Hyperfine characterization of SrTixHf1−xO3

ARTICLE IN PRESS

Physica B 389 (2007) 111–115 www.elsevier.com/locate/physb

Hyperfine characterization of SrTixHf1xO3 Roberto E. Alonso, Martin Falabella, Alberto R. Lo´pez-Garcı´ a Departamento de Fı´sica, Fac. Cs. Exactas, Universidad Nacional de La Plata, Calles 115 y 49, 1900 La Plata, Argentina

Abstract In order to study how the Hf replacement by Ti affects the crystalline structure of SrHfO3 pure perovskite, X-ray diffraction (XRD) studies and hyperfine characterizations of SrHf1xTixO3 for the compositions x ¼ 0.25, 0.50 and 0.75 are reported. The structure at room temperature (RT) was determined by XRD. Hyperfine electric quadrupole interaction at Ta probes was determined by perturbed angular correlation spectroscopy. For every sample, spin precession curves were measured from RT to 1000 1C, in 50 1C steps. From the data fit it was determined that for x ¼ 0.25 the compound is orthorhombic (Pnma) at RT and undergoes one first-order and one second-order phase transitions at 550 and 750 1C, respectively. For x ¼ 0.50 the compound is in the orthorhombic (Cmcm) phase at RT and undergoes ¯ from RT on. Also, comparison with other a second-order phase transition at about 250 1C. For x ¼ 0.75, the compound is cubic (Pm3m) perovskite-type compound studies is performed. r 2006 Elsevier B.V. All rights reserved. PACS: 77.80.Bh; 77.90.+k; 76.80.+y Keywords: PAC; XRD; Phase Transitions

1. Introduction The crystalline structure called ‘‘perovskite’’ takes its denomination from the mineral CaTiO3. In nature there exist a lot of compounds with this structure and chemical formula ABO3. For some A and B combinations compounds with highly desirable physical properties such as superconductivity, ferromagnetism and ferroelectricity have been found. In the last two decades, perovskites have gained a great interest not only from the scientific but also from the technological point of view, because some of them present drastic anomalies in their dielectric constant at a given temperature that indicate the occurrence of a phase transition. In fact, at these temperatures they exhibit a paraelectric-ferroelectric or else a paraelectric–antiferroelectric phase transition. In the ferroelectric phase, the crystal presents an spontaneous electric polarization which can be inverted on applying an external electric field. The most representative compounds with such type of transition are BaTiO3, PbTiO3 and KNbO3 [1,2]. Corresponding author. Tel.: +54 221 424 6062; fax: +54 221 425 2006.

E-mail address: alonso@fisica.unlp.edu.ar (R.E. Alonso). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.07.034

Nowadays, a new variety of compounds are being synthesized by making partial substitutions of the A or B cations by others homovalent A0 and B0 , thus obtaining combinations of general chemical formula AyA0 1y BxB0 1xO3, with 0pyp1 and 0pxp1. These perovskites have a great basic and technological interest because the cation substitution can enhance their properties and performances as compared with the pure ABO3 ones [3]. Examples of such materials are Pb(ZrTi)O3 and Ba (TiHf)O3 [4]. At high temperatures the perovskite-type compounds present cubic symmetry, with the B (or B0 ) cation in the center of the cubic unit cell, the A (or A0 ) cation at the vertices of the cube and the O anions at the center of the faces and forming a regular octahedron that contains the B cations. When the temperature is lowered, these compounds suffer phase transitions to structures of lower symmetry, and their phase diagrams are strongly dependent on the characteristics of the A, A0 , B and B0 specimens, together with the x and y concentrations. In general, very small atomic displacements from their positions in the cubic array occur in the structures of lower symmetry.

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Simple perovskites SrHfO3 (SH) and SrTiO3 (ST) present different crystalline structures [5,6]. On one hand, at room temperature (RT), SH is orthorhombic Pnma and with increasing temperatures it exhibits the following structural transitions: at about 600 1C it becomes orthorhombic Cmcm, then tetragonal I4/mcm at about 730 1C ¯ at about 1080 1C. While the first and lastly cubic Pm3m phase transition is not continuous, the other two have been reported as continuous [7]. On the other hand, ST is already cubic at RT. Then, it is clear that the nature of the B cation plays an important role in the resulting crystalline structure of these oxides. In fact, the partial substitution of Hf by Ti, occurring in SrHf1xTixO3 produces a series of new compounds since a competition between Ti–O and Hf–O bonds appears due to the different ionic radii (RTi ¼ 0.68 A˚ and RHf ¼ 0.81 A˚) and covalence grade. As a consequence, different crystalline structures will be formed in correspondence with the x variation. It was observed that in A1xA0 xBO3 and AB1xB0 xO3 perovskites, for small and large x values the crystalline structure is mainly determined by the most abundant oxide. For intermediate compositions, different situations have been observed. In Sr1yBayZrO3 where one of the pure perovskites is cubic at RT and the other pure one exhibits several lower-symmetry structures above this temperature, the change of composition seems to have the same effect as the change of temperature in the noncubic pure oxide [8]. In this way, by varying the composition, different structures can be observed keeping the temperature constant. This effect can in principle be also valid for ABB0 O3 combinations. In this work the study of the effect of a gradual Hf substitution by Ti in SrHf1xTixO3 (x ¼ 0.25, 0.50 and 0.75) with X-ray diffraction (XRD) and perturbed angular correlation (PAC) spectroscopies is reported. After characterizing the crystalline structures of the compounds by XRD at RT, the electric field gradient (EFG) tensor was measured by PAC from RT to 1000 1C for each Hf concentration in order to analyze the occurrence of phase transitions. 2. Experimental Powder samples of SrHf1xTixO3 were prepared by the solid-state reaction procedure using stoichiometric quantities of high purity strontium carbonate and hafnium and titanium oxides to form the compounds corresponding to x ¼ 0.25, 0.50 and 0.75. The starting material was mixed, ground and heated at 1000 and 1200 1C for 24 h and finally at 1500 1C for 3 h. The thermal evolution of the powder was monitored by XRD. Powder XRD data were collected at RT on a Philips PW1700 diffractometer using Cu-Ka radiation and a graphite monocromator. The collection time was 10 s in steps of 0.021 and within the angular range 201p2yp1001. XRD data were analyzed by the Rietveld method using the FullProf program [9].

Then the samples were irradiated with thermal neutrons to produce the 181Hf isotope by the nuclear reaction 180 Hf+n-181Hf* (180Hf is one of the stable isotopes of hafnium), that decays to 181Ta probes by b involving the 5 2+ excited state with nuclear quadrupole moment Q ¼ 2.80 b. The spin precession curves were determined using a twodetector PAC spectrometer with time resolution of 0.7 ns. To measure the temperature dependence of the hyperfine interaction in 50 1C steps, the samples were located in an oven with thermal stability of 1 1C. The hyperfine parameters, i.e. the fraction f of probes perturbed by an EFG, the quadrupole frequency oQ ¼ eQVzz/[4I(2I1)_], the asymmetry parameter Z ¼ [(VxxVyy)/Vzz] satisfying Vxx+Vyy+Vzz ¼ 0 and the line width or distribution parameter d that describes the effect of imperfections, are obtained by fitting the perturbation factor X G22 ðtÞ ¼ f S G S22 ðtÞ, S

with GS22 ðtÞ ¼ s20 þ

X

s2n cosðon tÞexpðon dtÞ,

n

to the data, ‘‘S’’ being each nonequivalent site occupied by the probes. Here a static electric quadrupole interaction and a Lorentzian profile for the frequency distribution have been assumed. The coefficients s20, s2n and the frequencies on (where n runs from 1 to 3) are known functions of Z. The frequencies on are also function of oQ. The detailed formalism of this spectroscopy can be found elsewhere [10]. 3. Results and discussion 3.1. XRD In Fig. 1 the obtained diffractograms of SrHf1xTixO3 at RT for the compositions x ¼ 0.25, 0.50 and 0.75 are shown. All of them were fitted according to different crystallographic structures selected between those that most frequently appear in these kinds of perovskites: ¯ Pm3m, P4 mm, I4/mcm, Imma, Pnma and Cmcm. Using these test-structures, the best fits obtained were the ¯ following: for x ¼ 0.75 the cubic Pm3m structure (RP ¼ 6.06, R-WP ¼ 7.81); for x ¼ 0.50 the orthorhombic Cmcm structure (RP ¼ 6.92, R-WP ¼ 9.12) and for x ¼ 0.25 the orthorhombic Pnma (RP ¼ 9.71, RWP ¼ 14.6). In Table 1 are shown the obtained lattice constants, together with the atomic positions. 3.2. PAC First-order phase transitions are characterized by a discontinuous change in the lattice constants and atomic positions as a function of temperature. As a result, the phase transition will be detected by PAC by a

ARTICLE IN PRESS R.E. Alonso et al. / Physica B 389 (2007) 111–115

Intensity (a.u.)

x = 0.25

x = 0.50

x = 0.75

20

40

60

80

100

2θ

Fig. 1. XRD spectra of SrTixHf1xO3 for x ¼ 0.25, 0.50 and 0.75 at RT.

discontinuous change in the quadrupolar frequency. On the other hand, second-order phase transitions are produced by a continuous change of one order parameter that diminishes towards zero as temperature increases, augmenting the symmetry of the structure at the temperature were the zero value is achieved. In perovskite-type compounds the order parameter usually is related with the tilt angle of the oxygen octahedron. Such type of phase transitions will not produce a discontinuous change in the value of the quadrupolar frequency, but will vary the slope of oQ(T) at the corresponding temperature. In Fig. 2 some spin-precession curves obtained for the sample with x ¼ 0.50 together with their respective fits can be seen. The asymmetry and distribution parameters are almost constant with temperature and composition, with mean values of 0.50 and 30%, respectively. Fig. 3 shows the fitted values of the quadrupole frequency as a function of temperature and composition. The overall decrease of this hyperfine parameter with temperature is commonly associated with cell expansion. Dotted lines joining the experimental points indicate thermal regions of uniform behavior. For x ¼ 0.25, three regions can be visualized. One from RT to 550 1C, in which the quadrupole frequency oQ

113

diminishes uniformly with temperature. At about 550 1C a jump from 28–31 Mrad/s in oQ suggests the presence of a first-order phase transition. From 550–750 1C, oQ is again a uniformly decreasing function. At 750 1C, the slope of oQ(T) changes, suggesting the presence of a second-order phase transition. For x ¼ 0.50 two regions can be distinguished: from RT to 250 1C, oQ(T) is a slightly diminishing function, but at 250 1C the slope changes, becoming somewhat steeper. As no jump in the quadrupole frequency is observed, the slope variation at 250 1C could be identified, in principle, with a second-order phase transition. From 250 1C onwards, no abrupt changes in the value or the slope of oQ can be observed. For x ¼ 0 oQ is a smoothly diminishing function of temperature over the whole temperature range of measurements. It was reported that in Sr1xBaxZrO3 the effect of the substitution of cation A ¼ Sr by Ba leads to a behavior similar to that of increasing the temperature in pure SrZrO3 [8]. In the present case, the substitution of cation B ¼ Hf by Ti in SrHf1xTixO3 seems to produce the same effect. When 25% of Hf is substituted by Ti it would be expected that the whole transition series should diminish the transition temperatures because ST is cubic at RT. XRD best fit was achieved with an orthorhombic Pnma structure for x ¼ 0.25, and PAC spectroscopy shows a first-order phase transition at 550 1C and a second-order phase transition at 750 1C. According to the previous discussion, the first one would be related to the PnmaCmcm phase transition. The second one could not be related with a Cmcm-I4/mcm transformation, because the temperature of this transition is higher than the corresponding one in pure SH. Thus, the second continuous change at 750 1C could be associated with the ¯ phase transition. In the middle, there I4=mcm ! Pm3m should exist a Cmcm-I4/mcm transition that is not observed by PAC spectroscopy. This fact is generally explained by the great similarity between the different structures exhibiting only very tiny distortions from the ideal cubic structures common to all perovskites-type compounds. These tiny structural differences might produce no measurable changes in the EFG at the probe site. Following the previous argument, when 50% of Hf is substituted by Ti it should be expected that the temperatures of the phase transitions be lower than in the x ¼ 0.25 case. The XRD data fit shows a Cmcm structure, suggesting that the Pnma-Cmcm transition had occurred below RT. PAC spectroscopy shows a secondorder phase transition at 250 1C, that could be associated to the Cmcm-I4/mcm structural change. The lack of ¯ transition observance by PAC of the I4=mcm ! Pm3m should be justified on the same hypothesis as in the x ¼ 0.25 case. ¯ cubic For x ¼ 0.75, XRD spectroscopy reveals a Pm3m structure at RT. This means that the whole phase transition series would have taken place below RT for this Hf

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Table 1 Space group, lattice constant, and atomic positions determined for x ¼ 0, 0.25, 0.50, 0.75 in SrHf1xTixO3. x

Hf (%)

Space group

a(A˚)

b(A˚)

c(A˚)

Atomic positions

0

100 [6]

Pnma

5.7516

8.1344

5.7646

0.25

75

Pnma

5.5928

7.8939

5.6053

0.50

50

Cmcm

8.0123

7.9862

8.0232

0.75

25

¯ Pm3m

4.0461

4.0461

4.0461

1

0

¯ Pm3m

3.9050

3.9050

3.9050

Sr (12+0.016, 14, 0.004) Hf/Ti (0,0,0) O1 (0.063, 14, 0.014) O2 (14+0.0289, 14, 0.0311, 0.0335) Sr (12+0.00413, 14, 0.00316) Hf/Ti (0,0,0) O1 (0.04864, 14, 0.03718) O2 (14+0.03455, 0.03008, 140.04504) Sr1 (0, 0.002, 14) Sr2 (0, 0.003, 14) Hf/Ti (14, 14, 0) O1 (14+0.01646, 0,0) O2 (0, 140.02810, 0.05086) O3 (14+0.0303, 140.5590, 14) Sr (0,0,0) Hf/Ti (12, 12, 12) O1 (12, 12, 0) Sr (0,0,0) Hf/Ti (12, 12, 12) O1 (12, 12, 0)

Data for x ¼ 1 are from Ref [6].

0.18

initial RT

0.12 0.06 0.00 0.18 500° C 0.12

-A22G22(t)

0.06 0.00 0.18 1000° C 0.12 0.06 0.00 0.18 final RT 0.12 0.06 0.00 0

10

20

30

40

50

t [ns]

Fig. 2. PAC spectra of SrTi0.50Hf0.50O3 at several temperatures.

concentration. As expected, no phase transition is observed by PAC above RT. Further investigations on the SrHf1xTixO3 family using specific experimental techniques for structural determina-

tion (HTXRD, Raman, etc) should be desirable to confirm the phase diagram here proposed based on the determination of the EFG as a function of Hf concentration and temperature. The present results are also a fundamental

ARTICLE IN PRESS R.E. Alonso et al. / Physica B 389 (2007) 111–115

SrTixHf1xO3 for x ¼ 0.25, 0.50 and 0.75. For x ¼ 0.25 the compound is orthorhombic Pnma at RT and undergoes one first-order and one second-order phase transitions at 550 and 750 1C, respectively. For x ¼ 0.50 the compound is in the orthorhombic Cmcm phase at RT and undergoes a second-order phase transition at about 250 1C. For ¯ at all temperatures. x ¼ 0.75, the compound is cubic Pm3m In analogy to what occurs in other ABO3 perovskites-type compounds under substitution of the A cation (like SrxBa1xZrO3) it can be concluded that Hf substitution by Ti lowers the temperatures of the phase transitions occurring in pure SrHfO3.

40 x = 0.25 35

Quadrupolar frequency ωQ [MRad/s]

30 25 40 x= 0.50

35

115

30

25

References

35 30 x = 0.75 25 20 15 0

200

400

600

800

1

Temperature [C]

Fig. 3. Quadrupole frequency as a function of temperature in SrTixHf1xO3 for x ¼ 0.25, 0.50 and 0.75.

starting point for computational simulations of the different structures involved in this family of compounds. 4. Conclusions XRD spectroscopy at RT and PAC spectroscopy between RT and 1000 1C were applied on samples of

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