A new family of ternary intermetallic superconducting/magnetic stannides

A new family of rare earth-rhodium-tin intermetallic compounds, with the representative formula (RE)Rh~Sn~,has been synthesized in single crystal form. The compounds containing the heavier rare earths are superconducting and those with the lighter rare earths are generally magnetic. The compound ErRh Sn36 exhibits reentrant superconductivity with T~ = 0.97 K and Tm = 0.57 K as determined from ac magnetic susceptibility measurements. The synthesis and X-ray characterization of the series are described and the results ofelectrical resistivity, upper critical magnetic field, magnetic susceptibility, specific heat and neutron scattering measurements on the Er compound are given.


Introduction Synthesis and Chemical Analysis
Considerable research activity has recently been Single crystals of all compounds were grown by disfocused on the interaction between superconductivity and solving the constituent metals in an excess of tin.The prolong-range magnetic orderP 4) Two classes of ternary corn-cess was carried out in evacuated, sealed fused silica tubes.pounds exhibiting these phenomena have been reported: the The tin used was 5 N's purity from Cominco American, Inc.; Chevrel phases typified by RE,Mo 6S8(5) and RE~MosSes (6) rare earth metals were of 3 to 4 N's purity obtained from and the tetraborides such as (RE)Rh4B4.
t7> In this Letter we Research Chemicals.Noble metals were of the highest purity report preparative, crystallographic, analytical, and transition obtainable from Engelhard Industries.All other materials temperature data for a new family of compounds.As in the used were of at least equivalent purity.A typical growth setetraborides, the Er compound displays reentrant supercon-quence, say for ErRh 11Sn36, was carried Out by weighing ductivity and for the adjacent members of the RE series the 0.2788 gms Er, 0.1715 gms Rh, and 5.00 gms Sn into a silica superconducting transition temperature increases to the right tube 7.0 mm I.D. X 9.0 mm O.D. The tube was evacuated, and the magnetic transition temperature increases to the left sealed off to a length of 8 cm, and supported vertically in a of Er.A comparison of resistivity, upper critical magnetic horizontally loaded, resistively heated furnace.Controlled field, ac magnetic susceptibility, neutron scattering, and heat temperature was held at 1050'C for 2 hrs to effect reaction capacity measurements on the Er compound is presented.and solution.A cooling rate of 5 to lOC/hr was initiated and The new family of ternary intermetallic stannides re-the program continued down to 575C when the tube was ported here employs no metalloids, and is thus the first class removed from the furnace and allowed to cool to room ternof compounds exhibiting superconductivity and magnetism perature.This 'quench' was done to preserve good crystal wherein all constituents are metals.Furthermore all the corn-surfaces, whereas if slow cooling was carried on to pounds have been prepared as single crystals.There are three solidification, crystal surfaces were often marred by precipidistinct crystal phases which are compositionally similar.The tates.When the solidified boule was placed in concentrated compounds were synthesized as single crystals by crystalliza-HCI, care was taken to remove the crystals as soon as they tion from an excess of liquid tin employed as a solvent, were loose, as prolonged exposure to HCI resulted in a black While the use of molten metals as solvents to effect crystalli-surface rather than the highly polished metallic surface otherzation is not new, this procedure should be a productive way wise obtained.We have determined by X-ray diffraction and to explore the periodic system for new compounds.
fluorescence that the black surface material is an amorphous ____________________________ composition of rhodium and tin, the erbium having been leached out by the HCI.Chemical analysis gave a composi-Supported by NSF/DMR76-24l78-A0I.
selected compounds, all of which were done in duplicate, are #Supported by US DOE Contract EY-76-C-02-0016.
given in  the resistivity at 300 K and the resistivity ratio (p300/p42) are given for representative members of the series in the last two columns of Table I.As can be seen the room temperature z resistivity is lower and the resistivity ratio higher for the cornpounds in phase I.This suggests a high degree of disorder in (UI) the crystals of phases II and III, but further study will be 2 needed to understand the microscopic origin of this difference.

Reentrant Superconductivity z
- To establish in more detail the reentrant superconduc-5 tive behavior of the Er compound, a number of different The measurements were performed with an He3 calorimeter tional Laboratory on a crystal 3-4 mm on a side which was using a standard heat pulse technique.There is a clear mounted in an hhl scattering plane in an He3-He4 dilution re-lambda-type anomaly which exhibits a peak at T = 0.47 K in zero applied magnetic field.A rough estimate of the entropy, vrel phases, large single crystals can be grown.Work on cry-S.below 3.5 K is Consistent with a value S = R in 2 per mole stal chemistry,~4 X-ray characterization,~5ãnd X-ray strucof Er suggesting that the ground state of the Er3~may be a ture determination~'6ĩs continuing and will be be reported doublet, elsewhere.In summary, we have presented the synthesis and critical field, ac magnetic suscepti-0 bility, neutron scattering and heat capacity; these measure-2 H • I KOs . .ments are displayed in Fig. 1.They clearly establish that at •--._......_ low temperatures the Er compound has fairly long-range fer-u I ..-.--.romagnetic order that destroys the superconductivity; much as 0 The low frequency (15 Hz) ac electrical resistivity measurements in zero and applied magnetic fields and the low Fig.I frequency (15 Hz) ac magnetic susceptibility measurements were made, respectively, on bars and powders prepared from a single crystal of ErRh 11Sn36.The measurements were frigerator.Bragg intensities were measured in 0-20 scans made at temperatures between 70 mK and 2.0 K in an He 3-through the various positions with integral h and I, and also He4 dilution refrigerator cryostat.The temperatures at which some half integral values both at T = 0.07 K and 0.8 K.The the ac electrical resistance dropped to 50% of its normal state only magnetic intensity (1(0.07K)-I(0.8K)) appeared at value (taken just above T~) in each applied magnetic field Bragg positions allowed in the f.c.c.lattice.It is clear therewere used to define the values of T 0 and Tm in determining fore that the ordering is ferromagnetic.The intensity of the H02.The x00 data give the values T0 = 0.97 K and Tm = (111) peak was measured on heating and cooling and the 0.57 K, while the ac electrical resistivity and upper critical results are shown in Fig. I.The transition appears to be confield measurements give T0 = 1.22 K and Tm = 0.34 K.No tinuous and no hysteresis could be observed.However, the thermal hysteresis was observed in the transition at Tm in width of the magnetic contribution to the intensity of the these measurements, in Contrast to the behavior previously (111) reflection was slightly broader than resolution suggestreported for ternary RE compounds which exhibit reentrant ing that true long-range magnetic order had not developed to superconductive behavior.0~12) A neutron scattering meas-temperatures ofT = 0.07 K. urement gives Tm = 0.61 K.The disparity in the values of The heat capacity (Cr) data for a single crystal specithe transition temperatures may be due to varying degrees of men of the Er compound between 0.4 and l.4'K are shown at sublattice order.~3~The neutron scattering measurements the bottom of Fig. 1, (measurements were taken up to 40 K). were done at the High Flux Beam Reactor at Brookhaven Na-

TABLE I
Yb and ErRhaSny compounds Compounds with Co and Ru replacing Rh have been using filtered Mo K0 radiation.The crystallographic data for prepared which also fall into this new class.A large number the series are reported in columns 2, 3 and 4 of Table 1. of compounds were also made with Ir replacing Rh.Of these, The results are given in the fifth and sixth columns of Table I.The compounds containing Yb and Ca o have superconducting transition temperatures greater than 8 K.There are magnetic transitions for RE compounds contaming Eu to Er with Tm decreasing the heavier the rare Phase I is primitive cubic with a0 9.7 A, and is found in La, Ca and Sr were of phase 1, (described later), and super-those compounds where RE is La to Gd and also with Yb, conducting with T0's at 2.6, '-' 7.1, and '-~5.! K respective-Th, Ca or Sr.The YbRh14Sn46 compound is distinguished from the other compounds of phase I by showing a broaden-20