C. XIANG et al.: ELECTRONIC AND OPTICAL PROPERTIES OF THE SPINEL OXIDES ... 735–743 ELECTRONIC AND OPTICAL PROPERTIES OF THE SPINEL OXIDES MgxZn1-xAl2O4 BY FIRST-PRINCIPLES CALCULATIONS ELEKTRONSKE IN OPTI^NE LASTNOSTI SPINELNIH OKSIDOV MgxZn1-xAl2O4, IZPELJANE IZ TEORETI^NIH OSNOV Chao Xiang1, Jianxiong Zhang1, Yun Lu2, Dong Tian3, Cheng Peng1 1Yangtze Normai University, School of Mechanical and Engineering, Fuling 408000, China 2Chiba University, Institute of Material Science and Engineering, Chiba 2790000, Japan 3Kunming University of Science and Technology, Faculty of Science, Kunming 650093, China 1254618608@qq.com Prejem rokopisa – received: 2016-10-07; sprejem za objavo – accepted for publication: 2017-02-10 doi:10.17222/mit.2016.296 The structural, electronic and optical properties of perfect MgxZn1-xAl2O4 oxides have been studied by first-principles calculations within the generalized gradient approximation of the density functional theory. It is interesting to note that a linear increase of cell volume (V) with increasing doping amount (x) occurs. The band gap increases in the series from 3.851 eV to 5.079 eV, which is in agreement with theoretical and experimental values. In addition, a blue shift of the absorption shoulder is observed in the UV region with the increase of x, as predicted by the imaginary part 2() of the dielectric function at zero frequency as well as bandgap. This can be explained by the threshold of the electronic transition from O-2p to the empty Mg-3p electron states due to the substitution of Zn with Mg. The real part 1() of the dielectric function located at zero frequency has a square fit relationship with refractive index n(0), which is 1.71–1.77 from x=0 to x=1. The energy-loss function shows that the replacement of Zn by Mg is responsible for a decrease in the intensity of the sharp peaks. The reflectivity shows that a higher coefficient of reflectivity (R(0)) at zero frequency corresponds to a smaller bandgap. Keywords: electronic transitions, dielectric function, refractive index, adsorption shoulder Preiskovali smo strukturne, elektronske in opti~ne lastnosti idealnih MgxZn1-xAl2O4 oksidov, izpeljane iz teoreti~nih osnov znotraj posplo{ene gradientne aproksimacije funkcionalne teorije gostote. Opazili smo linearno pove~anje celi~nega volumna (V) z nara{~ajo~o koncentracijo Mg (x). [irina prepovedanega pasu nara{~a od 3.851 eV to 5.079 eV v skladu s teoreti~nimi in eksperimentalnimi vrednostmi. Poleg tega ob nara{~anju dele`a Mg opazimo modri premik dodatnega absorpcijskega vrha v UV obmo~ju, kakor napovedujeta vrednosti imaginarnega dela 2() dielektri~ne funkcije pri frekvenci 0 in prepovedanega pasu. To je mogo~e pojasniti s pragom elektronskega prehoda elektronov iz O-2p v prazen Mg-3p zaradi nadomestitve Zn z Mg. Kvadrat realnega dela 1() dielektri~ne funkcije pri frekvenci 0 se ujema z lomnim koli~nikom n(0), ki je 1.71–1.77 za x=0 do x=1. Funkcija izgube energije, ka`e, da zamenjava Zn z Mg povzro~a zmanj{anje intenzitete ostrih vrhov. Reflektivnost ka`e, da vi{ji koeficient refleksije (R(0)) pri frekvenci 0 odgovarja manj{i {irini prepovedanega pasu. Klju~ne besede: prehodi elektronov, dielektri~na funkcija, lomni koli~nik, dodatni absorpcijski vrh 1 INTRODUCTION Spinal oxides with the general chemical formula AB2O4 have a close-packed, face-centered-cubic struc- ture (space group Fd3m) characterized by two symmetrically distinct polyhedra: a tetrahedron and an octahedron. They are widely used in various fields such as catalysis, gas sensor, semiconductor, biomedical, catalyst carrier, as well as electroluminescent displays owing to their catalytic, physical, structural, electronic and optical properties.1,2 Among them, MgAl2O4 and ZnAl2O4 have high-temperature resistance,3,4 and they are highly reflective for wavelengths in the ultraviolet (UV) region, which make them candidate materials for reflective optical coating in aerospace applications.5 In particular, MgAl2O4 is one of the potential candidates for the full wave band transparent window materials with high transmittance in IR- and visible-wavelength even extending to microwave ranges,6,7 and it also can be used as lamps and lasers,8 transparent ceramic material for high-temperature,9 transparent armor and glass.10 Similarly, ZnAl2O4 can be used as a ceramic material similar to MgAl2O4. It can be used as a transparent conductor, optical material and dielectric material,11,12 and it is suitable for UV photoelectronic applications.13 Simultaneously, much work has been done on the structural, electronic and optical properties of MgAl2O4 and ZnAl2O4 over the past few years.14–27 The effect of point vacancies on the spectral properties of MgAl2O4 has been studied by S. L. Jiang et al.14 They revealed that the absorption peak at 5.3 eV is attributed to the neutral oxygen vacancy Vo 0 , while two peaks at 3.2 eV and 4.75 eV are attributed to the 1+ charged oxygen vacancy Vo 1 + . A related mechanism of transparency in MgAl2O4 nano- ceramics prepared by sintering under high pressure and low temperature has been studied by J. Zhang et al.15, who suggested that the decrease in the transparency with increasing temperature (>700 °C) is therefore a result of the light scattering at large pores. The low-temperature, high-pressure preparation of transparent nanocrystalline MgAl2O4 ceramics has been investigated by T. C. Lu et Materiali in tehnologije / Materials and technology 51 (2017) 5, 735–743 735 MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS UDK 67.017:621.3.011.5:535.327 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(5)735(2017) al.,16 indicating that the nanoceramics are highly transparent even though their relative densities are all less than 99 %, owing to the low or negligible light scattering from the nanosized grains and pores. The optical properties of ZnAl2O4 nanomaterials obtained by the hydrothermal method have been investigated by Miron and co-workers,17 demonstrating that the band gap is determined from the absorbance spectra, and it de- pends strongly on the temperature used for further heating the samples. A first-principles study on struc- tural, electronic and optical properties of spinel oxides ZnAl2O4, ZnGa2O4 and ZnIn2O4 has been carried out by F. Zerarga and co-workers,18 implying that the peaks and structures in the optical spectra are assigned to interband transitions. The fabrication of transparent polycrystalline ZnAl2O4 – a new optical bulk ceramic – has been inve- stigated by Goldstein and co-workers19, who suggested that specimens have a high transparency (ILT78\%;  = 800 nm; t = 2 mm). Plus, the differences in struc- tural, electronic and optical performance between alumi- num spinel MgAl2O4 and ZnAl2O4 have been presented.28–30 To the best of our knowledge, the struc- ture, electronic and optical properties of MgxZn1-xAl2O4 (0