Weigang Cao,
Qiang Li,
Kun Lin,
Zhanning Liu,
Jinxia Deng,
Jun Chen and
Xianran Xing*
Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, China. E-mail: xing@ustb.edu.cn; Fax: +86-10-62332525; Tel: +86-10-62334200
First published on 29th September 2016
New materials with suitable negative thermal expansion (NTE) are much desired to control thermal expansion in solids. In the present study, we determined the phase transition temperature of Dy2W3O12 from the monoclinic phase to the orthorhombic phase at 996 °C. The orthorhombic phase could be retained by quenching, and further high temperature XRD and synchrotron radiation X-ray powder diffraction (SXRD) experiments revealed that Dy2W3O12 shows NTE (−2.6 × 10−5 °C−1) in the temperature range of 150–500 °C, which is the largest negative thermal expansion in the A2W3O12 family (A = rare earth element). A possible NTE mechanism and enhanced NTE were elucidated by the transverse thermal motion of the bridge oxygen in A–O–W linkages accompanied by distortion of the polyhedra with large Dy3+ on the A site.
In a previous report, the phase diagrams of A2W3O12 with different rare earth elements have been investigated by Nassau.30 These tungstates can be classified into three groups according to their structure change with changes in the ionic radius and temperature, and these are shown in Fig. 1. In group I, A2W3O12 (A = La to Eu) presents the monoclinic phase without transformation to the orthorhombic phase. This group exhibits positive thermal expansion (PTE). In group III, A2W3O12 (A = Er to Lu) adopts an orthorhombic structure and has no phase transition above room temperature, which shows NTE. Group II is a special group, which presents a monoclinic phase at low temperature and transforms to an orthorhombic phase at high temperature. This means that this group shows PTE at low temperature and possesses NTE at high temperature. Owing to difficulty controlling the high temperature phase, seldom have studies on group II investigated the thermal expansion.
Dy2W3O12 is a typical example of group II. Although some studies have been carried out on the photoluminescence properties,31 electrical transport properties32 and magnetic properties33 of its monoclinic phase, there are few investigations into orthorhombic Dy2W3O12. Furthermore, previous studies have shown that the size of the A3+ ion is correlated to thermal expansion in orthorhombic A2W3O12.25 A larger size of A3+ leads to more enhanced NTE, and the cation radius of Dy3+ is large in the orthorhombic A2W3O12 family. So it is significant to investigate orthorhombic Dy2W3O12, which possibly exhibits stronger NTE.
In this paper, we synthesized Dy2W3O12 with an orthorhombic phase by high temperature quenching, investigated the mechanism of synthesis (see in Fig. S1†) and characterized the sample by XRD and TG-DSC. The as-synthesized orthorhombic Dy2W3O12 can be stabilized by 500 °C. The thermal expansion was studied by high temperature XRD and synchrotron radiation X-ray powder diffraction (SXRD). The photoluminescence and the magnetic properties of hydrated orthorhombic Dy2W3O12 were also examined (see the ESI†), which is helpful for the further study of orthorhombic Dy2W3O12.
Phase identification and high temperature X-ray powder diffraction (XRD) were carried out on a PANalytical diffractometer with Cu Kα radiation. In order to exhibit the thermal expansion of the quenched sample, high temperature XRD data were collected using an HTK1200 accessory, and recorded in the temperature range from 150 °C to 500 °C at intervals of 50 °C, and between 10° and 80°. SXRD was also used to study the thermal expansion. Variable temperature SXRD data were collected at the BL44B2 beamline of the Spring-8 synchrotron radiation facility, Japan, using a constant wavelength of λ = 1.08017 Å. TG-DSC (SETARAM, Labsys Evo) data of the quenched sample were collected under an air atmosphere to investigate the weight change and the phase transition from room temperature to 1000 °C at a rate of 10 °C min−1. Meanwhile, high temperature XRD was also used to investigate the phase transition from 200 °C to 800 °C. Photoluminescence was investigated with a F-4500 spectrometer, and the magnetic data was recorded by SQUID in the temperature range from −268 °C to 127 °C.
Fig. 4(a) displays the TG-DSC curves of the quenched sample with one thermal cycle. According to the slope of the weight loss, it can be identified that there are two regions of weight loss between 100 °C and 200 °C. The first stage of the weight loss is attributed to desorption of H2O, which is adsorbed on the surface. The second weight loss is due to the crystal water, which is equivalent to 44 g mol−1. The formula of the quenched sample is described as Dy2W3O12·2.7H2O by calculating the weight loss of crystal water. There is nearly no weight change above 200 °C in the heating process. Furthermore, there is also no obvious weight change in the cooling process. The DSC curve and the high temperature XRD measurements obviously show the phase transformation as a function of temperature, which is explicitly illustrated in Fig. 4(c). In the DSC curve, the endothermic peaks of 107 °C and 163 °C are in compliance with the weight loss, indicating that Dy2W3O12·2.7H2O changes to orthorhombic Dy2W3O12. The broad exothermic peak at 697 °C is ascribed to the phase transition from the orthorhombic phase to the monoclinic phase, which is identified by the high temperature XRD measurement shown in Fig. 4(b). The high temperature XRD measurement indicated that the phase transition occurs between 500 °C and 600 °C, and total change to the monoclinic phase occurs at 700 °C. This change trend is consistent with the DSC results. The peak at 996 °C is attributed to the phase transition from the monoclinic phase to the orthorhombic phase, which corresponds with the previous report.30 In addition, the peak at 974 °C may be due to the recrystallization of the amorphous phase. Furthermore, the exothermic peak at 911 °C is due to the phase transition from the orthorhombic to monoclinic phase during cooling. At low temperature, the monoclinic phase of Dy2W3O12 is the thermodynamically stable phase. From the TG-DSC and high temperature XRD, we can conclude that the orthorhombic phase is a metastable phase, and exists stably below 500 °C.
The thermal expansion of the quenched sample is investigated by the high temperature XRD and high temperature SXRD methods. Based on the conclusion of the TG-DSC results, the quenched sample was measured below 500 °C. Meanwhile, the hydrated sample doesn’t exhibit negative thermal expansion due to H2O hindering the rocking motion of the bridge oxygen. So the thermal behaviour of the quenched sample is studied above 150 °C. Therefore, high temperature XRD data were collected in the range from 150 °C to 500 °C, in which the quenched sample retains the orthorhombic phase. Furthermore, the quenched sample totally excludes water until 250 °C in the process of collecting SXRD data, which is due to operating in a sealed environment.
The lattice constant and volume of orthorhombic Dy2W3O12 as a function of temperature are shown in Fig. 5. The refined results of the XRD and SXRD are given in Tables S1 and S2, respectively (see the ESI†), and the coefficients of the thermal expansion for the axes and volume were calculated from those data, which are shown in Table 1. From Fig. 5, it is clear to see that the quenched sample shows shrinkage along three axes, and the a-axis and c-axis have stronger negative thermal expansion than the b-axis. The overall volume thermal contraction of the quenched sample is αv = −2.6 × 10−5 °C−1, which is the largest negative thermal expansion in a A2W3O12 system so far, as shown in Table 1. This is because there is a close correlation between the magnitude of the thermal expansion and the size of the A cation, as a larger A cation has a more negative coefficient of thermal expansion. Dy3+ has the largest size in the A2W3O12 family (A = rare earth element) with an orthorhombic phase at present. High temperature quenching is a useful technology to explore the negative thermal expansion.
Fig. 5 Plots of the evolution of unit cell parameters and cell volume of the quenched sample as a function of temperature. |
Rare earth tungstates | A3+ (Å) | αa (×10−6 °C−1) | αb (×10−6 °C−1) | αc (×10−6 °C−1) | αv (×10−5 °C−1) | Reference |
---|---|---|---|---|---|---|
Sc2W3O12 | 0.68 | −6.3 | 7.5 | −5.5 | −0.4 | Evans14 |
Y2W3O12 | 0.88 | −10.4 | −3.1 | −7.6 | −2.1 | Woodcock15 |
Lu2W3O12 | 0.848 | −9.7 | −2.9 | −5.7 | −1.8 | Sumithra16 |
Yb2W3O12 | 0.858 | −10.2 | −2.7 | −6.4 | −1.9 | Sumithra16 |
Er2W3O12 | 0.881 | −10.1 | −3.4 | −6.7 | −2.0 | Sumithra16 |
Dy2W3O12 (XRD) | 0.908 | −11.9 | −4.7 | −9.2 | −2.6 | This work |
Dy2W3O12 (SXRD) | 0.908 | −11.6 | −4.9 | −9.2 | −2.6 | This work |
Many detailed and elegant analyses have been carried out on the mechanism of NTE in A2W3O12.13,18,19,25,34–36 The root cause is the transverse thermal motion of the bridge oxygen in A–O–W linkages accompanied by distortion of the polyhedra to shorten the distance between A and W atoms. As far as we know, the magnitude of the thermal expansivity is correlated with the size of the A cation. This is because the rigidity of the polyhedra hinders the rocking motion of the bridging oxygen and then decreases the negative thermal expansion. A detailed description is shown in Fig. 6. With a small A3+, the oxygen–oxygen repulsion makes the polyhedra of AO6 rigid, which decreases the rocking motion of the bridging oxygen. Meanwhile a large A3+ makes the polyhedra of AO6 more flexible. With increasing the ionic size of A3+, the oxygen–oxygen interaction diminishes, and the polyhedra of AO6 appear to be less rigid, which enhances the negative thermal expansion. This is the reason that Dy2W3O12 with the largest cation Dy3+ has the most enhanced thermal expansion in the A2W3O12 (A = rare earth element) family.
Footnote |
† Electronic supplementary information (ESI) available: The mechanism of quenching, Le Bail fit of XRD data of the quenched sample at 200 °C, the cell parameters of orthorhombic Dy2W3O12 versus temperature from XRD results and SXRD results, the photoluminescence and the magnetic properties of the quenched sample. See DOI: 10.1039/c6ra21136d |
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