Article
Reference
Crystal growth and x-ray structure of metastable a-KCoPO
4LUJAN PEREZ, Marcos Luis, KUBEL, Frank, SCHMID, Hans
Abstract
Crystal of KCoPO4 were obtained by growth in a gel of tetramethoxysilane/H2O at 64 Deg.
The hexagonal crystals have space group P63, a 18.206(1), c 8.5135(8) .ANG., Z = 24. At.
coordinates are given. The structure is isotypic with a-KZnPO4. It comprises an ordered three dimensional network of alternating CoO4 and PO4 tetrahedra, which has rings of six tetrahedra of six tetrahedra in the xy plane. These rings from tunnels in the [001] direction, where the K ions are located. Upon heating, the compd. undergoes a phase transition at .apprx.565 Deg; on cooling, the phase transition occurs at 449 Deg, where it transforms into another structure which is yet unknown. The hexagonal structure is a metastable phase.
LUJAN PEREZ, Marcos Luis, KUBEL, Frank, SCHMID, Hans. Crystal growth and x-ray structure of metastable a-KCoPO4. Zeitschrift für Naturforschung. B, 1994, vol. 49, no. 9, p.
1256-1262
Available at:
http://archive-ouverte.unige.ch/unige:31348
Disclaimer: layout of this document may differ from the published version.
1 / 1
Crystal Growth and X-Ray Structure of Metastable a-KCoP0
4Marcos Lujan*, Frank Kubel, Hans Schmid
Departement de Chimie Minerale, Analytique et Appliquee, Universite de Geneve, 30 Quai Ernest~Ansermet, CH~1211 Geneva, Switzerland
Z. Naturforsch. 49b, 1256-1262 (1994); received May 4, 1994
Potassium Cobalt Orthophosphate, Crystal Structure, Crystal Growth, Metastable Phase, Phase Transition
Crystals of KCoP04 were obtained by growth in a gel of tetramethoxysilaqe/water at 64
oc
lp.e hexagonal crystals have space group P63 , a= 18.206(1), c = 8.5135(8) [A], V= 2443.8(4) [A3], Z = 24. The structure was solved by single crystal X-ray diffraction methods. The struc- ture is isotypic with a~KZnP04. lt comprises an ordered three dimensional network of alter- nating CoO, and P04 tetrahedra, which has rings of six tetrahedra in the xy plane. These rings form tunnels in the [001] direction, where the potassium ions arc located. Upon heating, the compound undergoes a phase transition at about 565 'C; on cooling, the phase transition occurs at 449 °C, where it transforms into another structure which is yet unknown. This indicates that the hexagonal structure is a metastable phase.
Introduction
Phosphates of the type M1M11P04 , where M1 stands for a monovalent cation (Li, Na, K, Cs, Rb) and MJI for a divalent cation (Mn, Fe, Ni, Co, Cd, Sr, Ba, Pb), are known to show ferroic properties- [1-3]. Phosphates in which Mu is a transition metal ion with unpaired electrons may exhibit also magnetoelectric effects [4, 5].
TI1e structures of many phosphates of this type are known [2, 3, 6]. Most of the Li+ and some Na+
phosphates crystallize in the olivine type. The tridymite type is found in structures with large M1 ions (K, Rb, Tl, Cs). Arcanite and beryllonite types are also found in this family of phosphates [7].
For KCoP04 different descriptions for the unit cell are found in the literature [1, 2, 3, 7]. This may be due to the difficulties encountered in the growth of single domain crystals. In this paper we show that KCoP04 actually exists in two forms at room temperature: a metastable form called a- KCP, and a stable form called y~KCP. Phase tran~
sitions of these two forms have been studied. A method for growing a~KCP crystals is described and the crystal structure of these has been deter~
mined by X~ray methods.
*
Reprint requests to Dr. Marcos Lnjan.Crystal Growth of a-KCP and y-KCP High temperature flux growth of y~KCP
Crystals of y~KCP were obtained by a method similar to that given by A. Kryukova et al. [8], who reacted MC12 (M
=
Ni, Pd) with M'3P04 (M'=
Na, K) in a molten phase to precipitate M'MP04
phosphates. In the present study the starting materials were anhydrous CoC12 and K3P04 .
When a mixture of these compounds is heated, the following reaction takes place:
CoCh + K3P04 ---. KCoP04 + 2 KC!.
Complete dissolution of the reacting materials was ensured by adding KCI.
Experimental. - A mixture of CoCl2 , K3P04
and KCI was prepared with the final molar ratio KCP:KCl
=
1:3. The mixture was placed into a sealed platinum crucible, to avoid evaporation of the KCI, and heated to 1100 oc_ After soaking for 3 h at that temperature, the flux was slowly cooled (3 °C/h) to 800 °C and finally to 20 °C. Deep blue needle shaped crystals separated from the flux upon dissolving it in hot water. The size of the largest crystals obtained was 0.5x0.5x2.0 mm3 in the form of a parallelepipedic prism elongated in one direction.Analysis. - The cobalt content of the crystal was analysed by complexometric methods (EDTA);
found: 30.46%, calculated 30.53%. The crystals have been identified by comparing their X~ray
powder diffraction pattern to the one obtained for
y~KCP synthesized by thermal treatment of a~
KCP (cf Fig. 2 b).
0932-0776/94/0900-1256 $06.00 © 1994 Verlag der Zeitschrift fiir Naturforschung. All rights reserved.
'
M. Lujan et al. · X-Ray Structure of Mctastablc a-KCoP04
Examination of the crystals. both by polarised light mieroscopy and X-rays, showed that they are almost invariably twinned. The domains can clearly be seen in one crystallographic direction only; for this purpose, one has to view the crystal in the direction parallel to the bisector of the ob- tuse angle formed by the facets parallel to the long side of the crystal. Under observation in the other directions the crystal appears to be single domain.
Growth of a-KCP from aqueous solurion
Very small spherulitic crystals can be obtained from aqueous solution by a method first described by H. Besset et al. [Y]. A dilute solution of CoCh is slowly added tu a concentrated solution of K2HP04 at 70-90 °C. Then the mixture is stirred for a few hours, maintaining the same tempera- ture. Thus a- KCP is obtained as a deep blue pow- der, the grains being less than 3 )lm in diameter.
Toda et al. [10] obtained crystals of about 7 pm by optimising the [CoCb ]: [K2HP04 ] ratio and leaving the gently stirred mixture for 120 h at 90 °C. This seems to he the maximum size of a- KCP crystals that can be obtained under the con- ditions given above. If the mixture is left for a few days, a-KCP transforms completely into the mono-hydrated form KCoP04 · H20. This method is very simple and offers the possibility of growing a-KCP below the phase transition. Therefore, we searched for a method to stabilize growth of n- KCP crystals in aqueous solution.
Experimental. - After several modifications of the method, we found that adding EDTA to the K2HP04 solution significantly increases the size of the crystals. A 400 m! solution was prepared con- taining 2.5 M of K2HP04 and about 5 mM of EDTA. This solution was heated to 90 °C and, keeping this temperature, 200 m! of a 0.3 M CoC12 solution was very slowly added with gentle agi- tation. The largest crystals obtained by this meth- od, blue hexagonal needles, were 0.03 x 0.03 x 0.1 mm' in size. Addition of EDT A to the K2HP04 solution has also the effect of catalysing the trans- formations of a-KCP into the mono-hydrated form, this transformation taking place in about 14 has compared to a few days without EDTA.
Analysis. - The crystals were identified by cum- paring their powder diffraction pattern to that given in the literature [10] and hy analysing the Co content by complexometric methods.
Growth of a-KCP in a gel obtained from tetramethoxysilane!water
Cobalt potassium phosphate is slightly soluble in water. Its solubility was estimated to he less
1257 than
w-•
M by analysing the Co2- concentration in the aqueous solution under the synthesis con- ditions given above. TI1is low solubility explains spherulitic growth of KCP in aqueous solution, be- cause growth takes place at high supersaturation [11].Crystal growth in gels is a method suitable for growth of very insoluble crystals. 'lbe gel reduces nucleation and stabilizes crystal growth; the qual- ity of the crystals obtained is also generally better than that of crystals grown from aqueous solutions [12]. Therefore, we tried to grow crystals of KCP in a gel of tetramethoxysilane (TMS)/water.
Ammonium phosphates of nickel and cobalt were grown in a silica gel by P. Kurz [13]. who obtained e. g. Ni2+ phosphates as apple-green, long needles. These crystals were not identified com- pletely, and represented probably a hydrated form.
Cobalt phosphates were grown under the same conditions, but in this case reddish-purple spherul- itic crystals were obtained. Ibis result shows that crystal growth of cobalt phosphates is unstable even in gels. Considering also the results described above, one can foresee that EDTA may also stabi- lize the growth in the gel as the medium is basi- cally a water solution.
.Experimental. - A gel was prepared by adding 7.2 g of TMS to 40 m! of a solution containing I 00 mM of CuCl2 · 6 H20 and 5mM of EDTA. Both liquids were mixed with a magnetic stirrer until only one phase was visible. Three 20 ml test tuhes were filled with 7 ml of the mixture and were left at room temperature until gelling was completed ( -24 h). Then the test tubes were put into a water bath kept at 65
cc.
To each tube 8 ml of a solution 2.5 M in K2HP04 and 2 mM in EDTA was added.For more details on the preparation and proper- ties of "TMS gels" see the paper published by H.
Arend et al. [14].
A few hours after the addition a bluish sol formed in the upper part of the gel which ex- tended slowly to the lower part, until all the gel became turbid. After a few days crystals of n-KCP began to grow in the gel, mainly in the upper part.
As the crystals grew, the sol around them disap- peared and the gel became clear. After 15 d the sol disappeared completely, and crystals of u-KCP became visible in the gel; they were regularly dis- tributed over the whole gel volume. They were separated from the gel mechanically. Largest crys- tals are 0.6 x 0.4 x 0.4 mm3 in size ( cf Fig. 1 a) and form regular polyhedra with well developed facets.
In one of the tubes no u-KCP crystals were found, but small, pink, plate-shaped crystals of KCoP04 · H20 were obtained. They were iden-
1258 M. Lujan et al. · X-Ray Structure of Metastable a-KCoP04
Fig. 1. a) a-KCP crystals obtained in a gel generated by hydrolysis of tetramethoxysilane, b) crystal of a-KCP partially dissolved, showing the presence of the gel inside the crystal.
tified by comparing their X-ray powder diffraction pattern to that measured by Toda et al. [10]. Crys- tals of KCoP04 · H20 were found also in the other tubes but to a much lesser extent.
Analysis. - The crystals of a-KCP were ana- lysed by means of a polarised light microscope and appeared to be single domains, but inclusions of the gel were visible inside the crystal. In an at- tempt to test the presence of the gel in the interior of the crystals, a few of these were dissolved in 0.1 M HN03. Figure 1 b shows an a-KCP crystal which was partially dissolved. Around the crystal a residual network can be seen, which has the same shape as the original crystal. The upper right hand pan of the figure shows the entire residual network of a crystal that has been completely dis- solved. This residual structure must be the skel- eton of the gel that has been included throughout the whole volume of the crystal during growth.
This phenomenon is aJso observed in other crys- tal synthesized in gels like CaC03 [15]. Neverthe- less, the crystals seem to keep their short and long range periodicity as their X-ray powder diffraction pattern is identical to that obtained for a-KCP crystals synthesized in aqueous solution. where there is no gel present.
DTA Studies
The crystals were studied by gifferential lhermal
~nalysis. A sample of264 mg of a-KCP was heated
to 1100 °C, cooled to 300
o c
and heated again to 1100a c.
A first order phase transition was ob- served at 576o c
during the first heating, with a- KCP transformed to a orthorhombic structure that was called ~-KCP by G. Engel [7]. On cooling the phase transition occurs at 449o c.
This large hys-teresis indicates a high activation energy for the
a) 10
8
0 0 0 ... 6 -...
.0 en -0 4
2
0
10 20 30 40 50 60 80 2 Theta [deg]
g J
b)8l 7
8
60
::::5
~4 en
3 2 1
0 I I
10 20 30 40 50 60 70 80 2 Theta [deg]
Fig. 2. X-ray powder diffraction patterns of KCP synthe- sized in aqueous solution: a) before (a-KCP) and b) after heating through the phase transition (y-KCP). No- tice that the two patterns are different. Pattern b) corre- sponds to the stable form of KCP.
M. Lujan et al. · X-Ray Structure of Metastable a-KCoP04 1259
transition. During the second heating the tran- sition begins at 558
oc.
Figure 2 gives the X-ray powder diffraction patterns of a-KCP before (a) and after (b) the DTA. The two structures are clearly different.A DTA was also carried out on a sample of y- KCP synthesized by the flux method (above) and X-ray powder diffraction patterns were measured before and after the DTA. In this case both pat- terns are the same and identical to that obtained for a-KCP synthesized from aqueous solution and measured after DTA (cf Fig. 2b). This means that the structure obtained in water solution (a-KCP) represents a metastable phase which transforms to the stable one ( y-KCP) upon heating through the transition phase and subsequent cooling. The y-KCP form is obtained by high temperature flux growth because the growth temperature is higher than the temperature of the phase transition.
Upon cooling through the transition the stable y- KCP form is adopted hy the crystals. This tran- sition is also the cause of twinning of the y-KCP crystals.
Determination of the Structure of the a-Form ofKCP
Precession photographs of a single crystal of a- KCP showed a hexagonaL 6/m Lane symmetry and the extinction law I
=
2n for the 000 I reflec- tions. Two space groups arc possible in this case.P63 and P63/m; P63 was chosen, because it gave a significantly better results.
A single crystal of a-KCP of the type shown in Fig. 1 (size; 0.23x0.23x0.40 mm) was selected for crystal structure determination. The cell param- eters were determined using 25 reflections with 18°<0<25°, and refined by diffractometer tech- niques. A least-squares refinement was also ap- plied giving final values a
=
b=
18.206(1) and c=
8.5135(8)
A.
The density of the crystals was meas- ured by pycnometric methods, a value of 3.14 g/cm3 was obtained, giving Z = 24 formula units per unit cell.
The intensity of 9096 reflections was measured on a CAD 4 automatic diffractometer (radiation Mo, A = 0.71073
A)
at 300 K. Among these reflec- tions, 2055 were independent, and 1991 were con- sidered observed according to the criterion I> 3 a( I). Two standard reflections were moni-tored at 1 h interval during the measurements; the maximum variation of the intensity of these reflec- tions was 4.4%. Absorption corrections were ap- plied based on the morphology of the crystal.
Table I. Crystallographic data for a-KCoP04 .
C:rysta 1 system
Space group ,
Unit cell dimensions [A]
Cell volume [A')
Density (measured) [g/cm3] Density (X-rays) [g/cm3]
Formula units (Z)
F(OOO)
Reflections measured
Observed rcncctions (l>3u) Radiation source
R
s
Rw(L1Q ),riru'mnx [ e/A 3]
Hexagonal P63 (N" 173)
a= 1R.206(1) c = 8.5135(8) c/a = 0.4676 2443.8(4) 3.14 3.147 24 2232
2055 (from 9096) 1991
Mo(K,J 0.025 0.024 2.236
0.76. +2.10
Table II. Atomic positional and isotropic displacement parameters [A 2].
Atom X V ,, Uiso
Co(1) 0.00113(3) 0.33046(3) 0.2012(2) 0.01 Ofl( 2) K( I) 0.003H0(5) 0.52414(6) 0.017H(2) o.o2m(3) P(1) 0.00951 (7) 0. ]()5%(6) 0.2247(2) 0.0090( 4) 0(1) 0.0467(2) 0.1886(2) 0.0577(4) 0.017(1) 0(2) 0.0831(2) 0.1931(2) 0.3410(4) 0.018(1) 0(3) 0.1159(2) 0.4050(2) 0.2803(4) 0.020( I) 0(4) 0.1233(2) 0.0528(2) 0.2353(4) 0.017(1) 0(5) 0.1302(2) 0.5461 (2) 0.2150(4) 0.019(1) Co(2) 0.17274(3) 0.16366(3) 0.3361(2) 0.0116(2) P(2) 0.17522(6) 0.49408(6) 0.2148(2) 0.0099(4) K(2) 0.19666(6) 0.33432(5) 0.0141(2) ().()219(4) 0(6) 0.2019(2) 0.4804(2) 0.0482( 4) [1.020( 1) 0(7) 0.2517(2) 0.0383(2) 0.2(,72( 4) 0.015(1) 0(8) 0.2561 (2) 0.5396(2) 0.3167(4) 0.017(1) 0(9) 0.2774(2) 0.2506(2) 0.2366( 4) 0.021(1) 0(10) 0.3139(2) 0.4027(2) 0.2344(4) 0.020(1) 0(11) 0.3206(3) 0.3357(2) 0.4804(6) 0.044(2) K(3) 0.33155(5) 0.18045(5) 0.0221(2) 0.0194(3) P(3) 0.33572(6) 0.33893(6) 0.3065(2) 0.0107(4) Co(3) 0.34741(3) 0.51347(3) 0.3264(2) 0.0109(2) 0(12) 0.4280(2) 0.3680(2) 0.2692(4) 0.030(1) 0(13) 0.4424(2) 0.0774(2) 0.2864(4) 0.019(1) 0(14) 0.4664(2) 0.2205(2) 0.2078( 4) 0.018(1) Co(4) 0.50171(3) 0.32384(3) 0.3287(2) 0.0109(2) P(4) 0.50609(6) 0.16368(6) 0.2148(2) 0.0094(4) 0(15) 0.5320(2) 0.1516(2) 0.0491-1(4) 0.017(1) 0(16) 0.5866(2) 0.2070(2) 0.317')(4) [).015(1)
K(4) 1/3 2/3 0.0444(2) 0.0155(4)
K(5) 1/3 2/3 0.5451(2) 0.0159(4)
K(6) 0 0 0 0.0176(4)
1260 M. Lujan et al. · X-Ray Structure of Metastable a-KCoP04
Table I summarises the general results and other data.
The positions of the Co and P atoms were found from density maps calculated by direct methods.
Calculations were made using the program Xwl 3.2 [16]; the K and 0 atoms were located from series of difference-Fourier syntheses. A full-ma- trix, least-squares refinement of the atomic co-or- dinates with isotropic thermal parameters con- verged to a residual R
=
0.036. Anisotropic refinement gave a final R = 0.025 (Rw = 0.024). No additional symmetry elements were found when the final atomic positions were checked with the program MISSYM. Table II gives the results of the refinement as fractional atomic positions and anisotropic thermal parameters*. The atomic posi- tions are given after standardization of the struc- ture with the program STRUCTURE TIDY (17).Description of the Structure of a-KCP
The structure of a-KCP is isotypic with the structure of a-KZnP04 [18], Fig. 3 shows a projec- tion of the structure along the c axis. Basically, the
* Further crystal and refinement details may be ob- tained from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen (Germany) on quoting the depository number CSD 58227. the names of the author and the literature quotation.
~ Co04 tetrahedron
,A.
down }
.A
up P04 tetrahedronQ
PotassrumFig. 3. Projection of the structure of a-KCP along the c axis.
structure consists of a three dimensional network of alternating P04 and Co0 4 tetrahedra which has rings of six tetrahedra in the al\b plane. These rings in turn form tunnel which run parallel to the c axis. Two types of ring can be easily dis- tinguished; those in which the P04 and Co04 tetrahedra point, alternatively, up (U) and down (D), relative to the a/\b plane (UDUDUD), and those in which three adjacent tetrahedra are pointing in one direction and the other three are pointing in the other direction (UUUDDD). The potassium atoms are located in the tunnel and are distributed in layers parallel to the a/\b plane. The layers are at a distance of about 4.26
A
in the di- rection of the c axis. The potassium atoms can beTable Ill. Bond distances [A] and angles [degrees] for Co04 and P04 tetrahedra.
Bond distances [A]
Col-0 11 1.897(5) Co3-0 lO 1.955(3) p 1-01 1.539( 4) P3-0 10 1.531(5) Co1-013 1.966( 4) Co3-0 l5 1.972(4) P1-02 ] .536(6) P3-0 11 1.502(5) Co 1-0 3 l.956(6) Co3-05 1 .954(3) Pl-04 1.531 (6) P3-0 12 1.521( 4) Co l-0 7 1.961 (5) Co3-0 8 1.947(6) Pl-0 7 1.544(3) PJ-09 1.536(3)
Co2-01 1.979( 4) Co4-0 l2 1.944(4) P2-0 3 1.535(4) P4- 0 13 1.537(6)
Co2-02 1.955(6) Co4-0 14 l.950(3) P2-05 1.532(6) P4-0 1-l 1.531 (5) Co2-04 1.950(6) Co4-0 16 1.952(4) P2-06 1.528( -l) P4-0 15 1.532(4)
Co2-09 1.959(3) Co4-06 1.970(4) P2-08 1.545(5) P4-0 16 1.545(3)
Bond angles (degrees)
03-Co 1-07 I 09. (2) 05-Co3-0 15 103.8(2) 0 1-Pl-02 10 .2(3) 09-P3-0 I 1 108.9(2) 03-Col-0 11 117.9(2) 05-Co3-08 120.4(2) 01-PI-04 109.6(2) 09-P3-0 12 110.4(2) 03-Co 1-0 13 I 10.3(2) 05-Co3-0 10 100.3(3) 01-PI-07 111.3(2) 09-P3-0 10 109.5(2) 07-Co1-0ll I 03.1 (2) 0 15-Co3-0 8 102.9(2) 02-PI-04 1 1 l.0(2) OII- P3-012 111.6(2) 07-Col-0 13 108.2(2) 015-Co3-0 LO I 18.3(2) 02-Pl-0 7 108.3(3) 0 11- P3-0 10 108.3(3) 0 I I -Co I -0 13 107.0(2) 08-Co3-0 10 111.8(3) 04-P L-07 108A(3) 0 12-P3-0 10 108.3(3) OI-Co2-09 113.2(1) 06-Co4-0 16 10-l.l(l) 03-P2-0 5 110.0(3) 0 13- P-l-0 16 109.6(2) 0 I-Co2-0-l I 0-l.5(2) 06-Co4-0 1-l 103.4(2) 03-P2-06 109.2(2) 0 13-P4-0 15 110.2(3) Ol-Co2-0 2 105.1(2) 06-Co4-0 12 113.3(2) 03-P2-08 110.0(3) 0 13- P-l-0 14 109.7(2) 09-Co2-04 113.9(1) 0 16-Co4-0 14 118.3(2) 05-P2-06 I 1 0.5(3) 0 16- P-l-0 15 108.2(2) 09-Co2-02 113.3(2) 016-Co4-0 12 110.3(2) 05-P2-08 109.2(2) 0 16- P4-0 14 109.0(3) 04-Co2-02 106.0(2) 014-Co4-0 12 I 07.4(2) 06-P2-08 108.0(3) 0 15-P4-0 14 11 0.2(3)
-~
M. Lujan el al. · X-Ray Structure of Metastahle a-KCoP04 1261 considered to have a co-ordination number CN =
6, the K-0 distances varying from 2.65 to 2.95
A.
The phosphate tetrahedra have a very regular form. T11e 0- P- 0 angles deviate very little from 109.5° (about 1°, mean deviation), the P-0 dis- tances vary from 1.502-1.545
A.
1he cobalt tetra- hedra are much more irregular, the mean devi- ation of the 0- Co --0 angles being about 6° from 109.5°. The Co-O distances range between 1.897 and 1.972A.
A more detailed list of bonding dis- tances and angles is given in Table Ill.It is interesting to note that the atom 0(11) has a relatively high isotropic displacement parameter in this structure, the anisotropic displacement par- ameters indicating an oblate shaped spheroid with its axis nearly parallel to the c axis. 0(11) is bonded to Co(1) and to P(3) atoms, with bond dis- tances of UlY7 and 1.502
A,
respectively, which arc the shortest bonding distances for these two types of bonds (cf Table Ill). This indicates that the Co(l)-0(11) and 0(11)-P(3) bonds are rela- tively stronger than the other bonds of the same type. In Fig. 3 the Co(l)04 tetrahedra can easily he recognized because they are pointing down- wards relative to the a·~-b plane; the P(3)04 tetra- hedra are pointing upwards. The two types of tetrahedra are joined by 0(11).Discussion
Crystals of KCP synthesized from aqueous solu- tion have a metastahle structure referred to as a- KCP and isotypic with a-KZnP04 . The structure of the stable form of KCP (y-KCP) at room tem- perature is still unknown. Crystals of y-KCP are heavily twinned because of a first order phase
transition occurring at about 565 oc_ According to B. Elouadi er al. [2], it has the trydimite structure, but so far no complete structural determination is available.
Crystal growth of KCP from aqueous solution is greatly stabilized by complexation of Co2+ ions with EDTA. 1hc mechanism of the stabilization is still unknown but complexation must play an important role as EDTA strongly complexes Co2+ in the conditions of the synthesis. lt would be interesting tu see if EDTA has the same effect on growth of other slightly soluble salts which contain a ion prone to form complexes with EDTA in water solution.
The a-KCP structure belongs to the polar point group P63 , which is acentric and chiral. As a consequence pyroelectricity, piezoelectricity, op- tical rotatory power, etc. are permitted; the com- pound is also a potential magnetoelectric material [19]. Magnetic ordering of the Co2' ions may oc- cur with the magnetic interaction pathway running through a phosphate tetrahedron, the shortest path between two Co atoms being through the oxygen located on the edge of a phosphate tetra- hedron (Co-0-0-Co). This interaction should he very weak and the magnetic ordering should occur at relatively low temperatures. Similar inter- actions have also he en proposed for Nbi370 13I3r and for different sodium phosphates of Fe3• [20].
Acknowledgement
Thanks arc due to R. Cros for the drawing, to E. Burkhardt for the photographs, to H. Lartigue for the DTA measurements, and to the Swiss National Science f<oundation for financial support.
1262 M. Lujan et al. · X-Ray Structure of Metastable a-KCoP04
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