. Evans, H.E. Lorenzana, K.A. Goettel, IF. Silvera, Phys. Rev. Len. 66 (1991) 193. M. Hanfland, R.J. Hemley, H.K. Mao, Phys. Rev. Lett. 70 (1993) 3760. B. Baranowski, Polish J. Chem. 66 (1992) 1737. B. Baranowski, Polish J. Chem. 69 (1995) 981. B. Baranowski, in: Chemical Approach to the Possibility of Hydrogen Hydride Formation, Proceedings of the 15th AIRAPT Conference, Warsaw, 1995. R.J. Hemley, Z.G. Soos, M. Hanfland, H.K. Mao, Nature 369 (1994) 384. Z.G. Soos, J.H. Eggert, R.J. Hemley, M. Hanfland, H.K. Mao, Chem. Phys. 245 (1995) 194. Z.G. Soos, D. Mukhopadhyay, Chem. Phys. Lett. 245 (1995) 194. R.J. Hemley, H.K. Mao, Science 249 (1990) 391. B. Baranowski, M. Tkacz, S. Majchrzak, Pressure dependence of hydrogen volume in some metallic hydrides, in: R. Pucci. G. Piccito (Eds.), Molecular Systems under High Pressure, Elsevier Science, 1991, pp. 139-156. A.L. Ruoff, in: Hydrogen at Multimegabar Pressures, Proceedings of the 15th AIRAPT Conference, Warsaw, 1995. N.W. Ashcroft, in: S. Ichimam, S. Ogata (Eds.), Elementary Processes in Dense Plasmas. Addison-Wesley, New York. 1995, p. 251. F. Siringo, R. Pucci, G.G.N. Angilella (preprint).