Preparation and electrical properties of inorganic electride [Y2Ti2O6]2+(2e−)

Oxygen-depleted samples [Y2Ti2O7−x]2x+(2xe−) (0 ≤ x ≤ 1.0) were prepared by reducing Y2TiO7 powders at 500 °C to 650 °C using CaH2 as a reductive agent, where x represents the content of , which was determined by thermogravimetric analysis. Powder X-ray diffraction patterns illustrate that the pure pyrochlore phase is kept for the samples with x ≤ 1.0, whereas the apparent x values surpass 1.0, and the impurity phase Y2O3 appears. The electride [Y2Ti2O7−x]2x+(2xe−) (x ≈ 1.0) can be obtained under a reductive condition, in which the concentration of VO is 7.75 × 1021 cm−3. The electron paramagnetic resonance measurements gave the concentration of unpaired electrons in the electride as 1.30 × 1021 cm−3, indicating that the degree of the ionization of is less than 10%. Conductivity measurements for a sintered pellet sample (relative density ∼ 70%) indicate that the electride has quite high conductivity (∼1.09 S cm−1 at 300 K). The conduction was interpreted by using the variable range hopping mechanisms.


Introduction
Electrides are a kind of material in which electrons serve as anions. 1 It was found that electrides were formed when the alkali metal and alkali earth metals were dissolved into liquid ammonia at low temperatures. As an example, when the metal Na is dissolved into liquid ammonia, the Na atoms are ionized and form solvated Na + [Na(am) n + , am ¼ ammonia] as cations. At the same time, the generated electrons are also solvated [e(am) m − ], which play the role of anions. The reactions between alkali metals and crown ethers also lead to the formation of electrides. In the crystal of Cs(18crown-6) 2 complex, Cs + is coordinated by two 18crown-6, forming [Cs(18crown-6) 2 ] + as cations, while the "electron-anions" generated from the ionization of the Cs atoms take the interstitial positions constructed by the [Cs(18crown-6) 2 ] + cations. Due to the existence of the solvated electrons, the electrides are very strongly reductive agents. 1 However, these electrides are only stable at low temperatures and very sensitive to air and moisture. Thus, their applications are impeded.
Recently, Matsushi et al. reported an inorganic solid electride-fully reduced mayenite [Ca 12 Al 14 O 32 ] 2+ (2e − ), 2 which is stable at ambient temperature and atmosphere. The original composition of mayenite can be represented as Ca 12 Al 14 O 33 , which is a neutral material. This material has a tetragonal unit cell, containing two molecules (Ca 12 Al 14 O 33 Â 2). The lattice framework is constructed by [Ca 12 Al 14 O 33 Â 2] 4+ with the positive charge composing 12 cages, 2 relatively loosely bonded O 2− take the positions in 2 out of 12 cages. 2,3 When the material is thermally treated in a reductive atmosphere, part of the loosely bonded O 2− ions may escape as O 2 from the lattice and leave electrons in the cages, considered as the electron captured oxygen vacancy V Â O , or F-center. With the proper reductive condition, all of the loosely bonded O 2− ions can be removed from the lattice. In this case, the relatively free electrons captured at V Â O play the role of anions. Thus the material is considered as the electride. The concentration of the "free electrons" is high in the lattice. Therefore, the electride has high conductivity. Since some of the "free electrons" are unpaired, the magnetic properties are also attractive. The inorganic electrides would be potentially used as reductive agents, electronemitters, etc 2,4 . Additionally, it was also reported 5,6 that these materials were applied as catalysts for the NH 3  Actually, the research to prepare the O-depleted pyrochlore could be found in the 1960s. In Goodenough's work, 9 the Pbnoble metal pyrochlores (Pb 2 M 2 O 7−x , M ¼ Ru, Ir, Re) with Odepletion were prepared, and the materials with x ¼ 1.0 were obtained. However, in this work, the concept of electride was not mentioned, and the functional properties (i.e. conductive or magnetic properties) were not studied. The research for the Odepleted Y 2 Ti 2 O 7 (Y 2 Ti 2 O 7−x ) appeared in this century, 10,11 in which the materials with x z 1.0 were prepared, and the conductivities and magnetic susceptibilities were measured. It is a pity that the concept of the electride was not applied to interpret the work either. Therefore, further research on the Odepleted Y 2 Ti 2 O 7 is imperative.
In this work, the reductive conditions were explored for preparing the O-depleted Y 2 Ti 2 O 7, and the electride with the composition Y 2 Ti 2 O 6 (x z 1.0) was obtained, which would be represented as [Y 2 Ti 2 O 6 ] 2+ (2e − ). The contents of V O and the concentration of unpaired electrons in the lattice were measured. The conductive properties of the electride Y 2 Ti 2 O 6 were characterized.

Experimental
Powder samples of the starting material Y 2 Ti 2 O 7 were synthesized by solid-state reactions from stoichiometric mixtures of the raw materials TiO 2 (A. R.) and Y 2 O 3 (99.99%) at 1400 C for 13 h in air. Prior to heating, the raw materials were thoroughly mixed by grindings in an agate mortar with a pestle. Asprepared powder samples of Y 2 Ti 2 O 7 have a white (or pale) body color.
The powder samples of the O-depleted Y 2 Ti 2 O 7 were prepared from the powder of Y 2 Ti 2 O 7 by using the solid reducing approach, in which the solid reductive agent CaH 2 was applied. The powder mixtures of Y 2 Ti 2 O 7 ($0.5 g) and CaH 2 in various mass ratios were ground in a glovebox (Mikrouns, Ar atmosphere, O 2 < 1 ppm, H 2 O < 1 ppm), and then sealed in vacuumed quartz ampoules ($5 ml). The sealed samples were heated at various temperatures ranging from 500-650 C with various time 5-40 h. Aer heating, crash the quartz ampoules and take the samples out. The samples were washed using deionized water for several times until the supernatant approached to neutral in order to remove Ca 2+ and OH − ions. Finally, the washed samples were dried at 140 C for 2 h in an oven. The reduced treatments lead to the samples becoming dark.
For conductivity measurements, the pellets of the O-depleted samples were sintered. The powder samples were mixed with a few drops of 5% polyvinyl alcohol solution as a binder and then were pressed into pellets with 10 MPa pressure in stainless steel die. The pellets were sealed in the vacuumed quartz ampoules and sintered at 950 C for a short time $2 h, in order to avoid a large change in the sample status. The obtained pellets had the dimension $1.0 cm in diameter and $0.15 cm in thickness with a relative density of $70%.
The phase purity of samples was characterized by X-ray diffraction (XRD) using a Rigaku Dmax2000 X-ray powder diffractometer (Japan) with Cu Ka radiation (l ¼ 0.15418 nm) at 40 kV and 100 mA. The XRD patterns were collected in the 2q range 10 $ 80 with the scanning rate of 8 min −1 . Electron paramagnetic resonance (EPR) measurements were performed at $9.4 GHz (X-band) using a Bruker A300 10-12 EPR spectrometer (Germany). The concentrations of unpaired electrons in the samples were determined by using the method supplied in ref. 12, in which the areas of the microwave absorption bands were calculated by twice integrations of the original EPR spectra ("dA/dH-H ′′ plots, A is the microwave absorption of the samples and H is the strength of the magnetic eld), and CuSO 4 $5H 2 O, diluted by Al 2 O 3 powders with the mass ratio CuSO 4 $5H 2 -O : Al 2 O 3 ¼ 1 : 9, was applied as a standard. Both the XRD and EPR measurements were carried out at room temperature. Thermal analyses were conducted on a simultaneous differential scanning calorimeter-thermogravimetric analyzer (DSC-TGA, Q600STD, Thermal Analysis, USA). The data were collected from room temperature to 1200 C with the temperature increasing rate of 20 C min −1 and air as owing gas.
The conductivities of the sintered pellets were measured in the temperature range 80-450 K using a four-wire setup, and Pt wires were cemented by Ag paste on the two opposite surfaces of the pellet samples. The temperature environment was provided by a closed cycle refrigerator (Janis VPF-100, UAS) using the liquid N 2 as the refrigerant. The temperature was regulated by a M331 temperature controller (LakeShore, USA). A SB118 current source and a PZ158A voltmeter (Shanghai Qianfeng Electronic Instruments, China) were used for data collection.   indicating that the former is much higher than the later. Therefore, it might be deduced that the oxygen atoms in the OY 4 tetrahedrons are easier to be removed out from the lattice than the oxygen atoms in the TiO 6 octahedrons. If Y 2 Ti 2 O 7 is thermally treated in a reductive atmosphere, and the oxygen vacancies V O tend to take the sites at the centers in the OY 4 tetrahedrons.

Investigation of the reductive conditions
The starting material Y 2 Ti 2 O 7 was prepared by the routine solid state reactions, which was described in the "experimental" section. The XRD pattern of the as-prepared sample shown in Fig. 2 illustrates that the pattern matches well to the related JCPDS le 42-413. Thus, the as-prepared Y 2 Ti 2 O 7 is a pure phase sample. In order to change the starting material to the Odepleted material or the electride, thermally reducing treatments are necessary for Y 2 Ti 2 O 7 . The reducing process was also described in the "experimental" section. For obtaining high content of V O , the reductive conditions should be chosen as strong as possible, but the phase purity should be kept.
Firstly, the inuence of the mass of CaH 2 to the phase purity was investigated. The Y 2 Ti 2 O 7 powder samples (0.5 g for each) were mixed with various amounts of CaH 2 (0.25 g-1.25 g), and annealed at 500 C for 16 h. Aer the reducing treatment, the white powders of Y 2 Ti 2 O 7 became dark, which can be explained by the defect reactions and chemical reactions. When Y 2 Ti 2 O 7 is annealed at a high temperature (e.g. above 500 C), the defect reaction (1) occurs, and the oxygen vacancies V Â O are produced: In the ambient atmosphere, eqn (1) only shis to right slightly, and the V Â O content is not very high. In this research, the solid reductive agent CaH 2 was added, thus the below chemical reaction was involved in the process and O 2 produced in eqn (1) was reduced by CaH 2 : Thus the addition of CaH 2 lead to right-shi of eqn (1) in a large extent and the V Â O content increased signicantly. In this research, it is respected that 1 mole O atoms should be removed, and 1 mole V Â O should be produced. In addition, V Â O may be ionized and transfer an electron to Ti Â Ti : For brief discussion, the second step of the ionization Ti have loosely bound electrons, the levels of which may be located in the band gap below the conduction band. These loosely bound electrons may be excited to the conduction band by the visible light, resulting in the dark body color of the reduced samples.
The XRD patterns of the samples thermally treated with various amounts of CaH 2 are shown in Fig. 2. The patterns of all the samples are in good agreement with that of JCPDS le 42-413 and no clear peaks related to any impurity phases were observed, thus, all the reduced samples are phase pure, which indicated that the mass of CaH 2 was not very sensitive to the phase purity of the reduced samples and the CaH 2 weights used in this work were all suitable. However, in order to keep a relatively strong reductive conditions, the CaH 2 weight of 0.75 g was chosen for the further work. Then optimized reducing temperature was investigated. Several powder samples of Y 2 Ti 2 O 7 (0.5 g for each) mixed with 0.75 g CaH 2 were annealed at various temperatures (500 C-650 C). The XRD patterns represented in Fig. 3 indicate that if the reducing temperature is below 600 C, the reduced samples are phase pure; whereas if it is above 650 C, impurity phase of Y 2 O 3 appears. The suitable temperature was decided as 600 C for the reducing annealing.
Furthermore, the inuence of annealing time to the phase purity was explored. The powder samples of Y 2 Ti 2 O 7 (0.5 g for each) mixed with 0.75 g CaH 2 were annealed at 600 C for various time (5-40 h). The XRD patterns (in Fig. 4) clearly shows that the reducing time 20 h may be considered as an optimized time. If the reducing time surpasses 20 h, the peaks of the impurity phase Y 2 O 3 appear.
In summary, to prepare electrides or O-depleted samples, the suitable reductive conditions are decided: the starting material Y 2 Ti 2 O 7 0.5 g mixed with the reductive agent CaH 2 0.75 g is annealed at 600 C for 20 h.

Measurements of the content of the oxygen vacancies V O
In 3.2 section, it was deduced that aer reducing treatment, the oxygen vacancies V O (including V Â O and V $ O ) may be formed. In this section, the existence of V O would be proved by TG measurements.
The TG measurements (Fig. 5) were conducted for the samples previously annealed with 0.75 g CaH 2 at 600 C for various time, 5-40 h. The TG data indicate that with the increase of the temperature from $320-$620 C, the weights of the samples enhance clearly and the weight increments are about several percent. The samples previously annealed longer time in the reductive conditions have larger increments.
In 3.2 section, it was deduced that aer reduction by CaH 2 , the samples have quit high contents of V O . In this case, the composition of the material can be represented by the formula Y 2 Ti 2 O 7−x . The x represents the content of V O in the lattice. The reaction that occurred during TG measurements can be represented as: Eqn (6)     larger the weight increment is. According to the values of the weight increments, the x values (the V O contents) can be calculated, showed in Fig. 5. For the sample previously annealed with CaH 2 for 5 h, the V O content x is 0.47. With an increase of the annealing time, the x value increases. When the annealing time increases to 20 h, the x value approaches 1.0, which illustrates that one mole of oxygen atoms are removed out from the lattice, and one mole V O is formed. As we know, V Â O has two loosely bound electrons (maybe, some of them are ionized and transferred to the Ti 0 Ti sites, but the total number of the loosely bound electrons are not changed), which may play the role of the anions. Thus this sample is considered as an electride, the composition of which can be represented as [Y 2 Ti 2 O 6 ] 2+ (2e − ). For the sample with annealing time 30 h, the apparent "x" value is 1.36 (over 1.0). However, the XRD data (Fig. 4) indicate that this sample is over reduced and is not phase pure, thus this "x" value is insignicance. The TG curves also show that the samples have slight weight loss above 620 C. It is mostly possible that eqn (1) slightly shis to right at higher temperatures since it is an entropy increasing reaction (solid O becomes gaseous O).
There is another possibility that the reducing treatments does not produce V O , but the loss of oxygen leads to the reduction of Ti 4+ to Ti 3+ . If this is the case, for the sample x ¼ 1, 1 O was lost, which would result in 2 Ti 4+ were reduced to Ti 3+ . In other words, in this sample, all Ti were reduced from Ti 4+ to Ti 3+ . XRD pattern (Fig. 4) revealed this sample has pure phase of pyrochlore. It could not be imaged that Ti 3+ compound could have the same structure as Ti 4+ compound. Therefore, it is believed that the reducing treatments produce V Â O in the lattice, rather than reduce Ti 4+ to Ti 3+ . It is known that the ionization of V Â O may produce Ti 3+ , described in eqn (4) or (5), but only small amount of Ti 4+ can be reduced (see 3.4 section), thus the pyrochlore structure can be kept. Actually, ref. 10

Measurements of the concentration of the unpaired electrons
In 3.2 section, it was presumed that once the oxygen vacancies V Â O were formed in the lattice, they would be ionized and transfer electrons to Ti, represented by eqn (3) and (4). In this case, two kinds of defects V $ O and Ti 0 Ti would exist, each of which has one unpaired electron. These defects can be characterized by using EPR.
A selected EPR spectrum for the sample annealed with 0.75 g CaH 2 at 600 C for 20 h is illustrated Fig. 6. The composition of the sample in Fig. 6 can be represented as Y 2 Ti 2 O 7−x (x z 1.0). This formula indicates that the sample has 1 mole of V O in the lattice, which is a quite high concentration of the defect. The high concentration of V O leads to lattice distortion, further it causes the broadening, deformation and overlapping of the EPR peaks, thus it is not easy to give clear assignments for this spectrum. Here, we would like to consider the peaks P 1 to P 3 . Refer to the assignments given in ref. [17][18][19][20] for some titania and titanates, the peak with g < 2.0 (P 2 ) is assigned to V $ O ; the peak with g < 2.0 (P 1 ) is assigned to Ti 3+ ðTi 0 Ti Þ. In fact, besides the electron transferring processes described in eqn (3)-(5), in the lattice, there are other electron transferring processes, e.g. the electrons transfer to the O 2 absorbed on the surface: P 3 has a g value larger than 2.0, which may be assigned to O 2 − . Actually, there are some peaks with poor resolution at lower magnetic elds, which might be related to some unknown defects containing unpair electrons, but they could not be assigned at this stage. The assignments for the above EPR peaks support the discussions in 3.2 and 3.2 sections about the formation of V O as well as the electron transfer processes, thus they also support that the sample related to Fig. 6 (Y 2 Ti 2 O 7−x , x z 1.0) is an electride.
A series EPR spectra were collected for the samples annealed with 0.75 g CaH 2 at 600 C for various times, i. e. for the samples Y 2 Ti 2 O 7−x with various values of x, represented in Fig. 7(a). The spectra in Fig. 7(a) have a similar prole to that in Fig. 6, which means these samples have the defects V $ O , Ti 0 Ti and O 2− . By using these EPR data, the concentrations of the unpaired electrons varied with annealing time (or x) can be calculated. The calculation equation was recommended by Weil, et al. 12 In these calculations, CuSO 4 $5H 2 O powder (mixed with Al 2 O 3 in the mass ratio W CuSO 4 $H 2 O : W Al 2 O 3 ¼ 1 : 9) was used as the standard. The EPR spectra for both the samples and the standard were collected in the same measuring conditions so as the parameters related to the instrument set-up, the signal amplication, and the scanning scale in the equation can be reduced. On the other hand, all of the defects V $ O and Ti 0 Ti , as well as Cu 2+ in the standard are the one-unpaired-electron centers and they all have the total spin quantum number S ¼ 1/2, thus the quantum number S also can be eliminated from the equation. The simplied equation for calculating the concentration of the unpaired electrons [e] can be represented as: Here N A is Avogadro's number; the subscript letter s denotes the samples and CuSO 4 denotes the standard CuSO 4 $5H 2 O; The parameter A is the area of the EPR absorption band. By integrating the original EPR spectra, which are in differential form (dA/dH-H), the absorption bands are obtained [ Fig. 7(b)]. The A values can be calculated by the second integration of the EPR spectra. g is the electron spin g factor, which can be calculated based on the EPR spectra. In this work, g CuSO4 was measured as 2.192 and the values of g s vary from 1.925 to 1.960, which is labelled for each spectrum in Fig. 7(a) m, FW and D represent the mass, formula weight and density, respectively. Since the resolutions of the spectra are poor, we could not calculate the concentration for each defect, but we may calculate the total concentration of the unpaired electrons approximately. The calculation results are illustrated in the inset in Fig. 7 Only less than 10% of V

DC conductivity measurements
Based on the above discussions, it is known that aer annealing with CaH 2 at 600 C for 20 h, Y 2 Ti 2 O 7 can be reduced to an electride, and its composition can be represented as [Y 2 Ti 2 O 6 ] 2+ (2e − ). Since the electride has loosely bound electrons (2e − ), it should have good conductivity. The DC conductivity of the electride was measured in the temperature range 100 K-500 K. Prior to the measurements. The pellet samples were sintered in vacuumed quartz ampoules at 900 C for 2 h (2.1 section). The data are illustrated in Fig. 8. The electride [Y 2 Ti 2 O 6 ] 2+ (2e − ) has relatively high conductivity, e.g. at 300 K, the conductivity s ¼ 1.09 S cm −1 . In this case, Arrhenius equation (s ¼ s A 0 e −E a / kT , E a is activation energy, k is Boltzmann constant and s A 0 is a constant) is not suitable to describe the relationship between the conductivity (s) and temperature (T). Nevertheless, the variable range hopping molds may be applied to this material. Following Chaudhuri's idea, 21 for the electride [Y 2 Ti 2 O 6 ] 2+ (2e − ), above 230 K, Mott equation is applied to analyze the "s $ T" data, while below this temperature, Efros-Shklovskii equation is used.
Since [Y 2 Ti 2 O 6 ] 2+ (2e − ) has quite high conductivity and the conductivity does not change very much with temperature, the lattice scattering to the electrons cannot be omitted. The preexponential factor in Mott equation should be modied from "s M 0 " to "s M 0 /T": Here both s M 0 and T O M are the constants. The data are represented in the "sT $ T 1/4 " form (Fig. 8). The line tting gave the value of 2.054 Â 10 5 K for TOM, by which other parameters for the conductivity were calculated following the equations below: 22 The density of state (DOS) at Fermi energy N M is represented as: a M is the decade rate of the wave function of the carriers, normally, the value of 2.0 nm −1 is given. Furthermore, the hopping distance (R M ) and the hopping energy (W M ) can be calculated as.
The calculated results (T ¼ 400 K) are listed in Table 1.
As the same reason, the pre-exponential factor of Efros-Shklovskii eqution is also modied as "s ES 0 /T": Here s ES 0 and T ES 0 are also the constants. By line tting, the value of T ES 0 was obtained as 717 K, and then, other parameters for the conductivity were calculated according to the following equations: The Coulomb gap D is calculated as: Here, T* is the temperature maximum that eqn (14) works. For the electride [Y 2 Ti 2 O 6 ] 2+ (2e − ), T* ¼ 230 K. DOS at the energy E ES F −D/2, N ES can be calculated as: Here, 3 r is the relative permittivity of Y 2 Ti 2 O 7 , ref. 23 gave the value of 54; 3 0 is the free space permittivity; e is the electron charge. The hopping distance (R ES ) can be calculated as: Here, a ES is the tunneling exponent, it can be calculated by: The value of N 2 is given as The hopping energy is given as: The calculated results are listed in Table 1 (T ¼ 200 K). Mott's theory and Efros-Shklovskii's theory are based on different DOS (density of state) features, which are schematically illustrated in the inset in Fig. 8. In Efros-Shklovskii's theory, the DOS varies as parabolic function near Fermi level, which is applied in low temperature range. At higher temperature range, DOS has at or constant feature by Mott's theory. The parameters listed in Table 1 are basically reasonable. Hopping energy W M is larger than W ES ; both W M and W ES are larger than the related kT. The hopping distance R M is shorter than R ES , but both of them are about 1.2 nm. According to the