Comment on “Theoretical investigations of the hyperfine interactions for Co2+ in the fluoroperovskites” [Chem. Phys. 348 (2008) 199]

Cr3+-Doped Fluorides and Oxides: Role of Internal Fields and Limitations of the Tanabe–Sugano Approach

“…For instance, a value E ( 4 T 2 ) = 13 870 cm –1 is found for KMgF 3 :Cr 3+ , while in the case of LiBaF 3 :Cr 3+ , the calculated value is E ( 4 T 2 ) = 16 008 cm –1 . This fact thus concurs with previous findings showing that E R ( r ) in the inverted perovskite enhances the value of E ( 4 T 2 ) with respect to what is found in the normal perovskite lattice. ,, The increase of E ( 4 T j ) ( j = 1, 2) on passing from KMgF 3 :Cr 3+ to LiBaF 3 :Cr 3+ is consistent with experimental findings , such as those shown in Table . Therefore, according to the discussion carried out in the introduction, the difference of 1 500 cm –1 between the experimental E ( 4 T 2 ) values of LiBaF 3 :Cr 3+ and KMgF 3 :Cr 3+ cannot be mainly due to the presence of a close vacancy in the first system.…”

“…It is worth noting that a situation similar to that collected in Table comes out when comparing the optical properties of KMgF 3 :M 2+ and LiBaF 3 :M 2+ (M = Ni, Co, Mn). − In all these cases, the experimental − and theoretical ,, results are consistent with a 10Dq value which is 10–15% higher for LiBaF 3 :M 2+ than when M 2+ enters KMgF 3 despite that the equilibrium M 2+ –F – distance is found to be the same within 1%.…”

“…Indeed, it is necessary to take into account the electric field, E R ( r ), created by the rest of the lattice ions on the active electrons of the complex . This explains that the 10Dq values measured for LiBaF 3 :M (M = Cr 3+ , Co 2+ , Ni 2+ , Mn 2 ) are found to be systematically about 10–20% higher than that in the case of KMgF 3 :M despite the fact that the impurity–ligand distance, R , is essentially the same for both lattices. − ,− Furthermore, the distinct electric field, E R ( r ), in ruby and emerald has been shown to be the main cause behind the different colors of such gemstones mainly determined by the energy, E ( 4 T 2 ), of the 4 A 2 (t 2g 3 ) → 4 T 2 (t 2g 2 e g 1 ) transition. , Indeed, accurate extended X-ray absorption fine structure (EXAFS) measurements carried out in the last years on both Al 2 O 3 :Cr 3+ and Be 3 Si 6 Al 2 O 18 :Cr 3+ prove that the mean Cr 3+ –O 2– distance is the same within the experimental uncertainty of 1 pm. , …”

Chemical Bonding, 10Dq Parameter and Superexchange in the Model Compound KNiF3

“…Indeed, the R distance is the same for KMgF 3 : M 2 + and LiBaF 3 : M 2 + (M = Ni, Mn), but 10Dq is about 0.1 eV higher for the second case as a result of the extrinsic contribution in the inverse perovskite structure. [36][37][38][39] In the same vein, in Al 2 O 3 : Cr 3 + (ruby gemstone) and Be 3 Si 6 Al 2 O 18 : Cr 3 + (emerald) the Cr 3 + -O 2À distance is the same but the extrinsic contribution in ruby is 0.24 eV and basically responsible for the distinct color displayed by ruby and emerald. [40][41][42][43][44] Therefore, as it was already pointed out by Sugano and Shulman [16] for a normal perovskite, the experimental 10Dq value for KNiF 3 should mainly arise from (10Dq) b reflecting the chemical bonding.…”

“…The internal electric field E ext (r), generated by the rest of lattice ions has been included into these calculations via a classical embedding of point charges, previously obtained through Ewald-Evjen summations. [39] However, in the normal perovskites, the effect of the internal electric field is negligible, as the electrostatic potential is fairly flat along the metal-ligand direction, [4,16] and therefore, it exerts no significant influence on the results.…”