Deuterium ordering found in new ferroelectric compound Co2(OD)3Cl

A detailed temperature-dependent Raman spectroscopic study revealed a new type of deuterium-order ferroelectrics in a geometrically frustrated magnet Co2(OD)3Cl at Tϵ = 229 K. Significant changes in the parameters of the Raman vibration modes were observed near Tϵ, suggesting a strong phonon-charge coupling. Additional asymmetric phonon bands appeared below around Tϵ, which are consistently interpreted by phonon folding processes due to a small local structural change resulting from the ordering of deuterium. The wavenumber and intensity changes of the Raman-active modes, as well as the normalized intensities of the additional bands, all follow a power-law fit Δω, ΔI, I ∝ (1 − T/TC)2β, wherein TC = 230 K ∼ Tϵ and β = 0.35(2), clearly demonstrating an ordering process below Tϵ. The critical exponent is reminiscent of a second order transition. Our study presents a rare and new type of multiferroic material with ferroelectricity arising from the deuterium ordering in geometrically frustrated magnets.


Introduction
Hydroxyl salts of the type M 2 (OH) 3 X have been known for a long time [1]. In recent years those with the magnetic ions of M = Cu 2+ , Ni 2+ , Co 2+ , Fe 2+ and Mn 2+ were shown to be geometrically frustrated magnets [2][3][4][5][6][7][8][9][10], and the substituted compounds of ZnCu 3 (OH) 6 Cl 2 [11,12], ZnCo 3 (OH) 6 Cl 2 [13] and MgCo 3 (OH) 6 Cl 2 [14] showed complete or partial spin liquid features resulting from the geometric frustration. Furthermore, measurements of the lattice parameters and dielectric constants in M 2 (OH) 3 X revealed simultaneous changes at their respective magnetic transition temperatures, demonstrating that those hydroxyl salts universally have strong magnetic-dielectric lattice couplings [15]. In special, the cobalt hydroxyl salts of Co 2 (OH) 3 Cl and Co 2 (OH) 3 Br, which possess the highest crystal symmetry of R m 3 among the hydroxyl salts, showed anomaly in dielectric constants at high temperatures of 229 and 224 K, respectively, when deuterated. Their crystal structure is similar to the spin-liquid compounds ZnCu 3 (OH) 6 Cl 2 , ZnCo 3 (OH) 6 Cl 2 and MgCo 3 (OH) 6 Cl 2 by having strong geometric frustration with alternatively stacked layers of regular kagome and triangle lattice planes. Ferroelectric transition was suspected and it was tentatively attributed to the modified hydrogen bonding due to a giant isotope effect, where geometric frustration provided a necessary condition so that a subtle change in the lattice/bonding produced dramatic change in their physical properties in Co 2 (OD) 3 Cl. However, because single crystals are not available at the present time and the usual sintering process cannot be applied to the hydroxyl salts, this subtle ferroelectricity has not been confirmed and its mechanism is unclear.
Ferroelectricity was first discovered in hydrogen-bonded materials and the ferroelectric transitions are characterized as either displacive (such as BaTiO 3 ) or order-disorder (such as NaNO 2 or KDP). In recent years, a different type termed as 'multiferroic' receives intense attention for scientific interest in their physical properties and potential for applications. Since Co 2 (OH) 3 Cl and Co 2 (OH) 3 Br showed magnetic transitions at low temperatures but with strong magnetic couplings even at 400 K [6,8,16], if the ferroelectric transition get confirmed, they can be grouped into multiferroic materials in the wide meaning of multiferroicity. Therefore, they can be viewed as unique multiferroic compounds linking geometric frustration and hydrogen-related ferroelectricity. However, since neither magnetic order nor obvious structure transition exists in this temperature range (e.g. in Co 2 (OD) 3 Cl, the ferroelectric response occurs at 229 K, which is much higher than its magnetic transition at T N = 10.5 K) [15], the origin for the observed 'ferroelectricity' in this insulating material remains mysterious and demands clarification. On this background, we carried out a detailed temperaturedependent Raman spectroscopic measurement to study the mechanism for the unusual ferroelectricity.

Experiments
Polycrystalline Co 2 (OD) 3 Cl synthesized using a hydrothermal reaction from CoCl 2 and NaOD at around 200°C, as previously reported, were pressed into a pellet and adhered to a silver plate with silver paste for Raman spectroscopic measurements. The Raman spectra were obtained with a computer-assisted Raman instrument HR800 HORIBA Jobin-Yvon using a special ×50 Olympus objective with an ultra-long working distance to improve the signal-to-noise ratio in the spectra, and a charge-coupled device (CCD) system to collect and process the scattering light. The scattering spectra were excited by a Spectra-Physics model 127 Ar + -ion laser (488.0 nm) with a resolution of 0.5 cm −1 between 4000 and 95 cm −1 . Low temperature measurements were performed using a liquid nitrogen dewar and a helium flow cryostat (Oxford Instruments) equipped with a temperature controller. The temperatures were maintained within a stability of ±0.2 K and a low laser power (5 mW) was used to minimize possible local heating.

Results and discussion
3.1. Mode assignment at room temperature The measured Raman spectra of Co 2 (OD) 3 Cl at typical temperatures are plotted in figure 1. Upon cooling to around 230 K, where the ferroelectric response occurred, significant changes in the Raman parameters as described below, as well as additional bands were observed. It is mentioned earlier that Co 2 (OD) 3 Cl has the same crystal structure with hydrogenated compound Co 2 (OH) 3 Cl, which is a highly symmetric rhombohedral structure in the space group R m 3 (No. 166) [1]. The magnetic ions Co 2+ form a three-dimensional network of linked tetrahedron with alternatively stacked layers of regular kagome and triangle lattice planes in the [001] direction. All Co 2+ ions are surrounded by six ligand ions, and surrounded by four oxygen ions at equal distances and two chlorine ions on the kagome lattice plane. On the other hand, Co 2+ ion on the triangular lattice plane is surrounded by six oxygen ions at equal distances. Another prominent structural feature is a notable distortion in the tetrahedron that the Co-Co distance on one side of the tetrahedron that has Cl − ion nearby is 3.42 Å, whereas those on the other three sides bonded with O 2− ions are 3.12 Å, indicating a 10% shorter Co-Co distance on the triangular lattice plane [15,16]. Thus, the symmetries were determined to be C s for the O site, C 3v for the Cl site, C 2h and D 3d for the Co K and Co T sites, wherein Co K and Co T denotes the Co 2+ ions on the kagome and the triangle lattice planes, respectively. Factor group analysis for this structure suggests 12 Raman active modes (5A 1g + 7E g ) and 17 infrared active modes (7A 2u + 10E u ), the Raman active modes of Co 2 (OD) 3 Cl are labeled M1 to M12. According to the previously reported assignment results of Co 2 (OH) 3 Cl/Br [17,18], and Cu 2 (OH) 3 Cl [19], the Raman spectra could be separated into four regions: (4)[Co 3 Cl] fingerprint-2 (FP 2 ) region: <200 cm −1 .
The assignment of various modes of Co 2 (OD) 3 Cl could been done as follows.
In the [OD] function group (FG) region, there are M1 (2630 cm −1 ) and M2 (2620 cm −1 ) two Raman bands with peak separations of several cm -1 . From the fact that all red-shift ratios are close to the theoretical value [μ OH /μ OD ] 1/2 = 72.8% (here, μ OH(D) is the [OH/D] reduced mass), M1 and M2 can be safely assigned to [OD] stretching modes.
The small band at 3555 cm -1 was confirmed to be due to contamination of a small amount of hydroxyl (strength ratio I OH /I OD ∼ 3%). The bands at 1600 and 1475 cm −1 (as denoted by asterisks) also exist in Co 2 (OH) 3 Cl, we could assign them to the combinations of multi-phonon modes, which are correlated with the residual H nuclei.
In  [20]. The assignment of the main modes has been summarized in table 1, and the mode displacements are illustrated in figure 2.

Evolution of Raman modes with temperature
The Raman scattering profile has been fitted using a Lorentz function to compute the exact wavenumber (Raman shift), line-width (full width at half maximum, FWHM) and intensity of each peak. In     widths decrease as a result of anharmonic effects and thermal changes. Here, we tried to fit the wavenumber and line-width variations using conventional formulae with lattice and phonon-phonon anharmonic interaction processes up to three phonons [21]: Here, x = hc ω 0 /2k B T and y = hc ω 0 /3k B T. As shown in figure 3, the wavenumber at high temperatures could be well fitted by the anharmonic equation. However, the selected modes showed clear deviation from the fitted curves below T ε , suggesting that there are additional interactions besides the expected lattice and phononphonon interaction. In order to explore the correlation between the vibrational and structural changes, we also considered the temperature dependence of the wavenumber differences for these modes. The temperature dependence of the wavenumber difference Δω 3 , Δω 8 and Δω 11 for mode M3, M8 and M11 between the observed and extrapolated wavenumbers below T ε could be well expressed in a power-law equation like that in the displacive-type ferroelectric Sn 2 P 2 Se 6 [22]. As exemplified in figure 3, the difference Δω 3 , Δω 8 and Δω 11 increased abruptly from T ε , following the equation Δω ∝ (1-T/T C ) 2β with T C = 230 K~T ε , β = 0.35(2) (β is the critical exponent for the order parameter, the error of the critical exponent is from the upper and lower limits in the fitting process). This suggests a subtle local structural change below T ε . Since the present material is geometrically frustrated, the variation in local [OD] unit vibration (M3) will induce subtle changes in the integral structure. Thus, [Co 3 O(D)] unit (M8) and [Co 3 Cl] unit (M11) exhibited nearly the same variation with that in the [OD] unit. The line-widths for these modes especially M8 involving the magnetic Co 2+ ions, showed obvious anomalies near T ε deviating from the expected behavior, as illustrated in figure 4. In general, the change in Raman parameters with temperature can be caused by several factors, such as phonon-phonon anharmonic interaction, spin-phonon coupling, or phonon renormalization resulting from electron-phonon coupling [23,24]. The latter one can be neglected when the carrier concentration is low. Since the line-widths are not susceptible to the subtle lattice volume changes due to magnetostriction, the variation in the line-width of M8 should be induced by the spin-phonon coupling. This result is needed to be confirmed by further studies.
Since the Raman scattering intensity is susceptible to the change of ferroelectricity, we also considered the integrated intensities and intensity differences below T ε for the selected modes. As shown in figure 5, the intensities below T ε deviated from the straight lines extrapolated from the data above T ε and saturated toward 150 K. The intensity difference (absolute value) ΔI 3 , ΔI 8 and ΔI 11 between the observed and extrapolated intensities can be well fitted in the same way as that in the well-known proton-ordering ferroelectric PbHPO 4 [25] and order-disorder ferroelectric NaNO 3 [26,27], similarly following the power-law expression ΔI ∝ (1-T/ 230) 2 * 0.35 . The results suggest an ordering process below T ε accompanying the appearance of ferroelectricity. This ordering is strongly coupled with the Co tetrahedron, and gives rise to the structural change. Direct evidence for the transition near 230 K appeared as the additional spectral bands labeled N1 to N7, as shown in figure 1, which are highly asymmetric in their line shapes. The Raman spectra of Co 2 (OH) 3 Cl (not shown here), measured at the same conditions as that on the deuterated compound, showed no additional spectral bands as on the deuterated one in the same frequency and temperature ranges. Thus, considering the fact that their lattice structures are quite the same, it is apparent that these additional bands below 230 K on Co 2 (OD) 3 Cl have close relation with the deuterium motion. Asymmetric Raman line shapes have once been attributed to a Fano effect, which are often found in charge-ordered ferroelectric semiconductors [29]. The temperature-dependent normalized intensities for the additional bands in the present material, as illustrated in figure 6, can also be well fitted by the above ordering expression I ∝ (1-T/230) 2 * 0.35 . Apparently, the intensities of the additional bands could be used conveniently to describe the ordering process for the ferroelectric transition below T C = 230 K, as in other materials [22,[25][26][27]. The additional bands arising below 230 K might be infrared active modes, which become Raman active due to the symmetry breaking at the phase transition. Inspection of their frequencies suggests that the additional bands can be assigned to the combination or overtone of anharmonic vibrations of the [Co 3 O(D)] units, as shown in table 1. This kind of anharmonic overtone and combination were rare, however, they have been observed in a proton-ordering ferroelectric material NaNH 4 SO 4 ·2H 2 O [28]. The fact of the occurrence of ferroelectricity with deuteration in Co 2 (OD) 3 Cl and the overtone and combination of the [Co 3 O(D)] unit modes strongly suggest a kind of deuterium ordering near the T ε . Thus, the additional asymmetric bands, as well as the prominent changes of the selected modes can be consistently explained by changes in lattice and anharmonicities arising from the deuterium ordering in the insulating Co 2 (OD) 3 Cl. Several additional modes are still recognizable in the paraelectric phase at 240 K, showing remaining phonon-charge couplings as observed in proton-ordering ferroelectric compounds [25].
Through the analyses in the present work, we obtained the critical exponent β = 0.35 (2). As is well known, second order phase transitions follow a power-law expression with universal critical exponents, wherein the β values are calculated to be around 0.31 ∼ 0.35 for different models [30]. The present β value is consistent with the value predicted for intrinsic second-order transitions. This interpretation is consistent with the experimental facts that no obvious structural phase transition was observed in the present material. On the other hand, rearrangement of all [OD], [Co 3 O(D)] and [Co 3 Cl] units occurred accompanying the D sub-lattice ordering. This kind of rearrangement, we suspect, gave rise to the ferroelectricity. The nature of the ferroelectric phase transition in the present material is nearly the same as that showed in KDP, wherein both displacive and orderdisorder type transitions were involved.
Another relevant feature is the temperature dependence of a band at ∼210 cm −1 (marked by the vertical arrows in figure 1) in the vicinity of the mode M10. This band decreased upon heating and completely disappeared at about 150 K. This change, together with the tendencies of saturated wavenumbers and intensities of the phonon modes at lower temperatures (see figures 3 and 5), may suggest that the system reached a new equivalent state for the deuterium ordering below 150 K. This feature deserves further investigation. In summary, strong evidences of deuterium-order ferroelectricity in compound Co 2 (OD) 3 Cl are obtained from a Raman spectroscopic study. A critical scaling is found near the T ε ∼ 230 K with a critical exponent β = 0.35(2), suggesting a second order transition. Since prominent changes occurred in the A 1g and E g modes associated with D and O, the deuterium ordering should occur near the three side planes of the Co tetrahedron in the structure. This ordering is strongly coupled with the lattice vibration modes involving the [Co 3 O(D)] units, and would bring out a structural transition, as is exemplified in the well-known ferroelectric KDP. However, the structural change in Co 2 (OD) 3 Cl should be exceptionally small or unusual, thus it was overlooked in previous structural studies. The present work shows that Co 2 (OD) 3 Cl can be viewed as a unique prototypical multiferroic compound combining the geometric frustration and deuterium ordering ferroelectricity. The present material should belong to the mixed-type ferroelectrics that an ordering arrangement of deuterium and an instability of the lattice vibration occur simultaneously, thus manifesting both the displacive-type and order-disorder type ferroelectric features.