CommunicationStudy of the vibrational, dielectric and infrared properties of CdSiP2 via first principles
Introduction
Mid-IR lasers can be used widely in fields of remote sensing, spectroscopy, science and medicine. Unfortunately, there are no suitable working materials to emit mid-IR light longer than 3 μm directly for solid state lasers, so they can only be obtained through frequency conversion by using nonlinear optical (NLO) crystals [1]. In search of NLO materials, researchers find that CdSiP2 has many superior properties: a large birefringence, a desirable nonlinear coefficient, a high thermal conductivity and a high melting point [2]. All these advantages make it a promising material for practical use.
Recently, large CdSiP2 single crystal with super optical quality was grown by Zawilski et al. [3] using stoichiometric melts approach, which allows us to measure some of its intrinsic bulk properties, such as its wave length dependent birefringence and dispersion. For CdSiP2, its electronic and optical properties and their behavior under pressure were studied by Chiker et al. [4], [5] using the full potential linearized augmented plane wave method. These properties were also detailedly studied by He et al. [6] and Xiao et al. [7]. In our previous work, we investigated the electronic, intrinsic hardness and elastic properties of CdSiP2 by first principles calculation [8]. Sellmeier dispersion formula of CdSiP2 was ever constructed by different research groups [9], [10] and its anisotropic thermal anharmonicity was recently studied [11], [12]. Besides, electron paramagnetic resonance experiments were also carried out to study the native defects within it [13], [14].
It is known that vibrational and dielectric properties are important for infrared NLO crystals because vibrational frequencies of the phonons in these crystal commonly locate in the infrared spectral range. For CdSiP2, vibrational frequencies of its optical phonons at the Brillouin zone center with LO–TO splitting were previously measured by Bettini et al. [15] and its Raman spectrum under pressure was experimentally studied by Shirakata [16]. Although ab initio phonon calculations have been involved in the course of investigating its Sellmeier dispersion formula [10] and anisotropic thermal anharmonicity [12], there is still no systematically theoretical study on its vibrational, dielectric and infrared properties though it is essential for interpreting the relevant properties. To this end, here we carry out a comprehensive first principles study on these properties of CdSiP2.
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Computational details
In this work, first principles calculations are carried out by utilizing the CASTEP code [17]. After testing, we selected LDA [18] and GGA-PBESOL [19] exchange-correlation functionals to perform the study. In these calculations, a 3×3×4 mesh is chosen for Brillouin zone integration according to the Monkhorst-Pack method [20]. Norm-conserving pseudopotentials constructed by Lin et al. [21] are used to substitute for the interactions between nuclei and electrons, in which the valence electron
Crystal structure relaxation
CdSiP2 is a tetragonal crystal with the space group I-42d (point group ), see Fig. 1. Its lattice constants at 298 K are a=5.680 Å and c=10.431 Å, in which the ions occupied three different Wyckoff positions: Cd at 4a (0, 0, 0), Si at 4b (0, 0, 0.5) and P at 8d (u, 1/4, 1/8), where u=0.2968 [23]. The cell parameters relaxed by the LDA and PBESOL functionals, together with the experimental data, are listed in Table 1.
The experimental data provided by Abrahams [23] and Mughal [24] are very
Conclusions
In the present work, the vibrational, dielectric and infrared properties of CdSiP2 have been studied by using density functional perturbation method. The calculated Born effective charge of Si has the largest anomaly indicating that covalent bonds are formed between Si and P. Investigation of the phonons of CdSiP2 reveals that the splitting of the B2 modes is commonly larger than that of A1 modes owing to their different vibrational patterns. The dielectric and reflectance spectra are simulated
Acknowledgments
The authors would like to thank the support by the National Natural Science Foundation of China under Grant no. 11504089, the Key Scientific Research Projects of Henan Colleges and Universities under Grant no. 15B140002 and the Doctoral Scientific Research Foundation of Henan University of Science and Technology under Grant no. 13480039.
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