The series LnSrVO4: synthesis, crystal structure, crystal chemistry and magnetic susceptibilities

Journal of Alloys and Compounds, 180 (1992) 281-287 JAL 8081

281

The Series LnSrVO4: synthesis, crystal structure, crystal chemistry and magnetic susceptibilities J. E. G r e e d a n a n d W e n h e G o n g Institute f o r Materials Research, McMaster University, Hamilton, Ont. L8S 4M1 (Canada)

Abstract The synthesis of several members of the series LnSrV04 can be achieved by hydrogen reduction of mixtures of Ln203, SrCO3 and V205 or SrCOa and LnV04 at 1200 °C for 24 h. The series exists from lanthanum to terbium inclusive. The range of existence is discussed in terms of the ratio of radii of the structure fields and the large cation size mismatch ratios. Crystal structures were refined from powder neutron diffraction data for L n - L a , Ce Pr and Nd. Cell constants are reported for all members from X-ray Guinier data All compounds are isostructural with K2NiF4, I 4 / m m m . Magnetic susceptibility data from 4.2 K to 300 K are described. It is possible that a local moment of about 2.6 /~B per V3+ exists for Ln~-Ce, Pr and Nd but Ln--La represents a different case.

1. I n t r o d u c t i o n I n t h e w a k e o f t h e d i s c o v e r y o f s u p e r c o n d u c t i v i t y in K2NiF 4 s t r u c t u r e copper oxides [1-3], much attention has been focused on other transition m e t a l o x i d e s w i t h r e l a t e d s t r u c t u r e s . One s e r i e s w h i c h h a s h e r e t o f o r e r e c e i v e d s c a n t a t t e n t i o n is LnSrVO4 w h e r e Ln is a lanthanide. In this c a s e o n l y t h o s e c o m p o u n d s w h e r e L n - L a , Ce a n d Nd h a v e b e e n r e p o r t e d [4, 5]. N o d e t a i l e d s t r u c t u r a l i n f o r m a t i o n e x i s t s e x c e p t f o r LaSrVO4 w h e r e p o s i t i o n p a r a m e t e r s w e r e d e r i v e d f r o m X-ray p o w d e r d a t a b y t h e i n t e g r a t e d i n t e n s i t y m e t h o d [5]. Also, n o p h y s i c a l p r o p e r t i e s h a v e b e e n r e p o r t e d . In t h i s s t u d y w e r e p o r t r e s u l t s c o n c e r n i n g t h e e x i s t e n c e limits o f t h e LnSrVO4 series, o f crystal s t r u c t u r e r e f i n e m e n t s b y p o w d e r n e u t r o n diffraction a n d m a g n e t i c s u s c e p t i b i l i t y data.

2. E x p e r i m e n t a l

details

2.1. M a t e r i a l s s y n t h e s i s

S t a r t i n g c o m p o u n d s w e r e r e a g e n t g r a d e SrCO3 a n d V205 a l o n g w i t h

Ln203 ( L n - - L a , Nd, Sin, E u a n d Gd) 9 9 . 9 9 % p u r e ( R e s e a r c h C h e m i c a l s ) , a n d CeO2, Tb407 a n d ProOzl also 9 9 . 9 9 % p u r e ( R e s e a r c h C h e m i c a l s ) . All r a r e e a r t h o x i d e s , e x c e p t PrsOzl a n d Tb4OT, w e r e p r e f i r e d at 1 0 0 0 °C in air f o r 12 h t o d e c o m p o s e a n y h y d r o x i d e s o r c a r b o n a t e s . CeSrVO4 c o u l d b e

Elsevier Sequoia

282

prepared by direct reaction of CeO2, SrCO3 and V205 in a hydrogen stream at 1200 °C for 24 h. The phases L n - - P r to Eu were prepared by firing well ground and well mixed powders of LnVO4 and SrCO8 also under hydrogen at 1200 °C for 24 h. The preparation of GdSrVO4 and TbSrVO4 required a temperature of 1250 °C. The LnVO4 phases were obtained by firing Ln2Oa and V205 for 10 h at 8 0 0 - 9 0 0 °C in air except for PrVO4 which was prepared under argon. The Pr208 resulted from reducing Pr6011 in hydrogen at 700 °C for 24 h. Th2O3 Was made by slow heating over 12 h of Tb40~ from 200 °C to 900 °C under a 60% CO-N2 gas mixture. In all cases alumina crucibles were used to contain pressed pellets of the reactants.

2.2. X-ray diffraction Phase identification was carried out with data obtained from a Nicolet I2 automated diffractometer and Cu Ka radiation. Precise lattice constants were refined from data collected with a Guinier camera (IRDAB model XDC700), Cu Kal radiation and a silicon internal standard.

2.3. Neutron diffraction Neutron powder diffraction data were obtained at the McMaster Nuclear Reactor with 1.3915/~ neutrons. Details of the diffractometer and the data analysis methods have been described previously [6].

2.4. Magnetic susceptibility A Quantum Design SQUID magnetometer was used to obtain magnetic susceptibility data over a temperature range from 5 K to 300 K at a variety of applied fields.

3. R e s u l t s

and discussion

Single-phase samples of LnSrVO4 compounds could be prepared by the methods described above for L n - - L a to Tb inclusive. All efforts to obtain corresponding materials for L n = D y and beyond resulted in multiphase mixtures which contained a phase of probable composition Sr2LnV20~. This is currently under investigation. The X-ray powder diffraction patterns for all single-phase materials could be indexed on a tetragonal cell of the K2NiF4 type. Table 1 shows the cell constants obtained along with the c/a ratio and the radius ratio rA/rB for the two types of cation. Here rA is the mean value of the IX-fold crystal radii for Sr 2+ and Ln 3+ and rB is the VI-fold radius for Vs+ [7]. The relative stability of the K2NiF4 structure to other structure types found for A2BO4 compounds is normally discussed in terms of structure field maps [8-12]. Poix [13] has proposed a somewhat different approach based on an attempt to extend the perovskite tolerance factor to the more complicated KsNiF4 structure. From structure-field considerations Ganguli has proposed an existence range for K2NiF4 of 1.7 < rA/rB < 2.4. Clearly, the existing members

283 TABLE 1 Unit cell constants, axial ratios and cation radius ratios for

LnSrVO 4

compounds

Ln

a (/~)

c (/~)

V (/~3)

c/a

rA/rs

La Ce Pr Nd Sm Eu Gd Tb

3.8828(4) 3.8694(3) 3.8643(3) 3.8612(3) 3.8558(2) 3.8505(3) 3.8504(4) 3.8378(5)

12.662 (2) 12.577(2) 12.518(1) 12.476(2) 12.372(1) 12.329(1) 12.327(2) 12.326(2)

190.89 188.29 186.93 186.00 183.94 182.79 182.75 181.55

3.26 3.25 3.24 3.23 3.21 3.20 3.20 3.21

1.80 1.78 1.77 1.76 1.75 1.74 1.73 1.72

TABLE 2 Structural parameters and agreement indices for LnSrV04 LaSrV04

CeSrVO4

PrSrVO4

NdSrVO 4

Positional parameters (z) Ln/Sr 0.3570(2) O1 0.1685(3)

0.3580(2) 0.1690(3)

0.3585(2) 0.1691 (2)

0.3580(2) 0.1697(2)

Isotropic temperature factors (/~2) Ln/Sr 1.27(7) V 0.5000 O1 1.42(9) 02 0.64(7)

0.71(6) 0.5000 1.19(7) 0.73(6)

0.66(6) 0.5000 1.58(8) 0.83(6)

0.54(5) 0.5000 1.73(9) 0.82(6)

Agreement indices Rw 5.49 Rs 5.10 Rp 4.30 Rz 2.41

6.18 5.96 4.69 2.57

6.01 5.98 4.58 2.66

5.50 5.24 4.02 1.80

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o f t h e L n S r V O 4 s e r i e s a p p r o a c h t h e l o w e r l i m i t o f t h i s r a n g e b u t in f a c t t h e c a l c u l a t e d rA/r~ f o r m o s t o f t h e h e a v y L n m e m b e r s , w h i c h a r e n o t f o u n d , e x c e e d t h e l i m i t o f 1.7. A n o t h e r f a c t o r w h i c h m a y i n f l u e n c e s t a b i l i t y i s t h e s i z e m i s m a t c h b e t w e e n S r 2+ a n d L n s+ w h i c h m u s t o c c u p y t h e s a m e c r y s tallographic site. The size mismatch percentage extends from 7% (lanthanum) to 17% (terbium) for the known phases and from 19% (dysprosium) to 24% ( l u t e t i u m ) f o r t h o s e n o t f o u n d . A t p r e s e n t i t is n o t c l e a r w h y t h e e x i s t e n c e r a n g e f o r t h i s s e r i e s t e r m i n a t e s a t L n S+ - - T b b u t t h e a b o v e f a c t o r s m a y p l a y a m a j o r r o l e . I t i s w o r t h c o m p a r i n g t h e e x i s t e n c e r a n g e o f t h e LnSrVO4

284 TABLE 3 Observed bond lengths (A) in LnSrVO4 from the neutron refinements Bond

LaSrV04

CeSrVO4

PrSrV04

NdSrVO4

Ln/Sr-OI(X 1) Ln/Sr-Ol(× 4) Ln/Sr-O2(x 4)

2.386(4) 2.764(1) 2.655(2)

2.377(4) 2.757(1) 2.633(2)

2.371(4) 2.754(1) 2.621 (2)

2.349(3) 2.752(1) 2.620(2)

V-OI(x 2) V--O2(X 4)

2.134(4) 1.941(1)

2.126(3) 1.935(1)

2.117(3) 1.932(1)

2.117(3) 1.931(1)

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Fig. 1. Neutron powder data for CeSrgO4. The crosses represent the data points, the solid line ~ the calculated prone and the lower curve N the difference plot. p h a s e s with t h o s e reported for LnSrCrO4 and LnSrFeO4 [ 1 4 ]. For the c h r o m i u m series the range is f r o m Ln----La to D y and for the iron c o m p o u n d s it is the s a m e as for vanadium, L n - L a to Tb. The octahedral ionic radius o f V s+ is

285

0 . 7 8 / ~ which is greater than that of Cra+ , 0 . 7 5 5 / ~ but nearly identical with that of high spin Fe 3+, 0.785 /~. The results of the crystal structure refinements for Ln--La, Ce, Pr and Nd are shown in Table 2 and the derived bond lengths in Table 3. Figure 1 displays a typical data set, for CeSrVO4, and the fit to the structural model. All refinements were carried out in space group I4/mmm with Ln/Sr in site 4e (00z) O(1) also in 4e, 0 ( 2 ) in 4c (0tO) and V in 2a (000). Neutron scattering lengths in femtometers were lanthanum 8.24, cerium 4.84, praseodymium 4.45, neodymium 7.69, strontium 7.02, vanadium - 0 . 3 8 2 , and oxygen 5.805 [15]. Owing to the small contribution of vanadium to the overall cross-section, its isotropic temperature factor was held fixed at 0.50 (~2). Refinement of the lanthanide to strontium ratio resulted in values not significantly different from 1.00.

4. M a g n e t i c s u s c e p t i b i l i t i e s The results of the susceptibility measurements for L n - L a , Ce, Pr and Nd are shown in Figs. 2 and 3. For LaSrVO4 the susceptibility is nearly temperature independent in the range 300 K to 100 K followed by a relatively sharp increase (decrease in 1/X) to 4.2 K. Measurements of the electrical resistivity in the same temperature range on single crystals of LaSrV04 indicate semiconducting behavior [16]. Therefore the susceptibility cannot be interpreted as some form of Pauli paramagnetism. An attempt to fit the high temperature part to a Curie-Weiss law yields a Curie constant of about 4.0 cm ~ m o l - 1 K which is four times the value of a spin only V 3 + ion. At present there is no obvious explanation for the magnetic properties. One possibility is that short-range magnetic correlations dominate the magnetism in this temperature range. In such a case there should be a susceptibility maximum at higher temperatures. To investigate this hypothesis susceptibility mea"~850

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300

286 surements were attempted up to 900 K. No such feature was observed up to 700 K. Unfortunately, owing to a leak in the apparatus, the sample began to oxidize rapidly above 700 K so it was not possible to extend further the temperature range of the measurements. For the other three compounds the data are very different, owing in part to the presence of paramagnetic lanthanide ions. Clearly, there are some common features, including a Curie--Weiss-like part at high temperatures followed by deviations below 1 2 0 - 1 0 0 K. The deviations are in the direction of an enhanced susceptibility (decreasing X-1) relative to the Curie--Weiss region. To a first approximation the high temperature susceptibility can be analyzed in terms of non-interacting Ln a+ and Va+ ions in which case Cobs=C(Lna+)+C(V3+). Curie--Weiss constants derived from the data of CeSrVO4 above 120 K are C - 1 . 7 cm a mo1-1 K -1 and 0 ¢ = - 1 9 0 K. These should be compared with the sum of the spin only value for Va+ of 1.0 and the free ion value of Ce 3+ of 0.80 and the agreement is seen to be good. Thus, there appears to be a major change in the susceptibility contribution from the Vs + sublattice upon exchanging cerium for lanthanum in the LnSrVO4 series. For the remaining compounds, PrSrVO4 and NdSrVO4, an attempt was made to separate the two contributions to the total susceptibility. In these cases convenient isostructural model compounds could be found or synthesized where V 3+ was replaced by Ga s+ which has a similar ionic radius, 0 . 7 6 / ~ v s . 0.78 /~ for Va+ [7]. NdSrGaO4 and PrSrGaO4 were prepared by firing Ln2Oa, SrO and GaeO8 mixtures at 1200 °C for 24 h. The neodymium compound could be prepared in air while the praseodymium compound was fired in argon to discourage oxidation to Pr(IV). PrSrGaO4 was a light green color consistent with Pr(III). Unit cell constants in ¢mgstrSms were a = 3 . 8 2 2 7 ( 3 ) , c = 12.582(1) for PrSrGaO4 and a = 3 . 8 1 7 3 ( 4 ) , c--12.531(1) for NdSrGaO4. In the case of the neodymium phase these cell constants are in good agreement with literature values [17] while the praseodymium compound is reported for the first time. In both cases the cell constants are close to those for the corresponding vanadium compounds. If it is assumed that the Ln 3+ susceptibility for the gallium phases is a good model for the Ln 3+ contribution in the vanadium compounds and that LnS+-V a+ interactions are relatively weak, then by subtracting the susceptibilities, LnSrVO4-LnSrGa04, at each temperature the temperature dependence of the V3+ contribution can be isolated. Plots obtained by this procedure are shown in Fig. 4. Note the Curie--Weiss behavior at high temperature followed by an anomaly below 120 K in both cases. The derived Curie-Weiss constants are C = 0.86 cm a m o l - i K and 0c = - 408 K for PrSrVO4 and C = 0 . 8 4 cm 8 mo1-1 K and Oc = - 163 K for NdSrV04. The Curie constant values are quite close to the spin-only value of 1.00 for Va+. Thus, among the LnSrVO4 phases Ln--Ce, Pr and Nd appear to exhibit distinctly different magnetic behavior for the Va + sublattice than does LaSrVO4 in the temperature range investigated. Further studies including electrical properties and neutron scattering are in progress.

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Acknowledgments We thank G. Hewitson for assistance in obtaining the magnetic susceptibility data in the range 300 K to 5 K. We thank F. DiSalvo and M. Hornbostel for collecting data on LaSrVO4 up to 900 K. Financial support from the Natural Sciences and Engineering Research Council of Canada and the Ontario Centre for Materials Research is acknowledged.

References 1 J. B. Bednorz and K. A. Muller, Z. Phys• B, 54 (1986) 189. 2 S. Vchida, H. Takage, K. Kitazana and S. Tanaka, Jim. J. Appl. Phys., 25 (1987) L1. 3 R. J. Cava, R. B. VanDover, B. Batlogg and E. A. Rietman, Phys• Rev. Lett., 58 (1987) 408• 4 V. A. Fotiev and G. V. Bazuev, Russ. J. Inorg. Chem., 26 (1981) 474. 5 J. M. Longo and P. M. Raccah, J. Solid State Chem., 6 (1973) 526. 6 J. N. Reimers, J. E. Greedan and M. Sato, J. Solid State Chem., 72 (1988) 390• 7 R. D. Shannon, Acta CrystaUogr. A, 32 (1976) 751. 8 G. Gattow, Z. Anorg. AUg. Chem., 333 (1964) 134. 9 K. Kugimiya and H. Steinfink, Inorg. Chem., 7 (1968) 1762. 10 F. P. Glasser and L. S. Dent Glasser, J. Am. Ceram. Soc., 46 (1963) 377. 11 O. Muller and R. Roy, The Major Ternary Structural Families, Springer, Berlin, 1974, p. 76. 12 D. Ganguli, J. Solid State Chem., 30 (1979) 353. 13 P. Poix, J. Solid State Chem., 31 (1980) 95. 14 J. C. Joubert, A. Collomb, D. Elmaleh, G. Le Flem, A. Daoudi and G. Ollivier, J. Solid State Chem., 2 (1970) 343. 15 V. F. Sears, in K. Skold and D. L. Price (eels.), Methods of Experimental Physics, Vol. 23A, Academic Press, New York, 1986, p. 521. 16 W. Gong and J. E. Greedan, unpublished results, 1990• 17 C. E. Gooden and G. J. McCarthy, JCPDS File 24-1191, 1972.