2O3

Journal of Alloys and Compounds 288 (1999) 65–75

L

Spin-glass behaviour of disordered perovskite LaNi 1 / 2 Rh 1 / 2 O 3 Carsten Schinzer ` Condensee ´ de Bordeaux, I.C.M.C.B. –UPR CNRS 9048, 87, Avenue du Docteur A. Schweitzer, 33608 Pessac cedex, Institut de Chimie de la Matiere France Received 2 September 1998; received in revised form 1 March 1999

Abstract The new, potentially ferromagnetic perovskite type oxide LaNi 1 / 2 Rh 1 / 2 O 3 has been synthesized by conventional ceramic techniques. ˚ b57.846(3) A, ˚ c55.554(2) A ˚ in space group Pnma (No. 62). The crystal symmetry is orthorhombic with lattice constants a55.580(2) A, Rietveld refinements on the powder pattern give clear evidence for a disordered B-site occupancy. Magnetic measurements reveal two transitions. In the paramagnetic domain an effective magnetic moment meff of 1.53(8)mB is accompanied by a Pauli paramagnetic susceptibility of xtip 56.0(3)310 24 emu mol 21 . Two magnetic transitions are observed, one around 40 K and one at 10 K. The one at |40 K is explained by the complete localization of electrons on LS-Ni III whereas the lower one represents the glass temperature of a spin-glass phase. The results imply that the charge distribution is actually not Ni II / Rh IV, but Ni III / Rh III with Rh thus being diamagnetic. The electrical conductivity and EPR spectra of the compound are presented.  1999 Elsevier Science S.A. All rights reserved. Keywords: Perovskites; Magnetism; Spin-glass; Rhodium oxides; Nickel oxides

1. Introduction The solid state chemistry of rhodium mixed metal oxides has in the past not been subject to many publications. Most papers deal with trivalent rhodium in perovskite-type [1,2], K 2 NiF 4 -type [3] or spinel-type [4] mixed metal oxides. Only few reports on tetravalent rhodium exist, e.g. in BaRhO 3 [5] or BaRu 12x Rh x O 3 [6]. Recently, zur Loye and coworkers published structure reports on a series of new, perovskite-related compounds that contain rhodium in the trivalent [7,8] or tetravalent [9] state. Other papers concentrate on Rh III / Rh IV mixed-valence compounds, e.g. La 12x A x RhO 3 (A5Ca, Sr, Ba) [10,11] or Sr 22x La x RhO 4 [12,13]. None of these publications pays attention to the fact that Rh IV may provoke a parallel spin alignment in materials whose magnetic properties are controlled by super-exchange. Strong ferromagnetic exchange interactions are expected from the Goodenough–Kanamoori rules, when cations with half-filled e g -orbitals interact with cations possessing empty or full e g -orbitals [14,15]. Such an interaction is possible in perovskites that contain a 4d or 5d metal cation, e.g. Ru III , Rh IV or Ir IV, in the low-spin state and a 3d metal in the high spin-state at the same time. La 2 MnIrO 6 is an example of a ferromagnetic ordered

perovskite with a Curie temperature around 130 K [16]. Other ferromagnetic perovskites containing the higher homologue Ir IV such as La 2 CoIrO 6 or La 2 NiIrO 6 [17,18] have been reported, although the nature of magnetic interactions present in the latter compound is not entirely clear. Thus, combined with the right partners, Rh IV can lead to ferromagnetic oxides, a property that is not often found in mixed metal oxides. However, a serious problem is involved in the chemistry of Rh IV oxides. The tetravalent oxidation state of Rh is less stable than the (diamagnetic) trivalent one. For M II -cations that can exist in the trivalent state, too, a M II / Rh IV charge distribution is not a priori valid but subject to competing effects. In the case of M5Mn, the trivalent state is a strong Jahn–Teller-ion and the Mn II / Rh IV charge distribution is the more stable one [19]. In the case of M5Fe, the opposite is valid, i.e. the Fe III / Rh III charge distribution is preferred due to the stability of Fe III in octahedral coordination [20]. As for other cations, e.g. M5Co or Ni, that both do exist in the divalent and trivalent state we must therefore confirm whether the oxidation states do really correspond with our expectations. Within the framework of our studies on Rh-containing perovskites [19,21] the new compound LaNi 1 / 2 Rh 1 / 2 O 3 was prepared and investigated. The pure materials, LaNiO 3

0925-8388 / 99 / $ – see front matter  1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00126-7

C. Schinzer / Journal of Alloys and Compounds 288 (1999) 65 – 75

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and LaRhO 3 , both obey distorted variants of the perovskite structure [22,23]. It is well known that LaNiO 3 exhibits metallic conductivity and it is classified as a narrow bandwidth semimetal where Ni III is in the low-spin state [24]. It undergoes a stoichiometry-controlled metal-insulator-transition upon substitution on the B-site, LaNi 12x B x O 3 , where the critical concentration x c is a function of the substituting cation, e.g. x c 50.2 for Mn, x c 50.3 for Fe [25], x c 50.55 for Co [26], x c 50.05 for Sb [27] or x c #0.25 for W [28]. 2. Experimental details

2.1. Preparation and analysis Powder samples of LaNi 1 / 2 Rh 1 / 2 O 3 were prepared by the mixing and intimate grinding of stoichiometric amounts of La 2 O 3 , NiO and RhO 2 . Two firings (20 h, 48 h) in a furnace at 11008C were performed with an intermediate grinding. The stoichiometry has been verified by EDX microanalysis averaging several points on the sample surface. The oxygen content which is usually not detected by EDX methods was determined by iodometric titration using the Bunsen-method.

2.2. X-ray diffraction Rietveld analyses were performed on a powder X-ray diffractogram registered on a Philips PW 3040 / 00 X’pert MPD system using Cu K a radiation from a ceramic tube. A Bragg–Brentano set-up equipped with an analysator crystal (PG) was used.

2.3. Magnetic properties 2.3.1. DC and AC susceptibility Magnetic measurements were carried out on a SQUID magnetometer (Quantum Design) equipped with the AC option. DC measurements were performed by cooling the sample to 5 K in a magnetic field of approximately zero (estimated error 60.1 G) and then applying the desired field strength. Zero-field-cooled (ZFC) curves were obtained from measurements during heating. The field-cooled (FC) curves arise from measurements made during the recooling of the sample. AC magnetisations were measured using a driving field

amplitude of 3 G at field alternation frequencies of 1.25 Hz, 12.5 Hz, 125 Hz and 1.25 kHz. Molar susceptibilities were calculated according to the given stoichiometry of LaNi 1 / 2 Rh 1 / 2 O 3 and corrected for the diamagnetic contribution assuming cation charges of Ni III and Rh III and using the values given by Selwood [29]. All values arising from magnetic measurements are given in cgs units.

2.3.2. Electron paramagnetic resonance EPR spectra of LaNi 1 / 2 Rh 1 / 2 O 3 have been recorded in the X-band at |9.50 GHz on a Bruker spectrometer. An Oxford helium flow cryostat system was used to cool the sample to the desired temperature. The sweep width was set to 6000 G with a center field of 3375 G. After placing about 25 mg of the sample in a quartz tube with a diameter of 2 mm the tube was evacuated to avoid oxygen condensation. 2.4. Electrical conductivity Four probe measurements were performed on pellets of LaNi 1 / 2 Rh 1 / 2 O 3 with |1 mm thickness. An automated sampling system based on a personal computer connected with a Keithley 220 current source and a Keithley 181 nanovoltmeter were used to measure the electrical conductivity. Pellets were compacted at a pressure of 1 t and recalcined for 4 h under reaction conditions.

3. Results A single phase product is obtained by the described preparation process. No changes of the X-ray pattern were observed after the first and the second calcination. The analytical data reported in Table 1 confirm the nominal stoichiometry LaNi 1 / 2 Rh 1 / 2 O 3 within the acceptable experimental errors.

3.1. X-ray diffraction The diffraction pattern of LaNi 1 / 2 Rh 1 / 2 O 3 is entirely indexable in a cubic unit cell with a57.864(8). However, the known cubic superstructures of the perovskite do all show extinctions corresponding to an F-centered unit cell, a fact that is not observed here. It is therefore concluded, that a very slight GdFeO 3 -type distortion of the cubic

Table 1 Results of chemical analysis for LaNi 1 / 2 Rh 1 / 2 O 3 EDX-microanalysis % atom La

% atom Ni

% atom Rh

Iodometry oxygen per formula

% atom O

Found

Calc

Found

Calc

Found

Calc

Found

Calc

20.5(5)

20

8.9(8)

10

10.3(8)

10

60.3(3)

60

3.005(8)

C. Schinzer / Journal of Alloys and Compounds 288 (1999) 65 – 75

67

Fig. 1. X-ray diffraction pattern of LaNi 1 / 2 Rh 1 / 2 O 3 . Tick marks below the pattern show allowed positions of reflexions in the assigned space group Pnma (No. 62).

perovskite is present. Fig. 1 shows the diffraction pattern of the compound with tick marks indicating the allowed reflections in space group P 2 1 / n 2 / m 2 / a (No. 62) with ˚ b57.846(3) A, ˚ c5 lattice parameters a55.580(2) A, ˚ 5.554(2) A. Rietveld refinements have been performed using the fractional coordinates given by Geller [30]. The cation positions could be obtained in quite good precision. An ordered cation arrangement has been reported for La 2 MgRhO 6 and La 2 ZnRhO 6 [21] and because of the rather similar radii of Mg II , Zn II and Ni II another refinement using this set-up (space group P 2 1 / n, No. 14) has been undertaken. The final R-values of the refinement of the ordered model is lowest when the occupancy on the Ni and Rh positions is completely statistic, i.e. both positions are occupied by half Ni and half Rh. We find R Bragg 5 6.32% and R Bragg 56.08% for the refinements in space groups Pnma and the P2 1 / n, respectively. Consequently, there is no evidence for cation ordering on the B-sites. Unfortunately, the quality of the data set is too poor to obtain reliable values for the positional parameters especially of the oxygen sites: the statistically expected minimum R-value is |13.5% and less than R p 516.2% and R wp 521.2% could not be obtained. The results of the refinements are therefore not presented in more detail.

3.2. Magnetic properties 3.2.1. DC susceptibility Fig. 2 shows the molar susceptibility xmol as a function of temperature at 1 kG. The zero field curve exhibits a maximum at 10 K which is not present in the field cooled curve, thus indicating an ordering phenomenon around 10 K. The inverse susceptibility curves x 21 mol are presented in the lower graph. A slight deviation of the ZFC and FC curves is visible at temperatures above 180 K. This effect may be due to a very small impurity; however, magnetic

starting materials (NiO, RhO 2 ) can be excluded. It is 21 furthermore evident that x mol is not a linear function of temperature. A temperature-independent paramagnetic contribution xtip has therefore been allowed for in the 21 Curie–Weiss law and x mol was corrected for this term. The 21 resulting corrected x mol is presented as a function of temperature in Fig. 3. Obviously, there are two linear regions: one between 10 K and 45 K and the other one between 50 K and 300 K. The ZFC and FC curves still separate above 180 K, which is the reason why only the FC curve is accounted for in the fit of the Curie–Weiss parameters as reported in Table 2. Two points of interest were investigated more closely in further DC measurements: the maximum of magnetisation around 10 K and the change of slope around 45–50 K. Additional measurements at 10 and 100 G have been performed in a temperature range of 3 to 60 K. The resulting x vs. T plots are shown in Fig. 4. The 10 G curves are slightly elevated due to a relatively high error in the center of the sample in the magnetometer and the measured magnetisation. These curves indicate two transition temperatures: a separation of ZFC and FC curves with a coinciding change of slope at |40 K. The maximum exhibits a second ordering phenomenon at |10 K. Hysteresis loops have been registered at 5 K, 10 K and 50 K. They are presented in Fig. 5. The loops at 5 and 10 K show a very small hysteresis. The 50 K loop is linear and it is therefore concluded that this temperature is situated in the paramagnetic domain.

3.2.2. AC susceptibility An effort was made to elucidate the nature of the two transitions by AC measurements at different field alternation frequencies nAC and at zero field as well as in an external field of 1 kG. The obtained curves for the real and 99 imaginary part of the molar susceptibility, x 9AC and x AC are presented in Fig. 6. x 99 at n 5125 Hz and AC AC

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C. Schinzer / Journal of Alloys and Compounds 288 (1999) 65 – 75

Fig. 2. Molar magnetic susceptibility at 1 kG (DC). The upper part shows the zero field cooled and field cooled curves (ZFC, FC); the lower part shows the inverse susceptibility as a function of temperature.

Fig. 3. Inverse molar susceptibility corrected for the temperature-independent paramagnetic contribution. The lines indicate the linear fits to the FC curve; their intersection is at T548 K.

C. Schinzer / Journal of Alloys and Compounds 288 (1999) 65 – 75 Table 2 Results of Curie–Weiss fits for LaNi 1 / 2 Rh 1 / 2 O 3 (1 kG curve) T range C meff Q xPauli

10–45 K 0.187(3) emu K mol 21 1.22(2)mB 26.7(5) K 6.0(3)310 24 emu mol 21

50–300 K 0.2939(8) emu K mol 21 1.533(4)mB 238.0(5) K

69

nAC 51250 Hz are very noisy due to high detector errors and are not presented here. It is worth saying that their shape resembles the one of the low-frequency curves. In the left part of Fig. 6, measurements at zero field are presented whereas the right part shows the results obtained in an applied field of 1 kG. On either side, both x 9AC and x 99 AC reflect similar properties: in zero field two transitions

Fig. 4. Molar magnetic susceptibility as a function of temperature at different field strengths.

Fig. 5. Hysteresis loops on LaNi 1 / 2 Rh 1 / 2 O 3 at different temperatures.

C. Schinzer / Journal of Alloys and Compounds 288 (1999) 65 – 75

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Fig. 6. AC susceptibilities for LaNi 1 / 2 Rh 1 / 2 O 3 at different field alternation frequencies. Upper plots show the real part x 9AC , lower plots the imaginary part x 99AC at zero field (left) and 1 kG (right).

are obvious from maxima and deviations of the curves at different field alternation frequencies. In 1 kG applied field, the transition at |10 K is still visible but the other one has vanished. Detailed results, such as temperatures of maxima, deviation and onset of effects are reported in Table 3. The maxima for the transition around 10 K lie about identical in the real and imaginary part of magnetisation in either zero or 1 kG external field. The slope and absolute 9 is about identical to xmol,ZFC in the DC value of x AC measurements. The maximum shifts slightly to higher temperatures when frequency or field strength are increased. These are typical observations for the glass temperature of spin-glasses [31]. 9 The transition at |40 K is reflected as a shoulder in x AC and it corresponds to the onset of a broad maximum or a

9 plateau-like behaviour in the imaginary part. The x AC curves separate in this temperature regime with increasing separation temperatures by increasing nAC . Moreover, this effect has completely disappeared in the measurements at 1 kG. This transition is therefore attributed to domain effects rather than to a beginning spin-freezing. 3.2.3. Electron paramagnetic resonance The recorded EPR spectra at various temperatures are presented in Fig. 7. All spectra seem to consist of one intense, broad line with nearly unchanged intensity up to 50 K and a continuous decrease of intensity up to 290 K. The spectrum at 4 K possibly exhibits a fine structure (additional peaks at H¯1900 G and H¯3350 G). The integrated spectra show clearly that the strong absorption peak possesses a shoulder and the spectra consist of at

Table 3 Details of AC magnetic measurements on LaNi 1 / 2 Rh 1 / 2 O 3 Field alternation frequency Zero field 9 T 1max x AC 9 a T dev x AC 99 T 1max x AC T onset x 99AC b

1.25 Hz

12.5 Hz

125 Hz

1250 Hz

9.9(3) K – 8.2(5) K 40 K

10.2(3) K 34 K 8.8(4) K 40 K

10.3(3) K 36 K –c –c

10.3(3) K 38 K –c –c

1000 G 9 T 1max x AC 99 T 1max x AC

9.84(4) K (5 K)

10.15(4) K 9.0(5) K

10.24(4) K –c

10.64(4) K –c

a

Onset of the deviation relative to the 1.25 AC-curve. Onset of the plateau. c Not determined. b

C. Schinzer / Journal of Alloys and Compounds 288 (1999) 65 – 75

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Fig. 7. EPR spectra of LaNi 1 / 2 Rh 1 / 2 O 3 at various temperatures.

least two superimposed lines that could be fitted using a Lorentz shape function. The results of the fits are reported in Table 4 where the according Lande´ factors for the peak positions, the half-width parameters of the Lorentz function and the relative intensities are listed. The latter have been normalized to the sum of the peak areas at 40 K, because the total area has its maximum at that temperature. Note that the half-width parameter of the Lorentz function does not correspond to the observed half-width of the peak.

Table 4 EPR peak positions (pos), Lorentz half-width parameters (hw) and relative intensity (int) for LaNi 1 / 2 Rh 1 / 2 O 3 T

4 10 20 30 35 40 50 100 200 290

Peak 1

Peak 2 a

pos / g

hw / G

int

3.072(8) 3.118(8) 3.113(7) 3.139(8) 3.143(7) 3.148(8) 3.148(8) 3.10(2) 3.02(2) 2.84(2)

11966 13866 14766 13466 13566 13666 13566 13666 11968 125614

24% 43% 61% 47% 56% 59% 56% 40% 6% 3%

pos / g

hw / G

int a

2.286(8) 2.274(7) 2.254(7) 2.293(7) 2.286(7) 2.286(7) 2.292(7) 2.26(3) 2.25(3) 2.12(5)

203612 174612 164612 191612 188612 187612 188612 176612 223614 280620

21% 27% 33% 35% 40% 41% 40% 27% 5% 3%

a Individual peak areas from the Lorentz fit were normalized to the sum of peak areas at 40 K.

Two general positions are observed at g|3.1 and g| 2.27. Intensity and g-shift of both lines have maxima around 40 K and then decrease with increasing temperature. The half-width parameters remain nearly unchanged, but the precision of the values determined at 200 K and 290 K is inferior mainly because of the very low signal intensity. Former EPR measurements on octahedrally coordinated Ni III in oxides show isotropic line positions at g|2.15 in the low-spin state [32]. The Lande´ factor of the high-spin state is about g|4 in the high-spin state [33]. The absorption lines of the EPR spectra in LS-Ni III are usually split due to the strong Jahn–Teller-effect of the 2 E g state typically leading to values guu |2.1 and g' |2.2 [34]. However, a splitting of a much higher extent has been reported for isoelectronic Co II ( guu |2.3, g' |4.9) [35]. At low temperatures splitting fields of |3500 G are reported for Ni III in the corundum matrice, the splitting field of LS-Ni III being a function of the sample orientation in the magnetic field, and the behaviour is relatively complex giving rise to four or more lines in the spectrum at very low temperatures [32]. The broadness of the lines observed here is explained by several coexisting effects: (i) a powder sample is investigated and thus the integral over all orientations is made, (ii) the samples are very concentrated and the fine line-shapes observed for less concentrated samples are not observed, (iii) a possible dipolar inter-

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C. Schinzer / Journal of Alloys and Compounds 288 (1999) 65 – 75

Fig. 8. Specific electrical conductivity of LaNi 1 / 2 Rh 1 / 2 O 3 as a function of inverse temperature. The fit to the temperature law is shown as a continuous line.

action between neighbouring cations will increase the line broadness [36].

3.3. Electrical conductivity The observed specific resistivity r of LaNi 1 / 2 Rh 1 / 2 O 3 is shown in Fig. 8. The specific conductivity at 280 K s280 K is 1.48(2) V cm. The compound is semiconducting on the whole temperature range investigated here. The slope of 21 the Arrhenius plot is nearly linear up to 0.025 K (ca. 40 K) and exhibits a clear change around this temperature. A fit to the Arrhenius law was performed in the range of 60–280 K, but the residual showed that the linear relation between log r and T 21 is not valid. The behaviour is best described by a modified Arrhenius law of the form Ea r ~exp]n kT A fit to this law in the range of 40 to 280 K resulted in values of Ea 50.59(2) kJ mol 21 (6.1(2) meV) and n5 0.66(1) ( x 2 50.0733; N599; no points excluded). The statistics could not be significantly improved by reducing the temperature range and they were drastically worse when the temperature range was extended to temperatures below 40 K. The resulting power law is very close to the power law claimed for disordered compounds with long-range Coulomb interactions where n51 / 2 ideally [37].

4. Discussion Analytical data leave no doubt that the stoichiometry of the compound is the expected one. Moreover, the magnetic measurements suggest a charge distribution of Ni III / Rh III

rather than Ni II / Rh IV. The properties of LaNi 1 / 2 Rh 1 / 2 O 3 are discussed in the light of these results first and are in a second part of the discussion compared with those of LaNiO 3 and LaRhO 3 . In a third part the spin-glass behaviour in LaNi 1 / 2 Rh 1 / 2 O 3 is compared to other oxide spin-glass systems. The orthorhombic distortion of perovskites LaB 1 / 2 Rh 1 / 2 O 3 is very weak. This has been shown for other perovskites LaM 1 / 2 Rh 1 / 2 O 3 , too [19,21]. A statistiIII III cal distribution of Ni and Rh on the B-positions is not very surprising since the only remaining driving force for a possible cation ordering is the difference in cation size. ˚ in octahedral The radius of the LS-Ni III cation is 0.56 A coordination according to Shannon’s data [38]. This makes ˚ a difference of |20% compared with the radius of 0.665 A reported for Rh III . Although this value is beyond the usual critical value (|15% difference in size) the size effect alone is not strong enough to enhance an ordering of the cations. The electronic structure of LaNi 1 / 2 Rh 1 / 2 O 3 as found from the conductivity measurement is close to a disordered compound with long-range Coulomb interactions over a broad temperature range. The high specific conductivity is reflected by the relatively high Pauli paramagnetic contribution of 6.0(3)310 24 emu mol 21 . Dipolar interactions between neighbouring Ni III cations can consequently be regarded as a major mechanism that contributes to the broadness of the EPR lines [36]. The magnetic transition at |40 K coincides with the change of the temperature law in the conductivity data. The magnetic behaviour of LaNi 1 / 2 Rh 1 / 2 O 3 is characterized by two transitions, a transition temperature at |40 K and the glass temperature T g at 10 K. Above the transition temperature at |40 K, a paramagnetic behaviour is observed with an elevated effective magnetic moment in

C. Schinzer / Journal of Alloys and Compounds 288 (1999) 65 – 75

this domain. The Weiss temperature Q indicates a weak antiferromagnetic interaction between the spins. The expected spin-only value is mSO 51.225mB and the determined meff from the Curie–Weiss fit is about five quarters of this value. A thermally excited spin-transition of LSNi III to HS-Ni III can however be excluded with respect to the EPR data, that remain practically unchanged over the whole temperature range. The elevated moment may be attributed to spin-orbit coupling that vanishes at T ,40 K. The effective magnetic moment detected by the Curie– Weiss fit at 1 kG in the temperature range of 10–40 K corresponds exactly to the spin-only value of mSO 5 1.225mB expected for half a LS-d 7 cation per mole. The Weiss constant decreases significantly and all xFC curves exhibit a ferromagnetic slope below this transition temperature even at lower fields. We therefore conclude that a change to mainly ferromagnetic interactions occurs at T # 40 K. Such an interaction is possible from the Goodenough–Kanamoori rules if the orientation of the LS-Ni III cations is attributed. Considered an isotropic octahedron, only two of six possible orientations lead to strong antiferromagnetic interactions (half-occupied and half-occupied e g ) whereas the other orientations lead to ferromagnetic interactions (empty and half-occupied e g ). Since both interactions are of the same type (s ), their strength is consequently the same. If there is no preferred axis present in the compound due to crystal symmetry, which of course is to be verified, the number of ‘ferromagnetic orientations’ exceeds the number of ‘anitferromagnetic’ ones. The imaginary part of the AC susceptibility measurements reveals strong interactions between the spins below 40 K and the interactions continue when the sample is cooled further. However, this effect is only visible in the zero field measurements and has disappeared in the measurements at 1 kG. This behaviour is attributed to domain effects present at low fields and the difference between FC and ZFC curves in the 10 G and 100 G measurements probably have the same reason. Domain effects are usually visible as broad intense absorption lines in the EPR spectra. In the spectra observed for LaNi 1 / 2 Rh 1 / 2 O 3 there is only a slight maximum of EPR line intensity at the ordering temperature. The expected effect is not very strong with respect to the sensitivity of the effect in xDC and xAC . It is therefore assumed that such an effect is hidden under the other EPR transition lines and the slight change in intensity around 40 K is the only hint that such an effect is present. A more precise interpretation of the EPR spectra could be undertaken if diluted spectra of LaNi 1 / 2 Rh 1 / 2 O 3 in a neutral matrix (e.g. MgO, ZnO etc.) could be achieved in order to reduce the line broadening effects. A spin glass transition with a glass temperature of T g 510 K is clearly confirmed by AC measurements and cannot be explained otherwise. The value of T g implies a short range magnetic order with weak exchange interactions. The anisotropy in xDC around 185 K has no

73

corresponding effect neither in the EPR spectra nor in the conductivity measurement and there is no evidence that any of the properties reported and discussed are due to an impurity phase of considerable amount. The volume of the primitive perovskite unit cell of ˚3 LaNiO 3 as calculated from its lattice parameters is |57 A 3 ˚ for the pure [22]. The corresponding values are 62.2 A ˚ 3 for rhodium compound, LaRhO 3 [23], and 60.8 A LaNi 1 / 2 Rh 1 / 2 O 3 which is only a small difference. This might be explained by the fact that more electrons are localized on Ni in these compounds than in the metallic LaNiO 3 . The magnetic behaviour of LaNiO 3 is characterized by a strong Pauli paramagnetism xPauli 52.8310 26 emu mol 21 [39] and the specific conductivity at room temperature is reported by several authors and is in the range of 3–9 mV cm [40,41]. On the other hand the room-temperature specific conductivity sRT of LaRhO 3 is in the range of 3.5–30 V cm [10,11,20]. Its activation energy Ea of 0.038–0.06 eV [10,20] is higher than the one found here and its thermal behaviour is well described by a simple Arrhenius law. Differences between Ea and the optically determined gap-energy (|1.5 eV) imply that LaRhO 3 is an extrinsic semiconductor with thermally excited defect centers [20]. The electronic properties of LaNi 1 / 2 Rh 1 / 2 O 3 thus represent an intermediate state between the metallic conductor LaNiO 3 and the semiconductor LaRhO 3 . As cited in Section 1 the substitution of other cations on the B-site in metallic LaNiO 3 usually leads to a stoichiometrycontrolled metal–insulator-transition. It is evident from our measurements that in the case of M5Rh III , the critical concentration x c is less than 0.5. Spin-glass behaviour has been evidenced for another ¨ Rh-containing compound, BiFe 1 / 2 Rh 1 / 2 O 3 , by Mossbauer spectroscopy and magnetic measurements [42]. Its spin freezing is observed at higher temperatures, T g |250 K, and an important fraction of long-range magnetic interactions is considered to be responsible for this high freezing temperature. Investigations of the semiconducting behaviour of BiFe 1 / 2 Rh 1 / 2 O 3 reveal a higher activation energy (0.067(1) eV) and a lower Pauli paramagnetic contribution (5310 25 emu mol 21 ). The optical gap-energy is identical to the Arrhenius activation energy and the compound is thus considered as an intrinsic semiconductor [20]. These results imply that the fraction of delocalized electrons is higher in LaNi 1 / 2 Rh 1 / 2 O 3 than in Bi 2 FeRhO 6 . Moreover the exchange interaction between neighbouring HS-Fe III cations is probably more important than between neighbouring LS-Ni III and this might explain the very low glass temperature found in LaNi 1 / 2 Rh 1 / 2 O 3 . Perovskite-type spin-glass systems have been studied earlier and it is a challenge to determine the crystalchemical factors that influence the glass-transition. Battle et al. have studied several Fe-containing spin-glass systems [43]. A common feature of all these perovskite-systems is a disordered arrangement of Fe III and another, diamagnetic

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C. Schinzer / Journal of Alloys and Compounds 288 (1999) 65 – 75

cation on the B-sites. Two groups of such Fe perovskites showing spin-glass transitions are distinguished, one with relatively high freezing temperatures (T g |200 K) and another one obeying glass temperatures in the range of 15–25 K. The two groups possess different features concerning composition and crystal-chemistry: AA’FeBO 6 (A5Ca, Sr, Ba; A’5La or RE; B5Ti IV, Sn IV ) generally have high freezing temperatures combined with a partial long-range magnetic order whereas A 2 FeBO 6 (A5Ca, Sr; B5SbV, NbV, TaV ) have low glass temperatures and exhibit short-range magnetic order [43]. It has been concluded that specific structural features are responsible for this difference: (i) the presence of two different A cations in the first group along with (ii) a distorted A cation array and (iii) a relatively low charge difference of one unit in the first group [42]. Filoti et al., however, noticed that the observations made on BiFe 1 / 2 Rh 1 / 2 O 3 contradict these assumptions in that (i) there is no charge difference and (ii) there is only one type of A cations in a probably non-distorted array [42]. The absence of a steric effect of the Bi III -lone pair, that has been reported for pyrochlores [44], has not been verified so far. The spinglass transition in LaNi 1 / 2 Rh 1 / 2 O 3 does not clarify the case, since, again, there is neither a distorted A cation array nor a charge difference present. The only possible explanation for the low glass temperature observed here is the different nature of the spin carrying species, LS-Ni III .

5. Conclusion We have successfully synthesized the new perovskite material LaNi 1 / 2 Rh 1 / 2 O 3 . The compound is a paramagnetic semiconductor from 40–300 K with an ordering temperature at 40 K that probably reflects a ferromagnetic ordering. The temperature law of electronic conductivity is explained by the disordered arrangements of the cations together with long-range Coulomb interactions. EPR spectra show at least two lines for Ni III ions at g|2.3 and g|3.1. A spin-glass transition is observed at 10 K. Of the homologous compounds with respect to Rh only LaNi 1 / 2 Co 1 / 2 O 3 behaves similarly in that its electronic properties are also those of a disordered compound with long-range Coulomb interactions [26]. LaNi 1 / 2 Ir 1 / 2 O 3 (5La 2 NiIrO 6 ) is an ordered compound with a charge distribution of Ni II / Ir IV [17,18] and there are thus no similarities to be found with LaNi 1 / 2 Rh 1 / 2 O 3 .

cal measurements and electron spin resonance) is gratefully acknowledged. The author is indebted to Prof. G. Demazeau (E.N.S.C.P.B., Bordeaux), Dr. B. Chevalier (I.C.M.C.B., Bordeaux) and Prof. P.D. Battle (University of Oxford) for valuable discussions.

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Acknowledgements The author holds a TMR Marie-Curie Research Training Grant of the European Union (Contract No. ERB FMB ICT 961859). The assistance of E. Lebraud (X-ray diffraction), I. Alves (microanalysis) and E. Marquesteaut (electri-

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