Synthesis, structure and magnetic susceptibility of KCrP₂O₇, a potential antiferromagnetic magnetoelectric

Article

Reference

Synthesis, structure and magnetic susceptibility of KCrP

O

, a potential antiferromagnetic magnetoelectric

GENTIL, Sandrine, et al.

Abstract

During the synthesis by flux evapn. in KCl at 930° of KCrIIPO4, oxidn. of Cr(II) was obsd., leading to the formation of KCrIIIP2O7. The crystal lattice of KCrP2O7 is monoclinic, space group P21/c with a = 7.3470(7), b = 9.9090(10), c =8.1730(8)Å, β = 106.806(11)°, Z = 4 and V

= 569.59(10) Å3, F(000) = 516, 1981 obsd. reflections with I > 3σ(I), R = 0.023, Rw = 0.026.

The magnetic susceptibility was measured between 3.6-300 K at different magnetic field strengths. The neg. value of the Curie-Weiss temp. and the behavior of magnetic susceptibility indicate an antiferromagnetic spin order in this compd. with a Neel temp. TN = 8.5 K.

GENTIL, Sandrine, et al . Synthesis, structure and magnetic susceptibility of KCrP

O

, a potential antiferromagnetic magnetoelectric. Ferroelectrics , 1997, vol. 204, no. 1, p. 35-44

DOI : 10.1080/00150199708222186

Available at:

http://archive-ouverte.unige.ch/unige:31138

Disclaimer: layout of this document may differ from the published version.

1 / 1

Ferroelectrics, 1997, Vol. 204, pp. 35-44 Reprints available directly from the publisher Photocopying permitted by license only

© 1997 OPA (Overseas Publishers Association) Amsterdam B.V. Published under license under the Gordon and Breach Science Publishers imprint.

Printed in India.

SYNTHESIS, STRUCTURE AND MAGNETIC SUSCEPTIBILITY OF KCrP

0

A POTENTIAL ANTIFERROMAGNETIC MAGNETO ELECTRIC

S. GENTIL, D. ANDREICA, M. LUJAN, J.-P. RIVERA, F. KUBEL and H. SCHMID

Departement de Chimie Minerale, Ana/ytique et Appliquee, Universite de Geneve, 30, quai Ernest-Ansermet,

CH-1211 Geneve 4, Switzerland (Received in final form 15 May 1997)

During the synthesis by flux evaporation in KCl at 930°C of KCr ¹¹ P04 an oxidation of chromium II was observed leading to the formation of chromium pyrophophate KCr m P207 •

The crystal lattice of KCr Iii P20_7, was found to be of monoclinic centrosymmetric space group P2₁₁c with a=7.3470(7), b=9.9090(10), c=8.1730(8)A, /9= 106.806(11)⁰, Z=4 and VM=569.59(10)A³• The magnetic susceptibility has been measured between 3.6 and 300K at different magnetic field strengths. The negative value of the Curie-Weiss temperature and the behaviour of magnetic susceptibility indicate an antiferromagnetic spin order in this compound with a Nee! temperature TN=8.5 K.

Keywords: Potassium pyrophosphate of chrmnium; potassium orthophosphate of chromium;

crystal structure; crystal growth; magnetic properties; magnetoelectric effect

1. INTRODUCTION

Recently several alkaline metal orthophosphates M+M2+P0₄ (M²+ = Mn, Fe, Co, Ni) were explored for their potential ferroic (ferroelectricity, magnetic ordering and possibly magnetoelectric effects) behaviour lll. Up to now the potassium orthophosphate of chromium II (KCrP04) has not been studied.

Indeed, during the synthesis of KCrP04, we observed in most of the cases an oxidation of this orthophosphate (Cr²+) into pyrophosphate KCrP207

35

36 S. GENTIL et a/.

(Cr3+) and metallic chromium (Cr^0). Recently, Galelica-Robert et a/Pl calculated the lattice parameters of KCrP207 from the d values published by Medvedev eta/. [JJ The latter authors prepared this compound and studied the structure by powder X-ray diffraction and IR spectroscopy.

In this paper, different methods for the synthesis of KCrP207 are studied.

The structure of the pyrophosphate was determinated on one of the crystallites synthesized (using the methods presented in this paper). The results obtained on the magnetic properties of KCrP207 powder samples are also presented.

2. SYNTHESIS OF KCrP207

Different experimental variants of synthesis at high temperature are available for obtaining the pyrophosphate.

2.1. High Temperature Synthesis of KCrP207 in KCI Flux

Following Kryukova's method f⁴¹, a mixture of anhydrous CrCh, K3P04

and KCl (molar ratio CrCh, K3P04 : KCl = 1 : 3) was placed into a sealed platinum crucible for avoiding evaporation of KCI. After heating to 950°C and soaking for 5 h, the flux was slowly cooled (2.SOC/h) to 750°C and finally to 20°C. Thus we obtained a green powder with needle shaped crystals which were separated by dissolving the flux in hot water.

2.2. Oxidation Process of Cr2+ in KCI Flux

By a method similar to that of item 2.1, it was possible to mix anhydrous CrCh with K3P04, with molar ratio CrCh, K3P04 : KCl =I : 3, in a molten phase. This mixture was heated to 1050°C in a sealed platinum crucible which has a small hole in the lid for assuring pressure equilibration and for allowing oxygen to oxidize Cr¹¹to Crm. After soaking for 4h, the flux was slowly cooled (4°C/h) to 750°C and finally to 20°C.

The formation of chromium metal powder and small parallelepiped shaped crystals of chromium III pyrophosphate, dispersed in an amorphous phase, suggest a reaction of oxidation of chromium II.

2.3. Oxidation of Potassium Orthophosphate to Potassium Pyrophosphate

KCrP207 can be obtained by reaction between KCrP04.xH20 (SJ (synthe- sized from aqueous reaction) and molten KCI. The mixture was put into an

l

1

J

37

open platinum or zirconium crucible and kept at 900°C for 24 h. Due to the evaporation, pyrophosphate in the form of a powder, contaminated by chromium metal, was obtained. The formation of single crystals, even with slow cooling, was never observed by this technique.

The hydrated form of KCrP04 has been synthesized by a method similar to one of Bassett et a/. 161 In order to avoid oxidation of Cr2+ ions, the synthesis was performed under nitrogen atmosphere. It consist in slowly adding a 0.3 M CrC12 solution to a solution of K2HP0⁴ 2 M which was kept at 70°. In this way a green powder of KCrPO; xH20 was obtained. A thermal treatment of this powder at 900°C give a crystallized powder of an anhydrous orthophosphate KCrP0_4.

2.4. Reaction with Dipotassium Hydrogen-Phosphate in KCI Flux

A mixture of anhydrous CrC13, K2HP04 and KCl (20 g% in KCl) was placed into an open zirconium crucible and heated to 840°C for 15 h. This synthesis was very successful for obtaining a homogeneous powder without any contamination.

3. STRUCTURE DETERMINATION 3.1. The Structure of KCrP₂0₇

In a first work, Medvedev et a/. 131 reported X-ray powder data and IR- spectroscopic data for some ternary pyrophospates M+CrP207 (M+ =Li, Na, K, Rb, Cs, NH4, Tl, Ag). Until now single crystal data are only reported for NaCrP207 I7J and TlCrP207 181 . In agreement with Medvedev's 131 results, the pyrophosphate KCrP207 is isotypic with TlCrP207 and not with NaCrP207 .

The cell dimensions, in space group P2dc, calculated by Gabelica-Robert et a/.121 are: a=7.337(5), b=9.892(9), c=8.151(6) and (3= 106.532(1).

We decided to refine the structure, using a single crystal. A STOE diffractometer with MoKa radiation (.X=0.71073A) was used for data collection.

The unit cell was found to be monoclinic with dimensions similar to those reported 121 for powders: a=7.3470 (7), b=9.9090 (10), c=8.1730 (8), (3=

106.806 (11), VM=569.59 (10) and Z=4.

The intensity of 2056 reflections out of 4403 measured reflections was considered for the resolution and refinement of the structure. Absorption

38 S. GENTIL et a/.

correction was made by analytical integration. All calculations were made with the program Xtal 3.2 191 • The crystallographic data are summarized in Table I.

The position of the Cr and P atoms was found from the density maps, calculated with the program Xtal 3.2. Subsequent series of difference Fourier synthesis yielded the potassium and oxygen positions. A full-matrix refinement, first made with isotropic and then with anisotropic atomic displacement parameters, converged to a final R = 0.023 (Rw = 0.026). The fractional atomic positions and isotropic thermal parameters are given in the Table II. A drawing of the structure is shown in Figure 1.

3.2. The Structure of KCrPO ₄¹⁵¹

Small crystals (average size of86x85x10 Jlm) ofKCrP04 were obtained by recrystallization of the dehydrated green powder in a KCl flux at 1050°C and was soaked for 4 h. The flux was slowly cooled (at a rate of 2. 7°Cjh) to 750°C.

The crystallites of KCrP04 in the form of parallelepiped shaped prisms, elongated along the a-axis, were obtained by the above mentioned synthesis.

Since the crystals are too small for single crystal diffraction work, the powder X-ray diffraction technique was used for solving the structure. The intensity of reflections in the range h = ± 11, k = ± 7 and I=± 12 was measured on a CAD4V%CF X-ray diffractometer with CuKa radiation (.A= 1.54181A).

TABLE I Crystallographic data for KCrP₂0₇ Crystal System

Space group

Unit cell dimensions [A) and degrees

Cell volume [A³)

Density (X-rays) (gfcm³]

Formula unitz (Z) F(OOO)

Reflections measured R internal

Observed reflections (I > 3a) Radiation source

R Rw

s

(~Q)min./maxJe/A³)

monoclinic P2J(c a=7.3470(7) b=9.9090(10) c=8.1730(8) ,8= 106.806(11) 569.59(10) 3.0906 4 516 4403 1.7%

1981 Mo 0.023 0.026 3.042 -0.68/0.69

KCrP207 39

TABLE II Atomic positional and isotropic displacement parameters [A²] (e.s.d.'s are given in brackets)

Atom xfa yfb zfc Uiso

K(1) 0.32159(7) 0.32044(5) 0.44643(6) * 0.0189(1)

CR(l) 0.73533(4) 0.10049(3) -0.23839(4) * 0.00659(8)

P(l) 0.63247(7) 0.09638(5) 0.33161(6) * 0.0074(1)

P(2) 0.94091(7) 0.13644(5) 0.19219(6) * 0.0074(1)

0(1) 0.9519(2) 0.2852(1) 0.2442(2) * 0.0109(4)

0(2) 0.1403(2) 0.0792(1) 0.2349(2) * 0.0116(4)

0(3) 0.8191(2) 0.1059(2) 0.0134(2) * 0.0149(4)

0(4) 0.8393(2) 0.0579(1) 0.3154(2) * 0.0109(4)

0(5) 0.5042(2) 0.0051(1) -0.2211(2) * 0.0094(4)

0(6) 0.5817(2) 0.2640(1) -0.2471(2) * 0.0108(4)

0(7) 0.6426(2) 0.0913(2) 0.5155(2) * 0.0148(4)

The unit cell (determined from 11 reflections with 10° < () < 40°) was found to be monoclinic centrosymmetric with space group P2Ifc and a= 10.564(4), b=6.3709(8) and c= 11.488(5), (3= 117.15(2)^0, VM=684.1(4)

A

density of the powder estimated by pycnometric method is 2.89 g/cm³ which gave Z = 6 formula units per unit cell.

4. MAGNETIC PROPERTIES

Magnetization measurements were carried out on powder samples in order to determine whether the magnetic moment of Cr³ + undergo ordering at low temperature.

A vibrating sample magnetometer (model 155 of Princeton Applied Research) was used Tor this purpose. A liquid helium cryostat was allowed to cool the samples down to 3.6 K. The samples were cooled without applied magnetic field. The magnetization was measured, in different magnetic fields, during the heating process at about 6 K/min. The intensity of the magnetic field was measured using a Hall Effect Gaussmeter.

The molar susceptibility, was computed from the usual expression:

_ Ms(MW)s XM- H

w

a s

where Ms and (MW)s are the sample magnetization and molecular weight respectively, Ha the applied field and ws the weight of the sample.

40 S. GENTIL et a/.

OK

@Cr

• p oO

c

1 - - - b

-l.

FIGURE I Atomic arrangement in KCrP207.

KCrPz01 41 TABLE III Atomic Displacement Parameters

Vll U22 U33 Ul2 U13 U23

K(l) 0.0168(2) 0.0224(2) 0.0160(2) -0.0013(2) 0.0021(2) -0.0009(2) CR(l) 0.0068(1) 0.0064(1) 0.0066(1) -0.0002(1) 0.0019(1) -0.0000(1) P(l) 0.0077(2) 0.0079(2) 0.0069(2) -0.0012(2) 0.0026(2) -0.0001(2) P(2) 0.0073(2) 0.0076(2) 0.0078(2) 0.0004(2) 0.0026(2) -0.0003(2) 0(1) 0.0093(6) 0.0081(6) 0.0154(7) 0.0004(5) 0.0038(5) -0.0018(5) 0{2) 0.0079(6) 0.0098(6) 0.0179(7) 0.0017(5) 0.0048(5) -0.0003(5) 0(3) 0.0163(7) 0.0188(7) 0.0075(6) 0.0001(6) 0.0004(5) -0.0016(5) 0(4) 0.0091(6) 0.0116(6) 0.0133(7) 0.0021(5) 0.0050(5) 0.0043(5) 0(5) 0.0091(6) 0.0102(6) 0.0095(6) -0.0026(5) 0.0035(5) -0.0012(5) 0(6) 0.0095(6) 0.0075(6) 0.0153(7) 0.0008(5) 0.0034(5) -0.0006(5) 0(7) 0.0153(7) 0.0222(8) 0.0074(6) -0.0054(6) 0.0042(5) -0.0011(5)

TABLE IV Bond distances [A] and bond angles [degrees] for KCrP207 at room temperature (e.s.d.'s are given in brackets)

bond distances [A]

Kl-01 2.764(1)· KL-02 3.183(2) Pl-07 1.485(2)

Kl-02 3.024(1) CRl-01 1.990(2) Pl-06 1.520(1)

Kl-03 3.059(2) CRl-07 1.932(2) Pl-06 1.525(1)

Kl-04 3.170(2) CRl-02 1.932(1) P2-0l 1.528(1)

Kl-05 2.893(1) CRl-03 1.972(2) P2-02 1,.514(1)

Kl-06 2. 735(1) CRl-05 1.983(2) 02-03 1.508(1)

Kl-05 2.778(2) CRL-06 1.963(1) P2-04 1.615(2)

Kl-06 2.932(2) Pl-04 1.608(2)

bond angles [degrees] for Cr04 and P04 groups

01-CRl-07 90.11(6) 02-CRI-03 90.29(6) 07-Pl-05 114.28(9) 01-CRI-02 97.81(6) 02-CRl-05 88.42(6) 07-Pl-06 113.16(9) 01-CRI-03 92.51(6) 02-CRl-06 172.50(6) 05-Pl-06 107.99(7) 01-CRl-05 173.77(6) 03-CRl-05 87.39(6) 01-P2-02 109.07(8) 01-CRI-06 89.50(6) 03-CRl-06 91.10(6) Ol-P2-03 115.61(8) 07-CRl-02 89.75(6) 05-CR!-06 84.27(6) Ol-P2-04 106.66(9) 07-CRl-03 177.36(7) 04-Pl-07 107.61(8) 02-P2-03 114.35(9) 07-CRl-05 89.97(6) 04-Pl-05 105.80(8) 02-P2-04 105.42(8) 07-CRl-06 88.53(6) 04-Pl-06 107.56(8) 03-P2-04 104.85(8)

The plot of xi} as a function of temperature, Figure 2, shows that at temperatures higher than lOK the compound follows a Curie-Weiss law given by:

where T is the temperature, C ^M is the Curie constant and () is the Curie- Weiss temperature. From the slope of the curve, the calculated effective

100~---~---r---~---~---~---~

m

"'U 80 ~---~

JJ

0

r

(J)

c

(J) (")

m

-i

r -i -<

...

3

40

20

KCrP₂0₇

a

=

I ....

~

.

!§

!;;""

z

j..,

ow

/ '

,, ' _'

H

Without ield cooling

t

0

.

.

^..

' 0 ~~~~~~~~~~~~~~~~~~~~~~~~~~

CD 0

3

^{100 150 200}

...._.

c TEMPERATURE [K]

FIGURE 2

43

magnetic moment is about 3.5 f.LB that is a little bit lower than the spin only magnetic moment of the Cr³ +ion (3.87 JLB).

Several factors allow to affirm than below the Neel temperature TN=

8.4 K the compound ia antiferromagnetically ordered : the shape of the temperature dependence of the susceptibility, the negative value of the Curie-Weiss temperature (0 = -35 K), the same temperature dependence of the inverse susceptibility when measured with and without field cooling of the sample down to 3.6 K, and the absence of a hysteresis loop in the field dependence of the magnetisation.

5. POTENTIAL MAGNETOELECTRIC (ME) PROPERTIES

By admitting that the paramagnetic centrosymmetric monoclinic phase with Shubnikov point group 2/ml ¹ (space group P2J/cl ¹⁾ orders magnetically below the Neel temperature to give an antiferromagnetic monoclinic commensurate phase, the classification of the 122 Shubnikov point groups according to ME types (Tab. II of reference 8) shows us that there exist only two magnetic point groups which satisfy the requirements: 2/m ¹ and 2¹ jm.

Both groups allow the linear ME effect. With the twofold axis taken as the y-axis, the tensor forms ^{(I OJ} are:

Thus the difference in permitted ME tensor coefficients would allow by ME measurements on single crystals to decide between these two groups, the more so since both of them do not allow any type of superposed bilinear ME effect [Ill. Even ME measurements. on polycrystalline samples 1¹²¹, with preliminary cooling through the Neel temperature (annealing) in the presence of simultaneous parallel and mutually perpendicular magnetic and electric fields, would be expected to show the presence or absence of diagonal coefficients.

Acknowledgements

Thanks are due to R. Boutellier for technical help and the Swiss National Science Foundation for support.

44 S. GENTIL et a!.

References

[I) Lujan, M. Ph. D. Thesis (1995). No2776, University of Geneva.

[2) Gabelica-Robert, M. and Tarte, P. (1982). Solid State Chemistry: Studies in inorganic chemistry, 3, 475-478.

[3) Medvedev, A. A., Lavrov, A. V., Chudinova, N. N. and Tananaev, I. V. (1970). Izv.

Akad. Nauk. SSSR, Neorg. Mater., 6, 1650-1656 [English traduction: Inorganic Materials].

[4) Kryukova, A. 1., Korshunov, I. A. and Egorova, L. Y. (1979). Fiz. Khim. Electrokhim.

Rasplavl Tverd. Electrolitov. Tezisy, Dokl. Vses. Konf Fiz. Khim. Ionnyich Raspalov Tverd.

Electrolitov. 7th, 1, 131-132.

[5) Gentil, S. et a!., to be published.

[6) Bassett, H. and Bedweell, W. (1933). J. Chern. Soc., 854-882.

[7) Bohaty, L., Liebertz, J. and Frohlich, R. (1982). Z. Kristallogr., 161, 53-59.

[8) Bensch, W. and Koy, J. (1995). Z. Kristallogr., 210, 455.

[9) Hall, S. D., Flack, H. D. and Stewart, J. M. (1992). Editor of Xtal 3.2: User's Manual.

[10) Rivera, J.-P. (1994). Ferroelectrics, 161, 165-180.

[II) Schmid, H. (1973)./nt. J. Magn., 4, 337-361 also in Freeman, A. J. and Schmid, H. Eds., Magnetoelectric Interaction Phenomena In Crystals, Gordon and Breach Science Publishers, London, New-York, Paris (1975).

[12) Hornreich, R. M. (1974). Int. J. Magnetism, 6, 47.