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    Structural and microwave dielectric properties of temperature-stable Ca1-xEuxMo1-xNbxO4 ceramics for dielectric resonator antenna applications
    (2025-10-15)
    Gu, Xiaolong
    ;
    Tang, Ying
    ;
    Wu, Jiangtao
    ;
    Zhang, Ning
    ;
    Zhang, Siying
    Ca<inf>1-x</inf>Eu<inf>x</inf>Mo<inf>1-x</inf>Nb<inf>x</inf>O<inf>4</inf> (0.1 ≤ x ≤ 0.7) ceramics were synthesized through the co-substitution of Ca<sup>2+</sup> and Mo<sup>6+</sup> in CaMoO<inf>4</inf> using Eu<sup>3+</sup> and Nb<sup>5+</sup>. All samples exhibited tetragonal scheelite structures within the range of 0.1 ≤ x ≤ 0.6. However, at x = 0.7, a monoclinic fergusonite structure was observed, as the [Nb/MoO<inf>4</inf>] tetrahedron at the B-site transforms into a [Nb/MoO<inf>6</inf>] octahedron. As the substitution amount increases, ε<inf>r</inf> (11.39–16.40), Q×f (21,950–82,930 GHz), and τ<inf>f</inf> (−47.72 to +8.01 ppm/℃) all show a gradual increase. Notably, the τ<inf>f</inf> value of this system can shift from negative to positive. The structural influence mechanism on the microwave dielectric properties in Ca<inf>1-x</inf>Eu<inf>x</inf>Mo<inf>1-x</inf>Nb<inf>x</inf>O<inf>4</inf> ceramics was clarified through various analytical methods, including ion polarizability, P-V-L chemical bond theory, bond valence theory, density of states (DOS), and electron localization function (ELF). Additionally, a resonant antenna designed and simulated using Ca<inf>0.4</inf>Eu<inf>0.6</inf>Mo<inf>0.4</inf>Nb<inf>0.6</inf>O<inf>4</inf> meets the requirements for satellite communication.
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    First principles study on structural and electronic properties of Li, Na and K - Intercalated zinc phthalocyanine
    (2018-01-01)
    Nuleg, Witoon
    ;
    Thanomngam, Pitiporn
    ;
    Phacheerak, Kanoknan Sarasamak
    Recently, alkali metals doped zinc phthalocyanine (ZnPc) was found to be an effective method for tuning electronic properties of ZnPc. In this research, the effect of alkali metal atoms (Li, Na, and K) intercalation on structural and electronic properties of ZnPc in β structure was investigated by first principles calculations based on density function theory (DFT). The generalized gradient approximation (GGA) is used to describe the exchange-correlation with the projector-augmented wave (PAW) method. The calculated lattice constants, bond lengths and bond angles of pristine ZnPc are in good agreement with other calculations and experimental data. The calculated band gap energy of pristine ZnPc is found to be 1.327 eV. The lattice constants, molecular arrangement, bond lengths and bond angles are sensitive to intercalating alkali metal atoms. It was found that intercalating alkali metal atoms induce significant changes in the band gap energy of ZnPc. The changes in band gap energy with intercalating alkali metal atoms are discussed.
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    First Principles Calculations of Structural and Elastic Properties of Perovskite Crystals: The Case of SrTiO3
    (2016-06-01)
    Pandech, Narasak
    ;
    Sarasamak, Kanoknan
    ;
    Limpijumnong, Sukit
    The structural and elastic properties of perovskite crystals can be independently calculated by using first principles calculations without bias. The detailed calculation of cubic perovskite (Pm3m) crystal by first-principles calculations both LDA and GGA approaches was shown, using SrTiO<inf>3</inf> as an illustration case. The equilibrium volumes and bulk modulus of SrTiO<inf>3</inf> were evaluated by fitting to Birch-Murnaghan equation of states. The C<inf>11</inf>, C<inf>12</inf>, and C<inf>44</inf> elastic constants were obtained from a polynomial fit to the calculated energy-strain relations. For SrTiO<inf>3</inf>, the calculated lattice constants, bulk modulus, and all three elastic constants by both LDA and GGA calculations are in good agreement with experimental values. Because LDA gives a slightly smaller lattice constant compared to that obtained from GGA, the bulk modulus and all three elastic constants obtained from LDA are slightly larger than those values obtained from GGA. The calculation steps illustrated here can be used as a guideline to study elastic properties of other novel materials.
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    Sound velocities and elastic properties of PbTiO3 and PbZrO 3 under pressure: First principles study
    (2013-05-01)
    Pandech, Narasak
    ;
    Sarasamak, Kanoknan
    ;
    Limpijumnong, Sukit
    The elastic constants and sound velocities as a function of pressure for perovskite materials PbTiO<inf>3</inf> (PTO) and PbZrO<inf>3</inf> (PZO) were investigated by first principles calculations. Under ambient pressure, the calculated structural parameters were calculated and found to be in good agreement with known values. To study properties under pressure, PTO and PZO were calculated at several reduced volumes, each of which corresponds to the system under pressure. The C<inf>11</inf>, C<inf>12</inf> and C<inf>44</inf> elastic constants are all found to increase with pressure for the pressure range studied. Because the sound velocities are directly derived from the elastic constants, the relationships between the sound velocities and pressure also follow similar trends. The longitudinal modes are all larger than those of the transverse modes. © 2012 Elsevier Ltd and Techna Group S.r.l.