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Crystal structure, phase transitions, and magnetic properties of titanium doped La0.5Sr0.5MnO3 perovskites
Author’s Accepted Manuscript Crystal structure, phase transitions, and magnetic properties of titanium doped La0.5Sr0.5MnO3 perovskites M. Hazzez, N. Ihzaz, M. Boudard, M. Oumezzine www.elsevier.com/locate/physb
PII: DOI: Reference:
S0921-4526(16)30043-6 http://dx.doi.org/10.1016/j.physb.2016.02.007 PHYSB309359
To appear in: Physica B: Physics of Condensed Matter Received date: 10 June 2015 Revised date: 18 January 2016 Accepted date: 2 February 2016 Cite this article as: M. Hazzez, N. Ihzaz, M. Boudard and M. Oumezzine, Crystal structure, phase transitions, and magnetic properties of titanium doped La0.5Sr0.5MnO3 perovskites, Physica B: Physics of Condensed Matter, http://dx.doi.org/10.1016/j.physb.2016.02.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Crystal structure, phase transitions, and magnetic properties of titanium doped La0.5Sr0.5MnO3 perovskites M. Hazzez a, N. Ihzaz a,1, M. Boudard b, M. Oumezzine a a
Laboratoire de Physico–Chimie des Matériaux, Département de Physique, Faculté des Sciences de Monastir,
5019 Monastir, Université de Monastir, Tunisia b
Laboratoire des Matériaux et du Génie Physique (CNRS UMR 5628), Minatec Bâtiment INPG, parvis Louis
Néel, BP 257, 38016 Grenoble Cedex 1, France
Abstract The current paper investigates the effect of titanium substitution on the structure as well as the magnetic properties of La0.5Sr0.5Mn1-xTixO3 (0≤x≤0.5) polycrystalline powder. The samples studied crystallize in a distorted perovskite structures of tetragonal (space group I4/mcm) symmetry with octahedral tilting scheme ( a 0 a 0c ), leading to the absence of octahedral tilting all along two perovskite main directions and to an out-of-phase along the third direction, or rhombohedral (space group R 3 c) symmetry with octahedral tilting scheme ( a a a ) yielding to out-of-phase along the three perovskite main directions. As the Ti content increases, a better matching of the (Mn/Ti)-O distances and (Mn/Ti)-O-(Mn/Ti) bond angle occurs. This phenomenon is created by an elongation of the (Mn/Ti)-O distance, as Mn4+ is substituted by the larger ion Ti4+. In the whole compositional range, the symmetry-adapted to atomic displacements, responsible for the out-of-phase tilting of the (Mn/Ti)O6 octahedra, stays active, anticipating tetragonal-to-rhombohedral phase transition. Taking in to account what has been explained above, measurements of magnetic properties show a decrease of magnetic ordering temperature when Ti content increases, which in turn leads to the diminution of the exchange interaction caused by reducing the FM coupling and the replacement of neighbouring manganese Mn3+-O-Mn4+ by Mn3+-O-Ti4+ bonds. This phenomenon results in broadening of the paramagnetic to ferromagnetic phase transition range. Further changes in magnetic properties with the increase in Ti concentration are studied. Keywords: Manganites, Phase transition, Tilt angle, Magnetic properties.
1
Corresponding author. E-mail address: [email protected] (N.Ihzaz).
1. Introduction Recently, systematic studies of Mn site doping effect in perovskite-related manganese oxides Ln1−xAxMnO3 (Ln : rare earth, A: alkaline-earth metals) have been carried out, emphasis has been put on structural phase transitions and magneto-electric properties [1–9], which triggered a broad interest in electrical and magnetic properties due to the existence of spin-state transitions and the unusual magnetic properties. These compounds (Ln1−xAxMnO3) exhibit a close correlation between structural and magnetic properties [10-13]. However, their disadvantage is that their Curie temperatures, TC are far below room temperature. The introduction of larger ions such as Sr, Ba and Pb in A-site, has been investigated by several researchers [14-20], the general deduction has been that partial doping results in an increase in TC, which makes Ln1−xAxMnO3 better candidates for applications at near room temperatures. This phenomenon can be achieved by an appropriate substitution of Mn ion by a nonmagnetic one, as a reduction in FM interactions enhances non-metallic behavior. In B-site, substitution of manganite family by tetravalent titanium Ti4+ ion decreases the overlap between the O2p and Mn3d orbitals, which in turn reduces the double exchange DE interaction between mixed valence Mn3+/Mn4+, thus decreasing the exchange coupling of neighbouring manganese and the magnetic ordering temperature TC, as well as increasing the resistivity [21-23]. A similar phenomenon has been observed in the La0.7Ca0.3Mn1−xTixO3 [21], and in La0.67A0.33BO3 (A = Sr, Ca and B = Mn, Ti) specimen [24]. In the current paper, we will study the effect of Ti substitution on structure as well as phase transition of La0.5Sr0.5Mn1-xTixO3 (x=0, 0.1, 0.3 and 0.5) polycrystalline powder, and their relative magnetic properties. 2. Experimental La0.5Sr0.5Mn1-xTixO3 (x=0, 0.1, 0.3 and 0.5) samples have been prepared by using a conventional solid state reaction method. The room temperature X-ray powder diffraction pattern was recorded on a Bruker D8 diffractometer (λCuKα1 = 1.54056 Å) in Bragg angle range 20°≤2θ≤120° with a step of 0.02° and counting time of 18 s per step. The structural refinement has been performed using the FullProf program [25], as well as the tetragonal setting of I4/mcm space group (No. 140) and rhombohedral structure where standard hexagonal setting of the R 3 c space group (No. 167) has been used. Magnetization (M) versus temperature (T) and magnetization versus magnetic field (H) have been measured by using two extracting sample magnetometers at high and low temperatures respectively. M(T) data
were obtained in 10 K- 400 K temperature range with an applied magnetic field of 500 Oe in field-cooled (FC) regime. Isothermal M(H) data were measured at 10 K under an applied magnetic field varying from 0 to 50 kOe. Crystallographic images have been made with the program CRYSTALMAKER [26]. 3. Results and discussions 3.1. Structural properties Fig.1. displays Rietveld analysis of X-ray diffraction data at room temperature for La0.5Sr0.5Mn1-xTixO3 (x=0, 0.1, 0.3 and 0.5) polycrystalline powder. The best correspondence between the observed and calculated XRD patterns for La0.5Sr0.5MnO3 (x=0) is achieved by tetragonal symmetry I4/mcm (space group No. 140, a
2a p , c
2a p , Z=4) yielding lower
agreement factor ( 2 1.52 % ) and confirming earlier reports for x=0 [27,28]. For the Ti concentration x=0.1, 0.3 and 0.5, the Rietveld refinements were successfully modeled using the rhombohedral and centrosymmetric space group R 3 c (No. 167, a
2a p , c 2 3a p ,
Z=6). The crystallographic transition from tetragonal to rhombohedral symmetry is identified by referring to the primitive cubic aristotype (h00)p-type reflections (the subscript p denotes the primitive cell) and the examination of their possible peak splitting. (h00)p reflections are split into doublets (220)T et (004)T suggesting the tetragonal distortion of the cubic lattice for the La0.5Sr0.5MnO3 specimen. Whereas, in samples with x=0.1, 0.3 and 0.5, the (h00)p reflections remain single, they are identified as (024)R indicative of rhombohedral symmetry (fig.2.). The basic (024)R reflections shift to lower 2θ values as Ti content increases, indicating that the lattice parameters slightly increase (see table 1, fig.4). We have concluded that the structural changes in La0.5Sr0.5Mn1-xTixO3 are undoubtedly associated with the replacement of Mn4+ ( rMn4 =0.53Å) by larger Ti4+ ( rTi4 =0.605Å) in B-site (ionic radii are taken for 6-fold coordination [29]). As expected, the Ti4+/Mn4+ substitution leads to a gradual increase in unit-cell dimensions and mean cation-anion distances through the series (fig.4.). From what has been discussed above, it is interesting to compare the phase sequence I4/mcmR 3 c. We notice that, according to Glazer’s classification scheme, La0.5Sr0.5MnO3 stabilizes in tetragonal structure with space group I4/mcm, which is derived from the cubic aristotype by antiphase rotation of the MnO6 octahedra about a fourfold axis. The split model corresponds to the tilt system ( a 0 a 0c ) in Glazer notation (G) [30,31] and (
0 x
0 y
z ) in Aleksandrov
notation (A) [32,33] which implies the absence of octahedral tilting for x and y-axes and an out-of-phase tilt angle of MnO6 octahedron for z-axis, thus the oxygen atoms are displaced in the opposite directions in successive layers of octahedra (fig.3(c), (d) and (e).). The MnO6 octahedra with two distinct oxygen positions (two apical: O1 and four equatorial: O2) is found to be slightly elongated along c axis. The Mn–O1–Mn bond angle being 180°, which explains that octahedra are not distorted along the c axis. The out-of-phase tilt
z is due to the
displacement of O atoms at the O2 site from its ideal position (1/4, 3/4, 0) to (1/4+u, 3/4+u, 0). The tilt angle
z has been calculated using the formula given in [34]. Mn–O2–Mn rotates
around the c axis roughly by
z =8.4°. These tilts can cause distortions of the MnO6
octahedra, although such distortions can be quantified by the octahedral distortion parameter d
=0.51 10 , where Δd -4
2
1 6 dn d 6n 0 d
and d is the average Mn-O bond length. The
selected bond lengths and bond angles, calculated from the structural parameters, are listed in table 1. In the series with x=0.1, 0.3 and 0.5, the perovskite lattice continues to “expand” through the rhombohedral structural range R 3 c with tilt system ( a a a ) (G) and ( x
y
z ) (A) standing for out-of-phase tilt angle of (Mn/Ti)O6 octahedron along three
perovskite main directions; x, y and z-axes. Hence, the increase in cell size involves a threecomponent rotation of the (Mn/Ti)O6 polyhedra that can be interpreted as a single tilt about a threefold axis of the cubic cell and with a unique (Ti/Mn)–O bond length d (fig.4.). According to the relationships between the tetragonal and rhombohedral cell and the cubic one, both tetragonal ( a 0 a 0c ) and rhombohedral ( a a a ) can be viewed as one tilt system _
involving a single octahedral rotation about [001]p. The tilt system corresponds to [00 1 ]T in _
I4/mcm symmetry and equivalent to [2 2 1]R using standard hexagonal setting of the R 3 c _
space group (fig.3). For [100]p and [010]p corresponding to [110]T and [1 1 0]T in tetragonal system, there is no octahedral tilting. However, in R 3 c symmetry, two identical out-of-phase _ _
tilts about [100]p and [010]p corresponding respectively to [241]R and [ 4 2 0]R axes, are observed ( x = y = z ). In R 3 c symmetry, the amplitude of the tilting angle
of the
octahedron along the three-fold axis cH, is controlled by the displacement of O atoms from
their ideal position (0.5, 0, 1/4) to (xO, 0, 1/4) and to the (Mn/Ti)–O–(Mn/Ti) bond angle δ. Using the formula given in [34], we have calculated the tilt angles from xO (table 1). A better matching of the (Mn/Ti)-O distances and δ bond angle occurs. Our results are analogous to previous studies of La1−ySry Mn1−xTixO3 (y = 0.3 or y = 0.33) compounds [35, 36] where an increase in the Ti content results in an increase in the (Mn/Ti)–O bond length d and lattice parameters. Therefore, the non-doped perovskite La0.5Sr0.5MnO3 (x=0) crystallizes with tetragonal (I/4mcm) symmetry. The increase in the nominal content of Ti4+ cations (x=0.1, 0.3 and 0.5) results in the transition towards the rhombohedral (R 3 c) phase, characterized by the formation of a larger average ionic radius of the perovskite B-site, which explains the increase of dMn–O, the cell parameters, the lattice volume and the decrease of the octahedral rotation angle. 3.2. Magnetic properties The inset of fig. 5 shows the temperature dependence of the magnetization curves M(T) of La0.5Sr0.5Mn1-xTixO3 (0≤x≤0.5) at 500 Oe magnetic field. We notice that the x = 0 sample shows an obvious ferromagnetic-paramagnetic FM-PM phase transition related to an increase in the magnetization (M) when temperature decreases. The Curie temperature TC is defined as the inflection point of the function dM/dT and estimated to be 352 K (confirming TC values reported for La0.5Sr0.5MnO3 samples in the literature [37,38]). In the doping level of 0.1≤x≤0.5, the ferromagnetism is severely suppressed, indicated by a drastic decrease of TC. The transition temperature range widens. A noticeable difference in the nature of the thermal magnetic evolution M(T) curves in the doped samples are due to the quadrivalent nonmagnetic titanium element Ti4+ doping, which strongly affects the measured TC values, lowering them continuously. The M(H) curves measured at 10 K for all samples exhibit a rapid increase when H is at low magnetic fields. It implies that a ferromagnet with a longrange FM ordering corresponds to the rearrangement of magnetic domains. The magnetization M increases constantly with no sign of saturation at higher fields, suggests a superposition of FM and AFM components. The saturation magnetization M sm easwill be referred to as the extrapolation of the M versus 1/H curve based on the law of approach given in [39]. As given in table 2, we have found out, the measured saturation moment magneton per atomic formula unit as follows:
meas expressed s
in Bohr
meas( / f .u.) s B
M smeas M m Na B
(1)
where N a is the Avogadro number, M m is the molecular mass per unit formula and Bohr magneton. The calculated saturation moment
cal s values
B
is the
have been obtained assuming a
quenched orbital moment, S = 2 for Mn3+, S = 3/2 for Mn4+ and g = 2 for both Mn3+ and Mn4+. cal s is
directly related to the Mn3+ rate (i.e. 1−x) and Mn4+ rate (i.e. x) and their corresponding
magnetic moments ( M Mn3 , M Mn4 ) as follows: cal s B / f .u.
1 x M Mn3
x M Mn4
2 B 1 x
1 1 4 x 3 2 2
(2)
The observed moments are relatively lower than those calculated for a full spin alignment, which confirms that, even with undoped specimen, an antiferromagnetic state AFM is present in all samples. We notice that this comparison can only be taken as qualitative processing. Both M(T) and H(T) measurements suggest that the ferromagnetic transition broadens as TC decreases. This is attributed to the disorder induced by the random substitution of Ti4+ in the manganese sublattice. This results in regions of different TC. Fig. 5 shows the inverse of the susceptibility as a function of temperature. For a ferromagnet, it is well known that in the PM range, the relation between
m
and the temperature T can be fitted by a Curie-Weiss law and
can be then written: C
m
(3)
T
where
is the Curie-Weiss temperature and C is the Curie constant defined as:
0 g 2J J 1 2 B 3k B
C
where 0 4
0 2 3k B eff
10 7 Hm 1 : is the permeability,
(4)
kB
1.38 10 23J/K is the Boltzmann
constant, g is the gyro-magnetic ratio, J is the total moment, μB 9.27 10 24 J/T is the Bohr magneton and
eff
is the effective paramagnetic moment. The obtained
values, as
well as the temperature range of fit, are given in table 2. The paramagnetic Curie–Weiss temperature
is found to be positive; it decreases with Ti4+ content which indicates that
mean FM interactions are dominant in the temperature range of the paramagnetic behavior. According to La0.5Sr0.5Mn1-xTixO3 composition, the calculated effective paramagnetic moment is: cal eff
2 Mn3 1 x eff
2 Mn4 x eff
(5)
where g =2,
eff
=4.90 μ B for Mn3+ ion and
eff
=3.87 μ B for Mn4+one [40 ]. From Table2, we
conclude that the measured effective magnetic moments
cal eff
in the PM regime are
significantly higher than the calculated ones. This may be the origin of the short-range FM correlation in the PM state [41]. The decrease in magnetic ordering temperature with increasing Ti content results in the diminution of the exchange interaction, which is caused by reducing the FM coupling between neighbouring manganese and the replacement of Mn3+-O-Mn4+ by Mn3+-O-Ti4+ bonds. This process leads to a widening of the paramagnetic to ferromagnetic phase transition range. The large radius of Ti4+ causes an increase in the average (Mn/Ti)-O distance and a slight growth in the bond angle (Mn/Ti)-O-(Mn/Ti). The overlap between the Mn3d and O2p orbital characterizes the bandwidth (W) empirically by: W∝cos[(1/2)(π-θMn/Ti–O–Mn/Ti)]/(dMn–O)3.5 [42]. The obtained values are listed in Table 1. We clearly notice that the decrease of the bandwidth W causes a decrease in the overlap between the O2p and Mn3d orbitals, thus reducing the DE interaction between mixed valence Mn, decreasing the exchange coupling of neighbouring manganese and the magnetic ordering temperature TC; hence increasing the resistivity [43, 44]. A similar phenomenon has been observed in the La0.7Ca0.3Mn1−xTixO3 specimen [43]. The dilution of the Mn lattice seriously disturbs the electron transfer through the Mn3+-O-Mn4+ network. The decrease in TC is associated to the decrease in the bandwidth W [44], likewise, the decrease of FM state is related to the loss of metallic character [43]. The existence of FM clusters plays a crucial role for the physical properties of perovskites at low temperatures. For different Ti content, the DE range bonds collapse with the possible appearance of FM clusters originating from persisting short-range FM correlations. In the PM regime, the anomalous measured effective magnetic moments may also be a result of the remaining short-range FM correlations in the PM state. Finally, considering the solid solution La0.5Sr0.5Mn1-xTixO3, we have expected that titanium is in its tetravalent state and consequently the non-magnetic Ti4+ (Ar, 3d0) ion substitutes the magnetic Mn4+ (Ar, t 32ge0g ) one. When titanium substitutes for manganese the charge compensation occurs via the Mn4+→Ti4+ model, as found in previous published work on La0.7Sr0.3Mn1-xTixO3
manganites [45]. This isovalent substitution is interesting since it
should modify the crystalline structure due to the difference between the tetravalent ionic radii of Mn4+ and Ti4+. Hence, the variation of Mn/Ti ratio shows a structural transition from tetragonal I4/mcm (tilt system a 0 a 0c ) to rhombohedral R 3 c phase (tilt system a a a ).
These structural modifications result in a decrease in the charge carrier bandwidth W (table 1), a replacement of some Mn3+-O2--Mn4+ bonds by Mn3+- O2--Ti4+ bonds and a progressive decrease of the tilt angle of the MnO6 octahedra. The variation of (Mn/Ti)-O bond distances and the exchange process occurring an anti-ferromagnetic ordering if neighbour atom is Ti+4, fully control the tilt and deformation of MnO6 octahedra and play an essential role in temperature dependent magnetic properties of La0.7Sr0.3Mn1-xTixO3
systems. Thus, this
feature leads to gradual decreasing both magnetization and Curie temperature. By analyzing M(T) and M(H) curves (Figs. 5, 6) we have concluded that the compounds with x ≥ 0.1 have a strongly pronounced spin-glass component. The long range network breaks down with the possible appearance of FM clusters due to remaining short-range FM correlations. Hence, we think that the distortion of rhombohedral cell leads to the increase of the canting angle of moments, which is considered like a competition between DE ferromagnetism and superexchange anti-ferromagnetism [46]. The dilution of the Mn sublattice is assumed to result in weakening of the effective exchange interaction between the Mn magnetic moments as some of them have a reduced number of magnetic next neighbours. Thus, it creates nonferromagnetic regions around the substituted Mn sites. A similar phenomenon has been observed in the same systems with x=0.15~0.2 [47] occurring a growth of the FM component at the expense of the AFM matrix. 4. Conclusion The effect of titanium-doping in manganites La0.5Sr0.5Mn1−x TixO3 (0≤x≤0.5) specimen have been investigated via X-ray diffraction and magnetization measurements. The variation of Mn/Ti ratio shows a structural transition from tetragonal I4/mcm (tilt system a 0 a 0c ) to rhombohedral R 3 c phase (tilt system a a a ) at x≥0.1. For both structures, one tilt system involving a single octahedral rotation about [001]p is in situ observed. A better matching of the (Mn/Ti)-O distances and (Mn/Ti)-O-(Mn/Ti) bond angle occurs. A decrease of magnetic ordering temperature with increasing Ti content is emphasized. A reduction in the saturated magnetic moment, when Ti-doped increases, is mainly an outcome of the dilution Mn4+ ions and the weakened FM double exchange interaction. This process leads to a widening of the paramagnetic to ferromagnetic phase transition range. The large radius of Ti4+ causes an increase in the average (Mn/Ti)-O distance, the cell parameters, the lattice volume and the decrease of the octahedral rotation angle.
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Figure captions: Fig. 1. XRD patterns of La0.5Sr0.5Mn1-xTixO3compounds, (a) x=0, (b) x=0.1 (c) x=0.3 and (d) x=0.5. Dots indicate the experimental data and the calculated data is the continuous line overlapping them. The lowest curve shows the difference between experimental and calculated patterns. The vertical bars indicate the expected reflection positions. Fig.2. A section of room-temperature diffraction patterns showing the evolution of the basic (h00)p reflections as function of Ti content in the 46-48° 2 range, which are indexed as (024)R in R 3 c, (220)T and (004)T in I4/mcm structure (in which the Gaussian fittings are also presented in dashed line, indicative of doublet peaks). The significance of the peak splitting and the change of their relative intensities are discussed in the text. Fig.3. Schematic representation of the crystal structure of La0.5Sr0.5Mn1-xTixO3 compounds showing the (Mn/Ti)O6 octahedra and the (La/Sr) atoms. (a) Tetragonal setting (I4/mcm space group), (b) hexagonal setting (R 3 c space group). A projected view of crystal structures of the sample (x=0) along: the [100]p (c), the [010]p (d) and The [001]p (e). The antiphase ( a 0 a 0c ) tilting of the adjacent octahedral layers can be clearly seen. A projected view of samples (x≥0.1), with the tilt system ( a a a ) involving a octahedral rotation about the pseudocubic axis [100]p (f), the [010]p (g) and The [001]p (h); rhombohedral phase using standard hexagonal setting of the R 3 c space group. Fig.4. Variation in unit-cell parameters, volumes (reduced to pseudocubic subcell), variation in mean interatomic distances d=(Mn/Ti)-O and bond angle δ=(Mn/Ti)-O-(Mn/Ti)
as a
function of the nominal Ti content x. Fig.5. The temperature dependence of the inverse of the magnetic susceptibility for La0.5Sr0.5Mn1-xTixO3 (0≤x≤0.5) measured at H= 500 Oe. The inset shows the M(T) curves measured at H= 500 Oe magnetic field; (a) M(T) below 300 K, with BS2 magnetometer and (b) M(T) above 300K with BS1 magnetometer. Fig.6. M(H) curves for La0.5Sr0.5Mn1-xTixO3 (0≤x≤0.5) at 10 K.
Fig.1.
.)
(024)R
x =0.5
Fig.2.
Fig.3.
Fig.4.
M (emu/g)
1/ (emu/mol/Oe)
-1
120 100 80 60 40
30 25 20 15 10 5 0
(a) - BS2
(b) - BS1
H = 500 Oe
0
100 200 300 400 500 T (K)
x =0 x =0.1 x =0.3 x =0.5
20 0 0
100
200
T (K)
Fig.5.
300
400
70
M (emu/g)
60 50 40 30 20
x =0 x =0.1 x =0.3 x =0.5
10 0 0
20
40
H (kOe)
Fig.6.
60
80
Tables: Table 1: Room temperature structural parameters (Rietveld refinement) for La0.5Sr0.5Mn1-xTixO3 specimen: tetragonal phase I4/mcm (No. 140) for x=0 and rhombohedral R 3 c (No. 167) phase for 0.1≤x≤.0.5.
x 0 Space group
0.1
0.3
0.5
R3c
I4/mcm
Cell a (Å)
5.4512 (2)
5.4660 (1)
5.4998 (1)
5.5400(3)
c (Å)
7.6748 (4)
13.3151 (6)
13.3787 (3)
13.5048(2)
Volume (Å3)
228.06 (1)
344.52 (3)
350.47 (1)
358.95 (4)
x(O)
0.2870 (11)
0.4575 (6)
0.4616 (5)
0.4720 (2)
d (Å)
1,9384
1,9429
1,9515
1,9617
(°)
163,200
166,270
167,606
170,900
(°)
8.400
6,865
6,197
4,550
0,09756
0,09712
0,09576
0,09428
Rp(%)
2.63
3.17
3.09
9.92
Rwp(%)
3.38
4.21
4.18
13.2
1.52
2.38
2.00
4.03
Structural parameters
W Discrepancy factors
2
Note: Atomic positions in the I4/mcm structure: (La/Sr) 4b (0 ½ ¼), (Mn/Ti) 4c (0 0 0), O1 4a (0 0 ¼); O2 8h (x x+½ 0); and in the R 3 c structure: (La/Sr) 6a (0 0 ¼), (Mn/Ti) 6b (0 0 0), O 18e (x 0 ¼)
Table 2: Magnetic
transition
temperature
TC;
experimental
and
calculated
magnetic
saturation
exp exp cal effective moments S and Scal ; Curie-Weiss temperature ; experimental eff and calculated eff
paramagnetic moments for La0.5Sr0.5Mn1-xTixO3 samples.
cal S μ B/f.u.
x
exp S μ B /f.u.
TC(K)
(K)
cal eff μ B
exp eff μ B
Range of fit (K)
0
3.5
2.84
352
367
4.37
4,54
369-400
0.1
3.2
2.77
348
345
4.20
4,45
380-400
0.3
2.6
2.16
265
296
3.85
4,02
358-400
0.5
2
1.63
80
170
3.46
3,94
288-400
PII: DOI: Reference:
S0921-4526(16)30043-6 http://dx.doi.org/10.1016/j.physb.2016.02.007 PHYSB309359
To appear in: Physica B: Physics of Condensed Matter Received date: 10 June 2015 Revised date: 18 January 2016 Accepted date: 2 February 2016 Cite this article as: M. Hazzez, N. Ihzaz, M. Boudard and M. Oumezzine, Crystal structure, phase transitions, and magnetic properties of titanium doped La0.5Sr0.5MnO3 perovskites, Physica B: Physics of Condensed Matter, http://dx.doi.org/10.1016/j.physb.2016.02.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Crystal structure, phase transitions, and magnetic properties of titanium doped La0.5Sr0.5MnO3 perovskites M. Hazzez a, N. Ihzaz a,1, M. Boudard b, M. Oumezzine a a
Laboratoire de Physico–Chimie des Matériaux, Département de Physique, Faculté des Sciences de Monastir,
5019 Monastir, Université de Monastir, Tunisia b
Laboratoire des Matériaux et du Génie Physique (CNRS UMR 5628), Minatec Bâtiment INPG, parvis Louis
Néel, BP 257, 38016 Grenoble Cedex 1, France
Abstract The current paper investigates the effect of titanium substitution on the structure as well as the magnetic properties of La0.5Sr0.5Mn1-xTixO3 (0≤x≤0.5) polycrystalline powder. The samples studied crystallize in a distorted perovskite structures of tetragonal (space group I4/mcm) symmetry with octahedral tilting scheme ( a 0 a 0c ), leading to the absence of octahedral tilting all along two perovskite main directions and to an out-of-phase along the third direction, or rhombohedral (space group R 3 c) symmetry with octahedral tilting scheme ( a a a ) yielding to out-of-phase along the three perovskite main directions. As the Ti content increases, a better matching of the (Mn/Ti)-O distances and (Mn/Ti)-O-(Mn/Ti) bond angle occurs. This phenomenon is created by an elongation of the (Mn/Ti)-O distance, as Mn4+ is substituted by the larger ion Ti4+. In the whole compositional range, the symmetry-adapted to atomic displacements, responsible for the out-of-phase tilting of the (Mn/Ti)O6 octahedra, stays active, anticipating tetragonal-to-rhombohedral phase transition. Taking in to account what has been explained above, measurements of magnetic properties show a decrease of magnetic ordering temperature when Ti content increases, which in turn leads to the diminution of the exchange interaction caused by reducing the FM coupling and the replacement of neighbouring manganese Mn3+-O-Mn4+ by Mn3+-O-Ti4+ bonds. This phenomenon results in broadening of the paramagnetic to ferromagnetic phase transition range. Further changes in magnetic properties with the increase in Ti concentration are studied. Keywords: Manganites, Phase transition, Tilt angle, Magnetic properties.
1
Corresponding author. E-mail address: [email protected] (N.Ihzaz).
1. Introduction Recently, systematic studies of Mn site doping effect in perovskite-related manganese oxides Ln1−xAxMnO3 (Ln : rare earth, A: alkaline-earth metals) have been carried out, emphasis has been put on structural phase transitions and magneto-electric properties [1–9], which triggered a broad interest in electrical and magnetic properties due to the existence of spin-state transitions and the unusual magnetic properties. These compounds (Ln1−xAxMnO3) exhibit a close correlation between structural and magnetic properties [10-13]. However, their disadvantage is that their Curie temperatures, TC are far below room temperature. The introduction of larger ions such as Sr, Ba and Pb in A-site, has been investigated by several researchers [14-20], the general deduction has been that partial doping results in an increase in TC, which makes Ln1−xAxMnO3 better candidates for applications at near room temperatures. This phenomenon can be achieved by an appropriate substitution of Mn ion by a nonmagnetic one, as a reduction in FM interactions enhances non-metallic behavior. In B-site, substitution of manganite family by tetravalent titanium Ti4+ ion decreases the overlap between the O2p and Mn3d orbitals, which in turn reduces the double exchange DE interaction between mixed valence Mn3+/Mn4+, thus decreasing the exchange coupling of neighbouring manganese and the magnetic ordering temperature TC, as well as increasing the resistivity [21-23]. A similar phenomenon has been observed in the La0.7Ca0.3Mn1−xTixO3 [21], and in La0.67A0.33BO3 (A = Sr, Ca and B = Mn, Ti) specimen [24]. In the current paper, we will study the effect of Ti substitution on structure as well as phase transition of La0.5Sr0.5Mn1-xTixO3 (x=0, 0.1, 0.3 and 0.5) polycrystalline powder, and their relative magnetic properties. 2. Experimental La0.5Sr0.5Mn1-xTixO3 (x=0, 0.1, 0.3 and 0.5) samples have been prepared by using a conventional solid state reaction method. The room temperature X-ray powder diffraction pattern was recorded on a Bruker D8 diffractometer (λCuKα1 = 1.54056 Å) in Bragg angle range 20°≤2θ≤120° with a step of 0.02° and counting time of 18 s per step. The structural refinement has been performed using the FullProf program [25], as well as the tetragonal setting of I4/mcm space group (No. 140) and rhombohedral structure where standard hexagonal setting of the R 3 c space group (No. 167) has been used. Magnetization (M) versus temperature (T) and magnetization versus magnetic field (H) have been measured by using two extracting sample magnetometers at high and low temperatures respectively. M(T) data
were obtained in 10 K- 400 K temperature range with an applied magnetic field of 500 Oe in field-cooled (FC) regime. Isothermal M(H) data were measured at 10 K under an applied magnetic field varying from 0 to 50 kOe. Crystallographic images have been made with the program CRYSTALMAKER [26]. 3. Results and discussions 3.1. Structural properties Fig.1. displays Rietveld analysis of X-ray diffraction data at room temperature for La0.5Sr0.5Mn1-xTixO3 (x=0, 0.1, 0.3 and 0.5) polycrystalline powder. The best correspondence between the observed and calculated XRD patterns for La0.5Sr0.5MnO3 (x=0) is achieved by tetragonal symmetry I4/mcm (space group No. 140, a
2a p , c
2a p , Z=4) yielding lower
agreement factor ( 2 1.52 % ) and confirming earlier reports for x=0 [27,28]. For the Ti concentration x=0.1, 0.3 and 0.5, the Rietveld refinements were successfully modeled using the rhombohedral and centrosymmetric space group R 3 c (No. 167, a
2a p , c 2 3a p ,
Z=6). The crystallographic transition from tetragonal to rhombohedral symmetry is identified by referring to the primitive cubic aristotype (h00)p-type reflections (the subscript p denotes the primitive cell) and the examination of their possible peak splitting. (h00)p reflections are split into doublets (220)T et (004)T suggesting the tetragonal distortion of the cubic lattice for the La0.5Sr0.5MnO3 specimen. Whereas, in samples with x=0.1, 0.3 and 0.5, the (h00)p reflections remain single, they are identified as (024)R indicative of rhombohedral symmetry (fig.2.). The basic (024)R reflections shift to lower 2θ values as Ti content increases, indicating that the lattice parameters slightly increase (see table 1, fig.4). We have concluded that the structural changes in La0.5Sr0.5Mn1-xTixO3 are undoubtedly associated with the replacement of Mn4+ ( rMn4 =0.53Å) by larger Ti4+ ( rTi4 =0.605Å) in B-site (ionic radii are taken for 6-fold coordination [29]). As expected, the Ti4+/Mn4+ substitution leads to a gradual increase in unit-cell dimensions and mean cation-anion distances through the series (fig.4.). From what has been discussed above, it is interesting to compare the phase sequence I4/mcmR 3 c. We notice that, according to Glazer’s classification scheme, La0.5Sr0.5MnO3 stabilizes in tetragonal structure with space group I4/mcm, which is derived from the cubic aristotype by antiphase rotation of the MnO6 octahedra about a fourfold axis. The split model corresponds to the tilt system ( a 0 a 0c ) in Glazer notation (G) [30,31] and (
0 x
0 y
z ) in Aleksandrov
notation (A) [32,33] which implies the absence of octahedral tilting for x and y-axes and an out-of-phase tilt angle of MnO6 octahedron for z-axis, thus the oxygen atoms are displaced in the opposite directions in successive layers of octahedra (fig.3(c), (d) and (e).). The MnO6 octahedra with two distinct oxygen positions (two apical: O1 and four equatorial: O2) is found to be slightly elongated along c axis. The Mn–O1–Mn bond angle being 180°, which explains that octahedra are not distorted along the c axis. The out-of-phase tilt
z is due to the
displacement of O atoms at the O2 site from its ideal position (1/4, 3/4, 0) to (1/4+u, 3/4+u, 0). The tilt angle
z has been calculated using the formula given in [34]. Mn–O2–Mn rotates
around the c axis roughly by
z =8.4°. These tilts can cause distortions of the MnO6
octahedra, although such distortions can be quantified by the octahedral distortion parameter d
=0.51 10 , where Δd -4
2
1 6 dn d 6n 0 d
and d is the average Mn-O bond length. The
selected bond lengths and bond angles, calculated from the structural parameters, are listed in table 1. In the series with x=0.1, 0.3 and 0.5, the perovskite lattice continues to “expand” through the rhombohedral structural range R 3 c with tilt system ( a a a ) (G) and ( x
y
z ) (A) standing for out-of-phase tilt angle of (Mn/Ti)O6 octahedron along three
perovskite main directions; x, y and z-axes. Hence, the increase in cell size involves a threecomponent rotation of the (Mn/Ti)O6 polyhedra that can be interpreted as a single tilt about a threefold axis of the cubic cell and with a unique (Ti/Mn)–O bond length d (fig.4.). According to the relationships between the tetragonal and rhombohedral cell and the cubic one, both tetragonal ( a 0 a 0c ) and rhombohedral ( a a a ) can be viewed as one tilt system _
involving a single octahedral rotation about [001]p. The tilt system corresponds to [00 1 ]T in _
I4/mcm symmetry and equivalent to [2 2 1]R using standard hexagonal setting of the R 3 c _
space group (fig.3). For [100]p and [010]p corresponding to [110]T and [1 1 0]T in tetragonal system, there is no octahedral tilting. However, in R 3 c symmetry, two identical out-of-phase _ _
tilts about [100]p and [010]p corresponding respectively to [241]R and [ 4 2 0]R axes, are observed ( x = y = z ). In R 3 c symmetry, the amplitude of the tilting angle
of the
octahedron along the three-fold axis cH, is controlled by the displacement of O atoms from
their ideal position (0.5, 0, 1/4) to (xO, 0, 1/4) and to the (Mn/Ti)–O–(Mn/Ti) bond angle δ. Using the formula given in [34], we have calculated the tilt angles from xO (table 1). A better matching of the (Mn/Ti)-O distances and δ bond angle occurs. Our results are analogous to previous studies of La1−ySry Mn1−xTixO3 (y = 0.3 or y = 0.33) compounds [35, 36] where an increase in the Ti content results in an increase in the (Mn/Ti)–O bond length d and lattice parameters. Therefore, the non-doped perovskite La0.5Sr0.5MnO3 (x=0) crystallizes with tetragonal (I/4mcm) symmetry. The increase in the nominal content of Ti4+ cations (x=0.1, 0.3 and 0.5) results in the transition towards the rhombohedral (R 3 c) phase, characterized by the formation of a larger average ionic radius of the perovskite B-site, which explains the increase of dMn–O, the cell parameters, the lattice volume and the decrease of the octahedral rotation angle. 3.2. Magnetic properties The inset of fig. 5 shows the temperature dependence of the magnetization curves M(T) of La0.5Sr0.5Mn1-xTixO3 (0≤x≤0.5) at 500 Oe magnetic field. We notice that the x = 0 sample shows an obvious ferromagnetic-paramagnetic FM-PM phase transition related to an increase in the magnetization (M) when temperature decreases. The Curie temperature TC is defined as the inflection point of the function dM/dT and estimated to be 352 K (confirming TC values reported for La0.5Sr0.5MnO3 samples in the literature [37,38]). In the doping level of 0.1≤x≤0.5, the ferromagnetism is severely suppressed, indicated by a drastic decrease of TC. The transition temperature range widens. A noticeable difference in the nature of the thermal magnetic evolution M(T) curves in the doped samples are due to the quadrivalent nonmagnetic titanium element Ti4+ doping, which strongly affects the measured TC values, lowering them continuously. The M(H) curves measured at 10 K for all samples exhibit a rapid increase when H is at low magnetic fields. It implies that a ferromagnet with a longrange FM ordering corresponds to the rearrangement of magnetic domains. The magnetization M increases constantly with no sign of saturation at higher fields, suggests a superposition of FM and AFM components. The saturation magnetization M sm easwill be referred to as the extrapolation of the M versus 1/H curve based on the law of approach given in [39]. As given in table 2, we have found out, the measured saturation moment magneton per atomic formula unit as follows:
meas expressed s
in Bohr
meas( / f .u.) s B
M smeas M m Na B
(1)
where N a is the Avogadro number, M m is the molecular mass per unit formula and Bohr magneton. The calculated saturation moment
cal s values
B
is the
have been obtained assuming a
quenched orbital moment, S = 2 for Mn3+, S = 3/2 for Mn4+ and g = 2 for both Mn3+ and Mn4+. cal s is
directly related to the Mn3+ rate (i.e. 1−x) and Mn4+ rate (i.e. x) and their corresponding
magnetic moments ( M Mn3 , M Mn4 ) as follows: cal s B / f .u.
1 x M Mn3
x M Mn4
2 B 1 x
1 1 4 x 3 2 2
(2)
The observed moments are relatively lower than those calculated for a full spin alignment, which confirms that, even with undoped specimen, an antiferromagnetic state AFM is present in all samples. We notice that this comparison can only be taken as qualitative processing. Both M(T) and H(T) measurements suggest that the ferromagnetic transition broadens as TC decreases. This is attributed to the disorder induced by the random substitution of Ti4+ in the manganese sublattice. This results in regions of different TC. Fig. 5 shows the inverse of the susceptibility as a function of temperature. For a ferromagnet, it is well known that in the PM range, the relation between
m
and the temperature T can be fitted by a Curie-Weiss law and
can be then written: C
m
(3)
T
where
is the Curie-Weiss temperature and C is the Curie constant defined as:
0 g 2J J 1 2 B 3k B
C
where 0 4
0 2 3k B eff
10 7 Hm 1 : is the permeability,
(4)
kB
1.38 10 23J/K is the Boltzmann
constant, g is the gyro-magnetic ratio, J is the total moment, μB 9.27 10 24 J/T is the Bohr magneton and
eff
is the effective paramagnetic moment. The obtained
values, as
well as the temperature range of fit, are given in table 2. The paramagnetic Curie–Weiss temperature
is found to be positive; it decreases with Ti4+ content which indicates that
mean FM interactions are dominant in the temperature range of the paramagnetic behavior. According to La0.5Sr0.5Mn1-xTixO3 composition, the calculated effective paramagnetic moment is: cal eff
2 Mn3 1 x eff
2 Mn4 x eff
(5)
where g =2,
eff
=4.90 μ B for Mn3+ ion and
eff
=3.87 μ B for Mn4+one [40 ]. From Table2, we
conclude that the measured effective magnetic moments
cal eff
in the PM regime are
significantly higher than the calculated ones. This may be the origin of the short-range FM correlation in the PM state [41]. The decrease in magnetic ordering temperature with increasing Ti content results in the diminution of the exchange interaction, which is caused by reducing the FM coupling between neighbouring manganese and the replacement of Mn3+-O-Mn4+ by Mn3+-O-Ti4+ bonds. This process leads to a widening of the paramagnetic to ferromagnetic phase transition range. The large radius of Ti4+ causes an increase in the average (Mn/Ti)-O distance and a slight growth in the bond angle (Mn/Ti)-O-(Mn/Ti). The overlap between the Mn3d and O2p orbital characterizes the bandwidth (W) empirically by: W∝cos[(1/2)(π-θMn/Ti–O–Mn/Ti)]/(dMn–O)3.5 [42]. The obtained values are listed in Table 1. We clearly notice that the decrease of the bandwidth W causes a decrease in the overlap between the O2p and Mn3d orbitals, thus reducing the DE interaction between mixed valence Mn, decreasing the exchange coupling of neighbouring manganese and the magnetic ordering temperature TC; hence increasing the resistivity [43, 44]. A similar phenomenon has been observed in the La0.7Ca0.3Mn1−xTixO3 specimen [43]. The dilution of the Mn lattice seriously disturbs the electron transfer through the Mn3+-O-Mn4+ network. The decrease in TC is associated to the decrease in the bandwidth W [44], likewise, the decrease of FM state is related to the loss of metallic character [43]. The existence of FM clusters plays a crucial role for the physical properties of perovskites at low temperatures. For different Ti content, the DE range bonds collapse with the possible appearance of FM clusters originating from persisting short-range FM correlations. In the PM regime, the anomalous measured effective magnetic moments may also be a result of the remaining short-range FM correlations in the PM state. Finally, considering the solid solution La0.5Sr0.5Mn1-xTixO3, we have expected that titanium is in its tetravalent state and consequently the non-magnetic Ti4+ (Ar, 3d0) ion substitutes the magnetic Mn4+ (Ar, t 32ge0g ) one. When titanium substitutes for manganese the charge compensation occurs via the Mn4+→Ti4+ model, as found in previous published work on La0.7Sr0.3Mn1-xTixO3
manganites [45]. This isovalent substitution is interesting since it
should modify the crystalline structure due to the difference between the tetravalent ionic radii of Mn4+ and Ti4+. Hence, the variation of Mn/Ti ratio shows a structural transition from tetragonal I4/mcm (tilt system a 0 a 0c ) to rhombohedral R 3 c phase (tilt system a a a ).
These structural modifications result in a decrease in the charge carrier bandwidth W (table 1), a replacement of some Mn3+-O2--Mn4+ bonds by Mn3+- O2--Ti4+ bonds and a progressive decrease of the tilt angle of the MnO6 octahedra. The variation of (Mn/Ti)-O bond distances and the exchange process occurring an anti-ferromagnetic ordering if neighbour atom is Ti+4, fully control the tilt and deformation of MnO6 octahedra and play an essential role in temperature dependent magnetic properties of La0.7Sr0.3Mn1-xTixO3
systems. Thus, this
feature leads to gradual decreasing both magnetization and Curie temperature. By analyzing M(T) and M(H) curves (Figs. 5, 6) we have concluded that the compounds with x ≥ 0.1 have a strongly pronounced spin-glass component. The long range network breaks down with the possible appearance of FM clusters due to remaining short-range FM correlations. Hence, we think that the distortion of rhombohedral cell leads to the increase of the canting angle of moments, which is considered like a competition between DE ferromagnetism and superexchange anti-ferromagnetism [46]. The dilution of the Mn sublattice is assumed to result in weakening of the effective exchange interaction between the Mn magnetic moments as some of them have a reduced number of magnetic next neighbours. Thus, it creates nonferromagnetic regions around the substituted Mn sites. A similar phenomenon has been observed in the same systems with x=0.15~0.2 [47] occurring a growth of the FM component at the expense of the AFM matrix. 4. Conclusion The effect of titanium-doping in manganites La0.5Sr0.5Mn1−x TixO3 (0≤x≤0.5) specimen have been investigated via X-ray diffraction and magnetization measurements. The variation of Mn/Ti ratio shows a structural transition from tetragonal I4/mcm (tilt system a 0 a 0c ) to rhombohedral R 3 c phase (tilt system a a a ) at x≥0.1. For both structures, one tilt system involving a single octahedral rotation about [001]p is in situ observed. A better matching of the (Mn/Ti)-O distances and (Mn/Ti)-O-(Mn/Ti) bond angle occurs. A decrease of magnetic ordering temperature with increasing Ti content is emphasized. A reduction in the saturated magnetic moment, when Ti-doped increases, is mainly an outcome of the dilution Mn4+ ions and the weakened FM double exchange interaction. This process leads to a widening of the paramagnetic to ferromagnetic phase transition range. The large radius of Ti4+ causes an increase in the average (Mn/Ti)-O distance, the cell parameters, the lattice volume and the decrease of the octahedral rotation angle.
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Figure captions: Fig. 1. XRD patterns of La0.5Sr0.5Mn1-xTixO3compounds, (a) x=0, (b) x=0.1 (c) x=0.3 and (d) x=0.5. Dots indicate the experimental data and the calculated data is the continuous line overlapping them. The lowest curve shows the difference between experimental and calculated patterns. The vertical bars indicate the expected reflection positions. Fig.2. A section of room-temperature diffraction patterns showing the evolution of the basic (h00)p reflections as function of Ti content in the 46-48° 2 range, which are indexed as (024)R in R 3 c, (220)T and (004)T in I4/mcm structure (in which the Gaussian fittings are also presented in dashed line, indicative of doublet peaks). The significance of the peak splitting and the change of their relative intensities are discussed in the text. Fig.3. Schematic representation of the crystal structure of La0.5Sr0.5Mn1-xTixO3 compounds showing the (Mn/Ti)O6 octahedra and the (La/Sr) atoms. (a) Tetragonal setting (I4/mcm space group), (b) hexagonal setting (R 3 c space group). A projected view of crystal structures of the sample (x=0) along: the [100]p (c), the [010]p (d) and The [001]p (e). The antiphase ( a 0 a 0c ) tilting of the adjacent octahedral layers can be clearly seen. A projected view of samples (x≥0.1), with the tilt system ( a a a ) involving a octahedral rotation about the pseudocubic axis [100]p (f), the [010]p (g) and The [001]p (h); rhombohedral phase using standard hexagonal setting of the R 3 c space group. Fig.4. Variation in unit-cell parameters, volumes (reduced to pseudocubic subcell), variation in mean interatomic distances d=(Mn/Ti)-O and bond angle δ=(Mn/Ti)-O-(Mn/Ti)
as a
function of the nominal Ti content x. Fig.5. The temperature dependence of the inverse of the magnetic susceptibility for La0.5Sr0.5Mn1-xTixO3 (0≤x≤0.5) measured at H= 500 Oe. The inset shows the M(T) curves measured at H= 500 Oe magnetic field; (a) M(T) below 300 K, with BS2 magnetometer and (b) M(T) above 300K with BS1 magnetometer. Fig.6. M(H) curves for La0.5Sr0.5Mn1-xTixO3 (0≤x≤0.5) at 10 K.
Fig.1.
.)
(024)R
x =0.5
Fig.2.
Fig.3.
Fig.4.
M (emu/g)
1/ (emu/mol/Oe)
-1
120 100 80 60 40
30 25 20 15 10 5 0
(a) - BS2
(b) - BS1
H = 500 Oe
0
100 200 300 400 500 T (K)
x =0 x =0.1 x =0.3 x =0.5
20 0 0
100
200
T (K)
Fig.5.
300
400
70
M (emu/g)
60 50 40 30 20
x =0 x =0.1 x =0.3 x =0.5
10 0 0
20
40
H (kOe)
Fig.6.
60
80
Tables: Table 1: Room temperature structural parameters (Rietveld refinement) for La0.5Sr0.5Mn1-xTixO3 specimen: tetragonal phase I4/mcm (No. 140) for x=0 and rhombohedral R 3 c (No. 167) phase for 0.1≤x≤.0.5.
x 0 Space group
0.1
0.3
0.5
R3c
I4/mcm
Cell a (Å)
5.4512 (2)
5.4660 (1)
5.4998 (1)
5.5400(3)
c (Å)
7.6748 (4)
13.3151 (6)
13.3787 (3)
13.5048(2)
Volume (Å3)
228.06 (1)
344.52 (3)
350.47 (1)
358.95 (4)
x(O)
0.2870 (11)
0.4575 (6)
0.4616 (5)
0.4720 (2)
d (Å)
1,9384
1,9429
1,9515
1,9617
(°)
163,200
166,270
167,606
170,900
(°)
8.400
6,865
6,197
4,550
0,09756
0,09712
0,09576
0,09428
Rp(%)
2.63
3.17
3.09
9.92
Rwp(%)
3.38
4.21
4.18
13.2
1.52
2.38
2.00
4.03
Structural parameters
W Discrepancy factors
2
Note: Atomic positions in the I4/mcm structure: (La/Sr) 4b (0 ½ ¼), (Mn/Ti) 4c (0 0 0), O1 4a (0 0 ¼); O2 8h (x x+½ 0); and in the R 3 c structure: (La/Sr) 6a (0 0 ¼), (Mn/Ti) 6b (0 0 0), O 18e (x 0 ¼)
Table 2: Magnetic
transition
temperature
TC;
experimental
and
calculated
magnetic
saturation
exp exp cal effective moments S and Scal ; Curie-Weiss temperature ; experimental eff and calculated eff
paramagnetic moments for La0.5Sr0.5Mn1-xTixO3 samples.
cal S μ B/f.u.
x
exp S μ B /f.u.
TC(K)
(K)
cal eff μ B
exp eff μ B
Range of fit (K)
0
3.5
2.84
352
367
4.37
4,54
369-400
0.1
3.2
2.77
348
345
4.20
4,45
380-400
0.3
2.6
2.16
265
296
3.85
4,02
358-400
0.5
2
1.63
80
170
3.46
3,94
288-400