Electronic structure of cubic ScF3 from first-principles calculations

Abstract

Abstract. The first-principles calculations have been performed to investigate the ground state properties of cubic scandium trifluoride (ScF3) perovskite. Using modified hybrid exchange-correlation functionals within the density functional theory (DFT) we have comprehensively compared the electronic properties of ScF3 obtained by means of the linear combination of atomic orbitals (LCAO) and projector augmented-waves (PAW) methods. Both methods allowed us to reproduce the lattice constant experimentally observed in cubic ScF3 at low temperatures and predict its electronic structure in good agreement with known experimental valence-band photoelectron and F 1s X-ray absorption spectra. IntroductionScandium trifluoride (ScF 3 ) is a promising material attracted much attention due to recently discovered strong negative thermal expansion (NTE) coefficient over a wide range of temperatures from 10 to 1100 K [1].At atmospheric pressure ScF 3 has cubic (space group P m3m) ReO 3 -type structure ( Fig. 1) down to at least 10 K [1]. However, opposite to the metallic ReO 3 , which has very week NTE effect [2,3], scandium trifluoride is an insulator with band gap of more than 8 eV [4]. A cubicto-rhombohedral phase transition occurs in ScF 3 at high pressure (P >0.5 GPa at T ∼300 K or P =0.1-0.2 GPa at 50 K) as determined from X-ray and neutron diffraction studies [1,5]. Raman spectroscopy confirms that upon pressure increase cubic ScF 3 undergo a phase transition to the rhombohedral (space group R3c) phase [5,6].The electronic structure of ScF 3 thin films has been investigated by means of resonant photoemission spectroscopy at the Sc 2p and F 1s absorption edges in Ref. [4]. Two chargetransfer-type satellites, the first one at about 13 eV below the main peaks in the valence-band and the second one with an energy separation of about 9-10 eV, were observed and indicate the strong hybridization effect between Sc 3d and F 2p states [4].The phonon properties of ScF 3 have been studied using a combination of inelastic neutron scattering experiment with ab initio calculations of lattice dynamics [7]. It was shown that a description of NTE within the quasi-harmonic approximation is not reliable in the case of ScF 3 . The authors in [7] have demonstrated that the R4+ mode (the one with the lowest energy at R-point of Brillouin zone (BZ)) has quartic potential and proposed the mechanism for NTE based on this anharmonicity. The transition between the ground state and the first excited

Low-temperature luminescence of ScF3 single crystals under excitation by VUV synchrotron radiation

“…Utilizing the phenomenological approach, we can estimate the band gap value (E g ) in ScF 3 as E g = 1.08E exc , where E exc is the energy of the excitonic transitions [36]. The band gap value is 11.02 eV and it is slightly higher than the theoretically predicted result [37]. On the other hand, the obtained band gap value is close to the value obtained before by means of resonant photoemission spectroscopy [38].…”

Entropic elasticity and negative thermal expansion in a simple cubic crystal

“…A modest hybridization (|5a 1 ≈ 0.83|F 2p + 0.16|Sc3d ) does not lead to sizeable charge transfer, such that Sc 3+ and F − appear to be close to their nominal ionic oxidation states. We performed DFT electronic structure calculations in ScF 3 , which further support these findings, indicating slightly hybrid valence bands below ∼ −4 eV and strong anisotropy of the effective potential of F ions [26,30].…”

“…The exceptional longitudinal rigidity of the Sc-F bond, which underlies the NTE mechanism in ScF 3 is rooted in covalence, where the hybridization of Sc and F electronic orbitals that lie deep inside the valence band is responsible for the large energy cost of changing the Sc-F distance [26][27][28][29][30]. Such Lewis-type dative bonding where paired electrons delocalize between ions to lower their kinetic energy has recently been described as a "charge transfer bond" [28].…”

Low-temperature radiation effects in wide gap materials

“…New materials, such as the ScF 3 single crystals with negative-expansion [5], the Er 3+ -doped LaInO 3 providing upconversion luminescence [8] as well as BaZrO 3 -based polysterene-perovskite hybrid composites [13] are also studied and reported in the present issue. Besides many conventional experimental methods, such as optical absorption, luminescence, magnetic resonance, etc.…”

“…Where necessary, the obtained results were supported by other experimental methods such as SEM, TEM and x-ray diffraction. Special attention was also paid to modern theoretical methods of the electronic band structure calculation [4,5]. A new quantum field approach for the description of highly-excited insulator materials was developed in paper [7].…”