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Solvated Electrons in High-Temperature Melts and Glasses of the Room-Temperature Stable Electride [Ca24Al28O64]4+⋅4e

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Science  01 Jul 2011:
Vol. 333, Issue 6038, pp. 71-74
DOI: 10.1126/science.1204394

Abstract

Solvated electrons in alkali metal-ammonia solutions have attracted attention as a prototype electronic conductor and chemical reducing agent for over a century. However, solvated electrons have not been realized in a high-temperature melt or glass of an oxide system to date. We demonstrated the formation of persistent solvated electrons in both a high-temperature melt and its glass by using the thermally stable electride [Ca24Al28O64]4+⋅4e (C12A7:e) and controlling the partial pressure of oxygen. The electrical and structural properties of the resulting melt and glass differ from those of the conventional C12A7:O2− oxide, exhibiting metallic and hopping conduction, respectively, and a glass transition temperature that is ~160 kelvin lower than that of C12A7:O2− glass. Solvated electrons reside in cage structures in C12A7:e and form a diamagnetic paired state.

Solvated electrons in alkali metal-ammonia solution have been a diverse stem for scientific and applied researches (1). Furthermore, alkali metal-amine solutions containing solvated electrons can be condensed into ionic solids, known as electrides, in which electrons are trapped in well-defined structural cavities and/or channels in matrices (2). Organic electrides can be created in which organic complexants, such as crown ethers, bind electrons, but these compounds are thermally unstable (2). This drawback provoked the development of thermally stable electrides based on inorganic compounds, such as calcium aluminum oxide (12CaO⋅7Al2O3, abbreviated as C12A7:O2−). In this compound, O2− is a caged species that compensates for the positive charge of the ~0.4-nm cage, and an electride can be formed by displacing this anion (3, 4). The caged electrons are protected from air and moisture even near room temperature (RT) because of the small size of the windows (diameters of ~0.1 nm) connecting the cages.

C12A7:e can be synthesized by means of various chemical and physical processes. We reported that strongly reduced C12A7:O2− melt crystallizes to form C12A7:e and that single crystals of C12A7:e can be grown from polycrystalline C12A7:e by the floating zone (FZ) melting method under a strongly reducing atmosphere (5, 6). These findings imply that the electrons persist in the cages just below melting point (Tm) under a strongly reducing atmosphere. Because the excess electrons in alkali metal-molten salt solutions at high temperature have been studied extensively (7), it is of both fundamental and applied interest whether a high concentration of solvated electrons persist in high-temperature melts of typical refractory oxides because the precursor C12A7:O2−, which is a by-product from iron smelting processes, exhibits Tm ~ 1688 K (8). Because CaO and Al2O3 are both stable oxides and typical electrical insulators, the conventional melt of C12A7:O2− does not contain carrier electrons in the molten state. Thus, the question arises: Do solvated electrons exist in the melt of refractory oxide-based C12A7:e—that is, like the solvated electrons in alkali metal-ammonia solutions, but at much higher temperatures? To answer this, we studied the physical properties of the melt of C12A7:e electride under a low-oxygen partial pressure (PO2). In addition, we examined structural and electrical properties of glasses prepared by rapidly quenching the C12A7:e melt so as to obtain structural information about the melt and realize a new type of amorphous semiconducting oxide.

The electrical conductivity (σ) of C12A7:e at elevated temperature decreases monotonically as the temperature increases up to 1750 K (Fig. 1A). Although a small decrease in σ is observed around the melting point (Tm ~ 1500 K) (fig. S3), the dependence does not change below and above Tm, so the C12A7:e melt exhibits metallic conduction. In contrast, σ increases with temperature in a C12A7:O2− melt, like that of a conventional oxide. The C12A7:e electride melt exhibits a higher density (ρ) than that of a C12A7:O2− melt, by ~10% at 1773 K (Fig. 1B).

Fig. 1

(A) The temperature dependence of σ of C12A7:e and C12A7:O2− melts. The σ of C12A7:e melt decreases as the temperature increases, showing metallic conduction. In contrast, a melt of the mother compound, C12A7:O2− oxide, shows ionic conduction. Upon melting, the σ of C12A7:e electride decreases to several siemens, which is an order of magnitude lower than that of the crystalline electride just below Tm. The sudden decrease in the σ of C12A7:O2− on melting is ascribed to the disappearance of free O2− ions accommodated in the cages because of the collapse of the three-dimensional network of sub-nanometer-sized cages that serve as the conduction pathway for oxide ions. The activation of ionic conduction in the C12A7:O2− melt (6.3 eV) is related to viscosity (19), implying that dissociation of O2− ions from the polymerized network structures determines both the rate of ionic conduction and viscous flow. (B) Density of C12A7:e and C12A7:O2− melts as a function of temperature. All data were acquired during heating. The red and pink circles represent the densities of C12A7:e melts measured under PO2 ~10−24 and ~10−16 atm, respectively. The error range was ±3% for the red and ±2% for the pink and blue data points.

This difference suggests that there are marked structural differences between the melts of C12A7:e and C12A7:O2−. To clarify this possibility, we rapidly quenched the C12A7:e melt from ~1873 K using a twin-roller at RT and examined the resulting glasses. When the C12A7:e melt is quenched, it is imperative to maintain a low PO2 in order to prevent oxidation of the melt (figs. S1, S4, and S5). Thus, we attached a homemade chamber containing a twin-roller to the melting system. The quenched black sample (C12A7:e electride glass) had a thickness of ~50 μm (Fig. 2A, inset). The x-ray diffraction (XRD) pattern of the glass shows a broad halo, and we only observed a ring pattern of transmision electron microscopy (TEM) electron diffraction (fig. S2), indicating that the glass was fully amorphous. Figure 2A shows the differential thermal analysis (DTA) of the C12A7:e glass, revealing a distinct shift in the base line at ~973 K that corresponds to the glass transition temperature (Tg). Tg is ~160 K lower than that of the conventional C12A7:O2− glass. This difference strongly suggests that the network structure of C12A7:e glass differs substantially from that of C12A7:O2− glass.

Fig. 2

(A) DTA comparison of the samples obtained by quenching C12A7:e (red line) and C12A7:O2− melts (black line). The left inset shows a photograph of the C12A7:e glass, which was sealed in a silica capsule (right inset) under a vacuum to avoid oxidation during heating. The combined XRD, TEM (fig. S2), and DTA results reveal that a glass with a Tg of 973 K was obtained. (B) The σ of C12A7:e glass. The σ increases as the temperature increases, showing semiconducting behavior following log σ ∝ T−1/4. When the glass is crystallized under an atmosphere of PO2 ~10−24 atm, σ shows the same temperature dependence as that of the parent electride at high temperature. The σ of C12A7:O2− glass is lower than the detection limit (10−10 S cm−1) at RT. (C) Correlation of the Ne in C12A7:e glasses and that in the starting C12A7:e electride used for melting. The dashed line shows the value of Ne in the starting C12A7:e electrides. (D) Optical absorption spectra of C12A7:e glass (red line) and C12A7:O2− glass (black line). The band at 3.3 eV is assigned to F+-like centers containing trapped electrons; their concentration was determined to be ~5 × 1018 cm−3 from an ESR spectrum (inset).

Figure 2B shows the temperature dependence of σ of C12A7:e glass. The conductivity near RT is ~10−7 S cm−1 and increases with increasing temperature. The temperature dependence of log σ is proportional to T−1/4 rather than T−1, indicating that electronic conduction is controlled by variable-range hopping (VRH). This temperature dependence implies that the C12A7:e glass contains a high concentration of localized electrons. Moreover, this change of conduction mechanism—from metallic conduction in the melt to VRH conduction in the glass—is similar to that reported in an amorphous thin film of metallic sodium-ammonia solution containing solvated electrons (9).

Next, we used iodometry (10) to confirm the presence of electrons and quantify the electron concentration (Ne) in different C12A7:e glasses. We prepared glasses by quenching melts of C12A7:e containing different Ne under an atmosphere of PO2 ~10−24 atm. The relation between Ne in the polycrystalline C12A7:e used to form the melts and Ne in the resulting C12A7:e glasses is shown in Fig. 2C. The observed linear relation shows that Ne is retained in C12A7:e glasses. That is, C12A7:e glasses exhibit a maximum Ne of 1.1 × 1021 cm−3, which is almost the same as that of recrystallized C12A7:e from a melt of C12A7:e with Ne of 1.7 × 1021 cm−3. We verified Ne values by measuring the weight gain caused by oxidation using thermogravimetry (TG). We checked whether solvated electrons were generated in the preparation of glasses by comparing Ne of a glass obtained from a C12A7:O2− oxide melt under the present PO2 atmosphere and a recrystallized sample. However, Ne was negligible in both the glass and crystal of C12A7:O2−. In other words, the electrons present in the electride glasses and recrystallized electrides are not newly generated but come from the electride melt. This result suggests that electrons caged in C12A7:e crystals persist in the melt without a severe degradation of Ne if a low PO2 of ~10−24 atm is maintained during melting and quenching.

Optical absorption spectra of C12A7:e and C12A7:O2− glasses are shown in Fig. 2D. The C12A7:O2− glass shows no distinct absorption band, whereas the C12A7:e glass exhibits a broad absorption band at 4.6 eV with a shoulder at 3.3 eV. The band at 3.3 eV is attributed to electrons forming F+-like centers, similar to the C12A7:O2− glass obtained by melting C12A7:O2− under a strongly reducing atmosphere in a carbon crucible (11). Electron spin resonance (ESR) measurement (Fig. 2D, inset) of the C12A7:e glass reveals a weak signal at g = 1.998 that we assigned to the F+-like centers where an electron is trapped at an oxygen ion vacancy and is coordinated by Ca2+ ions (11). ESR measurements gave an Ne of ~5 × 1018 cm−3, which is just 0.5% of the Ne (~1.1 × 1021 cm−3) of the C12A7:e glass. From these results, we assume that most of the electrons in the C12A7:e glasses are responsible for the optical absorption at 4.6 eV, existing in a spin-paired state.

Raman spectra of the C12A7:e glasses with different Ne provide structural information to compare with those of C12A7:O2− oxide glasses (Fig. 3). There are three distinct differences: First, a sharp band appears at 186 cm−1 in the spectra of the C12A7:e glasses, and its intensity increases as Ne increases. Second, the band at 430 cm−1, which is weak in C12A7:O2− glass, becomes distinct, and its intensity relative to the 780 cm−1 band increases with Ne. Third, the intensity of the 780 cm−1 band, which is assigned to the stretching mode of Al−O of a tetrahedral AlO4 unit (12), is greater relative to the 560 cm−1 band. The sharp band at 186 cm−1 in the spectra of the electride glasses should be associated with trapped electrons because this band is observed only for the electride glasses with Ne > 3 × 1020 cm−3 and its intensity is proportional to Ne. To confirm the relation between the 186 cm−1 band and trapped electrons, we measured Raman spectra of the electride glass with a Ne of 1.1 × 1021 cm−3 using several excitation lasers of different wavelengths. As shown in the inset of Fig. 3, the intensity of the 186 cm−1 band relative to the 780 cm−1 band increases as the excitation photon energy increases, agreeing well with the optical absorption spectrum. This observation strongly suggests that the 186 cm−1 band originates from the resonance Raman scattering associated with the absorption band at 4.6 eV. Several bands associated with cage vibration are present at around 200 cm−1 in Raman spectrum of a C12A7 crystal (13). Thus, we attribute the sharp band at 186 cm−1 to vibration associated with the cages containing trapped electrons that generate the absorption band at 4.6 eV. The sharpness of this Raman band may be understood in terms of selective excitation of the cages containing trapped electrons at the specific excitation wavelength of the laser.

Fig. 3

Raman spectra of a C12A7:O2− crystal (c-C12A7:O2−), C12A7:O2− glasses (g-C12A7:O2−), and C12A7:e glasses (g-C12A7:e) with different Ne. The C12A7:O2− glasses (B) and (C) were obtained by quenching a melt of C12A7:O2− oxide under an oxidizing atmosphere of PO2 ~ 1 atm by using an alumina crucible and a reducing atmosphere of PO2 ~ 10−16 atm by using a carbon crucible (11), respectively. The wavelength of the excitation laser was 457 nm. (Inset) The relationship of the Raman intensity ratio (I186/I780) of the 186 cm−1 band to the 780 cm−1 band measured with excitation lasers of various wavelengths. The optical absorption spectrum of C12A7:e glass [sample (G)] is shown to clarify the resonance effect.

Following a previous Raman study on calcium aluminate glasses (12), we assigned the band at 560 cm−1 to the transverse motion of bridging oxygen within 4Al−O−4Al (superscript denotes oxygen coordination number) linkages. The increase in the intensity of the 780 cm−1 band relative to that of the 560 cm−1 band is explained by the higher concentration of depolymerized tetrahedral AlO4 units in C12A7:e glasses as compared with C12A7:O2− glass. Such an anion unit is the dominant structure in a CaO-rich composition. The band at 430 cm−1 is attributed to edge- or face-sharing AlO4 units that are present in glasses with Al2O3-rich compositions (12). The presence of these structural units is consistent with the higher density of the C12A7:e glass compared with the C12A7:O2− glass. That is, C12A7:e glass contains heterogeneous anion structures corresponding to those in CaO- and Al2O3-rich compositions (the fraction of such anion structures is very low in conventional C12A7:O2− glass) in addition to structures with the nominal composition—highly depolymerized structures and a highly polymerized network structure connected by edge- or face-sharing.

On the basis of the above results, we propose a structural model for the melt and glass of C12A7:e electride, as shown in Fig. 4. The densities of both electride and oxide melts are greater than the 2.68 g cm−3 of the crystal composed of sub-nanometer-sized cages connected in three dimensions, indicating that the crystallographic cages lacking caged species collapse upon melting. Indeed, no cage structure has been observed in many structural analyses of C12A7:O2− melts and glasses (14, 15). However, because the density of the C12A7:e melt is ~10% greater than that of the C12A7:O2− melt we need to consider a denser network structure for the electride melt. The appearance of a distinct Raman band from edge- or face-sharing AlO4 groups in the electride glasses is consistent with the observed difference in density. Edge- or face-sharing AlO4 groups form a dense network structure that not only leads to a lower Tc of the electride glass than that of C12A7:O2− glass (Fig. 2A) but also facilitates crystallization when the electride melt is quenched. This difference forced us to use a roller-quenching process, even though the viscosity of the C12A7:e melt is higher than that of the C12A7:O2− melt (the stable glass formation range for CaO–Al2O3 systems is restricted to CaO 62 to 65%) (16).

Fig. 4

Model of the melt and glass of C12A7:e electride. (A and B) The crystal structures of C12A7:O2− and C12A7:e electride. The free oxygen anions [blue sphere in the cage of (A)] of C12A7:O2− can be selectively extracted by several reducing processes, and electrons [green sphere in the cage of (B)] are trapped to maintain charge neutrality, yielding C12A7:e electride. (C and D) Feasible structures of the melt and glass of C12A7:e electride, respectively. Upon melting, empty cages collapse to form a dense network structure, and the cages containing electrons remain but decrease in size. The wavefunctions of electrons trapped in the cages of the melt percolate to allow metallic conduction. In the glassy state, electrons in cages have different trapping potential because of structural randomness. The majority of electrons in the glass are trapped in cages to form a diamagnetic state (bipolaron) with a nearby electron, whereas a minority of electrons (~1018 cm−3) are trapped at interstitial sites and coordinated with Ca2+ ions to form a paramagnetic state similar to an F+-like center in CaO.

Next, we considered the structure that traps electrons. The absorption peak from electrons trapped in cages of crystalline C12A7 is located at ~2.8 eV. Given that a caged electron can be modeled as a “particle in a box” (17), we considered that the cages containing trapped electrons that give rise to the 4.6 eV band are smaller than the cages in the crystal. Observations of the spin-paired state and hopping conduction in the glass indicate that two electrons are trapped at adjacent sites. Because of large Coulombic repulsion, it is unrealistic that a single shrunken cage accommodates two electrons, so it is more probable that two shrunken cages each containing a trapped electron are connected to each other, forming a peanut-shaped structure: a bipolaron, which is reminiscent of solvated electrons in alkali metal-ammonia solutions (18), although this peanut-shaped bipolaronic structure is still speculative. The metallic conduction of the melt suggests that the shrunken cages containing trapped electrons are connected and span the melt, leading to the percolation of the wave functions of the electrons over the melt. Thus, we consider that the melt of C12A7:e electride at high temperature adopts a similar state to an alkali metal-ammonia solution containing solvated electrons in which the electron density is delocalized over the liquid by strongly associated bipolaron structures. That is, solvated electrons persist in the high-temperature melt because they are trapped in sub-nanometer-sized cages like those in a C12A7:e crystal. The present semiconductive glasses, which were obtained by quenching of C12A7:e melts, contain a high concentration of interstitial electrons and may be categorized as a previously unidentified class of amorphous electronic materials.

C12A7:e electride is a light metal oxide–based material and is a representative constituent of slag as a vitreous byproduct of the smelting process. The present C12A7:e melt exhibiting metallic conductivity—metallic slag—and C12A7:e glass displaying electro-conductivity will provide new applications for oxide melts and glasses. We anticipate that our present work will stimulate further research to exploit similar liquids in other inorganic-based electrides and elemental electrides under high pressure.

Supporting Online Material

www.sciencemag.org/cgi/content/full/333/6038/71/DC1

Materials and Methods

Figs. S1 to S5

Movie S1

References and Notes

  1. Acknowledgments: We thank K. Hayashi and T. Yoshizumi of Tokyo Institute of Technology for the help of iodometric titration. S. Matsuishi and S. Ito of Tokyo Institute of Technology are thanked for useful discussions. Help by T. Tomoda for Raman spectra measurements is acknowledged. This study was supported by the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST), Japan Society for the Promotion of Science, Japan.
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