Magnetic properties of heavy-rare-earth orthomanganites

J. Phys.Chem. Solids, 1973,Vol,34, pp. 859-868. PergamonPress. Printedin Great Britain

MAGNETIC

PROPERTIES OF HEAVY-RARE-EARTH ORTHOMANGANITES*

V. E. WOOD, A. E. AUSTIN, E. W. COLLINGS and K. C. BROG Battelle, Columbus Laboratories, 505 King Avenue, Columbus, Ohio 4320 I, U.S.A. (Received 20 July 1972)

A b s t r a c t - T h e orthorhombic phases of YMnOa, HoMnO3 and YbMnO3 were made by transformation of the normal hexagonal phases at 1000~ and 35-40 kbar. The magnetic susceptibilities were measured from 4.2 to 400 K. Antiferromagnetic ordering is evident at 42 K for YMnO3 and 9 K for HoMnO3. The paramagnetic Curie temperatures were decreased considerably in magnitude from those of the hexagonal phases, being --67 K for YMnO3, --23 K for HoMnO3 and - 8 3 K for YbMnO3. HoMnOa is metamagnetic below 9 K. In YbMnOa and HoMnOa, both the rare-earth and Mn ions carry approximately their full moment. The magnetic structures are discussed in comparison with the orthoferrites. It is shown that strong Jahn-Teller distortion around the Mn 3+ ion leads to a slight reduction in the lattice parameter c (b'), and thence to the low magnetic ordering temperatures. 1. INTRODUCTION

THE MANGAN1TESof yttrium and of the heavy rare earths (of formula RMnO3 where R = Ho, Er, Tm, Yb, Lu or Y) are normally hexagonal[l], probably with space group P6a cm. They are both ferroelectric [2] and antiferromagnetic [3]. (A weak ferromagnetism associated with rare-earth ordering below 10 K may also be observed.) Under high pressure and temperature, these compounds may be transformed to an orthorhombic form similar to that of the lighter manganites [4]. Other than X-ray determinations of lattice parameters, no physical property measurements have been previously reported on these high-pressure forms. In the present paper, we discuss preparation of orthorhombic YMnOa, HoMnOa and YbMnO3 and the results of magnetic susceptibility measurements on both the orthorhombic and on the corresponding hexagonal forms. 2. PREPARATION AND CHARACTERIZATION

The most satisfactory method of preparing the hexagonal-phase rare-earth manganites in *This research was supported by the Advanced Research Projects Agency of the U.S. Department of Defense and was monitored by the U.S. Army Missile Command under Contract No. DAAH01-70-C- 1076. 859

powder form was found to be the following: High-purity Mn metal and 99.9 or 99.99 per cent rare-earth oxide are dissolved in an acid solution which is then evaporated to dryness. This is followed by calcining in air at 1175~ for 15-20 hr. Calcining at higher temperatures tends to lead to formation of MnaO4. Samples of normal-form powder were als/3 produced by solid-state reaction of MnOz and the rareearth oxide in oxygen at 1250-1300~ Some of this normal hexagonal-phase material was transformed into the more dense orthorhombic phase using both girdle and piston-cylinder high-pressure equipment[5]. Powders of the compounds were sealed in platinum and heated to 1000~ for 2 hr at pressures from 35 to 60 kbar. The specimens were quenched from 1000~ while kept under high pressure. Hexagonal YMnO3 and HoMnO~ were transformed completely at pressures greater than 35 kbar, while hexagonal YbMnO3 required a pressure of 40 kbar at 1000~ for transformation. This procedure yielded specimens in the form of sintered discs 2-3 mm in dia. X-ray diffraction studies of both hexagonal and orthorhombic forms revealed no second phases other than occasional traces of unreacted rare-earth oxides. It was judged that

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V . E . WOOD et al.

these would have no significant influence on are all narrow-gap p-type semiconductors. the interpretation of the magnetic measure- Accurate activation energies have not been ments. The magnetic results themselves indic- determined, owing to the polycrystalline naated the presence of some minor phases not ture of the samples and the likelihood of strucdetected by X-rays. These are discussed tural and chemical changes at high temperabelow.) Crystal-structure data for both forms tures. High-frequency (100 MHz) capacitance are given in Table 1. The hexagonal space measurements indicate that the relative dielecgroup is assumed to be P63cm on the basis of tric constant of ortho-YMnO3 at 300 K is less the observed antiferromagnetism and X-ray than 3. data. The trigonal space group P3cl cannot be Further details on sample preparation and ruled out on the basis of the X-ray data alone. electrical properties may be found in a recent In the orthorhombic phase the possible space report [9]. groups are centrosymmetric Pbnm, as in the 3. MAGNETIC SUSCEPTIBILITY lighter manganites and the orthoferrites[6], The magnetic susceptibilities of selected and acentric P21nb as in NdGaO3[7]. These are Buerger groups which cannot be distin- samples of both hexagonal and orthorhombic guished by X-ray diffraction alone. The results forms of yttrium, holmium, and ytterbium of a variety of physical property tests for manganites were measured between 4.2 and acentricity (piezoelectricity, pyroelectricity, 300 or 400 K in fields up to about 10 kOe. second harmonic generation) were incon- Susceptibility was measured by the Curie clusive because of the high conductivity of technique using a Cahn R G electronic microthe samples. The transformation from the balance and an electromagnet fitted with 7-in.hexagonal to the orthorhombic phase is re- dia. 'constant-force' pole caps. The magnetic constructive and involves a volume decrease force field (H "c~H/az) at the sample reference of about 9 per cent. This is related to an in- position was calibrated using a 486.77mg crease in cation-oxygen coordination in which piece of high-purity Pt, the susceptibility the number of oxygen near neighbors of a (0-977tzemu/g at 293 K) and susceptibility manganese ion increases from 5 to 6 and that temperature dependence (6-3 x 10-1~ of a rare-earth ion increases from 7 to 12. The within 260-300 K) of which have been measunit-cell parameters are close to those ured by Budworth et al.[10]. Ferromagnetic contamination of the specimen could be detecpreviously reported [ 1,4]. Electrical resistivity measurements from ted, as well as corrected for, by the H o n d a room temperature to around 1100 K indicate Owen method [11] in which the nonferrothat, like their hexagonal counterparts [8], the magnetic component was obtained as the orthorhombic heavy-rare-earth manganites intercept, X~, in a reciprocal-field plot based Table 1. Crystal structure data on rare-earth manganites. Unit cell parameters in angstroms* at 25~

Compound

a0

Hexagonal Co

fit

ao

YMnO3 HoMnO3 YbMnO3

6.12 6.13 6.05

11.39 11"43 l 1.36

61.6 62.0 60.0

5-24 5-26 5 "22

*Error _+0-01 ,~ tVolume per formula unit,/~s ~

Orthorhombic b0 co 5-84 5.84 5.81

7.36 7.35 7 "30

l)t 56.3 56.5 55.3

M A G N E T I C PROPERTIES OF H E A V Y - R A R E - E A R T H O R T H O M A N G A N I T E S

on the equation X=

NoP,B~ /x2 e~f 3k T-Op"

861

(1)

X = X~o+MH -1, where M represents the magnetization of the ferromagnetic component. For successful application of the Honda-Owen method, M must be constant; that is, the ferromagnetic component must be saturable in fields lower than those used in the susceptibility measurement. The Honda-Owen method is not only a correction technique. Study of the behavior of M during a susceptibility experiment can yield useful information regarding the weightfraction and properties of the ferromagnetic contaminant itself. The specimen temperature could be varied between 1.5 K and temperatures in excess of those used in this investigation. The 1.5-4 K range was covered by pumping on liquid He. Temperatures between 4 and 78 K were obtained by allowing a He-cooled granulated charcoal ballast to warm slowly to equilibrium with a liquid-nitrogen bath. The range 78300 K was obtained by bucking the temperature of the specimen tube against the liquid nitrogen bath; while temperatures above 300 K were obtained by resistive heating. In the high temperature region (above 150 or 200 K) all the suceptibilities could be fitted to a Curie-Weiss law of the form

The parameters /Xeffand 0p, determined by a least-squares fit of the high temperature data to the linear function X-I(T), are given in Table 2. That Table also contains, for comparison, the set of 'theoretical' values obtained by assuming the spin-only moment for the Mn a+ ions and the strong LS coupling moments for the rare earths. Below 150 K there are significant deviations from Curie-Weiss behavior. In all cases these deviations occur well above the Mn ordering temperatures. The results of susceptibility measurements continued to lower temperatures on the above samples are shown in Figs. 1-5 (note scale changes). Some of the samples contained small amounts of ferrimagnetic MnsO4 as discussed below. The effects of this impurity on the apparent values of/-*elf and Op were insignificant in those compounds in which it occurred. In all cases, the results of our measurements on the hexagonal forms are in good agreement with those of Pauthenet and Veyret [3] The following generalizations may be made about the high-temperature propertie.s of the orthomanganites: 1. The effective moments are close to the 'theoretical' values, as was previously found to be true of the hexagonal phases,

Table 2. High-temperature magnetic properties of rare-earth manganites

Material

Preparation method ta~

Crystal system

Paramagnetic Curie temp., 0p(K)

YMnOs YMnOs HoMnOs HoMnO3 YbMnOa YbMnOs

E E SS SS E E(SS)

hex ortho hex ortho hex ortho

--550 (-502 tb~) -67 - 2 3 (-35 tb]) --23 --200 (-219 tb]) - 8 3 (-79 tel)

Effective moment, ~en Bohr magnetons 5.37 (5.34 tbl) 4-98 11.1 ( 11.4tb~) 11 "3 6-43 (6-74 tb]) 6.72 (6-70 tel)

'Theoretical' moment, P'th Bohr magnetons 4.90 4-90 11.68 11-68 6.68 6.68

ta~Method of preparation of original material-E: evaporation to dryness of Mn metal-R203 acid solution; SS: solid state reaction of MnO2 and RzOs. tblResults of Pauthenet and Veyret [3]. It]Results on sample prepared by solid state reaction using lower parity starting materials.

JPCS Vol. 34 No. 5 - G

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V . E . WOOD et al.

2. The paramagnetic Curie temperatures are reduced in magnitude compared to the corresponding hexagonal manganites, and 3. the dependence of 0p on rare-earth ion is similar to that found in the hexagonal manganites and in the orthochromites [ 12]. The low-temperature susceptibility measurements will be discussed for each compound in turn. 1. YMnO3. The interpretation of the results of earlier susceptibility measurements on hexagonal YMnO3 was complicated by the presence of small amounts of ferrimagnetic Mn304, which orders at 42 K[13]. This contaminant does not present such a problem in the other manganites where it is less likely to form and where the total susceptibility is much higher. As mentioned above, by use of relatively low reaction temperatures, we have managed to produce YMnO3 in both hexagonal and orthorhombic forms substantially free of Mn304. The susceptibility data in Fig. 1 include points measured in fields of both 5-7 and 10.1 kOe. As has been noted previously [3], the susceptibility temperature-dependence

of the hexagonal form does not show any noticeable break at 80 K, which is known from neutron diffraction measurements to be the N6el temperature. In contrast, ortho-YMnO3 has a typical lambda-like antiferromagnetic transition at 42 K. 2. HoMnO3. The samples of HoMnO3 discussed here were initially prepared by solidstate reaction; no samples of this compound have been made by the evaporation method. The low-temperature susceptibility of a sample of the hexagonal form containing about 2.9 wt. % Mn~O4 is shown in Fig. 2. Pauthenet* found a large magnetization associated with ordering of the H e 3§ ions below about 5 K. The presence of Mn304 in our sample prevented us from confirming this unambiguously. Neutron diffraction[14] shows that the Mn ions order at 76 K. Figure 3 shows the low-temperature susceptibility of a sample of orthorhombic HoMnO3 uncontaminated with Mn304. The low-field (5.671 kOe) data indicate an antiferromagnetic transition at about 9 ___1 K. The susceptibility anomaly at that temperature is similar in shape to that found at 42 K in ortho YMnO3. The high-field (10.120kOe) data show scarcely any break at 9 K, indicating that the material

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~o

I00 9(

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0

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~o

80

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T(K)

Fig. 1. Low-temperature magnetic susceptibility of YMnO3. Circles: orthorhombic form. Squares: susceptibility of hexagonal form multiplied by 3 to fit on same axes. Data shown for applied fields of both 5.67 and 10.12 kOe.

i

i

i

i

i

20

40

60

80

too

T

i

p

~zo

(K)

Fig. 2. Low-temperature, high-field magnetic susceptibility of hexagonal HoMnOa. This sample contained about 2-9 wt. % Mn304, the only noticeable effect of which is the slight bump in the data at about 45 K. *Private communication. The column labeled 0N2 in Table 3 of Ref. [3] has unfortunately been misprinted.

M A G N E T I C P R O P E R T I E S OF H E A V Y - R A R E - E A R T H O R T H O M A N G A N I T E S

863

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50

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50

T (K)

Fig. 3. Low-temperature magnetic susceptibility of orthorbombic HoMnO3. Circles: applied field 5.67 kOe. Squares: applied field 10.12 kOe.

is metamagnetic. This magnetic transition in ortho-HoMnO3 is in all likelihood associated with the Ho a§ ions, which order at comparable temperatures in similar materials, e.g. 12 K in the orthochromite [12], 6-5 K in the orthoferrite [15], 2-4K in the orthocobaltite[16] and, as just mentioned, 5 K in the hexagonal manganite. The ordering of the Mn 3§ ions which presumably occurs at a higher temperature is masked by the large susceptibility of the Ho 3§ ions. Figure 4 shows the susceptibility and magnetization at 4.2 K as functions of the applied field. The apparent slight increase in X at low fields, which might indicate a residual magnetization, is not statistically significant. The magnetization is not dissimilar to that in FeClz well below the metamagnetic ordering temperature [ 17]. The large low-temperature susceptibility of the holmium ion makes necessary the use of very small samples of HoMnO~, about 10-4g in our apparatus. This in turn leads to possible errors in the absolute values of the susceptibility owing to uncertainty as to the exact value of the sample mass. Of course, this uncertainty is only in the scale of the ordinate. The same remarks apply to a lesser degree, to the ytterbium manganites to be discussed next.

I00

60

o/

/

o/

o/

/ 0 0

2

4

6

8

I0

12

14

(kOe)

Fig. 4. Susceptibility (left scale) and magnetization (right scale) of ortho-HoMnO3 at 4.2 K as a function of applied magnetic field.

864

V . E . WOOD et al.

rather than the acentric P21nb, since Pbnm allows the antiferromagnetic groups Pbn'm' and Pb'nm' as well as ferromagnetic Pb'nm'. The space group P21nb allows only (weakly) ferromagnetic groups[19] (P21n'b', P21'n'b, and P21'nb'). Although a transition to a structure with those magnetic symmetries is not required to yield a state with a net moment, one might anticipate some moment if it were allowed since weak ferromagnetism is found in the corresponding orthochromites and orthoferrites (which also might have either space group P21nb or Pbnm) [6, 8]. (2) Interpretation of the low-temperature 4. DISCUSSION data for ortho-HoMnOa is complicated both In this section we consider briefly (1) the by uncertainty as to which ions are involved crystallographic implications of the antifer- in the observed ordering and by the limitations romagnetic ordering in the orthomanganites, of existing theories of metamagnetism. It (2) the metamagnetic properties of ortho- seems most probable (see below) that the order HoMnOn, and (3) the probable spin arrangement of the Mn ions is Gu (Bertaut's notation[12]). and nature of the exchange in ortho-YMnO3. Since this type of order consists of interpen(1) It seems reasonable to suppose that anti- etrating lattices, it is less likely to be associferromagnetic ordering of the Mn 3+ ions occurs ated with metamagnetism, which is more in all three orthomanganites investigated. This commonly found in layer structures. This may ordering provides some indication that the be considered an additional argument that the crystallographic space group is centrosym- Ordering observed at 9 K is that of the Ho 3+ metric (and, of course, paraelectric) Pbnm, ions. If the Mn 3+ ions have ordered antiferromagnetically at some higher temperature and if the rare-earth ions and 3d-ions are coupled magnetically when they both order, as is the case in most similar compounds, then the order of the holmium ions must be Cz in zero E field [ 12]. This type of order consists of spins E .4 lying in {110} planes with planes with all spins pointing along the positive c-axis alter~e .2 nating with planes of opposite spins. Metamagnetism then may involve field-induced rotation of whole planes (figuratively speaking) L i 6~0 20 40 8JO I00 of unfavorably-oriented spins against the anisotropy forces tending to align them along the T (K) Fig. 5. Low-temperature, high-field susceptibility of c-axis. The observed metamagnetism, of hexagonal (circles) and orthorhombic (squares) forms of course, involves an average of such effects YbMnO3. The hexagonal-phase sample contained about over some distribution of crystallite orienta0.6 wt. % MnsO4. tions. Metamagnetism occurs at around 2 K in *Pauthenet (Ref. [3] and private communication) the neighboring, normally orthorhombic found weakly ferromagnetic ordering of tho Yb a+ ions at 3"8 K in hexagonal YbMnO3. compounds TbMnOs and DyMnOa, but the 3. YbMnO3. In Fig. 5 we show the low-temperature susceptibility of samples of both hexagonal and orthorhombic forms of YbMnO3 prepared using the evaporation method. No ordering above 4.2 K is apparent in either case, although in the hexagonal form the Mn 3+ ions are known to order at 87.3 K from observation of the exchange splitting of spectral lines [ 18]. The hexagonal-form specimen contains about 0.6 wt. % of MnsO4. Measurements down to 1-6 K on a different sample of ortho-YbMnO3 indicated an antiferromagnetic transition at about 2 K.*

MAGNETIC PROPERTIES OF HEAVY-RARE-EARTH ORTHOMANGANITES

effect is not pronounced and requires fields in the 20 kOe range for its observation[3]. In HoCoO3 and TbCoO3, metamagnetism has been observed at 1.5 K in fields o f 7 - 1 0 kOe [16]. In ErCrO3, metamagnetism is found below 9 K in relatively weak fields [20]. It has also been reported in several orthoferrites [21 ] and in DyFe~Crl_xO3 solid solutions[22]. Ortho-HoMnO3 appears to have a considerably higher rare-earth ordering temperature than any of these other compounds except for ErCrOa, in which the ordering o f the rare earth (at 16.8K) is induced by the weak ferromagnetism of the Ct a+ ions. (3) T h e Mn ions in ortho-YMnO3 may be considered to occupy the vertices of the pseudomonoclinic unit cell shown in Fig. 6. I f we neglect the differences in distances to neighbors on the same and the other sublattices, we can analyze this as a simple two-sublattice antiferromagnet. T h e group theory for this situation has been discussed by Bertaut[12].

865

T h e r e are three independent antiferromagnetic spin configurations; we assume ortho-YMnO3 has one o f them. We find ' G ' - t y p e order is most reasonable; in this case G = 3C (vl/2 + v2), and

(2)

TN = 3C(--vl/2 + v2).

Taking TN = 42 K, 0p = - - 6 7 K and C for Mn 3+ 3 cm3-deg/mole, we find v, = - 12.1, v2 = - 1.4. The parameter ~1 is about half as large as in the hexagonal form[3]. T h e absence of weak ferromagnetism r6quires the order to be Gu; this ordering is also illustrated in Fig. 6. Since M n 3§ is an ion which is expected to undergo strong J a h n - T e l l e r distortion [23], one might suppose that the magnetic properties of the R E orthomanganites could be understood in terms of the effects of such a lattice distortion. T h e r e is a larger distortion in the orthomanganites than in the orthoferrites. In the pseudomonoclinic cell (Fig. 6) this shows up as an increase in a' and c' and in the angle /3' and as a decrease in b' (c). Specifically, for ortho-YMnO3, a' = c' = 3.92A, b' = 3.68A, / 3 ' = 9 6 . 2 ~ while for the orthoferrite[6], a' = c' = 3.846A, b' = 3.802A, /3' = 93.2 ~ This distortion, with b' < a' = c'. is of the type expected for a J a h n - T e l l e r ion[23]. T h e relative distortion ( a ' - b ' ) / b ' remains at about 6.5 per cent for all the heavy-rareearth manganites from G d (ionic radius 1.06 A) to Yb (0.98 .A). In the corresponding orthoferrites and orthochromites the distortion is considerably smaller, being about 1 per cent, and is again relatively independent of rareearth-ion radius. In Fig. 7 we show the atomic volume as a function of RE-ion radius (for octahedral coordination[24]) for the orthomanganites, -ferrites, -chromites, and -cobaltites. It is seen that in all cases the volume " ~ ~ ~'c increases more or less linearly with ionic radius for the heavier rare earths, but that as one gets to the light-rare-earth manganites, the volume starts to decrease, reflecting a conFig. 6. Pseudomonoclinic unit cell of orthomanganites, siderable tipping of the oxygen octahedra showing G~-ordering of Mn 3§ ions at comers of cell and lattice parameters for ortho-YMnO3. around the rare earth and an increase in the

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V . E . WOOD et al. 6O 58 56 54 i

5o 48 46-44 42 Lu

4ol

097

Yb

Tm

Er

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I

0.98

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1.00

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Y Ho

Dy

Tb

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1.02

1.03

1.04 Ionic

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1,05

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1.06

L07

Nd

Sm

I

1.08

I

1.09

l

1.10

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I,II

1.12

Radii (/~)

Fig. 7. Dependence of volume per formula unit on rare-earth octahedral coordination ionic radius for orthomanganites (triangles), orthoferrites (squares), orthochromites (open circles), and orthocobaltites (solid circles), data from Refs. [4], [6l and [16], from B E R T A U T F. and F O R R A T F.,J. dePhys. 17, 129 (1956), from Q U E Z E L A M B R U N AZ S, and M A R E S C H A L M., Bull. Soc. Franc. Min. Crist. 86, 204 (1963), and from present work.

effective RE-O ~- coordination. This occurs in spite of the relatively large atomic volume because of the severe distortion. Of other perovskite-like compounds, only KCrF3 and KCuF3 have similar large tetragonal distortions[25], 6.0 and 5.2 per cent respectively; strong Jahn-Teller effects of the Cr 2+ and Cu 2§ ions are presumably responsible. A different point of view is that of Havinga [26], who concluded from a study of compounds in which Mn was partially replaced by Ti or Nb that magnetic properties of many perovskite-like Mn 3+ compounds might be understood solely on the basis of packing effects, without any necessity of an appeal to the Jahn-Teller effect. Specifically, Havinga assumed that stiff oxygen octahedra were oriented in such a way as to preserve the M n O distance at a minimum value (found to be 2.00 A) compatible with ionic sizes. A similar suggestion had previously been made for the orthoferrites by Coppens and Eibschutz [6, 27]. They were concerned only with the packing around the rare-earth ion, though. Similar effects might also play a role in com-

pounds containing both Mn 3+ and Mn 4+ ions[28]. But in none of these compounds (except) those with a preponderance of Mn 4+ ions) is there the strong tetragonal distortion found in the orthomanganites. Assuming only near-neighbor interactions, Havinga showed that in low-conductivity LaMnO3, the M n - O - M o exchange is antiferromagnetic in the c-direction but positive between neighboring Mn's in the 'basal' plane. Estimating the magnitude of these interactions from the measurements on the other compounds, he was able to account reasonably well for the Nrel and paramagnetic Curie temperatures in this compound and in BiMnO3. We have tried to apply the same ideas to orthoYMnO3. Continuing to assume G-type order, we find from the experimental results that the near-neighbor-only contribution to the characteristic temperatures should be T u n n = --Opnn = 55 K. In the pseudomonoclinic unit cell (Fig. 6), the bond angles are y, = 157.5~ and y2 = 134~ (both _+1.5~ for 'stiff' oxygen octahedra with M n - O distances of 2.00A. From Havinga's results, expressed in terms of partial

M A G N E T I C P R O P E R T I E S OF H E A V Y - R A R E - E A R T H O R T H O M A N G A N I T E S

Curie temperatures, we find 01 (corresponding to yl) = 36 K, 02 = --144 K. Then TN= 21011 + 1021=216K, 0 p = 2 0 1 + 0 2 = - 7 2 K . While drastic extrapolation from Havinga's data is necessary to estimate 0z, the value - 1 4 4 K is about the maximum that could be expected and leads to as good calculated values of TN and 0p as can be obtained with this model. The assumption of stiff oxygen octahedra also requires the octahedra be tilted 24 ~ from the c-axis, compared to 19 ~ in the orthoferrites. If we make the contrary assumption that the tilt remains at 19 ~ but that the oxygen octahedra are distorted along with the rest of the unit cell, we find a reduced Mn-O~ distance of 1.94,~, together with Mn-O~ distances of 2-00 and 2.04 fi~. The corresponding bond angles are now y2 = 141 ~ and Yl = 152 ~. If we make the usual assumption that to a first approximation the M n - O - M n exchange depends only on these angles and if we use Havinga's data again to estimate strength of the exchange, we find 0~ = 10.5 K, 02 = - 7 7 K, TN= 98 K, 0v-~--56 K. Thus it is easier to account for the magnetic properties in terms of this model than in terms of purely steric effects. Such steric effects might of course play a role in preventing cubic packing, thus allowing the Jahn-Teller mechanism to operate at full strength[23], distorting the lattice further. The principal value of this type of phenomenological analysis is in relating crystallographic changes to magnetic properties. In the heavy-rare-earth orthomanganites it is clearly the moderate reduction in the lattice parameter c (compared, say, to the orthoferrites), rather than the sizable expansion along b, which is primarily responsible for the conspicuously low transition temperatures and the occurrence of purely antiferromagnetic ordering. This is not the case in the hexagonal forms of these compounds, though, where planes of Mn 3+ ions are spaced further apart along the c-axis and where the triangular magnetic ordering in the planes seems to favor antiferromagnetism[3]. T h u s both forms of these

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compounds are quite different from the corresponding orthoferrites and orthochromites. Acknowledgement- We wish to thank J. F. Miller for preparing the starting materials, Ralph Smith and R. D. Baxter for their assistance in the magnetic and electric measurements, respectively, and R. P. Kenan for helpful discussions. We also wish to thank Prof. R. Pauthenet for sending us the results of his measurements on the rare-earth ordering temperatures in the hexagonal forms of the rare-earth manganites. REFERENCES 1. Y A K E L H. L., K O E H L E R W. D., B E R T A U T E. F. and F O R RAT F., A cta crystaIlogr. 16, 957 (1963). 2. B E R T A U T E. F., F O R R A T F. and F A N G P., Compt. Rend. 256, 1958 (1963); I S M A I L Z A D E I. G. and K I Z H A E V S. A., Soviet Phys.-solid St. 7, 236 (1965). 3. P A U T H E N E T R. and V E Y R E T C., J. de Phys. 31, 65 (1970). 4. W A I N T A L A. and C H E N A V A S J., Compt. Rend. 264, B168 (1967); Mat. Res. Bull. 2, 819 (1967). 5. Y O U N G A. P., R O B B I N S P. B. and S C H W A R T Z C. M., p. 262 and LaMORI P. N. p. 321 In High Pressure Measurements (Edited by A. A. Giardini and E. C. Lloyd), Butterworths, Washington (1963). 6. C O P P E N S P. and E I B S C H U T Z M., Acta crystallogr. 19, 524 (1965); also M A R E Z I O M., R E M E I K A J. P. and D E R N I E R P. D., Acta crystallogr- B26, 300, 2008 (1970). 7. BRUSSET H., G I L L I E R - P A N D R A U D H. and B E R D O T J. L., Bull. Soc. Chim. France 8, 2886 (1967). 8. SUBBA RAO G, V., W A N K L Y N B. M. and RAO C. N. R., J. Phys. Chem. Solids 32, 345 (1971). (In contrast to the usage of the present paper, these authors refer to the normal hexagonal forms of the compounds as orthomanganites.) 9. W O O D V. E., B R O G K. C., A U S T I N A. E., M I L L E R J. F., J O N E S W. H., V E R B E R C. M., C O L L I N G S E. W. and B A X T E R R. D., Annual Technical Report to Advanced Research Projects Agency, Contract D A A H 0 1 - 7 0 - C - 1 0 7 6 (June 1971), A D 726201; also A U S T I N A. E., M I L L E R J. F. and W O O D V. E., pp. 786-794 in Proc. 9th Rare Earth Research Conf., Blacksburg, Va., October 1971 (National Technical Information Service, U.S. Dept. of Commerce, Springfield, Va. 22151; CONF-711001). 10. B U D W O R T H D. W., H O A R E F. E. and PRESTON J., Proc. R. Soc. Lond. A257, 250 (1960). 11. BATES L. F., Modern Magnetism, p. 133, Cambridge University Press, England ( 1951). 12. B E R T A U T E. F., M A R E S C H A L J . , D E VRIES G., A L E O N A R D R., P A U T H E N E T R., R E B O U I L LAT J. P. and Z A R U B I C K A V., I E E E Trans. Magnetism 2, 453 (1966). 13. D W I G H T K. and M E N Y U K N., Phys. Rev. 119, 1470 (1960); N I E L S E N O. V., J. de Phys. 32, C1-51 (t971).

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