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Proton conductors: nanometric cavities, H2 precipitates under pressure, and Rydberg matter formation

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  • International Institute for Hydrogen Materials Research

Abstract and Figures

Proton conductors (PC) are metal oxides often used as solid electrolyte with hydrogen above 400 K, in which anomalous presence increase of several chemical elements and excess heat would have been obtained from near-surface locations. Near the surface of other metal oxides, closely spaced hydrogen at a distance of only 2 pm at least during a fraction of the time has been detected, and has been proposed to be in the form of hypothetical ultradense Rydberg matter H(0). How can H(0) form in PC near the cathode interface? Nanometric cavities (NC) were observed in the PC near their cathode interfaces. These NC would contain H2 precipitates with impurities, under a pressure of the order of 0.1 GPa. Since PC are crossed by a large flux of protons, a simple mechanism is proposed to increase the H2 pressure in these NC rapidly and temporarily well above the PC tensile strength. A second mechanism is then described to turn this H2 into a metallic-molecular state, form a Rydberg matter H(1) and then H(0) with a pressure decrease. In NC, the presence of impurities and the entry of the hydrogen atoms in the form of Rydberg atoms are proposed to decrease the pressure required to form metallic-molecular hydrogen. Finally, different experiments are proposed to test this research approach, particularly by transmission electron microscopy and Raman micro-spectroscopy.
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J. Condensed Matter Nucl. Sci. 21 (2016) 26–39
Research Article
Proton Conductors: Nanometric Cavities, H2Precipitates under
Pressure, and Rydberg Matter Formation
François de Guerville
i2-HMR International Institute for Hydrogen Materials Research, 56 Rue Estelle, 34 000 Montpellier, France
Abstract
Proton conductors (PC) are metal oxides often used as solid electrolyte with hydrogen above 400 K, in which anomalous presence
increase of several chemical elements and excess heat would have been obtained from near-surface locations. Near the surface of
other metal oxides, closely spaced hydrogen at a distance of only 2 pm at least during a fraction of the time has been detected, and
has been proposed to be in the form of hypothetical ultradense Rydberg matter H(0). How can H(0) form in PC near the cathode
interface? Nanometric cavities (NC) were observed in the PC near their cathode interfaces. These NC would contain H2precipitates
with impurities, under a pressure of the order of 0.1 GPa. Since PC are crossed by a large flux of protons, a simple mechanism is
proposed to increase the H2pressure in these NC rapidly and temporarily well above the PC tensile strength. A second mechanism
is then described to turn this H2into a metallic-molecular state, form a Rydberg matter H(1) and then H(0) with a pressure decrease.
In NC, the presence of impurities and the entry of the hydrogen atoms in the form of Rydberg atoms are proposed to decrease the
pressure required to form metallic-molecular hydrogen. Finally, different experiments are proposed to test this research approach,
particularly by transmission electron microscopy and Raman micro-spectroscopy.
c
2016 ISCMNS. All rights reserved. ISSN 2227-3123
Keywords: H2precipitates, Impurities, Large hydrogen flux, Nanometric cavities, Partial metallization of hydrogen, Pressure,
Proton conductors, Rydberg states
1. Introduction
Some metal oxide crystals with perovskite structure are excellent Proton Conductors (PC). The PC can be used as
solid electrolytes with H2above 400 K with porous electrodes. They are then penetrated by a very large hydrogen flux
(proton conductivity up to 0.4 S/cm at 900 K) [1–3], and incorporate large concentrations of protons (several atomic
percent) [4].
The PC are among the simplest and the most effective systems to study Condensed Matter Nuclear Science
(CMNS). Mizuno et al. carried out CMNS experiments with polycrystalline PC based on Y-doped SrCeO3used
as solid electrolytes at 650 K with deuterium passing through Pt porous electrodes, as shown in Fig. 1. Voltage and
E-mail: francois.de.guerville@i2-hmr.com
c
2016 ISCMNS. All rights reserved. ISSN 2227-3123
François de Guerville / Journal of Condensed Matter Nuclear Science 21 (2016) 26–39 27
Figure 1. Mizuno’s experimental system to study CMNS using a polycrystalline proton conductor based on Y-doped SrCeO3as solid electrolyte
with D2. From [8].
intensity were as low as 18 V and 40 µA. The unit cell of Y-doped SrCeO3keeps a centrosymmetric structure and thus
there is no additional electric polarization of the material which could give additional kinetic energy to the hydrogen
ions. Mizuno et al. have reported the observation of chemical elements in the PC near their electrode interfaces which
were not present before electrolysis [5,6], and anomalous heat production during electrolysis [7–9], as shown in Fig. 2.
Similar results were reported by other researchers with other PC [10–12].
Holmlid and coworkers’ experiments have shown that, near the surface of highly porous metal oxide crystals based
on K-doped Fe2O3exposed to hydrogen, a part of the hydrogen atoms are separated from each other by only 2 pm
at least during a fraction of time, as shown in Fig. 3a [13–18]. Other experiments are consistent with the presence
of a significant population of compact pairs of hydrogen atoms in other hydrogenated materials [19]. This closely
spaced hydrogen might be a new form of hydrogen, called H(0), shown in Fig. 3b. H(0) is a hypothetical ultradense
form of Rydberg matter with a phenomenal density of 105g/cm3. It might be a promising nuclear fuel, superfluid
(a) (b)
Figure 2. (a) Elemental analysis for impurities in the proton conductor Y-doped SrCeO3, showing the presence increase of several chemical
elements after electrolysis. (b) Anomalous heat production during electrolysis. From [6,8].
28 François de Guerville / Journal of Condensed Matter Nuclear Science 21 (2016) 26–39
Figure 3. (a) Laser induced Coulomb explosions with time-of-flight mass spectroscopy near the surface of a metal oxide based on K-doped Fe2O3.
The two first peaks correspond to closely spaced deuterons (2 pm). From [13]. (b) Hypothetical ultradense hydrogen Rydberg matter H(0). From
[18].
and superconductive at room temperature. The presence of H(0) in a PC near its cathode interface could explain the
observation of new chemical elements and excess heat.
H(0) would form almost spontaneously from classical hydrogen Rydberg matter H(1). H(1) can be viewed as a
generalized metal [20–24]. Moreover, H(0) could be formed directly from H2at ultrahigh pressures. Thus, the route
explored in this article points toward metallization of hydrogen and high pressures. The main question of this article is:
How to form hydrogen Rydberg matter in a PC used as solid electrolyte with H2at 650 K near its cathode interface?
2. State of the Art in Condensed Matter Physics
2.1. Nanometric cavities and H2precipitates under pressure
There has not been much research into nanoscale structure of PC with large densities of incorporated protons. How-
ever, crystalline silicon and metal oxides with perovskite structure implanted by large quantities of protons have been
intensely studied, and this knowledge can be adapted to PC.
Figure 4. Ionic implantation of H+in silicon crystals.
François de Guerville / Journal of Condensed Matter Nuclear Science 21 (2016) 26–39 29
Figure 5. Nanometric cavities in H+implanted silicon, after an annealing at 720 K, observed by TEM. (a) Plane view and (b) cross-section side
view. From [26].
2.1.1. In silicon implanted with H+
Silicon does not conduct protons. With the ionic implantation technique, crystal surfaces are temporarily penetrated by
a large flux of hydrogen ions. This technique is used to obtain large hydrogen local concentrations in these materials,
typically at a depth of several hundred nanometers under the surface, as shown in Fig. 4. During material implantation,
many point defects are created among which vacancy-hydrogen complexes VnHm[25].
During annealing, when hydrogen concentration locally overtakes the solubility limit, these mobile complexes
VnHmagglomerate to form disk-shaped cavities of about ten nanometers in diameter [26–28]. This co-precipitation
of vacancies and hydrogen, followed by the competitive growth (“Ostwald ripening”) of the Nanometric Cavities
(NC) and their coalescence, allow the crystalline matrix-defects system to minimize its elastic energy. Such NC are
observable by Transmission Electron Microscopy (TEM) after an annealing at 720 K [26]. Figure 5a shows some NC in
their disc plane (plane view). Figure 5b, obtained from a TEM cross-section of the sample, shows a NC perpendicularly
Figure 6. Nanometric cavities in H+implanted silicon, after an annealing at 870 K for 30 min, observed by cross-sectional TEM. From [29].
30 François de Guerville / Journal of Condensed Matter Nuclear Science 21 (2016) 26–39
Figure 7. (a) Raman spectroscopy on Si surface implanted with hydrogen ions at 470 K showing the presence of H2molecules in nanometric
cavities. From [31]. (b) Diagram of H2molecules trapped in Si nanometric cavities. From [29].
to its disc plane (side view). Figure 6 shows microcracks formed from a large density of NC in the implantation zone
after an annealing at 870 K [29].
These NC contain precipitates of molecular hydrogen H2[29–32]. They are traps for hydrogen and for impurities
[33], and they contain most of the implanted hydrogen [32]. The presence of H2in these NC was established by Raman
spectroscopy. In Fig. 7a, the top Raman spectrum is composed of three peaks. The first two correspond to Si–Si and
Si–H vibrations. The third one corresponds to the H–H vibration of H2(vibron) and has a characteristic position when
the H2is fluid, as it is the case here [30,31]. Figure 7b shows hydrogen molecules H2in a NC [29].
The H2filling a NC is typically pressured to a dozen of GPa at 300 K [34–37]. This value is above the tensile
strength of silicon, around 7 GPa. It is expected that this internal pressure of H2is much larger during annealing above
650 K [34]. This pressure decreases when the diameter of the NC increases. Reciprocally, this H2under pressure
generates stress on the crystalline matrix. The resulting strain field contrast surrounding a NC can be observed by
TEM, as shown in Fig. 8a [28]. A diagram showing the pressure of H2in NC on the Si crystalline matrix is presented
in Fig. 8b [29].
Figure 8. (a) Cross-section TEM image showing a nanometric cavity pressurized by its internal H2, and the long-range surrounding strain field,
deep under the free surface of a hydrogenated Si wafer below 470 K. From [28]. (b) Diagram showing the pressure of H2in nanometric cavities on
the Si crystalline matrix. From [29].
François de Guerville / Journal of Condensed Matter Nuclear Science 21 (2016) 26–39 31
Figure 9. TEM images of high local densities of hydrogen related nanometric cavities (side view) in metal oxides with perovskite structure near
their surfaces. These samples were implanted with hydrogen at 300 K for SrTiO3(a) and 570 K for LaAlO3(b). From [38].
2.1.2. In metal oxides with perovskite structure implanted with H+
The same phenomena are observed in metal oxides with perovskite structure, such as SrTiO3, BaTiO3, LaAlO3and
LiTaO3[38–40]. The two TEM images in Fig. 9 show large densities of NC under the surfaces of SrTiO3and LaAlO3
[38].
Figure 10. TEM images of nanometric cavities in polycrystalline proton conductors. Red arrows highlight some nanometric cavities. (a) Plane
view in Gd-doped BaCeO3used below 770 K. From [41]. (b) Cross-section side view in Y-doped BaZrO3used at 590 K. From [42].
32 François de Guerville / Journal of Condensed Matter Nuclear Science 21 (2016) 26–39
Figure 11. (a) Phase diagram PTof pure hydrogen, established by Raman spectroscopy. From [44, 46–51]. (b) Metallic-molecular phase IV
of hydrogen. From [51].
In hydrogen-implanted BaTiO3, the internal pressure of H2in microcracks was evaluated between 0.004 and
0.5 GPa [39]. This pressure should be considerably higher in NC. The tensile strength of polycrystalline BaTiO3is
0.06 GPa.
2.1.3. In proton conductors used as solid electrolytes with hydrogen
High densities of NC related to hydrogen incorporation have been observed by TEM in three polycrystalline PC used
as solid electrolytes with hydrogen [41–43]. Figure 10 shows TEM images of Gd-doped BaCeO3used below 770 K
(Fig. 10a) and Y-doped BaZrO3used at 590 K (Fig. 10b). For Y-doped BaZrO3, these NC were found within a depth
Figure 12. Fully metallic phase of SiH4at only 113 GPa at 300 K. From [53].
François de Guerville / Journal of Condensed Matter Nuclear Science 21 (2016) 26–39 33
Figure 13. (a) Hydrogen Rydberg atom. (b) Hydrogen Rydberg matter H(1).
of 100 nm from its cathode interface, whereas for Ba3Ca1.18Nb1.82 O9δused at 1020 K, these NC were found in its
grain boundaries.
The phenomena observed in PC near their cathode interface when they are used as solid electrolyte with hydrogen
at around 650 K can be similar to those observed in silicon and in metal oxides with perovskite structure implanted
with H+.
2.2. Partial metallization of hydrogen
2.2.1. Hydrogen with impurities under high pressures
Pure hydrogen can be compressed to high pressures in disk-shaped diamond anvil cells, whose dimensions are typically
100 µm in diameter and 5 µm in thickness [44–49]. The pressure–temperature phase diagram of hydrogen, obtained
by Raman spectroscopy, is presented in Fig. 11a [44,46–51]. Hydrogen is a molecular solid in phase I above 5 GPa at
300 K, and above 23 GPa at 650 K. Above 250 GPa at 650 K, the phase of hydrogen has not been studied yet.
Above 250 GPa and at 300 K, hydrogen is a metallic-molecular solid in phase IV and/or V. As shown in Fig. 11b,
the structures of these phases are constituted of alternating layers of :
(1) H2molecules forming sheets of graphene type by intermolecular coupling, and whose state is intermediate
between metallic state and molecular state,
(2) normal hydrogen molecules H2.
In metallic-molecular H2, the intramolecular length of H2, initially equal to 74 pm, increases with pressure [51].
Although whether it is possible to produce metallic hydrogen in the laboratory is still debated, above 450 GPa and
at room temperature (not shown), hydrogen is supposed to be fully metallic and superconducting.
The presence of some impurities (Li, Si, S, . ..) significantly decreases the pressure required to approach a metallic
state when hydrogen is compressed in a diamond anvil cell [52–55]. Figure 12 shows the structure of metallic SiH4
under 113 GPa at 300 K, but SiH4is fully metallic above only 50 GPa [53]. In this configuration, hydrogen atoms are
separated from each other by 154 pm.
2.2.2. Rydberg states
Different Rydberg states are shown in Fig. 13. A hydrogen Rydberg atom (Fig. 13a) is a hydrogen atom with its
electron orbiting very far from the proton (>5nm), quasi-circularly [56]. It results from recombination of a proton
34 François de Guerville / Journal of Condensed Matter Nuclear Science 21 (2016) 26–39
and an electron, and it easily forms on metal oxide surfaces.
Hydrogen Rydberg matter H(1) is a hexagonal plane cluster of circular Rydberg atoms (Fig. 13b), whose electrons
are strongly excited and delocalized [20–24]. In this phase, hydrogen is a generalized metal which has properties
similar to covalent bonding. Protons are separated from each other by 150 pm at the fundamental energy level.
3. Research Approach
3.1. Rapid and temporary increase of H2pressure in nanometric cavities of proton conductors
This research approach starts with the facts that PC, submitted to electrolysis with H2at 650 K, can contain a high
local density of NC near their cathode interfaces and are crossed by a large flux of hydrogen.
Hypothesis 1: The NC contain H2and hydrogen combined with impurities under a pressure on the order of 0.1 GPa.
Hypothesis 2: The NC are penetrated by a large flux of hydrogen.
Hypothesis 3: The entering hydrogen is trapped in the NC in the form of H2, and the outgoing hydrogen flux is
negligible compared to the entering flux.
Hypothesis 4: The internal pressure of the hydrogen in the NC increases rapidly and temporary well above the PC
tensile strength. The questions arising from this hypothesis are discussed in Section 4.1.
3.2 Ultradense Rydberg matter formation in nanometric cavities of proton conductors with decrease of H2pressure
From now on, H(0) is supposed to exist and to possibly form in PC. The proposed H(0) formation mechanism is
illustrated in Fig. 14.
Hypothesis 5: In NC with a diameter greater than 40 nm, hydrogen penetrates in the form of circular Rydberg atoms
(n20, l=m=n1).
Figure 14. Proposed mechanism for H(1) formation in nanometric cavities consisting of circular hydrogen Rydberg atoms bombarding metallic-
molecular hydrogen sheets, followed by the formation of the hypothetical H(0) and decrease of pressure.
François de Guerville / Journal of Condensed Matter Nuclear Science 21 (2016) 26–39 35
Hypothesis 6: The H2with impurities in the NC turns temporarily into a metallic-molecular phase. The required
temporary internal pressure could be only on the order of tens GPa.
Hypothesis 7: Circular Rydberg atoms penetrating into the NC transfer their excitation energy to the metallic-
molecular sheets of H2in the immediate proximity of the NC wall, and together, they turn into Rydberg matter
H(1).
Hypothesis 8: H(1) spontaneously turns into H(0) under the conditions prevailing in the NC. This transition is accom-
panied by a pressure decrease in the NC.
4. Discussion
4.1. Rapid and temporary increase of H2pressure in nanometric cavities well above the tensile strength of the
proton conductor
It should take less than 1 min to increase the pressure of pure hydrogen from 0.1 to 20 GPa at 650 K, in a disk-shaped
NC with a thickness of 1.5 nm penetrated by a flux of five hydrogen atoms per nm2per second at normal incidence,
regardless of its diameter [57]. Nowadays, better PC (for instance, Y-doped BaZrO3single crystals) can achieve much
larger fluxes than the one considered here, estimated from [8]. Otherwise, since NC trap impurities, they might acquire
a global electrical charge, and the incoming proton flux in NC might be different from what was envisaged.
In NC, what mechanisms limit this pressure increase? From what is known for H+implanted silicon during
annealing, H2pressure increase in NC should accelerate the NC growth by Ostwald ripening. Yet, the mechanical
properties of the crystalline matrix change locally within the vicinity of a single pressurized NC [39]. Pressurized NC
could implement compressive stress on the nearby PC matrix, thus having an inhibiting effect on the growth on each
other [29].
Morevover, H2pressure increase in NC should accelerate the coalescence of a tiny part of the NC into microcracks,
in which the internal pressure is lower. At a given annealing temperature, depending on the local concentration of
hydrogen, the characteristic time needed to form microcracks can be on the order of 1 h. Consequently, the proposed
rapid internal pressure increase may be little limited by the slower NC coalescence into microcracks. Finally, an open
question is: Why have only Samgin et al. [11] reported the observation of cracks in their PC samples after CMNS
experiments?
4.2. Rydberg matter formation
Other open questions follow. Is it possible to decrease the pressure required for partial metallization of hydrogen in
NC down to several tens of GPa, thanks to the presence of impurities and the entry of hydrogen atoms in NC in the
form of Rydberg atoms?
What is the NC optimum size to form H(1)? If the NC are too small, circular Rydberg atoms cannot enter them. If
they are too large, the H2internal pressure may be too low to turn H2into metallic-molecular phase. I propose 40 nm
in diameter and 1.5 nm in thickness is the optimum size. The best NC orientation should be parallel to the interfaces.
Furthermore, could the formation of H2Rydberg molecules [58] play a role in H(1) formation?
5. Experimental Tests to Validate this Approach
5.1. Transmission electron microscopy
The TEM is the best tool to observe NC in a crystalline matrix, and study their locations, their sizes, their shapes, their
orientations and their density. Strain field contrast is also observable around NC containing hydrogen under pressure.
36 François de Guerville / Journal of Condensed Matter Nuclear Science 21 (2016) 26–39
Together with simulations, it is possible to deduce the H2pressure within the NC [34]. The TEM with electron energy-
loss spectroscopy enables accurate mapping of hydrogen in the samples, and provides information on its bounding
[59,60].
5.2. Raman micro-spectroscopy
Raman micro-spectroscopy reveals hydrogen vibrations and is the most adapted tool to detect the presence of H2with
impurities trapped in the NC near the cathode interface. Moreover, this technique enables us to identify and study
the hydrogen phase: molecular fluid, molecular solid, metallic-molecular solid, or Rydberg matter H(1). For H(1),
it is possible to study electronic excitation and vibrational shifts of its partially covalent bonding [20]. However, the
presence of impurities in the NC should complicate the deciphering of the Raman spectra. It may then be difficult to
identify the hydrogen phase and evaluate its internal pressure.
Otherwise, stresses undergone by the crystalline matrix, generated by H2in the NC, are also roughly assessable by
Raman spectroscopy.
Sample structural characterizations under the surface up to a depth of about 1 µm can be carried out, after removing
the cathode by chemical etching in an acid bath. By using a laser excitation wavelength below the optical absorption
threshold, the probed depth can be decreased to about several hundreds of nm or less [61], which is the ideal depth to
probe the NC. Furthermore, the use of confocal microscopy also limits the probed depth to 500 nm.
5.3. X-ray diffraction
X-ray diffraction enables us to measure strain perpendicular to the surface plane in a layer of a crystalline matrix,
generated by a large density of in-plane NC containing H2under pressure. These measurements can be carried out
in-situ [42]. Probed depth is around 2 µm.
5.4. Neutron scattering
Neutron diffraction can detect long range ordered structure of deuterium. It does not work so well on protium, due to
its lower mass. If there is enough ordered deuterium in NC, a deuterium lattice may be detectable inside. Moreover,
information could be obtained about the molecular dynamics and the collective dynamics of hydrogen (phonons) in
NC. Yet, the probed depth is very large and the bulk of the sample is probed.
5.5. Nuclear magnetic resonance spectroscopy
An NMR spectrum with anomalously large shift in a proton NMR experiment would provide unambiguous independent
confirmation of the presence of closely spaced hydrogen.
5.6. Laser-induced Coulomb explosions with time-of-flight mass spectroscopy
This technique allows to detect closely spaced hydrogen near a material surface, and evaluate the initial distance
between them.
6. Conclusion
In proton conductors, used as solid electrolytes with hydrogen around 650 K, large densities of nanometric cavities can
form near their cathode interfaces. Assuming these nanometric cavities contain H2precipitates with impurities under
François de Guerville / Journal of Condensed Matter Nuclear Science 21 (2016) 26–39 37
a pressure on the order of 0.1 GPa, a simple mechanism is proposed to increase rapidly and temporarily the H2internal
pressure well above the tensile strength of proton conductors. Then, assuming hydrogen can exist as ultradense H(0), a
second mechanism is proposed to make the H2with impurities in the nanometric cavities turn into a metallic-molecular
phase, form Rydberg matter H(1) and then form H(0) with a pressure decrease. In nanometric cavities, the presence
of impurities and the entry of the hydrogen atoms in the form of Rydberg atoms are proposed to decrease the pressure
required to form metallic-molecular hydrogen. Different experiments are proposed to study the hydrogen trapped in
these nanometric cavities, particularly by transmission electron microscopy and Raman micro-spectroscopy.
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