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CRYSTAL FIELD EFFECTS IN RARE EARTH IONS
Abstract
L'étude par spectrométrie Mössbauer d'isotopes de terre rare placés dans des environnements cubiques peut donner des informations détaillées sur le champ cristallin. L'étude de 170Yb3+ dans TmPd3, YPd3, YbBe13, TmBe13, TmAuNi4 et YbAuNi4 montre que l'état fondamental de Yb3+ est Γ7 dans les deux premiers composés et Γ6 dans les trois derniers. La structure paramagnétique bien résolue des spectres Mössbauer dans TmPd3, TmBe13, TmAuNi4 et surtout dans YbAuNi4 sont les premiers à avoir été observés dans des systèmes paramagnétiques concentrés, c'est la preuve que Tm3+ dans les composés de Tm a un état fondamental qui est un singulet bien défini. L'étude de 166Er dans LaAl2, YPd3 et YCu aurait dû conduire à des spectres de type Γ8 purs, pour lesquels le rapport A6 < r6 > / A4 < r4 > aurait pu être réduit. Les spectres ne correspondent pas à une structure pure et peuvent être expliqués par une distorsion de la symétrie cubique qui sépare l'état Γ8 en deux doublets de Kramer. Le spectre Mössbauer de la transition à 122keV du 152Sm dans SmBe13 conduit à A4 < r4 > < 2cm-1. Mössbauer studies of rare earth isotopes located in cubic environments may yield quite detailed information on the crystalline fields. Studies of 170Yb3+ in TmPd3, YPd3, YbBe13, TmBe13, TmAuNi4 and YbAuNi4 show that the Yb3+ ground state is Γ7 in the first two compounds and Γ6 in the last three. The well resolved cubic paramagnetic hyperfine structure Mössbauer spectra in TmPd3, TmBe13, TmAuNi4 and in particular in YbAuNi4 are the first to be observed in concentrated paramagnetic rnetallic systems. They prove that Tm3+ in the Tm compounds has a well isolated singlet ground state. Studies of 166Er in LaAl2, YPd3 and YCu were expected to yield pure Γ8 spectra from which the ratio A6 < r6 > / A4 < r4 > could be deduced. The spectra do not correspond to pure Γ8 spectra and are explained in terms of local distortions from cubicity which split the Γ8 state into two Kramers' doublets. The Mössbauer spectra of the 122 keV transition of 152Sm in SmBe13 (first 152Sm metallic system to show an observable Mössbauer effect) yield A4 < r4 > < 2 cm-1.
The structural chemistry of beryllium intermetallic compounds is governed by the small atomic radius, low valence electron count and covalent bonding character of beryllium. The combination with the large rare earth and actinide ions leads to high coordination numbers in the MBe13 compounds, with NaZn13 structure. The cage-like structure of the beryllium framework around Ln atoms gives rise to low-energy Einstein modes in the phonon spectra. In the heavy lanthanide beryllides, the competition between indirect exchange interaction and magnetocrystalline anisotropy leads to complex helimagnetic order. The binary beryllides compounds of Ce, U and Np show heavy fermion behavior. In the ternary rare earth-beryllium compounds, beryllium can serve as interstitial atom or take the place of p-block and transition metal atoms. The rare earth-transition metal beryllides are closely related to the rare earth-iron boride phases, where beryllium can take the place of boron as well as iron in the structure. The beryllium-rich compounds typically show a high degree of high space filling and high coordination numbers for the rare earth atoms. Most beryllium-rich compounds with non-magnetic ions are superconductors with critical temperatures below 1 K.
Although beryllium is widely used as alloying component in diverse light-weight alloys, the crystal chemistry of beryllium containing Zintl phases and intermetallic compounds is only scarcely developed and only few phase diagrams, mostly the industrially relevant ones, have been studied in detail. The present review summarizes the crystal chemical data of binary and ternary beryllium intermetallic compounds along with the results of the few documented physical property studies.
Mössbauer studies of 237Np in NpO2+x at temperatures 4.2 to 77 K were performed. The spectra show spin-relaxation phenomena which strongly depend on x and temperature. They are analyzed in terms of spin relaxation in a split cubic Gamma(2)8 quartet of Np4+(4I92). It is concluded that the Orbach-Blume mechanism (1tau~T5) is the dominant spin-lattice-relaxation process.
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