- Email: [email protected]
Structural and magnetic studies on polycrystalline Ni doped ZnV2O4
Physica B: Condensed Matter 563 (2019) 101–106
Contents lists available at ScienceDirect
Physica B: Condensed Matter journal homepage: www.elsevier.com/locate/physb
Structural and magnetic studies on polycrystalline Ni doped ZnV2O4 a,∗
Mrittika Singha , Rajeev Gupta a b
T
a,b
Material Science Programme, IIT Kanpur, 208016, India Department of Physics, IIT Kanpur, 208016, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Vanadates Vegard's law Raman Ni doping
In the present investigation, we report the effect of Ni doping on the structural and magnetic properties of polycrystalline Zn1-xNixV2O4 (0 ≤ x ≤ 0.2) samples, synthesized using the solid state reaction route. X-Ray diffraction studies show that the samples are single phase and devoid of any impurity phases. Further, the Rietveld analysis of the X-ray diffraction data reveal that lattice parameter decreases with increasing Ni content with a linear dependence on Ni concentration. We find additional phonon modes in the Raman spectra of the prepared samples in comparison to the predicted modes by group theoretical analysis, which is suggestive of the migration of Zn2+ ions and presence of Ni2+ ions on the octahedral sites. Interestingly, we find that on doping with Ni the magnetic transition (TN = 41 K) and the structural transition (TS = 57 K) in ZnV2O4 are suppressed. In presence of Ni the system exhibits a spin glass like state at low temperature possibly due to changes in the orbital and spin ordering in the system.
1. Introduction Metal oxides are always scientifically and technologically important and interesting material for study especially after the discovery of high TC superconductors of barium, lanthanum, copper, and oxygen [1]. The reasons behind their immense importance are as follows: metal oxides are abundant, cost effective materials and rich in excellent physical properties. The wide range of electronic and magnetic properties are observed in transition metal oxides due to strong correlation among spin, orbital and charge ordering in such oxides which in turn give rise to a variety of exotic phenomena like ferromagnetism, superconductivity, ferroelectricity, multiferroicity etc. [2]. In this respect perovskites are always the matter of prime concern. Besides perovskites spinel oxides have emerged as a centre of attention as they display fascinating physical properties like heavy fermion behavior in LiV2O4 [3], charge ordering in AlV2O4 [4], superconducting state in LiTi2O4 [5], half metallic state in Fe3O4 [6], or transparent high conducting state in Cd2SnO4 [7] etc. Presently, Vanadates with chemical formula AV2O4 in which divalent A site is occupied by Mn, Fe, Co, Zn, Mg, Cd etc. have been investigated extensively mainly because of its unusual magnetic ordering. These compounds show strong coupling among orbital, spin and lattice degrees of freedom which is quite complex interaction process compared to the conventional interactions. In AV2O4 spinel structure, V3+ (3d2) ions with triply degenerate t2g orbital are located at the centre of each VO6 octahedron [8] and each V3+ ion
∗
Corresponding author. E-mail address: [email protected] (M. Singha).
https://doi.org/10.1016/j.physb.2019.03.037 Received 14 February 2019; Accepted 26 March 2019 Available online 28 March 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.
form a pyrochlore lattice of corner sharing tetrahedra which are coupled by antiferromagnetic super exchange interaction [9]. The geometry of pyrochlore lattice makes the antiferromagnetic interaction between V3+ ions magnetically frustrated. Such kind of frustrated magnetic systems are the origin of many interesting magnetic phases such as spin glass, noncollinear and incommensurate spin ordering and unusual critical ordering [10]. The properties of these vanadates depends on type of the cation occupying the tetrahedral A site which is surrounded by four O2− ions. When nonmagnetic cations such as Zn occupies the A site, a very interesting behavior is observed in such a strongly correlated system. In the present investigation, we have attempted to probe the effect of magnetic ion such as Ni2+ incorporation on structure as well as on magnetic behavior of ZnV2O4 with Ni doping in Zn site. Apart from this, our interest in working with Ni came from the notion of non-existence of NiV2O4 compound [11] because of appreciably small V3+- V3+ separation (R) with respect to critical separation RC (i.e. R < RC). Hence, our motive was to put an effort whether we can stabilize at least a small percentage of Ni in ZnV2O4 or not. In this respect, a close examination of structural and magnetic properties could reveal the interplay among them in terms of tuning the properties of the systems. ZnV2O4 has normal cubic spinel structure with space group Fd3¯m at room temperature and the cubic phase persists down to ∼55 K and at low temperature it undergoes two successive phase transitions. At TS = 50 K, ZnV2O4 exhibits a structural phase transition from symmetric cubic to
Physica B: Condensed Matter 563 (2019) 101–106
M. Singha and R. Gupta
angle with the increase of Ni content. The peak shifting towards higher angle implies a lattice contraction. From the refinements we obtained lattice parameter, a, and unit cell volume, a3 for each of the doping. The plots of these parameters as a function of Ni content are shown in Fig. 1(c). We observe a monotonous decrease in lattice parameters with increasing Ni doping as expected due to smaller ionic radii of Ni2+ (0.69 Å) compared to Zn2+ (0.74 Å) except for 2% doping concentration. This fact can be attributed to the cation migration in tetrahedral and octahedral sites. As mentioned before Zn atom is tetrahedrally and V atom is octahedrally coordinated by O2− ions and as the size of tetrahedral void is smaller but the bond strength is higher than octahedral void (V site), it is therefore sensitive to the small changes in the cation size occupying A site and Ni atom could (as it is present in very small percentage i.e. 2%) migrate to V site which resulted into a proportional migration of V3+ ion in tetrahedral site. The above assumption could also be verified from the increase in lattice parameter due to larger size of V3+ ion (0.78 Å) than Zn2+ ion (0.74 Å) for x = 0.02 sample. Nevertheless, the linear variation of lattice constant in the composition region 0 ≤ x ≤ 0.2 implying that the solid solutions follow Vegard's law, ai (i = 1,2, ..)Zn1 − x Nix V2 O4 = (1 − x ) ai (i = 1,2, ..)ZnO + xai (i = 1,2, ..)NiO .
less symmetric tetragonal structure with the compression of c axis followed by a paramagnetic (PM) to antiferromagnetic (AFM) transition at TN = 40 K [10]. As far as ZnV2O4 is concerned, two ordering processes- orbital ordering and orbital driven spin ordering are responsible for structural and subsequent magnetic transition at low temperature. Various groups have proposed different theoretical models to explain the orbital and spin ordering in ZnV2O4. One is ‘antiferro-orbital’ ordering proposed by Tsunetsugu and Motome [8] in which dxy orbital is occupied by one electron and dxz and dyz orbitals are alternately occupied along c axis. Second one is ‘ferro-orbital’ ordering proposed by Tchernychyov [12]. He took into account the concept of complex orbital state dxz ± idyz at each V site. Third competing model is ‘orbital-Peierls’ ordering in which dyz and dzx ordinals are aligned as [13] dyz - dyz-dxz - dxz chains along [101] or [011] directions. We have shown here, lattice parameters of polycrystalline (PC) Zn1xNixV2O4 (0 ≤ x ≤ 0.2) follow Vegard's law and in this composition range the systems form stable single phase compounds. Further, EDS analyses reveal the sample stoichiometry is maintained with some deviation for 20% Ni doped sample while structure remains same. Micro Raman study depicts the effect of cation migration of Zn2+ and Ni2+ ions into the octahedral site. On the other hand, magnetic measurements show that doping with Ni in Zn site in ZnV2O4 has influenced the orbital and spin ordering and as a result of which structural transition has suppressed completely and a cusp like feature has appeared in further lowering the temperature, typically below 20 K.
+ ai (i = 1,2, ..)V2 O3 We also found the maximum change in unit cell volume is about 0.4% for 0 ≤ x ≤ 0.2. This number is quite small compared to the size difference between Zn and Ni ions (6.8%). Moreover, we obtained the ZneZn and VeV separation of the samples from the refined data and plotted in Fig. 1(d). Upon increasing the Ni content, ZneZn separation as well as VeV separation decreases although the changes are very small. For maximum doping the changes are found to be 0.12% and 0.11% respectively. And as discussed by Mukherjee et al. [16], the contrasting behavior in polyhedral volume change could possibly be responsible for small change in overall cell volume for various compositions. To get the immediate quantitative understanding of consistency in sample composition, EDS measurements were carried out. The measured elemental analyses in units of atomic percent are listed in Table 1. The measurements reveal that the atomic percentage of V deviates more as compared to those of Ni and Zn from the ideal value. In general, EDS measurements are sensitive to three factors, viz. atomic number difference of the elements, absorption and fluorescence. In the present case, the effects of absorption and fluorescence can be neglected since the samples are not layered material and free from fluorescence generation. Therefore, the deviation could be explained due to the significant difference in atomic numbers of V (23) as compared to Ni (28) and Zn (30). However, the data depicts the reasonable consistency in composition. To investigate the variation in local structure due to compositional modulation we performed Raman scattering measurements since it is very sensitive to the deformation of local crystal structure. Raman measurements on these samples at room temperature reveal the presence of many Raman active modes, different from those predicted by factor group analysis by considering zone centre phonons as shown in Fig. 2(a). It exhibits vibrational modes in the frequency range 100–1200 cm−1. Factor group theory predicts the irreducible representation, Гirr = 5F1u+3F2g+2A2u+2Eu+2F2u + A1g + Eg + F1g by taking into account cubic normal spinel structure with space group (Oh7), 8 formula unit (f.u.) per unit cell but the Bravais lattice is rhombohedral which contains 2 f.u./unit cell. The A atoms i.e. Zn and Ni atoms occupy 8a sites, whereas, B atom i.e. V atoms occupy 16d sites and O atoms occupy 32e sites [17]. Among these vibrational modes 5 modes are Raman active (3F2g, Eg, A1g), out of five F1u modes four are infrared (IR) active and one is an acoustic mode. Since, the space group contains a centre of inversion, the Raman active and IR active modes are mutually exclusive for the same vibrational mode. Moreover, Eg and F2g modes are doubly and triply degenerate respectively. In this
2. Experimental PC Zn1-xNixV2O4 samples were prepared by the conventional solid state reaction method [10,11,14] using ZnO (99.9%), NiO (99%) and V2O3 (99.99%) as starting precursors and mixing them in proportional molar ratio. The mixtures were pressed into pellets after prolonged grinding. The pellets were then put into evacuated quartz ampoules and heat treated at 800 °C for 60 hrs. Thereafter, calcined pellets were sintered in the temperature range 850–1000 °C following the same procedure as stated above to obtain single phase compounds. The samples were characterized first by X-ray diffraction (XRD) measurement at room temperature using PANalytical, X'Pert Powder system with Ni filter, CuKα radiation (λ = 1.54056 Å). We performed Energy Dispersive Spectroscopic (EDS) analysis which is attached with the tungsten filament Scanning Electron Microscope (W-SEM) (JEOL, JSM-6010LA) to check the sample homogeneity. Micro-Raman scattering measurements were conducted in back scattering geometry using NIR laser (785 nm) as excitation source and a microscope with 50X objective using single monochromator (model: Labram HR Evaluation, JY Horiba, France) equipped with a peltier cooled charge coupled detector. All magnetic measurements were carried out in the temperature range 4.2 K–300 K using a vibrating sample magnetometer (Quantum Design) with an applied field of 100 Oe. 3. Results and discussions Fig. 1(a) shows the room temperature X-ray diffraction patterns of Zn1-xNixV2O4 (0 ≤ x ≤ 0.2). The figure also includes the Reitveld refinement curves (shown as solid lines) generated using the FullProf software [15]. The reliability factors are shown in this figure and the goodness of fit (GOF) indices are 0.91, 1.0, 1.1, 1.1, 1.1 and 0.98 respectively. The room temperature diffraction pattern for ZnV2O4 matches well with the reported JCPDS data (File No. 01-075-0318) and the Ni doped samples with x = 0.02 to 0.2 also exhibit the single phase ZnV2O4 like structure. All patterns can be fitted with cubic phase with Fd3¯m space group. Absence of any additional peaks in all patterns reveals the phase purity of the prepared samples. Fig. 1 (b) shows the enlarged view of the XRD patterns with 2θ in a narrow window of 42–44°. We observed a gradual shifting of (400) peak towards higher 102
Physica B: Condensed Matter 563 (2019) 101–106
M. Singha and R. Gupta
Fig. 1. (a) Rietveld refinement of the XRD pattern at room temperature for Zn1-xNixV2O4 samples. (b) Enlarge XRD pattern in the 2θ range 42-44° showing peak shift with Ni doping. (c) Variation of lattice parameter along with unit cell volume as a function of Ni content. (d) Variation of Zn-Zn separation and V-V separation with increasing Ni content.
(b) (a)
(c) (d)
Table 1 EDS data of the samples obtained from W-filament SEM. Sample
ZnV2O4 Zn0.98 Ni0.02V2O4
Zn0.95 Ni0.05V2O4
Zn0.90 Ni0.1V2O4
Zn0.85 Ni0.15V2O4
Zn0.8 Ni0.2V2O4
Chemical elements
Zn V Ni Zn V Ni Zn V Ni Zn V Ni Zn V Ni Zn V
(a)
(b)
Atom% Ideal
Measured
14 29 0.3 14 29 0.7 14 28.6 1 13 29 2 12 29 3 11 29
17 ± 0.4 33 ± 0.2 0.25 ± 0.08 17.0 ± 0.3 35 ± 0.2 0.8 ± 0.01 16 ± 0.05 29 ± 0.1 1 ± 0.02 12 ± 0.06 24 ± 0.04 3 ± 0.2 16 ± 0.4 34 ± 0.3 4±1 16 ± 1 35 ± 2
(c)
Fig. 2. (a) Room temperature Raman spectra of Zn1-xNixV2O4 showing crystal symmetry is preserved even after doping with Ni in Zn site and (b)–(c) shifting of mode position as a function of increasing doping level of Ni.
approach, White et al. considered the vibrations of octahedral cations are negligible; therefore, they do not contribute to the lattice vibrations. Judging from this point, the spectra in Zn1-xNixV2O4 are not straight forward to explain. A close examination of the spectra (Fig. 2(a)) shows two kinds of anomalies: (i) the splitting of few modes in two or more sub bands and (ii) presence of extra bands in higher Ni doped samples. The mode at 992 cm−1 which is observed in x = 0, 0.05 and 0.1 samples is identified as the signature of unreacted V2O5 phase. The pattern also shows that the number of Raman active modes remains same up to x = 0.1. However, we observed that two additional modes have appeared which are marked by asterisks beyond x = 0.1. Further, the mode R1 has gradually shifted to lower wave number along with the decrease in relative intensity with respect to its nearest neighbor with the increase of Ni content. It favors the fact that R1 mode could be
related to the vibration of ZneO bond whose intensity goes down with the increase of Ni concentration. This fact agrees well with most of the literature on spinel lattice according to which the lowest frequency Raman active mode (F2g (1)) is related to the total translation of AO4 unit [17,18]. Likewise the mode R4 gradually softens as the Ni content goes up. However, the variation in mode frequency as a function of composition is depicted in Fig. 2 (b) and (c). As discussed by Mukherjee et al. [16] among many possible reasons for shifting of vibrational modes, one straight forward possibility is due to the change in cation size. Since the size difference between Zn2+ and Ni2+ is 7% (ionic radii of Zn+2–0.74 Å and Ni+2–0.69 Å respectively), this factor will have significant effect on the Raman spectra. As shown by XRD data the 103
Physica B: Condensed Matter 563 (2019) 101–106
M. Singha and R. Gupta
length as a result of increase in octahedral volume due to Ni incorporation in tetrahedral site. While R6 band undergoes gradually blue shifting with the increase of Ni content. The increasing trend in R6 might come from the Ni contribution in the octahedral site due to cation migration tendency. As discussed by various groups [19,27], Ni has octahedral site preference which strengthen our interpretation that the hardening of R6 mode as a result of migration of Ni from tetrahedral to octahedral site. In addition to this, Fig. 2(a) clearly shows that relative intensity of R6 vibration increases with respect to R7 vibration as a function of Ni content and above x = 0.15, R7 mode is suppressed completely. This fact can be correlated to the change in bond strength. As we know, the electronegativity of Ni is higher than Zn (Ni∼1.91 and Zn~1.65 according to Pauling scale), NieO bonds are stronger than ZneO bonds, therefore, it is possible that high frequency modes remain unaffected while increasing the Ni content. The weak bands at 602 cm−1 and 916 cm−1 (marked by asterisks) which are visible above 10% doping are possibly arising due to chemical disorder in the samples and are due to the accompanying relaxation of selection rules in such cases. Further, to investigate the composition dependence of the magnetic transition as well as quality of the samples we carried out magnetization (M) measurement as a function of temperature (T) at 100 Oe applied field for Zn1-xNixV2O4 samples under zero field cooled (ZFC), field cooled condition (FC) (Fig. 3(a)). The M-T plot of ZnV2O4 shows a sharp rise of magnetization at 57 K and a less pronounced curvature in FC curve at 41 K while decreasing temperature. According to previous reports, the higher transition temperature corresponds to structural transition from cubic to tetragonal phase and the second transition is associated with magnetic transition from PM to AFM state [28]. Moreover, below 10 K the increase in magnetic moment with decreasing temperature has not been observed here as reported by Vasiliev et al. [29]. In our case, the difference between ZFC and FC curves persists till 150 K which is at intermediate position as observed by Ebbinghaus et al. [30] and Ueda et al. [31]. The difference between ZFC and FC is possibly due to the magnetic frustration at V site. Doping with Ni at Zn sites shifts the anomaly to lower temperature and the structural transition disappears as well. In case of ZnV2O4, as the temperature is lowered, independent V spins start forming short range correlated units, called clusters via antiferromagnetic correlation and interestingly, the short range spin correlation became stronger with Ni doping. With further lowering of temperature the clusters grow gradually and develop a weak moment until the spin freezing temperature is reached [32]. For 2% Ni doped sample, the magnetic measurement reveals a weak canted antiferromagnetic state. However, for x ≥ 0.05 the systems exhibit clear cusps at low temperature (Fig. 3(a)). These cusps are the signature of spin glass like state. Fig. 3 (b) demonstrates the variation of Curie-Weiss temperature, ϴCW as a function of composition. Susceptibility (χ) for pure sample (x = 0) is found independent of temperature above 100 K. Therefore, it does not follow Curie Weiss law and prevents the exact estimation of ϴCW. However, for doped samples χ (not shown here) shows temperature dependence in paramagnetic region. Therefore, ϴCW was obtained by fitting the susceptibility data in the temperature range 80–250 K to the Curie-Weiss law χ = χ0 + [C/ (T+ϴCW)], where χ0, C and ϴCW are the diamagnetic susceptibility, Curie constant and Weiss temperature respectively. IϴCWI decreases with increasing Ni content which indicates that molecular field is promoting the applied field and therefore, increasing the magnetization as a result of canting of neighboring moments instead of pure antiferromagnetic spin alignment, which in turn increases the strength of exchange interaction. In addition, based on the values of IϴCWI and cusp's temperature the frustration parameter, f has been calculated from the ratio between the two (=IϴCWI/spin glass temperature) as 22.8, 16.4 and 5.7 for x = 0.05, 0.1, 0.15 respectively. Thus Ni incorporation in the pure compound gradually reduces the frustration from highly frustrated to weakly frustrated system with increasing Ni content. Quantitatively the
increase in Ni content results in lattice contraction as well as change in unit cell volume due to the change in bond length (l). The decrease in the bond length causes an increase in phonon frequency (ω) as ω α l−1/ 2 . Experimentally, we observed that the theory is followed by R2 and R3 modes. By comparison with other spinels, we made a tentative mode assignment for ZnV2O4. As previously explained A1g + Eg+3F2g modes are five Raman active modes. Thus the modes R1, R2, R3 and R4 might be attributed as F2g (1), Eg, F2g (2) and F2g (3) modes respectively and whole of the high frequency band could be attributed to different forms of A1g mode. According to group theory, only one A1g mode is expected since this mode is assigned as the symmetric breathing motion of the AO4 tetrahedron. In this assignment, it has been taken into consideration that either the AO4 units are isolated from the rest of the lattice or the A-O bonds are much stronger than BeO bonds [19]. However in our case ZneO and VeO bond strengths are expected to remain similar as electronegativities of Zn and V are almost close to each other. Therefore, we can say that there is a possibility of octahedral cation to take part in high frequency vibration. However, Preudhome et al. [20] have shown in an extensive experimental work on infrared studies on II-III spinels that the high frequency vibrations exclusively depend on tetrahedral unit is incorrect. In fact Gupta et al. [21,22] in their study on sulphides and selenides, ACr2X4, have shown that B-X interaction dominate over A-X interaction. Therefore, bonding with octahedral cation is equally important as that with tetrahedral cation for A1g mode frequency. In addition to this, Verwey and Heilmann [23] have shown that although normal II-III spinel is the most stable configuration but many of them has a tendency to exist in intermediate state, a state between normal and inverse spinal (0 < t < 1). For normal spinel t = 0 and for inverse spinel t = 1 for the general structural formula of the spinel A1-tBt (B2-tAt)O4. Thus, t denotes the fraction of A cation in octahedral site. Among many other possibilities [24], cation migration in local scale could be responsible for multiplicity of A1g mode. Inversion parameter, t was determined from the relative integrated intensity (I) ratio between 220 and 440 reflections of the XRD patterns. For that the required effective Debye Waller factor, B was obtained from the Rietveld refinement data. According to Datta and Roy [25] I220/I440 is very sensitive to change in t. The values of B and t are listed in Table 2. Table 2 shows that Zn1-xNixV2O4 has a probability of cation disorder although this tendency is very small and does not differ much among doped and undoped samples. However, all the spectra (Fig. 2(a)) are very similar and depict a large band in the frequency range 750–1000 cm−1; which is split into four sub bands named as R5, R6, R7 and R8 respectively. Fig. 3 shows that R5 and R6 change monotonously while R7 and R8 remain undispersed. In spinel structure, each oxygen is four fold coordinated with one divalent and three trivalent cations [26]. Therefore, VO6 octahedron is regular only for ideal geometry for which oxygen parameter, u = 0.25; whereas ZnO4 tetrahedron is always regular. Hence, there is a possibility of octahedral distortion. Moreover, the broadness of A1g mode provides a signature that it is evolved either by cation-anion bond length and/or, by polyhedral distortion as discussed by many groups [24]. The cation disorder in Zn1-xNixV2O4 form a dominant defect on octahedral site as we found so far, which can explain the broadness of the A1g mode. R5 mode could be attributed to vibrations involved in VO6 unit on octahedral site. Monotonous decrease of R5 mode frequency is possible due to increase of VeO bond Table 2 B and t parameters of spinels Zn1-xNixV2O4. Ni content, x
B
Inversion parameter, t
0 0.02 0.05 0.1 0.15 0.2
0.694 0.638 0.360 0.604 0.947 0.738
0.22 0.18 0.16 0.17 0.21 0.21
104
Physica B: Condensed Matter 563 (2019) 101–106
M. Singha and R. Gupta
(c)
(a)
(d)
(e)
(b)
Fig. 3. (a) ZFC and FC dc magnetization vs. T plot for Zn1-xNixV2O4 (0 ≤ x ≤ 0.15) samples at 100 Oe external fields. (b) Absolute value of calculated ϴCW plot as a function of composition. (c) Schematic presentation of the model based on which canting angle has been calculated. (d) Variation of canting angle and exchange integral with increasing Ni content and (e) MS vs. x plot at 5 K.
this limit, the lattice parameter decreases linearly as a function of doping following Vegard's law. EDS data confirms that sample stoichiometry is maintained with little deviation in 20% doped one. This work shows the formation of defect structure due to cation migration in tetrahedral and octahedral site which is responsible for additional phonon modes as observed in Raman spectra than those predicted by factor group theory analysis. The modes involved with polyhedral entities in structures are clearly identified which thus help to describe the multiplicity of bands. We assign the low frequency modes to be associated with tetrahedral cations whereas high frequency modes to be associated with vibrations of octahedral cations. Magnetic measurements reveal that with the increase of doping, structural transition has suppressed completely and a spin glass like ground state has appeared at low temperature. Substitution of Ni only by 2% for Zn in Zn1xNixV2O4 system is sufficient to suppress the structural transition. The formation of spin glass like state as a result of absence of structural transition is a consequence of presence of high degree of disorder which is confirmed by Raman scattering study. However, the net magnetization has increased hundred to thousand times with increasing Ni content.
strength of exchange interaction was obtained by calculating the value of exchange constant, J from molecular field approximation, TN = 2zJS (S+1)/3kB, where z is the number of nearest neighbors of a magnetic ion, S is the total spin quantum number and kB is Boltzmann constant. And J is plotted as a function x as shown in Fig. 3(d). The very small value of magnetization for ZnV2O4 at or below TN gives an indirect signal for perfect AFM type of ground state. However, with doping the magnetization has changed drastically. Only 2% Ni is required to increase the magnetization by 103 times. Based on this observation, we tried to calculate the canting angle between moments associated with Ni atoms which belong to two different sub lattices. For simplicity we assume that one moment is pointed along the crystallographic axis and another one is at an angle with the axis. The calculated canting angles are plotted in Fig. 3(d). The schematic of the assumption is shown in Fig. 3(c). The increase in saturation magnetization, MS as a function of doping near the lowest temperature (5 K) has been summarized in Fig. 3(e). 4. Conclusions In conclusion, XRD and Raman measurement clearly show that specimens are single phase in the composition range 0 ≤ x ≤ 0.2 which means that Zn1-xNixV2O4 are successfully synthesized in this range. In 105
Physica B: Condensed Matter 563 (2019) 101–106
M. Singha and R. Gupta
Acknowledgement
[13] D.I. Khomskii, T. Mizokawa, Phys. Rev. Lett. 94 (2005) 156402. [14] Z. Zhang, D. Louca, A. Visinoiu, S.H. Lee, J.D. Thompson, T. Proffen, A. Llobet, Y. Qiu, S. Park, Y. Ueda, Phys. Rev. B 74 (2006) 014108. [15] J. Rodríguez-Carvajal, Phys. B Condens. Matter 192 (1993) 55–69. [16] S. Mukherjee, V. Ranjan, R. Gupta, A. Garg, Solid State Commun. 152 (2012) 1181–1185. [17] W.B. White, B.A. DeAngelis, Spectrochim. Acta Mol. Spectros 23 (1967) 985–995. [18] R.D. Waldron, Phys. Rev. 99 (1955) 1727–1735. [19] M.A. Laguna-Bercero, M.L. Sanjuán, R.I. Merino, J. Phys. Condens. Matter 19 (2007) 186217. [20] J. Preudhomme, P. Tarte, Spectrochim. Acta Mol. Spectros 27 (1971) 1817–1835. [21] H.C. Gupta, M.M. Sinha, K.B. Chand, Balram, Phys. Status Solidi B 169 (1992) K65–K68. [22] H.C. Gupta, A. Parashar, V.B. Gupta, B.B. Tripathi, Phys. B Condens. Matter 167 (1990) 175–181. [23] E.J.W. Verwey, E.L. Heilmann, J. Chem. Phys. 15 (1947) 174–180. [24] Z.V. Marinković Stanojević, N. Romčević, B. Stojanović, J. Eur. Ceram. Soc. 27 (2007) 903–907. [25] R.K. Datta, R. Roy, J. Am. Ceram. Soc. 50 (1967) 578–583. [26] K.E. Sickafus, J.M. Wills, N.W. Grimes, J. Am. Ceram. Soc. 82 (1999) 3279–3292. [27] R.F. Cooley, J.S. Reed, J. Am. Ceram. Soc. 55 (1972) 395–398. [28] M. Reehuis, A. Krimmel, N. Büttgen, A. Loidl, A. Prokofiev, Eur. Phys. J. B Condens. Matter Complex Syst. 35 (2003) 311–316. [29] A.N. Vasiliev, M.M. Markina, a. M. Isobe, Y. Ueda, J. Magn. Magn. Mater. 300 (2006) e375–e377. [30] S.G. Ebbinghaus, J. Hanss, M. Klemm, S. Horn, J. Alloy. Comp. 370 (2004) 75–79. [31] Y. Ueda, N. Fujiwara, a.H. Yasuoka, J. Phys. Soc. Jpn. 66 (1997) 778–783. [32] H. Mamiya, M. Onoda, Solid State Commun. 95 (1995) 217–221.
We are thankful to Indian Institute of Technology Kanpur for financial support. References [1] J. Cava Robert, J. Am. Ceram. Soc. 83 (2008) 5–28. [2] B.J. Aylett, Appl. Organomet. Chem. 13 (1999) 476–477. [3] S. Kondo, D.C. Johnston, C.A. Swenson, F. Borsa, A.V. Mahajan, L.L. Miller, T. Gu, A.I. Goldman, M.B. Maple, D.A. Gajewski, E.J. Freeman, N.R. Dilley, R.P. Dickey, J. Merrin, K. Kojima, G.M. Luke, Y.J. Uemura, O. Chmaissem, J.D. Jorgensen, Phys. Rev. Lett. 78 (1997) 3729–3732. [4] K.-i. Matsuno, T. Katsufuji, S. Mori, Y. Moritomo, A. Machida, E. Nishibori, M. Takata, M. Sakata, N. Yamamoto, H. Takagi, J. Phys. Soc. Jpn. 70 (2001) 1456–1459. [5] D.C. Johnston, H. Prakash, W.H. Zachariasen, R. Viswanathan, Mater. Res. Bull. 8 (1973) 777–784. [6] A. Yanase, K. Siratori, J. Phys. Soc. Jpn. 53 (1984) 312–317. [7] S.B. Zhang, S.-H. Wei, Appl. Phys. Lett. 80 (2002) 1376–1378. [8] Yukitoshi Motome, Hirokazu Tsunetsugu, Phys. Rev. B 70 (2004) 184427. [9] V.O. Garlea, R. Jin, D. Mandrus, B. Roessli, Q. Huang, M. Miller, A.J. Schultz, S.E. Nagler, Phys. Rev. Lett. 100 (2008) 066404. [10] Y. Ueda, N. Fujiwara, H. Yasuoka, J. Phys. Soc. Jpn. 66 (1997) 778–783. [11] D.B. Rogers, R.J. Arnott, A. Wold, J.B. Goodenough, J. Phys. Chem. Solids 24 (1963) 347–360. [12] O. Tchernyshyov, Phys. Rev. Lett. 93 (2004) 157206.
106
Contents lists available at ScienceDirect
Physica B: Condensed Matter journal homepage: www.elsevier.com/locate/physb
Structural and magnetic studies on polycrystalline Ni doped ZnV2O4 a,∗
Mrittika Singha , Rajeev Gupta a b
T
a,b
Material Science Programme, IIT Kanpur, 208016, India Department of Physics, IIT Kanpur, 208016, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Vanadates Vegard's law Raman Ni doping
In the present investigation, we report the effect of Ni doping on the structural and magnetic properties of polycrystalline Zn1-xNixV2O4 (0 ≤ x ≤ 0.2) samples, synthesized using the solid state reaction route. X-Ray diffraction studies show that the samples are single phase and devoid of any impurity phases. Further, the Rietveld analysis of the X-ray diffraction data reveal that lattice parameter decreases with increasing Ni content with a linear dependence on Ni concentration. We find additional phonon modes in the Raman spectra of the prepared samples in comparison to the predicted modes by group theoretical analysis, which is suggestive of the migration of Zn2+ ions and presence of Ni2+ ions on the octahedral sites. Interestingly, we find that on doping with Ni the magnetic transition (TN = 41 K) and the structural transition (TS = 57 K) in ZnV2O4 are suppressed. In presence of Ni the system exhibits a spin glass like state at low temperature possibly due to changes in the orbital and spin ordering in the system.
1. Introduction Metal oxides are always scientifically and technologically important and interesting material for study especially after the discovery of high TC superconductors of barium, lanthanum, copper, and oxygen [1]. The reasons behind their immense importance are as follows: metal oxides are abundant, cost effective materials and rich in excellent physical properties. The wide range of electronic and magnetic properties are observed in transition metal oxides due to strong correlation among spin, orbital and charge ordering in such oxides which in turn give rise to a variety of exotic phenomena like ferromagnetism, superconductivity, ferroelectricity, multiferroicity etc. [2]. In this respect perovskites are always the matter of prime concern. Besides perovskites spinel oxides have emerged as a centre of attention as they display fascinating physical properties like heavy fermion behavior in LiV2O4 [3], charge ordering in AlV2O4 [4], superconducting state in LiTi2O4 [5], half metallic state in Fe3O4 [6], or transparent high conducting state in Cd2SnO4 [7] etc. Presently, Vanadates with chemical formula AV2O4 in which divalent A site is occupied by Mn, Fe, Co, Zn, Mg, Cd etc. have been investigated extensively mainly because of its unusual magnetic ordering. These compounds show strong coupling among orbital, spin and lattice degrees of freedom which is quite complex interaction process compared to the conventional interactions. In AV2O4 spinel structure, V3+ (3d2) ions with triply degenerate t2g orbital are located at the centre of each VO6 octahedron [8] and each V3+ ion
∗
Corresponding author. E-mail address: [email protected] (M. Singha).
https://doi.org/10.1016/j.physb.2019.03.037 Received 14 February 2019; Accepted 26 March 2019 Available online 28 March 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.
form a pyrochlore lattice of corner sharing tetrahedra which are coupled by antiferromagnetic super exchange interaction [9]. The geometry of pyrochlore lattice makes the antiferromagnetic interaction between V3+ ions magnetically frustrated. Such kind of frustrated magnetic systems are the origin of many interesting magnetic phases such as spin glass, noncollinear and incommensurate spin ordering and unusual critical ordering [10]. The properties of these vanadates depends on type of the cation occupying the tetrahedral A site which is surrounded by four O2− ions. When nonmagnetic cations such as Zn occupies the A site, a very interesting behavior is observed in such a strongly correlated system. In the present investigation, we have attempted to probe the effect of magnetic ion such as Ni2+ incorporation on structure as well as on magnetic behavior of ZnV2O4 with Ni doping in Zn site. Apart from this, our interest in working with Ni came from the notion of non-existence of NiV2O4 compound [11] because of appreciably small V3+- V3+ separation (R) with respect to critical separation RC (i.e. R < RC). Hence, our motive was to put an effort whether we can stabilize at least a small percentage of Ni in ZnV2O4 or not. In this respect, a close examination of structural and magnetic properties could reveal the interplay among them in terms of tuning the properties of the systems. ZnV2O4 has normal cubic spinel structure with space group Fd3¯m at room temperature and the cubic phase persists down to ∼55 K and at low temperature it undergoes two successive phase transitions. At TS = 50 K, ZnV2O4 exhibits a structural phase transition from symmetric cubic to
Physica B: Condensed Matter 563 (2019) 101–106
M. Singha and R. Gupta
angle with the increase of Ni content. The peak shifting towards higher angle implies a lattice contraction. From the refinements we obtained lattice parameter, a, and unit cell volume, a3 for each of the doping. The plots of these parameters as a function of Ni content are shown in Fig. 1(c). We observe a monotonous decrease in lattice parameters with increasing Ni doping as expected due to smaller ionic radii of Ni2+ (0.69 Å) compared to Zn2+ (0.74 Å) except for 2% doping concentration. This fact can be attributed to the cation migration in tetrahedral and octahedral sites. As mentioned before Zn atom is tetrahedrally and V atom is octahedrally coordinated by O2− ions and as the size of tetrahedral void is smaller but the bond strength is higher than octahedral void (V site), it is therefore sensitive to the small changes in the cation size occupying A site and Ni atom could (as it is present in very small percentage i.e. 2%) migrate to V site which resulted into a proportional migration of V3+ ion in tetrahedral site. The above assumption could also be verified from the increase in lattice parameter due to larger size of V3+ ion (0.78 Å) than Zn2+ ion (0.74 Å) for x = 0.02 sample. Nevertheless, the linear variation of lattice constant in the composition region 0 ≤ x ≤ 0.2 implying that the solid solutions follow Vegard's law, ai (i = 1,2, ..)Zn1 − x Nix V2 O4 = (1 − x ) ai (i = 1,2, ..)ZnO + xai (i = 1,2, ..)NiO .
less symmetric tetragonal structure with the compression of c axis followed by a paramagnetic (PM) to antiferromagnetic (AFM) transition at TN = 40 K [10]. As far as ZnV2O4 is concerned, two ordering processes- orbital ordering and orbital driven spin ordering are responsible for structural and subsequent magnetic transition at low temperature. Various groups have proposed different theoretical models to explain the orbital and spin ordering in ZnV2O4. One is ‘antiferro-orbital’ ordering proposed by Tsunetsugu and Motome [8] in which dxy orbital is occupied by one electron and dxz and dyz orbitals are alternately occupied along c axis. Second one is ‘ferro-orbital’ ordering proposed by Tchernychyov [12]. He took into account the concept of complex orbital state dxz ± idyz at each V site. Third competing model is ‘orbital-Peierls’ ordering in which dyz and dzx ordinals are aligned as [13] dyz - dyz-dxz - dxz chains along [101] or [011] directions. We have shown here, lattice parameters of polycrystalline (PC) Zn1xNixV2O4 (0 ≤ x ≤ 0.2) follow Vegard's law and in this composition range the systems form stable single phase compounds. Further, EDS analyses reveal the sample stoichiometry is maintained with some deviation for 20% Ni doped sample while structure remains same. Micro Raman study depicts the effect of cation migration of Zn2+ and Ni2+ ions into the octahedral site. On the other hand, magnetic measurements show that doping with Ni in Zn site in ZnV2O4 has influenced the orbital and spin ordering and as a result of which structural transition has suppressed completely and a cusp like feature has appeared in further lowering the temperature, typically below 20 K.
+ ai (i = 1,2, ..)V2 O3 We also found the maximum change in unit cell volume is about 0.4% for 0 ≤ x ≤ 0.2. This number is quite small compared to the size difference between Zn and Ni ions (6.8%). Moreover, we obtained the ZneZn and VeV separation of the samples from the refined data and plotted in Fig. 1(d). Upon increasing the Ni content, ZneZn separation as well as VeV separation decreases although the changes are very small. For maximum doping the changes are found to be 0.12% and 0.11% respectively. And as discussed by Mukherjee et al. [16], the contrasting behavior in polyhedral volume change could possibly be responsible for small change in overall cell volume for various compositions. To get the immediate quantitative understanding of consistency in sample composition, EDS measurements were carried out. The measured elemental analyses in units of atomic percent are listed in Table 1. The measurements reveal that the atomic percentage of V deviates more as compared to those of Ni and Zn from the ideal value. In general, EDS measurements are sensitive to three factors, viz. atomic number difference of the elements, absorption and fluorescence. In the present case, the effects of absorption and fluorescence can be neglected since the samples are not layered material and free from fluorescence generation. Therefore, the deviation could be explained due to the significant difference in atomic numbers of V (23) as compared to Ni (28) and Zn (30). However, the data depicts the reasonable consistency in composition. To investigate the variation in local structure due to compositional modulation we performed Raman scattering measurements since it is very sensitive to the deformation of local crystal structure. Raman measurements on these samples at room temperature reveal the presence of many Raman active modes, different from those predicted by factor group analysis by considering zone centre phonons as shown in Fig. 2(a). It exhibits vibrational modes in the frequency range 100–1200 cm−1. Factor group theory predicts the irreducible representation, Гirr = 5F1u+3F2g+2A2u+2Eu+2F2u + A1g + Eg + F1g by taking into account cubic normal spinel structure with space group (Oh7), 8 formula unit (f.u.) per unit cell but the Bravais lattice is rhombohedral which contains 2 f.u./unit cell. The A atoms i.e. Zn and Ni atoms occupy 8a sites, whereas, B atom i.e. V atoms occupy 16d sites and O atoms occupy 32e sites [17]. Among these vibrational modes 5 modes are Raman active (3F2g, Eg, A1g), out of five F1u modes four are infrared (IR) active and one is an acoustic mode. Since, the space group contains a centre of inversion, the Raman active and IR active modes are mutually exclusive for the same vibrational mode. Moreover, Eg and F2g modes are doubly and triply degenerate respectively. In this
2. Experimental PC Zn1-xNixV2O4 samples were prepared by the conventional solid state reaction method [10,11,14] using ZnO (99.9%), NiO (99%) and V2O3 (99.99%) as starting precursors and mixing them in proportional molar ratio. The mixtures were pressed into pellets after prolonged grinding. The pellets were then put into evacuated quartz ampoules and heat treated at 800 °C for 60 hrs. Thereafter, calcined pellets were sintered in the temperature range 850–1000 °C following the same procedure as stated above to obtain single phase compounds. The samples were characterized first by X-ray diffraction (XRD) measurement at room temperature using PANalytical, X'Pert Powder system with Ni filter, CuKα radiation (λ = 1.54056 Å). We performed Energy Dispersive Spectroscopic (EDS) analysis which is attached with the tungsten filament Scanning Electron Microscope (W-SEM) (JEOL, JSM-6010LA) to check the sample homogeneity. Micro-Raman scattering measurements were conducted in back scattering geometry using NIR laser (785 nm) as excitation source and a microscope with 50X objective using single monochromator (model: Labram HR Evaluation, JY Horiba, France) equipped with a peltier cooled charge coupled detector. All magnetic measurements were carried out in the temperature range 4.2 K–300 K using a vibrating sample magnetometer (Quantum Design) with an applied field of 100 Oe. 3. Results and discussions Fig. 1(a) shows the room temperature X-ray diffraction patterns of Zn1-xNixV2O4 (0 ≤ x ≤ 0.2). The figure also includes the Reitveld refinement curves (shown as solid lines) generated using the FullProf software [15]. The reliability factors are shown in this figure and the goodness of fit (GOF) indices are 0.91, 1.0, 1.1, 1.1, 1.1 and 0.98 respectively. The room temperature diffraction pattern for ZnV2O4 matches well with the reported JCPDS data (File No. 01-075-0318) and the Ni doped samples with x = 0.02 to 0.2 also exhibit the single phase ZnV2O4 like structure. All patterns can be fitted with cubic phase with Fd3¯m space group. Absence of any additional peaks in all patterns reveals the phase purity of the prepared samples. Fig. 1 (b) shows the enlarged view of the XRD patterns with 2θ in a narrow window of 42–44°. We observed a gradual shifting of (400) peak towards higher 102
Physica B: Condensed Matter 563 (2019) 101–106
M. Singha and R. Gupta
Fig. 1. (a) Rietveld refinement of the XRD pattern at room temperature for Zn1-xNixV2O4 samples. (b) Enlarge XRD pattern in the 2θ range 42-44° showing peak shift with Ni doping. (c) Variation of lattice parameter along with unit cell volume as a function of Ni content. (d) Variation of Zn-Zn separation and V-V separation with increasing Ni content.
(b) (a)
(c) (d)
Table 1 EDS data of the samples obtained from W-filament SEM. Sample
ZnV2O4 Zn0.98 Ni0.02V2O4
Zn0.95 Ni0.05V2O4
Zn0.90 Ni0.1V2O4
Zn0.85 Ni0.15V2O4
Zn0.8 Ni0.2V2O4
Chemical elements
Zn V Ni Zn V Ni Zn V Ni Zn V Ni Zn V Ni Zn V
(a)
(b)
Atom% Ideal
Measured
14 29 0.3 14 29 0.7 14 28.6 1 13 29 2 12 29 3 11 29
17 ± 0.4 33 ± 0.2 0.25 ± 0.08 17.0 ± 0.3 35 ± 0.2 0.8 ± 0.01 16 ± 0.05 29 ± 0.1 1 ± 0.02 12 ± 0.06 24 ± 0.04 3 ± 0.2 16 ± 0.4 34 ± 0.3 4±1 16 ± 1 35 ± 2
(c)
Fig. 2. (a) Room temperature Raman spectra of Zn1-xNixV2O4 showing crystal symmetry is preserved even after doping with Ni in Zn site and (b)–(c) shifting of mode position as a function of increasing doping level of Ni.
approach, White et al. considered the vibrations of octahedral cations are negligible; therefore, they do not contribute to the lattice vibrations. Judging from this point, the spectra in Zn1-xNixV2O4 are not straight forward to explain. A close examination of the spectra (Fig. 2(a)) shows two kinds of anomalies: (i) the splitting of few modes in two or more sub bands and (ii) presence of extra bands in higher Ni doped samples. The mode at 992 cm−1 which is observed in x = 0, 0.05 and 0.1 samples is identified as the signature of unreacted V2O5 phase. The pattern also shows that the number of Raman active modes remains same up to x = 0.1. However, we observed that two additional modes have appeared which are marked by asterisks beyond x = 0.1. Further, the mode R1 has gradually shifted to lower wave number along with the decrease in relative intensity with respect to its nearest neighbor with the increase of Ni content. It favors the fact that R1 mode could be
related to the vibration of ZneO bond whose intensity goes down with the increase of Ni concentration. This fact agrees well with most of the literature on spinel lattice according to which the lowest frequency Raman active mode (F2g (1)) is related to the total translation of AO4 unit [17,18]. Likewise the mode R4 gradually softens as the Ni content goes up. However, the variation in mode frequency as a function of composition is depicted in Fig. 2 (b) and (c). As discussed by Mukherjee et al. [16] among many possible reasons for shifting of vibrational modes, one straight forward possibility is due to the change in cation size. Since the size difference between Zn2+ and Ni2+ is 7% (ionic radii of Zn+2–0.74 Å and Ni+2–0.69 Å respectively), this factor will have significant effect on the Raman spectra. As shown by XRD data the 103
Physica B: Condensed Matter 563 (2019) 101–106
M. Singha and R. Gupta
length as a result of increase in octahedral volume due to Ni incorporation in tetrahedral site. While R6 band undergoes gradually blue shifting with the increase of Ni content. The increasing trend in R6 might come from the Ni contribution in the octahedral site due to cation migration tendency. As discussed by various groups [19,27], Ni has octahedral site preference which strengthen our interpretation that the hardening of R6 mode as a result of migration of Ni from tetrahedral to octahedral site. In addition to this, Fig. 2(a) clearly shows that relative intensity of R6 vibration increases with respect to R7 vibration as a function of Ni content and above x = 0.15, R7 mode is suppressed completely. This fact can be correlated to the change in bond strength. As we know, the electronegativity of Ni is higher than Zn (Ni∼1.91 and Zn~1.65 according to Pauling scale), NieO bonds are stronger than ZneO bonds, therefore, it is possible that high frequency modes remain unaffected while increasing the Ni content. The weak bands at 602 cm−1 and 916 cm−1 (marked by asterisks) which are visible above 10% doping are possibly arising due to chemical disorder in the samples and are due to the accompanying relaxation of selection rules in such cases. Further, to investigate the composition dependence of the magnetic transition as well as quality of the samples we carried out magnetization (M) measurement as a function of temperature (T) at 100 Oe applied field for Zn1-xNixV2O4 samples under zero field cooled (ZFC), field cooled condition (FC) (Fig. 3(a)). The M-T plot of ZnV2O4 shows a sharp rise of magnetization at 57 K and a less pronounced curvature in FC curve at 41 K while decreasing temperature. According to previous reports, the higher transition temperature corresponds to structural transition from cubic to tetragonal phase and the second transition is associated with magnetic transition from PM to AFM state [28]. Moreover, below 10 K the increase in magnetic moment with decreasing temperature has not been observed here as reported by Vasiliev et al. [29]. In our case, the difference between ZFC and FC curves persists till 150 K which is at intermediate position as observed by Ebbinghaus et al. [30] and Ueda et al. [31]. The difference between ZFC and FC is possibly due to the magnetic frustration at V site. Doping with Ni at Zn sites shifts the anomaly to lower temperature and the structural transition disappears as well. In case of ZnV2O4, as the temperature is lowered, independent V spins start forming short range correlated units, called clusters via antiferromagnetic correlation and interestingly, the short range spin correlation became stronger with Ni doping. With further lowering of temperature the clusters grow gradually and develop a weak moment until the spin freezing temperature is reached [32]. For 2% Ni doped sample, the magnetic measurement reveals a weak canted antiferromagnetic state. However, for x ≥ 0.05 the systems exhibit clear cusps at low temperature (Fig. 3(a)). These cusps are the signature of spin glass like state. Fig. 3 (b) demonstrates the variation of Curie-Weiss temperature, ϴCW as a function of composition. Susceptibility (χ) for pure sample (x = 0) is found independent of temperature above 100 K. Therefore, it does not follow Curie Weiss law and prevents the exact estimation of ϴCW. However, for doped samples χ (not shown here) shows temperature dependence in paramagnetic region. Therefore, ϴCW was obtained by fitting the susceptibility data in the temperature range 80–250 K to the Curie-Weiss law χ = χ0 + [C/ (T+ϴCW)], where χ0, C and ϴCW are the diamagnetic susceptibility, Curie constant and Weiss temperature respectively. IϴCWI decreases with increasing Ni content which indicates that molecular field is promoting the applied field and therefore, increasing the magnetization as a result of canting of neighboring moments instead of pure antiferromagnetic spin alignment, which in turn increases the strength of exchange interaction. In addition, based on the values of IϴCWI and cusp's temperature the frustration parameter, f has been calculated from the ratio between the two (=IϴCWI/spin glass temperature) as 22.8, 16.4 and 5.7 for x = 0.05, 0.1, 0.15 respectively. Thus Ni incorporation in the pure compound gradually reduces the frustration from highly frustrated to weakly frustrated system with increasing Ni content. Quantitatively the
increase in Ni content results in lattice contraction as well as change in unit cell volume due to the change in bond length (l). The decrease in the bond length causes an increase in phonon frequency (ω) as ω α l−1/ 2 . Experimentally, we observed that the theory is followed by R2 and R3 modes. By comparison with other spinels, we made a tentative mode assignment for ZnV2O4. As previously explained A1g + Eg+3F2g modes are five Raman active modes. Thus the modes R1, R2, R3 and R4 might be attributed as F2g (1), Eg, F2g (2) and F2g (3) modes respectively and whole of the high frequency band could be attributed to different forms of A1g mode. According to group theory, only one A1g mode is expected since this mode is assigned as the symmetric breathing motion of the AO4 tetrahedron. In this assignment, it has been taken into consideration that either the AO4 units are isolated from the rest of the lattice or the A-O bonds are much stronger than BeO bonds [19]. However in our case ZneO and VeO bond strengths are expected to remain similar as electronegativities of Zn and V are almost close to each other. Therefore, we can say that there is a possibility of octahedral cation to take part in high frequency vibration. However, Preudhome et al. [20] have shown in an extensive experimental work on infrared studies on II-III spinels that the high frequency vibrations exclusively depend on tetrahedral unit is incorrect. In fact Gupta et al. [21,22] in their study on sulphides and selenides, ACr2X4, have shown that B-X interaction dominate over A-X interaction. Therefore, bonding with octahedral cation is equally important as that with tetrahedral cation for A1g mode frequency. In addition to this, Verwey and Heilmann [23] have shown that although normal II-III spinel is the most stable configuration but many of them has a tendency to exist in intermediate state, a state between normal and inverse spinal (0 < t < 1). For normal spinel t = 0 and for inverse spinel t = 1 for the general structural formula of the spinel A1-tBt (B2-tAt)O4. Thus, t denotes the fraction of A cation in octahedral site. Among many other possibilities [24], cation migration in local scale could be responsible for multiplicity of A1g mode. Inversion parameter, t was determined from the relative integrated intensity (I) ratio between 220 and 440 reflections of the XRD patterns. For that the required effective Debye Waller factor, B was obtained from the Rietveld refinement data. According to Datta and Roy [25] I220/I440 is very sensitive to change in t. The values of B and t are listed in Table 2. Table 2 shows that Zn1-xNixV2O4 has a probability of cation disorder although this tendency is very small and does not differ much among doped and undoped samples. However, all the spectra (Fig. 2(a)) are very similar and depict a large band in the frequency range 750–1000 cm−1; which is split into four sub bands named as R5, R6, R7 and R8 respectively. Fig. 3 shows that R5 and R6 change monotonously while R7 and R8 remain undispersed. In spinel structure, each oxygen is four fold coordinated with one divalent and three trivalent cations [26]. Therefore, VO6 octahedron is regular only for ideal geometry for which oxygen parameter, u = 0.25; whereas ZnO4 tetrahedron is always regular. Hence, there is a possibility of octahedral distortion. Moreover, the broadness of A1g mode provides a signature that it is evolved either by cation-anion bond length and/or, by polyhedral distortion as discussed by many groups [24]. The cation disorder in Zn1-xNixV2O4 form a dominant defect on octahedral site as we found so far, which can explain the broadness of the A1g mode. R5 mode could be attributed to vibrations involved in VO6 unit on octahedral site. Monotonous decrease of R5 mode frequency is possible due to increase of VeO bond Table 2 B and t parameters of spinels Zn1-xNixV2O4. Ni content, x
B
Inversion parameter, t
0 0.02 0.05 0.1 0.15 0.2
0.694 0.638 0.360 0.604 0.947 0.738
0.22 0.18 0.16 0.17 0.21 0.21
104
Physica B: Condensed Matter 563 (2019) 101–106
M. Singha and R. Gupta
(c)
(a)
(d)
(e)
(b)
Fig. 3. (a) ZFC and FC dc magnetization vs. T plot for Zn1-xNixV2O4 (0 ≤ x ≤ 0.15) samples at 100 Oe external fields. (b) Absolute value of calculated ϴCW plot as a function of composition. (c) Schematic presentation of the model based on which canting angle has been calculated. (d) Variation of canting angle and exchange integral with increasing Ni content and (e) MS vs. x plot at 5 K.
this limit, the lattice parameter decreases linearly as a function of doping following Vegard's law. EDS data confirms that sample stoichiometry is maintained with little deviation in 20% doped one. This work shows the formation of defect structure due to cation migration in tetrahedral and octahedral site which is responsible for additional phonon modes as observed in Raman spectra than those predicted by factor group theory analysis. The modes involved with polyhedral entities in structures are clearly identified which thus help to describe the multiplicity of bands. We assign the low frequency modes to be associated with tetrahedral cations whereas high frequency modes to be associated with vibrations of octahedral cations. Magnetic measurements reveal that with the increase of doping, structural transition has suppressed completely and a spin glass like ground state has appeared at low temperature. Substitution of Ni only by 2% for Zn in Zn1xNixV2O4 system is sufficient to suppress the structural transition. The formation of spin glass like state as a result of absence of structural transition is a consequence of presence of high degree of disorder which is confirmed by Raman scattering study. However, the net magnetization has increased hundred to thousand times with increasing Ni content.
strength of exchange interaction was obtained by calculating the value of exchange constant, J from molecular field approximation, TN = 2zJS (S+1)/3kB, where z is the number of nearest neighbors of a magnetic ion, S is the total spin quantum number and kB is Boltzmann constant. And J is plotted as a function x as shown in Fig. 3(d). The very small value of magnetization for ZnV2O4 at or below TN gives an indirect signal for perfect AFM type of ground state. However, with doping the magnetization has changed drastically. Only 2% Ni is required to increase the magnetization by 103 times. Based on this observation, we tried to calculate the canting angle between moments associated with Ni atoms which belong to two different sub lattices. For simplicity we assume that one moment is pointed along the crystallographic axis and another one is at an angle with the axis. The calculated canting angles are plotted in Fig. 3(d). The schematic of the assumption is shown in Fig. 3(c). The increase in saturation magnetization, MS as a function of doping near the lowest temperature (5 K) has been summarized in Fig. 3(e). 4. Conclusions In conclusion, XRD and Raman measurement clearly show that specimens are single phase in the composition range 0 ≤ x ≤ 0.2 which means that Zn1-xNixV2O4 are successfully synthesized in this range. In 105
Physica B: Condensed Matter 563 (2019) 101–106
M. Singha and R. Gupta
Acknowledgement
[13] D.I. Khomskii, T. Mizokawa, Phys. Rev. Lett. 94 (2005) 156402. [14] Z. Zhang, D. Louca, A. Visinoiu, S.H. Lee, J.D. Thompson, T. Proffen, A. Llobet, Y. Qiu, S. Park, Y. Ueda, Phys. Rev. B 74 (2006) 014108. [15] J. Rodríguez-Carvajal, Phys. B Condens. Matter 192 (1993) 55–69. [16] S. Mukherjee, V. Ranjan, R. Gupta, A. Garg, Solid State Commun. 152 (2012) 1181–1185. [17] W.B. White, B.A. DeAngelis, Spectrochim. Acta Mol. Spectros 23 (1967) 985–995. [18] R.D. Waldron, Phys. Rev. 99 (1955) 1727–1735. [19] M.A. Laguna-Bercero, M.L. Sanjuán, R.I. Merino, J. Phys. Condens. Matter 19 (2007) 186217. [20] J. Preudhomme, P. Tarte, Spectrochim. Acta Mol. Spectros 27 (1971) 1817–1835. [21] H.C. Gupta, M.M. Sinha, K.B. Chand, Balram, Phys. Status Solidi B 169 (1992) K65–K68. [22] H.C. Gupta, A. Parashar, V.B. Gupta, B.B. Tripathi, Phys. B Condens. Matter 167 (1990) 175–181. [23] E.J.W. Verwey, E.L. Heilmann, J. Chem. Phys. 15 (1947) 174–180. [24] Z.V. Marinković Stanojević, N. Romčević, B. Stojanović, J. Eur. Ceram. Soc. 27 (2007) 903–907. [25] R.K. Datta, R. Roy, J. Am. Ceram. Soc. 50 (1967) 578–583. [26] K.E. Sickafus, J.M. Wills, N.W. Grimes, J. Am. Ceram. Soc. 82 (1999) 3279–3292. [27] R.F. Cooley, J.S. Reed, J. Am. Ceram. Soc. 55 (1972) 395–398. [28] M. Reehuis, A. Krimmel, N. Büttgen, A. Loidl, A. Prokofiev, Eur. Phys. J. B Condens. Matter Complex Syst. 35 (2003) 311–316. [29] A.N. Vasiliev, M.M. Markina, a. M. Isobe, Y. Ueda, J. Magn. Magn. Mater. 300 (2006) e375–e377. [30] S.G. Ebbinghaus, J. Hanss, M. Klemm, S. Horn, J. Alloy. Comp. 370 (2004) 75–79. [31] Y. Ueda, N. Fujiwara, a.H. Yasuoka, J. Phys. Soc. Jpn. 66 (1997) 778–783. [32] H. Mamiya, M. Onoda, Solid State Commun. 95 (1995) 217–221.
We are thankful to Indian Institute of Technology Kanpur for financial support. References [1] J. Cava Robert, J. Am. Ceram. Soc. 83 (2008) 5–28. [2] B.J. Aylett, Appl. Organomet. Chem. 13 (1999) 476–477. [3] S. Kondo, D.C. Johnston, C.A. Swenson, F. Borsa, A.V. Mahajan, L.L. Miller, T. Gu, A.I. Goldman, M.B. Maple, D.A. Gajewski, E.J. Freeman, N.R. Dilley, R.P. Dickey, J. Merrin, K. Kojima, G.M. Luke, Y.J. Uemura, O. Chmaissem, J.D. Jorgensen, Phys. Rev. Lett. 78 (1997) 3729–3732. [4] K.-i. Matsuno, T. Katsufuji, S. Mori, Y. Moritomo, A. Machida, E. Nishibori, M. Takata, M. Sakata, N. Yamamoto, H. Takagi, J. Phys. Soc. Jpn. 70 (2001) 1456–1459. [5] D.C. Johnston, H. Prakash, W.H. Zachariasen, R. Viswanathan, Mater. Res. Bull. 8 (1973) 777–784. [6] A. Yanase, K. Siratori, J. Phys. Soc. Jpn. 53 (1984) 312–317. [7] S.B. Zhang, S.-H. Wei, Appl. Phys. Lett. 80 (2002) 1376–1378. [8] Yukitoshi Motome, Hirokazu Tsunetsugu, Phys. Rev. B 70 (2004) 184427. [9] V.O. Garlea, R. Jin, D. Mandrus, B. Roessli, Q. Huang, M. Miller, A.J. Schultz, S.E. Nagler, Phys. Rev. Lett. 100 (2008) 066404. [10] Y. Ueda, N. Fujiwara, H. Yasuoka, J. Phys. Soc. Jpn. 66 (1997) 778–783. [11] D.B. Rogers, R.J. Arnott, A. Wold, J.B. Goodenough, J. Phys. Chem. Solids 24 (1963) 347–360. [12] O. Tchernyshyov, Phys. Rev. Lett. 93 (2004) 157206.
106