Structure of a Novel Spinel Li_0.5Zn_5/3Sb_2.5/3O₄ by Neutron and Synchrotron Diffraction Analysis

Abstract

Zn_7/3Sb_2/3O₄ is a secondary phase in ZnO-based varistors. Acceptor impurities, such as Li⁺, increase the resistivity. This effect is produced by a modification of the grain boundary barriers. The role of the cationic distribution in the mentioned events is worth clarifying. The Li_0.5Zn_5/3Sb_2.5/3O₄ room-temperature structure was determined by means of a neutron diffraction and synchrotron X-ray diffraction investigation. The title compound was prepared by conventional ceramic process. The elemental composition of the investigated sample was verified by means of electron microscopy—energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The neutron experiment was performed at the high-intensity neutron diffractometer with position-sensitive detector at the D1B beamline of the Laue-Langevin Institute, Grenoble. The high resolution synchrotron measurement was carried out at MCX beamline of Elettra Sincrotrone Trieste. Rietveld analysis was performed with the FullProf program. Li_0.5Zn_5/3Sb_2.5/3O₄ belongs to the spinel family, space group $Fd\overline{3}m$ (227). The measured lattice parameter is a = 8.5567(1) Å. The Li⁺¹ and Zn⁺² ions are randomly distributed among the tetrahedral and octahedral sites as opposed to Sb⁺⁵ ions which have preference for octahedral sites. Fractional coordinate of oxygen, u = 0.2596(1), indicates a slight deformation of the tetrahedral and octahedral sites. The data given in this paper provide structural support for further studies on measurements and microscopic explanations of the interesting properties of this family of compounds.

1. Introduction

Spinels motivate continuous scientific interest due to their physicochemical properties and their application in a wide spectrum of fields. They are used in catalysis, power management, telecommunications, medicine, and physical-chemical sensing, among other disciplines [1,2,3,4,5,6].

An important current field of spinel application is as a component of multiphase systems, such as ZnO ceramic varistors, to which small amounts of (Bi, Sb, Co, Mn, Cr, Al, Ni) oxides are added to improve their electrical characteristics. Particularly zinc antimony spinel, Zn_7/3Sb_2/3O₄, is a well-known secondary phase in ZnO-based varistor ceramics. It has been reported to inhibit ZnO grain growth and to diminish the sintering temperature, therefore attaining better stability at higher voltages. The mentioned effect is due to segregation effects at grain boundaries, leading to the formation of separate spinel grains. The electrical properties of Zn-based spinels are sensitive to variations in the crystallographic structure [7,8,9].

Undoped Zn_7/3Sb_2/3O₄ has two polymorphic phases. According to Harrington [10], the transition from the low-temperature β-polymorph to the high-temperature α-polymorph occurs at 1225 °C. The α-phase is cubic inverse spinel [11,12,13]. Its configuration shows 8 tetrahedral positions occupied by Zn and 16 octahedral sites randomly occupied by Sb and Zn. The occupancy ratio of Sb and Zn at the octahedral positions is 1:2 [14]. The geometry of the structural tetrahedra and octahedra vary weakly with changes in environmental and crystallization conditions [15]. The β-phase adopts a complex orthorhombic structure [10,16], possibly forming a superstructure [17].

It has been published that a small addition (ppm order) of monovalent cations—such as Li⁺, Na⁺, or K⁺—to Zn_7/3Sb_2/3O₄ has an influence on the ZnO varistors’ electric properties, presumably because it acts as a grain growth inhibitor, decreases the donor density, and thus increases the potential barrier. In addition, the small size effect facilitates the diffusion of lithium cations into the ZnO crystal lattice [18,19,20].

Studying the binary system Zn_7/3Sb_2/3O₄–LiZnSbO₄, we have found the formation of a solid solution Li_xZn_(7−4x)/3Sb_(2+x)/3O₄ (0.3 < x < 0.6). Therefore, the aim of the present study is to determine the crystal structure of characteristic compound Li_0.5Zn_1.67Sb_0.83O₄ (x = 0.5) with focus on the elucidation of the ionic distribution. The investigation was performed by means of neutron diffraction (ND) and synchrotron X-ray diffraction (SyXRD).

2. Experimental

Polycrystalline Li_0.5Zn_5/3Sb_2.5/3O₄ was prepared by solid-state reaction process. The purity of the compounds was as follows: (1) Li₂CO₃ (99.99% Sigma Aldrich, St. Louis, MO, USA) dehydrated at 600 °C, (2) ZnO (99.99% Sigma Aldrich) dehydrated at 200 °C, and (3) Sb₂O₅ (99.95% Sigma Aldrich). The compounds were ground in an agate mortar with acetone to favor a homogeneous mixture and then heated in Pt foil boats up to a temperature of 750 °C for six hours to achieve decarbonation of Li₂CO₃. The final heat treatment consisted of a five-cycle process, with 24 h per cycle, at rising temperatures from 800 °C to 1000 °C (ΔT = 50 °C per cycle), in normal oxidizing atmosphere. At the end of every heat treatment the sample was removed from the furnace and cooled in air, weighed, and reground prior each treatment. In order to confirm the formation of the desired phase, the obtained sample was characterized by means of conventional XRD.

To verify the elemental composition of the investigated sample, a scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDXS) observation was performed using a JEOL JSM-7600F (JEOL, Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) was carried out in a Thermo Scientific K-alpha (Thermo Fisher Scientific, East Grinstead, England), in ultrahigh vacuum with a monochromatic Al Kα X-ray source with an energy of 1486.6 eV.

High resolution SyXRD experiment was carried out at MCX beamline of Elettra Sincrotrone Trieste (Trieste, Italy). Taking into consideration the output spectrum of the MCX beamline, the value E = 13.048 keV, leading to λ = 0.9500 Å was chosen. The experimental setup was calibrated by the measurement of a silicon standard. The sample was measured with reflection geometry on a flat sample-holder plate. The measured angular domain was 10° < 2θ< 90°, with a recording step of Δ(2θ) = 0.005°.

The ND experiment was carried out on the high-intensity D1B beamline of the Laue–Langevin Institute, Grenoble. The sample was measured inside of a vanadium tube with a position sensitive detector diffractometer, with 0.1° step in 2θ, in a range of 1° ≤ 2θ ≤ 128°, with λ = 1.28 Å.

The structural refinement of Li_1.5Zn₅Sb_2.5O₁₂ was carried out using FullProf program [21]. In a first stage of refinement, the scale factor, zero error, and background were adjusted. Next, lattice parameters, profile, and structural data were refined. Within the adopted structural model, the only atomic coordinates adjusted were the oxygen fractional coordinates. The Debye-Waller factors were treated as isotropic. Two restraints were considered. The first one is that the tetrahedral and octahedral spaces are fully occupied and the second one is that the antimony ions are located at the octahedral sites [22,23,24,25].

3. Results and Discussion

The Li_0.5Zn_5/3Sb_2.5/3O₄ compound shows spinel structure, as described in Figure 1. The space group is $Fd\overline{3}m$ (227) and the lattice parameter is a = 8.5567(1) Å. Crystal data and structure refinement details of the Rietveld refinements of ND and SyXRD were combined and summarized in

Table 1. Crystallographic data and refinement of the compound Li_0.5Zn_5/3Sb_2.5/3O₄.

Crystal Data
			Radiation
Cubic, $Fd\overline{3}m$ (227)			Synchrotron X-rays, λ = 0.9500 Å
			Constant wavelength Neutron, λ = 1.28 Å
a = 8.5567(1) Å			Particle morphology: equiaxial powder
V = 626.5078(2) Å3			Color: White
Z = 8
Rietveld reliability factors
ND Data			XRD Data
Rp = 0.05			Rp = 0.06
Rwp = 0.06			Rwp = 0.08
Rexp = 0.05			Rexp = 0.05
RBragg = 3.928			RBragg = 5.377
χ2 = 1.631			χ2 = 4.0342
1150 data points			9794 data points
Excluded region(s): none			Excluded region(s): 1 (23° ≤ 2θ ≤ 24°)
	x	y	z	Uiso	Occupancy
Zn1	0.12500	0.12500	0.12500	0.0107(10)	0.82(4)
Li1	0.12500	0.12500	0.12500	0.0107(10)	0.18(4)
Li2	0.50000	0.50000	0.50000	0.0121(6)	0.16(2)
Zn2	0.50000	0.50000	0.50000	0.0121(6)	0.42(2)
Sb1	0.50000	0.50000	0.50000	0.0121(6)	0.42(2)
O1	0.2596(1)	0.2596(1)	0.2596(1)	0.0131(3)

. Figure 2 shows the Rietveld refinement of the ND study, in which the ionic distributions were adjusted. Figure 3 shows the SyXRD Rietveld refinement, focused on the lattice parameters determination.

The material SEM observation is shown in Figure 4a,b. Observed particles show regular morphologies, with smooth surfaces and defined edges; the particle sizes are around 200 nm. Figure 4c shows the elemental analysis by EDXS of a representative area; the atomic percentage ratio for Zn:Sb is 2:1, in agreement with the given significant figures in the Li_0.5Zn_1.67Sb_0.83O₄ formula. The assumption of total occupation for the tetrahedral and octahedral sites is consistent with the compound electrical neutrality and the obtained refinement reliability coefficients. Further, the cation Sb⁵⁺ has the electronic configuration as t_2g⁶ e_g², which leads to octahedral occupancy.

Further evidence of the composition of the sample was obtained from XPS analysis. The binding energies were corrected for specimen charging to the carbon C 1s position at 284.8 eV. C 1s line came from adventitious surface carbon. The XPS spectrum of Li_1.5Zn₅Sb_2.5O₁₂ powder (Figure 5a), confirms the presence of zinc, antimony, lithium, and oxygen. O 1s and Sb 3d_5/2 peaks are overlapped. They have similar binding energies of 539.28 eV and 539.48 eV, respectively (Figure 5c). To circumvent the mentioned obstacle, the Sb concentration was determined via the Sb 3d_3/2 peak (530.07 eV) [6,26,27]. The high-resolution spectra for Zn 2p, Sb 3d_3/2, O 1s, and Li 1s are shown in Figure 5b–d. The atomic concentrations were calculated by means of Equation (1)

$$C_x=\frac{I_x/S_x}{{{\displaystyle \sum }}_i{I}_i/S_i}$$

(1)

C_x and I_x are, respectively, the atomic fraction and the peak area of element x and S_x is the relative sensitivity of the considered photoelectron peak [28]. The atomic percentage for each element was Li (7.3%), Zn (22.4%), Sb (12.7%), and O (57.6%) which are in a good agreement with the composition of Li_1.5Zn₅Sb_2.5O₁₂.

The lattice parameter of the investigated phase is larger than that of the spinel Li_0.64Fe_2.15Ge_0.21O₄ (a = 8.2903(3) Å) [29], similar to Zn₂Co₅Sb₂O₁₂ (a = 8.55917(9) Å) and smaller than the ones of spinel Zn₇Sb₂O₁₂ (a = 8.6047 Å) [16].

The site-occupation refinement indicates that Li⁺ and Zn⁺² are distributed on tetrahedral and octahedral sites. In the work of Harrington for Zn₇Sb₂O₁₂ doped with Cr and Ni, Zn⁺² was located only in tetrahedral positions [6,12]. However, Ezhilvalavan & Kutty reported that Zn²⁺ ions are located in both octahedral and tetrahedral positions [30].

The determined value of the oxygen fractional coordinate (u = 0.2596(1)) is slightly higher than the ideal spinel value. To explore the geometrical implications of this result, the cation–oxygen distances were calculated by means of the program Vesta [31] and by the equations of O’Niell and Talanov [1,32]. Our geometrical results are compared with those reported by other authors and with the ones calculated by mean of the Shannon radii, weighting with the occupation factors. The values obtained via Shannon radii were b_t(Shannon) = 1.998 Å and b_o(Shannon) = 2.088 Å. Our experimental bond length in the tetrahedral sites is b_t = 1.986(4) Å. This value is comparable with those of Zn₂Co₅Sb₂O₁₂ (b_t = 1.99807(2) Å) [16], Zn_7/3Sb_2/3O₄ (b_t = 2.015(8) Å) [10] and Li_0.64Fe_2.15Ge_0.21O₄ (b_t = 1.857(2) Å) [29]. In the octahedral sites, the bond length is b_o = 2.065(4) Å. Our value is slightly less than those corresponding to the aforementioned reports by Redhammer and Harrington (b_o = 2.0373(11)) Å [29]; b_o = 2.05950(2) Å [10]; b_o = 2.072(5) Å [16]. The obtained ratio b_o/b_t = 1.039 is smaller than the one corresponding to an ideal spinel (b_o/b_t(ideal) = 1.155). This result implies an increase in the size of the tetrahedral sites at the expense of the octahedral ones. The observed changes in the site sizes are due mainly to the distribution of Zn²⁺ and Li⁺ and the difference in the effective ionic radius [33].

4. Conclusions

Doping Zn_7/3Sb_2/3O₄ with Li⁺ introduces slight modifications of its spinel structure. Mentioned variations have been investigated by taking full advantage of experiments with synchrotron radiation and neutrons. Synchrotron light allowed high resolution measurement of the cubic cell parameter. Neutrons led to the determination of the coordination polyhedra changes and sites’ occupancies, including Li⁺ ions’ distribution. Zn⁺² ions are partly replaced by Sb⁺⁵ and Li⁺ in octahedral sites and by Li⁺ in tetrahedral sites.

The present structure analysis represents a configurational basis for first principles calculations, leading to quantitative interpretation of experimental results, related with thermo-electrical and optical properties of the considered materials, results which publication is being prepared.