Electron Paramagnetic Resonance of Trivalent-Rare-Earth-Monovalent-Alkaline-Earth Ion Pairs in CaF2

Abstract

Paramagnetic resonance from rare-earth ions in sites having rhombic symmetry have been observed from calcium fluoride crystals doped with rare-earth and sodium fluorides. The ions investigated were Ce 3+ , Nd 3+ , Dy 3+ , and Yb 3+ , and for each the axes of the g tensor were found to lie along the [110], [110], and [001] directions. In the case of the ytterbium-doped crystals, replacing the sodium fluoride by lithium or potassium fluoride generated new, but similar, rhombic spectra. It is proposed that all these spectra arise from trivalent-rare-earth-monovalent-alkaline-earth ion pairs, the ions occupying adjacent Ca 2+ ion sites. New tetragonal spectra have also been observed from Ce 3+ and Nd 3+ ions in crystals heavily doped with NaF, and it is suggested that these might arise from the association of the rare-earth ions with sodium ions on the next nearest Ca 2+ ion sites, though this is not firmly established.

Superionicity and Polymorphism in Calcium Fluoride at High Pressure

“…As it can be appreciated, the agreement between experiments and calculations is remarkably good. Our results are also in accordance with previous data obtained by others at zero pressure [T s = 1400(90) K and the melting temperature T m = 1660(50) K] [8][9][10][11][12]. We attempted to estimate the α − γ boundary by performing DFT free energy calculations within the quasi-harmonic approximation however, as we will explain later, this approach turns out to be inadequate for CaF 2 .…”

“…In α-CaF 2 a strong increase of the ionic conductivity is observed as T is raised up to ∼ 1400 K [8][9][10]. At this point the mobility of the fluorine anions is comparable to that of a molten salt, while the melting temperature of the system lies ∼ 300 K higher [11,12].…”

High-Pressure, High-Temperature Phase Diagram of Calcium Fluoride from Classical Atomistic Simulations

“…where a = 2044.05 K, b = 1.2049 GPa and c = 0.1202 are the optimal values for the cubic phase, and a = 389.19 K, b = 0.0180 GPa and c = 0.3159 for the orthorhombic. Our predicted melting temperature at P = 0 is 2044 (100) K, which turns out to be slightly larger than the experimental value 1690 (20) K [41,42]. At this temperature the calculated bulk modulus of the superionic phase is ∼ 57 GPa, which is significantly smaller than the obtained for the perfect cubic crystal (see Table II).…”

Coloring of natural fluorites

“…McLaughlan [21] showed that when fluorite was doubly doped with sodium and rare-earth elements (Ce3+ and Nd3+), its EPR (electron paramagnetic resonance) spectrum showed two lines, one with tetragonal symmetry, the other with rhombic synimetry. When the sodium concentration was increased, he observed that the rhombic spectra were enhanced, while the intensity of the tetragonal spectruni was suppressed, and another tetragonal symmetry spectrum appeared.…”