Ab initio, crystal field and experimental spectroscopic studies of pure and Ni2+-doped KZnF3 crystals

Pressure dependence of the crystal-field spectrum of KNiF3 : Single and double excitations

“…The results will be compared with well-known correlations already established in ABF 3 : Mn 2+ [26,27]. Also, they allow us to compare the experimentally obtained n exponent for KNiF 3 with that obtained for KZnF 3 : Ni 2+ (n = 6) from DFT methods [13], which is different from that spectroscopically derived for Mn 2+ along the ABF 3 : Mn 2+ series (n = 4.7) [26,27] or that reported for Co 2+ in KCoF 3 (n = 5.1) through high-pressure measurements [21]. Precise measurements of n are deserving of adequate theoretical methods to explain the slight departure of n from the CF theory estimates of n = 5, due to bonding effects related to specificities of the transition-metal cation and the ligand (crystal structure) [13,28,29].…”

“…In addition, some * Corresponding author: rodriguf@unican.es weak absorption peaks appearing below the band gap in the UV range have been assigned either to SE or to double excitation (DE) transitions but their assignment still remains unclear [7,11]. Besides, KNiF 3 has been taken together with other fluoroperovskites KBF 3 as a prototype system to check the suitability of DFT methods to calculate structural properties mainly related to the equation of state and elastic constants [9,10,[12][13][14][15][16]. In spite of it, different values of the bulk modulus ranging from 60 to 115 GPa have been recently reported in KNiF 3 [9,10,17,18], showing important discrepancies with the experimental value derived from acoustic measurements of K 0 = 84 GPa [18,19].…”

“…This dispersion of data points out that precise theoretical methods for determining structural properties in such simple systems are yet to be improved. A study on the crystallographic and elastic properties of KZnF 3 using density functional theory (DFT) based methods using both general-gradient approximation (GGA) and local-density approximation (LDA) exchange correlation functionals, although it provides fair agreement with experimental data, illustrates the dispersion of calculated data depending on the employed functional [13]. In addition, this work tackles the study of the electronic structure of Ni 2+ introduced as a substitutional impurity in KZnF 3 , and what is more challenging, its pressure dependence [13].…”

“…A study on the crystallographic and elastic properties of KZnF 3 using density functional theory (DFT) based methods using both general-gradient approximation (GGA) and local-density approximation (LDA) exchange correlation functionals, although it provides fair agreement with experimental data, illustrates the dispersion of calculated data depending on the employed functional [13]. In addition, this work tackles the study of the electronic structure of Ni 2+ introduced as a substitutional impurity in KZnF 3 , and what is more challenging, its pressure dependence [13]. Besides difficulties arising to calculate suitable CF energies of Ni 2+ as an impurity inside KZnF 3 , it must be noted that the study first requires an adequate structural description of the lattice relaxation around Ni 2+ , i.e., the NiF 6 octahedron must relax inward with respect to the host ZnF 6 octahedron due to the shorter ionic radius of Ni 2+ (0.69 Å) than Zn 2+ (0.74 Å) [24].…”

Large Differences in the Optical Spectrum Associated with the Same Complex: The Effect of the Anisotropy of the Embedding Lattice

“…Experimentally, this property is observed, for example, spectroscopically when studying the optical properties of a particular octahedral complex embedded in different cubic lattices. Good examples of this behavior are found, for instance, for MF6 4-complexes (M = Ni, Mn) in pure and doped cubic fluoroperovskites [14][15][16][17][18][19][20] or in CrX6 3units (X = F, Cl) formed in cubic elpasolites [21][22][23][24][25][26][27] . In such series of isomorphous lattices the optical absorption spectrum of a given complex is nearly unmodified with the lattice substitution as only the transitions that are dependent on the crystal-field splitting parameter, 10Dq, experience shifts smaller than 0.2 eV due to changes of the metal-ligand distance, R, smaller 14,16 than 5%.…”