Gingerich, discussing the thermodynamic properties of gaseous rare earth metals and their gaseous compounds. Zuckermann, Transport Properties (Electrical Resistivity, Thermoelectric Power, and Thermal Conductivity) of Rare Earth Intermetallic Compounds 117 .
Synthesis of intermetallic compounds
Often some initial refinement of the melt is advisable before subjecting it to SSE. Two main techniques form the basis of most preparation methods used for the synthesis of intermetallic rare-earth metals: arc or induction melting.
Crystal growth 1. Introduction
Transformation growth
But for a shear transformation, the grain growth of new grains from the low temperature phase necessitates prolonged annealing just below the transition temperature, where the critical stress is caused by the shear and associated volume change. The single crystal of the high temperature phase crystallizes from the molten zone and as the zone passes through the rod, the transformation front follows.
SSE crystal growth
This topic will be returned to in the next section in the context of SSE crystal growth. Of the successful SSE crystal growth experiments performed so far, two involve the transformation cycle method: Pr and Nd.
Liquid-solid growth
- Introduction
- Bridgman technique
Some early attempts to grow rare earth crystals used zone melting. Schematic representation of the Czochralski technique: (a) seeded growth from a hot pot; (b) random orientational growth with a 'neck'; (c) induction heated cold crucible and (d) triple arc cold crucible.
Vapour-solid growth
Indeed, Shi and Fort (1986) recently observed a high density of twinning in the rhombohedral phase. However, with the temperature difference much larger, platelet crystals grew directly in the fcc phase on {111}.
Specimen fabrication
Bulk magnetization measurements have been reported on samples consisting of 4 0 4 layers of Dy and Y (Sinha et al. 1986) and similar Gd-Y alloy layers (Kwo et al. 1986) fabricated by this technique. For the elements, the best results have been obtained with SSE crystals (Chapman et al. 198!, Clark et al. 1982) and for what little work has been done on joints, a combination of annealing and careful fabrication has resulted in some success. (Goddard et al. 1982, Lord et al. 1982).
Characterisation
All three methods were used by Beyss et al. 1980) to assess some RA12 compounds, and similar measurements on these phases were made by Abell et al. Auger and energy loss spectroscopy are also employed to provide detailed analytical data (Verhoeven et al. 1985).
Acknowledgements
Introduction
In such situations and in the more usual case where the compound melts incongruently, it is useful and often necessary to have other techniques available for the preparation of single crystals. It is not our intention to provide an introduction to the theory and practice of crystal growth.
Description of technique
- Examples
This cracking can be prevented by flattening the bottom of the quartz tube so that the pot can sit there without touching the sides, or you can use a short piece of quartz in the bottom of the tube to bounce the pot away from the nesting site. Even at the p p m level, 02 can be recovered from the gas stream by metal flux.
CRUe,BLE I ll]h ol
MULLITE TUBE
VALVES EXHAUST GAS
Further aspects of crystal growth from metallic fluxes 1. Diagnostics
- Separation oJ" crystals' and flux
In view of the exceptional stability of rare-earth oxides, this indicates that the interaction between rare-earths and tin is quite strong, substantially reducing the activity of rare-earths in the melt. Ir dissolved in Cu will attack Ta, but the speed of this attack depends on which rare earth is in the melt. When welding a melt charge confined in Ta in the arc furnace, it is helpful to keep the wide, flattened portion of the tube confined in a clamp made of two Cu rods.
U- and rare-earth-Be13 crystals from At, lamellae A1 are often found in growth. When growing rare-earth rhodium stanides from excess Sn, we were never able to completely remove Sn inclusions, except in smaller crystals from the equilibrium melt. Similarity in chemical or atomic size of the elements in the stream to one of the constituents of the compound should raise this possibility.
In growing rare-earth rhodium stannides from excess Sn, the crystals can be leached with dilute HC1. The advantage of this technique is that the voltage used in erosion is easily controlled. This can be avoided by pre-reacting the c o m p o u n and using it in the load, instead of the elements.
Extensions of the technique 1. Use of temperature gradient
This can be done approximately by soaking arc molten pellets of the mixture in given amounts of flux at fixed temperatures, quenching to room temperature, and then measuring the weight loss of the compound leached pellet. We have made little mention of the use of seed melts, or of the provision of purposeful germination sites. For the most part we have only discussed the simplest of procedures, but it is clear that almost all technology adapted to produce large single crystals of oxides and similar materials can be suitably adapted to the growth of crystals from molten metal streams.
Appendix 1
Appendix 2
- Phase diagrams and crystal structures 1. Phase diagrams
- Preparation of the alloys
- Magnetic properties of RzFel4B compounds
- Anisotropy fields
- Magnetic properties of R2Fe14B hydrides
- Magnets' obtained by splat-cooling
- Magnetic properties 1. R C r 2 X 2 compounds
As a result of the low Curie points, large variations in the temperature dependence of the magnetic properties are shown. An improvement of the magnetic properties around room temperature can be obtained by increasing the Curie points. The compositional dependence of the Curie temperatures in PrzFe14xCOxB alloys was also analyzed in the above models (Pedziwiatr et al. 1986b).
Arajs (1965) shows that the composition dependence of Curie temperatures in iron-silicon alloys is not linear. In the case of the Sm2FeleB single crystal (Hiroyoshi et al. 1985) the easy direction of magnetization lies along [100] in the tetragonal structure. This is attributed to the competing effect of Nd sublattice anisotropy and 3d sublattice anisotropy (Pedziwiatr et al.
For T = A1, the reduction of the anisotropy field at room temperature is mainly due to thermal effects. Dependence of the anisotropy field on composition in Y2Fet4 xT~B alloys (T-AI, Co, Si, Cu or Ni). 22 shows the dependence of the magnetic properties of Nd15Fe7w_2xCo2xB 8 alloys on the composition.
The magnetic properties of the melt-spun R - F e - B magnets have been correlated with the microstructure and domain structure (Hadjipanayis et al. 1985d, Aly et al. 1986). Data on the magnetic properties of ThCr2Si 2 type compounds with chromium are scarce.
Ferro ~ Ferro
Measurements with a differential scanning calorimeter show that the phase transition near 150 K is endothermic, with the heat of transition L = 0.04(1)cal/g (Gyorgy et al. 1987). The temperature dependence of the hyperfine magnetic field shows that the magnetic order in the Gd sublattice disappears at 65 K (~gtka 1987a). The magnetic data (Felner and Nowik 1978, Shigeoka 1984, Iwata et al. 1986) indicate that GdMn2Ge2 is ferrimagnetic at low temperatures.
Below this temperature, the magnetic moment is localized on the Gd and Mn sublattices, and above it only on the Mn sublattice. A temperature dependence of the magnetization for (Gd, Tb, Dy)Mn2Ge 2 indicates that the magnetic transition at Tm (see Fig. 8) is the first-order ferrimagnetic-antiferromagnetic transition (Shigeoka 1984). The temperature dependence of the electrical resistivity and the thermal expansion of RMn2Ge 2 compounds have been measured to explain the nature of magnetic transitions.
The temperature dependence of the Al/l thermal expansion of polycrystalline samples RMn2Ge 2 is shown in fig. The magnetic structure in TbMn2Si 2 was determined during magnetization and neutron diffraction studies. The magnetic moment of both sublattices in the ferro- and antiferromagnetic phase is parallel to the c-axis.
The magnetic phase diagram obtained on the basis of the magnetization and the susceptibility measurements in weak external magnetic fields is shown in fig. In the La-rich range (x->0.4) the ferromagnetic coupling between the ferromagnetic Mn sublayers becomes dominant. Magnetic properties of the Cel_xLaxMnzSi2 system were investigated by neutron diffraction and magnetometry (Szytula and Siek 1982).
Over the past fifteen years, almost all compounds of the RFe2Si 2 and RFe2Ge 2 groups have been studied by the magnetometric, the M6ssbauer effect and the neutron diffraction methods. The magnetic structure of NdFezSi 2 is antiferromagnetic of a collinear + - - + type, with a magnetic unit cell twice as large as the chemical one in the direction of the c-axis (so-called type AFII - see fig. 23). (Pinto and Shaked 1973). The magnetic moment of Er atoms is perpendicular to the c axis (Leciejewicz et al. 1984).
There are no data on the magnetic properties of this compound (Bauminger et al. 1973). The absence of a quadrupole splitting favors the low-spin divalent iron with the 3d 6 configuration contrary to that of 3d ~° in similar compounds as suggested by Felner (1975). The magnetic data for the RFe2Si 2 and RFe2Ge 2 systems are listed in table 2.
![Fig. 8. Temperature dependence of the magnetiza- tion along the [001] direction at 7.2 kOe for single crystal GdMn2Ge 2 (Iwata et al](https://thumb-ap.123doks.com/thumbv2/123dok/10275004.0/153.702.82.318.100.377/temperature-dependence-magnetiza-direction-single-crystal-gdmn-iwata.webp)
GdFe2SI 2
Influence of physical properties on magnetism in RT2X ~ compounds
The available experimental data clearly indicate that the terms H~xch and Hcf are dominant in describing the magnetic properties of the R ion in a crystal, so we will discuss them in detail. The other factors are the interaction of the f-electron shells with the crystal field C E F and the interactions with valence electrons in the anionic lattice via hybridization orbitals. Similar effect is observed in magnetically isostructural ErT2Si 2 (Leciejewicz et al. 1983), EuTzGe 2 and GdTzGe 2 (Felner and Nowik 1978) compounds, suggesting that although the magnetic interactions can be discussed in terms of the R K K Y model, the number of free electrons donated to the conduction band depends on the number of d electrons.
The dependence of the N6el temperatures and the atomic volumes for TbT2X 2 compounds (T = 3d, 4d and 5d transition metal, X = Si or Ge) in the function of the atomic numbers of T elements. A plot of the F(k) function against the wave vector k for different values of 2k F (Fermi vector) in TbRu2Si 2 (Slaski et al. 1984). The Fourier map of the electron density recently obtained for DyCo2Si 2 (Szytula et al. 1988) shows S i - C o - S i layers forming an independent sandwich with saturated electric charge.
T values of the B" coefficient can be derived from experimental data obtained from magnetic susceptibility, Schottky specific heat, hyperfine structure, magnetic form factor, and neutron inelastic scattering measurements. The orientation of the magnetic moment with respect to the tetragonal axis can be related to sign of the coefficient B~.These data show that the correlation between the signs of the coefficient B~ and the orientation of the magnetic moments agrees with that d e d u c e d by n e u t r o n diffraction experiments.
