ESR Investigation of Phase Transitions in Some Elpasolite‐Type Fluorides

Abstract

Structural phase transition temperatures are determined for some elpasolite‐type fluorides Rb2KMF6 (M = Ga, Cr, Fe) through electron spin resonance. The results are compared with low temperature X‐ray diffraction and calorimetric results. For the new elpasolite Rb2KGaF6 a transition temperature of T ≈ 120 K is detected using Cr3+ and Fe3+ as paramagnetic probes. In the case of Rb2KGaF6:Fe3+ the isotropic superhyperfine constant As is well resolved in the powder spectrum. An analysis of experimental As values found for Fe3+ in different fluoride lattices is reported. In the case of paramagnetic elpasolites, the variation of the linewidth of the ESR spectra of Rb2KCrF6 and Rb2KFeF6 with temperature confirms the occurrence of a phase transition at T(Cr) ≈ 150 K and T(Fe) ≈ 170 K. The Cr3+ or Fe3+ doped K2NaGaF6 elpasolite which remains cubic down to very low temperatures is also checked. As expected no change in the ESR spectra occurs in this case.

A complex defect studied through DFT: The Fe3–O2− pair in KMgF3

“…With the exception of this case, where f s (eq)/f s (ax) = 3.5, the other results collected in Table II lead to values of f s (eq)/f s (ax) lying between 1.8 and 2.1. These figures are thus close to the A s (eq)/A s (ax) values 5, 8–11 measured for FeF 5 O 4− in KMgF 3 (2.4), KZnF 3 (1.8), and RbCdF 3 (2.0).…”

“…Transition‐metal impurities in insulating materials often give rise to centers where unpaired electrons are highly localized 1, 2. Some of the simplest examples of this behavior are Ni 2+ 1–3, Mn 2+ 4, or Fe 3+ 5, 6 centers in cubic fluoroperovskites or cations such as Cr 4+ or Fe 6+ in oxides 7. In all these cases the electronic properties associated with the impurity can be understood to a long extent only on the basis of an MX N complex formed by the impurity, M, and the N nearest anions X.…”

“…For definite clarification of the structure of this center it is necessary to first account for the only available experimental data obtained by means of EPR and ENDOR techniques. So, all centers ascribed to FeF 5 O 4− units in lattices such as KMgF 3 5, 8, KZnF 3 , or RbCdF 3 5, 9–11 exhibit an isotropic superhyperfine constant corresponding to the four equatorial fluorines (A s (eq)), which is about twice that coming from the apical fluorine (A s (ax)). Searching to clarify the structure of the center in KMgF 3 , DFT calculations have been carried out, considering the good results obtained on cubic centers of Fe 3+ and Mn 2+ impurities in fluoroperovskites 4, 6.…”

Key Role of Deep Orbitals in the dx2–y2–d3z2–r2 Gap in Tetragonal Complexes and 10Dq

“…In the same vein as for octahedral NiF 6 4– , MnF 6 4– , or FeF 6 3– units in cubic fluoroperovskites, ,− both A s and q s quantities, corresponding to the e g (σ) orbital, are strongly dependent upon the metal–ligand distance, while A p is much less sensitive.…”

The dependence of 10Dq upon the metal–ligand distance, R, for transition‐metal complexes. What is its microscopic origin?

“…(7) points out that the significant dependence of lODq upon R is, in fact, related to the strong dependence displayed by f and A, well demonstrated though the analysis of experimental results found for several 3d impurities (Mn2+, Ni' , Fe3+, etc.) in fluoride lattices [11,12,15,16]. Nevertheless, as the first contribution to lODq arising from the d-p…”