Dear Researcher,
I found Ni2+ emission red-shifted in the order BaZrO3>CaZrO3>SrZrO3 perovskites, where Ni ions are located at the center of 6 coordinate octahedra. It means Ni emits at longer wavelength in CaZrO3 than in SrZrO3. However, Ca (0.112nm) is smaller than Sr (0.142nm), it means, Ca-O should be shorter than Sr-O and crystal filed experienced by Ni in the CaTiO3 should be larger than Sr counterpart. If so d-d splitting of Ni is larger in CaTiO3; i.e. high energy d-d transition? short wavelength emission? It obey in CaZrO3 vs BaZrO3 but in SrZrO3 case is reverse?
I am looking some suggestions.
Regards,
Hom
My friend it involves a bit of academic discussion as:
[1] The radii of four ions are given as:
0.112- Ca(II)
0.142-Sr(II)
0.149-Ba(II)
0. 83-Ni (II)
[2] I represent the three perovskites as ABO3[A= Ca(II), Sr(II) Ba(II) ; B=Zr(IV)]
[3] We are substituting Ni(II) at A and this is called A substitution/ doping.
[4] If we compare the radius of Ni(II) with those of Ca(II), Sr(II) Ba(II), we find Ni(II) is much shorter than all the three alkaline earth ions. This leads to:
(a) Compression at A sites
(b) Change of porosity of crystals
( c) Change the parameters of the doped crystal wrt undopped crystals
( d) Stretching of Ni-O and by thereby of the remaining M-O[M= Ca(II), Sr(II, Ba(II)] bonds
[5] These factors differ in the three crystals
[6] Ideally speaking all these four factors would cause change in the crystal field meaning thereby the electron density in all the five d orbitals will change and so will be extent of crystal field.
[7] The situation can be looked at analogous to COMPRESSION or Z- IN case of a an octahedral stereo chemistry where the Jahn – Teller effect will make the t2g and eg sets to loose their degeneracy as shown in attachment( Zoom 200%) so that dz2 becomes the highest in energy and right below in energy is dx2-y2 . The CFSE, will , then be given by difference between these two orbitals.
[8] So the conclusion is that this CFSE and thus the crystal field is not dependent on one factor,i.e the relative sizes of the alkaline metal ions rather on four factors which affect the electron density of the three crystals differently.
The facotrs as described by Manohar Sehgal is right, but i want to add one thing more that is the spin configuration of the cation also affect crystal field. According to spin configuraiton the electron distribution in the t2g and eg will change and in turns CFSE also.
First of all, how do you prove that Ni is really +2 in the heterovalent substitution? It can become and is stabilized as Ni+3 in perovskites. Second, the crystal field strength is increasing from Ca to Ba, since the ionicity of Ca-O bond is smaller than for other p-elements, therefore the Ni-O bond becomes more and more covalent thus increasing the crystal field. Third? heterovalent doping very often results in clustering of doping atoms, which also can be different for various perovskites/ Too many factors influencing the crystal field to make conclusions only from spectra.
B.Ya.Brach, N.V.Chezhina, B.N.Dudkin. Magnetic properties of the solid solutions with perovskite structure containing d-elements in trivalent state. J. Inorg. Chem. 1979.V.24 N 8,pp.2064-2067 (in Russian); N.Chezhina, Yu.Yablokov, et al. About the nature of Ni3+ magnetic centers in the LaSrAl1-xNixO4 ceramics. Applied Magnetic Resonance, 1991, v.2, p.547-558; We have a number of works with perovskites and perovskite like compounds (lanthanum gallate for example) where Ni introduced as NiO becomes Ni(+3)
Superexchange interactions are a main point in perovskite ABO3 oxides, especially doped with d-elements, they make a great impact on various properties (magnetic, electric, etc.) of these complex oxides. We study such exchange interactions widely. Their character and energy depend on many factors - the sizes of oxygen octahedra, the nature of d-element, the sizes and nature of diamagnetic elements forming perovskite structure, local distortions in the structure, clustering of magnetic atoms in the lattice and so on. If you are interested I could give you references to our works. Some of them you may see in the research gate. For example Chezhina, Korolev, Solid state Ionics 2012, Chezhina, Fedorova. Russian Journal of general chemistry, 2009-2015 and many others. Good luck
Dear Manohar and Priyanath, thanks for suggestions and theoritical concepts, I will study in detail.
Dear Chezhina, I appreciate your great effort, these papers will be guidelines for my research.
Anyway we have used reduction (10%H2/N2) atmosphere and confirmed Ni2+ states (Major) by EXAFS and XAFS but there might be Ni3+ states (minor).
I am mainly interested on local distortions because the ZrO6 octahedra distort remarkably depending on the dodecahedrom cavity of AO12 unit. Do you have some experience on it? Of course I will study study superexchange interactions, but its new for me.
I do not understand how you confirmed Ni+2 state by reduction in H2/N2 atmosphere. What was the temperature? What about ortahedra distortion? we have a large experience about this/ d-elements introduced into aluminum sites in LaAlO3 or LaSrAlO4 produce essential distortions, the same is in bismuth niobates with various structure but the same principle of octahedra junction - vie the apexes.
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