Effect of inversion degree on the optical spectra of transition element doped spinel lattice transparent ceramic hosts
GOLDSTEIN A. 1, ZHANG G. 2, WU Y. 2, SHAMES A. 3
1 CERAMOSAURUS REX, HAIFA, Israel; 2 N Y state college of ceramics, Alfred university, Alfred , United States; 3 Department of physics, Ben-Gurion University, Beer Sheva, Israel
Spinel type lattices are characterized by various inversion degrees ("i" ) . The parameter`s value affects the way transition metals are accomodated into a lattice (oxidation and coordination states). As aresult the profile of optical absorption or emission and EPR spectra-of transparent ceramic hosts based on spinel lattice- may be affected by "i" . Not enough spectral data exist for elucidating the correlation between "i' and the spectra. Lacking is data on spinels with high "i" value. This work intended to contribute to the understanding of the correlation mentioned above.For this, first a transparent ceramic spinel host possesing a high value of "i" ( "i" up to 0.8 was developed ) . It coresponds to a MgGa2O (MGO) composition. The intention is to dope the host with a number of transition metals large enough to allow trends detection.For this one has to compare with spectral results in a low"i" host MgAl2O4 (MAO; "i" around 0.2 ). The MGO was made by SPS of an in-house made powder. Densifcation levels of 99.84 % could be achieved. Plates 0.85 mm thickness attained thus transmission levels of up to 70% ( at a wavelength of 600 nm).To the best of our knowledge these are the first transparent ceramic specimens derived from MgGa2O powders. MgAl2O4 plates ( t=2mm) attained transmission around 80%, at the same wavelength.Ni doping of MGO produced an absorption spectrum prduced bands at 1020,620 and 380 nm. By the aid of the appropiate Tanabe-Sugano diagram the bands were assigned to the spin allowed transitions f Ni2 + in a site of aproximately octahedral symmetry.Using the Tauc method the host`s band gap was calculated to some 4.6 eV and a wide luminescence band, centerd on 1260 nm was measured.In MgAl2O4 host the optical absorption and emission spectra had a profile similar to that in MGO.No significant effect could be linked to the significant "i" variation between the two hosts. Another dopant examined was Ti. Especially fluorescence data revealed that ,like in MAO in MGO a small amount of Ti 4 + can be accomodated with fraction of Ti 3+ being larger. The spectra and the EPR revealed that in MAO that a fraction of Ti 3+ appears in octahedral coordination while the rest in tetrahedral coorddination. In MGO all Ti 3+ is located in tetrahedral sites. Thus in the case of Ti 3+ the difference in "i" has a notable spectral effect as opposed to the case of Ni 2+. It is highly probable that the difference in behaviour is owed, mostly, to the differnce in crystal field stabilization energy (CFSE) of a d1 ( Ti 3+ ) configuration (low) to that corresponding to a d8 ( Ni 2+) configuration(high). A high CFSE confers high stability to six coordination of the TM cation.
It results that an increase of"i" tends to facilitate taking of Td sites by dopants;factors like CFSE may oppose the tendency. Aditional dopants behaviour needs examination; near future work.