Rare Earth Nitrate Hybrid Double Perovskites [Me4N]2[MLn(NO3)6] (M = Na-Cs; Ln = La-Gd, ex. Pm).

An exploration of the synthetic and structural phase space of rare earth hybrid double perovskites A2B'BX6 (A = organocation, B' = M+, B = M3+, X = molecular bridging anion) that include X = NO3- and B' = alkali metal is reported, complementing earlier studies of the [Me4N]2[KB(NO3)6] (B = Am, Cm, La-Nd, Sm-Lu, Y) (Me4N = (CH3)4N+) compounds. In the present efforts, the synthetic phase space of these systems is explored by varying the identity of the alkali metal ion at the B'-site. Herein, we report three new series of the form [Me4N]2[B'B(NO3)6] (B = La-Nd, Sm-Gd; B' = Na, Rb, Cs). The early members of the Na-series crystallize in the trigonal space group R3̅ from La to Nd where a phase transition occurs in the phase between 273 and 300 K, going from R3̅ to the high-symmetry, cubic space group Fm3̅m. The preceding trigonal members of the Na-series also undergo phase transitions to cubic symmetry at temperatures above 300 K, establishing a decreasing trend in the phase-transition temperature. The remainder of the Na-series, as well as the Rb- and Cs-series, all crystallize in Fm3̅m at 300 K. The temperature-dependent phase behavior of the synthesized phases is studied via variable-temperature spectroscopic methods and high-resolution powder X-ray diffractometry. All phases were characterized via single-crystal and powder X-ray diffraction and Fourier transform infrared (FT-IR) and Raman spectroscopic methods. These results demonstrate the versatility of the perovskite structure type to include rare earth ions, nitrate ions, and a suite of alkali metal ions and serve as a foundation for the design of functional rare earth hybrid double perovskite materials such as those possessing useful multiferroic, optical, and magnetic properties.

Electronic Spectroscopy and Photoionization of LiBe.

LiBe has been the subject of several theoretical investigations and one spectroscopic study. Initially, these efforts were motivated by interest in the intermetallic bond. More recent work has explored the potential for producing LiBe and LiBe+ at ultracold temperatures. In the present study, we have advanced the spectroscopic characterization of several electronic states of LiBe and the ground state of LiBe+. For the neutral molecule, the 12Π, 22Σ+, 32Σ+, and 42Π(3d) states were observed for the first time. Data for the 22Σ+-X2Σ+ transition support a theoretical prediction that this band system is suitable for direct laser cooling. Photoelectron spectroscopy has been used to determine the ionization energy of LiBe and map the low-energy vibrational levels of LiBe+ X1Σ+. Overall, the results validate the predictions of high-level quantum chemistry calculations for both LiBe and LiBe+.

Electronic Spectroscopy and Photoionization of LiMg.

Dimers consisting of an alkali metal bound to an alkaline earth metal are of interest from the perspectives of their bonding characteristics and their potential for being laser cooled to ultracold temperatures. There have been experimental and theoretical studies of many of these species, but spectroscopic data for LiMg and the LiMg+ cation are sparse. In this study, rotationally resolved electronic spectra for LiMg are presented. The ground state is confirmed to be X12Σ+ and observations of low-lying electronically excited states are reported for the first time. Reexamination of transitions in the near-UV spectral range indicates that previous band assignments should be revised. Two-color laser excitation techniques were used to determine an ionization energy of 4.7695(4) eV. This value is 1.2 eV below the previously reported experimental estimate. Vibrationally resolved spectra were obtained for LiMg+, yielding molecular constants that were consistent with a substantial strengthening of the bond on ionization.

Spectroscopy and electronic structure of the hypermetallic oxide, MgOMg.

Electronic spectra for the hypermetallic oxide MgOMg have been observed in the 21 100 cm-1-24 000 cm-1 spectral range using laser induced fluorescence and two-photon resonantly enhanced ionization techniques. Rotationally resolved data confirmed the prediction of a X̃1Σg + ground state. The spectrum was highly congested due to the optical activity of a low-frequency bending mode and the presence of three isotopologues with significant natural abundances. Ab initio calculations predict a bent equilibrium structure for the Ã1B2 upper state, consistent with the observation of a long progression of the bending vibration mode. However, the vibrational intervals were not reproduced by the theoretical calculations. In part, this discrepancy is attributed to strong vibronic coupling between multiple electronically excited states. Two-photon ionization measurements were used to determine an ionization energy of 6.5800(25) eV.

Low energy states of NdO+ probed by photoelectron spectroscopy.

The ionization energy (IE) of NdO and the low-energy electronic states of NdO+ have been examined by means of two-color photoionization spectroscopy. The value obtained for the IE, 5.5083(2) eV, is 0.54 eV higher than previous estimates. This leads to the conclusion that the autoionization reaction Nd + O → NdO+ + e- is exothermic by 1.76(10) eV. Thirty vibronic levels of NdO+ arising from eight electronic states were observed with partial rotational resolution. The energy level pattern and supporting electronic structure calculations indicated that all of the observed states correlated with the Nd3+(4f3, 4I)O2- configuration. The structure was consistent with a ligand field theory model where the electronic states of the Nd3+(4f3, 4I) atomic ion define a repeated motif in the electronic state energy intervals of the molecular ion. Comparisons with UO+ show close similarity in the electronic structures of these isoelectronic species.

Bond dissociation energies of TiSi, ZrSi, HfSi, VSi, NbSi, and TaSi.

Predissociation thresholds have been observed in the resonant two-photon ionization spectra of TiSi, ZrSi, HfSi, VSi, NbSi, and TaSi. It is argued that because of the high density of electronic states at the ground separated atom limit in these molecules, the predissociation threshold in each case corresponds to the thermochemical bond dissociation energy. The resulting bond dissociation energies are D0(TiSi) = 2.201(3) eV, D0(ZrSi) = 2.950(3) eV, D0(HfSi) = 2.871(3) eV, D0(VSi) = 2.234(3) eV, D0(NbSi) = 3.080(3) eV, and D0(TaSi) = 2.999(3) eV. The enthalpies of formation were also calculated as Δf,0KH°(TiSi(g)) = 705(19) kJ mol-1, Δf,0KH°(ZrSi(g)) = 770(12) kJ mol-1, Δf,0KH°(HfSi(g)) = 787(10) kJ mol-1, Δf,0KH°(VSi(g)) = 743(11) kJ mol-1, Δf,0KH°(NbSi(g)) = 879(11) kJ mol-1, and Δf,0KH°(TaSi(g)) = 938(8) kJ mol-1. Using thermochemical cycles, ionization energies of IE(TiSi) = 6.49(17) eV and IE(VSi) = 6.61(15) eV and bond dissociation energies of the ZrSi- and NbSi- anions, D0(Zr-Si-) ≤ 3.149(15) eV, D0(Zr--Si) ≤ 4.108(20) eV, D0(Nb-Si-) ≤ 3.525(31) eV, and D0(Nb--Si) ≤ 4.017(39) eV, have also been obtained. Calculations on the possible low-lying electronic states of each species are also reported.

Bond dissociation energies of diatomic transition metal selenides: TiSe, ZrSe, HfSe, VSe, NbSe, and TaSe.

Predissociation thresholds have been observed in the resonant two-photon ionization spectra of TiSe, ZrSe, HfSe, VSe, NbSe, and TaSe. It is argued that the sharp onset of predissociation corresponds to the bond dissociation energy in each of these molecules due to their high density of states as the ground separated atom limit is approached. The bond dissociation energies obtained are D0(TiSe) = 3.998(6) eV, D0(ZrSe) = 4.902(3) eV, D0(HfSe) = 5.154(4) eV, D0(VSe) = 3.884(3) eV, D0(NbSe) = 4.834(3) eV, and D0(TaSe) = 4.705(3) eV. Using these dissociation energies, the enthalpies of formation were found to be Δf,0KHo(TiSe(g)) = 320.6 ± 16.8 kJ mol-1, Δf,0KHo(ZrSe(g)) = 371.1 ± 8.5 kJ mol-1, Δf,0KHo(HfSe(g)) = 356.1 ± 6.5 kJ mol-1, Δf,0KHo(VSe(g)) = 372.9 ± 8.1 kJ mol-1, Δf,0KHo(NbSe(g)) = 498.9 ± 8.1 kJ mol-1, and Δf,0KHo(TaSe(g)) = 562.9 ± 1.5 kJ mol-1. Comparisons are made to previous work, when available. Also reported are calculated ground state electronic configurations and terms, dipole moments, vibrational frequencies, bond lengths, and bond dissociation energies for each molecule. A strong correlation of the measured bond dissociation energy with the radial expectation value, ⟨r⟩nd, for the metal atom is found.