Interconfiguration Fluctuation Research Articles

Scaling properties for the first RE-like mixed valence examples in uranium compounds: U 2Ru 2Sn and U 2RuGa 8

The present study was motivated by the scaling characterization of the first example of mixed valence (MV) RE-like behaviour found recently among intermetallic ternary uranium compounds. The χ( T) function for both title compounds has been fitted to the interconfigurational fluctuation (ICF) model of Sales and Wohlleben in order to determine the characteristic fluctuation temperatures T sf and interconfigurational excitation energies E ex. A good scaling, with similar values of T sf like from those derived from the ICF model, has been achieved for both these ternaries by plotting Tχ( T)/ C against the reduced T/ T sf . Moreover, this scaling follows almost exactly those found earlier in a number of MV- RE compounds.

Intermediate valence behaviour of the ternary cerium–nickel–phosphide Ce20Ni42P30

We have investigated the magnetic properties and the X-ray absorption spectra of the ternary compound Ce20Ni42P30, which crystallises in the hexagonal Sm20Ni41.6P30 structure type, space group P63/m. The cerium atom adopts an intermediate valence state ν=3.19. The high-temperature dependence of the magnetic susceptibility was analysed in terms of the interconfiguration fluctuation model resulting in a characteristic spin fluctuation temperature TSF=249K and an energy difference between the corresponding 4f0 and 4f1 states Eex/k=879K.

EuMn2Ge2andEuMn2Si2:Magnetic structures and valence transitions

The magnetic structures of ${}^{153}{\mathrm{EuMn}}_{2}{\mathrm{Ge}}_{2}$ and ${}^{153}{\mathrm{EuMn}}_{2}{\mathrm{Si}}_{2}$ have been determined by neutron diffraction measurements (\ensuremath{\sim}1.8--723 K). The Mn sublattices of both ${\mathrm{EuMn}}_{2}{\mathrm{Ge}}_{2}$ and ${\mathrm{EuMn}}_{2}{\mathrm{Si}}_{2}$ order antiferromagnetically $[{T}_{N}=667(9)\mathrm{K};$ space group ${I4}^{\ensuremath{'}}{/m}^{\ensuremath{'}}{m}^{\ensuremath{'}}m$ and ${T}_{N}=391(5)\mathrm{K};$ space group ${I}_{p}{4/m}^{\ensuremath{'}}{m}^{\ensuremath{'}}{m}^{\ensuremath{'}},$ respectively] with no evidence for ordering of the ${\mathrm{Eu}}^{+2}$ ions in ${\mathrm{EuMn}}_{2}{\mathrm{Ge}}_{2}$ down to 1.5 K. ${\mathrm{EuMn}}_{2}{\mathrm{Si}}_{2}$ exhibits a thermally driven valence transition around ${T}_{v}\ensuremath{\sim}527\mathrm{K}$ with a pronounced increase in the unit cell volume of \ensuremath{\sim}7% from the ${\mathrm{Eu}}^{3+}$ state at low temperatures $(Tl\ensuremath{\sim}150\mathrm{K})$ to an average valence state of \ensuremath{\sim}2.5 at high temperatures $(Tg\ensuremath{\sim}600\mathrm{K}).$ The valence-induced volume changes have been analyzed for the first time in terms of the interconfigurational fluctuation model with occupation probabilities for the ${\mathrm{Eu}}^{2+}$ state in ${\mathrm{EuMn}}_{2}{\mathrm{Si}}_{2}$ and the average ${\mathrm{Eu}}^{\mathrm{val}}$ values found to agree well with our reanalysis of earlier ${}^{151}{\mathrm{EuMn}}_{2}{\mathrm{Si}}_{2}$ isomer shift data. The strength of the magnetic interaction for antiferromagnetic ordering of the Mn sublattice is decreased by \ensuremath{\sim}10%--15% as a result of the change in the electronic configuration of ${\mathrm{EuMn}}_{2}{\mathrm{Si}}_{2}.$

Specific heat anomaly due to valence transition in Eu(Pd 1− xPt x) 2Si 2

Specific heat was measured for Eu(Pd 1− x Pt x ) 2Si 2 with 0≤ x≤0.10, in which the Eu valence sharply varies with temperature. A large specific heat peak due to the valence change was observed in each compound. Using the specific heat of LaPd 2Si 2 as a normal part, we estimated the entropy associated with valence transition. It was found that the entropy increases abruptly around the valence transition temperature and reaches a saturation value of 22–23 J/K mol at room temperature. The temperature dependence of the entropy associated with the valence transition was calculated in terms of the interconfigurational fluctuation (ICF) model. We will demonstrate that the experimental results are well described by the ICF model.

Field-induced valence transition in EuNi 2(Si 1− xGe x) 2

Magnetic susceptibility and high-field magnetization have been examined for the intermediate valence system EuNi 2(Si 1− x Ge x ) 2. It was found that the compounds with 0.79 ⩽ × ⩽ 0.82 show a first-order valence transition under high magnetic field. The results are discussed in terms of the interconfigurational fluctuation (ICF) model.

Temperature- and field-induced valence transitions of

The magnetic properties of the system were studied in which an intermediate-valence state of Eu is realized at around x = 0.75. It was found that the compounds with show a temperature-induced valence transition below room temperature, while those with are antiferromagnetic with a stable state. A first-order valence transition induced by high magnetic field was observed for . Such valence transitions against temperature or field are discussed on the basis of the interconfigurational fluctuation (ICF) model. It is shown that the observed linear relation between the transition field and the transition temperature is explained by the present model.

Field-induced valence transition of Eu(Pd1−xPtx)2Si2

The magnetic susceptibility and high-field magnetization have been measured for the intermediate valence system Eu(${\mathrm{Pd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Pt}}_{\mathrm{x}}$${)}_{2}$${\mathrm{Si}}_{2}$ with 0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.15. A first-order valence transition is observed for all the compounds under high field of 100 T at low temperatures. This valence transition is of first order accompanied with a large hysteresis, which is in contrast to a continuous valence change against temperature. Based on the interconfigurational fluctuation (ICF) model, the temperature- and field-induced valence transitions are discussed. It is found that a first-order valence transition can be induced by magnetic field, even if the system shows a continuous valence transition against temperature. Metamagnetic behavior at finite temperatures is also understood qualitatively by the ICF model.

First-Order Valence Transition of EuPd2Si2Induced by High Magnetic Fields

In order to study the magnetic field effects on the intermediate valence of the Eu system, magnetization was measured for EuPd 2 Si 2 and Eu(Pd 0.95 Pt 0.05 ) 2 Si 2 up to 100 T. A first-order metamagnetic transition was observed for both samples under high magnetic fields. The transition field is 93T for EuPd 2 Si 2 at 6 K. The saturation magnetization close to 7µ B indicates that this metamagnetism is a field-induced valence transition from an intermediate valence state to a divalent state of the Eu ion. Based on the interconfigurational fluctuation (ICF) model, the excitation energy to convert an Eu 3+ ion into Eu 2+ was estimated. Temperature dependence of the metamagnetic transition field of EuPd 2 Si 2 was also studied.

Generalized gradient approximation and its effects on the magnetic moments and magneto-optics of f-electron systems

We have been developing an understanding of the role of correlation (configuration fluctuation) effects on the giant magneto-optic properties of a class of unusual highly anisotropic, magnetically ordered, highly correlated f-electron materials. While orbital polarization in the band calculations improved the ordered moment agreement with experiments for the less correlated materials, the magneto-optic of calculation and experiment remained distinctly different. Moreover, the more highly correlated the material, the worse the agreement of the two quantities. We have now done similar calculations within local-density approximation with gradient (nonlocal) corrections included. While the nonlocal effects introduced by the generalized gradient approximation significantly modify the calculated equilibrium moment and magneto-optic behavior, comparison with experiment reinforces our earlier conclusion that band theory, which treats fluctuations in a time-averaged sense, is inadequate for capturing key effects of interconfigurational fluctuations in correlated-electron systems. Explicit treatment of correlation effects analogous to those we have already reported for the magnetically ordered moments and ordering temperature is necessary for the magneto-optic behavior.

EPR linewidth of local magnetic moments diluted in Ce intermediate-valence compounds.

Anomalous thermal behavior on the EPR linewidths has been observed for Gd impurities diluted in ${\mathrm{Ce}}_{\mathit{x}}$${\mathit{A}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{B}}_{\mathit{n}}$ (A=La,Y, B=Ir,Os,Rh,Pd) intermediate-valence compounds. In this work we show that the exchange interaction between the local magnetic moments and the intermediate-valence host ions has an important contribution to the relaxation rates of the local moments. We calculated the relaxation, using the Redfield formalism and the ideas contained in the interconfigurational fluctuation model of Hirst. We show that the exchange interaction contribution has an exponential dependence on the excitation energy of the intermediate-valence ions.

Europium valence transition in EuCu 2Si 2 and EuPd 2Si 2 investigated with 119Sn mössbauer spectroscopy

The influence of the europium ion valence instability on the electronic structure of the compounds EuCu 2Si 2 and EuPd 2Si 2 is observed by the Mössbauer effect with diluted 119Sn probes. The effects are unambiguously visible for the compound EuPd 2Si 2 where the valence transition spans a much narrower temperature range than in the case of EuCu 2Si 2. The temperature variation of the electric field gradient at the 119Sn nucleus sitting in the silicon sublattice of the former compound was successfully described in the framework of the interconfigurational fluctuation model yielding a form of the temperature dependence of the excitation energy and the level width of the divalent europium configuration. The step-like turn of the Mössbauer line position observed in addition to the Doppler temperature shift is a consequence of the variation in electronic density at 119Sn in the Si site which in turn is due to the change in the spatial distribution of the Eu electrons.

Magnetism and crystal chemistry of cerium stannides

The system Ce-Sn has been reinvestigated in the temperature range from 400 to 800 °C using X-ray powder diffraction techniques. From a characterization of its crystal structures the existence of nine binary compounds has been confirmed. Magnetic investigation of the binary cerium stannides in the temperature range from 4.2 to 600 K revealed a systematic tendency for ferromagnetic ordering of the ceriumrich compounds up to a ratio Ce:Sn ≈ 1, followed by antiferromagnetism up to a ratio of Ce:Sn ≈ 0.4, towards interconfigurational fluctuation in the compound richest in tin, CeSn 3. Candidates for heavy fermion behaviour found in the transition regions, besides CeSn 3, are α-Ce 5Sn 3 and Ce 11Sn 10.

Magnetism and structural chemistry of ternary borides RECo 2B 2 (RE  Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ho or Er) and boron substitution in (Y,Ce)Co 2Si 2 − xB x

The structural chemistry and magnetochemical behaviour of the ternary borides (RECo 2B 2) have been investigated. All compounds (RE  Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ho or Er) were found to be isotypic. From X-ray powder diffraction (Guinier) data the body-centred crystal symmetry and atom order were confirmed in all cases (ThCr 2Si 2-type structure, space group I4/ mmm). Non-stoichiometry (cobalt deficiency) was revealed by quantitative electron microprobe analysis for TbCo 1.92B 2 and HoCo 1.96B 2. There were no indications of the existence of the phases “CeCoB 3” and “CeCo 2B 2” either in the as-cast alloys or in the samples annealed at 800 °C or 1000 °C respectively. The phase stability for the incongruently melting borides (Pr,Nd,Sm)Co 2B 2 is limited to T < 950 °C. The magnetic properties of these alloys were studied in the temperature range 1.5 K < T< 900 K and in fields up to 10 T revealing a typical Van Vleck paramagnetism of free RE 3+ ions at temperatures T > 200 K. The magnetic results for SmCo 2B 2 ( μ eff = 1.76 μ B) compare well with the ideal Van Vleck behaviour of Sm 3+ ions with a J = 5 2 ground state and a lowlying excited first level, J = 7 2 . Magnetic susceptibilities of (Y,La)Co 2B 2 are practically temperature independent. At temperatures below 30 K antiferromagnetic ordering is found for RE ≡ Tb ( T M = 13 K), Dy (9.3 K), Ho (8.5 K) and Er (3.3 K), whereas the borides (Pr,Nd,Gd)Co 2B 2 order ferromagnetically at T M = 19.5 K, 28 K and 26.5 K respectively. Substitution of silicon by boron was investigated along the isopleths RECo 2B 2 − x Si x (ThCr 2Si 2-type structure) and RECoSi 3 − x B x (BaNiSn 3-type structure) for RE ≡ Y, Ce. Whereas a rather restricted mutual solid solubility resulted for the YCo 2Si 2 − x B x alloys, boron was practically insoluble in CeCoSi 3. From the magnetic behaviour of CeCoSi 3 at low temperatures, a non-magnetic cerium ground state is proposed, which was interpreted in terms of the interconfiguration fluctuation (ICF) model ( T SF = 230 K, E EX = 630 cm −1).

The electronic conditions in the 5d1layer-metal LaI2making comparison with the iso-electronic tantalum dichalcogenides, with the other RE di-iodides, and with the RE monochalcogenides

Magnetic and resistivity measurements made here reveal an absence of charge-density wave formation in the d1 metallic layer di-iodide LaI2, despite the authors' band structural work showing the Fermi surface of this material to have strong nesting characteristics. The absence of CDWs in these circumstances casts interesting light on the formation of CDW/PLDs in the analogous layer dichalcogenides 5d1IT- and 2H-TaS2, etc. Comparison is made also with other La2+ metallic compounds, like LaS and LaH2. Within the rest of the RE di-iodide family there arise further features of significance concerning spin-density waves magnetic frustration and interconfiguration fluctuation phenomena.

Nuclear magnetic resonance studies in rare earth ternary phosphides

The results of31P Knight shift (KS) and spin-lattice relaxation time (T1) measurements in the temperature interval 4.2–300 K are reported for the compounds RENi2P2(RE=Ce, Eu, Yb) in order to understand the nature of the ground state in these compounds. KS results conclusively establish that all these compounds exhibit non-magnetic ground states. The temperature dependence of spin-fluctuation temperature (Tsf) in each case is estimated from the measured data. For EuNi2P2 the values ofTsf above 77 K qualitatively agree with those obtained from Mossbauer and susceptibility data employing ionic interconfigurational fluctuation model, but disagree at lower temperatures.

Pressure-dependent hyperfine interactions in NpAl 2

237Np Mössbauer data of NpAl 2 are reported for pressures up to 90 kbar in a temperature range from 1.5K to 77K. Application of pressure yields a continuous reduction of the Curie temperature T c, of the saturation magnetic hyperfine field and of the isomer shift indicating an increasing delocalization of 5f electrons. Severe relaxational broadenings of the magnetic patterns are found especially for high pressures and close to T c. In part these may be connected with interconfigurational fluctuations but a major role play fluctuations within the lowest electronic levels of Np at rates in the range of 1–10 GHz.

Investigation of 4f-magnetism in CeNi 2P 2, EuNi 2P 2 and YbNi 2P 2 by susceptibility and NMR studies

The results of 31P NMR Knight shift (KS) and spin-lattice relaxation time (T 1) as well as magnetic susceptibility ( ξ) measurements in the temperature interval 4.2–300K are reported for the compounds RENi 2 P 2( RE = Ce, Eu and Yb) in order to understand the nature of the 41 magnetism in these compounds. The KS results conclusively establish that all these compounds exhibit non-magnetic ground states. There is a low-temperature upturn in ξ for CeNi 2P 2 and YbNi 2P 2 presumably due to magnetic impurities. The estimated spin fluctuation temperatures (T sf) for EuNi 2P 2 above 50K agree qualitatively with those obtained earlier from Mössbauer and ξ data employing ionic interconfigurational fluctuation model, but seem to disagree at low temperatures. Implications of this result are discussed.

Magnetic behaviour of ternary silicides CeT 2Si 2 (T ≡ Ru, Rh, Pd, Os, Ir, Pt) and boron substitution in Ce{Ru, Os} 2Si 2− xB x

The ternary silicides CeT 2Si 2 have been investigated with respect to crystal chemistry and magnetochemical behaviour. Ce{Ru, Rh, Pd, Os} 2Si 2 crystallize with the body-centred tetragonal ThCr 2Si 2-type structure, whereas CePt 2Si 2 and h-CeIr 2Si 2 adopt the primitive tetragonal CePt 2Si 2-type structure and CaBe 2Ge 2-type structure respectively. h-CeIr 2Si 2 exhibits a phase transition into a ThCr 2Si 2-type low temperature modification ( T u ≈ 920 ° C). The magnetic behaviour at low temperatures is characterized by the onset of antiferromagnetic ordering in the case of CeRh 2Si 2 whereas a non-magnetic cerium ground state is proposed for the remaining platinum metal members Ce{Ru, Pd, Os, Ir, Pt} 2Si 2, which was interpreted in terms of the interconfiguration fluctuations model. Boron solubility in Ce{Ru, Os} 2Si 2− x B x at 1000 °C was found to be limited at x ≈ 1 and resulted in a rather small change in the susceptibility values as compared with those of the ternary silicides CeRu 2Si 2 and CeOs 2Si 2.

Spectroscopic observation of intra- and inter-configurational excitations in the intermediate valence compound EuCu 2Si 2

Intraionic spin-orbit split multiplet levels have been observed by electronic Raman scattering in the intermediate valence compound EuCu 2Si 2. From the temperature dependence of the Raman peak positions and widths, respectively, the interconfigurational excitation energy E x and an upper limit of the interconfigurational fluctuation temperature T f have been obtained.

Intermediate valence of europium in EuxLa1−xBe13

Mossbauer studies of the 21.6 keV transition of151Eu in cubic EuBe13 and Eu1/2La1/2Be13 in the temperature range 100 K to 600 K have been performed. The position of the absorption line in EuBe13 moves from 0.60 mm/s at 100 K to 0.04 mm/s at 575 K. In Eu1/2La1/2B13 the line moves from 0.50 mm/s at 100 K to −0.03 mm/s at 500 K. We conclude that Eu in EuxLa1−xB13 is in an intermediate valence state and we analyze the temperature dependence of the isomer shift in terms of an interconfigurational fluctuation model. The model contains temperature independent parameters Eexc, the interconfiguration excitation energy, Tf, the valence fluctuation temperature, and S3, the isomer shift of pure Eu3+ in EuBe13. Assuming S3−S2= 13 mm/s the analysis yields S3=0.70 mm/s, Tf=400 K and Eexc=3000 K for EuBe13, whereas Tf=460 K and Eexc=2750 K for Eu1/2La1/2B13.