High-pressure Raman scattering and an anharmonicity study of multiferroic wolframite-type Mn0.97Fe0.03WO4
M Mączka1, M Ptak1, K Pereira da Silva2, P T C Freire2 and J Hanuza3
Published in J. Phys.: Condens. Matter 24 345403
doi:10.1088/0953-8984/24/34/345403
Abstract
A high-pressure Raman scattering study of wolframite-type Mn0.97Fe0.03WO4 is presented up to 10.4GPa. The phonon wavenumbers vary linearly with pressure. The mode Grüneisen parameters are larger for many bending and lattice modes when compared to the stretching modes due to the larger compressibility of Mn(Fe)O6 octahedra when compared to WO6 octahedra. Combining the pressure-dependent Raman data of this work with the temperature-dependent Raman data on this crystal previously reported by us has allowed estimation of the temperature-dependent pure lattice and intrinsic anharmonic contributions to the observed total Raman shifts as a function of temperature. It has been found that the observed unusual hardening of the 884, 698 and 674cm−1 stretching modes upon heating from 4 to about 150–200K followed by the usual softening above 150–200K is a result of a positive intrinsic anharmonic contribution and a negative pure lattice contribution; i.e.,up to about 150–200K the anharmonic contribution surpasses the lattice contribution and the total Raman shift is slightly positive whereas above 150–200K the lattice contribution becomes dominant and the Raman bands exhibit the usual softening with increasing temperature.
Parte superior do formulário
Parte inferior do formulário
1.Introduction
Wolframite-type simple tungstates of general formula AWO4, where A=Zn, Mg, Cd, Ni, Co, Fe, Mn or Cu, are of great interest due to their various properties and applications including laser crystals, scintillation detectors and catalysis[1–3]. In particular, MnWO4 is a promising material for humidity sensors[4] and high-gain Raman lasers[5]. Moreover, it has attracted attention due to its electrochemical and multiferroic properties[6–8].
High-pressure studies have been reported for a number of wolframite-type tungstates such as ZnWO4[9–11], CdWO4[12, 13] and MgWO4[11, 13]. These studies have shown that the wolframite-type structure is quite stable and the pressure-induced phase transitions in these tungstates were detected beyond 16–17GPa[9–11]. Very recently, experimental and calculated Raman wavenumbers as well as the pressure coefficients dω/dT have been reported also for CuWO4, NiWO4, FeWO4 and CoWO4[14]. Regarding MnWO4, high-pressure x-ray diffraction studies have been performed up to 8.0GPa[13]. Moreover, experimental and calculated Raman wavenumbers as well as the pressure coefficients dω/dT have been reported, without showing experimental Raman spectra[14]. It is worth adding that in spite of many reported studies on wolframite-type tungstates, anharmonicity analysis has not yet been reported for any member of this family of compounds.
Mn0.97Fe0.03WO4 crystal has recently been studied by us through temperature-dependent Raman scattering. This study revealed an unusual downturn of phonon shifts below 150–200K for some phonons[15]. We suggested that these anomalies might originate from some subtle structural changes due to the anisotropic character of thermal expansion[15]. In addition to these anomalies, we observed weak anomalies below 20K due to the onset of magnetic order[15].
In order to further improve the understanding of the phonon properties of Mn0.97Fe0.03WO4 crystal and the origin of the observed temperature-dependent anomalies, it is necessary to access its properties as a function of both temperature and pressure since a combined temperature- and pressure-dependent study allows the wavenumber shifts arising due to pure lattice and pure anharmonic effects to be separated. This paper reports a study of Mn0.97Fe0.03WO4 single crystal as a function of pressure through Raman scattering and anharmonicity analysis of the phonon modes. We will show that the obtained results provide new information on the anharmonicity and the origin of the temperature-dependent anomalies in Mn0.97Fe0.03WO4.
2.Experimental details
Single crystals of Mn0.97Fe0.03WO4 were grown by the flux method. Crystal growth details can be found elsewhere[15].
The high-pressure Raman spectra were obtained with a WiTeC Alpha 300 confocal Raman microscope on an about 10μm thick platelet cleaved from a Mn0.97Fe0.03WO4 single crystal in backscattering geometry. The 532nm line of a Nd:YAG laser was used as the excitation source. No analyser was used and therefore our Raman spectra correspond to both diagonal and off-diagonal components of the polarizability tensor. For high-pressure measurements, the samples were loaded into a homemade diamond anvil cell of the National Bureau of Standards (NBS) type, with diamonds of 0.6mm of culets. A stainless steel gasket with a thickness of 210μm was pre-indented to 90μm, and a 200μm hole was drilled in the centre of the indentation by means of an electric discharge machine from EasyLab. The mineral oil Nujol was chosen as the pressure transmitting fluid. Pressures were measured based on the shifts of the ruby R1 and R2 fluorescence lines. The spectrometer slits were set for a resolution of 2cm−1.
3.Results and discussion
3.1.High-pressure Raman scattering
The crystal structure of MnWO4 is monoclinic, space group P2/c (#13, ), with the unit cell parameters a=4.830(1), b=5.7603(9), c=4.994(1)Å, β=91.14 and Z=2[13]. It consists of edge-sharing WO6 and MnO6 octahedra that form chains along the c axis (see figure1). The WO6 octahedra within a chain are connected by a double oxygen bridge with shorter W–O1 and longer W⋯O1 distances of 1.9101 and 2.1377Å, respectively[13]. The remaining O2 oxygen atoms form a short W–O2 distance of 1.7847Å. The manganese and tungsten atoms are arranged in alternating sheets parallel to (100)[13].
Figure1.View of the MnWO4 structure along the a axis.
The monoclinic unit cell of MnWO4 comprises 12 atoms, which have 36 zone-centre degrees of freedom described by irreducible representations of the factor group C2h as 8Ag+8Au+10Bg+10Bu. Three of these modes, Au+2Bu, are acoustic modes. The optic modes can be subdivided into 2Ag+Au+4Bg+2Bu translational modes of the Mn and W atoms and 6Ag+6Au+6Bg+6Bu modes involving oxygen atoms. Among these modes the Ag and Bg modes are Raman-active whereas the Au and Bu modes are IR-active. This analysis shows that one expects to observe 18 Raman modes.
Pressure-dependent Raman spectra of Mn0.97Fe0.03WO4 crystal are presented in figures 2 and 3. A detailed assignment of the Raman modes has been reported in[15, 16] and here we recall only the few most important conclusions. First, the Raman bands in the 770–890cm−1 range can be assigned to symmetric and asymmetric stretching modes of the short W–O2 bonds. Second, the bands in the 670–700cm−1 range can be assigned to stretching modes of the double oxygen bridge involving W–O1 bonds. Third, translational motion of the W and Mn atoms contributes strongly to the modes observed below 290cm−1 and the remaining modes in the 550–290cm−1 range can be assigned to the WO6 bending modes[15]. Figures 2 and 3 show also a very weak band at 924cm−1. This band has been assigned by Iliev etal to some unknown impurity[16].
Figure2.Raman spectra of Mn0.97Fe0.03WO4 recorded at different pressures during compression experiments.
Figure3.Raman spectra of Mn0.97Fe0.03WO4 recorded at different pressures during decompression experiments.
By increasing the pressure the Raman bands shift towards higher wavenumbers (see figure2). As can be noticed, the Bg modes are not observed at ambient pressure. According to the selection rules, for monoclinic MnWO4 with the C2 axis parallel to the lattice parameter b, the Bg modes cannot be observed in the backscattering geometry applied here only if the incident laser beam is parallel to the C2 axis, because in this case the Raman spectrum corresponds to the αxx, αzz and αxz components of the polarizability tensor (Ag modes). Our previous polarized spectra of isostructural Mn0.85Co0.15WO4 showed that the Ag band at 328cm−1 is very weak when the incident and scattered light polarizations are parallel to the a axis but relatively strong when they are parallel to the c axis[15]. Our spectrum at ambient pressure is, therefore, close to the b(cc+ca)b scattering geometry. Figure3 shows that Bg modes become clearly visible above 5GPa. This result can be most likely attributed to some reorientation of the crystal in the diamond anvil cell, i.e.above 5GPa the laser beam is no longer parallel to the b axis. However, the Raman spectra remain qualitatively the same up to 10.4GPa, the highest pressure reached in our experiment. This result proves that Mn0.97Fe0.03WO4 does not exhibit any structural phase transitions up to 10.4GPa.
Figure4.Wavenumber versus pressure plots of the Ag (circles) and Bg (triangles) modes observed in Mn0.97Fe0.03WO4 crystal for compression (solid symbols) and decompression (empty symbols) experiments. The solid lines are linear fits on the data.
Further insights into the pressure dependence of phonons can be tracked in Raman studies of Mn0.97Fe0.03WO4 crystal during decompression. Upon releasing the pressure the spectrum of the starting phase was recovered, as can be observed in figure3. However, the intensities of some bands of the starting phase are different before increasing the pressure and after releasing the pressure. This difference is due to some reorientation of the sample during the pressure increase and release as well as the creation of defects in the studied sample.
The overall changes in the Raman spectra can be followed in detail by analysing the frequency (ω) versus pressure (P) plot shown in figure4. For all peaks the ω(P) behaviour is linear. The results for pressure coefficients (dω/dP) and wavenumber intercepts at zero pressure (ω0), obtained from fitting of the experimental data through the least squares method, are listed in table1. Table1 also summarizes the mode Grüneisen parameters γi=B0/ωi∂ωi/∂P, where B0 is the bulk modulus. The mode Grüneisen parameters have been calculated using B0=131GPa obtained from high-pressure x-ray diffraction studies[13].
Table1. Experimental Raman wavenumbers along with pressure coefficients dω/dP obtained from the linear fits on the data and the mode Grüneisen coefficients γ for Mn0.97Fe0.03WO4, MnWO4 and ZnWO4. T' denotes translational modes and ρ, τ, ω, δ and ν denote rocking, twisting, wagging, bending and stretching modes of the WO2 groups and double oxygen bridges.
Mode / Mn0.97Fe0.03WO4 / MnWO4[14] / ZnWO4[10] / Assignment[15] /ω0 / dω/dP / γ / ω0 / dω/dP / ω0 / dω/dP / γ /
Bg / 89 / 0.73 / 91 / 0.95 / 1.45 / T'(W)
Ag / 129 / 0.02 / 123.1 / 0.65 / 0.74 / T'(W)
Bg / 159.1 / 0.73 / 0.60 / 160 / 0.22 / 145.8 / 1.20 / 1.15 / T'(Mn) + T'(W)
Bg / 165.0 / 1.34 / 1.06 / 166 / 0.78 / 164.1 / 0.72 / 0.61 / T'(Mn) + T'(W)
Bg / 177 / 1.03 / 189.6 / 0.67 / 0.49 / T'(Mn)
Ag / 207.6 / 2.69 / 1.70 / 206 / 2.01 / 196.1 / 2.25 / 1.61 / T'(Mn)
Bg / 272 / 2.03 / 267.1 / 1.32 / 0.69 / ρ(WO 2)+T'(Mn)
Ag / 257.0 / 0.32 / 0.16 / 258 / 0.30 / 276.1 / 0.87 / 0.44 / τ(WO 2)
Bg / 292.3 / 1.84 / 0.82 / 294 / 2.02 / 313.1 / 1.74 / 0.78 / ω(WO 2)
Ag / 325.3 / 2.05 / 0.83 / 327 / 1.50 / 342.1 / 1.74 / 0.71 / δsc (WO 2)+δs (W–O–W)
Bg / 356.8 / 3.82 / 1.40 / 356 / 4.09 / 354.1 / 3.87 / 1.53 / δs (W –O –W )+ρ(WO 2)
Ag / 396.7 / 1.60 / 0.53 / 397 / 1.69 / 407 / 1.65 / 0.57 / νs (W–O–W)
Bg / 509.9 / 2.29 / 0.59 / 512 / 2.86 / 514.5 / 3.18 / 0.86 / νs (W–O–W)
Ag / 545.4 / 2.62 / 0.63 / 545 / 2.39 / 545.5 / 3.0 / 0.77 / δs (W–O–W)
Bg / 673.9 / 3.56 / 0.69 / 674 / 4.20 / 677.8 / 3.9 / 0.80 / νs (W–O–W)
Ag / 698.5 / 2.95 / 0.55 / 698 / 3.08 / 708.9 / 3.3 / 0.65 / νs (W–O–W)
Bg / 774.4 / 4.07 / 0.69 / 774 / 3.58 / 786.1 / 4.4 / 0.78 / νas (WO 2)
Ag / 884.2 / 2.48 / 0.37 / 885 / 1.63 / 906.9 / 3.7 / 0.57 / νs (WO 2)
The dω/dp values for Mn0.97Fe0.03WO4 obtained by us are in reasonable agreement with those reported by Ruiz-Fuertes etal for MnWO4[14]. The only exceptions are the modes at 884, 325 and 159cm−1 for which we observe significantly larger values than reported previously for MnWO4[14]. Here it is important to add that Nujol is rigorously hydrostatic up to 4GPa[17]. However, it is quasihydrostatic above 4GPa, up to the highest pressure reached in our experiments (10.4GPa). Our results show that the ruby luminescence band presented no significant modification even at the highest pressure reached in our experiments. Therefore, the eventual non-hydrostaticity is small and should have a weak effect on our results. To check whether this assumption is correct, we performed additional linear fitting using data for Mn0.97Fe0.03WO4 obtained only in the 0–4GPa range, where the conditions with Nujol as the pressure transmitting medium are perfectly hydrostatic. We observed that the dω/dP values did not change significantly for any mode, i.e.the linear fittings to the experimental wavenumbers are not significantly modified due to the quasihydrostaticity above 4GPa. It is interesting to notice that the dω/dp values reported previously for MnWO4 were obtained from high-pressure study using Ne as the pressure transmitting medium[14]. Ne is hydrostatic up to 15GPa[17] but the authors also obtained data above 15GPa, where the conditions are quasihydrostatic. We suppose, therefore, that the observed differences in some dω/dp values estimated in this paper for Mn0.97Fe0.03WO4 and previously for MnWO4 can mainly be attributed to the different chemical compositions of the studied materials (the presence of Fe3+ ions in our case). Since, however, some of the experimental points for Mn0.97Fe0.03WO4 and MnWO4 were obtained above the hydrostatic limits of Nujol and Ne, respectively, an additional weak contribution due to non-hydrostaticity cannot be excluded.
Inspection of table1 shows that the mode Grüneisen parameters of the stretching modes are relatively small (0.37–0.69). Larger values are found for many bending and lattice modes. For instance, for the 207cm−1 mode corresponding to translational motion of Mn2+ ions, the mode Grüneisen parameter is 1.7. The large mode Grüneisen parameter for this mode indicates that under applied pressure the Mn–O bonds shorten more significantly than the W–O bonds. In other words, the observed behaviour can be attributed to the larger compressibility of AO6 octahedra when compared to WO6 octahedra, as proposed previously for other wolframite-type compounds[10–12, 14]. Table1 also shows that the mode Grüneisen parameters found for the 770–890cm−1 modes (stretching modes of the short W–O2 bonds) and the 670–700cm−1 modes (stretching modes of the double oxygen bridges involving W–O1 bonds) are comparable. This behaviour suggests that the changes in the W–O1 and W–O2 bond lengths are comparable. This conclusion is consistent with an x-ray diffraction study of MnWO4, which revealed that when the pressure increases to 5.9GPa the W–O1 and W–O2 bond lengths decrease by 0.63 and 0.53%, respectively[13].