Intermediate Valence Behavior Research Articles

Low temperature magnetic behavior of Ce7Ni3

Abstract Low-temperature specific heat ( C p ) (70 mK T χ ) (70 mK T 7 Ni 3 (under applied magnetic fields up to B = 6 T) are presented. The maxima in C p (at 1.55K) and in χ (at 1.85 K) are related to an antiferromagnetic (AF) type transition. The experimental results are interpreted in terms of three Ce sublattices (1Ce I , 3Ce II and 3Ce III atoms per formula unit) of the Th 7 Ni 3 -type structure. These sublattices correspond to the three different Ce local environments. According to the entropy gain the AF order is related to the Ce I sublattice and the Ce II atoms behave as heavy fermions, with γ LT ≈ 2.7 J/Ce II -at K 2 and a characteristic temperature of T K ≈ 4 K. The Ce III atoms are responsible for an intermediate valence behavior with γ Hf ≈ 0.3J/Ce III -at K 2 and a characteristic temperature of T 0 ≈ 70 K, which is confirmed by the ln T dependence of the electrical resistivity.

Anomalous magnetic behavior of cerium in Ce 2Sn 5 and Ce 3Sn 7, two superstructures of CeSn 3

The two magnetic compounds Ce 2Sn 5 and Ce 3Sn 7 have very similar crystallographic structures. They are both superstructures of CeSn 3 with two different cerium sites: one, similar to the cerium site in CeSn 3, is “non-magnetic” or in intermediate valence state, and the other carries a well-defined moment and dominates the magnetic properties at low temperatures. Although their macroscopic magnetic properties under the effect of an applied field look quite similar, the magnetic structures of the two compounds in zero field are totally different. In fact, Ce 2Sn 5 presents a modulated structure with moments of maximum value 1.3μ B aligned along a, whereas Ce 3Sn 7 is an antiferromagnet with moments of only 0.36μ B parallel to c. This difference cannot be attributed to different Crystal Electric Field effects, which, from inelastic neutron spectroscopy measurements, are found to be fairly close in the two compounds. A strong anisotropy of the interionic exchange coupling is evidenced in both compounds from the magnetization curves and neutron data, within a crystal field model. Especially, in Ce 3Sn 7, a negative value is found for the c component of the exchange coefficient, leading to strong negative interactions when moments are aligned along c. The effects of hybridization of the cerium 4f-electron with the band electrons play an important role in Ce 2Sn 5 and Ce 3Sn 7. In both compounds, besides the intermediate-valent behavior of cerium in the CeSn 3-type site, Kondo effects are present on the magnetic cerium site, leading to an extra reduction of the cerium magnetic moments. These hybridization effects are also likely to be at the origin of the anisotropy of the exchange interactions and of the resistivity.

Crystal chemistry and magnetic behaviour of the ternary compounds RE 3(Au, Pt) xGa 11− x (RE = Y, Gd, Tb, Dy, Ho, Er, Tm and Yb)

Phases of composition RE 3Au x Ga 11− x (RE = Y, Gd, Tb, Dy, Ho, Er, Tm and Yb) and RE 3Pt x Ga 11− x (RE = Y, Tb, Dy, Ho, Er, Tm and Yb) have been synthesized from the elements by arc- or HF-melting followed by heat treatment at 600 °C. Yb 3Au 5.5Ga 5.5 crystallizes with a unique structure type, all other compounds belong to the La 3Al 11 structure type. Isotypism of the new compound Yb 3Au 4.8Ga 6.2 with La 3Al 11 has been established from X-ray powder diffraction data using full profile refinement (space group Immm, a = 4.3684 A ̊ , b = 12.976 A ̊ , c = 9.4686 A ̊ ). Only partially ordered substitution of the Al-sites by Ga and Au has been found. The magnetic properties have been studied over the temperature range 4–550 K. Above liquid nitrogen temperature (LNT), the magnetic behaviour of all compounds investigated, with the exception of the ytterbium phases, corresponds to the paramagnetism of tripositive rare earth ions. Below LNT, antiferromagnetic ordering has been found for Gd 3Au 3.6Ga 7.4 and (Tb, Ho) 3Pt 2.2Ga 8.8; a metamagnetic transition occurs for (Tb, Dy, Ho) 3Au 3.8Ga 7.2 as well as Dy 3Pt 2.2Ga 8.8, whereas (Er, Tm) 3Au 3.8Ga 7.2 appear to be ferromagnets. The yttrium compounds are very weak, temperature-independent paramagnets. For both Yb 3Au x Ga 11− x compounds, ytterbium atoms adopt the non-magnetic Yb 2+ ground state, resulting in temperature-independent magnetic susceptibility over the whole temperature range studied. Yb 3Pt 2Ga 9, however, reveals a small magnetic moment (1.1 μ B per Yb atom), possibly due to intermediate valence behaviour.

Magnetic behaviour of Ni 3P Ni 2P, NiP 3 and the series Ln 2Ni 12P 7 (Ln = Pr, Nd, Sm, GdLu)

The magnetic properties of the title compounds were investigated with a SQUID magnetometer between 2 K and 300 K. Ni 3P, Ni 2P, NiP 3 and Lu 2Ni 12P 7 are Pauli paramagnetic. Sm 2Ni 12P 7 shows Van Vieck paramagnetism, while the magnetic susceptibilities of the other ternary phosphides obey the Curie-Weiss law resulting in magnetic moments corresponding to the free ion values of the lanthanoid atoms. Thus, the nickel atoms of these ternary phosphides do not carry magnetic moments. The ytterbium atoms in Yb 2Ni 12P 7 show mixed or intermediate valence behaviour. No magnetic order down to 2 K was observed for the praseodymium, neodymium, holmium, erbium and thulium compound. Gd 2Ni 12P 7 is antiferromagnetic with a Néel temperature of T N = 15 K, Dy 2Ni 12P 7 is ferromagnetic ( T c = 9K), while Tb 2Ni 12P 7 ( T N =12 K) is metamagnetic. The existence of EuNi 5P 3 is confirmed; Eu 2Ni 12P 7 with Zr 2Fe 12P 7-type structure was not obtained.

Volume dependence of f-electron structure in intermetallic compounds of Eu and Np from Mössbauer high pressure studies

Mössbauer spectroscopy on EuAl 2 between 4.2 and 300 K at pressures up to 50 GPa coupled with X-ray diffraction studies in the same pressure range show — contrary to expectation — that this Laves phase material possesses a fully localized 4f 7 configuration even under a volume reduction of ∼30%. The results on EuAl 2 will briefly be compared to data on EuCu 2Si 2, a material exhibiting intermediate valence behavior. For a series of Np intermetallics it is shown that systematics of the volume coefficients ∂lnT mag −∂lnV and ∂lnμ −∂lnV (where T mag = magnetic ordering temperature and μ = ordered momemt on Np) allows the distinction between compounds having a localized, a narrow band or a wide band 5f-electron structure.

New ytterbium compounds with the BaAl 4 type of structure: crystal chemistry and magnetic properties of Yb(Cu,Ag,Au,Pd,Pt) xGa 4 - x

Ternary samples with the composition Yb(Cu,Ag,Au,Pd,Pt) xGa 4 - x have been synthesized from the elements by high frequency melting followed by heat treatment at 600 °C. The crystal structure of all compounds was investigated using X-ray single-crystal or powder techniques and belongs to the BaAl 4 type of structure. The homogeneity regions of the Yb(Cu,Ag,Au,Pd,Pt) xGa 4 - x phases have been established from X-ray powder diffraction analyses and lie in the range x = 0.30 –0.95 for Cu, 0.25 –0.43 for Ag, 0.22 –1.45 for Au, 0.23 –0.65 for Pt and near x = 0.25 for Pd. The unit cell dimensions vs. composition reveal a significant deviation from Vegard's rule for Cu and Au and a slight deviation for Ag and Pt. A partially ordered occupation of the 4 d and 4 e sites in the BaAl 4 structure type has been found depending on the M/Ga ratio in the homogeneity range. The weak temperature-dependent paramagnetism of these compounds is due to the non-magnetic ground state of ytterbium (Yb 2+). However, for YbPt 0.5Ga 3.5 a rather intermediate valence behaviour (Yb 2+ → Yb 3+) is encountered.

Coexistence of heavy fermion and intermediate valence behaviour in Ce24Co11

Low temperature specific heat, magnetic susceptibility andLIII-XAS measurements on Ce24Co11 are reported. The electronic specific heat can be decomposed into two contributions: a low temperature one (arising atT 5 K, characterized by a γHT = 1.8 J/K2 mole coefficient. The χ·T product changes continuously with temperature, from 0.48 emu K/mole at 2 K to 7.85 emu K/mole at 200 K. TheLIII-mean valence is ν=3.15.

Structural study of Tm on Mo(110)

Rare earth metals and alloys have been the focus of considerable interest as a consequence of their unusual electronic and structural behaviour, associated with partially filled and highly localised 4f states. Adsorbed layers of RE metals on non-alloying substrates present a valuable opportunity for studying the effects of strong atomic-like correlations perturbed by the presence of a non-localised conduction band, in well-defined atomic configurations. These include intermediate or mixed valence and heavy fermion behaviour. We report an initial study of the growth and atomic structure of Tm adsorbed on Mo(110), at room temperature, using Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), work function changes (Δφ) and secondary electron emission crystal current (SEECC). After initial formation of a two-dimensional overlayer gas, an ordered arrangement with a 10 × 2 unit mesh is seen at 0.6 monolayer coverage. At one monolayer the structure is hexagonal; it persists up to three monolayers effective coverage. This behaviour is similar to that observed for Yb on this surface.

Effect of high pressure on the electrical resistivity of the heavy-fermion compound UBe13.

The electrical resistivity \ensuremath{\rho} of the heavy-electron superconductor ${\mathrm{UBe}}_{13}$ was investigated over temperatures ranging from 1 to 300 K and at high pressures from 12 to 152 kbar. The temperature, ${\mathit{T}}_{\mathrm{max}}$, at which \ensuremath{\rho} attains a maximum increases linearly with pressure at two limiting rates: 0.25 K/kbar below \ensuremath{\sim}30 kbar and 0.65 K/kbar above \ensuremath{\sim}30 kbar. At low temperatures, the \ensuremath{\rho}-versus-T data can be described by the expression \ensuremath{\rho}=${\mathrm{\ensuremath{\rho}}}_{0}$+${\mathit{AT}}^{2}$ over a temperature interval of \ensuremath{\sim}1 K at 12 kbar to more than 16 K at 152 kbar. The coefficient A can be related to the Fermi-liquid energy scale ${\mathit{T}}^{\mathrm{*}}$ by ${\mathit{A}}^{\mathrm{\ensuremath{-}}1/2}$\ensuremath{\propto}${\mathit{T}}^{\mathrm{*}}$. The pressure dependence of ${\mathit{A}}^{\mathrm{\ensuremath{-}}1/2}$ also increases linearly with pressure at two limiting rates, with a crossover at about 50 kbar. The similarity in the pressure dependences of ${\mathit{T}}_{\mathrm{max}}$ and ${\mathit{A}}^{\mathrm{\ensuremath{-}}1/2}$ suggests that both parameters are proportional to a single energy scale, while the changes in pressure dependence of ${\mathit{T}}_{\mathrm{max}}$ and ${\mathit{A}}^{\mathrm{\ensuremath{-}}1/2}$ may represent a crossover from heavy-fermion to intermediate-valence behavior.

Magnetic susceptibility of intermediate valent CeRh and heavy fermion Ce 24Co 11 intermetallic compounds

Temperature and field dependence of the magnetic susceptibility of CeRh and Ce 24Co 11 intermetallic compounds were measured between 1.5 and 300 K and up to 1 T. Experimental data are discussed within the available models for rare earth intermetallic compounds with intermediate valent- and heavy fermion properties. Evidences of intermediate valent behaviour in CeRh and of heavy fermion behaviour in Ce 24Co 11 are reported.

Effect of pressure on the valence state of Yb in YbPd2Si2

The intermediate valent behaviour of YbPd2Si2 has been studied under pressure in the temperature range from 1.2 K to 90 K by using the 84 keV Mossbauer transition in170Yb. At 54 kbar and 4.2 K we obtain an increase of the electric field gradient (EFG) by a factor of ∼3. In addition, the EFG varies strongly with temperature, in contrast to the behaviour at ambient pressure. At 1.2 K a change of the hyperfine pattern is observed indicating a magnetic character of the Yb ion. These results provide evidence of a pressure induced change of the valence state close to 3+.

Intermediate valency I: Couplings, Hamiltonian, and electrical resistivity

A discussion is given of the couplings between the three sets of variables (conduction electrons,f-electrons, and lattice degrees of freedom) involved in intermediate valent behaviour and the resulting Hamiltonian is formulated. Emphasis is put on couplings between electronic and lattice variables, in particular on “spin”-flip terms resulting from spin orbit coupling. Concerning anomalies in the electrical resistivity, an explanation which doesnot rely on the Kondo effect is given.

Elastic properties of single crystal uranium compounds

Measurements of the elastic stiffness constants C ij and the bulk moduli of USb and USe have been performed at room temperature. Small or even negative Poisson's ratios are observed, which point to a possible intermediate valent behavior. Low-temperature sound velocity experiments on USe and UTe are presented. Whereas the transverse mode C 44 of USe and UTe shows close to the Curie temperature T c a strong temperature dependence accompanied by a rapidly changing attenuation, the longitudinal mode C 11 of USe behaves much less dramatic. We attribute this to a different coupling behavior of the longitudinal and the transverse modes with the rhombohedral lattice distortion that takes place below T c. For USe an elastic Debye temperature of 178 K could be derived.

Pressure effects on dense Kondo ferromagnets and intermediate valence states in CeNi xPt 1- x alloys

We report magnetization measurements carried out under hydrostatic pressures on isostructural CeNi x Pt 1- x alloys. Rich platinum alloys (x ⩽ 0.9) are dense Kondo ferromagnets whereas rich nickel alloys present intermediate valence behaviour. The large pressure effects observed for ferromagnetic alloys are consistent with the Kondo lattice model. Alloys with intermediate valence behaviour have also very large pressure effects. Moreover, CeNi exhibits versus pressure a first-order transition associated with the 4f instability analogous to the γ- transition of cerium.

Intermediate valence in TmxY1-xSe using MV x-ray absorption spectroscopy.

We have applied x-ray absorption spectroscopy (XAS) in the region of the Tm ${M}_{\mathrm{V}}$ edge to study the intermediate-valence behavior of the Tm ions in the series of mixed chalcogenides ${\mathrm{Tm}}_{\mathrm{x}}$${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$Se, with 0.004\ensuremath{\le}x\ensuremath{\le}1. The clearly distinguishable spectral features resulting from divalent and trivalent Tm ionic states can be fitted with good accuracy to yield the mean Tm valence v\ifmmode\bar\else\textasciimacron\fi{}. We discuss the precision of the resulting values critically and consider the surface sensitivity of the technique. The results for concentrated ${\mathrm{Tm}}_{\mathrm{x}}$Se are in good agreement with lattice-constant systematics. In the dilute materials, we confirm the intermediate valence of Tm down to x=0.4 at. % and find v\ifmmode\bar\else\textasciimacron\fi{} to be nearly constant below x=20 at. %, but to be lower than in TmSe.

Semimetallic behaviour of SmS at high pressure

Abstract The transition from semiconducting to metallic intermediate-valence behaviour occurring at 20 kbar in SmS in studied by resistivity and Hall coefficient measurements (1.2 K ⪕ T ⪕300 K, 4 kbar ⪕ P ⪕21 kbar). The data are analyzed in a two-band-conduction model. It is concluded that an indirect band gap closes at 20 kbar and that a direct one subsist above this transition.

Intermediate valence behavior of ternary cerium and uranium transition metal borides

Low-temperature specific heat, electrical resistivity, ac magnetic susceptibility, and dc magnetization measurements were made on ternary cerium and uranium transition metal borides with the general formula CeT3B2 (T=Co, Ru, Rh, and Ir) and UT3B2 (T=Co, Ru, and Ir). The compound CeRu3B2 was found to exhibit superconductivity below 0.68 K. The values of the electronic specific heat coefficient range from 9.7 mJ/mole-K2 for CeCo3B2 to 64 mJ/mole-K2 for UIr3B2. The electrical resistivity versus temperature curves of all of the compounds exhibit negative curvature and are reminiscent of valence fluctuation behavior. In the case of CeIr3B2, the electrical resistivity attains a maximum value at 180 K, while the dc magnetic susceptibility has a temperature dependence that is typical of intermediate valence Ce compounds, approaching a finite value at zero temperature. The electrical resistivity of the ferromagnetic compound CeRh3B2 reveals a rapid decrease in spin disorder resistivity below 120 K. The dc magnetic susceptibility of this material can be described as the sum of a constant term and a Curie-Weiss contribution with an effective magnetic moment of 1.01 µB per formula unit and a Curie-Weiss temperature of 119 K. Magnetization measurements on CeRh3B2 yield a saturation magnetic moment of 0.37 µB per formula unit and a Curie temperature of 113 K.

Time dependent hyperfine fields in intermediate valent140CeTb

The hyperfine fields of highly diluted cerium in ferromagnetic terbium were measured with the technique of γ-γ angular correlation, and their temperature dependence is explained in terms of an extended Jaccarino-Walker-Wertheim model, which by incorporation of a nonmagnetic state describes the intermediate valent behaviour of CeTb. Below 50 K strong relaxational effects are observed, which are understood as passage of the transition rates through the time window of TDPAC.

Crystal fields and pressure dependence of spin dynamics in CeAg measured by inelastic neutron scattering

We have studied polycrystalline CeAg and LaAg by inelastic neutron scattering at various temperatures betweenT=2 and 300 K and at different hydrostatic pressures up to 2.3GPa (23kbar). We point out the difficulties in the interpretation of the magnetic part of the measured spectra of CeAg at zero pressure and compare our results and interpretation with previous neutron scattering experiments. Applying pressure we find a broadening of the magnetic quasielastic line, i.e. the Ce3+ ion becomes more unstable. We observe still a strong temperature dependence, which means there is no indication of a classical intermediate-valent behaviour of the Ce ions up to 2.3 GPa.

Pressure induced transition from intermediate valence to metallic behaviour in collapsed SmS

The resistivity ϱ and Hall coefficient R h of nominally stoichiometric pressure collapsed SmS single crystals were measured as a function of temperature T (1.5 K< T < 300 K) at several pressures P from 4 to 22 kbar. With increasing pressure we observed a change in the ϱ (T) and R h (T) variations from an intermediate valence behaviour (P < 20 kbar) to a metallic behaviour with Kondo like scattering (P > 20 kbar).