Aluminum to germanium inversion in mullite‐type RAlGeO₅: Characterization of a rare phenomenon for R = Y, Sm–Lu

Abstract

Mullite-type RMn 2 O 5 (R = Y, rare-earth element) ceramics are of ongoing research attentions because of their interesting crystal-chemical, physical, and thermal properties. We report a detailed structural, spectroscopic and thermal analysis of the series of mullite-type RAlGeO 5 (R = Y, Sm-Lu) phases. Polycrystalline samples are prepared by solid-state synthesis methods. Each sample is characterized by X-ray powder diffraction followed by Rietveld refinements, showing that they are isotypic and crystallize in the space group Pbam. The change of the metric parameters is explained in term of the lanthanide contraction effect. A rare inversion of Al/Ge between octahedral and pyramidal sites have been observed for these mullite-type so called O10 compounds, and the inversion parameter found to be between 0.22(1) and 0.30(1) for different R-cations. The <Al/Ge-O> bond distances and their bond valence sums (BVSs) support the respective inversions.Density functional theory (DFT) calculated phonon density of states (PDOS) and electronic band structures are compared for the vibrational and electronic band gap features respectively. Analysis of UV/Vis absorption spectra using both derivation of absorption spectra fitting (DASF) and Tauc's methods demonstrates that each of the RAlGeO 5 O10 compounds is high bandgap semiconductor, possessing direct transition between 4.1(1) and 5.4(1) eV. Both Raman and Fourier transform infrared spectra show clear red shift (quasi-harmonic) of the vibrational wavenumbers with respect to the ionic radii of the R-cations. Selective Raman bands at higher wavenumber region further complement the inversion of Al/Ge between two coordination sites. The higher decomposition temperature of the RAlGeO 5 compounds, compared to those of RMn 2 O 5 phases, is explained in terms of higher bond strength of Al/Ge-O than those of Mn-O. Irrespective to the inversion between Al-and Ge-sites, the decomposition temperature also depends on the type of R-cation in RAlGeO 5 .

Advantages and developments of Raman spectroscopy for electroceramics

“…Theoretical Raman spectroscopy helps to assign experimentally observed modes and allows the prediction of Raman spectra that have not yet been measured. Results have been reported for simple substances-γ-Al 2 O 3 143 and h-ZnO 144 ; minerals-dolomitic lime 145 , silicates 146 ; transparent conductive oxides (TCO)-SrSnO 3 147 , CaSnO 3 148 ; non-linear optic (NLO) materials-LiTaO 3 and LiNbO 3 149 , Bi 12 SiO 20 150 , ZnTiO 3 151 , KSbGe 3 O 9 152 ; potassium antimonyl silicate (relaxor) 153 ; multiferroics-BiFeO 3 154 , RAlGeO 5 (R = Y, Sm-Lu) 155 ; and photovoltaic double perovskite Cs 2 AgSbCl 6 156 . Theory offers rich insights into the connection between the Raman signal and the stress state of the material.…”

On red tin (II) oxide: temperature‐dependent structural, spectroscopic, and thermogravimetric properties

“…Taking the extinction coefficient as approximation of the Kubelka‐Munk function F(R) the relation between band‐gap energy, transition type and absorption is given by the equation: $${F\left(R\right)=B{\left(h\nu -{E}_{g}\right)}^{n}}$$ which could be transformed to $${ln\ F\left(R\right)=ln\ B+\ n{\rm ln}\left(h\nu -{E}_{g}\right)}$$ . From the first‐derivative expression, [46,47] the band‐gap value can be calculated from the peak‐shape of the UV/Vis data. The DASF results are compared with those obtained from the Tauc method, and the nature of transition can be figured out from the combined approach known as RATD method (reflection, absorption, Tauc and DASF) [48] …”

“…From the firstderivative expression, [46,47] the band-gap value can be calculated from the peak-shape of the UV/Vis data. The DASF results are compared with those obtained from the Tauc method, and the nature of transition can be figured out from the combined approach known as RATD method (reflection, absorption, Tauc and DASF).…”

Mullite‐type EMBO4: synthesis, structural, optical, and vibrational properties of rare tin(II) borates with M = Al and Ga

“…Direct and indirect band gaps were calculated using the Tauc 35,36,37 method and compared with the value obtained from the absorption spectrum fitting (DASF) method 38 . This leads to a full analysis of the band gap transition type and energy combined in the so‐called RATD (reflectance–absorbance–Tauc–DASF) analysis 39,40 …”