(Invited) "Structure-Property", "Composition-Property" and "Property-Property" Relations As Useful Tools for Understanding of Optical Materials Performance
Authors: Brik Mikhail G. , Srivastava Alok M , Camardello Sam Jospeh 
Published: February 2020

Continuously growing demand for optical materials used in various applications (such as solid state lighting, lasers, sensors, scintillators etc) stimulates active research aimed at the development of new materials and optimization/improvement of the existing ones. Systematic comparative studies of large groups of isostructural compounds or isoelectronic impurities can help in uncovering different “structure-property”, “composition-property”, and “property-property” relations, which can lead to a deeper understanding of optical materials operation and can be used as reliable guidelines identifying directions for a smart search for new materials with better performance.
In the present work we discuss our recent results of application of several different approaches (crystal field theory, density functional theory (DFT)-based computational techniques, configurational coordinate model etc) to various groups of materials – pyrochlores, perovskites, garnets, spinels, chalcopyrites, III-V and II-VI semiconductors, pure and doped with transition metal and rare earth ions.
Firstly, an empirical model was developed to express the lattice constants of isostructural cubic and tetragonal compounds in terms of ionic radii and electronegativities of constituting ions [1]. The average deviation between the experimental values of the lattice constants and those estimated from the proposed theory is of the order of 1 % only. The model not only allows to fit the lattice constants of the considered crystals, but also gives an opportunity to predict the lattice constants of new compounds and analyze the stability criteria for existing materials.
Secondly, DFT-based calculations were employed to explain peculiar features of the Cr4+ and Mn4+ spectra in Y2Ti2O7 and Y2Sn2O7 pyrochlores. In particular, it has been shown that the blue shift of the 3d-ions absorption peaks in Y2Sn2O7 in comparison with Y2Ti2O7 (in spite of larger interionic separations in the former case) can be explained by increased ligands charge in Y2Sn2O7 because of a weaker overlap of the oxygen orbitals with completely filled 4d10 orbitals of Sn4+ [2].
Thirdly, careful examination of variation of energies of the spin-forbidden transitions of transition metal ions (Mn4+, Cr3+, Ni2+) in various solids allowed to relate those energies with the covalent effects experienced by those ions. In this connection, a clear link between the degree of covalency of chemical bonds and spatial arrangements of crystal lattice ions was established. These findings can help in search for new efficient red phosphors based on the Mn4+ ions [3].
Fourthly, DFT-based calculations of the structural, optical, electronic properties of chalcopyrites and II-VI (III-V) semiconductors revealed important changes of the band gap character (direct or indirect) or enhanced absorptivity with composition, which has direct influence on perspectives of applications of these materials for solar cell materials and/or LED devices [4].
All obtained results are compared with the corresponding experimental data; agreement between both sets of data is discussed. Predictive potential of the presented calculations and possibility of applications of the developed models to other systems are highlighted.
References:
[1] M.G. Brik, A.M. Srivastava, “Pyrochlore Structural Chemistry: Predicting the Lattice Constant by the Ionic Radii and Electronegativities of the Constituting Ions”, Journal of the American Ceramic Society 95 (2012) 1454-1460.
[2] A.M. Srivastava, M.G. Brik, “Comparative ab initio study of electronic, optical and chemical bonding properties of pyrochlores, Y2B2O7 (B=Ti4+, Sn4+)”, Journal of Luminescence 130 (2010) 2368–2376.
[3] M.G. Brik, S.J. Camardello, A.M. Srivastava, N.M. Avram, A. Suchocki, “Spin-Forbidden Transitions in the Spectra of Transition Metal Ions and Nephelauxetic Effect”, ECS Journal of Solid State Science and Technology 5 (2016) R3067-R3077.
[4] M.G. Brik, M. Piasecki, I.V. Kityk, “Structural, Electronic and Optical Features of CuAl(S1-xSex)2 Solar Cell Materials ”, Inorganic Chemistry 53 (2014) 2645-2651.