Optical Conductivity of Mayenite: From Insulator to Metal

Abstract

Mayenite-based oxides [12CaO·7Al 2 O 3 ] starting from [Ca 24 Al 28 O 64 ] 4+ + 2O 2− (insulator) and subsequently annealed so as to obtain [Ca 24 Al 28 O 64 ] 4+ + 4e − (metal) were studied by reflectance and transmission on seven samples with dc conductivities spanning the range 10 −10 to 1500 Ω −1 cm −1 . Three of them are essentially insulating. The remaining display an increasing dc conductivity as the electron concentration increases. A set of phonons in the infrared below 0.15 eV and an intense line in the ultraviolet at 6.5 eV account for the optical conductivity of the insulating samples. As the samples become increasingly metallic, we observe an enhancement of spectral weight in the visible range. Simultaneously, the reflectance and the resulting optical conductivity develop a stronger component in the infrared, characteristic of mobile electrons. This electronic response appears to be strongly coupled to the phonons, as shown by their Fano profiles present in the metallic samples. Our results indicate that free carriers promote the formation of polarons and that these two excitations contribute to the dc conducting properties of mayenites. This overall electronic picture is consistent with first principles calculations of the density of states in mayenites. The observed strong electron−phonon coupling may be of interest in view of the superconductivity properties found in the metallic samples.

Structure Property Relationships and Cationic Doping in [Ca24Al28O64]4+ Framework: A Review

“…Optical reflectance data show sharp phonon lines in the infrared regime as well as absorption in the visible and ultraviolet range typical of insulators. Limited Drude response is observed and contributions to the reflectance pattern can be fit with a Lorentzian model further indicating the dominate carrier type is localized opposed to free carriers [51,52]. Temperature dependent DC conductivity measurements when ∂ < 0.5 reveal a temperature dependence of log(σ) of T −1 suggesting that at low concentrations the electrons conduct as polarons [37].…”

“…Optical reflectance studies find that as electron concentration is increased a Drude peak and broadening of phonon peaks in the IR region, characteristic of free electron carriers, is observed in conjunction with the characteristic Lorentzian transitions associated with localized carriers [51,52]. The dominant conductivity type changes from VRH conduction to band conduction, but the Lorentzian features suggests both forms of conduction occur concurrently [51,52]. Lobo et al identified an increase in asymmetry of phonon peaks with increased carrier concentration in the metallic regime.…”

“…There have only been EPR studies conducted just up to the metal transition, and paramagnetic electron species are still observed [37]. These experimental results along with the theoretical calculations point to the presence of localized and delocalized electrons existing in the C12A7 leading to concurrent and cooperative types of electrical conduction [43,52].…”

“…Theoretical calculations of the band structure corresponding to [Ca Al O ] : (4 * ∂)e (2 − ∂)O with various levels of ∂ have proven difficult due to the conversion of the system from an insulating to a metallic state, however, similar trends are observed from a variety of different functionals [3,34,51,52]. Sushko et al performed band structure calculations from the stoichiometric insulating state to the electride state using two different density functionals; B3LYP works well with insulating systems and LDA works well with metallic systems [3].…”

“…These 12 states of the CCB account for all 12, unoccupied and occupied, cages in the unit cell of C12A7, and the structure of the cage correlates to the energy of these states giving rise to the interesting electrical properties. Due to the large band gap, C12A7 single crystals exhibit >90% optical transparency in the visible range, and at low electron concentration C12A7 combines optical transparency with electrical conductivity leading to potential applications as a transparent conductive oxide that is both inexpensive and earth abundant [52][53][54]. At low and medium electron concentration, C12A7 transfers into a semiconducting state.…”