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1

Magnetic interaction in RTxX2 ternary compounds

Gil A.

Materials Science Poland

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2006

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Vol. 24, No. 3

577--583

EN

This work is an attempt of systematization the magnetic properties of the RTxX2 (R-rare earth, T-transition metal, X=Si, Ge, Sn) compounds crystallizing in the orthorhombic CeNiSi2 type structure. All silicides crystallize in the stoichiometric structure RTX2 while germanides and stannides form mainly defected structures RTxX2 (where 0

2

Struktura, właściwości magnetyczne i pole krystaliczne w potrójnych chalkogenkach lantanowców i talu TlLnx2 (X = S, Se lub Te)

Duczmal M.

Prace Naukowe Instytutu Chemii Nieorganicznej i Metalurgii Pierwiastków Rzadkich Politechniki Wrocławskiej. Monografie

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2003

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Vol. 67, nr 34

3--68

PL

Praca zawiera wyniki badań magnetycznych potrójnych chalkogenków lantanowców i talu TlLnX2, gdzie Ln oznacza lantanowiec, a X - siarkę, selen lub tellur. Wszystkie one, poczynając od neodymu, krystalizują w trygonalnej strukturze typu a-NaFeO2. Jest to struktura warstwowa, w której jony Ln3+ występują w otoczeniu sześciu jonów X2- tworzących nieznacznie zdeformowany (spłaszczony wzdłuż osi trójkrotnej) oktaedr. Z zależności namagnesowania od indukcji magnetycznej (0-14 T) wyznaczono parametry pola krystalicznego, stosując prosty model, w którym deformacja pola oktaedrycznego wyraża się jednym parametrem pola drugiego rzędu B20. Zależny od symetrii otoczenia czynnik geometryczny tego parametru maleje wyraźnie przy przejściu od lekkich do ciężkich lantanowców, zgodnie ze zmniejszaniem się krystalograficznej deformacji poliedrów koordynacyjnych LnX6. Dowodzi to słuszności zastosowanego prostego modelu pola krystalicznego. Związki gadolinu badano także metodą elektronowego rezonansu paramagnetycznego. Porównanie wyznaczonej eksperymentalnie dla TlGdSe2 i obliczonej granicy wysokotemperaturowej szerokości linii EPR sugeruje występowanie w tym związku helikoidalnego uporządkowania magnetycznego, wcześniej wykrytego wśród izostrukturalnych związków chromu(III).

EN

A large group of the ternary lanthanides and thallium dichalcogenides TlLnX2 (Ln = lanthanide, X = S, Se, or Te) were investigated. Because of the high volatility of thallium chalcogenides (the compounds can decompose during a long heating above 670 K) the unique methods of synthesis were elaborated. TlCeSe2 and TlCeTe2 are tetragonal, whereas the compounds containing neodymium and the heavier lanthanides crystallize in the rhombohedral structure of the alpha-NaFeO2 type. Praseodymium compounds were not obtained as a pure phase - all experiments gave a mixture of rhombohedral and tetragonal phases. The high field magnetization of the compounds (up to 14 T) was measured. From the magnetic induction dependence of magnetization the crystal field acting on the lanthanide ions were estimated. The gadolinium compounds TlGdS2 and TlGdSe2 were investigated by the EPR method. The first coordination sphere of Ln3+ ions in alpha-NaFeO2 type structure contains six chalcogenide ions X2-. It was proved by X-ray analysis that they form an octahedron only slightly shrinked along the threefold axis. Thus, the octahedral crystal field model was assumed with the additional parameter B20 containing the entire deformation. The parameters of crystal field were calculated from the magnetic field dependence of magnetization. Such a method is rarely used because high fields are hardly available and the calculations are much more laborious than e.g. magnetic susceptibility calculations. This requires a simultaneous diagonalization of the crystal field and magnetic field (Zeeman effect) interactions, whereas Van Vleck formula is usually sufficient for calculating the magnetic susceptibility. The octahedral part of the Hamiltonian (described by the geometrical coefficients changes rather little for sulfides, selenides and tellurides. On the other hand, the geometrical coefficients of the second order crystal field parameters decrease markedly on going from the light to heavy lanthanides, along with diminishing the deformation of LnX6 polyhedra. This proves that the simple crystal field model, employed to interpret the magnetic properties of the rhombohedral TlLnX2 compounds, explains satisfactorily the relationship of the crystallographic distortion and the deformation of the octahedral crystal field influencing the Ln3+ ions. The magnetic susceptibility measurements did not reveal directly a magnetic order in TlLnX2 compounds, According to de Gennes rule, the gadolinium compounds should have the highest ordering temperatures. In fact, the temperature dependences of reciprocal susceptibility of TlGdS2 and TlGdSe2 deviate from straight lines below 20 K. The alpha-NaFeO2 structure is a typical layer structure and a long-range magnetic order may not appear despite a relatively strong exchange interactions. On the other hand, these interactions strongly influence the EPR lineshape. The temperature dependence of the EPR linewidth was measured for TlGdS2 and TlGdSe2, but the interpretation of the results for the sulfide failed because of irregularities in TlGdS2 spectra. The TlGdSe2 resonance linewidth decreases below 15 K likewise in nonmetallic layer ferromagnets (K2CuF4, CrBr3, CdCr2Se4). From the DELTA B(T) dependence the temperature of magnetic ordering was estimated. Then the high-temperature limit of the EPR linewidth was calculated, using the classical Van Vleck method, taking into account magnetic exchange and dipole interactions. Two possible models were considered: (i) three-dimensional, with the exchange integral J1 between the nearest neighbours in the Gd3+ layer and J2 with the nearest neighbours in adjoint layers, and (ii) by analogy to the isostructural chromium compounds - the model of a helicoidal spins arrangement with the modulation period noncommensurate with the lattice constants. The second model gave the better agreement of the calculated and the experimental TlGdSe2 linewidths. The results may be improved after taking into account the preferred orientation of the sample (confirmed by the X-ray measurements).

3

Crystal field distribution in disordered materials doped with chromium.

Grinberg M.

Optica Applicata

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2000

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Vol. 30, nr 4

657-662

EN

An analysis of the crystal field distribution and distribution of the electron lattice coupling of disordered materials doped with Cr/sup 3+/ ions is presented. The theoretical background contains the extended crystal field model based calculations of the emission line-shape and kinetics. The multi-site effect in the gallogermanate and lithium tantalate crystals and aluminosilicate glasses is analysed in detail.