Spectroscopic Study of Structural Phase Transition and Dynamic

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Spectroscopic Study of Structural Phase Transition and Dynamic Effects in [(CH]NH][Cd(N)] Hybrid Perovskite Framework 3

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Monika Trzebiatowska, Miroslaw Maczka, Maciej Ptak, Laisvydas Giriunas, Sergejus Balciunas, Mantas Šim#nas, Daniel Klose, and Juras Banys J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01121 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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The Journal of Physical Chemistry

Spectroscopic Study of Structural Phase Transition and Dynamic Effects in [(CH3)2NH2][Cd(N3)3] Hybrid Perovskite Framework Monika Trzebiatowska,*,† Mirosław Mączka,† Maciej Ptak, †Laisvydas Giriunas,‡ Sergejus Balciunas,‡ Mantas Simenas,‡ Daniel Klose,§ and Juras Banys‡

†Institute

of Low Temperature and Structure Research, Polish Academy of Sciences, Box 1410,

50-950 Wrocław 2, Poland ‡Faculty

of Physics, Vilnius University, Sauletekio av. 9, LT-10222 Vilnius, Lithuania

§Department

of Physical Chemistry, ETH Zürich, Vladimir-Prelog-Weg 2, 8093 Zürich,

Switzerland ABSTRACT. We present a combined study of [(CH3)2NH2][Cd(N3)3] hybrid perovskite dense metal-organic framework by IR, Raman, continuous-wave electron paramagnetic resonance (CW EPR), phase-transition-induced current and dielectric measurements. The Raman and IR study has shown that the phase transition observed near 175 K is associated with a pronounced narrowing and intensity increase of bands related to the

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NH2 group of dimethylammonium (DMA+) cations located in the cavities of the framework. This observation has proved that the phase transition is associated with an ordering of DMA+ cations. The dielectric response of [(CH3)2NH2][Cd(N3)3] single crystal has revealed a weakly expressed anomalous behavior of the dielectric constant at the phase transition point. In the low-temperature phase a pronounced dispersion in the kilohertz frequency range has been observed, which might be assigned to the two-fold reorientation dynamics of the DMA+ cations. The phase-transition-induced current measurements of DMACd(N3)3 single crystal reveal a small anomaly at the phase transition temperature demonstrating non-ferroelectric behavior. The CW EPR experiments of manganese-doped DMACd(N3)3 disclose that the paramagnetic Mn2+ probe ions have been successfully incorporated in the crystal structure and form MnN6 octahedra. The temperature evolution of the CW EPR spectrum reveals that these centers are sensitive to the structural changes of the cadmium-azide framework occurring during the phase transition. The temperature evolution of the spin Hamiltonian parameters and CW EPR linewidth of the Mn2+ ions have allowed to detect the effects of the DMA+ cation dynamics and to characterize the type of the phase transition.


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INTRODUCTION

The dense metal-organic frameworks (MOFs) of general formula ABX3, where A denotes protonated amine, B is divalent metal cation (Cd2+, Mn2+, Co2+, Ni2+, etc.) and X is short organic linker (HCOO-, N3-, CN-, SCN-, N(CN)2- (dca-), Au(CN)2- and HPO2-), have received enormous interest in recent years due to their interesting physicochemical properties.1-36 Most efforts have been devoted to the studies of metal-formate frameworks because some of these compounds show coexistence of magnetic and electric order that make them promising materials for applications in spintronics.3,5-7,11,15,18-20 Although multiferroic properties have not been reported up to now for perovskite-like azides of general formula AB(N3)3, these compounds have also received a lot of interest because manganese analogues exhibit long-range magnetic order up to 92 K,22 i.e., the magnetic order is observed at much higher temperatures than for perovskite-like formate frameworks, in which the manganese analogues showed magnetic order near 8-9 K.2,3,5,8,9,19 Furthermore, some azide structures may exhibit polar order (polar structure has been reported for [(CH3)2NH2][Mn(N3)3]), and they often exhibit switchable dielectric properties.22,23,25,26 The electric order and switchable dielectric properties as well as dielectric relaxation phenomena in the family of formate- and azide-based frameworks are associated with



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structural phase transitions and dynamics of organic cations.1,3-5,8-10,13,17,19,22,23,25,26 Therefore, many efforts have been undertaken to study the phase transition mechanisms in these dense MOFs using X-ray diffraction, differential scanning calorimetry

and

dielectric

spectroscopy.1,3-5,8-10,13,17,19,22,23,25,26

The

frameworks

constructed of azide ions are expected to induce structural transitions to even greater extent than in the case of formate-based networks because such frameworks are more flexible and deformable.22-27 The mechanisms of the phase transitions in the family of metal-azides templated by protonated amines are, however, still poorly understood. Nevertheless, the available literature data show that the molecular dynamics of protonated amines and, in some compounds, also N3- ions, plays an important role in this mechanism.22-27 For instance, it has been reported that the phase transition in [(CH3]2NH2][Cd(N3)3] (abbreviated as DMACd(N3)3), taking place at T0 = 172 K during cooling, is associated with an ordering of dimethylammonium (DMA+) cations and deformation of the [Cd(N3)3]- framework.26 It is therefore of great interest to probe azidobridged frameworks by methods sensitive to changes in hydrogen bond (HB) strength, local structural distortion and order-disorder phenomena.


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In the high-temperature (HT) phase (T > T0), DMACd(N3)3 crystalizes in the trigonal R

3 space group, in which the cadmium cations are linked by the N3- ions to form a threedimensional framework with pseudo-cubic cages.26 Each such cage contains a single DMA+ cation, which is H-bonded with the [Cd(N3)3]- framework and can occupy one of the six preferential positions (see Figure 1a). The space group of DMACd(N3)3 in the low-temperature (LT) phase (T < T0) is centrosymmetric triclinic P 1 , and the DMA+ cation shows a two-fold disorder (incomplete ordering) (Figure 1b).

Figure 1. Crystal structure of DMACd(N3)3 in the (a) HT and (b) LT phases. The protons in the HT phase are omitted for clarity. The structural data are taken from Ref. 26.

The methods sensitive to dynamics of molecular units and structural distortion are Raman and IR spectroscopies. They have not been employed yet in the studies of



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azide-based MOFs but they have been shown to provide deep insight into temperatureand pressure-induced structural changes in the formate- and dicyanamide-based frameworks.8-10,13,19,33 The dielectric spectroscopy is another method that proved to be a highly useful tool to probe the dynamic effects of hybrid frameworks including DMACd(N3)3.4,26,37-39 This method allows to characterize the phase transition properties as well as dynamics of the organic cations in different structural phases of a compound. The single crystal dielectric measurements of DMACd(N3)3 are still lacking, as the previous study has reported the measurements on a pressed powder sample resulting in a smeared phase transition anomaly.26 Another powerful method of choice to study the local structural changes and dynamic effects in perovskite frameworks is the electron paramagnetic resonance (EPR) spectroscopy.40-43 This technique can monitor the local environment (order parameter) of the paramagnetic transition metal probes that are introduced into the diamagnetic frameworks. Previously, we employed the continuous-wave (CW) and pulsed versions of this technique to investigate the framework deformation

and

molecular

cation

motion

in

[(CH)3NH2][Zn(HCOO)3],

[CH3NH2NH2][Zn(HCOO)3] and [NH3(CH2)4NH3][Zn(HCOO)3]2 compounds doped with a tiny amount of paramagnetic Mn2+ and Cu2+ ions.38,40,41,44,45 EPR spectroscopy has not been, however, employed in study of any azido-bridged framework. In this work we utilize IR, Raman, dielectric and EPR spectroscopic techniques as well as measurements of the phase-transition-induced current to investigate DMACd(N3)3 framework. The temperature dependent experiments allow us to characterize comprehensively the DMA+ cation motion, the framework deformation and the structural phase transition in this compound.


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EXPERIMENTAL SECTION

Synthesis. All reagents (analytical grade) used for the synthesis were commercially purchased from Sigma-Aldrich and used without further purification. In order to obtain DMACd(N3)3 : 0.05 Mn2+ mol%, HNO3 acid was added to the mixture containing 2 mL of 2.0 M solution of dimethylamine in methanol and 5 mL of water until the pH value was close to 7. Then 5 mmol of NaN3 dissolved in 10 mL of water was slowly added under stirring, the mixture was heated to 60 C and 10 mL of water solution containing cadmium nitrate tetrahydrate (0.49975 mmol) and manganese nitrate tetrahydrate (0.00025 mmol) was added. The resulting clear solution was kept at 60 C for half an hour and allowed to stand at room temperature. The pure (undoped) DMACd(N3)3 crystals were synthesized in the same manner with the use of 0.50000 mmol of cadmium nitrate tetrahydrate. The colorless crystals were harvested after one week. Doping of DMACd(N3)3 with such a small amount of paramagnetic Mn2+ ions was necessary for EPR spectroscopy.



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Caution! Although our samples have never exploded during handing, metal azide compounds are potentially explosive. Only a small amount of material should be prepared and handled with great caution.

IR and Raman spectroscopy. The temperature-dependent Raman spectra on pure (undoped) sample were measured using a Renishaw InVia Raman spectrometer equipped with a confocal DM 2500 Leica optical microscope, a thermoelectrically cooled CCD as a detector and an argon laser operating at 488 nm. The temperaturedependent IR spectra were measured using a Nicolet iN10 stand-alone Infrared Microscope. The temperature was controlled using Linkam cryostat cell, and the spectral resolution was 2 cm-1.

Dielectric spectroscopy. The dielectric spectroscopy measurements of a single crystal sample were performed in the kHz frequency and 125 - 300 K temperature ranges using a HP 4284A precision LCR meter. Silver paste electrodes were deposited on the sample to ensure a good electrical contact. The measurements were performed during cooling at a rate of 1 K/min. Phase-transition-induced current. The phase-transition-induced current measurements were performed by cooling the single crystal sample from 300 to 150 K with an applied external electric field of 2.5 kV/cm. Then the field was removed, and a 10 kΩ resistor was used to short


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the sample. The current was measured using a 6514 Keithley electrometer during heating at a rate of 2 K/min EPR spectroscopy. The CW EPR experiments of Mn2+-doped DMACd(N3)3 powder were performed at X- and Q-band microwave frequencies using Bruker E580 and Bruker E600 EPR spectrometers, respectively. For the measurements, a modulation field of 0.6 mT and 100 kHz was used. The temperature was measured using a T-type thermocouple placed near the sample within the EPR tube. The spectral simulations were performed using EasySpin 5.2.21 software.45 RESULTS AND DISCUSSION

Raman and IR studies. The correct interpretation of Raman and IR data requires the knowledge of the crystal structure. In the HT phase, the N3- ions are ordered and there is only one crystallographically unique ion in the primitive cell (see Fig. 1a). The free N3ion has D∞h symmetry, and its fundamental internal vibrations consist of the symmetric stretching mode 1, asymmetric stretching mode 3 and bending mode 2.47 The correlation diagram, presented in Table S1, shows that the 1 mode should split in the R 3 phase into two Raman-active modes Ag+Eg, whereas the 3 and 2 modes should

give rise to IR-active modes Au+Eu and 2Au+2Eu, respectively. Since the N3- ions are located at an inversion center, all translational modes are IR-active, while all librational



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modes are Raman-active (see Table S1). The Cd2+ cations are also located at the inversion center. Thus their translational motions are IR-active only (Table S1). There are three N3- anions in the primitive cell the LT P 1 structure, but all of them are crystallographically independent. Thus one may expect to observe 3 and 9 Ramanactive modes for the 1 and librational modes as well as 6, 3 and 9 IR-active modes for the 2, 3 and translational modes of N3-, respectively. The vibrations of free DMA+ cation can be subdivided into symmetric stretching (s(NH2)), antisymmetric stretching ((as(NH2)), scissoring ((NH2)), rocking ((NH2)), wagging ((NH2)) and twisting ((NH2)) modes of the NH2 group, symmetric stretching (s(CNC)), antisymmetric stretching (as(CNC)) and bending ((CNC)) modes of the CNC group as well as symmetric stretching (s(CH3)), antisymmetric stretching ((as(CH3)), bending ((CH3)), rocking ((CH3)) and torsion ((CH3)) modes of the CH3 groups.8 Since the DMA+ cations exhibit six-fold disorder in the R 3 phase and two-fold disorder in the P 1 phase (see Fig. 1b), we do not present the distribution of the vibrational modes of DMA+ among the irreducible representations.


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The IR and Raman spectra of the studied compounds are presented in Figs. 2, 3, S1 and S2. Table S2 lists wavenumbers of the observed bands and their assignment. The assignment of internal modes of N3- ions can be fairly easily done based on literature data for other azides.47,48 In particular, the 3 and v2 modes give rise to IR bands at 2054 cm-1 and 662+612 cm-1, respectively. These modes are not observed in the Raman spectra of the HT phase, which is in agreement with the selection rules. The 1 mode gives rise to strong Raman band at 1360 cm-1. It is worth noting that this mode should not be active in the IR spectra, but a weak band is observed at 1354 cm-1. This band can be, therefore, assigned to a combination mode. The internal modes of DMA+ cation can also be easily assigned by comparison with the data obtained for related dense MOFs with formate ligands.8,49 It is worth noting that the (NH2) and (NH2) modes are observed in the room-temperature IR spectrum at 1581 and 823 cm-1, respectively (Table S2), i.e. at much lower wavenumbers than the corresponding modes in dimethylammonium metal formates (near 1630 and 910 cm-1, respectively).8,49 This behaviour points to significantly weaker strength of HBs in DMACd(N3)3 azide than in the related formates. Such a large difference between azides



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and formates is due to a different nature of the HBs, i.e., there are HBs in formates of NH…O type and of N-H…N type in azides present. According to the selection rules the translational modes of Cd2+ and N3- ions do not contribute to the Raman spectra (Table S1). The studies of formate frameworks have shown that the librational and translational modes of DMA+ cation are observed as weak Raman bands.8,49 The librational modes of N3- linker have been, however, observed for NaN3 and AgN3 as strong Raman bands in the 70-220 cm-1 range.47,48 We attribute, therefore, all Raman bands of DMACd(N3)3 below 230 cm-1 to L(N3) modes.

Figure 2. The details of the temperature-dependent Raman spectra of DMACd(N3)3. The asterisks indicate bands that exhibit a sudden increase in intensity below T0.


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Figure 3. The details of the temperature-dependent IR spectra of DMACd(N3)3. The asterisks indicate bands that exhibit a sudden increase in intensity below T0.

The spectra exhibit weak changes on cooling from 300 to 175 K (Figures 2 and 3). When the temperature decreases below 175 K, the spectra experience abrupt changes. Firstly, the bands related to vibrations of the N3- ions, i.e., 3(N3) IR band at 2054 cm-1, 1(N3) IR band at 1354 cm-1 and the 2(N3) IR bands at 662 and 612 cm-1 exhibit splitting (Figures 3c and S2, Table S2). Splitting is also observed for the Raman-active N3- librational mode at 105 cm-1 (Figure 2d). This behavior is consistent with symmetry lowering and increase of non-equivalent N3- ions from one in the HT phase to three in 13 ACS Paragon Plus Environment


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the triclinic LT phase. Secondly, the internal modes of the N3- ion exhibit weak shifts at T0 and weak changes in full width at half maximum (FWHM) (see for instance behavior of the 1(N3) mode in Figure S3c). This behavior is consistent with the ordered nature of the azide ions in both phases. Thirdly, the modes related to the CNC group of DMA+ cation (see for instance the s(CNC) mode in Figure S3b) do not show any sudden changes at T0. This behavior points to weak changes in the C-N bond length and CNC bond angles during the structural phase transition. It is worth noting that significantly different behavior was observed for DMAM(HCOO)3 frameworks (M=Mn,Ni,Fe,Zn), i.e., where the s(CNC) mode showed abrupt increase (decrease) of its wavenumber (FWHM) at the phase transition temperature on cooling.8,49 Fourthly, Raman and IR bands at 1461 and 1460 cm-1 split unto doublets below T0 (Figures 2b, 3c and S3d). Since these bands correspond to the (CH3) modes and there are two CH3 groups in one DMA+ cation, the observed splitting indicates that these CH3 groups have significantly different C-H bond lengths in the LT phase. Fifthly, very broad and hardly observable Raman bands at 1581 and 1422 cm-1, corresponding to the ω(NH2) and τ(NH2) modes, show pronounced increase in intensity just below T0 (Figure 2b).


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Furthermore, a new Raman band appears at 843 cm-1 below T0 (Figure 2c) that corresponds to the (NH2) mode and four new bands corresponding to the (NH2) can be observed at 3152, 3068, 2831 and 2781 cm-1 at 170 K (Figure 2a). A strong intensity increase can also be noticed for the IR bands at 1435 and 1421 cm-1 corresponding to the ω(NH2) and τ(NH2) modes, respectively (Figure 3c). The IR-active (NH2) mode exhibits sudden upshift from 826 to 838 cm-1 and narrowing from 23.5 to 12.9 cm-1 when going from the HT to LT phase (Figure 3d, 4c and 4d). This behavior confirms strongly firstorder character of the phase transition, significant increase in the HB strength and ordering of the DMA+ ions in the LT phase. It is worth adding that although similar behavior has been reported for metal formate frameworks templated by DMA+ cations, FWHM value measured at 80 K for the (NH2) mode of DMACd(N3)3 (13.1 cm-1) is significantly larger than FWHM values of DMAM(HCOO)3 frameworks (M=Mn,Ni,Zn) (ca. 5 cm-1).8,49 This behavior is consistent with a complete order of DMA+ cations in formate frameworks and a presence of partial (two-fold) disorder in the LT phase of the azide system studied here.



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Figure 4. The temperature dependence of the wavenumbers and FWHM values of IRactive modes: (a) and (b) (NH2), (c) and (d) (NH2). The vertical lines correspond to the phase transition temperature T0.

Dielectric spectroscopy. To investigate the dynamics of the DMA+ cations, we performed dielectric spectroscopy experiments of DMACd(N3)3 single crystal sample. The obtained temperature dependence of the complex dielectric permittivity ε* = ε′ − iε″ (ε′ and ε″ denote the real and imaginary parts, respectively) measured along the [01-1] direction in the kilohertz frequency range is presented in Figure 5. The real part of the dielectric permittivity increases with decreasing temperature and reaches a maximum value of about 28 at the phase transition temperature. Below T0 the real part starts to decrease, while ε″ suddenly increases and shows a 16 ACS Paragon Plus Environment

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pronounced dispersion indicating dipolar dynamics on the microsecond time scale. Based on the available XRD data, this relaxation might be assigned to the two-fold reorientation of the DMA+ cations, while an abrupt increase of the dielectric losses demonstrates first-order character of the transition. Note that a similar anomalous behavior of ε* and an appearance of the dispersion in the LT phase was also observed in the previously reported dielectric study of the pressed DMACd(N3)3 powder sample.26 However, in contradiction to the previous investigation, our single crystal measurements do not show any dynamics in the HT phase. Thus, we believe that the reported HT relaxation is not related to the DMA+ cation dynamics, but likely originates from the intergrain conductivity, which might occur due to the defects or adsorbed water on the grain surfaces. We expect that the motion of the DMA+ cations is faster in the HT phase and hence should be visible at higher frequencies.

Figure 5. The temperature dependence of the complex dielectric permittivity of DMACd(N3)3 single crystal probed at different frequency along the [01-1] direction.



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The frequency dependence of the complex dielectric permittivity at several selected temperatures in the LT phase is presented in Figure 6a revealing a typical dipolar relaxation. The observed dependences were approximated using the empirical ColeCole model:50

 *     *   

 . 1 1  i 

(1)

Here ω = 2πν denotes the probing frequency, and τ is the mean relaxation time of the process. Δε is the dielectric strength of the relaxation, and ε(∞) is the dielectric permittivity in the infinite-frequency limit. The width of the relaxation is determined by the parameter 0 ≤ α < 1. For α = 0, the relaxation reduces to the Debye model. The best fits of our DMACd(N3)3 data has been obtained for α increasing from 0.34 at 170 K to 0.41 at 125 K. The determined inverse temperature dependence of the mean relaxation time is presented in Figure 6b revealing an activated Arrhenius process. It has been further approximated using the Ea kT

Arrhenius equation    0 e , where Ea and τ0 denote the activation energy and attempt time of the dipolar relaxation, respectively, and k is the Boltzmann constant. The obtained Ea and τ0 values for the DMA+ cation dynamics are accordingly 96(4) meV and 110(10) ps. This result is in a good agreement with the previous dielectric study of the pressed DMACd(N3)3 powder.26 Note, that the determined activation energy is roughly three times smaller than obtained for the 18 ACS Paragon Plus Environment

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cation dynamics in the HT phases of the related formate frameworks.37,38 This may indicate weaker HBs in the azide framework in agreement with the IR data. However, further NMR or quasielastic neutron scattering studies are needed to unambiguously determine the origin of this motion.

Figure 6. (a) The frequency dependence of the complex dielectric permittivity of DMACd(N3)3 single crystal at several selected temperatures measured along the [01-1] direction. The solid curves are approximations by the Cole-Cole equation. (b) The inverse temperature dependence of the mean relaxation time of the dipolar process in the LT phase.

Phase-transition-induced current. It has been reported that the space group of the LT phase of DMACd(N3)3 is centrosymmetric triclinic P-1,26 which makes a ferroelectric behavior and an appearance of the spontaneous electric polarization impossible in this compound. To verify this result, we also performed measurements of phase transition induced current, which would correspond to the pyrocurrent for a ferroelectric



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compound. The obtained temperature-dependent current of DMACd(N3)3 single crystal is presented in Figure S4 (Supporting Information) revealing a narrow anomaly at the phase transition point. The obtained current is very small and corresponds to the minute electric polarization of 0.07 μC/cm2. Such a value is significantly smaller compared to the polarization of the related non-centrosymmetric formate frameworks.6,20,51 This indicates that the origin of the observed current in DMACd(N3)3 is likely the release of some charged defects during the phase transition confirming centrosymmetric nature and non-ferroelectric behavior of the LT phase. EPR spectroscopy. To further probe the phase transition properties, we employed CW EPR spectroscopy on DMACd(N3)3 doped with a small amount of paramagnetic Mn2+ ions. The obtained temperature-dependent X-band EPR spectra of DMACd(N3)3:Mn2+ powder are presented in Figure 7 revealing typical powder patterns of the Mn2+ ions in the 3d5 electronic configuration (6S5/2 electronic ground state).52 In this state, the total electron spin of Mn2+ is S = 5/2, which results in five fine-structure ΔmS = ±1 EPR transitions (here mS is the magnetic electron spin quantum number).53 If a zero-field splitting is present, the resonance fields of these transitions are different, providing central (mS = −1/2 ↔ 1/2) and outer (mS = ±3/2 ↔ ±1/2 and ±5/2 ↔ ±3/2) fine-structure lines. Each line is further split into six hyperfine lines, due to the interaction between the unpaired electrons and nuclear spin I = 5/2 of

55Mn

nucleus. In the

powder spectrum, the outer transitions occur at the extrema of the angular dependence of the


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fine-structure providing a significantly more complicated spectrum compared to the single crystal case.

Figure 7.

The temperature dependent X-band CW EPR spectra of Mn2+ ions in

DMACd(N3)3. The emphasis on the (a) central and (b) outer fine-structure transitions. The spectrum changes drastically at 178 K.

The CW EPR spectrum of DMACd(N3)3:Mn2+ exhibits a drastic change at the phase transition temperature of 178 K (upon cooling) indicating that the Mn2+ centers are susceptible to the phase transition. In the HT phase, the spectrum shows unresolved outer fine-structure transitions (Figure 7b), although complex lineshape of the central hyperfine lines suggests the presence of the zero-field splitting (Figures 7a and 8b). The pronounced broadening of the outer fine-structure lines may be assigned to a broad distribution of the zero-field splitting parameters. Below the phase transition point, the 21 ACS Paragon Plus Environment


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outer fine-structure transitions become clearly resolved indicating a much narrower distribution of these parameters. Upon further cooling, the lines from the outer transitions are shifting away from the central transition demonstrating an increase of the zero-field splitting. Note that a very similar temperature evolution of the CW EPR spectrum

was

also

observed

for

the

Mn2+

probe

ions

incorporated

in

[(CH)3NH2][Zn(HCOO)3] framework, which contains the same DMA+ molecular cations.40 To further characterize the Mn2+ centers in DMACd(N3)3, we performed simulations of the experimental CW EPR spectra using the following spin Hamiltonian:53

H   e gBS  AisoSI  H FS ,

(2)

where the first and second terms describe the electron Zeeman and hyperfine interactions, respectively, characterized by the isotropic g-factor g and hyperfine coupling constant Aiso of the Mn2+ ion. B and βe denote the external magnetic field and Bohr magneton. The last term describes the fine-structure of the spectrum and can be expressed using the extended Stevens operators O qk S  (k = 2, 4, 6 and q = +k,..., −k) as54


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H FS   Bkq O qk S  . k

(3)

q

Here the coefficients Bkq express the magnitude of the corresponding zero-field splitting. The ordinary axial D and orthorhombic E parameters, which in our case measure the distortion of the MnN6 octahedra, are related to these coefficients as D  3B20 and

E  B22 . The ordinary fourth-order cubic a and axial F zero-field splitting parameters can be expressed as a  24B44 and F  180 B40  36 B44 . In our simulations, the frames of the operators corresponding to a and F zero-field splitting coincide with the four-fold symmetry axis of the cubic system. The simulations of the spectra obtained at 292 K (HT phase) and 134 K (LT phase) are presented in Figure 8. The simulation of the HT phase spectrum was performed using the following spin Hamiltonian parameters: g = 2.0003(1), Aiso = −244(1) MHz and D = −29(1) MHz. Other zero-field splitting parameters were set to zero. To account for the poorly resolved fine-structure (Figure 8a,b), we used a Gaussian distribution of the axial and orthorhombic zerofield splitting parameters with the FWHM ΔD = 38(1) and ΔE = 7.5(5) MHz. Note that the same set of the parameters was also used to successfully simulate the Q-band CW EPR spectrum of DMACd(N3)3:Mn2+ (see Figure S5a,b). The determined value of Aiso is typical for the MnN6 octahedron55 confirming successful incorporation of the Mn2+ ions at the Cd2+ lattice sites. The non-zero value of D indicates that the MnN6 octahedra are already slightly distorted in the HT phase. Note that the experimental XRD study of DMACd(N3)3 indicates almost ideal CdN6 octahedra in the HT phase,26 suggesting that 23 ACS Paragon Plus Environment


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the observed slight distortion of the MnN6 units originates from the internal strains generated by the defect-nature of the Mn2+ probes in the DMACd(N3)3 lattice. We relate the origin of the zerofield splitting distribution to the constantly changing Mn2+ ion environment due to the hopping DMA+ cation motion, which is tightly coupled by the HBs to the MnN6 octahedra. Note that a recent EPR and DFT study43 has revealed the same broadening mechanism in the HT phase of the related [(CH3)2NH2][Zn(HCOO)3]:Mn2+ framework.

Figure 8. The simulation of the X-band CW EPR spectrum of DMACd(N3)3:Mn2+ recorded at (a) 292 and (c) 134 K. (b) Enlarged mI = 5/2 hyperfine line revealing poorly resolved finestructure in the spectrum obtained at 292 K. (d) The emphasis on the simulation of the outer finestructure transitions of the spectrum recorded at 134 K. The simulation of the LT phase X-band CW EPR spectrum (see Figure 8c,d) was performed using g = 2.0004(1), Aiso = −245(1) MHz and the following zero-field splitting parameters: D = −558(1), E = 135(1) and a = 25(1) MHz. The used distributions of D


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and E parameters were ΔD = 15(1) and ΔE = 16(1) MHz. Our simulation also successfully reproduced the so called half-field transitions of the CW EPR spectrum (see Figure S6). We further measured and simulated the Q-band CW EPR spectrum of DMACd(N3)3:Mn2+ (see Figure S5c,d). The simulation using the same set of the spin Hamiltonian parameters revealed a good agreement with the experiment validating the determined parameter values. The much higher values of the axial and orthorhombic zero-field splitting parameters in the LT phase indicate that the MnN6 octahedra are significantly more distorted being in agreement with the XRD study, which shows a substantial framework deformation during the phase transition.26 In addition, the narrower distributions of D and E parameters compared to their absolute values demonstrate that in the LT phase the MnN6 octahedra are almost uniformly distorted (i.e. Mn2+ probe ions sense the same local environment) due to the partially established long-range order. This also indicates that in the LT phase the DMA+ cation motion observed in the kilohertz frequency range by the dielectric spectroscopy has a small effect on the framework deformation.



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The temperature dependence of the peak-to-peak CW EPR linewidth Γpp of the central transition hyperfine line of the Mn2+ ions in DMACd(N3)3 is presented in Figure 9a. A sudden anomalous increase of Γpp at 178 K followed by a maximum at 174 K indicates a first-order character of the phase transition in agreement with the dielectric and IR/Raman results as well as a previous study.26 An anomalous maximum of the CW EPR linewidth at the phase transition point was also observed in the related [(CH3)2NH2][Zn(HCOO)3] and [NH3(CH2)4NH3][Zn(HCOO)3]2 frameworks and other compounds exhibiting structural phase transitions.44,45,56,57 The origin of this behavior is usually related to the decrease of the relaxation times due to the order parameter fluctuations at the phase transition point.57 Note that for DMACd:Mn2+ the temperature of the linewidth maximum is slightly lower than the phase transition temperature (Figure 9a) suggesting that we observe a combined effect of the order parameter fluctuations and a sudden change of the zero-field splitting. Note that the distribution of the zerofield splitting parameters only weakly affects the hyperfine lines of the central transition, while it is the main broadening mechanism of the outer transitions in the HT phase.


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Figure 9. The temperature dependences of (a) the peak-to-peak CW EPR linewidth of the mI = −5/2 hyperfine line of the central transition, and (b) D and E zero-field splitting parameters of the Mn2+ centers in DMACd(N3)3. The error bars are smaller than data points.

The temperature dependences of the axial and orthorhombic zero-field splitting parameters of the Mn2+ probe ions in DMACd(N3)3 are presented in Figure 9b. A sharp increase of these parameters at 178 K confirms the first-order character of the structural phase transition in DMACd(N3)3, while the whole temperature dependences resemble a behavior typical for an order parameter η of the phase transition.58 The low-temperature CW EPR spectrum of DMACd(N3)3:Mn2+ does not exhibit a splitting into two fine-structure branches, which would correspond to the positive and negative values of the order parameter. Thus, it can be assumed that the axial zero-field



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splitting parameter D is proportional to the square of the order parameter: D = D0 + Cη2, where D0 is the value of D just above the phase transition temperature. For a weak firstorder phase transition (close to the second-order), the Landau theory provides the following expression for η2:58

   2 2

  4 T  T  c 1  1  2   

1/ 2

  

 , 

(4)

where Tc denotes the LT limit of the disordered phase existence. β < 0 and α, γ > 0 are constants, and the maximum temperature hysteresis can be expressed as β2/4αγ. We tried to describe the experimental temperature dependence of D using Eq. 4, but the fit quality was poor (not shown), and we obtained β2/4αγ > 100 K. Such a high value of the maximum temperature hysteresis indicates that the simple Landau model for a weak first-order phase transition is not suitable to describe the ordering of DMACd(N3)3. The most probable reason for this discrepancy is that the phase transition in this compound has a strong first-order character far away from the tricritical limit. Note that we observed a very similar behavior of the phase transition for the Mn2+-doped [(CH)3NH2][Zn(HCOO)3] framework containing the same molecular cation.44


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SUMMARY AND CONCLUSIONS In this work, we used different experimental techniques to probe the structural phase transition, DMA+ cation dynamics and cadmium-azide framework deformation in DMACd(N3)3 hybrid perovskite. The IR and Raman data have exposed significantly weaker HBs in DMACd(N3)3 compared to formate analogues. This behavior is attributed to different acceptor atoms in the HBs: oxygen in the formates and nitrogen in the azide. The temperaturedependent studies indicate that the phase transition has an order-disorder character. The dielectric spectroscopy of DMACd(N3)3 single crystal sample displays a maximum of the real part of the complex dielectric permittivity at the phase transition point followed by the pronounced dispersion in the LT phase. The observed relaxation in this phase might be assigned to the two-fold reorientation dynamics of the DMA+ cations, although other dipolar motion mechanisms cannot be rejected. The determined activation energy of this Arrhenius process is 96(4) meV, which is in a good agreement with the previously reported measurements of a pressed powder sample.26



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The phase-transition-induced current measurements of DMACd(N3)3 single crystal sample have revealed a small and very narrow anomaly at the phase transition temperature. The corresponding electric polarization is found to be very small confirming the centrosymmetric nature of the low-temperature phase of DMACd(N3)3 and absence of the ferroelectric behavior. The CW EPR experiments of manganese-doped DMACd(N3)3 have shown that the Mn2+ centers have successfully replaced Cd2+ ions and formed MnN6 octahedra. A sudden change of the EPR spectrum at the phase transition point demonstrates that these paramagnetic probes are sensitive to the structural changes occurring during the transition. In the HT phase, small and distributed values of the zero-field splitting parameters of the Mn2+ ions have been observed indicating many differently distorted MnN6 octahedra, which are directly influenced by the DMA+ cation motion. Below the phase transition point, the MnN6 octahedra become significantly more distorted. The temperature dependences of the Mn2+ CW EPR linewidth and zero-field splitting parameters indicate strong first-order character of the phase transition. The overall temperature evolution of the DMACd(N3)3:Mn2+ EPR spectrum is very similar to the one


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observed for the related [(CH3)2NH2][Zn(HCOO)3]:Mn2+ perovskite40 demonstrating a tight relation between the DMA+ cation dynamics and framework deformation in both compounds.

ASSOCIATED CONTENT

Supporting Information. Tables S1 and S2: The vibrational selection rules and experimental wavenumbers together with the proposed assignments. Figures S1-S4: The IR and Raman spectra at different temperatures, wavenumber vs. temperature plots of a few Raman modes, temperature dependence of the phase-transition-induced current. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ; phone: +48-713954184

Author Contributions



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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources Narodowe Centrum Nauki under project 2017/25/B/ST5/00160.

ACKNOWLEDGMENT This research was supported by the National Science Center (Narodowe Centrum Nauki) in Poland under project no. 2017/25/B/ST5/00160.

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Spectroscopic Study of Structural Phase Transition and Dynamic

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