Infrared study of the magnetostructural phase transition in correlated CrN

Abstract

We report on the pressure and temperature dependence of the electronic and vibrational properties of polycrystalline CrN studied by optical transmission and reflection measurements over the frequency range 0.012-2.48 eV. The optical conductivity spectrum of CrN at ambient conditions shows a phonon mode at ≈55 meV with a shoulder at ≈69 meV, a pronounced midinfrared absorption band centered at 123 ± 2 meV, and a high-energy absorption band at ≈1.5 eV. The absorption bands are discussed in terms of the charge-transfer insulator picture. Following the reflectance spectrum with increasing pressure, the activation of an additional phonon mode above 0.6 GPa indicates the occurrence of a pressure-induced structural phase transition. Furthermore, the absorption spectrum exhibits significant changes in the far-infrared range with decreasing temperature: The phonon mode shows a sudden broadening followed by a splitting below 270 K. These changes observed under pressure or while cooling down can be associated with the magnetostructural phase transition reported previously.

Quasiparticle self-consistent GW band structure of CrN

“…2. Optical studies were also reported by Ebad-Allahet al 15 and confirm the results of Zhang and Gall but extend them by studying the temperature and pressure dependence.…”

“…A similar approach to modeling paramagnetic phases by means of SQS supercells has recently been used by Trimarchi et al 17 for various transition metal oxides and by Varignon et al 18,19 for perovskites. Besides these questions of the band gap, another controversy emerged over CrN, namely the reported strong reduction or even "collapse" of the bulk modulus under pressure, reported by Rivadulla et al 20 While this softening of the bulk modulus under a pressure induced transition was later invalidated, 3 their work and subsequent work by Ebad-Allah et al 15 does show the presence of a magneto-elastic transition under pressure. The coupling of the magnetic ordering to elastic distortion was already well explained by Filipetti et al 21,22 Steneteg et al 23 , using LSDA+U calculations including randomness in the paramagnetic state, also found only a small reduction in bulk modulus under the cubic to orthorhombic phase transition.…”

Electronic correlations in epitaxial CrN thin film

“…Similarly, Herwadkar et al 26 performed first-principles calculations using the local spin-density approximation corrected by the Hubbard Coulomb term (LSDA + U) and showed a small spin separation of states near the Fermi-level (E ) which open a small charge-gap of less than 1 eV between the N-2 and Cr-3 bands hinting that the CrN is a charge-transfer type insulator. Moreover, Allah et al 33 studied the electronic and vibrational properties of polycrystalline CrN using optical transmission and reflection measurements. They reported different absorption bands in the frequency range of 0.012–2.48 eV and explained these bands in terms of the charge-transfer insulator picture.…”

Electronic Correlations in Epitaxial CrN Thin Film

“…Similarly, Herwadkar et al [26] performed first-principle calculations using the local spin-density approximation corrected by the Hubbard Coulomb term (LSDA+U) and showed a small spin separation of states near the Fermi-level (E F ) which open a small charge-gap of less than 1 eV between the N-2p and Cr-3d bands hinting that the CrN is a charge-transfer type insulator. Moreover, the electronic and vibrational properties of polycrystalline CrN studied by Allah et al [33] using optical transmission and reflection measurements. They observed different absorption bands in the frequency range of 12-248 meV and explained these bands in terms of the charge-transfer insulator picture.…”