FRANCIS GALASSO AM) WILDADARBY
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Vol. 66
PREPARATION, STRUCTURE, AND PROPERTIES OF KZNb03F BYFRANCIS GALASSO AND WILDADARBY United Aircraft Corporation, Research Laboratories, East Hartford, Connecticut Received J a n u a r y 88, 196%
An oxyfluoride, KtNbOaF with the K2NiF4 structure, has been repared. Unlike most compounds with this structure, &Nb03F was found to have a c/u ratio greater than the calculate% 3.41 value which satisfies the geometrical conditions of touching spheres. From a study of interatomic distances (obtained from single crystals), it was found that the high c / a ratio was related to an elongation of the anion octahedra in the “c’’ direction. The thermal coefficient of expansion for KziYb03Fwas found to be more than twice as large in the “e” direction as in the ‘La”direction, a fact which may be responsible for the positive temperature coefficient of resistance exhibited by this material.
Introduction In 1955 Balz and Plieth described the K2NiF4 structure and showed its relation to the perovskite structure.l Since that time several ternary oxides and fluorides having the K2NiF4 structure have been reported in the literature and results of a survey of these compounds (presented in Table I with their cell data) revealed that there was a tendency for compounds with the KzNiF4 structure to exhibit a smaller c/a ratio than the calculated 3.41 value which satisfies the geometrical conditions of touching spheres. Recently, however, the oxyfluoride K2n’b03F, of the K2XiF4 type, was prepared at the Research Laboratories and was shown to have a c / a ratio of 3.46. As a consequence of this interesting observation, a study was initiated LATTICECONSTANTS
TABLE I COMPOUNDS STRUCTURE
FOR
a@.)
KzU04 BazPb04 SrzSnOb RbzU04 BazSn04 RbzZnF4 SrzTi04 K2ZnF4 K2NiF4 La2Ni04 Sr2Mn04 Sr2Mo04 Ca 2Mn 04 SrzRuO4 KzMgF4 SrzIrOn SrLa A104 8rzRh0’! CS~UO~
KqNbOaF
4.34 4.30 4.04 4.34 4.13 4.10 3.88 4.01 4.01 3.86 3.79 3.92 3.67 3.87 3.98 3.89 3.75 3 85 4.38 3.96
d.) 13.10 13.30 12.53 13.83 13.27 13.25 12.60 33.02 13.08 12.65 12.43 12.84 12.08 12.74 13.16 12.92 12.50 12.90 14 79 13.67
to investigate in more detail the interatomic distances and the properties observed for K2Xb03F. Experimental Investigations Preparation of KzNbOaF.-Powder samples of KzPibOIF were prepared by heating mixtures of potassium carbonate and niobium pentoxide in a 1:1 molar ratio with a large excess of potassium fluoride in a platinum arucible a t 750” for 24 hr. After the reaction, excess potassium fluoride waa removed by washing the product with water. Single crystals of KzNb03F were obtained when the same mixtures used to prepare powder samples were heated t o 1000” and then slowly cooled to 600’ at a rate of 3O0/hr. When the product was washed with water, small crystals in the shape of thin rectangular plates remained. Chemical Analysis and Density.-The ffuorescent X-ray spectrographic method using a borax bead matrixll was em-
TABLE I1 POWDER X-RAYDIFFRACTIOS DATAFOR &Nb03F hkl
WITH THE
K2NiF4
c/a
Rei.
3.02 3.10 3.10 3.18 3.21 3.24 3.24 3.25 3.26 3.28 3.28 3.28 3.29 3.29 3.31 3.32 3.33 3.36 3.88 3.46
2 3 3 2 3 4 5,6
4 1 7 1 I 5 8 9 10 5 8 2
(1) D. Balz and K. Plieth, 2. Elektrochem., 69, 6,545 (1955). (2) L. M. Kovba, E. A. Ippolitova, Yu. P. Simanov, and Vikt. -4. Spitsyn. Dokl. A k a d . Nauk U S S R , 120, 5, 1042 (1958). (3) R. Weiss and R. Faivre, Compt. rend., 248, 106 (1959). ( 4 ) 0. Schmitz-Dumont and H. Rornefeld, 2. anorg. allgem. Chem., 287, 120 (1956). ( 5 ) 9. N. Ruddlesden and P. Popper, Acta Cryst., 10, 538 (1957). (6) K. Lukaszewicz, Roczniki Chem., 33, 239 (1959). (7) V. A. Rabenau and P. Eckerlin, Acta Cryst., 11,304 (1958). (8) J. J. Randall and R. Ward, J . Am. Chenz. Soe., 81, 2629 (1959). (9) B. Brehler and H. G. 1‘. Winkler, Heidelbwger Beitr. M i n e m I . Fetmg., 4, 6 (1954). (10) J. J. Randall, L. Iiata, and R . Ward, J. Am. Chem. Soc., 79, 266 (1057).
002 101 004 103 110 112 006 105 114 200 202 116 211 107 204 008 213 206 215 118 109 220 222 0010 301 217 224 208 303 310 312 11,10 226
d/n (obsd.)
d / n (calcd.)
6.79 3.78 3.42 2.98 2.80 2.58 2.276 2.248 2.162 1.974 I.898 1.768 1.751
6.84 3.80 3.42 2.97 2.80 2.59 2.278 2.250 2.164 1.978 1.900 I . 767 1.754 1.751 1.712 1.709 1.649 1.494 1.485 1.458 1.418 1 398 1.370 1.367 1.313 1.311 1.294 1 293 I . 267 1.251 1.230
1.712 1.648 1.494 1.459 1.418 1.397 1 370 1.314 1.295 1.267 1.251 1 233
1.228
1.193
1.192
I (obsd.)
5
SW-
S+ 5
WM MW-
S-
w+ M
MW-
M+ M
w+
w w+ www+ w w+ w--
w+
(11) n. F. Fornwalt and J. Komisarek, “8nalytical Chem. in Nuclear Reactur Technology,” TID-7568,PT. l , U.S.A.E.C. (1959).
July, 1962
PREPARATION, STRUCTURE, AXD PROPERTIES O F
I x 104
- 20
40
100
160 220 T E M P E R A T U R E , ‘C
200
340
Fir. l.-AltPrn~.t,inP rurrent, resistmrct of K,NhO,F. 50 c.n.8
ployed to determine potassium content, while wet chemical analysis was used to determine the niobium and fluoride content. Results of these analyses indicated the presence of 33.52% IC, 38.03% Nb, and 7.70% F ; the theoretical values for K:NbOzF are 32.8% K, 39.05% S b , and 7.98y0 F. Density was determined pycnometrically and found to be ?.64 g,/rc. as compared t o 3.68 g./cc. as calculated from the lormula. X-Ray Analysis.-Powder X-ray diffraction photographs were taken using tl 57.3 mm. radius Phillips powder camera with copper K a radiation. The X-ray patterns of powder samples and ground crystals, which wei-e found to be t,he same, were indexed on the basis of B tetragonal unit eel1 of
1319
a = 3.956, c = 13.670. Table I1 presents the indexing data for KzNbOaF. For single crystal X-ray analysis, a long thin rectangular crystal plate was mounted on a Buerger precession camera and molybdenum KCYwas used as the source of radiation. -4diffraction pattern obtained when the X-ray beam entered perpendicular to the crystal plate revealed the fourfold symmetry expected in the hkO net. The crystal then was rotated 90’ and hOE data obtained. Intensities for the hkO and h0Z reflections were visually estimated using a multiple film technique and a calibrated film strip. The similarities in lattice parameters, reflections observed, and the sizes of the constituent atoms for KzNb03F and KZNiF4 resulted in the adoption of the space group I-4/mmm and the atomic ositions found in the K2NiF4 structure for the initial calcuktions. These atomic position parameters were used in the structure factor calculations for K2Nb03F to determine the signs that would be placed before the 9bserved structure factors in order to obtain an hO1 Fourier projection. The electron density map was calculated using the Sly and Shoemaker program written for the IBM 704 computer. Using the atomic parameters taken from the Fourier projection peaks, the structure factors were recalculated, and it was found that the signs obtained for these structure factors were the same m those obtained from the previous calculation. The atomic positions from the Fourier analysis were, therefore, corrected for series-termination effects and used for subsequent refinement calculations which involve minimizing the reliability factor by a trial and error method. The atomic positions adopted in space group I-4/ mmm (No. 139) are the following: two niobium ions are at
TEMPERATURE,
Fig. 2.-Thermal
KJTb03F
O C
expansion data. 0,0,0 and
l/~,l/Z,l/~; four potassium ions are at O,O,zx; O,O,~!K; ~/Z,~/Z,~/Z XK; and ~/z,~/z,~/z - t~ with ZK = 0.350; two fluorine and two oxygen ions are at O,O,zr; O,O,EF; l/z,l/z,l/z ZF; l/z,l/~,l/z - XF where ZF = 0.151 and four more oxygen ions are a t l/2,0,0; O,l/z,O; O,’/Z,~/Z;l/~,O,l/2. For these calculations the fluoride ions were considered t o be randomly distributed onlv in the 0.0.z. etc., positions after considering the thermal expansion data for KgNb03F in the “a” and “c” axial directions. It is possible that there is an ordering of the fluoride ions existing even within the O,O,z positions, but this could not be determined from the present data. Table 111, which contains the‘obsetwad and calculated struoture factors, will be deposited with
+ +
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FRANCIS GALASSO AND WILDADARBY
Vol. 66
powders using a Norelco diffractometer with an attached Tem-Pres heater. Tracings were obtained a t room temperature, 100,300, and 500” for the three compounds,.and also a t TOO, 900, and 1000° for SrzTi04. Due to dissociation of theAe compounds, tr?ciags for K2NiF4 and KtNbQ3F were not obtained a t h g h temperatures. Figure 2 presents the “a” and “c” axis expansion for these three compounds. It was found that the “a” direction thermal coefficient of expansion was nearly the same as the coefficient in the “c” direction for both KtNiFd and SrsTiOd and that the coefficients in both directions are larger for the fluoride than for the oxide, (for K2NiF4 olg = 2.87 X 10-6 and aC = 2.91 X while for SrzTiOl cya = 1.46 X and cyc = 1.44 X On the other hand, the thermal coefficient of expansion for KzNbQaF in the “c” direction is 3.77 X 10-6, which is more than twice the value in the “a” direction, 1.52 X 10-6.
Discussion of Results From Fig. 3 it can be seen that the central cube with cell edge a’ in the K2XiF4structure is the perovskite structure with the A position taken as the origin. Thus, for an ideal structure with -x touching spheres, a’ should equal “a” of the K2NiF4 structure. Using the same reasoning of touching spheres, (B-X), = (E-X), = a/2 and (A-X), = (A-x)& = u/2/Z SO that the distance along the 42) or c/u K2NiF4 “c” axis should be 4 2 should equal 3.41. Results from this study show that the c/a ratio for K2Xb03F is greater than this ideal value, whereas all other compounds in this category have a c/a value less than 3.41. In an attempt to understand the difference between K2Kb03F and the other compounds with this structure, the interatomic distances marked in Fig. 3 were calculated. The results are a = 3.96, a’ = 4.10, (A-x), = 2.72, (A-X)ab = 2.80, and (B-X), = 2.06, (B-X), = 1.98. It can be noted that the large c / a ratio for this compound is due to a‘ being longer than a and the elongated octahedra Fig. 3.-KzNiF4 structure. in the “c” direction, (B-X),/(B-X), = 1.04. If the other compounds presented in Table I have the Library of Congress. The over-all reliability factor calcuelongated octahedra, it could only be a t the exlated for these reflections was found to be 0.097. pense of a shortening of other bonds in the “c” Property Measurements. Electrical Resistance .-Pellets for resistance measurements were prepared by pressing 1.5 g. direction. From consideration of these facts, it is of powdered KzNb03Fin a 6 / 8 ineDdiameterdie under 10,000 highly probable that K2NbO8F differs from other p.s.i. followed by sintering a t 500 . The pellets were sanded compounds with the KzSiF4structure, because it to flatten the surface and then plated with evaporated gold. Powder X-ray patterns of the sanded material corresponded contains fluoride ions which are preferentially situwith the standard KzNbOdFpattern, thus indicating that the ated in the apical (unshared) positions in the octasintering temperature was not high enough to cause dissocia- hedra, while the oxygen ions bridge the octahedra tion of the material. Alternating current resistance DS. temperature measure- to give a two-dimensional layer. This conclusion ments were obtained using a partial parallel substitution is substantiated by the thermal expansion data method utilizing a General Radio type 715 capacitance which are so markedly anisotropic. bridge. A standard capacitor in parallel with a decade It is felt that the rapid change of unit cell dimenresistor is placed in one arm of the bridge; the sample sions with temperature also may cause a change of capacitor, the bridge capacitor, and a decade box were placed in parallel in the opposing arm of the bridge. Meas- contact resistance between crystallites in the sinurements were taken a t 50 C.P.S. over a temperature range tered pellet, which in turn produces the positive from approximately -18 to 316”. The data curve (Fig. 1) temperature coefficient of resistance exhibited by shows a drop in resistance from 3.0(10)6 ohms a t -18’ to this material, It is noted that similar reasoning 0.6( 10)s a t -l.Oo, followed by an increase in resistance with temperature up to approximately 180” and then a drop in was applied by Peria, Bratschun, and Fenity to resistance a t higher temperatures. The curve resembles explain the YTC effect in semiconducting alkaline those obtained for barium titanate thermistor materials earth titanates.lZ - A
+
which are doped with rare earth ions. Thermal Expansion.-High temperature X-ra diffractometer tracings were made d KpNiFq, SrzTiOl, andkNbOaF
(12) W. T. Peria, W. R. Bratschun, and R. D. Fenity, J. Am. Cersram. Soo., 44, 249 (1961).
