ZnxMn1âxO Solid Solutions in the Rocksalt Structure: Optical, Charge

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ZnxMn1-xO Solid Solutions in the Rocksalt Structure: Optical, Charge Transport, and Photoelectrochemical Properties Venkata S. Bhadram, Qian Cheng, Candace K. Chan, Yiqun Liu, Stephan Lany, Kai M. Landskron, and Timothy A. Strobel ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00084 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 5, 2018

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ACS Applied Energy Materials

ZnxMn1-xO Solid Solutions in the Rocksalt Structure: Optical, Charge Transport, and Photoelectrochemical Properties Venkata S. Bhadram1,*, Qian Cheng2, Candace K. Chan2,*, Yiqun Liu3, Stephan Lany4, Kai Landskron3 and Timothy A. Strobel1,* 1

Geophysical Laboratory, Carnegie Institution for Science, 5251 Broad Branch Rd., NW, Washington D.C., 20015 USA

2

Materials Science and Engineering, School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, AZ 85287 USA 3

Department of Chemistry, Lehigh University, Bethlehem, PA 18015, USA 4

National Renewable Energy Laboratory, Golden, CO 80401, USA

AUTHOR INFORMATION Corresponding Author(s) * [email protected], * [email protected], * [email protected] KEYWORDS: Metastable oxides; high-pressure synthesis; charge transport; band gap bowing; photoelectrochemical water-splitting.

ACS Paragon Plus Environment

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Theoretical predictions of ZnO:MnO solid solutions (abbreviated here as ZMO) with the rocksalt-type structure suggest improved visible light absorption and suitable band edge positions for the overall water splitting reaction, but experimental efforts to produce such phases are limited by the low solubility of Zn within this structure type. Here, we produce solid solutions of ZnxMn1-xO, with x = 0.5 and 0.3 in the metastable rocksalt phase using high-pressure and high-temperature (HPHT) techniques. X-ray diffraction and electron microscopy methods were employed to determine the crystal structure, chemical composition and homogeneity on submicron scale. The solid solutions exhibit increased optical absorbance in the visible spectral range as compared to the parent oxides ZnO and MnO. Our theoretical calculations for ZnxMn1xO

with x = 0.5, 0.25 predict band gaps of 2.53 and 2.98 eV, respectively with an unusually large

bandgap bowing. Our calculations also show small effective electron mass for these materials indicating their potential for solar energy applications. Initial photoelectrochemical tests reveal that ZMO solid solutions are suitable for water oxidation and warrant further experimental optimization.

Semiconducting metal oxides comprise of a vast group of materials with extensive diversity in cation constituents, crystallographic arrangements and functionalities that include light-emitting diodes, microelectronics, photovoltaics and photocatalysts.1-4 Given the substantial range of cation solubility for many oxides, seemingly endless combinations of multinary metal/oxide compositions are possible for which material-specific properties can be tuned by varying stoichiometry.

Wurtzite (wz) ZnO is one of the well-studied functional oxides due to its high electron mobility, high thermal conductivity, wide and direct band gap (3.2 eV) and large exciton binding energy.


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These material properties make wz-ZnO suitable for wide range of devices including, thin-film transistors, photodetectors, light emitting diodes and laser diodes.5-9 wz-ZnO is particularly attractive for photocatalytic applications as it possesses a closed-shell d10 valence. This configuration has an additional advantage associated with dispersive conduction bands, which are composed of sp orbitals that can facilitate the generation of photoexcited electrons with high mobility.10 However, the applicability of wz-ZnO to photocatalysis is limited due to its wide band gap that allows absorption only in the UV region of the solar spectrum – a region that accounts for just 5% of the total solar irradiance. The large gap severely restricts the number of photons that can create charge carriers needed for the photochemistry. Thus, the development of novel band engineering strategies for wz-ZnO is essential to the realization of practical solar materials.

Alloying is a simple and effective strategy for wide band gap semiconductors.11-13 In contrast with doping, alloying may offer the benefit of altering electronic properties effectively without creating impurity states in the band gap that affect the decay of excited states. When alloyed with wz-ZnO, the resulting solid solution with wz-GaN exhibited a reduced band gap and was found to be very useful for the hydrogen evolution reaction under visible light irradiation.11 Recently, Kanan et al.,14 have predicted rocksalt (rs) ZMO alloys with reduced band gaps and band edge positions that are suitable for the water splitting reactions. The band edge positions were determined from calculations of the Fermi level for the undoped system using the most stable surface (001) of rocksalt ZMO. The largest band gap reduction was predicted for the rs-ZMO alloy with 50 at.% Zn, wherein the band gap energy shifts into the visible light region (2.61 eV). rs-ZMO with 25 at.% Zn was predicted to exhibit a direct band gap that may exhibit enhanced photoabsorption and favorable photocatalytic activity. Kanan et al.14 also predicted that the


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dielectric constants of ZMO solid solutions increase with Zn concentration, which, along with the reduced band gap, would lead to high carrier concentrations and carrier mobilities. They have also investigated ligand-metal charge-transfer interactions15 as well as doping strategies to enhance the charge carrier separation and transport.16 In addition, they have modelled the electrochemical water oxidation reaction on (001) surfaces of ZMO alloys.17 Despite the promising calculation results, alloying wz-ZnO with rs-MnO in bulk form at atmospheric pressure is hindered by the low solubility of Zn in rs-MnO. Although MnO remains in the rs phase up to pressures of 30-37 GPa,18-19 ZnO undergoes a structural transformation from wz to rs structure above 6 GPa.20-21 This high-pressure rs phase of ZnO is unstable below 2 GPa and cannot be quenched to ambient conditions.21 However, using HPHT technique and in situ synchrotron X-ray diffraction measurements, Sokolov et al 22 have demonstrated that phase-pure rs-ZnxMn1-xO (0.3≤x≤0.5) solid solutions of various stoichiometries can be stabilized at ambient conditions by quenching from 5 GPa and 1600 K. Solid solutions with high Zn content, i.e, ZnxMn1-xO (x > 0.5) cannot be stabilized in a single rs phase. On the other hand, ZnxMn1-xO alloys synthesized at low-pressure conditions exhibit a structural transition from rs single phase to wz single phase around x=0.32,12 suggesting that the rs structure alloys in the composition range 0.3≤x≤0.5 observed here after quenching to ambient conditions are metastable phases. Although synthesis of these novel alloys has previously been reported, at present, detailed experimental characterization and data on photoabsorption and photoelectrochemical properties are nonexistent.

In this communication, we present structural, optical absorption and

photoelectrochemical properties of rs-ZMO solid solutions synthesized using high-pressure and high temperature (HPHT) technique for two compositions, Zn0.5Mn0.5O (ZMO55) and Zn0.3Mn0.7O (ZMO37). We performed electronic structure calculations to estimate the band gaps


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and to understand bandgap reduction better. We have also computed dielectric functions and effective carrier masses for these systems which were non-existent in the literature.

The

complete details of theoretical methods are given in the supplementary information. The powder X-ray diffraction pattern of ZMO55 and ZMO37 solid solutions are shown in Figure1a. The solid solutions are phase pure and exhibit the cubic rs structure (space group 3) with lattice parameters a = 4.3769(1) Å (for ZMO55) and a = 4.4038(1) Å (for ZMO37). The slightly larger lattice volume of ZMO37 as compared to ZMO55 is probably due to the larger ionic radius of Mn2+ (0.080 nm) as compared to that of Zn2+ (0.074 nm), which is consistent with the earlier report.22 Energy dispersive X-ray spectroscopy analysis (EDS) collected on scanning electron microscope (SEM) is shown in Figure 1b,c. As shown, both the samples are compositionally uniform (on the micron scale) and the estimated atomic compositions of Zn and Mn are comparable to the starting mixtures. We did not observe any other elements other than Zn, Mn, and O in our EDS analysis, which confirms the purity of the samples. To further examine the structure and chemical purity of the materials on the submicron level, we performed transmission electron microscopy (TEM) measurements. The TEM analysis of the samples is shown in Figure 1d,e. The SAED patterns consist of sharp spots that indicate the polycrystalline grains are of comparable dimension to the beam aperture (200 nm). The SAED patterns can be easily indexed to rs-type lattice using the lattice parameters obtained from the powder X-ray diffraction measurements. The EDS analysis on the TEM instrument suggests that the samples are chemically pure and homogeneous on the submicron scale (see insets of Figure 1d,e). ZMO samples were tested for thermal stability by subjecting them to high temperatures and checking the phase stability using X-ray diffraction after cooling them to room temperature.


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The samples were found to be stable up to ≈500°C with a dwell time of 30 minutes (see Figure S1 in supplementary information).

Figure 1. (a) X-ray diffraction patterns of the starting mixture and ZMO pellets recovered from HPHT experiments. Colored tick marks indicate Bragg peak positions for the rs and wz phases. Inset shows images of the recovered sample pellets with diameters of ~2 mm. Scanning electron microscopy (SEM) analysis: energy dispersive X-ray analysis of (b) ZMO37 and (c) ZMO55. Transmission electron microscopy (TEM) analysis of (d) ZMO55 and (e) ZMO37. The insets show SAED patterns and EDS spectra collected from the encircled regions.

The diffuse reflectance (R) of finely ground wz-ZnO, rs-MnO and ZMO powders was measured between 0.6-6 eV range using a UV-Visible-near IR spectrophotometer utilizing a 150 mm integrating sphere. The Kubelka-Munk function (F(R) = K/S = (1-R)2/2R; where K = absorption coefficient, S = scattering coefficient), which is a representative of absorbance, is constructed from the R data and is shown in Figure 2a. Wz-ZnO absorbs strongly above its


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bandgap energy (3.2 eV) and displays a sharp band edge feature which is consistent with the earlier reports.23 In contrast, rs-MnO exhibits no sharp band edge feature as in agreement with earlier reports 24-26 but exhibits increased absorbance in the visible spectral region (1.5-3.1 eV) as compared with wz-ZnO. The absorption peaks around 3.0 eV and 2.1 eV in rs-MnO correspond to optical d→d transitions of Mn2+ ions in the octahedral crystal field provided by surrounding oxygen octahedra in the rs structure.24 The slow onset of the absorption edge is likely indicative of defect or phonon mediated absorption. The d→d transition peak observed in rs-MnO around 2.1 eV has almost no shift in ZMO solid solutions, which implies no change in the local order or electronic structure of Mn2+ ions in the oxygen octahedra. However, a significant drop in the intensity of this peak is observed with increasing Zn concentration which is due to the reduction in Mn concentration. This could also imply a decrease in long-range order between Mn2+ ions with increasing substitution of Zn. Detailed neutron diffraction analysis to quantify the cationic disorder in ZMO solid solutions would help in understanding this aspect of the optical absorbance spectra in detail. The optical absorbance spectra of the ZMO solid solutions in the 13 eV energy range were independently constructed from optical transmittance measurements through thin sample films (thickness ≈ 5µm) squeezed between two diamond anvils (see Figure S2 in supplementary information). In order to estimate the fundamental bandgaps in ZMO solid solutions, we have performed quasi-particle energy band structure calculations within the GW approximation for the supercell model using the VASP code,27,28 and a 16 atom special quasirandom supercell (SQS).29 The computational parameters are described in detail in Ref. [30]. The calculations were performed for the compositions ZnxMn1-xO, with x = 0.5 and 0.25 that are the closest to the experimental compositions that could be achieved using SQS model. The corresponding average lattice


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parameters of ZnxMn1-xO are 4.359 Å and 4.399 Å for x = 0.5 and 0.25, respectively, in agreement with the experimentally determined values. The bandgaps of these compositions (constructed using SQS model) were previously predicted by G0W0 calculations using input from ab initio PBE+U or HSE.14 Our calculated bandgap values (see Figure 2a) closely matched the previous HSE/G0W0 results. It should be noted that the bandgaps of ZMO are considerably smaller than the bandgaps of pure rs-MnO (3.46 eV) and rs-ZnO (3.40 eV), calculated in the same approach, indicating an unusually large band gap bowing parameter of about −3 eV. For comparison, in the wz structure alloys, the bowing parameter was only −1.5 eV.12 The origin of

Figure 2. (a) Kubelka-Munk function (F(R)) derived from the diffuse reflectance of wz-ZnO, rsMnO and rs-ZMO solid solutions. The theoretical bandgaps are indicated with red and blue vertical dashed lines. Real (Re) and imaginary (Im) parts of the dielectric function of (b) Zn0.5Mn0.5O and (c) Zn0.25Mn0.75O from GW calculations. the gap reduction was explained previously14 from the density of states at the band edges. It appears that the presence of Zn 4s states at the conduction band minimum (CBM) of the alloy brings the band edge down and causes the reduction in the bandgap. This is consistent with our


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electronic density of states calculations using GW approximation (See Figure S3 in supplementary information). Conversely, the occupied Mn 3d orbitals raise the valence band maximum (VBM) relative to rsZnO. However, the absorption onset in our experimental optical measurements occurs at still lower energies, about 1 eV below the calculated band gap, possibly indicating the presence of mid-gap states. We note that additional contributions to the mid-gap states and lower absorption onset could potentially originate from cation vacancies due to the presence of Mn3+ ions (MnO may contain a small Mn2O3 impurity), although valence state measurements are needed to validate this possibility. We have computed the dielectric function, ε, for ZMO using the G0W0 approximation. Figure 2b shows both the real (Re) and imaginary (Im) components. We see that Im ε begins to rise at energies above 3.5 eV, i.e., at energies higher than the bandgap. This behavior is indicative of an optically forbidden gap, consistent with the indirect character of the band gaps in pure rs-MnO and rs-ZnO (strictly speaking, in an alloy direct and indirect gaps cannot be distinguished). From the Re ε, the estimated dielectric constants (ε∞) are ≈ 4 for both compositions and are in agreement with the previously report.14 From the standpoint of photoelectrochemical water splitting, it is very important that a semiconductor possesses a high carrier lifetime and mobility. From our GW calculations, we have estimated the effective hole and electron masses for ZMO and they are presented along with other ZnO and MnO based systems in Table 1. The electron effective masses (

∗ ) of

ZMO compositions are low and are comparable to those of rs-ZnO, rs-MnO and wz-Zn0.5Mn0.5O. These light effective electron masses suggest that ZMO solid solutions are excellent electron transporters. However, the effective hole masses are higher than in the other stable ZnO and


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∗ MnO systems (see Table. I.), and the prototypical p-type oxide Cu2O ( = 3.7).31 These observations indicate that the ZMO alloys have a rather small valence band dispersion. It is noteworthy that the hole mass in Zn0.25Mn0.75O is smaller than that of ZMO55. ∗

∗

Table 1. Calculated effective electron ( ) and hole ( ) masses for the various stable ZnO and MnO systems. For the holes, the density of states effective mass 2 is given, whereas the band effective mass is given for the electrons. rs-MnO

wz-ZnO

rs-ZnO 12.10

wzZn0.5Mn0.5O 5.60

rsZn0.5Mn0.5O 8.40

rsZn0.25Mn0.75O 6.20

∗

3.70

3.20

∗

0.31

0.26

0.30

0.30

0.27

0.29

The superior optical absorption properties of ZMO samples in the visible spectral region led us to examine their photoelectrochemical response and to evaluate their potential as a photoanode for the oxygen evolution reaction (OER). Chopped light linear sweep voltammetry (LSV) measurements were performed on ZMO bulk pellets attached to a fluorine-doped tin oxide (FTO) coated glass substrate as working electrode in 0.5 M Na2SO4 (aq) with Ag/AgCl saturated KCl reference electrode and Pt as counter electrode. The schematic of the experimental set-up is shown in Figure 3a. The measured photocurrent under the illumination of white light (using a solar simulator) is plotted against the applied voltage as shown in Figure 3b. A maximum photocurrent on the order of 20 µA cm-2 was observed for the ZMO37 sample when the applied voltage was 0.95 V versus Ag/AgCl. There was no detectable photoresponse observed for the ZMO55 sample. It is possible that the absorbed light is either inefficiently separated or there is significant recombination of photogenerated charge carriers at defect sites. These observations are consistent with the high hole effective masses (as shown in Table 1), which not only leads to


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low hole mobilities, but also increase the likelihood of the formation of deep defect states and the self-trapping of hole carriers, thereby suppressing the photocurrent.[32] The high dark current observed for both the samples above 0.3 V (versus Ag/AgCl) suggests that ZMO can function as an electrocatalyst for the OER. Although ZMO37 showed measureable photoresponse (current) in the chopped light LSV measurements, the magnitude of the current made it difficult to detect O2 evolution.

Figure 3. (a) A schematic of the set-up used for Linear Sweep Voltammetry (LSV) measurements. Here Ag/AgCl saturated KCl and Pt wire were used as a reference and counter electrodes respectively. (b) Linear Sweep Voltammogram of ZMO pellets under chopped light. (c) Mott-Schottky plot of ZMO37 at 1 Hz with Ag/AgCl as reference electrode. (d) The band edge positions of ZMO37 as compared to those of pure ZnO[45] and MnO[14]. The band edge positions of ZMO37 are estimated in two different ways: (1) from the Mott-Schottky measurements (denoted as M-S) (2) from the atomic electronegativities (denoted as Cal.). To estimate the flat band potential (Efb) of ZMO37, Mott-Schottky (M-S) measurements33 were performed on bulk pellets. According to M-S relation,

(1)


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Cs is the junction capacitance, e is the charge of the electron and ε, Efb and ND are the relative permittivity, flat band potential and carrier density of ZMO37, respectively. Figure 3c shows the plot of the measured as a function of the applied voltage (E). A linear space-charge region with positive slope was observed for the sample. This suggests that ZMO37 is a n-type semiconductor. From our theoretical calculations, the low-frequency dielectric constant (ε) of rsZMO37 is ≈30 (including both electronic and ionic contributions). Thus, ND is estimated to be ~ 6.88 x 1016 /cm3. Extrapolation of the linear region to the abscissa (as shown in Fig. 3c) gives a flat band potential (Vfb) of ~0.696 V versus Ag/AgCl at 1 Hz (~0.893 V vs. NHE). Using the Vfb and the theoretically obtained band gap, the band edge positions were determined (see Figure 3d). The band edges were also determined from the atomic electronegativities of the constituent atoms using the method described elsewhere.34-35 Assuming that the Fermi level is in the middle of the bandgap, the conduction band edge (Ecb) of ZMO37 can also be estimated using the following equation,

-

!" #.% &'()" #.) &*() "&+),

(2)

where the electronegativity (χ) of each element was obtained from ref [36]. The estimated conduction band edge (Ecb) of ZMO37 is −3.989 eV versus vacuum which corresponds to 0.51 eV versus NHE. Using the calculated band gap value (2.98 eV) for this composition, the corresponding valence band edge position is 3.71 eV versus NHE. The location of the band edges is shown in Figure 3d. The band edge positions are not in good agreement with the theoretical report of Kanan et al.14 The presence of surface states or bulk deep-level states can lead to deviations from ideal Mott-Schottky behavior, which would affect the assignments of the band edge positions.[37] This would require further detailed study. Nevertheless, the band edge positions indicate that current ZMO37 samples cannot be used for the overall water splitting


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reaction without an external bias to raise the conduction band minimum above the potential for H2 evolution, much like other commonly investigated photoanodes WO3, Fe2O3, and BiVO4.38 However, ZMO could be used for the oxygen evolution reaction in the presence of a sacrificial electron scavenger or in a Z-scheme configuration as the O2 evolution photocatalyst.39 In summary, the structural, optical and photoelectrochemical properties of rocksalt ZnxMn1-xO (x= 0.5, 0.3) solid solutions were investigated in phase-pure bulk powders synthesized using HPHT techniques. The solid solutions were found to exhibit increased optical absorption in the visible spectral region as compared to the parent compounds ZnO and MnO. Our theoretical calculations predict bandgaps of 2.53 eV and 2.98 eV with an unusually large band gap bowing. Our calculations also show a small electron effective mass for this composition, which is an indication of good electron transport properties. However, the large hole effective mass may downgrade its applicability as a photoanode. The high dark current observed for the solid solutions in our photoelectrochemical tests suggests that ZMO could be a suitable electrocatalyst for OER. The samples with 30 at.% of Zn exhibited moderate photocurrent and the corresponding band edge positions suggest that it is suitable for photocatalytic water oxidation. Although the outcome of our photoelectrochemical tests on ZMO bulk pellets are encouraging, more experimental characterization and optimization of these materials are required to improve their photocatalytic performance. Fabricating nanostructured electrodes and investigating their functionality is one important direction. This is a feasible path forward as ZnO is known to be stable in the rs phase in nanostructures

40-41

and this will make it easier to stabilize ZMO

compositions in the rs structure. In addition, the hole transport properties of ZMO could be significantly enhanced by using nanostructures. In the past, similar strategies have been successfully implemented for hematite electrodes.42-44


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ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Experimental methods, XRD as a function of temperature between 100-500°C, optical transmission measured through diamond anvil cell, and calculated electronic density of states. ACKNOWLEDGMENT We thank Y. Fei for assistance with multi-anvil experiments. This work was supported by Energy Frontier Research in Extreme Environments (EFree) Center, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science under award No. DESC0001057. S.L. was supported by the EFRC Center for Next Generation of Materials by Design (CNGMD). REFERENCES (1)

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TOC GRAPHICS:


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Figure 1. (a) X-ray diffraction patterns of the starting mixture and ZMO pellets recovered from HPHT experiments. Colored tick marks indicate Bragg peak positions for the rs and wz phases. Inset shows images of the recovered sample pellets with diameters of ~2 mm. Scanning electron microscopy (SEM) analysis: energy dispersive X-ray analysis of (b) ZMO37 and (c) ZMO55. Transmission electron microscopy (TEM) analysis of (d) ZMO55 and (e) ZMO37. The insets show SAED patterns and EDS spectra collected from the encircled regions. 795x800mm (120 x 120 DPI)


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Figure 2. (a) Kubelka-Munk function (F(R)) derived from the diffuse reflectance of wz-ZnO, rs-MnO and rsZMO solid solutions. The theoretical bandgaps are indicated with red and blue vertical dashed lines. Real (Re) and imaginary (Im) parts of the dielectric function of (b) Zn0.5Mn0.5O and (c) Zn0.25Mn0.75O from GW calculations. 1102x567mm (120 x 120 DPI)



Figure 3. (a) A schematic of the set-up used for Linear Sweep Voltammetry (LSV) measurements. Here Ag/AgCl saturated KCl and Pt wire were used as a reference and counter electrodes respectively. (b) Linear Sweep Voltammogram of ZMO pellets under chopped light. (c) Mott-Schottky plot of ZMO37 at 1 Hz with Ag/AgCl as reference electrode. (d) The band edge positions of ZMO37 as compared to those of pure ZnO[42] and MnO[14]. The band edge positions of ZMO37 are estimated in two different ways: (1) from the Mott-Schottky measurements (denoted as M-S) (2) from the atomic electronegativities (denoted as Cal.). 470x459mm (120 x 120 DPI)


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ZnxMn1âxO Solid Solutions in the Rocksalt Structure: Optical, Charge

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ZnxMn1âxO Solid Solutions in the Rocksalt Structure: Optical, Charge