ALBERT

Notes: Heat capacity measurements of four lanthanide sesquisulfides La2S3, Ce2S3, Nd2S3, and Gd2S3, prepared in the γ phase, have been obtained between 6 and 350 K by adiabatic calorimetry. The total heat capacity has been resolved into lattice, electronic, magnetic, and Schottky components. The Schottky contributions agree well with the calculated values based on the observed splitting of the ground-state manifold of the rare earth ions occupying sites of S4 symmetry in the Th3P4 structure. The observed splitting is obtained from an analysis of the hot bands in the absorption spectrum and from direct observation of the Stark levels in the far infrared. The Stark levels (all doublets) for Ce2S3 (2F5/2) are 0, 185, and 358 cm−1; for Nd2S3(4I9/2), they are 0, 76, 150, 180, and 385 cm−1. For La2S3, which has no Schottky or magnetic contributions to the heat capacity, the thermal data can be extrapolated to 0 K. The entropy for La2S3 at 298.15 K (as S0/R) is 19.51. Schottky and magnetic ordering at lower temperatures in Ce2S3, Nd2S3, and Gd2S3 preclude such extrapolation techniques. Therefore the entropy at 298.15 K for these compounds {S0–S0(7 K)}/R, is 21.34, 22.38, and 20.05, respectively.

Notes: We report an analysis of new and previously existing optical absorption and fluorescence data, far-infrared data, and electronic Raman scattering data for Eu3+, Dy3+, and Er3+ in the C3i sites of Y2O3 and R2O3, where R=a rare earth. Our previous analysis of C2-site spectra yields an effective point-charge model for the host lattice that allows initial estimates to be calculated for the C3i-site crystal-field parameters Bkm. Best-fit values of B20, B40, and B43 are obtained for Eu, and best-fit values of all Bkm allowed by symmetry are obtained for Dy and Er. The best-fit Bkm are in relatively poor agreement with the model; in particular, B20 has the opposite sign from and B44 is much smaller than the model predictions. From the best-fit Bkm we obtain phenomenological crystal-field components Akm, from which we predict Bkm and C3i -site energy levels for the ground states of Tb3+, Ho3+, Tm3+, and Yb3+. While the effective point-charge model is apparently too crude to make accurate, quantitative, a priori predictions, the model and the data allow one to predict confidently the behavior of ions doped into C3i sites for which no data exist.

Notes: Heat-capacity measurements by adiabatic equilibrium calorimetry are reported for γ-phase Pr2S3, Tb2S3, and Dy2S3 between 5 and 350 K. Highly purified samples were prepared and their composition verified by chemical analysis. Precision lattice parameters were determined for each compound and are compared with literature values. The total heat capacity has been resolved into lattice, magnetic, and Schottky components by a volumetric approach. The experimental Schottky contributions accord with the calculated curves based on the crystal-field splitting of the 2S+1LJ ground state of the lanthanide ions occupying sites of S4 symmetry in the Th3P4 lattice. The individual crystal-field electronic energy levels have been obtained in part from an analysis of the hot-band data observed in the absorption spectra of Pr2S3, Tb2S3, and Dy2S3, and from a calculated splitting in which the crystal-field parameters Bkm, were determined from a lattice-sum calculation. Molar thermodynamic properties are reported for all three compounds. The entropy at 298.15 K {S0−S0 (7 K)}, is 22.78R, 22.93R, and 23.36R, for γ-phase Pr2S3, Tb2S3, and Dy2S3, respectively.

Notes: Absorption spectra of trivalent thulium (Tm3+, 4f12) doped double molybdate NaLa(MoO4)2 (NLM) are reported between 370 and 2000 nm at approximately 16 K. Laser-excited fluorescence spectra obtained at 4.2 K from multiplet manifolds 1G4 and 3H4 to manifolds 3F4 and the ground state 3H6 are also reported. The observed spectra are broad in this disordered host as compared with the spectra observed for Tm3+:YAG (yttrium aluminum garnet). The observed crystal field splitting of the multiplet manifolds in Tm3+:NLM is in reasonable agreement with the calculated splitting based on smoothed crystal field parameters obtained from an analysis of Nd3+:NLM and Er3+:NLM. The smaller crystal field splitting in Tm3+:NLM as compared with Tm3+:YAG may discourage cross-relaxation from the 3H4 to the 3F4 manifold in this host. This in turn may favor laser operation on the 3H4→3F4 transitions over the 3F4→3H6 transitions.

Notes: We report a detailed crystal-field splitting analysis of the energy levels of Dy3+(4f9) in single crystals of Dy2S3 that have the Th3P4 cubic defect structure. From an analysis of the temperature-dependent absorption spectra, we have identified seven of the eight crystal-field split energy (Stark) levels of the ground-state multiplet manifold, 6H15/2. Sixty-two experimental Stark levels from various multiplet manifolds of Dy3+ are compared with a calculated crystal-field splitting, whose initial crystal-field parameters, Bnm, were determined from lattice-sum calculations. The rms deviation between experimental and calculated levels is 7 cm−1. Both the experimental and calculated crystal-field splitting of the 6H15/2 manifold are compared with an assignment of Schottky levels obtained from a reassessment of heat capacity data reported earlier. Based on entropy considerations and verification of the Schottky level assignments by analyses of the optical and magnetic susceptibility data, we conclude that the anomaly observed in the heat capacity data near 3.4 K is due to antiferromagnetic ordering. © 1999 American Institute of Physics.

Notes: Polarized absorption and fluorescence spectra were analyzed to establish individual energy (Stark) levels of Nd3+ ions in host crystals of Sr5(PO4)3F (SFAP) and Ca5(PO4)3F (FAP). Site-selective excitation and fluorescence facilitated differentiation between Nd3+ ions in emitting sites associated with 1.06 μm stimulated emission, and nonemitting Nd3+ ions in other sites. Measurements were made on samples containing different concentrations of Nd3+ at 4 K and higher temperatures. Substitution of Nd3+ for Sr2+ or Ca2+ was accompanied by passive charge compensation during crystal growth. Crystal-field splitting calculations were performed according to site for Stark levels of Nd3+ ions identified spectroscopically. We obtained a final set of crystal-field parameters Bnm for Nd3+ ions in fluorescing sites with a rms. deviation of 7 cm−1 (52 levels in Nd:SFAP) and 8 cm−1 (59 levels in Nd:FAP). For one of the nonemitting sites in Nd:FAP we obtained a final set of Bnm parameters which gave a rms deviation of 6 cm−1 between 46 experimental and calculated levels. © 1996 American Institute of Physics.

Notes: Intrinsic structural disorder in scandium-substituted garnets, attributed to mixed occupancy of certain sites in the crystal lattice by different cations, has direct consequences for the optical spectra of rare-earth activator ions dispersed over multiple sites. In trivalent thulium-doped yttrium scandium gallium garnet (Tm3+:YSGG), site-selective laser excitation spectra reveal the presence of Tm3+ ions in regular D2 sites, disturbed regular sites, and in octahedral C3i sites. Absorption spectra obtained at 4 K between 0.26 and 1.85 μm are broader than those observed in more-ordered crystal hosts and include structure attributed to Tm3+ ions in sites of other than D2 symmetry. A crystal-field splitting calculation was carried out in which a parametrized Hamiltonian (including Coulombic, spin-orbit, and crystal-field terms for Tm3+ ions in D2 symmetry) was diagonalized for all manifolds of the Tm3+ (4f12) configuration. The rms deviation between 52 experimental and calculated Stark levels of Tm3+ in regular D2 sites was 5 cm−1.

Notes: Absorbance and site-selective fluorescence studies of Eu3+:Sr5(PO4)3F (Eu:SFAP) provide a nearly complete characterization of the Eu3+ energy levels. These results are compared with calculated energies generated by lattice-sum analysis. Eu3+ is found to occupy one site predominantly, but numerous minority sites are also evident; polarization studies of absorption and fluorescence lines provide evidence for the principle site having Cs symmetry. Vibronic transitions occur in spectra as well. Interpretation of the lattice location and charge compensation mechanism for the principal site is discussed in the context of obstacles arising from spectral interpretation, high crystal fields in SFAP, low site symmetry, low J-numbers in Eu3+ energy manifolds, and lattice covalency. Eu2+ emission upon ultraviolet excitation is also demonstrated; valleys in the emission profile are attributed to intracrystal absorption by Eu3+. © 1995 American Institute of Physics.

Notes: The spectroscopic properties of Cr-doped Ca5(PO4)3F known as Cr:FAP were examined using several different chemical and physical means. Results from analytical titration measurements indicate that chromium is tetravalent in the samples investigated. Based on electron-spin resonance measurements at temperatures as low as 11 K, no trivalent chromium and only a trace of pentavalent chromium (0.5–10 ppm) was detected in the samples used for optical studies. We postulate that Cr4+ is stabilized in the crystal as a result of the small amount of Y2O3 that was added during the crystal growth process and that charge balance is achieved when Y3+ replaces Ca2+ and Cr4+ replaces P5+. Assuming tetravalent chromium, absorption, and emission spectra were interpreted by analyzing phonon sidebands and zero-phonon transitions observed at various temperatures. The observed crystal-field splitting is compared with calculated levels based on a Tanabe–Sugano diagram for quartet and doublet states of Cr4+(3d2) in tetrahedral symmetry. The parametrized Hamiltonian includes the free-ion parameters for Cr4+(3d2) and an initial set of crystal-field parameters obtained from a point-charge lattice-sum calculation where ionic distances were obtained from x-ray crystallographic data. The parametrized Hamiltonian with crystal-field terms in C4 symmetry, when diagonalized within the complete 3d2 configuration, yields calculated Stark levels in reasonable agreement with levels established from the experiment. © 1995 American Institute of Physics.

Notes: The optical-absorption spectrum of Ho3+:Y3Al5O12 (Ho:YAG) has been analyzed between 220 and 2160 nm under variable temperature conditions between 4 and 60 K. Measurements between 220 and 470 nm were obtained from a Ho:YAG crystal that contained 15 at. % holmium. A crystal containing 0.5 at. % holmium was used to obtain the spectrum between 470 and 2160 nm. Within the temperature and total spectral range, more than 1500 transitions due to Ho3+(4f10) were observed and attributed to transitions between individual crystal-field (Stark) levels of the ground-state multiplet 5I8, and 49 different excited multiplet manifolds, 4f10(2S+1LJ), of trivalent holmium. From an analysis of these transitions it was possible to establish 419 of the 486 Stark levels predicted below 45 000 cm−1. This large number of experimentally characterized energy levels provided a suitable basis for a detailed analysis of the 4f10(Ho3+) electronic state structure in Ho:YAG. The analysis was carried out using a model Hamiltonian that assumes D2 site symmetry for the Ho3+ ions and incorporates all of the interactions known to have a significant influence on the 4fN electronic state structure of trivalent lanthanide ions in crystalline hosts. The radial-dependent parts of the interaction terms in the model Hamiltonian are represented in parametric form, and the resulting parameters are used as variables in fitting calculated-to-experimental energy-level data.The parametric fits of energy-level data yield a rms deviation of ∼11 cm−1 between the calculated and observed locations of the 419 experimentally assigned levels (between 0 and 45 000 cm−1), and both the isotropic and anisotropic (crystal-field) interaction parameters of the model Hamiltonian are well characterized by these data fits. Among the 50 4f10[SL]J multiplet manifolds represented in the energy-level analysis, only three show major discrepancies between calculated and observed crystal-field splitting energies. The three problematic multiplet regions are: (5G6, 5F1), centered at 22 258 cm−1, 5G5, centered at ∼24 040 cm−1; and (5G3, 3L9), centered close to 28 950 cm−1. These same multiplet regions have posed problems in energy-level analyses of Ho3+(4f10) in other systems. The possible influence of correlation crystal-field (CCF) interactions on these multiplet manifolds is explored in the present study, but CCF effects appear to be small. Overall, the model Hamiltonian derived gives an excellent representation of 4f10(Ho3+) energy-level structure in Ho:YAG, and the eigenvectors of this Hamiltonian should provide a satisfactory basis for future calculations and modeling studies of Ho:YAG optical and magneto-optical properties. © 1995 American Institute of Physics.