The crystallographic and magnetic structure of RbCoCl3 · 2 D2O were determined mainly by means of neutron diffraction measurements. Below TN = 2.79 K the magnetic moments are ordered in a canted antiferromagnetic pattern. A meta-magnetic phase transition is observed at unusually small field values (H = 18 Oe at T = 2 K)

Flokstra, J.

Frikkee, E.

Jonge, W.J.M. de

Vermeulen, A.J.W.A

English

The crystallographic and magnetic structure of RbCoCl3 · 2 D2O were determined mainly by means of neutron diffraction measurements. Below TN = 2.79 K the magnetic moments are ordered in a canted antiferromagnetic pattern. A meta-magnetic phase transition is observed at unusually small field values (H = 18 Oe at T = 2 K).</p

Vermeulen, A.J.W.A.

De Jonge, W.J.M.

Pure OAI Repository

Crystallographic and magnetic structure of RbCoCl<sub>3</sub> · 2 D<sub>2</sub>O

Flokstra, Jakob

de Jonge, W.J.M.

University of Twente Research Information

Physica 98B (1980) 205-208 © North-Holland Publishing Company LETTER TO THE EDITOR CRYSTALLOGRAPHIC AND MAGNETIC STRUCTURE OF RbCoCI3 • 2 D 2 0  A.J.W.A. VERMEULEN :~, E. FRIKKEE Energieonderzoek Centrum Nederland, Postbus 1, 1755 ZG Petten, The Netherlands J. FLOKSTRA Technische Hogeschool Twente, Postbus 217, 7500 AE Enschede, The Netherlands and W.J.M. DE JONGE Technische Itogeschool Eindhoven, Postbus 513, 5600 MB Eindhoven, The Netherlands Received 30 October 1979 The crystallographic and magnetic structure of RbCoCI 3 • 2 D20 were determined mainly by means of neutron diffrac- tion measurements. Below T N = 2.79 K the magnetic moments are ordered in a canted antiferromagnetic pattern. A meta- magnetic phase transition is observed at unusually small field values (H = 18 Oe at T = 2 K). 1. Introduction The interpretation of  the magnetic behaviour of  the linear-chain antiferromagnet RbCoC13 • 2 H20 (TN = 2.97 K) is severely hampered by the lack of  knowledge about the orientation of  the magnetic moments in the ordered phase. In a previous paper on the results of  dynamic-susceptibility measu.rements [1 ] a magnetic order within the chains was proposed similar to that in CsCoC13 • 2 aq. (aq. = H20,  D20) [2,3], i.e. a canted antiferromagnetic ordering with a net ferromagnetic moment along the chain direction (c-axis). The relative orientation of  the ferromagnetic components was assumed to be parallel within a bc-plane and anti- parallel in adjacent bc-planes. The same canted anti- ferromagnetic model (with ferromagnetic components close to the c*-direction) has been used by McElearney and Merchant [4] to analyse the results of  their zero- field susceptibility and heat-capacity measurements. They conclude that the intrachain exchange inter- ~t Present address: Philips Medical Systems Division, Best, The Netherlands. action is predominantly antisymmetric, and that con- sequently the angle between successive moments is almost 90 ° . During the past years we have investigated RbCoC13 • 2 H20  and the deuterated compound with a variety of  experimental techniques in order to obtain a more complete picture of  the magnetic properties. In this paper we report on the magnetic structure of  RbCoC13 • 2 D20 (TN = 2.79 K), which was determined by neutron diffraction experiments and additional sus- ceptibility and NMR measurements. 2. Experimental results The powder sample was prepared from single crys- tals grown at 35°C from a solution of  COC12 and RbCI (molar ratio 4 : I)  in D 2 0  and DC1 (20%) by slow evaporation. The neutron diffraction experiments were per- formed on the powder diffractometer installed at the HFR at Petten. The diffractometer was used in its standard operation mode, i.e. with a Cu (111) mono- 205 206 A.J. I¢.A. Vermeulen et al. / Crystallographic and magnetic structure o f  RbCoCI3 . 2 D20 chromator yielding a neutron wavelength X = 2.573 A, a pyrolytic graphite filter and 30' horizontal collima- tors in front of  the monochromator and the detector. Powder diffraction patterns were recorded at T 300 K, 4.2 K and 1.2 K in the range 0.02 A -  ~ < sin 0/X < 0.36 )~-1. The first two diagrams were used to complement and extend the previous X-ray struc- ture determination at 300 K [5], whereas the mag- netic structure in the antiferromagnetic phase was derived from the latter diagram. For the structure refinements, Rietveld's prof'de-analysis method [6] was applied with the following values for the scatter- ing lengths [7]: 0.708 (Rb), 0.250 (Co), 0.958 (C1), 0.6672 (D) and 0.5803 (O), in units 10 -12 cm. For the refinement of  the magnetic structure the Co 2+ form factor given by Watson and Freeman [8] was used. In the analysis of  the diagram recorded at 300 K, the starting values for the cell parameters and the frac- tional coordinates of  the heavy atoms were taken from the X-ray structure determination of  the hydrated compound at room temperature [5]. To obtain adequate starting values for the deuterium positions, it was assumed that deuterium bonds exist between the oxygen and chlorine ions. In that case the O-C1 distance should be about 3.2 A and the D - O - D  bonding angle approximately 110 °. Only two chlorine neighbours appear to satisfy these conditions. One of  these belongs to a neighbouring chain in the bc-plane, the other one to the nearest CoO2 C14-octa- hedron within the chain. The above criteria have also been used by McElearney and Merchant to indicate the most probable hydrogen-bond pattern in the hydrated compound [4]. The expected isomorphism with RbCoC13 • 2 H20 (space group C 2/c) was con- firmed in the refinement procedure, which yielded the final structural parameters listed in table I. The coor- dinates of  the heavy atoms are in good agreement with tile X-ray results [5], with the exception of  the y-coordinate for Rb which must be negative (cf. table I in ref. [5]). The analysis of  the diffraction pattern recorded at 4.2 K shows that the structure is the same as at 300 K, and that the contraction of  the unit cell between 300 and 4.2 K is slightly anisotropic: Aa/a = 0.010, Ab/b  = Ac /c  = 0.006 (cf. table I). In the powder diagram recorded at 1.2 K two weak additional peaks are observed at small scattering angles. These magnetic reflections may be indexed as (]-01) and (101). A comparison with the magnetic space groups that are compatible with the crystallographic space group C 2/c shows that only two of  those, viz. Cp 2/c and Cp 2'/c, lead to reflections with both (h + k) and l odd. Refinement computations were carried out for both cases and the most probable struc- ture was selected on basis of  the final magnetic R- Table I Structural parameters of RbCoCI 3 • 2 D20 at room temperature and at 4.2 K. Standard deviations based on statistics only are given in parentheses, in units of the last decimal place. R is defined in terms of observed and calculated intensities Yi : R = 100 X .::i I r°bs - gcalcL/r~iY~i bs. 300 K C 2/c 4.2 K C 2/c x y z x y z C o  Rb Cl(1) C1(2) D(1) D(2) O a b C R 0 0 0 0 0 0 1 1 1 1 - ~  0 ~ - ~  0 1 1 0 0.200(2) ~ 0 0.203(2) 0.1403(4) 0.243(2) 0.0517(9) 0.1428(4) 0.243(2) 0.0541(8) 0.1042(8) -0.266(2) 0.280(1) 0.1038(7) -0.271(2) 0.283(1) 0.0900(7) -0.420(3) 0.123(2) 0.0904(5) -0.431(2) 0.115(1) 0.0885(10) -0.273(3) 0.160(2) 0.0965(8) -0.274(3) 0.165(2) 15.724(2) A 15.563(2) h 5.6283(6) A 5.5957(5) A 8.748(1) A 8.6961(8) A 118.326(7) ° 118.444(6) ° 6.5 6.2 A.J. W.A. Vermeulen et al. / Crystallographic and magnetic structure o f  RbCoCl3 . 2 D20 2 0 7  T a b l e  II O r d e r e d  m a g n e t i c  m o m e n t s  o f  RbCoC-13 • 2 D 2 0  a n d  CsCoCI  3 - 2 D 2 0  [ 3 ] .  H = 0 ,  T = 1.2 K.  RbCoC13  • 2 D 2 0  CsCoCI  3 • 2 D 2 0  #Co 2.4(2) ~t B /~Co 2.5(2)/z B #b 2.3(1)/z B t~c 2.4(2) ~t B tZac 0.7(3) tz B ta a 0.7(2) #B ~a -0 .4(11)  #B #c 0.3(6) #B factor. The best fit was obtained with space group Cp 2'/c (R = 23, to be compared with R = 51 for Cp 2/c) and the magnetic moment  components listed in table II. Further evidence for the occurrence of  the magnetic space group Cp 2'/c was obtained from NMR measurements. Symmetry considerations, com- bined with the experimental result that the local field is zero at the Rb positions and unequal to zero at the Co sites, permit the groups Cp 2'/c and C 2'/c'. The latter group has to be rejected on the basis of  the neutron diffraction results. The small component/aac of  the magnetic moment  in the ac-plane was calculated from the total moment  and its component  along the b-axis (table II). Due to the large uncertainty in the components/a a and ttc the direction ofllac cannot be determined from the pow- der-diffraction data. Additional dynamic-susceptibility measurements [9] indicate that the principal axes of  -J r'1 -J ] i~x , i x \ ~/'~.--~L...NL.J ~- ",, , , ;Ix; . . . . .  X/.J Fig.  1. M a g n e t i c  s t r u c t u r e  o f  R b C o C 1 3  - 2 D 2 0 .  the susceptibility tensor in the ac-plane are very close to the crystallographic a and c*-axes, in accordance with the location of  the principal axes in the hydrated compound [4,9]. The direction c* where the suscepti- bility reaches its maximum value, may be identified with the direction ofttac. The combination of the sus- ceptibility and diffraction results yields the magnetic structure in zero field, shown in fig. 1. In each chain a canted antiferromagnetic ordering exists, with a cant- ing angle q~ = (17 -+ 5) ° between the moments  and the b-axis, and ferromagnetic alignment of  the compo- nents Itac- The resulting ferromagnetic moments  of  the chains are aligned parallel in the bc-plane, whereas the relative orientation in adjacent bc-planes is antiparallel. Dynamic-susceptibility measurements on RbCoC13 • 2H20 display the same characteristics as were found for the deuterated compound so it may be expected that the magnetic structures of  both  materials are the same. 3. Discussion The present determination of the magnetic struc- ture shows that RbCoC13 • 2 D20 is not a linear-chain system with almost exclusively Dzyaloshinsky-  Moriya exchange interactions, in contrast to the con- clusion of McElearney and Merchant for the hydrated compound. The magnetic structure is indeed very sim- ilar to that of  CsCoC13 • 2 D20 [3]. In both systems the canting angle is about 17 °, and the magnetic mo- ments of  the Co2÷-ion are equal within the experi- mental uncertainty (table II). On basis of  the present results it is not possible to distinguish between the two possible orientations of  the magnetic moment  with respect to the ligand octahedron. However, on the analogy of  the magnetic structure of  CsCoC13 • 2H20 [2] it is expected that the moment  is oriented along the direction close to the Co-O-C1(2)  plane. In this context it is noteworthy that in the compounds CsFeC13 • 2 H20 and RbFeCI3 • 2 H20 the magnetic moments are aligned almost parallel to the Fe-CI(I)  direction. Generally, when Co 2÷ and Fe 2+ ions are exposed to the same ligand field their magnetic mo- ments are perpendicular to each other [10]. In spite of  the similarity between the magnetic structures of  CsCoC13 • 2 aq. and RbCoCI3 • 2 aq. in zero field, there is a strikinlz difference between their 208 A.J.W.A. Vermeulen et  al. / Crystallographic and magnetic structure o f  RbCoCla . 2 D 20 behaviour in a magnetic field applied along the weak ferromagnetic component. In RbCoCI3 • 2 H20 and RbCoC13 • 2 D20 a metamagnetic phase transition is observed at unusually small field values, H = 37 Oe and 18 Oe, respectively, at T = 2 K. At the transition, the magnetic moments with a component lie. anti- parallel to the field are reversed, which results in a parallel alignment of~tc, in adjacent bc-planes. In CsCoC13 • 2 H20 the metamagnetic transition involves the same reversal of moments in alternate planes, but takes place at H(T = 0) ~ 2.85 kOe [2]. Obviously, the antiferromagnetic interaction between the chains in neighbouring bc-planes, which are separated by Rb ions, is very much weaker than the coupling between the ac-planes in the Cs compound. This is quite remarkable because the analogous interionic distances and bonding angles in the superexchange-interaction path C o - C I ( o - X - C 1 0 ) - C o  (X = Cs, Rb) differ by at most 3.4%. Possibly the different magnetic properties are related to different symmetry properties of the 3d wave functions in the ligand field, as the Co2+-ion is surrounded by a strongly distorted cis-octahedron in CsCoC13 • 2 aq. and an almost regular trans-octahedron in RbCoC13 • 2 aq. A detailed report about the mag- netic properties of RbCoC13 • 2 aq. will be published soon [9]. Acknowledgements We are indebted to Dr. J.A.J. Basten for valuable discussions on the neutron-diffraction results and to Mr. J. Boeijsma for preparing the deuterated crystals. References [ 1 ] J. Flokstra, G.J. Gerritsma, B. van den Brandt and L.C. van der Marel, Phys. Lett. 53A (1975) 159. [2] A. Herweijer, W.J.M. de Jonge, A.C. Botterman, A.L.M. Bongaarts and J.A. Cowen, Phys. Rev. B5 (1972) 4618. [3] A.L.M. Bongaarts and B. van Laar, Phys. Rev. B6 (1972) 2669. [4] J.N. McElearney and S. Merchant, Phys. Rev. B18 (1978) 3612. [5] S. Haxkema and W. van der Graaf, Inorg. Nucl. Chem. Lett. 11 (1975) 813. [6] H.M. Rietveld, J. Appl. Cryst. 2 (1969) 65. [7] C.G. ShuU, Compilation (1972), M.I.T. [8] R.E. Watson and A.J. Freeman, Acta Cryst. 14 (1961) 27. [9] J. Flokstra, G.J. Gerritsma, W.J.M. de Jonge, A.J.W.A. Vermeulen and E. Frikkee, to be published. [10] J.A.J. Basten, Q.A.G. van Vlimmeren and W.J.M. de Jonge, Phys. Rev. B18 (1978) 2179. 