Multiferroic materials under compression: How pressure modifies the structure of CuWO4
Tungstates of 3d-transition metals form an important family of inorganic materials with applications in various fields. CuWO4 is one of them, being well known as a semiconductor with technological applications in scintillator detectors, laser hosts, photoanodes, optical fibers, etc. In the last few years it has also attracted attention as a multiferroic material with an intriguing magnetic phase diagram. CuWO4 has been studied at ambient pressure to characterize its optical, magnetic, and crystallographic properties. It crystallises in a triclinic structure and can be described as a hexagonal close packing, with the cations occupying half of the octahedral sites. In earlier studies, the influence of the strong Jahn-Teller distortion of the CuO6 octahedra in the physical properties of CuWO4 was discussed. Recently there has arisen a large interest in the high-pressure effects on the physical properties of CuWO4 and related compounds. We have studied the effects of pressure on the structural, vibrational, and magnetic behaviour of cuproscheelite. We performed high-pressure powder X-ray diffraction and Raman spectroscopy experiments as well as ab initio calculations. For the diffraction experiments we used the I15 Extreme Conditions beamline at Diamond Light Source. Experiments provide evidence that the Jahn-Teller distortion is reduced under compression. In addition, a structural phase transition has been discovered at 10 GPa causing an enhancement of the crystal symmetry. Finally, experiments under different stress conditions show that non-hydrostatic stresses induce a second phase transition at 17 GPa and reduce the compressibility of CuWO4.
We performed high-pressure (HP) X-ray diffraction (XRD) experiments at beamline I15. We used a monochromatic X-ray beam (? = 0.61506 Å) focused down to 30 mm x 30 mm. A diamond-anvil cell (DAC) was used and XRD patterns were recorded on a MAR345 image plate. In the experiments we employed micron-size powders (purity > 99.5%, Mateck). Silicone oil (SO) or argon (Ar) were used as pressure-transmitting medium. The pressure was measured using the ruby-fluorescence technique. Prior to loading the DAC, XRD and Raman measurements confirmed that only the low-pressure triclinic phase (cuproscheelite) is present (see Fig. 1). The unit-cell parameters were a = 4.709 Å, b = 5.845 Å, c = 4.884 Å, a = 88.3°, ß = 92.5°, and ? = 97.2° 1.
Figure 1: Crystal structure of the low- (left) and high-pressure (right) phases. A picture with crystals in both phases is included. The distortion of the CuO6 octahedron can be clearly seen in the figure.
Figure 2: shows selected HP XRD patterns. Up to 7.8 GPa all the Bragg reflections can be indexed according to the CuWO4 triclinic structure. Beyond 9 GPa extra peaks appear pointing out the onset of a phase transition, to a phase with monoclinic symmetry. The triclinic and monoclinic structures coexist within a 6 GPa range. In the experiment performed under Ar, no evidence of additional structural changes or chemical decomposition of CuWO4 is found up to 20.3 GPa. In contrast, in the experiment performed under SO, beyond 16 GPa there is an extinction of the peaks of the triclinic phase and additional peaks are detected (see Fig. 2). In particular, new peaks located around 2? = 7.5° and 11.5° can be seen at 20.3 GPa. Both facts indicate the onset of a second transition. The structural changes are reversible. These conclusions are fully supported by Raman and optical measurements (see changes in single crystal CuWO4 in Fig. 1).
From the analysis of XRD patterns, we have obtained the pressure behaviour of the lattice parameters of the low-pressure phase of CuWO4. Figure 3: shows the evolution of the unit-cell parameters and volume up to 10 GPa. The c-axis is less compressible than the other axes. This anisotropic compression is higher in the experiment performed under Ar. Similar anisotropic behaviours were found in the structurally related CdWO4, MgWO4, MnWO4, and ZnWO4.2 This is caused by the different linking of octahedral units along different crystallographic directions, with the CuO6 octahedra being much more compressible than the WO6 octahedra. The unit-cell volume was analysed using a Birch–Murnaghan EOS (B0’ = 4). The unit-cell volume (V0) and the bulk modulus (B0) at zero pressure obtained for the triclinic phase are: V0 = 132.8(2) Å3 for both experiments, and B0 = 171(2) and 134(6) GPa for SO and Ar experiments, respectively. B0 is comparable with the theoretical value and with the bulk modulus of wolframite-structure tungstates2. Experiments show that CuWO4 is 18% more compressible under Ar than under SO. This medium-dependent behaviour can be explained considering that Ar is a better hydrostatic medium than SO.
Figure 2: Selected HP XRD patterns; silicone-oil experiment. Changes in diffraction patterns indicate the occurrence of two phase transitions. Pressures are indicated in the plot.
Additional information on the structural HP behaviour can be extracted from the study of the triclinic angles of CuWO4. Upon compression a clear symmetrisation is suffered by CuWO4 with all the angles getting closer to 90º (Fig. 3). Particularly interesting is the behaviour of a and ß angles. Both decrease under pressure, taking the same value at the transition pressure. These facts could be related with a symmetrisation of the CuO6 octahedra associated to a reduction of the Jahn-Teller (JT) distortion. This phenomenon can be quantified through the pressure effects on the JT distortion parameter, defined as:
where RCu-O are the six Cu-O distances of the CuO6 octahedra and <RCu-O> is their average value. We deduced that sJT decreases from 0.201 Å at ambient pressure to 0.160 Å at 10 GPa, approaching the value of sJT in wolframite-type CdWO4 (0.095 Å) and MnWO4 (0.088 Å). The symmetrisation of the CuO6 octahedra together with the fact that cuproscheelite is a symmetry-reduced version of wolframite, suggest that the HP phase might have a monoclinic wolframite-type structure. This hypothesis was used to analyse the diffraction data of the HP phase. We found that XRD patterns cannot be properly indexed considering only a wolframite-type phase (P2/c, Z = 2). However, we have been able to index the XRD patterns assuming the coexistence of both structures. The structural changes induced at the transition are illustrated in Figure 1. The unit-cell parameters of the HP phase at 16 GPa are: a = 4.524 Å, b = 5.529 Å, c = 4.896 Å, ß = 90.86°. A volume change of 1% occurs at the triclinic-monoclinic transformation. The transition from the triclinic to a monoclinic wolframite-like structure is confirmed by Raman experiments and ab initio calculations. In view of this evidence, the proposed monoclinic structure appears as the most probable for the HP phase. The coexistence of the low- and high-pressure phases is compatible with the domain formation detected by visual observation in single-crystals (see Fig. 1). Macroscopic fringes are systematically observed at pressures close to the transition. Since the peak profile in XRD experiments remained sharp throughout the coexistence range of phases, pressure inhomogeneities or uniaxial stresses can be excluded as the origin of phase coexistence. Instead, it could reflect a first-order character of the transition. This is further supported by the observed volume discontinuity at the phase transition. The reported structural sequence agrees with the systematic deduced for orthotungstates based upon crystal-chemistry arguments3. The phase transition is also consistent with the fact that in solid solutions of CuWO4 and ZnWO4 (NiWO4) an increase of the Zn (Ni) concentration induces a volume reduction and the transition from cuproscheelite to wolframite4. The detected structural changes induce a change in the magnetic order (from antiferromagnetic to ferromagnetic) and a reduction of the band gap of CuWO4.
Figure 3: Pressure evolution of the volume and unit-cell parameters obtained from experiments using different pressure-transmitting media. SO (silicone-oil), less hydrostatic. Ar (argon), quasi-hydrostatic. Worsening of hydrostatic conditions causes a reduction of crystal compressibility.
Ruiz-Fuertes, J., Errandonea, D., Lacomba-Perales, R., Segura, A., Gonzalez, J.,Rodriguez, F., Manjon, F.J., Ray, S., Rodriguez-Hernandez, P., Muñoz, A., Zhu, Zh. and Tu, C.Y. High-pressure structural phase transitions in CuWO4. Phys. Rev. B. 81, 224115 (2010)
- Kihlborg, L. and Gebert, E. Acta Cryst. B26, 1020 (1970).
- Ruiz-Fuertes, J. et al. J. Appl. Phys. 107, 083506 (2010).
- Errandonea, D. and Manjón, F.J. Prog. Mat. Sci. 53, 711 (2008).
- Schofield, P.F. and Redfern, S.A.T. J. Phys.: Condens. Matter. 4, 375 (1992).
Spanish MEC Grants MAT2007-65990-C03-01/03, MAT2010-21270-C04-01/03 and CSD-2007-00045.