Growth of large K2MnF4 single crystals and characterization by X-ray and neutron diffraction investigations

Jourttal

of’ Crystnl

Growth 21 (1974) 82-84 c_:North-Holland

GROWTH

Publishitzy

OF LARGE K,MnF,

BY X-RAY AND NEUTRON

Co.

SINGLE

CRYSTALS

DIFFRACTION

AND CHARACTERIZATION

INVESTIGATIONS

K. BITTERMANN II. Ph.,~sikalisches

lnstitut

der UnirersitBt,

7 Stuttgtrrt

1. Gerwrn~~ Fed. Rep.

and G. HEGER Institrct fiir Kristcrllqyrcrphie Received

6 April

der Utlicrrsitiit.

1973; revised

manuscript

Tiibitz,yrn, Germnuj, received

Fed. Rep.

15 September

1973

Single crystals of K2MnF, up to 100 mm3 were grown by the Bridgman method using a mixture of KI’ and 35 mol”,, KMnF,. To characterize these large crystals, neutron diffraction investigations were carried out. In agreement with X-ray diffraction measurements the layer type crystal structure of K2MnF, was found to have a significant anisotropy of the mosaic spread.

1. Introduction

2. Crystal growth of incongruently

K,MnF, is a typical example of a two-dimensional antiferromagnet’.2). It crystallizes in a face centered tetragonal structure, which belongs to the space group D~~(I4jmmm) with lattice constants a = 4.19 A and c = 13.30 A3). To grow extremely pure K,MnF, single crystals, KF and KMnF, were used as starting materials. The first compound, KF, was obtained from Merck, Darmstadt (West-Germany) in “suprapur” quality. K F single crystals were grown by the Bridgman method in a highfrequency heated furnace in a nickel ampoule. The second compound, KMnF,, was obtained starting from MnF, of technical quality (approximately 95 :/; pure) from Riedel de Ha&, Hannover (WestGermany). For purification purposes it was pressed to the form of small disks and sublimed. With the sublimate it was possible to grow MnF, single crystals by the Bridgman method using a graphite ampoule. Repeated growth of such a crystal resulted in very pure MnF, starting material. Always the upper part of the crystals was separated because it was enriched with impurities. A MnF, crystal treated this way seventeen times was used as a new material to grow KMnF, single crystals: KF and MnF, were carefully dried and mixed according to the equation of reaction KF+MnF, = KMnF,. KMnF, single crystals were grown by the Bridgman method in a high frequency heated furnace using a nickel ampoule.

Bridgman growth of incongruently melting compounds can be done by decreasing the temperature of the interface according to the crystallization curve of the phase diagram in question4). The composition of the melt changes during crystallization such, that the amount of the compound of which the grown single crystal consists decreases in the melt. As a consequence of the constant cross-section of the growth vessel the growing of the crystal falls continuously back behind the transport. This means that the growing of the crystal is slowed down while the speed of transport is kept constant. Regarding this, the Bridgman technique is difficult to handle in the case of incongruently melting compounds. The difficulties can be overcome to a certain extent with a large amount of melt of the starting materials. In this case it is possible to work with a linear transport rate. Large single crystals of the wanted incongruently melting compound can thus be obtained. Crystal growth was done by the Bridgman method in a vacuum vessel inside of which a graphite ampoule could be heated by high frequency current. Inside ot the graphite ampoule there was a crucible made of nickel in which the crystals were grown. This nickel crucible had an inside diameter of 20 mm and was about 80 mm long. One end of the crucible was shaped to a conical tip with an angle of 100 The single crystals of KF and KMnF, were crushed to small pieces 82

melting K2MnF,

OF

GROWTH

LARGE

K,MnF,

and mixed in the composition ratio KF+35 mol% KMnF,. This mixture was carefully vacuum dried with decomposing teflon at 450 “C for 12 h ; 46.7 g of this composition were used for one run. The growth vessel was evacuated for several hours at a pressure of 8 x lO-‘j Torr and then filled with 100 Torr of extrapure argon. Due to the form of the high-frequency coil the melting and annealing region have temperatures of 1000 “C and 650 “C, respectively. According to experience the moving rate was chosen to 0.7 mm/h and the temperature gradient was 50 “C/cm. Therefore a linear lowering of the temperature of about 3.5 “C/h was obtained. Clear and faint pink K,MnF, single crystals grew starting in the tip of the nickel crucible. With quite a sharp border to the K,MnF, region a solid mixture of K,MnF,-KF followed further to the end of the solid cylinder. 3. Investigations

of the K,MnF,

TABLE

Li Na Rb Mg Sr

content IS IO 14 88 5

K,MnF, and K,Mn,F, were found mixed together in different ratios. Forming larger single crystals additional KMnF, parts always could easily be seperated. As these three compounds mentioned have identical u-axes which have the same orientation in the solid mixture no splitting of the (hk0) reflections could be observed. Studying the (001) line in the reciprocal lattice, three systems of (001) reflections were found according to the three parallel c-axes: ckMnF3 = aKMnFJ = 4.16A, ckzMnF4 = 13.21 A, ~k+,,,~~, = 21.66 A. Different intensity ratios between these systems of (001) reflections indicate different composition ratios of the three components in the solid mixtures. Starting from a composition ratio of KF and 66 mol”/, KMnF, additionally to the KMnF, part a clear solid mixture of K,MnF, and K,Mn,F, was found. Starting from a composition ratio of KF+40 mol “/,

r;, 0'

,I IO'

209

30"

LO"

50'

60"

70"

DJffroction Angle 20 -

Fig. meter width

80"

90"

100”

110”

120'

-

I.

Resolution curve of the single crystal neutron diffractoP32 at the FR2 reactor and measured half maximum of the K2MnF4 sample crystal (I., = 1.0346 A).

1

in ppm of a K2MnF., Al Cr Fe Ni

83

CRYSTALS

single crystals

The clear K,MnF, monocrystalline part below the solid mixture had a length of about 12 mm and a diameter of 20 mm. The crystals cleave readily in planes perpendicular to the c-axis, they are homogeneous and show a light pink colour. With a Beckman atomic absorption spectrometer the content of impurity cations of the alkaline, alkaline-earth, and iron group elements was measured (listed in table 1). In order to determine the lattice constants at room temperature, X-ray Weissenberg photographs of small K,MnF, single crystals were used: a, = (4.1664 f 0.0008) A, c0 = (13.217 f 0.006) A. At composition ratios of the starting materials different from that given above, also clear crystalline parts were obtained which would not be distinguished in colour. In order to test these parts which were expected to be K,MnF, single crystals neutron diffraction measurements have been performed on samples up to 2 cm3. Depending on the composition ratio of the starting materials, three components as there are KMnF,,

Impurity

SINGLE

0.5 22 22 66

sample cu Ag Zn Pb

crystal 0.2 0.3 0.3 12

Fig. 2. x 3.5.

Photograph

showing

a KZMnF4

sample

crystal.

Magn.

84

K. BITTERMANN

KMnF,, crystals were obtained with single crystalline parts of KMnF, and K,MnF, in a sheet arrangement. From the growth described above (KF+ 35 mol”:; KMnF,), a K,MnF, sample crystal of 5 x 4 x 3 mm3 was used to study the mosaic spread by neutron diffraction measurements. In fig. 1 the half width of the reflexes (001) with I = 4, 6, 8, 10, 12, 14, 16 and (l&O) with hk = 20, 40, 60, 22, 42, 62, 44 is plotted together with the resolution curve of the single crystal diffractometer P32 (FR2, KFZ Karlsruhe)h). According to Dachs’) the mosaic spread of the K,MnF, sample crystal was computed. The medium half width of the mosaic spread perpendicular and parallel to the cleavage planes was found to be II~,,,~,,, = 0.8” and M P(OO/) = 2.44”, respectively. Such an anisotropic mosaic spread is typical for layer type structures as in the case of K,MnF,.

AND

G. HEGER

to Dr. K. R. Albrand, MPI fur Festkiirperforschung, Stuttgart, and Dr. G. Brand& Institut fur angewandte Festkorperphysik, Freiburg i.Br., who carried out the X-ray measurements and to Mr. K. Bahr for his help with regard to technical questions. One of us (G.H.) thanks the Gesellschaft fiir Kernforschung, Karlsruhe, especially the RB department, for the availability of the neutron diffraction facilities at the FR2 reactor. The financial aid of the Deutsche Forschungsgemeinschaft and the Bundesminister fur Bildung und Wissenschaft is gratefully acknowledged. References 1) Hironobu

2)

ldeka

33 (1972) 393. R. J. Birgenau.

and Kinshiro

Hirakawa,

H. J. Guggenheim

J. Phys. Sot. Japan

and G. Shirane,

Phys. Rev.

Acknowledgements

B l(1970) 2211. 3) J. C. Cousseins, Rev. Chim. Mineral. 1 (1964) 573. 4) A. Neuhaus, H. G. Holz and H. D. Klein, Z. Physik. Chem. NF 53 (1967) 163. 56 (1952) 576. 5) G. Wagner and D. Balz, Z. Elektrochem.

One of us (K.B.) is grateful to Professor H. Pick for the possibility to work in his institute. Thanks are due

6) G. Egert, H. Dachs. Justierung atom (unpublished, 1971). 7) H. Dachs, Z. Krist. 115 (1961)

eines Neutronenmonochrom80.