Thermal analysis, Raman spectroscopy and complex impedance analysis of Cu2+-doped KDP

Raman spectroscopy and differential thermal analysis (DTA) and thermogravimetric analysis have been carried out on Cu-doped KH2PO4 (Cu-KDP). X-ray diffraction powder data reveal that the structure of the KDP crystal does not change with the additive Cu2+ ion. DTA analysis and Raman study of Cu-KDP as a function of temperature reveal that this compound undergoes two phase transitions at about Ttr =453 and 473 K. The electrical conductivity measurements on polycrystalline pellet of Cu-KDP (5) are performed from room temperature (RT) up to 495 K. Only one phase transition is observed at 470 K. The activation energy in the migration is 0.42 eV in the temperature range from RT to 470 K. For temperature above 470 K, the activation energy of the superprotonic phase is 1.87 eV.


Introduction
The KDP family compounds (MH 2 PO 4 ; M=K, Cs, Rb, NH 4 ) are interesting for many reasons [1][2][3][4][5][6]. In particular, CsH 2 PO 4 (CDP ) as a fuel cell electrolyte at temperatures so-called superprotonic behaviour present an abrupt, several-order-ofmagnitude jump in its proton conductivity upon heating above the temperature Ttr≈508 K [7]. It is demonstrated that the abovementioned proton conductivity enhancement is associated with a polymorphic phase transition from its room temperature monoclinic (P2 1 /m) phase to a high-temperature dynamically disordered cubic (Pm-3m) CDP modification [8]. Interestingly, the RbH 2 PO 4 (RDP) compound also exhibits a superprotonic transition at 566 K, although at room temperature RDP is not monoclinic as CDP, but tetragonal (I 4 2d). Using synchrotron X-ray studies, Botez et al. [8] demonstrated that heating RDP towards its superprotonic transition leads to an intermediate temperature (Ttr =383 K) change of the RDP tetragonal phase into a monoclinic modification, isomorphic (crystallographically identical) to the monoclinic CDP phase. There is another phosphate, NH 4 H 2 PO 4 , which crystallizes at room temperature in tetragonal space group I 4 2d and upon heating does not show an abrupt enhancement of its proton conductivity [9].
Much less is understood about the structural, chemical and physical property changes that occur in KH 2 PO 4 (KDP) upon heating from room temperature toward its melting point. Thermal events observed around Ttr =458 K, for example, have been attributed by some authors to a polymorphic phase transition at an intermediate-temperature KDP modification [10], while others have claimed that the behaviour at Ttr was in fact due to chemical changes, such as dehydration and onset of partial polymerization of the room temperature of tetragonal KDP phase [11,12].
Previous investigations of the structural changes of KDP have been devoted, in the past few years, to understand the exact nature of this transition in KDP, which remained a controversial subject. Using X-ray diffraction measurements, Itoh et al. [13] concluded that on heating through 460 K, the crystal system changed from tetragonal to monoclinic, space group P2 1 or P2 1 /m. The lattice parameters at 468 K were given as a=7.47 Å, b=7.33 Å, c=14.49 Å, α=β=90°and γ=92.2°. The spots in the Weissenberg photograph at 468 K were only consistently indexed by assuming a twin structure in the monoclinic phase appearing above 460 K. The complete assignment of the reflections was difficult to do, and until now detailed structural analyses including atomic coordinates have not been done [11]. Moreover, it has been reported that the monoclinic phase of KDP is metastable at temperatures below Ttr and that it reverts to the stable tetragonal phase after being kept for some days in air at room temperature [10,[12][13][14]. Very recently, temperature-resolved synchrotron X-ray studies have demonstrated that heating KDP towards its superprotonic transition leads to an intermediate temperature (Ttr =463 K), where the KDP tetragonal phase changes into a monoclinic phase, the same as both isomorphic monoclinic RDP and CDP [15]. Moreover, they observed that monoclinic KDP is stable up to 508 K and indicated that a monoclinic (P2 1 /m)-cubic (Pm3m) transition upon further heating similar to the one responsible for the superprotonic behaviour of CDP and RDP is not precluded. They concluded that the reported lack of superprotonic behaviour in KDP [16] was most likely due to ion size effects and not due to crystal structure considerations.
In view of these results, it appears that further investigations aimed to clarify the structural changes undergone by KDP upon heating above Ttr are worth carrying out.
It is well known that the presence of a small amount of impurities can considerably influence the growth habit, optical properties and dielectric properties of KDP crystals [17][18][19][20][21].
Due to various physicochemical properties, impurities can be selectively incorporated at the surface of the crystal layer and into the kink sites. The presence of metallic impurities may form both isolated and interstitial defect centres [22].
This work presents the evolution of the Raman spectra of single crystal of pure KDP and Cu 2+ -doped KDP (Cu-KDP) when the temperature is raised from 303 to 503 K. Raman scattering method has been successfully used to reveal structural information about a wide variety of inorganic compounds: phosphate solid acids [23,24] and sulfate [18,25,26]. In particular, Raman spectroscopy is used to demonstrate the effect of heating on the partial decomposition of KDP [25,27]. Thus, this method is suitable for the present investigation, which presents also the influence of bivalent Cu 2+ impurity ions on the electrical properties.    (Table 1). Figure 1 compares the XRD patterns of Cu-KDP (1), Cu-KDP (2), Cu-KDP (3), Cu-KDP (4) and Cu-KDP (5) as well as from KDP references.
The characterization of compound is carried out from X-ray diffraction powder data. The intensities of the diffractograms are collected by using a diffractometer Bruker-AXS, type D8 with CuKα radiation (l(K α1 ) =1.54060 Å, l(K α2 ) =1.54443 Å). Diffraction intensities are measured between 10°and 60°, with a 2θ step of 0.02°for 2 s per point. The data are collected at room temperature. The unit cell parameters calculated by using the patterns matching the routine of the FULPROOF program [28] are a =7.45 Å and b =6.974 Å. Figure 2 shows the X-ray diffraction (XRD) pattern of Cu-KDP (5). All the reflection peaks of the XRD pattern of the sample are indexed in a single-phase tetragonal KDP structure with I 4 2d space group [29].
Thermogravimetric (TGA) and differential thermal analysis (DTA) are performed on polycrystalline with a TGA/DTA Q600 STD TA Instruments apparatus (sample with Pt crucibles,  Table 2 Raman spectra of pure KDP, Cu-KDP (1), Cu-KDP (2), Cu-KDP (3), Cu-KDP (4) and Cu-KDP (5)  The infrared absorption spectrum was recorded using a pellet of sample, which was prepared by mixing 1 mg sample in a total weight (samples+KBr) of 200 mg. A Perkin-Elmer FT-IR vs very strong, s strong, m medium, w weak Electrical conductivity measurements were carried out by means of impedance spectroscopy in dehydrated N 2 atmosphere in the temperature range from 315 to 495 K. The experiments were done out with ion-blocking sputtered Pt electrodes in a two-probe cell. A frequency response analyser (Solartron 1260) and a dielectric interface (Solartron 1296) were used in the frequency domain from 1 Hz to10 MHz. Before each measurement, electrochemical system linearity and stability were checked. An AC voltage of 300 mV is used, with a waiting time of 25 min for each 23-°C step (thermal equilibration). Impedance diagrams plotted in the Nyquist complex plane were fitted with a series combination of Rs and Rp//CPE elements of the Z-view 3.2c software assigned [30].

DTA and TGA studies
The thermal measurement (Fig. 3) of Cu-KDP (5) reveals the presence of two high-temperature phase transitions at 462 K (189°C) and 473 K (200°C). Thermal gravimetric analysis shows that the transition is not related to decomposition. Significant weight loss occurred from 498 K (225°C).

Infrared and Raman spectroscopy
The KDP crystal at room temperature may be considered as consisting of K + and (H 2 PO 4 ) − ions belonging to the space group I 4 2d. The H 2 PO 4 groups together with atoms of K lying between them on axis z (c) create columns shifted one against the other at c/4 along the direction z. Each PO 4 group is linked with four neighbouring PO 4 groups by four hydrogen bonds lying almost exactly in the planes perpendicular to the z-axis. The local symmetry of the (H 2 PO 4 ) tetrahedron is C 2 ,   Table 4 Wavenumbers (cm −1 ) of the bands in the Raman spectra of Cu-KDP (5) vs very strong, s strong, m medium, w weak not S 4 , which is the average site symmetry in the paraelectric phase, as determined by X-ray analysis [5]. At room temperature, the Raman spectrum of Cu-KDP is similar to that of KDP [26,31].  Tables 2 and  3. The analysis is based on the classical identification of external modes implying the whole lattice network, internal modes of the (H 2 PO 4 ) − tetrahedra and stretching and bending modes of OH bonds. The attribution is done with respect to the KDP attribution taken from [4,31,32] and calculated frequencies of free (H 2 PO 4 ) − ions.
To calculate the frequencies of the vibrational modes of the H 2 PO 4 entity, ab initio RHF/MP2 and DFT/B3LYP electronic structure calculations were carried out using the TZV standard basis set augmented by one diffuse and one polarization functions implemented in the GAMESS program [33]. To identify different vibrational modes, the MOLDEN package [34] was used. Vibrational levels obtained by RHF/MP2 and DFT/B3LYP calculation on free (H 2 PO 4 ) − and in the experimental result are compared with those measured [32] in aqueous solution. Figure 6 shows the correlation of the free (H 2 PO 4 ) − group vibrations in C 2v internal vibrations in C 2 factor group symmetry through the D 2d one in the crystal.
The spectroscopic characteristics of the free H 2 PO 4 ion with pseudo-symmetry C 2v can be deduced from the free tetrahedral PO 4 entity. The two stretching modes v 1 (~938 cm −1 ) and ν 3 (~1,017 cm −1 ) lead to four stretching modes (two modes ν(PO 2 ), one antisymmetrical (B 1 ) at about 1,033 cm −1 and one symmetrical (A 1 ) at 1,060 cm −1 , and two ν(P (OH) 2 ) around 732 and 759 cm −1 with symmetry A 1 and B 2 , respectively) (Fig. 6). The bending modes, arising from ν 2 and ν 4 , are expected at around 315 and 382 cm −1 and at 419, 475 and 484 cm −1 , respectively. Finally, the three calculated frequencies involving the OH group are 1,093, 1,299 and 3,830 cm −1 . All of these vibrations theoretically appear in Raman spectra, and only the vibrations with symmetry mode A 1 , B 1 , and B 2 appear in IR spectra.
As shown in Figs. 4 and 5, the additive has a considerable influence in the Raman and IR spectra. For example, the bands which appear at 3,600 and 3268 cm −1 in pure KDP and assigned to free O-H stretching [19] are absent from Cu-KDP (3). The absence of these peaks supports the adsorption of Cu 2+ in the surfaces of the crystal. The deviation of IR frequencies for O-H stretching observed at 3,435 and 3,516 cm −1 in pure KDP [19] to higher frequencies, respectively, at 3,483 and 3,565 cm −1 in Cu-KDP (3) (Tables 2 and 3).
In the Raman spectra, we observed two new bands centred at 253 and 235 cm −1 in the spectrum of Cu-KDP (3). We observed that these two peaks become more intense for Cu-KDP (5). These two bands are characteristic of the Cu-O bond [35].
External modes result from translational and vibrational modes of anions and translational ones of cations. The vibrational Fig. 10 The temperature dependence of the wavenumbers of some bands observed in the Raman spectra of the Cu-KDP (5) Fig. 9 Raman spectra of Cu-KDP (5) material in phase II, phase I-a and phase I-b and translational modes of anions can be seen as hydrogen bond stretchings and bendings inside the layers. They are observed below 300 cm −1 for KDP families [36]. According to Som et al., the lines observed at 116 and 154 cm −1 in pure KDP and at 118 and 155 cm −1 in Cu-KDP may be assigned to K-PO 4 translatory vibrations along the c-axis [37]. The 187 cm −1 in Cu-KDP (3) band can be assigned to the H 2 PO 4 rotational band. The band observed at 193 cm −1 from Cu-KDP (3) can be attributed to the stretching ν(O-H≡O) bridge vibration. This band is observed at about 191 cm −1 in Raman spectra of pure KDP.
The ABC-type broad bands of high-frequency H vibrations have been interpreted as O-H stretching modes in Fermi resonance with combination involving mainly O-H bending vibrations [38] or in terms of strong coupling between fast O-H and slow O=O stretching modes [38]. These spectral characteristics have been observed in a variety of strong hydrogen-bonded solids having O-O distances varying from 2.45 to 2.66 Å [39]. In the present case, the ABC-type bands appear in the Raman spectrum of pure KDP at 2,728, 2,370 and 1,817 cm −1 , respectively (Fig. 7). These bands appear at about 2,830, 2,383 and 11,758 cm −1 , respectively, in Cu-KDP (5) (Fig. 7). The substantial change in the position of these bands reflects the possible incorporation of Cu 2+ in the lattice site of KDP crystal.
In conclusion, in a low concentration of copper (Cu-KDP (1) and Cu-KDP (2)), all of the copper is practically substituted in volume, whereas in the high concentrations the copper is absorbed in both volume and the surface.
It is reported that at an increase of the temperature above 453 K, a monoclinic phase is obtained. The tetragonal→monoclinic transition usually occurs at a temperature between 453 and 493 K; the measured value depends strongly on sample preparation [8,40]. In the monoclinic phase, space group P2 1 /m (C 2h ), the atomic distribution and lattice parameters at 463 K are obtained (a=7.590 Å, b=6.209 Å, c=4.530 Å, β=107.36°) [8].
In Fig. 8, the increase of temperature from 303 to 503 K shows some modifications in the Raman spectra of Cu-KDP (5) but without any well-known bands due to P 2 O 7 entities [40]. These bands might indicate some dehydration (Table 4). Two-phase transitions at 453 and 473 K are related to Cu-KDP (5) (Fig. 9).
The increase of temperature in Cu-KDP (5), between 303 and 503 K, causes the disappearance of lines at low frequency, corresponding to the lattice modes and the band at 193 cm −1 , attributed to the stretching O==O bridge vibrations. The appearance in Cu-KDP (5) spectrum of a line at 61 cm −1 and another at 1,175 cm −1 corresponds to ν as (PO 2 ) vibrations. Also, there are substantial changes in the position and intensity of some bands corresponding to the internal vibrations of H 2 PO 4 .
The most important change concerns the Raman line associated to ν as P(OH) 2 mode. This strong line observed at 914 cm −1 at room temperature splits to two Raman lines near 901 and 960 cm −1 for the temperature between 453 and 463 K and reappears (from 473 K) with a single strong line at 922 cm −1 . The frequency of this vibration, ν as P(OH) 2 , estimated by Joost Vande Vondele et al. [32] from the calculation of free H 2 PO 4 and in aqueous solution are 795 and 944 cm −1 , respectively.
Appreciable changes are also observed for lines at 1,175 cm −1 due to ν as (PO 2 ) vibrations. This line appears between 453 and 473 K and disappears from 483 K.
A plot of the temperature behaviour of some Raman wavenumber is shown in Fig. 10, which unambiguously proves the existence of singularities in the temperature range 303-503 K. From these curves, the temperature of the II→I (a) transition is estimated at 453 and 473 K for I-a→I-b transition.  Fig. 10 and show that Cu-KDP (5) follows the Cole-Cole law. The bulk ohmic resistance relative to experimental temperature is the intercept on the real axis of the zero-phase angle extrapolation of the highest frequency curve. In Fig. 11, the equivalent circuit of the crystal under the ac electric field at lower temperatures can be modelled well as a solution resistance R s in series with the parallel combination of a polarization resistance R P with a constant phase angle element (cpe).
The conductivity of Cu-KDP (5) is shown in Arrhenius curve (Fig. 12). Only one transition, at Ttr=450 K, of the two transitions noted by the calorimetric and Raman studies is apparent in the conductivity results. This transition leads to a sharp increase of the conductivity. The second transition is not observed because it produces a small impact on the transport properties.
The activation energies are obtained as E a =0.42 and 1.87 eV above and below Tp=450 K for the ionic hopping of mobile ions (H + ) ( Table 5). In the KDP material, the activation energies are 0.42 and 0.68 eV for the temperature above and below 453 K, respectively. In KH 2 AsO 4 , the activation energies are 0.40 and 0.59 eV for the temperature above and below 453 K [41].
It is noteworthy that the superprotonic Cu-KDP (5) has a rather low activation energy for proton transport (0.42 eV) and a significantly higher conductivity (2.2×10 −7 Ω −1 cm −1 ) for the polycrystalline pellet compared to the pure KDP (10 −10 Ω −1 cm −1 ) [5,42]. It is possible that the higher conductivity is due to the proton defects in Cu-KDP caused by the substitution of K + by Cu 2+ .

General conclusions
The high temperature phenomena of Cu-KDP are not related to chemical change such as thermal decomposition but related to the physical change of the structural phase transition. Doping KDP with Cu 2+ significantly modifies its electric properties. The transport properties of protons in such crystals may be treated in terms of proton defects.