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The structure, thermal expansion and phase transition properties of Ho2Mo3−xWxO12 (x = 0, 1.0, 2.0) solid solutions
Materials Research Bulletin 70 (2015) 640–644
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
The structure, thermal expansion and phase transition properties of Ho2Mo3 xWxO12 (x = 0, 1.0, 2.0) solid solutions X.Z. Liu, L.J. Hao, M.M. Wu, X.B. Ma, D.F. Chen, Y.T. Liu * Department of Nuclear Physics, China Institute of Atomic Energy, Beijing 102413, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 December 2014 Received in revised form 1 May 2015 Accepted 12 May 2015 Available online 16 May 2015
Three solid solutions of Ho2Mo3 xWxO12(x = 0, 1.0, 2.0) were prepared by solid state reaction method, the temperature dependent in-situ X-ray diffraction and thermal analysis were performed to investigate their structure and thermal expansion. All samples have orthorhombic structure(space group Pbcn# 60) with negative thermal expansion at the room temperature. the substitution of W for Mo enlarges the lattice constant and slightly influences the negative thermal expansion. An irreversible phase transformation to the Pba2 phase(Tb2Mo3O12 structure) was observed at high temperature for Mo-rich samples. This ploymorphism could be effectively suppressed by the W-substitution for Mo, this phenomenon could be explained by the lower electronegativity of W6+ than Mo6+. ã2015 Elsevier Ltd. All rights reserved.
Keywords: Oxides X-ray diffraction Crystal structure Phase transitions Thermal expansion
1. Introduction In recent years, some molybdates and tungstates with a general chemical formula of A2M3O12 (A = transition metal and rare earth, M = Mo or W) were found to exhibit a negative or low positive thermal expansion coefficient(TEC) in a very wide temperature range [1–6]. This novel property could be used for achieving the desired TEC by forming the composites with the material that has a normal expansion. This will bring in important practical applications in many areas, for example, where the device suffer from immense thermal shocks or thermal mismatches, or where a temperature independent device size is needed [7]. One of major attractions of A2M3O12 compounds is that the great structural flexibility brings a large variety of TEC. The ionic radii of the A3+ may vary from Al3+(0.535 Å) to La3+(1.06 Å), providing lots of options to tailor the TEC [8,9]. The structure of A2M3O12 family has two rules: first, it shows a phase transition from low temperature monoclinic structure (P21/ a) with a positive thermal expansion (PTE) to high temperature Pbcn structure with negative thermal expansion (NTE) [10–13]. Many previous experiments have shown that the transition temperature depends on the electronegativity of A3+ [14,15], where a higher electronegativity corresponds to a reduced polyhedra rigidity and thus leads to a higher phase transition temperature. Second, its NTE property is closely related with the
* Corresponding author. Fax: +86 010 69357787. E-mail address: [email protected] (Y.T. Liu). http://dx.doi.org/10.1016/j.materresbull.2015.05.021 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.
ionic radius of A3+ [5,15], the larger A3+ leads to stronger NTE. This is due to the larger A3+ cation formes a polyhedron with less rigidity and allows a stronger transverse libration of oxygen anion between A3+ and M6+. To tailor the TEC efficiently, the material with strong NTE is highly desired. Ho3+ and Y3+ are largest cations at A position for Pbcn structure, the molybdates of them are found to form a Pbcn phase with strong NTE [16–20]. On the other hand, a polymorphism has also been reported for these two cations. The polymorgh with Pba2 phase usually coexist with Pbcn phase and they can transform into each other under proper temperature [21,22]. The Pba2 structure is ferroelectric [23] and a good candidate for laser doubling frequency material [24]. On the other side, instablility of this phase will greatly hamper the application of forming a composite with desired TEC, therefore how to stablize the Pbcn phase is an important question to answer. Furthermore, the tungstate counterpart of Ho2Mo3O12,i.e. Ho2W3O12, is isostructural with La2W3O12, which has a 8-coordinated A3+ with O2 and a space group of C2/c [19]. This structure difference makes Wsubstitution for Mo interesting. Considering also that W6+ has a lower electronegativity than Mo6+, the substitution may also have influences on phase transition property. In this work, we invesitgate the influence of W-substitution for Mo on the structure, thermal expansion and phase transition property. 2. Experiment The polycrystalline samples (Ho2Mo3 xWxO12, x = 0.0, 1.0, 2.0) were synthesized by conventional solid-state reaction with high purity oxides Ho2O3, WO3, MoO3 as starting materials. The
X.Z. Liu et al. / Materials Research Bulletin 70 (2015) 640–644
reactants were preheated at 500 C in the air for 6 h to remove the absorbed water. Stoichiometric amounts of the three oxides were thoroughly ground using an agate mortar for an hour. The mixtures were calcined in the air at 800 C for 24 h, then at 900 C for 24 h with intermediate regrinding. Finally, the samples were slowly cooled down to room temperature in the furnace. The in situ X-ray diffraction data was collected at various temperatures from a Bruker D8 X-ray diffractor with Cu Ka radiation in the angle range of 10–70 with a step of 0.02048 . The diffraction patterns of Ho2Mo3 xWxO12 (x = 0.0, 1.0, 2.0) are measured at room temperature (RT), 200 C, 300 C, 500 C, 700 C, 800 C and 850 C, respectively. The sample with x = 0 further experienced another heating cycle to study the phase stability. Before the mesasurement, the sample was held at 200 C in vaccum for an hour to remove the absorbed water mollecules from the framework and this process was monitored by the in-situ XRD. The heating rate is 30 C/min and each temperature was kept for 5 min before data were recorded. Samples experienced a natural cooling process from 850 C to RT taking about ten minutes. After first heating cycle, A XRD pattern of the Pba2 phase sample for x = 0.0 (see next section) is collected using a PANAlytical X’Pert3 Powder diffracmeter in the angle range of 5–120 . The structure analysis was performed by Reitveld refinements using Fullprof suite program [25]. Thermogravimetric (TG) and differential thermal analyses (DSC) curves were obtained in a nitrogen atmosphere in the temperature range from RT to 500 C using TA instruments SDT Q600 thermal analyzer. The heating rate was 10 C/min and Al2O3 powder is used as a reference material. 3. Result and discussion 3.1. Phase formation and thermal analysis All three samples with x = 0, 1.0, 2.0 are found to be formed a Pbcn phase with NTE property at RT, this is confirmed by the XRD patterns at RT after the dehydration treatment, which are given in Fig. 1. The TG and DSC curves are shown in Fig. 2(a) and (b), respectively. All samples show weight loss when heated, which is due to that samples are hygroscopic. It has been reported that the Pbcn phase is hygroscopic and forms a trihydrate when exposed to the moisture [26,27]. We note that for x = 0 and 1.0 (Mo-rich sample), the curve have two different inclinations corresponding to a two step process in the loss of water, while for x = 2.0 (W-rich sample), this process is only one step. It has been reported that the
Fig. 1. XRD patterns for Ho2Mo3 treatment.
xWxO12
(x = 0.0, 1.0, 2.0) at RT after dehydration
641
two and one step water losing process corresponds to molybdate and tungstate, respectively [31]. This can be explained that the trihydrate of molybdate is a mixture of amorphous and crystalline compounds, while that of tungstate is crystalline only. The number of water molecules released from samples with x = 0 and 1.0 are calculated to be 2.77 and 2.84 per unit cell, while this value for x = 2.0 is 3.05, which is consistent with those reported previously [29]. Furthermore, from Fig. 2(b), we note the endothermic peak temperature which corresponds to the water releasing process is lowered along with the W-substitution. The desorption energy of water molecule calculated from the area of endothermic peak for three samples with x = 0.0, 1.0, 2.0 are 2551 J/g, 2536 J/g and 2751 J/ g, respectively. Both the increasing water contents and lowered water releasing temperature indicate that the W-substitution enlarges the unit cell and thus the space in which the water molecule is contained. 3.2. In-situ XRD experiment for x = 0 The Ho2Mo3O12 is studied by two heating cycle at several temperatures, the in-situ diffraction patterns in the first cycle are shown in the Fig. 3. At the RT, it adopts the Pbcn structure, this is consistent with the previous reports [19]. With the increasing temperature, Bragg peaks move to higher angle, indicating a reduced lattice parameter with increasing temperature, i.e., a NTE property. These patterns are quantitatively analysed by structural Rietveld refinement and temperature dependent lattice parameteres are derived. TEC of aa = 4.47 10 6 / C, ab = 4.47 10 6 / C and ac = 4.47 10 6 / C along three principal directions are obtained, this result also very similars to the report by Xiao et. al. [19], and has a stronger NTE than molybdates with smaller A3+ [15]. With further heating, the transformation from Pbcn to Pba2 phase is observed. Our refinement gives that at 700 C there is 15.74% sample transfroms into Pba2 phase, and as the temperature reaching 800 C, this proportion increases to 44.43%. After holding the sample at 800 C for an hour, the Pbcn phase remarkably reduced, the refinment gives that the Pba2 phase increases to 83.23%. Finally when the sample is heated up to 850 C, it totally trasnforms into the Pba2 phase. Based on the XRD result of sencond heatig cycle (see upper part of Fig. 3), we found this transformation process is irreversiable, this phenomenon might stem from the metastable nature of Pba2 phase, a slow transformation and coexistence of two phase have also been observed in Y2Mo3O12 [21]. To adress the structure of Pba2 phase of Ho2Mo3O12, the structural Rietveld refinement is performed on the pattern recorded at the RT after the first heating cycle. The structure of Pba2 phase Y2Mo3O12[21] is chosen as the initial starting model. During the refinement process, with the scale factor, background parameters, lattic parameters, peak profile and position parameters varied while the occupied number and thermal parameters fixed. Due to the low symmetry of Pba2 structure, there are 51 atomic positoinal parameters were refined. Finally the refinement converges to, Rp = 10.1%, Rwp = 10.6% and Rexp = 1.8%, the result is shown in Fig. 4 and derived parameters are given in Table 1, respectively. The extracted lattice parameters are a = 10.327 Å, b = 10.303 Å and c = 10.570 Å, respectively. These are very close to the values in Y2Mo3O12 [21]. Pba2 phase is very different from Pbcn structure, this point has been shown in Ref. [21]. For Pbcn phase the corner shared polyhedron configuration cause a NTE property, while for Pba2 phase two neighboring HoO7 polyhedra share a common edge and form a Ho2O12 polyhedron, this hampers the libration motion of oxygen anion and destroys the novel NTE property in Pbcn phase. In addition, the 7-coordinated Pba2 structure has a denser structure than Pbcn phase which with 6-coordinated configuration, the unit
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Fig. 2. (a)TG and (b)DSC curves for samples with x = 0.0, 1.0, 2.0, the dashed lines are used for guiding eyes.
cell volume of Pba2 phase is 1125.83 Å3 while the Pbcn phase is 1385.81 Å3, The calculated density of two phase is 4.779 g/cm 3 and 3.882 g/cm 3, respectively. We note in Ref. [21] the density values for two phase are overestimated by a factor of 2. The giant lattice volume difference means that the transition from Pbcn to Pba2 phase causes a collapse of framework structure, this will destroy the composite and thereby device in the NTE application, so that this triansition should be avoided. 3.3. The W-substitution influence The W-substituted samples (x = 1.0, 2.0) also adopts a Pbcn structure at RT as shown in Fig. 1. The lattice constants from refinement are shown in Fig. 5, from which at first glance the Wsubstitution for Mo increases the lattice constants obviously, this attributes to the larger ionic radius of W6+ than Mo6+. Futhermore, it could be seen that both the substituted and unsubstituted samples show decreasing lattice constant and volume with the similar slope upon heating. We note that the slope below 200 C show a deviation from the whole temperature range, it is attributed into the incomplete dehydration process, so that when
Fig. 3. The in-situ XRD patterns of Ho2Mo3O12, patterns are recorded by the sequence from bottom to top.
calculating the TEC the data below 200 oC has not taken into account. The linear TEC derived from the tempearture denpendence of lattice volume are al = 7.275 10 6 / C, 7.931 10 6 / C and 8.249 10 6 / C for x = 0.0, 1.0, 2.0, respectively. This indicates that the W-substitution for Mo slightly increases the NTE property, illustrating that the ionic radius of M6+ position also has a influence on the NTE property. On the other side, the Wsubstituted samples show a different phase transition property from Ho2Mo3O12 at high temperature. Fig. 6 show parts of x = 1.0 and 2.0 XRD patterns at 850 C, we can see only a few proprotion of x = 1.0 sample transfromed into the Pba2 phase, the refinement gives that the content of 8.3%. For x = 2.0 sample, the Pba2 phase has never been observed in the temperature range of our study. The Pba2 phase contents for all three samples as a fucntion of temperature are shown in Fig. 7, from which we can see the W-substitution for Mo suppress the pahse transition and stablize the Pbcn phase effectively. As mentioned above, it has been raised that the electronegativity of A3+ always affects the phase transition property [28], The higher cation electronegativity reduces the effective charge of the
Fig. 4. Rietveld refinement result of Ho2Mo3O12 Pba2 phase at RT. The meanings of symbols are given in the legend.
X.Z. Liu et al. / Materials Research Bulletin 70 (2015) 640–644
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Table 1 The refinement resulted lattice parameters and atomic positions for Pba2 Ho2Mo3O12 at room temperature. Lattice parameters
a = 10.327 Å
b = 10.303 Å
c = 10.570 Å
Atom Name
x
y
z
0.5064 (0) 0.3131(0) 0. 4971 (0) 0.2041 (0) 0.2527 (1) 0.5384(2) 0.2937(2) 0.0069(5) 0.1514(2) 0.1526(3) 0.3351(4) 0.3758(2) 0.0666(3) 0.1740(5) 0.1120(2) 0.3182 (5) 0.3648(4)
0.7398(11) 0.2651(9) 0.3603 (12) 0.6469(13) 0.0018(15) 0.5208(18) 0.4844(19) 0.3168(13) 0.6846(18) 0.7063(15) 0.3122(17) 0.8301(16) 0.3044(15) 0.0860(16) 0.9804(18) 0.0842 (16) 0.9162(17)
Ho1 Ho2 Mo1 Mo2 Mo3 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12
0.1863 (0) 0.4969 (0) 0.2050(0) 0.0052(0) 0.2517 (1) 0.1920(3) 0.5152(4) 0.1203(2) 0.5065(6) 0.1260(18) 0.1559(3) 0.4577(2) 0.4560(3) 0.1435(4) 0.4025(2) 0.3711(4) 0.1752(4)
oxygen anion, and then reduces the O–O repulsion and lowers the rigidity of AO6 polyhedron, this makes the phase transition occurred at higher temperature and leads to an active phase transtion property. This picture might also suitable for the Mo/W site [18,28]. The Mo6+ has a relative stronger electronegativity than W6+, thus it leads to a more active phase transition property of molybdate than tungstate, this is consistent with our investigation that W substitution for Mo supresses the phase transition from Pbcn to Pba2 structure. There are also reports on Sc2Mo3O12 /Sc2W3O12 [28] and In2Mo3O12/In2W3O12 [29] systems to support
Fig. 6. The parts view of refinement results of (a) x = 1.0 and (b) x = 2.0 at 850 C, the meanings for each symbol is same as Fig. 3.
Fig. 5. The extracted lattice constants (a) a, (b) b, (c) c, and (d) V as the function of temperature for x = 0.0 (&), 1.0 (*) and 2.0 (~).
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Acknowledgements The financial support from NSFC (No. 11475268, No. 11275013) and “973 Project” (2010CB833101) are greatly appreciated. The authors are grateful to Prof. Fuhui Liao for technical support on in situ XRD and thermal analysis experiment. References
Fig. 7. The temperature dependent Pba2 phase contents for Ho2Mo3 (x = 0.0, 1.0, 2.0), the meaning for each symbol is given in the legend.
xWxO12
this point. We also note that Mo-substitution for W in La2W2O9 [30] and ZrW2O8 [31,32] leads to enhanced phase transition activity. Thus we address that the W-substitution for Mo could stablize the Pbcn structure by suppressing the phase transition property, this will be useful for improving the thermal stability of device. 4. Conclusion The solid solution Ho2Mo3 xWxO12 (x = 0.0, 1.0, 2.0) were perpared by solid state reaction method, and performed in-situ Xray diffraction from RT to 850 C. At RT all samples show a Pbcn structure with NTE property, with the temperature upto 700 C an irrevesible phase transition to Pba2 phase is observed. The Pba2 phase has a remarkable reduced lattice volume compared with Pbcn phase, so that this transition will destroy the composite matrix and further break the device, this will hamper the application of this compound as a NTE material. We found this transformation could be supressed effectively by the W-substitution for Mo, this can be explained by the W6+ has a lower electronegativity than Mo6+. This is consistent with the influence of electronegativity on A3+ site. We address that the W-substitution for Mo could stablize the Pbcn phase and thus useful for its application as a NTE material.
[1] J.S.O. Evans, T.A. Mary, A.W. Sleight, Phys. B Condens. Mater. 241–243 (1998) 311–316. [2] J.S.O. Evans, T.A. Mary, A.W. Sleight, J. Solid State Chem. 137 (1998) 148–160. [3] J.S.O. Evans, T.A. Mary, Int. J. Inorg. Mater. 2 (2000) 143–151. [4] S. Sumithra, A.K. Tyagi, A.M. Umarji, Mater. Sci. Eng. B 116 (2005) 14–18. [5] S. Sumithra, A.M. Umarji, Solid State Sci. 8 (2006) 1453–1458. [6] S. Sumithra, A.M. Umarji, Solid State Sci. 6 (2004) 1313–1319. [7] L.M. Sullivan, C.M. Lukehart, Chem. Mater. 17 (2005) 2136–2141. [8] M.M. Wu, J. Peng, S.B. Han, Z.B. Hu, Y.T. Liu, D.F. Chen, Ceram. Int. 38 (2012) 6525–6529. [9] M.M. Wu, X.Z. Liu, D.F. Chen, Q. Huang, H. Wu, Y.T. Liu, Inorg. Chem. 53 (2014) 9206–9212. [10] P.M. Forster, A. Yokochi, A.W. Sleight, J. Solid State Chem. 140 (1998) 157–158. [11] A.K. Tyagi, S.N. Achary, M.D. Mathews, J. Alloys. Compd. 339 (2002) 207–210. [12] A.W. Sleight, Inorg. Chem. 37 (1998) 2854–2860. [13] J.S.O. Evans, J. Chem. Soc. Dalton Trans. (1999) 3317–3326. [14] W.T.A. Harrison, Mater. Res. Bull. 30 (1995) 1325–1331. [15] M. Ari, P.M. Jardim, B.A. Marinkovic, F. Rizzo, F.F. Ferreira, J. Solid State Chem. 181 (2008) 1472–1479. [16] B.A. Marinkovic, P.M. Jardim, R.R. deAvillez, F. Rizzo, Solid State Sci. 7 (2005) 1377–1383. [17] L. Wang, F. Wang, P.F. Yuan, Q. Sun, E.J. Liang, Y. Jia, Z.X. Guo, Mater. Res. Bull. 48 (2013) 2724–2729. [18] B.A. Marinkovic, M. Ari, R.R. de Avillez, F. Rizzo, F.F. Ferreira, K.J. Miller, M.B. Johnson, M.A. White, Chem. Mater. 21 (2009) 2886–2894. [19] X.L. Xiao, Y.Z. Cheng, J. Peng, M.M. Wu, D.F. Chen, Z.B. Hu, R. Kiyanagi, J.S. Fieramosca, S. Short, J. Jorgensen. Solid State Sci. 10 (2008) 321–325. [20] Z.Y. Li, W.B. Song, E.J. Liang, J. Phys. Chem. C 115 (2011) 17806–17811. [21] S.D. Gates, C. Lind, J. Solid State Chem. 180 (2007) 3510–3514. [22] C. González-Silgo, C. Guzmán-Afonso, V.M. Sánchez-Fajardo, S. AcostaGutiérrez, A. Sánchez-Soares, M.E. Torres, N. Sabalisck, E. Matesanz, J. Rodríguez-Carvajal, Powder Diffr. 28 (2013) S33–S40. [23] W. Jeitschko, Acta Crystallogr. Sect. B Struct. Crystallogr. Cryst. Chem. 28 (1972) 60–76. [24] S. Kojima, T. Nakamura, Phys. Rev. B 18 (1978) 453–458. [25] J. Rodriguez-Carjaval, An Introduction to the Program FULLPROF 2006, in: J. Rodriguez-Carjaval (Ed.), Laboratoire Leon Brillouin (CEA-CNRS), France, 2007. [26] K. Nassau, H.J. Levinstein, G.M. Loiacono, J. Phys. Chem. Solids 36 (1965) 1805– 1816. [27] S. Sumithra, A.M. Umarji, Mater. Res. Bull. 40 (2005) 167–176. [28] J.S.O. Evans, T.A. Mary, A.W. Sleight, J. Solid State Chem. 133 (1997) 580–583. [29] V. Sivasubramanian, T.R. Ravindran, R. Nithya, A.K. Arora, J. Appl. Phys. 96 (2004) 387–392. [30] J.A. Collado, M.A.G. Aranda, A. Cabeza, P. Olivera-Pastor, S. Bruque, J. Solid State Chem. 167 (2002) 80–85. [31] C. Closmann, A.W. Sleight, J.C. Haygarth, J. Solid State Chem .139 (1998) 424–426. [32] H.F. Liu, K.M. Pan, Q. Jin, Z.P. Zhang, G. Wang, X.H. Zeng, Ceram. Int. 40 (2014) 3873–3878.
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
The structure, thermal expansion and phase transition properties of Ho2Mo3 xWxO12 (x = 0, 1.0, 2.0) solid solutions X.Z. Liu, L.J. Hao, M.M. Wu, X.B. Ma, D.F. Chen, Y.T. Liu * Department of Nuclear Physics, China Institute of Atomic Energy, Beijing 102413, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 December 2014 Received in revised form 1 May 2015 Accepted 12 May 2015 Available online 16 May 2015
Three solid solutions of Ho2Mo3 xWxO12(x = 0, 1.0, 2.0) were prepared by solid state reaction method, the temperature dependent in-situ X-ray diffraction and thermal analysis were performed to investigate their structure and thermal expansion. All samples have orthorhombic structure(space group Pbcn# 60) with negative thermal expansion at the room temperature. the substitution of W for Mo enlarges the lattice constant and slightly influences the negative thermal expansion. An irreversible phase transformation to the Pba2 phase(Tb2Mo3O12 structure) was observed at high temperature for Mo-rich samples. This ploymorphism could be effectively suppressed by the W-substitution for Mo, this phenomenon could be explained by the lower electronegativity of W6+ than Mo6+. ã2015 Elsevier Ltd. All rights reserved.
Keywords: Oxides X-ray diffraction Crystal structure Phase transitions Thermal expansion
1. Introduction In recent years, some molybdates and tungstates with a general chemical formula of A2M3O12 (A = transition metal and rare earth, M = Mo or W) were found to exhibit a negative or low positive thermal expansion coefficient(TEC) in a very wide temperature range [1–6]. This novel property could be used for achieving the desired TEC by forming the composites with the material that has a normal expansion. This will bring in important practical applications in many areas, for example, where the device suffer from immense thermal shocks or thermal mismatches, or where a temperature independent device size is needed [7]. One of major attractions of A2M3O12 compounds is that the great structural flexibility brings a large variety of TEC. The ionic radii of the A3+ may vary from Al3+(0.535 Å) to La3+(1.06 Å), providing lots of options to tailor the TEC [8,9]. The structure of A2M3O12 family has two rules: first, it shows a phase transition from low temperature monoclinic structure (P21/ a) with a positive thermal expansion (PTE) to high temperature Pbcn structure with negative thermal expansion (NTE) [10–13]. Many previous experiments have shown that the transition temperature depends on the electronegativity of A3+ [14,15], where a higher electronegativity corresponds to a reduced polyhedra rigidity and thus leads to a higher phase transition temperature. Second, its NTE property is closely related with the
* Corresponding author. Fax: +86 010 69357787. E-mail address: [email protected] (Y.T. Liu). http://dx.doi.org/10.1016/j.materresbull.2015.05.021 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.
ionic radius of A3+ [5,15], the larger A3+ leads to stronger NTE. This is due to the larger A3+ cation formes a polyhedron with less rigidity and allows a stronger transverse libration of oxygen anion between A3+ and M6+. To tailor the TEC efficiently, the material with strong NTE is highly desired. Ho3+ and Y3+ are largest cations at A position for Pbcn structure, the molybdates of them are found to form a Pbcn phase with strong NTE [16–20]. On the other hand, a polymorphism has also been reported for these two cations. The polymorgh with Pba2 phase usually coexist with Pbcn phase and they can transform into each other under proper temperature [21,22]. The Pba2 structure is ferroelectric [23] and a good candidate for laser doubling frequency material [24]. On the other side, instablility of this phase will greatly hamper the application of forming a composite with desired TEC, therefore how to stablize the Pbcn phase is an important question to answer. Furthermore, the tungstate counterpart of Ho2Mo3O12,i.e. Ho2W3O12, is isostructural with La2W3O12, which has a 8-coordinated A3+ with O2 and a space group of C2/c [19]. This structure difference makes Wsubstitution for Mo interesting. Considering also that W6+ has a lower electronegativity than Mo6+, the substitution may also have influences on phase transition property. In this work, we invesitgate the influence of W-substitution for Mo on the structure, thermal expansion and phase transition property. 2. Experiment The polycrystalline samples (Ho2Mo3 xWxO12, x = 0.0, 1.0, 2.0) were synthesized by conventional solid-state reaction with high purity oxides Ho2O3, WO3, MoO3 as starting materials. The
X.Z. Liu et al. / Materials Research Bulletin 70 (2015) 640–644
reactants were preheated at 500 C in the air for 6 h to remove the absorbed water. Stoichiometric amounts of the three oxides were thoroughly ground using an agate mortar for an hour. The mixtures were calcined in the air at 800 C for 24 h, then at 900 C for 24 h with intermediate regrinding. Finally, the samples were slowly cooled down to room temperature in the furnace. The in situ X-ray diffraction data was collected at various temperatures from a Bruker D8 X-ray diffractor with Cu Ka radiation in the angle range of 10–70 with a step of 0.02048 . The diffraction patterns of Ho2Mo3 xWxO12 (x = 0.0, 1.0, 2.0) are measured at room temperature (RT), 200 C, 300 C, 500 C, 700 C, 800 C and 850 C, respectively. The sample with x = 0 further experienced another heating cycle to study the phase stability. Before the mesasurement, the sample was held at 200 C in vaccum for an hour to remove the absorbed water mollecules from the framework and this process was monitored by the in-situ XRD. The heating rate is 30 C/min and each temperature was kept for 5 min before data were recorded. Samples experienced a natural cooling process from 850 C to RT taking about ten minutes. After first heating cycle, A XRD pattern of the Pba2 phase sample for x = 0.0 (see next section) is collected using a PANAlytical X’Pert3 Powder diffracmeter in the angle range of 5–120 . The structure analysis was performed by Reitveld refinements using Fullprof suite program [25]. Thermogravimetric (TG) and differential thermal analyses (DSC) curves were obtained in a nitrogen atmosphere in the temperature range from RT to 500 C using TA instruments SDT Q600 thermal analyzer. The heating rate was 10 C/min and Al2O3 powder is used as a reference material. 3. Result and discussion 3.1. Phase formation and thermal analysis All three samples with x = 0, 1.0, 2.0 are found to be formed a Pbcn phase with NTE property at RT, this is confirmed by the XRD patterns at RT after the dehydration treatment, which are given in Fig. 1. The TG and DSC curves are shown in Fig. 2(a) and (b), respectively. All samples show weight loss when heated, which is due to that samples are hygroscopic. It has been reported that the Pbcn phase is hygroscopic and forms a trihydrate when exposed to the moisture [26,27]. We note that for x = 0 and 1.0 (Mo-rich sample), the curve have two different inclinations corresponding to a two step process in the loss of water, while for x = 2.0 (W-rich sample), this process is only one step. It has been reported that the
Fig. 1. XRD patterns for Ho2Mo3 treatment.
xWxO12
(x = 0.0, 1.0, 2.0) at RT after dehydration
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two and one step water losing process corresponds to molybdate and tungstate, respectively [31]. This can be explained that the trihydrate of molybdate is a mixture of amorphous and crystalline compounds, while that of tungstate is crystalline only. The number of water molecules released from samples with x = 0 and 1.0 are calculated to be 2.77 and 2.84 per unit cell, while this value for x = 2.0 is 3.05, which is consistent with those reported previously [29]. Furthermore, from Fig. 2(b), we note the endothermic peak temperature which corresponds to the water releasing process is lowered along with the W-substitution. The desorption energy of water molecule calculated from the area of endothermic peak for three samples with x = 0.0, 1.0, 2.0 are 2551 J/g, 2536 J/g and 2751 J/ g, respectively. Both the increasing water contents and lowered water releasing temperature indicate that the W-substitution enlarges the unit cell and thus the space in which the water molecule is contained. 3.2. In-situ XRD experiment for x = 0 The Ho2Mo3O12 is studied by two heating cycle at several temperatures, the in-situ diffraction patterns in the first cycle are shown in the Fig. 3. At the RT, it adopts the Pbcn structure, this is consistent with the previous reports [19]. With the increasing temperature, Bragg peaks move to higher angle, indicating a reduced lattice parameter with increasing temperature, i.e., a NTE property. These patterns are quantitatively analysed by structural Rietveld refinement and temperature dependent lattice parameteres are derived. TEC of aa = 4.47 10 6 / C, ab = 4.47 10 6 / C and ac = 4.47 10 6 / C along three principal directions are obtained, this result also very similars to the report by Xiao et. al. [19], and has a stronger NTE than molybdates with smaller A3+ [15]. With further heating, the transformation from Pbcn to Pba2 phase is observed. Our refinement gives that at 700 C there is 15.74% sample transfroms into Pba2 phase, and as the temperature reaching 800 C, this proportion increases to 44.43%. After holding the sample at 800 C for an hour, the Pbcn phase remarkably reduced, the refinment gives that the Pba2 phase increases to 83.23%. Finally when the sample is heated up to 850 C, it totally trasnforms into the Pba2 phase. Based on the XRD result of sencond heatig cycle (see upper part of Fig. 3), we found this transformation process is irreversiable, this phenomenon might stem from the metastable nature of Pba2 phase, a slow transformation and coexistence of two phase have also been observed in Y2Mo3O12 [21]. To adress the structure of Pba2 phase of Ho2Mo3O12, the structural Rietveld refinement is performed on the pattern recorded at the RT after the first heating cycle. The structure of Pba2 phase Y2Mo3O12[21] is chosen as the initial starting model. During the refinement process, with the scale factor, background parameters, lattic parameters, peak profile and position parameters varied while the occupied number and thermal parameters fixed. Due to the low symmetry of Pba2 structure, there are 51 atomic positoinal parameters were refined. Finally the refinement converges to, Rp = 10.1%, Rwp = 10.6% and Rexp = 1.8%, the result is shown in Fig. 4 and derived parameters are given in Table 1, respectively. The extracted lattice parameters are a = 10.327 Å, b = 10.303 Å and c = 10.570 Å, respectively. These are very close to the values in Y2Mo3O12 [21]. Pba2 phase is very different from Pbcn structure, this point has been shown in Ref. [21]. For Pbcn phase the corner shared polyhedron configuration cause a NTE property, while for Pba2 phase two neighboring HoO7 polyhedra share a common edge and form a Ho2O12 polyhedron, this hampers the libration motion of oxygen anion and destroys the novel NTE property in Pbcn phase. In addition, the 7-coordinated Pba2 structure has a denser structure than Pbcn phase which with 6-coordinated configuration, the unit
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Fig. 2. (a)TG and (b)DSC curves for samples with x = 0.0, 1.0, 2.0, the dashed lines are used for guiding eyes.
cell volume of Pba2 phase is 1125.83 Å3 while the Pbcn phase is 1385.81 Å3, The calculated density of two phase is 4.779 g/cm 3 and 3.882 g/cm 3, respectively. We note in Ref. [21] the density values for two phase are overestimated by a factor of 2. The giant lattice volume difference means that the transition from Pbcn to Pba2 phase causes a collapse of framework structure, this will destroy the composite and thereby device in the NTE application, so that this triansition should be avoided. 3.3. The W-substitution influence The W-substituted samples (x = 1.0, 2.0) also adopts a Pbcn structure at RT as shown in Fig. 1. The lattice constants from refinement are shown in Fig. 5, from which at first glance the Wsubstitution for Mo increases the lattice constants obviously, this attributes to the larger ionic radius of W6+ than Mo6+. Futhermore, it could be seen that both the substituted and unsubstituted samples show decreasing lattice constant and volume with the similar slope upon heating. We note that the slope below 200 C show a deviation from the whole temperature range, it is attributed into the incomplete dehydration process, so that when
Fig. 3. The in-situ XRD patterns of Ho2Mo3O12, patterns are recorded by the sequence from bottom to top.
calculating the TEC the data below 200 oC has not taken into account. The linear TEC derived from the tempearture denpendence of lattice volume are al = 7.275 10 6 / C, 7.931 10 6 / C and 8.249 10 6 / C for x = 0.0, 1.0, 2.0, respectively. This indicates that the W-substitution for Mo slightly increases the NTE property, illustrating that the ionic radius of M6+ position also has a influence on the NTE property. On the other side, the Wsubstituted samples show a different phase transition property from Ho2Mo3O12 at high temperature. Fig. 6 show parts of x = 1.0 and 2.0 XRD patterns at 850 C, we can see only a few proprotion of x = 1.0 sample transfromed into the Pba2 phase, the refinement gives that the content of 8.3%. For x = 2.0 sample, the Pba2 phase has never been observed in the temperature range of our study. The Pba2 phase contents for all three samples as a fucntion of temperature are shown in Fig. 7, from which we can see the W-substitution for Mo suppress the pahse transition and stablize the Pbcn phase effectively. As mentioned above, it has been raised that the electronegativity of A3+ always affects the phase transition property [28], The higher cation electronegativity reduces the effective charge of the
Fig. 4. Rietveld refinement result of Ho2Mo3O12 Pba2 phase at RT. The meanings of symbols are given in the legend.
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Table 1 The refinement resulted lattice parameters and atomic positions for Pba2 Ho2Mo3O12 at room temperature. Lattice parameters
a = 10.327 Å
b = 10.303 Å
c = 10.570 Å
Atom Name
x
y
z
0.5064 (0) 0.3131(0) 0. 4971 (0) 0.2041 (0) 0.2527 (1) 0.5384(2) 0.2937(2) 0.0069(5) 0.1514(2) 0.1526(3) 0.3351(4) 0.3758(2) 0.0666(3) 0.1740(5) 0.1120(2) 0.3182 (5) 0.3648(4)
0.7398(11) 0.2651(9) 0.3603 (12) 0.6469(13) 0.0018(15) 0.5208(18) 0.4844(19) 0.3168(13) 0.6846(18) 0.7063(15) 0.3122(17) 0.8301(16) 0.3044(15) 0.0860(16) 0.9804(18) 0.0842 (16) 0.9162(17)
Ho1 Ho2 Mo1 Mo2 Mo3 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12
0.1863 (0) 0.4969 (0) 0.2050(0) 0.0052(0) 0.2517 (1) 0.1920(3) 0.5152(4) 0.1203(2) 0.5065(6) 0.1260(18) 0.1559(3) 0.4577(2) 0.4560(3) 0.1435(4) 0.4025(2) 0.3711(4) 0.1752(4)
oxygen anion, and then reduces the O–O repulsion and lowers the rigidity of AO6 polyhedron, this makes the phase transition occurred at higher temperature and leads to an active phase transtion property. This picture might also suitable for the Mo/W site [18,28]. The Mo6+ has a relative stronger electronegativity than W6+, thus it leads to a more active phase transition property of molybdate than tungstate, this is consistent with our investigation that W substitution for Mo supresses the phase transition from Pbcn to Pba2 structure. There are also reports on Sc2Mo3O12 /Sc2W3O12 [28] and In2Mo3O12/In2W3O12 [29] systems to support
Fig. 6. The parts view of refinement results of (a) x = 1.0 and (b) x = 2.0 at 850 C, the meanings for each symbol is same as Fig. 3.
Fig. 5. The extracted lattice constants (a) a, (b) b, (c) c, and (d) V as the function of temperature for x = 0.0 (&), 1.0 (*) and 2.0 (~).
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Acknowledgements The financial support from NSFC (No. 11475268, No. 11275013) and “973 Project” (2010CB833101) are greatly appreciated. The authors are grateful to Prof. Fuhui Liao for technical support on in situ XRD and thermal analysis experiment. References
Fig. 7. The temperature dependent Pba2 phase contents for Ho2Mo3 (x = 0.0, 1.0, 2.0), the meaning for each symbol is given in the legend.
xWxO12
this point. We also note that Mo-substitution for W in La2W2O9 [30] and ZrW2O8 [31,32] leads to enhanced phase transition activity. Thus we address that the W-substitution for Mo could stablize the Pbcn structure by suppressing the phase transition property, this will be useful for improving the thermal stability of device. 4. Conclusion The solid solution Ho2Mo3 xWxO12 (x = 0.0, 1.0, 2.0) were perpared by solid state reaction method, and performed in-situ Xray diffraction from RT to 850 C. At RT all samples show a Pbcn structure with NTE property, with the temperature upto 700 C an irrevesible phase transition to Pba2 phase is observed. The Pba2 phase has a remarkable reduced lattice volume compared with Pbcn phase, so that this transition will destroy the composite matrix and further break the device, this will hamper the application of this compound as a NTE material. We found this transformation could be supressed effectively by the W-substitution for Mo, this can be explained by the W6+ has a lower electronegativity than Mo6+. This is consistent with the influence of electronegativity on A3+ site. We address that the W-substitution for Mo could stablize the Pbcn phase and thus useful for its application as a NTE material.
[1] J.S.O. Evans, T.A. Mary, A.W. Sleight, Phys. B Condens. Mater. 241–243 (1998) 311–316. [2] J.S.O. Evans, T.A. Mary, A.W. Sleight, J. Solid State Chem. 137 (1998) 148–160. [3] J.S.O. Evans, T.A. Mary, Int. J. Inorg. Mater. 2 (2000) 143–151. [4] S. Sumithra, A.K. Tyagi, A.M. Umarji, Mater. Sci. Eng. B 116 (2005) 14–18. [5] S. Sumithra, A.M. Umarji, Solid State Sci. 8 (2006) 1453–1458. [6] S. Sumithra, A.M. Umarji, Solid State Sci. 6 (2004) 1313–1319. [7] L.M. Sullivan, C.M. Lukehart, Chem. Mater. 17 (2005) 2136–2141. [8] M.M. Wu, J. Peng, S.B. Han, Z.B. Hu, Y.T. Liu, D.F. Chen, Ceram. Int. 38 (2012) 6525–6529. [9] M.M. Wu, X.Z. Liu, D.F. Chen, Q. Huang, H. Wu, Y.T. Liu, Inorg. Chem. 53 (2014) 9206–9212. [10] P.M. Forster, A. Yokochi, A.W. Sleight, J. Solid State Chem. 140 (1998) 157–158. [11] A.K. Tyagi, S.N. Achary, M.D. Mathews, J. Alloys. Compd. 339 (2002) 207–210. [12] A.W. Sleight, Inorg. Chem. 37 (1998) 2854–2860. [13] J.S.O. Evans, J. Chem. Soc. Dalton Trans. (1999) 3317–3326. [14] W.T.A. Harrison, Mater. Res. Bull. 30 (1995) 1325–1331. [15] M. Ari, P.M. Jardim, B.A. Marinkovic, F. Rizzo, F.F. Ferreira, J. Solid State Chem. 181 (2008) 1472–1479. [16] B.A. Marinkovic, P.M. Jardim, R.R. deAvillez, F. Rizzo, Solid State Sci. 7 (2005) 1377–1383. [17] L. Wang, F. Wang, P.F. Yuan, Q. Sun, E.J. Liang, Y. Jia, Z.X. Guo, Mater. Res. Bull. 48 (2013) 2724–2729. [18] B.A. Marinkovic, M. Ari, R.R. de Avillez, F. Rizzo, F.F. Ferreira, K.J. Miller, M.B. Johnson, M.A. White, Chem. Mater. 21 (2009) 2886–2894. [19] X.L. Xiao, Y.Z. Cheng, J. Peng, M.M. Wu, D.F. Chen, Z.B. Hu, R. Kiyanagi, J.S. Fieramosca, S. Short, J. Jorgensen. Solid State Sci. 10 (2008) 321–325. [20] Z.Y. Li, W.B. Song, E.J. Liang, J. Phys. Chem. C 115 (2011) 17806–17811. [21] S.D. Gates, C. Lind, J. Solid State Chem. 180 (2007) 3510–3514. [22] C. González-Silgo, C. Guzmán-Afonso, V.M. Sánchez-Fajardo, S. AcostaGutiérrez, A. Sánchez-Soares, M.E. Torres, N. Sabalisck, E. Matesanz, J. Rodríguez-Carvajal, Powder Diffr. 28 (2013) S33–S40. [23] W. Jeitschko, Acta Crystallogr. Sect. B Struct. Crystallogr. Cryst. Chem. 28 (1972) 60–76. [24] S. Kojima, T. Nakamura, Phys. Rev. B 18 (1978) 453–458. [25] J. Rodriguez-Carjaval, An Introduction to the Program FULLPROF 2006, in: J. Rodriguez-Carjaval (Ed.), Laboratoire Leon Brillouin (CEA-CNRS), France, 2007. [26] K. Nassau, H.J. Levinstein, G.M. Loiacono, J. Phys. Chem. Solids 36 (1965) 1805– 1816. [27] S. Sumithra, A.M. Umarji, Mater. Res. Bull. 40 (2005) 167–176. [28] J.S.O. Evans, T.A. Mary, A.W. Sleight, J. Solid State Chem. 133 (1997) 580–583. [29] V. Sivasubramanian, T.R. Ravindran, R. Nithya, A.K. Arora, J. Appl. Phys. 96 (2004) 387–392. [30] J.A. Collado, M.A.G. Aranda, A. Cabeza, P. Olivera-Pastor, S. Bruque, J. Solid State Chem. 167 (2002) 80–85. [31] C. Closmann, A.W. Sleight, J.C. Haygarth, J. Solid State Chem .139 (1998) 424–426. [32] H.F. Liu, K.M. Pan, Q. Jin, Z.P. Zhang, G. Wang, X.H. Zeng, Ceram. Int. 40 (2014) 3873–3878.