Franz Weitzer

Reassessment of the Al–Mn system and a thermodynamic description of the Al–Mg–Mn system

Abstract A thermodynamic optimization for the Al – Mn system is performed by considering reliable literature data and newly measured phase equilibria on the Al-rich side. Using X-ray diffraction, differential thermal analysis, and scanning electron microscopy with energy dispersive X-ray spectroscopy methods, the melting behavior of λ-Al4Mn was correctly elucidated, and two invariant reactions associated with λ-Al4Mn (L + μ-Al4Mn λ-Al4Mn at 721 ± 2 °C and L + λ-Al4Mn Al6Mn at 704 ± 2 °C) are observed. The model Al12Mn4(Al, Mn)10 previously used for Al8Mn5 was modified to be Al12Mn5(Al, Mn)9 based on crystal structure data. In addition, the high-temperature form of Al11Mn4 is included in the assessment. Employing fewer adjustable parameters than previous assessments, the present description of the Al – Mn system yields a better overall agreement with the experimental phase diagram and thermodynamic data. The obtained thermodynamic description for the Al – Mn system is then combined with those in the Al – Mg and Mg – Mn systems to form a basis for a ternary assessment. The thermodynamic parameters for ternary liquid and ternary compound Mn2Mg3Al18 (τ) are evaluated on the basis of critically assessed experimental data. The enthalpy of formation for τ resulting from CALPHAD (CALculation of PHAse Diagrams) approach agrees reasonably with that via first-principles methodology. Comparisons between the calculated and measured phase equilibria in the Al – Mg – Mn system show that the accurate experimental information is satisfactorily accounted for by the present description. A reaction scheme for the whole ternary system is presented for practical applications.

Al, Ga substitution in RE2Fe17 (RE=Ce,Pr,Nd): magnetic behavior of RE2Fe15(Al,Ga)2 alloys

Alloys with the composition RE2Fe17 and RE2Fe15X2 (X=Al,Ga) have been synthesized by the arc‐melting technique followed by an 125‐h anneal at 900–1000 °C in evacuated silica capsules. The new compound Nd2Fe15Ga2 is unstable below 800 °C, and is only obtained by quenching from above 900 °C. From x‐ray powder diffraction analysis all alloys except Pr2 Fe15 Ga2 were found to crystallize with the hexagonal Th2 Zn17 type; indications for a preferential occupation of the (6c) sites by the X atoms are small. Substitution of Al, Ga atoms generally leads to a large increase of the Curie temperature and of the magnetocrystalline anisotropy, thereby enhancing the remanence as well as the coercivity. These effects are a consequence of the strongly distance‐dependent exchange interactions of the Fe‐Fe and Fe‐RE coupling mechanisms. As for binary Pr2 Fe17 , the easy axis of magnetization for Pr2 Fe15 Al2 was found to be in the basal plane.

Crystal chemistry and magnetism of neodymium stannides including compounds of the structural series REnSn3n−2

The binary system Nd−Sn has been investigated employing X-ray powder techniques and magnetic susceptibility measurements up to 600 K and 6 T. The formation of the compounds Nd 5 Sn 3 , Nd 5 Sn 4 , Nd 11 Sn 10 , NdSn, “Nd 3 Sn 5 ,” and NdSn 3 has been confirmed. New phases Nd 2 Sn 5 , Nd 3 Sn 7 , and NdSn 2 have been observed and their crystal structures were found to be isotypic with the structure types of Ce 2 Sn 5 , Ce 3 Sn 7 , and NdSn 2 (ZrGa 2 ), respectively. Further representatives of the Ce 2 Sn 5 -, the Ce 3 Sn 7 -, and the NdSn 2 -type have been synthesized for La, Pr, and Sm. The lanthanum-containing compounds are diamagnets above 80 K. La 2 Sn 5 and LaSn 2 become superconducting at 5.4 and 3.6 K, respectively. Pr 2 Sn 5 and PrSn 2 exhibit metamagnetism, whereas Pr 3 Sn 7 is a weak ferromagnet. Nd 5 Sn 3 and Nd 3 Sn 5 are metamagnets with Neel temperatures T N =40 K and T N =14 K, respectively, whereas Nd 5 Sn 4 , Nd 11 Sn 10 , NdSn, and Nd 3 Sn 7 order ferromagnetically at 67, 75, 32, and 10.5 K, respectively. NdSn 2 , Nd 2 Sn 5 , and NdSn 3 prove to be antiferromagnets at low temperatures. For the new samarium stannides Sm 3 Sn 7 and SmSn 2 , peaks in the susceptibility curves at 20 and 32 K give evidence of antiferromagnetic spin structures.

Magnetism of ternary compounds RE6Fe13X; RE=Pr, Nd; X=Cu, Ag, Au, Zn, Cd, and Hg

Novel compounds RE6Fe13X (X=Cu, Ag, Au, Zn, Cd, Hg) have been synthesized by arc melting (Cu, Ag, Au) or by reaction sintering (Zn, Cd, Hg), followed by heat treatment at 550 or 600 °C up to 350 h in evacuated silica capsules. From room temperature x‐ray powder diffraction analyses the alloys were found to crystallize with the ordered Nd6Fe13Si type. The Curie point of all the compounds is found to lie around room temperature. Due to their low magnetization values a ferrimagnetic coupling within the Fe sublattice is suggested. A canted RE‐spin structure cannot be ruled out. 57Fe Mossbauer spectra were recorded from the samples with X=Ag and Au. Additionally, 197Au spectra were taken from the two Au‐containing compounds. The 57Fe spectra are composed of various subpatterns, originating from the different Fe sites and were found to be essentially unaffected by the specific element RE and X. In all cases under investigation the spectra could only be fitted under the assumption of an easy axis of magnetizatio...

Phase diagrams of the ternary systems Mn, Fe, Co, NiSiN

Abstract Phase equilibria in the ternary systems Mn, Fe, Co, and NiSiN are investigated and isothermal sections at 900°C (FeSiN, NiSiN), at 1000°C (MnSiN, CoSiN) and at 1150°C (FeSiN) are presented. In the system MnSiN, Si3N4 coexists with MnSiN2, Mn3Si, Mn5Si3, MnSi, and MnSi2−x. In the systems Fe, Co, NiSiN, Si3N4 coexists with all binary silicides but reacts rapidly with iron above 1120 ± 10°C, and cobalt and nickel above 1170 ± 10°C to form binary silicides and nitrogen gas.

Magnetism and crystal chemistry in REFe12−xGax (RE=Y,Ce,Pr,Nd,Sm,Gd,Tb,Dy,Ho,Er,Tm,Yb,Lu and MM=mischmetal) and (Zr,Pr) (Fe1−xCox)12−yGay

The existence of ternary compounds according to the formula REFe12−x Gax which for x ∼6 represents the iron‐rich end of a homogeneous range has been confirmed. X‐ray powder analysis of alloys annealed at 800 °C or above generally reveal isotypism with the body‐ centered tetragonal ThMn12 ‐type structure. For the alloys with heavier rare‐earth elements from Gd to Lu a phase transition to a body‐centered orthorhombic structure type (ScFe6 Ga6 type) is observed, which has not been reported before. The transition corresponds to a crystallographic group‐subgroup relation (I4/mmm→t2 →Immm), and the transition temperature increases with the ordinal number of the rare earth, indicating the higher the stability of the ScFe6 Ga6 ‐type structure, the smaller the radius of the rare‐earth element. Accordingly, the ThMn12 ‐type structure is stable for the early rare‐earth members and no transition was observed as low as 400 °C. From magnetization curves it is shown that for REFe12−x Gax (RE=rare earth, Y) all magnetic ...

Structural chemistry, magnetism and electrical properties of binary Nd silicides

Abstract Binary praseodymium silicides have been thoroughly investigated with respect to their crystal chemistry and physical properties. Formation and structure types of all known binary phases have been confirmed. Moreover, the crystal structure determination of a phase, previously reported to exist with the approximate composition Pr 3 Si 4 , was successfully performed from X-ray powder data on the basis of the isotypic orthorhombic Ho 3 Si 4 type of structure. The magnetic properties of these Pr-silicides are characterized by the onset of ferromagnetic order below 110 K. Pr 5 Si 3 ( T C =44 K) shows a canted spin structure. Pr 5 Si 4 ( T C =40 K) and PrSi ( T C =51 K) reveal a weak aniosotropy. Whereas Pr 3 Si 4 ( T C =105 K) exhibits a magnetic behavior as typically observed in narrow domain wall ferromagnets favoring easy axis anisotropy, the reduced saturation moment suggests a ferrimagnetic spin structure. PrSi 2− x ( x =0.12, GdSi 2 type) exhibits an antiferromagnetic transition at a Neel temperature T N =11 K followed by a spin reorientation towards a parallel spin alignment at 9 K. The electrical resistivities resemble a temperature dependence of metals, with characteristic changes of slope of the ρ ( T ) curves at the Curie temperatures. The ρ ( T ) plots follow ∼ T 2 relations as predicted for isotropic ferromagnets in the ordered state for the compound PrSi 1.33 , only, whilst for the other compounds a ∼ T 2.8–3.7 dependency was observed.

On the reaction scheme and liquidus surface in the ternary system Al-Si-Ti

Abstract The constitution of the ternary system Al–Si–Ti is reinvestigated over the entire composition range using X-ray diffraction, energy dispersive X-ray analysis, differential thermal analysis up to 1500 °C and metallography. The ternary phases τ1 and τ2 are confirmed and their crystal structures and solid state phase equilibria are corroborated. The compositions observed confirm previous literature. The phase τ1 (I41/amd, Zr3Al4Si5-type) melts incongruently at 953 °C < T < 1000 °C into L + τ2 + TiSi2. The phase τ2 (Cmcm, ZrSi2-type) melts incongruently at 1338 °C into L + TiSi + TiSi2. The liquidus surface is characterized by a eutectic trough, which descends monotonically from the pseudobinary maximum emax1 L = βTi + Ti5Si3 at 1540 ± 5 °C through a series of transition reactions and ends in the ternary eutectic L = τ1 + (Si) + (Al) at 573 °C.

Magnetism and crystal chemistry of compounds REMn1−xSn2−y with RE = La, Ce, Pr, NdandSm

Abstract The homogeneous ranges of the ternary solid solutions REMn 1−x Sn 2−y , RE = La , Ce , Pr , Nd and Sm , have been investigated from arc melted alloys annealed at 800°C and quenched. From X-ray powder analyses the solid solutions were found to crystallize with the defect CeNiSi 2 -type structure (filled ZrSi 2 -type). LaMn 0.24 Sn 1.79 is a temperature dependent paramagnet due to a localized manganese moment. CeMn 1−x Sn 2−y alloys are ferrimagnets. Whereas (Pr,Nd,Sm)Mn 1−x Sn 2−y exhibit antiferromagnetic ordering, an umklapp-process occurs for the Pr-containing alloys at magnetic fields higher than 2.5 Tesla.

Samarium‐iron based magnet materials with the Th2Zn17‐type structure

Substitution mode and magnetic behavior in Sm2Fe17 have been investigated as a function of (a) the metal substitution (Fe replaced by Mn, Co, Al, Ga) and (b) the accommodation of interstitial carbon atoms. X‐ray powder analysis proved in all cases a consistency with the rhombohedral Th2Zn17‐type structure and with a random distribution of the Al‐atoms on the iron‐sublattice sites. Substitution of iron in Sm2Fe17 by Mn,Co,Al,Ga, and/or carbon leads to an anisotropy change from easy plane to easy axis geometry. In correspondence with the volume expansion, Curie temperatures tend to increase as a result of an enhanced ferromagnetic coupling mechanism. Magnetization values, however, are strongly reduced due to a change of collinear ferrimagnetism to a ferromagnetic type of order of the various magnetic sublattices. The magnetic anisotropy phase diagram at 300 K has been derived for the iron‐rich region of the quaternary system Sm2Fe17−x−yCoxAly (0≤x≤8, 0≤y≤3) revealing a wide region of spin reorientation betw...