The principal susceptibilities of neodymium salts have been measured
from room temperature down to liquid air temperature. The results are discussed in terms
of Crystalline Field Theory. It is found that (1) though the mean susceptibility can be
explained satisfactorily on the basis of a single cubic field even of the fourth degree, the
absolute magnitude of the fourth order cubic field estimated in this manner from the
magnetic data will not be correct, (2) the cubic part of the field in the various neodymium
salts estimated from magnetic data on the assumption that the cubic part of the field is
wholly of the fourth degree, is found to vary markedly from crystal to crystal. (3) The
influence of the fourth and six degree terms of the cubic field on the Stark splitting of the
rare earth ions and in crystals, and their large magnetic anisotropy go to show that the
low lying energy levels as observed in absorption spectra cannot be attributed to cubic
field alone.(4)The X1—axis of neodymium sulphate rotates by about 7a In the range
studied

Mookherji, A

English

IACS Institutional Repository

THE PRINCIPAL MAGNETIC SUSCEPTIBILITIES OF SINGLE CRYSTALS OF RARE EA R IH  SALTS AT LOW TEM- PERATURES. PART III—NEODYMIUM SALTS45By  a . MOOKHERJI{Received fot puhUcallon. June sy 1949)ABSTRACT. The principal susceptibilities of neodymium salts hai'e been measured from room temperature down to liquid air temperature. The results are discussed in t^niis of Crystalline Field Theory. It is found that (i) though the mean susceptibility cah be explained satisfactorily on the basis of a single cubic field even of the fourth degree, 'the absolute magnitude of the fourth order cubic field estimated in this manner froniWe magnetic data will not be correct, (2) the cubic part of the field in the various neodymitim salts estimated from magnetic data on the assumption that the cubic part of the field is wholly of the fourth degree, is found to vary markedly from crystal to crystal. I3) The influence of the fourth and six degree terms of the cubic field on the Stark splitting of the rare earth ions and in crystals, and their large magnetic anisotropy go to show that the low lying energy levels as observed in absorption spectra cannot be attributed to cubic field alone. (/\'i The x i—axis of neodymium .sulphate rotates by about 7* In the range studied.I N T R O D U C T I O NPeaney an<3 Schlapp (1932) have attempted to estimate the character and the size of the crystalline electric field acting on the Nd*''^ion in crystals. They find that a cubic field of fourth degree represented by P(A-^  + y V  2*) gives quite a good agreement with the measurement of NdjiSOiiaSHaO by Gorter and de Haas (1931). These values of magnetic measurements give a crys­talline electric field which is about three times higher than the field estimated from the magnetic measurements of Zernicke and James (1926) and St. Meyer (1925). In spite of these discrepancies one fact stands out clearly that the . effective mean square moment as determined by each set of workers can be explained on the assumption of a single suitable cubic field (different for different workers) which is the same at all temperatures inthe range studied.The cubic field referred to is generally regarded as of the fourth degree and though the mean of the three principal susceptibilities can be explained satisfactorily on the basis of such a field— b^ut the absolute magnitude of this fourth degree field ‘as estimated by Penney and Schlapp and others (Spedding, 1937) will not be correct, since the sixth degree terms of the cubic field also influences the susceptibility pf the Nd^‘'‘*ion, and the magnitudes of the fourth degree and six degree terms in the crystals of. Nd‘’“ ions are not known.In this communication is reported the magnetic measurements on single crystals of neodymium salts from room temperature to liquid air temperature. The results are discussed in terms of the ground level split by the cubic field of fourth and sixth degrees. The calculations of the low lying energy levels from the measurements on absorption spectra of the rare earth ions in crystals are also discussed.E X P E R I M E N T A LCrystals were grown out of aqueous solution of highly pure rare earth specimens, a gift from Professor Trombd of Paris University.The experimental methods used in these measurements were the same as adopted' in our previous measurements on cerium salts (Mookherji, 1949).1R E S U L T SThe results of measurements are collected in Tables I to IV. The nature of variation of magnetic anisotropy and of the square of the principal moments with absolute temperatures are shown in Figs, i to 5.Notations and diamagnetic corrections used in this paper arc the same as adopted in our previous paper on cerium salts.M A O N E T I C  U E H A v I^' »UR o f  Nd+++ . I O N  I N  F O U R T H  A N I> SII X T  II D E G R E E  C U B I C  F I E L DThe ground state of the ion i s ‘ 7»/u. The next higher level isseparated from it by about 3510 cm~  ^ and hence in neodymium salts at all ordinary temperatures, the higher level will have very little influence on the magnetic properties-Principal Magnetic Susceptibilities of Neodymium Salts, etc. 4 ft:5^i7iaP—9• F jg. INd*(S04», SHjpX i-X i412 A. MookherjiF ig . 2Nd*(S0 4 )3, 8H * 0F i g . 3Ndji(SO*):,, 8H2O . —  Gorter X —  Jackson o —  AuthorF i g . 4NdjMg3(NO,)i„ m HjOPrincipal Magnetic Su$ceptihilities of Neodymium Salts, etc. 4J3r a ­ilMi* 7soMX’M il*/100 150 200T*K-M250 300F ig. 5Nd^MgiCNOj),*, 24HjOP'lG. 6Nd*Mg3(N03)i,. 24Hi,0TABtB IMagnetic Anisotropy at so^CCrystalCrystallo­graphicdataMode of suspensionOrienta­tion in the fieldAx MagneticAnisotropyMouoclinic 6^’ axis vertical 0 -  +14*6 g6o Xi - X 2  =  96oN dg (804)3, 8H2C)prism*a* axis vertical 'i?’ axis along 188 X l - X s = - 1 2 3a:b:c2.983 : i : i .997 a = i x 8 '8 '(001) pi. hor.field‘0’ axis along ^1016 '(' =  13-5field C a l 1^1 = 1 2 .9N d jM g ilN O s ),^ , T r ig o n a l Trig. Axis Trig. Axis normals 636 X J .-X H  *»6s68 4 H *0 horizontal to field :414 A . Mtaafefierjf*, jT able IIAbsolute Susceptibility along a convenient directionCrystalNd.2(S04>3, 8H2ONdaMgsfNOglia.24H2ODirection along which susceptibility was measuredAlong xi-^ axisNormal to Trig, axisTemp. "C3^ -528.0Density of the crystal2.988 2.18sVol.Susceptibi­lity4t-9312.67Corres­ponding gm. mol. sucept.lOIlO9610Corl-esponding gm. mol. sucept. at 30*C10130'9550Unlike cerium, and praseodyraiuni,- the splitting of the jpuergy levels of Nd '^ '^*'-ion in a cubic field depends on whether the cubic field iijyolves only the fourth degree terms or, in addition, terms of the sixth degree also. This is particularly so with the seiT i^rations of the lower levels as will be seen from Table V  taken from a,, paper by Eynch (1937) which gives!the energy levels, under a cubic field involving fourth degree term.  ^ only and also under a cubic field involving both fourth degree and sixth degree terms. The magnitudes of these two groups being chosen on the basis dt known distance of the negatively charged atoms surrounding the Nd^ "* ^-ion in the crystal. >Hence in neodymium salts the estimate of the magnitude of the cubic part of the field from the observed mean susceptibility data will be very different according as we choose fourth degree terms alone or jncljude sixth degree terms also. Taking, for example, NdaCvSOJaj 8H2O in which the crys­talline field, in the nekhbohrhood_of„Nd'" different fromthat in the neighbourhpod of crystaj of PrafepJa, 8HaO,since the two crystals are isomorphous, find that, with a cubic field of the fourth degree, the energy levels will be given by (Penney and Schlapp, 1932) ^ = 20.95./4( i )W ^ ^ W ,^ ig .S 9 A  )where * A * is a constant of the fourth degree cubic field. If-the fields-are exactly the same i!n the praseodymium and neodymium ,salts, |^Mti/apr should be equal to 17.8 according to the calculation of Penney^n^ Kyndh (1936)- 'Now assuming a purely fouith degree cubic field for neodymium^ the mean of the three principal susceptibilities will be : given by (Penney and Schlapp, 1932) , i^^ [ (:I483ei9-59M~.2 3 9 c - - 9 ' .3879^- 0^.951/)'A+ (6 .o65ei9-39>' + 4 .03i e - 9 i:i>' + i . 68c-ao-9Sf)]+  [ 2 ipi9- " +  2 e- 9 -1 iv +  «■- 20.95*'] « .where v= A i k TPrincipal Magnetic Susceptibilities of Neodymium Saltst etc.T able III>Temperature variation of Magnetic Anisotropy ,Crystal Tamp.•KNdj(S04!3, 8HjO300280260240220200,18016014012010085Suspension used for measurement*1)’ axis vertical 'a’ axis verticalNd2Mg3(NQ3h2,24H2O300 280 260  240 1320 j206180i6o14012010085Xi-“Xa9981190141016S019802300270032003880454052005670X3-“ Xl COS^ «-X2T040116 013401540178030402S103270*373^42604660Trigonal axis horissoutalXA-Xii668 I807965nso !1360162019302360  3080 , 412054407040d'1039■ 45■ 87-138-19'1-331-438- 6 3 67^90-- 9 1 8+  1 4 .61 4 .413-9513-61 3 .61 3 .61 3 .613-2512 .4  11-3-“ -15:67 .04 t6 A .  Mookherji T able IV-I'eraperature variation of Principal SusceptibilitiesCrystal and direction along which measure- ment was takenNdj,(804)3 BH2O, Along Xi-axisNdaMgn(N03)ia,2/1H2OAlong xi-ftxisTemp.3002802602402202001801403 20 100 85300280260240220200180160X40120100^5105201146012400i34o<^>14590160201796020270232806^55030310353SOXj.1029010790113501226012840137401473015880171J0188302ogoo22900x'a x \ X'jA9520 10620 102201 12.73 11,52 13.89 12-3610270 11500 10740 12.94 11.60 12.98 12,50iiocio 12410 TI600 13-00 111.52 13.01 12.5011720 13350 IJ940 12.96 .” -.34 12.91 12.4312610 14500 13900 12.94 ;IT.1Q 12.86 i?*3313720 15890 15210 12.92 11.07 12,81 12.2715250 17760 16990 3^'03 ,11 .08 12.89 1^3317070 20050 19130 J3-07 ,11.01 12.91 >«*B319400 22850 21840 13-14 10.95 12.91 I2*3’322010 25910 24820 12.84 10.64 2^ p54 13,0125110 29520 28310 12.22 ,10.12 11.91 11*7529680 34430 33820 12.12 10.17 11.80 11.36X I t X' u 2 H-W r *9620 10070 12.45 11.63 12.189980 10520 12.18 11,15 11.8410380 U030 11,99 10.88 11.56111 10 ij8So 11.87 10,75 11.5011840 12840 11-39 10.18 lb.9912120 13200 II.08 9.772 lb.6412800 14090 10.69 9.387 10.3213520 T5090 10.24 8.722 9*73314030 1608a 9-658 7.980 9.07614710 1 20400 j  9-72i 7.120 8.45015460 26050 i 8.427i 6.3337.690T5860 28190 ' 7.H98 5-466 7.050Principal Magnetic Susceptibilities of Neodymium Salts, etc. 417T a b l e  V  'Only Fourth degree terms vSixth degree terms also included0, 63.8 278.6 cm"' c'* 33-7 304.8 cm’ 'The value of A which fits best with experimental data is A • the observed and calculated values are shown in Table VI.T m a  VI*A ^  - 7 .0  cm^'-“ 7.0 cm"*^  ;TemperatureCalculated Observed300 12.7 12.5200 12.6100 1 11.8 n.885 1 tt.6 X1.4V m . L t V  W *   ^ j .  VY AAX ^ V /A . X, ^ w  W  1^  m*whereas the actual value of ‘ a ’ for Pr*^  ^ ion deduced directly from the magnetic data for Pra(S04)3. 8HaO is ~ -5^5  cm“  ^ (Mookherji, 1949).The discrepancy is evidently due to the occurrence of the sixth degree terms in the actual fields (the second degree terms, which represent the rhombic part of the field, will not aft'ect the mean square moment appreciably).The discrepancy is even more striking in NdflMgs(Nog)ia, 24HaO of which A calculated from the magnetic measurements on the assumption of fourth degree cubic terms alone come out as A = —$z cm” * which will be equivalent to a = - i .8 c m ” * for Pr^*^ ion in the same field, whereas the value of ‘a’ calculated directly from the magnetic measurements on Pr® Mgs (NO ,)«, 24H2O (Mookherji, 1948) in which the crystal field is presumably the same as in Nd,Mga(N0s)i2i 24H3O is a=  -0.508 cm” *-It is clear from the foregoing discussions that while in cerium, and to a good approximation in praseodymium also, the estimate of the cubic field, assumed to be wholly of the fourth degree, from the magnetic data will be of correct magnitude, it will not be so in Nd^’^ '*' ion, and hence any estimate of the cubic field from the magnetic data will be unreliable in the absence of previous knowledge regarding the relative magnitudesof the two types of terms. . . ■ .This is evidently the explanation of the enormous variation m the magnetic behaviour of Nd^^^ion in different salts of neodymium ; whereas ip cerium and praseodymium salts the magnetic behaviour does not show4f8 A . Mookherjisuch marked variations : this is due to the fourth degree terms of the cubic field being of nearly the same order in all salts but not the sixth degree terms. ■ .For the octahydrated sulphate magnetic data at very low temperatures are available for the mean susceptibility from the measurements of Gdrter and de Haas (1931) on the powder crystal and for the principal susceptibilities from those of Jackson (1939) on single crystals. These data are given in Tables VII and V III for comparison.It has been observed that Xi^axis of Nd2(S04)a, 8H *0 crystals rotate through 7 degrees as the temperature is lowered from 3oo“K  to 85®K.T abt,e V IIT ‘ K 300 288.51T.5 11.4200 JOO10.7 9.120.44.86T able V IIIT H E  A B S O R P T I O N  S P E C T R A  I N  R E b A T I  O N  TO THE S E P A R A T I O N  O F  L E V E L S  I N  A C U B I C  F I E L DExtensive measurements have been made on the absorption spectra of the rare earth salts, both in solid state and in aqueous solutions, particularly b^ » Spedding , (1937). and Freed (1931) and their collaborators. From the absorption measurements the low lying energy levels of the various rare earth ions, have been deduced. Now the number of these levels, andtheir relative separations are found to accord closely with what one should expect to'occur in a cubic field of the fourth degree, ^ d  furthermore the overall separations in isoraorphous salts of different rare earths are of magnitudes ;to be expected from nearly the same magnitude of-the cubic flield. These results ar^ strikingly illustrated by the following Table I X  taken from a recent paper by Penney and Xynch (1939) in which'the calculated values refer to a cubic field of the fourth degree of the, same maj^i'tude in all caSeS, the magnitude of the field being so chosen as to give in the. olDserved overall splitting.of 260 cm "*. The agreemeintbetween the observed and the calculated values is indeed very impfesaiv^.Principal Magnetic Susceptibilities of Neoaymium Salts, etc. 419Ftotn this close fit, Spedditig concludes that the absotption measurements can give the energy levels much more accurately than the magnetic data—  which is readily conceded, since the calculations from the magnetic data are based on the observed deviations of 'p‘ from the Hund value and these deviations are in most cases small. For the same magnitude of the cubic field, the energy separations involve factors proportional to =  , where Z is the nuclear charge of the pai'amagneticatom, and o- is the screening constant for the 4/ electrons responsible for the observed magnetic moment of the ion. Since the energy levels can be deduced accurately from the absorption measurements, it should be possible to utilise this data to calculate the precise values of <r, which are not so easily evaluated even from the emission spectroscopic data for the ionized atom.T able IXIonLevels in cm"Observed CalculatedPj.+ + + 0, no, :>35, 500 0, I3I, 307, 4670, 77, 260 0, 76, 360Er+++ 0, 19, zli, 85 0, 19, 38, 85, 89Dy + ++ 0, 22, 57, So, T12 0, 53. 57, 84, 107In spite of the close inter-consistency of the arguments and the experi­mental fit, one should view with scepticism the separation of levels as deduced from the absorption spectra as representing the separation produced by a cubic field alone, and that of the fourth degi'ee. Firstly we know that most of these salts are strongly anisotropic magnetically, and the anisotropy of the individual ions should be greater than that of the crystal, since what is observed as the anisotropy of the crystal as a whole is merely an averaged effect of the anisotropies of the differently oriented individual groups. In any case the anisotropy of the individual ions cannot be less than that of the crystal, and even such anisotropy as has been observed for the Pr*** ion (Mookherji 1949) should correspond to a separation which is not much less than that produced by the cubic part of the field. Though the rhombic part of the field has little influence on the mean of the three principal susceptibilities of the crystal, its effect on the separation of the levels is quite.large.Moreover, even if the effect of the rhombic part of the field on the separation of the levels is neglected for the purpose of argument, the cubic part of the field cannot be regarded as consisting of the fourth degree terms alone. At least in the case of Nd^ "^^  ion tlie effect of the sixth degree terms is cpnsiderable; whereas the experimental fit suggested by the data given 6—1713P—9420 A . Mookherjiin Table IX  is nearly as good in neodymium salts as in the salts offor example, in which the sixth degree terms are not so effective as in Nd' '^^*.Lastly the observed absorption spectrum of Gd2(S04)a, 8H2O (Nutting, and Spedding, 1937), is by no means as simple as one should expect from the splitting in the crystalline field. The ground state of Gd*** ion is an S.-state, “S t/s . The splitting in the crystalline field even in strong and asymmetric ones will be quite small, of the order of a few cm~^. Whereas absorption spectra suggest a complicated and widely separated set of energy levels, near the ground level. All of which show that in spite of the apparent coincidence of the energy levels, the low lying energy levels observed in absorption spectra cannot be attributed to the cubic field alone. They may be electronic levels, as influenced by vibrational levels, which in the rare earth salts may be separated by energy differences of the order of a {few hundred cm“ ,^ IThe evidence from specific heat data is more decisive regarding the separations of the lowest levels, but can give no more information regarding the origin of the separation of levels than absorption measurements. \A C K N O W L E D G M E N T SThe work was carried out at the laboratories of the Indian Association for the Cultivation of Science, Calcutta, and the author wishes to express his thanks to its Committee of Managment for affording all facilities. The author also wishes to express his thauks to Sir K. S. Krishnan K t., D.Sc.i. F .R .S ., F .N .I., for his keen interest and valuable advice and help, lh an ks are also due to my friend, Dr. A. Bose, D.Sc., for his valuable discussions and to Prof. Tromb6 for his gift of pure rare earth specimens.R E F E R E N C E SFreed, S., 1931, Phys. Rev., 38, 2132.Gfirter, C. J. and de Haas W. J., 1931, Proc. Roy. Acad. Am., 3*, 1249. Jackson, L. C., 1939, Proc. Roy. Soc., A 170, 266.Kyuch, J. G„ 1937. Trans. Farad. Soc., 33, 1402.Meyer, St.. 1925, Z. Phys., 26, 81, 4-78.Mookherji, A, 1949,1. }onr. Phys., 23, 207, 309.Nutting, G. C. and Spedding F, H., 1937, J. Chem. Phys., 8, 33, Penney, W. G. and Kynch J. G., 1939, Proc. Roy. ,Soc., A,"170, 112. Penney, W. G. and Schlapp R, 1932, Phys. Rev., 41, 194.,, ,, I, ), ,, 42, 666.Spedding, F. H., 1937, ]^ . Chem. Phys., 8, 316.Spedding, and Howe, 1937, /. Chem. Phys,, 8, 415.Spedding, Hamlin and Nutting, 1937, J. Chem. Phys., 6, rgi. JSernick", J. and James, 1926, .imer. Chem. Soc., 18, 2827.

The Principal Magnetic Susceptibilities of Single Crystals of Rare Earth Salts at Low Temperatures. Part III—Neodymium Salts

http://arxiv.iacs.res.in:8080/jspui/bitstream/10821/1514/1/The%20Principal%20Magnetic%20Susceptibilites%20of%20Single%20Crystals_A%20Mookherji.pdf

The Principal Magnetic Susceptibilities of Single Crystals of Rare Earth Salts at Low Temperatures. Part III—Neodymium Salts

Abstract

Similar works

Full text

Available Versions

IACS Institutional Repository