Magnetic structure of binary fluorides containing Mn2+

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Magnetic structure of binary fluorides containing Mn2+

S.J. Pickart, H.A. Alperin, R. Nathans

To cite this version:

S.J. Pickart, H.A. Alperin, R. Nathans. Magnetic structure of binary fluorides containing Mn2+.

Journal de Physique, 1964, 25 (5), pp.565-566. �10.1051/jphys:01964002505056500�. �jpa-00205829�

565.

MAGNETIC STRUCTURE OF BINARY FLUORIDES CONTAINING Mn2+ (1)

By ^{S. J.} PICKART and H. A. ALPERIN

U. S. Naval Ordnance Laboratory, ^Silver Spring,

Maryland and Brookhaven National Laboratory, Upton, ^{New York.}

R. NATHANS

Brookhaven National Laboratory, Upton, ^{New York.}

Résumé.

La configuration ^des spins ^des fluorures binaires ayant pour formule chimique XMnF3 (X = Na, Rb, ^{Cs et} NH4) ^{a été} étudiée par diffraction neutronique. ^{La maille antiferro-} magnétique ^des composés ^{avec X}

Na, ^{Rb et} NH4 ^{est du même} type que celle trouvée pour

KMnF3 (type G). ^Pour CsMnF3, qui ^est hexagonal, les couches Mn2+ sont arrangées ^en séquence antiparallèle ^le long ^{de l’axe} c, ^{avec une} direction des spins perpendiculaire ^{à l’axe.}

Abstract.

Spin orderings ^{of the} binary fluorides with chemical formula XMnF3 (X

Na, Rb, Cs, ^and NH4) ^were investigated by ^neutron diffraction. The antiferromagnetic ^{lattices of the} compounds ^{with X}

Na, Rb, ^and NH4 ^{are found to be of the same basic} type ^{as found in} KMnF3 (G-type). ^In CsMnF3, ^{which is} hexagonal, ^{the Mn2+} layers ^are arranged ^{in an} antiparallel

sequence along ^the c-axis, ^{with the} spin direction normal to the axis.

LE JOURNAL ^DE PHYSIQUE ^TOME 25, ^MAI 196fit,

Introduction.

Binary fluorides of the type XMnF (X

Na, Rb, Cs, ^and NH4), ^{which were} investigated previously by ^{means of} x-ray [1, 2],

electron spin ^resonance [3, 4] specific ^heat [5], ^and magnetization [6, 7] measurements, ^{have been} reported ^{to be} antiferromagnetic. ^The present study reports powder ^neutron diffraction measu-

rements on these compounds undertaken to con-

firm the antiferromagnetism ^{and to} study ^details

of the magnetic ordering. Some information was

also obtained concerning ^{the nuclear} structures.

Experimental.

^The measurements were taken

on powdered samples, ^{which were} kindly ^furnished by ^{M. Shafer.} Temperatures intermediate bet-

ween 4.2 oK and 77 oK were attained by heating

the sample holder, ^{which was insulated from the} liquid ^{helium bath} by ^{thin teflon} sheets, ^or by pumping ^on liquid nitrogen. ^{In this} way ^the

sample ^{could be held at} any desired temperature,

which was measured by ^{a carbon} resistance thermo- meter in contact with the sample ^{holder. For}

small temperature changes, ^the sample ^{came into} equilibrium ^after about one-half hour.

Structure information.

All of the compounds

can be considered as based on perovskite.

RbMnF3 ^and NH4MnF3 ^{are cubic} (tolerance ^ratios

1.00 and 0.97, respectively), ^while NaMnF3 ^and CSNInF ^have tolerance ratios outside the cubic range. The Na compound ^is orthorhombic [1]

and has a pseudomonoclinic ^{cell derived from a}

(1) ^Work performed under the auspices of the U. S.

Atomic Energy Commission.

slight distortion of the cubic cell ; ^{the Cs} compound

is hexagonal [2], ^{with the ABCABC} stacking ^{of the}

cubic phase ^{altered to} ABCACB, ⁱⁿ analogy ^with

the hexagonal ^{form of} BaTio3’

In the case of RbMnF3 ^{we observe} only ^cubic perovskite reflections, ^as expected. However, ^if

the tabulated nuclear scattering amplitudes

~ bRb

^bF

0.55) ^are used, ^{there is a small} discrepancy between weak reflections which can

be removed only by making bRb > bF. Indica- tions are that bxb should be increased to about 0. 63.

The hydrogen positions ⁱⁿ NH4IVInF3 ^{should be}

obtainable from the neutron data. Good _agree- ment ~~

0 .044) ^{was obtained} by distributing

the hydrogen statistically ^{in the} twelvefold position 12(1) ^of Pm3m, i.e., along ^{the direction toward the} neighboring ^{F ions. The} N-H distance is 1.14 ~.,

which is slightly larger than that found [8] ⁱⁿ NH4Cl ^and ND4Cl.

The nuclear scattering ^{from the} CsMnF3 ^was

examined to see if any change ^{in the fluorine} para- meters might ^be required. Using ^the x-ray para- meters determined by ^{Zalkin et al.} [2], ^{we obtained}

an R factor of 0 . 053, which, since fluorine is the heaviest scatterer, ^{indicates little} necessity ^for change.

Although ^{the neutron data for} NaMnF3 ^are

still being analyzed, ^{we note that none} of the weak reflections that required ^an orthorhombic cell in the x-ray ^{case were} observed. However, contrary ^to

the x-ray data, ^{several of the} reflections with mixed indices that require doubling ^{the monoclinic}

cell are quite strong, indicating ^{that the} depar-

tures from the ideal perovskite ^structure may ^be

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01964002505056500

566

large. Because of the large ^{number of} overlapping lines, ^further single-crystal measurements will

probably ^be necessary ^to determine the structure.

IVIagnetie scattering. ^{- The} antif erromagnetic ordering ⁱⁿ RbMnF 3, N aMnF 3, ^and NH4MnF3 ^was

found to be of the " G-type " ^or (-7t7t7t), ^{as classified} by Gersch and Koehler [9]. ^{In this} arrangement

the lattice of Mn ions is doubled in three directions and each positive spin ^is surrounded by ^{six nega-}

tive neighbors. This is the structure previously

observed [10] ⁱⁿ KMnF3.

In the case of RbMnF ^{it is} impossible ^{to deter-}

mine the spin ^direction because of the cubic sym-

metry. ^The magnetic form factor previously

determined for Mn2+ in ^a variety ^of compounds [11] gave ^a good ^{fit to the data. The Neel} tempe-

rature as determined by disappearance ^of magnetic scattering ^{is 82} OK, ⁱⁿ good agreement ^with spe- cific heat measurements [5], ^{and the} temperature dependence ^of the sublattice magnetization closely

follows a Brillouin function.

Although ^the magnetic scattering ⁱⁿ NH4MnF3

exhibits the diffraction symmetry ^{of the} G-type structure, irregularities ^were observed in the rela- tive intensities which indicate either ^an unusual form factor dependence ^{or the} presence of a

multiaxis structure. These irregularities ^{were dif-}

ficult to put ^{on a} quantitative basis because of the

errors caused by ^the large background arising ^trom

the spin ^diffuse scattering ^{of the} hydrogen ; ^{it is} planned ^{for this reason to} repeat ^the measurements

on a deuterated sample. ^The temperature depen-

dence of the scattering ^{is also} quite ^{anomalous in}

this _case, showing ^a nearly ^linear relationship up to the Néel point ^{of 84} OK, ^{which is somewhat} higher than that given by resonance measure-

ments [12].

It was hoped at the outset that the spin ^direction

could be determined uniquely ⁱⁿ NaMnF3, ^{in view}

of the lowered symmetry. However, ^the depar-

ture from cubic symmetry proved ^{to be too small}

to provide the necessary resolution, ^and again single crystal measurements are indicated. The sublattice magnetization ^{follows a Brillouin} curve, and a Néel point ^{of 60 OK was} determined, ⁱⁿ good agreement ^{with the lower of the two transition} points reported ^for this material [3]. ^{No evidence}

was found for any higher transition, except ^that

an appreciable amount of short range order seems

to persist above the Neel point.

A spin ^{structure had been} proposed ^for CsMnF3

on the basis of anisotropy calculations [4], ^which

consists of an antiparallel sequence ^{of the} hexa-

gonal ^Mn2+ layers (which ^are approximately c/6 apart) ^{with the} spin ^axis lying within the layer.

It was found that this model is the only possible

one consistent with the neutron diffraction data.

The temperature dependence falls off somewhat faster than a Brillouin function, ^{with a Néel} point

of 64 OK, ⁱⁿ agreement ^with previous ^data [12].

A systematic discrepancy ^{with the} magnetic ^inten-

sities was noticed when calculations were made with the Mn2+ form factor ; this could be removed

by using ^a form factor close to, ^but slightly higher

than the free-atom one for Mn2+ (3d~) ^calculated by Freeman and Watson [13].

Discussion

Pr RUNDLE. - Nous avons essayé ^{de corr6ler}

les f requences ^de vibrations de translations de

aux distances N-H - F, ^mais peu d’exemples

étaient disponibles. Avez-vous obtenu une bonne distance N-H...F dans NHMnF ?

Dr PICKART. - Sur la base de notre mod6le

statistique ^de NH MnF ^{et avec les} parametres qui

resultent du raffinement des intensit6s obtenues a I’h6lium liquide, ^la longueur de liaison N-H est de 1,14 ^{A et} celle de H-F de 1,84 A.

REFERENCES [1] ^SIMANOV (Y.), ^BATSANOVA (L.) ^{and KOBA} (L.), ^J.

Inorg. Chem., ^{U. S. S.} R., 1957, 2, 207.

[2] ^ZALKIN (A.), ^LEE (K.) and TEMPLETON (D.), ^{J. Chem.}

Physics, 1962, 37, ^697.

[3] ^TEANEY (D.), ^BLACKBURN (J.) and STEVENSON (R.),

Bull. Am. Phys. Soc., 1962, 7, 201.

[4] ^LEE (K.) and PORTIS (A. M.), ^{Bull. Am.} Phys. Soc., 1962, 7, 612.

[5] ^MORUZZI (V.) and TEANEY (D.), ^{Bull. Am.} Phys. Soc., 1963, 8, ^382.

[6] McGUIRE (T. R.), ^{Bull. Am.} Phys. Soc., 1963, 8, ^55.

[7] LEE (K.), ^PORTIS (A.) ^{and WITT} (G.), ^{To be} published.

[8] ^LEVY (H.) and PETERSON (S.), Phys. Rev., 1952, 86,

766.

[9] ^GERSCH (H.) and KOEHLER (W.), ^J. Phys. ^Chem.

Solids, 1958, 5, ^180.

[10] ^SCATTURIN (V.), ^CORLISS (L.), ^ELLIOTT (N.) ^and

HASTINGS (J.), ^Acta Cryst., 1961, 14, 19.

[11] ^CORLISS (L.), ^ELLIOTT (N.) ^{and HASTINGS} (J.), Phys.

Rev., 1956, 104, ^924.

[12] ^TEANEY (D.), Private communication.

[13] ^FREEMAN (A.) and WATSON (R.), ^Acta Cryst., 1961,

14, 231.