Pressure induced Raman and fluorescence singularities in LiYF4LiYF_4

The pressure effect on the fluoride scheelite laser host $LiYF_4$ is studied at room temperature up to 26 GPa by Raman scattering and up to 40 GPa by $P^{3+}$ fluorescence of doped sample. The Raman spectra exhibit three singularities at the vicinity of 6 GPa, 10-12 GPa and 16-17 GPa. The samples pressurized to 21 GPa or higher do not recover the original phase after being released, giving more Raman lines than original samples. The luminescence spectra of $P^{3+}$ are collected in the energy range corresponding to following transitions $^3P_{0,1}--^3H_{4,5,6}$, $^1D_2--^3H_4$ and $^3P_0--^3F_2$. Singularities are observed in the vicinity of 6 GPa, 10 GPa, 16 GPa, 23 GPa in agreement with the Raman study. Moreover, an irreversible transition occurs at 23 GPa. The samples pressurized to above 26 GPa become amorphous when released and all the sharp lines disappear. Above 31 GPa, the spectra at high pressures show only some broad bands corresponding to transitions between two multiplets of the $^4F_2$ configuration of $Pr^{3+}$. These singularities suggest possible phase transformations leading to lowering of the lattice symmetry.Comment: 12 pages, 13 figures, 2 table, LaTe

DFT study of pressure induced phase transitions in LiYF4

An investigation of the pressure induced phase transition from the scheelite phase (I41/a, Z=4) to the fergusonite-like phase (I2/a, Z=4)/LaTaO(P21/c, Z=4) of LiYF4 is presented. Employing density functional theory (DFT) within the generalized gradient approximation, the internal degrees of freedom were relaxed for a pressure range of 0 GPa to 20 Gpa. The influence of pressure on the lattice vibration spectrum of the scheelite phase (I41/a, Z=4) was evaluated using the direct approach, i.e. using force constants calculated from atomic displacements. The transition volume is in good agreement with experiment, while the transition pressure is overestimated of 6 GPa. At 20 GPa, a P21/c structure with apentacoordinated lithium cation is found to be the most stable phase. This structure is compatible with a transition driven by a Bg zone-center soft optic mode linked to a soft-acoustic mode along the [11-1] direction as observed for the proper ferroelastic transition of BiVO4

Effect of pressure on crystal-field transitions of Nd-doped YLiF4

Luminescence spectra of Nd3+-related crystal field transitions of YLiF4 with 2% Nd doping were measured as a function of hydrostatic pressure at T=5 K. Specifically, transitions from the F-4(3/2) (1) excited level to the I-4(9/2) and I-4(11/2) = states have been studied. Most of the isolated-ion transitions show a redshift with increasing pressure up to 10 GPa, resulting from a decrease of the Slater free-ion and spin-orbit parameters and an increase of the crystal held interactions as pressure increases. Spectra near the F-4(3/2)(1) to I-4(9/2)(1) ground state emission evidence the presence of satellite lines assigned to Nd3+-Nd3+ ion pairs coupled by magnetic dipolar interaction. The splitting between the F-4(3/2) (1)-I-4(9/2)( 1) isolated-ion emission and its satellites is found to increase with pressure. This effect is attributed to stronger magnetic coupling as the Nd3+ pair distances decrease. The pressure dependence of the F-4(3/2)(1) to I-4(9/2) = (1) luminescence energy indicates subtle structural changes of the scheelite phase at around 5.5(5) GPa. Major spectral changes observed near 10 GPa indicate a first-order structural phase transition

Inelastic scattering of light by magnetic excitons in the pseudo Ising antiferromagnets K2CoF4 and Rb2CoF4

In addition to previously reported phonon Raman scattering, we have observed inelastic light scattering by magnetic excitons in K2CoF 4 and Rb2CoF4 ; the polarized spectra have been studied at low temperatures under an applied magnetic field up to about 5 teslas. The results are interpreted on the basis of a multilevel propagating exciton model deduced from a Hamiltonian including the single ion spin-orbit coupling, the low symmetry crystal field and a nearest neighbours Heisenberg exchange interaction, acting in the 4Γ+4 (Oh) ground state of Co 2+. From the lowest exciton study we derive a value of the tetragonal crystal field Δ (— 425 cm-1 and — 635 cm -1, respectively in K2CoF4 and Rb2CoF 4) and of the isotropic exchange parameter J ( — 11.55 cm-1 and — 9.65 cm-1, respectively in K 2CoF4 and Rb2CoF4). The remaining features of the spectra, which include one and two exciton scattering, are partly interpreted using the above model with these values. The derived J values are close to previous determinations in KCoF3 and RbCoF 3, as expected. The Ising character, which is caused by the low symmetry crystal field, is found to be less marked when evaluated from the magnon dispersion results than it was indicated by previous magnetic measurements, and this has been confirmed by recent inelastic neutron scattering data in Rb2CoF 4.Outre l'effet Raman dû aux phonons, antérieurement signalé, nous observons la diffusion inélastique de la lumière liée aux excitons magnétiques dans K2CoF4 et Rb2CoF4 ; les spectres polarisés ont été étudiés à basse température en présence d'un champ magnétique pouvant atteindre 5 teslas environ. Nous interprétons les résultats au moyen d'un modèle multi-excitonique déduit d'un Hamiltonien tenant compte du couplage spin-orbite, du champ cristallin de basse symétrie et d'une interaction d'échange de type Heisenberg, perturbant l'état fondamental 4Γ+4 (Oh) des ions Co2+. De l'étude de l'exciton de plus basse énergie, nous déduisons la valeur du champ cristallin quadratique Δ (— 425 cm-1 et — 635 cm -1, respectivement pour K2CoF4 et Rb2CoF 4) ainsi que celle du paramètre d'échange isotrope J (— 11,55 cm-1 et — 9,65 cm-1, respectivement pour K2CoF4 et Rb2CoF4). Le reste du spectre, qui contient des contributions à la diffusion par un et par deux excitons, est partiellement interprété, grâce au modèle ci-dessus décrit, pour les valeurs ainsi déterminées des paramètres Δ et J, dont il dépend. Les valeurs de J sont voisines de celles qui ont été antérieurement trouvées pour KCoF 3 et RbCoF3, comme prévu. Le comportement de type Ising, qui provient du champ cristallin de basse symétrie apparaît moins marqué lorsqu'on l'évalue à partir de la courbe de dispersion des magnons que ne le laissaient prévoir d'anciennes mesures magnétiques, résultat confirmé par une récente étude de diffusion inélastique de neutrons dans Rb2CoF4

Scheelite to fergusonite phase transition in YLiF4 at high pressures

Yttrium lithium orthofluoride YLiF4 with a tetragonal scheelite structure (I4(1)/a, Z=4) has been studied with angle-dispersive x-ray powder diffraction in a diamond anvil cell at room temperature. Upon compression to about 10.0 GPa, the c/a axial ratio increases, demonstrating that the tetragonal distortion of the fluorite superstructure is enhanced. At 10.6 GPa, there occurs a transformation to a fergusonite structure (I2/a, Z 54) that involves small distortions of the cation matrix and significant displacements of anions in the simple cubic packing. There is no detectable discontinuity in the evolution of the unit cell volumes during the I4(1) /a --> I2/a transformation. Changes of the lattice parameters and axial ratios are similar to those found for temperature-induced ferroelastic scheelite-fergusonite transitions in rare earth orthoniobates and orthotantalates. A second sluggish phase transition to an as yet unidentified polymorph of YLiF4 occurs above 17.0 GPa

Low-temperature high-resolution VUV spectroscopy of Ce3+ doped LiYF4, LiLuF4 and LuF3 crystals

RadiochFor LiYF4:Ce3+, LiLuF4:Ce3+ and LuF3:Ce3+ crystals UV/visible emission and time-resolved VUV/UV excitation spectra were recorded at liquid helium temperature with spectral resolution of 0.1 nm for excitation spectra and better than 0.3 nm for emission spectra. Well resolved fine structures due to zero-phonon lines were clearly observed in both excitation and emission spectra for LiYF4:Ce3+ and LiLuF4:Ce3+. For LuF3:Ce3+ crystal no fine structure was detected in the spectra even at the highest spectral resolution. Under the host excitation, the fine structure for high-energy emission band of Ce3+ (5d-F-2(5/2)) in LiLuF4:Ce3+ becomes well pronounced because of weaker reabsorption effect, as compared to Ce3+ 4f-5d absorption, due to small penetration depth for exciting radiation. As a result the crystal-field splitting for F-2(7/2) and F-2(5/2) levels of Ce3+ in LiLuF4 crystal was measured. First observation of zero-phonon lines at similar to81,550 and similar to82,900 cm(-1) as well as vibronic side bands due to interconfigurational 4f(14)-4f(13)5d transitions in Lu3+ is reported for excitation spectrum of LiLuF4:Ce3+