# Three-Dimensional Perovskite Methylhydrazinium Lead Chloride with Two Polar Phases and Unusual Second-Harmonic Generation Bistability above Room Temperature

Miroslaw Maczka, Anna Gagor, Jan Zaręba, Dagmara Stefanska, Marek Drozd, Sergejus Balciunas, Mantas Šimėnas, Juras Banys, Adam Sieradzki

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Miroslaw Maczka, Anna Gagor, Jan Zaręba, Dagmara Stefanska, Marek Drozd, et al. Three-Dimensional Perovskite Methylhydrazinium Lead Chloride with Two Polar Phases and Unusual Second-Harmonic Generation Bistability above Room Temperature. Chemistry of Materials, 2020, 32 (9), pp.4072-4082. 10.1021/acs.chemmater.0c00973. [hal-03526930](https://hal.science/hal-03526930)

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# Three-Dimensional Perovskite Methylhydrazinium Lead Chloride with Two Polar Phases and Unusual Second-Harmonic Generation Bistability above Room Temperature

Mirosław Mączka,* Anna Gagor, Jan K. Zaręba, Dagmara Stefanska, Marek Drozd, Sergejus Balciunas, Mantas Šimėnas, Juras Banys, and Adam Sieradzki

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Cite This: *Chem. Mater.* 2020, 32, 4072–4082

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## ACCESS

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Supporting Information

**ABSTRACT:** Formation of noncentrosymmetric three-dimensional (3D) lead halide perovskites has been a widely sought after goal because the polar structure opens up new vistas to properties of these materials, e.g., improved charge separation for photovoltaics arising from ferroelectric order. Here, we report growth and unique properties of a new highly distorted 3D perovskite, methylhydrazinium lead chloride (CH₃NH₂NH₂PbCl₃, MHyPbCl₃). This perovskite crystallizes in polar P2₁ structure at room temperature, which consists of two types of PbCl₆ octahedra: one weakly and another strongly deformed. The unusual deformation of every second perovskite layer is forced by the large size of methylhydrazinium cations and the ability of NH₂⁺ terminal groups of methylhydrazinium cations to form coordination bonds with Pb²⁺ metal centers. On heating, MHyPbCl₃ undergoes a phase transition at 342 K into another polar Pb2₁m phase with ordered organic cations. Temperature-resolved second-harmonic generation (TR-SHG) measurements confirm acentricity of both phases and show that second-harmonic response is enhanced for the high-temperature Pb2₁m phase. This intriguing property of MHyPbCl₃ has been employed to demonstrate an unprecedented kind of quadratic nonlinear optical switching in which a second-harmonic response is switched between a room-temperature, low-SHG state and a high-temperature, high-SHG state. X-ray diffraction shows that enhancement of polar properties is due to rearrangement of the perovskite's organic substructure. There is a clear pyrocurrent peak, but switching of the electric polarization could not be observed. Optical studies showed that MHyPbCl₃ is a wide-bandgap material with a bandgap of 3.4 eV (365 nm). At low temperatures, it exhibits weak UV emissions at 362 and 369 nm as well as a strong broadband white emission.

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## INTRODUCTION

Hybrid organic–inorganic compounds have been the subject of intense studies because their diverse structural and chemical variability offers unlimited opportunities for tuning their physical and chemical properties by chemical modification of the organic and/or inorganic parts. One of the most important subgroups of hybrid perovskites is metal halides, especially three-dimensional (3D) perovskites with formula ABX₃, where A is an organic or alkali metal cation, B is a divalent cation (typically Pb²⁺, Sn²⁺, or Ge²⁺), and X stands for halogen.¹⁻⁵ 3D halide perovskites, especially the iodide subfamily, have enjoyed tremendous attention in recent years because of their unique properties that raised high hopes regarding applications in solar cells.²,⁴,⁶ While 3D lead halides are promising photovoltaic materials, they also present many other interesting properties such as photoluminescence (PL), lasing, and scintillating properties.¹⁻³,⁵,⁷,⁸ Lead chlorides attracted less attention than bromides and iodides because their large bandgaps (>2.8 eV) make them unsuitable for single-junction solar-cell applications.

They are, however, suitable candidates for UV light-emitting devices and photodetectors.⁹,¹⁰

Excellent optoelectronic properties of 3D halides promoted broad interest in the search for new 3D analogues. Unfortunately, the structural tunability of 3D lead halides is highly limited because cavities formed in the lead halide frameworks may accommodate only the smallest organic cations. Thus, the 3D structure is predicted to be stable when the tolerance factor is between 0.8 and 1.0,¹ and this condition is satisfied only for cesium, methylammonium (MA⁺), hydroxylammonium (NH₃OH⁺), hydrazinium (NH₂NH₃⁺), azetidinium ((CH₂)₃NH₂⁺), formamidinium (FA⁺), and imidazolium

Received: March 5, 2020

Revised: April 12, 2020

Published: April 13, 2020

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($C_3N_2H_5^+$) cations.¹ Experimental studies show that the real situation is even worse because 3D perovskites containing corner-sharing PbX$_6$ octahedra thus far comprised only MA$^+$, FA$^+$, or Cs$^+$ as cations, while for the remaining small organic cations, a nonperovskite or layered perovskite structure were formed.¹¹⁻¹⁴ It is worth adding that, most recently, a new 3D lead halide perovskite-related ferroelectric [2-trimethylammonioethylammonium]Pb$_2$Cl$_6$ was reported that, however, has a different lead chloride framework of both corner- and edge-sharing PbCl$_6$ octahedra.¹⁵

It is well-known that the highest-temperature phases of MAPbX$_3$, FAPbX$_3$, and CsPbX$_3$ have cubic symmetry (space group *Pm$\overline{3}$m), but they undergo successive phase transitions on cooling to tetragonal and then orthorhombic or trigonal phases, which according to diffraction studies are centrosymmetric.²,¹⁶⁻²⁰ Very recently, we have employed methylhydrazinium cation ($CH_3NH_2NH_2^+$, MHy$^+$) in the construction of organic–inorganic hybrid compounds²¹⁻²³ and discovered that the lead halide framework may also accommodate MHy$^+$ in spite of the fact that this cation has a significantly larger ionic radius (264 pm) than FA$^+$ (253 pm).¹,²¹ This is evident in the formation of MHyPbBr$_3$ 3D perovskite, for which the tolerance factor is predicted to be 1.03.²³ The high-temperature (HT) phase of MHyPbBr$_3$ has the same cubic symmetry (space group *Pm$\overline{3}$m) as the highest-temperature phases of MAPbX$_3$, FAPbX$_3$, and CsPbX$_3$. However, unlike archetypal MAPbX$_3$, FAPbX$_3$, and CsPbX$_3$ analogues, the room-temperature (RT) structure of MHyPbBr$_3$ shows exceptionally large framework distortion and is strongly noncentrosymmetric, as evidenced by second-harmonic generation (SHG) studies.²³ We have also showed that MHyPbBr$_3$ features a switchable dielectric behavior, associated with phase transition at 418 K into the *Pm$\overline{3}$m phase as well as thermochromism and two-photon energy upconversion under 800 nm excitation.²³

Here, we report the synthesis of a second 3D perovskite comprising MHy$^+$ cation, i.e., MHyPbCl$_3$. This 3D perovskite is an analogue to MHyPbBr$_3$, but its properties related to polar order are even richer because, on either side of the above-room-temperature phase transition, this material exhibits non-centrosymmetric structure. Quite intriguingly, the high-temperature (HT) phase was found to be more polar than the RT phase. The unusual enhancement of the SHG response that occurs upon transition to the HT phase was employed to demonstrate an unprecedented type of quadratic nonlinear optical (NLO) switching in which the SHG response is switched between RT low-SHG state and HT high-SHG state. Insights on the optical and electrical properties as well as the phase-transition mechanism are provided, unequivocally showing that MHyPbCl$_3$ reveals disparate properties compared to the MHyPbBr$_3$ analogue, as well as different properties than those of FAPbCl$_3$, MAPbCl$_3$, and CsPbCl$_3$ chloride-based analogues.

## EXPERIMENTAL SECTION

**Synthesis.** PbCl$_2$ (98%, Sigma-Aldrich), methylhydrazine (98%, Sigma-Aldrich), hydrochloric acid (37 wt % in H$_2$O, Sigma-Aldrich), N,N-dimethylformamide (DMF, 99.8%), and methyl acetate (99.5%, Sigma-Aldrich) were commercially available and used without further purification. To grow single crystals, HCl was added dropwise to 2 mmol of methylhydrazine until pH = 7. Then 3 mL of DMF and 2 mmol of PbCl$_2$ was added under continuous stirring, and the mixture was heated to 40 °C and stirred for 1 h. Because after this time not all PbCl$_2$ was dissolved, a small amount of DMF was added until complete dissolution of PbCl$_2$. The solution was placed in a glass vial, and this vial was placed in a second larger glass vial containing methyl acetate. The

lid of the outer vial was thoroughly sealed, whereas the lid of the inner vial was loosened to allow diffusion of methyl acetate into the precursor solution. Colorless transparent crystals with dimensions up to 1.5 mm were harvested after 5 days, filtered from the mother liquid, and dried at room temperature. A good match of their powder X-ray diffraction (XRD) patterns with the calculated ones based on the single-crystal data (Figure S1 in the Supporting Information) confirmed the phase purity of the bulk sample.

Caution! Methylhydrazine is toxic and must be handled with extreme caution and the appropriate protective gear.

**DSC and Thermogravimetric Measurements.** Heat capacity was measured using a Mettler Toledo DSC-1 calorimeter with a high resolution of 0.4 µW. Nitrogen was used as a purging gas, and the heating and cooling rates were 5 K/min. The sample weight was 29.01 mg. The excess heat capacity associated with the phase transitions was evaluated by subtraction from the data the baseline representing variation in the absence of the phase transitions. A thermogravimetric analysis (TGA) study was performed in the temperature range 300–1123 K using a PerkinElmer TGA 4000. The sample weight was ca. 17.56 mg, and the heating speed rate was 10 K/min. Pure nitrogen gas as an atmosphere was used.

**X-ray Powder Diffraction.** A powder XRD pattern was obtained on an X'Pert PRO X-ray diffraction system equipped with a P1Xcel ultrafast line detector and Soller slits for Cu Kα$_1$ radiation ($\lambda$ = 1.54056 Å). The powder was measured in the reflection mode, and the X-ray tube settings were 30 mA and 40 kV.

**SHG.** SHG studies were performed using a laser system consisting of a Quantronix Integra-C regenerative amplifier operating as an 800 nm pump and a Quantronix-Palitra-FS BIBO crystal-based optical parametric amplifier (OPA). This system delivers wavelength-tunable pulses of ~130 fs length and operates at a repetition rate of 1 kHz. In this study we have used the output from the Quantronix-Palitra-FS femtosecond OPA set to 1300 nm wavelength.

Assessment of SHG efficiency of MHyPbCl$_3$ was performed with the use of the Kurtz–Perry technique. Potassium dihydrogen phosphate (KDP) was used as a SHG reference. The single crystals of MHyPbCl$_3$ and KDP were crushed with a spatula and sieved through a mini-sieve set (Aldrich), allowing for collection of a microcrystal size fraction of 125–177 µm. Next, size-graded samples of MHyPbCl$_3$ and KDP were fixed between microscope glass slides (forming tightly packed layers), sealed, and mounted to the sample holder. The Kurtz–Perry test was performed at 298 K for 1300 nm wavelength, at an average beam power of 120 mW. Spectra were collected for 750 and 1000 ms for KDP and MHyPbCl$_3$, respectively (averaged 5 times).

The laser beam was directed onto samples at 45° and was unfocused in all cases. Signal-collecting optics, mounted to the glass optical fiber, was placed perpendicularly to the plane of the sample (backscattering geometry). Scattered pumping radiation was suppressed with the use of a 700 nm short-pass dielectric filter (FESH0700, Thorlabs). All emission spectra were recorded by an Ocean Optics Flame spectrograph.

The examination of the SHG response in the function of the temperature phase was conducted in a separate measurement (using a 1300 nm beam, in which the sample was mounted to a thermostated heating plate (Heidolph) equipped with an external thermocouple). The sample-preparation protocol was the same as for the Kurtz–Perry test. For TR-SHG the average power used was 99 mW; for the switching study the beam was attenuated to 49 mW. Spectra were collected for 2500 ms at each temperature point and averaged two times.

Caution! Working with the high-power laser is dangerous to the eyes, especially in the spectral range in which the beam is invisible. Adequate eye protection should be used during measurements.

**Dielectric Properties.** For dielectric and pyrocurrent measurements, we used a cylindrical pellet sample (5 mm in diameter and 1.68 mm in thickness) pressed from MHyPbCl$_3$ powder using a uniaxial press (pressure < 200 MPa). Electric field-dependent polarization measurements were performed on a single-crystal sample of size 0.8 × 1 × 1 mm$^3$. In both cases, electrodes were deposited using a silver paste.

Dielectric measurements were performed in the 100 Hz–1 MHz frequency range using an HP 4284A LCR meter. A parallel-plate
<figure><img src="image_9.png" /><figcaption>**Figure 1.** Change in (a) C~p~ and (b) S related to the phase transition in the heating (red) and cooling (blue) runs.</figcaption></figure>

capacitor model was used to calculate the complex dielectric permittivity. Experiments were performed during cooling at a rate of 1 K/min.

Prior to pyrocurrent measurement, the pellet sample was heated to 355 K and then cooled down to 150 K. During cooling, an electric poling field of 180 V/cm was applied. At 150 K, the sample electrodes were shorted for 15 min using a 10 kΩ resistor. Current measurements were performed using a Keithley 6514 electrometer during heating of the sample from 190 to 347 K with a heating rate of 3 K/min.

An aixACCT instrument was used to study the electric field-dependent electric polarization. A periodic triangular signal was used for the measurements. A high voltage was obtained using a Trek 609E6 voltage amplifier.

**Electron Absorption and Steady-State Photoluminescence.**

The RT diffuse reflectance spectrum of the powdered sample was measured using a Varian Cary 5E UV-vis-NIR spectrophotometer. Temperature-dependent emission spectra under 266 nm excitation line from a diode laser were measured with the Hamamatsu photonic multichannel analyzer PMA-12 equipped with a BT-CCD linear image sensor. The temperature of the sample was controlled using a Linkam THMS 600 Heating/Freezing Stage. Time-resolved studies were performed using a femtosecond laser (Coherent Model Libra) as an excitation source.

**Single-Crystal X-ray Diffraction.** Single-crystal X-ray diffraction was collected at the Xcalibur diffractometer operating with an Atlas CCD detector and Mo Kα radiation. Empirical absorption correction using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm, was done using CrysAlis PRO 1.171.38.46 (Rigaku Oxford Diffraction, 2015). H atom parameters were constrained.

## ■ RESULTS AND DISCUSSION

**DSC and TGA.** The DSC measurements show the presence of heat anomalies at 342 and 329 K during heating and cooling, respectively (Figures 1 and S2). This anomaly is symmetric, suggesting a first-order character of the phase transition. The associated changes in enthalpy ΔH and entropy ΔS are about 0.97 kJ mol⁻¹ and 2.7 J mol⁻¹ K⁻¹, respectively. These values are an order of magnitude smaller than those reported for MHyPbBr₃ (9.9 kJ mol⁻¹ and 25.3 J mol⁻¹ K⁻¹).²³ For an order-disorder transition, ΔS = R ln(N), where R is the gas constant and N is the ratio of the number of configurations in the disordered and ordered phases. Thus, for N = 2, the phase-transition entropy should be 5.74. The small value of ΔS suggests, therefore, either a small contribution of an order-disorder process to the phase transition or a displacive character of this transition. When a sample is heated up to 480 K and cooled to RT, the anomaly due to the phase transition is still observed on cooling but a new anomaly starts to be visible above 470 K. The third experiment with heating up to 500 K and

cooling down to RT shows a clear peak near 490 K on heating and a lack of heat anomaly near 330 K on cooling (Figure S2). These data indicate that MHyPbCl₃ decomposes near ~490 K.

The TGA plot indicates that MHyPbCl₃ decomposes at ~494 K (Figure S3). In this plot, a weight loss of ~22.6% takes place between 494 and 620 K, corresponding to the release of methylhydrazinium chloride (the calculated value is 22.9%). On further heating, PbCl₂ starts to sublime at ~770 K, and this process ends above 1000 K.

**Single-Crystal X-ray Diffraction.** MHyPbCl₃ crystallizes in the monoclinic polar space group P2₁ (phase II) and at T_c = 342 K transforms to orthorhombic Pb21m (phase I) (Table S1). The transformation from I → II is translationengleiche, group-subgroup related; P2₁ is a maximal subgroup of Pb21m. Phase II is isotypic with its bromine analogue MHyPbBr₃.²³ Because of the presence of two inequivalent positions of lead ions that occupy alternating [001] layers, the 3D structure is highly anisotropic, as displayed in Figure 2b. It is composed of two types of inorganic layers. One layer is built up of moderately distorted Pb(1)Cl₆ octahedra (Pb-Cl bonds of 2.803–2.922 Å, Table S2) that exhibit out-of-plane tilting along the a-axis and in-plane tilting along the c-axis. The second layer can be regarded as being composed of massively deformed Pb(2)Cl₆ octahedra with Pb-Cl bonds of 2.818–3.170 Å (Table S2). However, one should take note of the ability of the NH₂⁺ terminal groups of MHy⁺ cations to form coordination bonds with Pb(2) metal centers, as pictured in Figure 2c and d. In II, the Pb(2)-NH₂ bonds are equal to 2.83 and 2.89 Å, which is below the maximum limit for this kind of interaction, as established based on the survey of the CCDC database; see Figures S4 and S5 in the Supporting Information. Thus, in MHyPbCl₃ the deformation of every second perovskite layer is forced by oversized MHy⁺ cations that possess the proper (terminal) placement of amine groups so that they can find their place in the Pb(2) coordination sphere, and this result is consonant with what was observed for MHyPbBr₃ recently.²³ It must be pointed out, however, that the strength of these interactions is greater in MHyPbCl₃ compared to that in the bromine analogue, which can be highlighted as follows.

If we consider the MHyPbBr₃ analogue, it is evident that all bonds between MHy⁺ and the organic substructure are broken above T_c, so that the lattice symmetry increases to the regular Pm$\overline{3}$m centrosymmetric space group, which is characterized by free rotation of MHy⁺ cations and a regular perovskite substructure. As a consequence, the octahedral deformation parameters decrease to 0. By contrast, in MHyPbCl₃ one notes
<figure><img src="image_10.png" /><figcaption>**Figure 2.** (a, b) Perovskite structures in I and II. The rose color represents Pb(1)Cl₆ octahedra, and gray represents Pb(2)Cl₆. To facilitate comparison with typical 3D lead halide perovskites, the two N atoms coordinating to Pb(2) centers are not shown. (c, d) Crystal structures of I and II. Dotted red lines denote hydrogen bonds, and dashed black lines show Pb-NH₂ coordinate interactions.</figcaption></figure>

that the phase transition does not bring such radical changes: neither breaking of Pb-NH₂ bonds occurs nor does the deformation of the lead cations coordination sphere disappear, as was the case of phase I of MHyPbBr₃. Indeed, phase I of MHyPbCl₃ is still ordered, with clearly discernible perovskite layers that feature deformation parameters comparable to those characterizing phase II. Here, the only exception is the Δd parameter that significantly increases for the deformed octahedra (Table 1). Finally, in I, the number of inequivalent lead positions is preserved and the Pb(1)Cl₆ octahedra exhibit in-plane tilt along the c-axis. In contrast to phase II, they are located at the mirror planes; thus, the monoclinic distortion vanishes. It is worth adding that stabilities of perovskite phases are often predicted based on the Goldschmidt's tolerance factor (GTF).¹ To use the concept of GTF for hybrid perovskites, two conditions must be satisfied, i.e., octahedral coordination of metal cations and free rotation of the organic cation.¹ These conditions are satisfied for the HT cubic phase of MHyPbBr₃,

for which GTF was 1.03.²³ However, they are not satisfied for MHyPbCl₃, and therefore, the concept of GTF cannot be applied for this compound.

The phase transition arises due to the reorientation of half of the MHy⁺ cations present in the structure combined with a rearrangement of the hydrogen-bond (HB) network. In both phases, MHy⁺ cations interact with chlorine ligands via N-H···Cl HBs of medium strength; see the geometrical parameters collected in Table S3. The atom numbering is given in Figure S6. At RT (phase II), there are two inequivalent MHy⁺ cations with all amine hydrogen atoms involved in N-H···Cl bonds. The N1-H1E···Cl5 bonding that stands at the origin of the Pb(1)Cl₆ out-of-plane octahedral tilting deserves special attention because it introduces the monoclinic distortion in phase II. In phase I this interaction is not present and the out-of-plane tilting of the Pb(1)Cl₆ octahedra vanishes. Application of AMPLIMODES software²⁵ indicates that the order parameter corresponds to lattice instability in the Brillouin zone center of Γ₂ symmetry. Table S4 confirms that octahedral rotation along the a-axis contributes significantly to the order parameter. However, shifts of lead atoms also contribute to this displacive mode. On transition to phase I, the number of crystallographically independent MHy⁺ cations is reduced to one, which demands a significant reorientation of MHy⁺. As a result, new HBs between the middle NH₂ groups are formed, and weak bonds between terminal NH₂⁺ groups disappear. At the same time, the length of the Pb-NH₂ coordination bond does not change and is equal to 2.88 Å.

The earlier-described modifications of the hydrogen-bonding system upon II → I phase transition are expected to bring about one important consequence pertaining to the polar properties of these phases. Namely, the rearrangement of the organic substructure causes the dipole moments of MHy⁺ cations to be more closely aligned along the polar b-direction (Figure 3), which strongly suggests that HT phase I might feature higher spontaneous polarization than RT phase II does. Given that enhancement of polar properties upon structural transition to HT phase is not generally a common phenomenon, in what follows we shall verify this crystallography-based guess using SHG measurements in a wide temperature range.

**Temperature-Resolved SHG and Bistability of Non-linear Response between Two SHG-Active States.**

Crystallographic studies strongly point to the polar nature of both phases I and II of MHyPbCl₃, with the former one presumably featuring more polar character. To test this hypothesis, and to collect broader evidence on the acentricity

<table><caption>**Table 1.** Comparison of Selected Geometrical Parameters for MHyPbCl₃ and Its Analogue, MHyPbBr₃ᵃ</caption><thead><tr><th>name</th><th>Ph.</th><th>symm.</th><th>T (K)</th><th>c.a.</th><th>Δd × 10<sup>-5</sup></th><th>σ<sup>2</sup> (deg)</th><th>X-Pb-X<sub>cis</sub> (deg)</th><th>X-Pb-X<sub>tr</sub> (deg)</th><th>Pb-X-Pb (deg)</th><th>Pb-NH<sub>2</sub> (Å)</th></tr></thead><tbody><tr><td rowspan="4">MHyPbBr<sub>3</sub></td><td>I</td><td>Pm&#773;3m</td><td>430</td><td>Pb</td><td>0</td><td>0</td><td>90</td><td>180</td><td>180<br>180<sup>b</sup></td><td></td></tr><tr><td rowspan="2">II</td><td rowspan="2">P2<sub>1</sub></td><td rowspan="2">293</td><td>Pb1</td><td>5.5</td><td>14</td><td>82-96</td><td>172-177</td><td>168-172</td><td></td></tr><tr><td>Pb2</td><td>112</td><td>301</td><td>71-132</td><td>145-157</td><td>145-151<br>157-170<sup>b</sup></td><td>2.915(1)<br>3.044(1)</td></tr><tr><td rowspan="4">MHyPbCl<sub>3</sub></td><td>I</td><td>Pb2<sub>1</sub>m</td><td>360</td><td>Pb1</td><td>8</td><td>14</td><td>81-96</td><td>170-176</td><td>170-175</td><td></td></tr><tr><td rowspan="2">II</td><td rowspan="2">P2<sub>1</sub></td><td rowspan="2">300</td><td>Pb2</td><td>146</td><td>302</td><td>71-133</td><td>143-157</td><td>143-154<br>163</td><td>2.883(1)</td></tr><tr><td>Pb1</td><td>22</td><td>21</td><td>80-97</td><td>170-173</td><td>166-170</td><td></td></tr><tr><td></td><td></td><td></td><td>Pb2</td><td>114</td><td>314</td><td>81-133</td><td>145-156</td><td>145-150<br>155-169<sup>b</sup></td><td>2.830(1)<br>2.893(1)</td></tr></tbody></table>

<footnote>ᵃX: Br, Cl. c.a., central atom; Δd, bond length distortion; σ², octahedral angle variance.²⁴ ᵇAngles between layers.</footnote>
ENHACEMENT OF SPONTANEOUS POLARIZATION AT HIGH TEMPERATURE!

<figure><img src="image_11.png" /><figcaption>**Figure 3.** Location of MHy⁺ dipole moments in both phases. Phase transition leads to rotation of every second MHy⁺ and reorganization of the dipoles setting. The polar properties are boosted at high temperature.</figcaption></figure>

of these phases, the temperature-resolved SHG measurements have been performed using 1300 nm femtosecond laser pulses.

Figure 4a provides a picture of the SHG signal evolution ($\lambda_{maxSHG}$ = 650 nm) for MHyPbCl₃ collected for heating and cooling runs (measurement range of 298–373 K), whereas in Figure S7 are collected the corresponding experimental spectra. During the heating run, the SHG signal gradually decreases up to ~337 K, at which point it abruptly rises by ~50% owing to the phase transition to I. With a further increase of temperature, the SHG signal decreases at an approximately constant rate up to 373 K. Upon cooling of the sample, one sees that the SHG response remains at the same level down to 332 K, where it drops by ca. 40%. Below 332 K, no substantial changes in SHG intensity are observed. The noticeable downward drift of integral intensities of SHG with temperature increase is due to the superposition of two effects: the increase of thermal motions of structure components and some degree of photochemical bleaching of the sample. Note that upon cooling this drift is much less pronounced since the freezing of thermal motions roughly compensates irradiation-induced deterioration of SHG activity. Taken together, these results provide an ample evidence that both phases feature noncentrosymmetric setting, and more importantly, demonstrate that phase II is more SHG-active than I, confirming what was deduced from X-ray diffraction data.

Next, to quantify the relative strength of the second harmonic of radiation generated by phase II, the Kurtz–Perry powder test²⁶ was performed using 1300 nm pumping at 298 K on MHyPbCl₃ sample sieved into 125–177 µm particle-size range. By comparison of the integral intensity of the SHG signal to the one collected for the KDP powder of the same particle size (Figure S8), we found that phase II of MHyPbCl₃ offers an SHG response approximately equal to 0.03 that of KDP. The relative efficiency of SHG produced by MHyPbCl₃ is 6 times lower when compared to the recently characterized MHyPbBr₃ analogue (0.18 that of KDP, also measured at 298 K using 1300 nm excitation).²³

It should be stressed that MHyPbCl₃ is the second example of 3D lead halide perovskite comprising corner-sharing PbX₆ octahedra that has noncentrosymmetric lattice at RT, a feature that is not present in archetypal 3D analogues comprising MA⁺,

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 4.** (a) Plots of integral intensities of SHG signal of MHyPbCl₃ for heating run (red squares) and cooling run (blue squares). (b) Experimental spectra of SHG signal obtained during switching experiment. Spectra were collected at 326 K (black traces) and 346 K (red traces). (c) Plot of integral intensities of SHG collected during switching experiment at 326 K (black squares) and 346 K (red circles).</figcaption></figure>

FA⁺, or Cs⁺ cations. Indeed, until the discovery of MHyPbBr₃ perovskite, the only reported noncentrosymmetric lead halide
perovskites were of layered type (e.g., Ruddelsden-Popper phases)²⁷,²⁸ and recently found 3D perovskite containing both corner- and edge-sharing PbCl₆ octahedra.¹⁵ The presence of the methylhydrazinium cation in both compounds therefore raises the question of whether their noncentrosymmetric structure is just an interesting coincidence or there really is some specific structural factor intrinsic to MHy⁺ ions that favors the formation of noncentrosymmetric perovskite phases. Bearing in mind that two examples of noncentrosymmetric 3D perovskites are far too few to make any broad generalizations, we suggest here that the formation of acentric 3D lead halide perovkites may be connected with high framework distortion, induced by the templating effect of an “oversized” cation such as MHy⁺. This hypothesis will seek experimental verification in the near future.

Recent years have witnessed a dramatic increase of interest in hybrid compounds that exhibit switchable physicochemical properties under the influence of external stimuli such as temperature or pressure. One of the emerging directions in that field is the design of materials that feature bistability of the NLO response (most commonly of SHG output) that could be achieved at around or above RT. Given that the increase of temperature generally favors structural disorder and structural phase transitions increasing crystal symmetry, frequently resulting in transition from noncentrosymmetric to centrosymmetric space groups, the vast majority of available quadratic NLO switches operate between two logic states: SHG-on and SHG-off, corresponding to temperatures below and above T_c, respectively.²⁹⁻⁴⁶ On the other hand, SHG switches that employ two different SHG-active states are a rarity, but still, the high-temperature phases usually feature lower SHG response than low-temperature ones.⁴⁷ Keeping in mind the unusual enhancement of the MHyPbCl₃ SHG response that occurs upon II → I phase transition, as evidenced by the TR-SHG results (Figure 4a), this 3D perovskite seems to be the right candidate for demonstration of NLO switching in which SHG is switched between RT low-SHG state and HT high-SHG state.

Figure 4b presents a set of experimental spectra of second-harmonic response obtained by irradiation of MHyPbCl₃ with 1300 nm femtosecond laser pulses at two temperatures: 326 and 346 K, at which phases II and I are stable, respectively. By toggling between these temperature points over several cycles, we were able to systematically increase upon heating and decrease upon cooling the SHG response of MHyPbCl₃. Integral intensities of obtained SHG signals are displayed in Figure 4c, and they show that the SHG response at 346 K is ~40–45% higher than that at the preceding (326 K) temperature point, which is in agreement with what was observed in TR-SHG measurements. Accordingly, MHyPbCl₃ may be considered the first example of quadratic NLO switching between two SHG-active states, with high T_c of 337 K, in which the HT phase corresponds to the high-SHG state. Note that the enhancement of the SHG response upon transition to the HT phase was only recently reported in the literature, yet material featuring this property operated well below RT (235 K); also, the switching behavior itself has not been proved directly.⁴⁸

A fair description of MHyPbCl₃ quadratic NLO switching properties also requires highlighting the weak points of this material that limit its practical use. Probably the most important drawback is some degree of sensitivity of that material to laser irradiation; this issue finds its reflection in TR-SHG data, visible as the downward drift of SHG intensities, as discussed earlier. Less-pronounced photochemical bleaching of the sample is also

apparent in single-temperature measurements; see Figure 4c. From the practical point of view, the contrast of SHG response also is important. Whereas “traditional” SHG switches operating between SHG-on and SHG-off states reach contrast rates as high as hundreds, the NLO switch based on MHyPbCl₃ features the contrast (defined as the ratio of integral intensity of the second-harmonic signals of high-SHG to low-SHG crystal phases) of ca. 1.5. Accordingly, although MHyPbCl₃ itself reveals an exotic type of SHG bistability, there is still a lot of room for improvements of properties of similar NLO switches employing two SHG-active states.

**Electrical Measurements.** We performed dielectric and electrical measurements of MHyPbCl₃ to further investigate the structural phase transition and properties of phase I. The temperature dependences of the real ε' and imaginary ε'' parts of the complex dielectric permittivity ε* = ε' - iε'' of the MHyPbCl₃ pellet sample are presented in parts a and b of Figure 5, respectively. A small anomaly of ε' can be observed at ~331 K

<figure><img src="image_14.png" /><figcaption>**Figure 5.** Temperature dependence of the (a) dielectric permittivity, (b) dielectric loss, (c) real M', and (d) imaginary M'' components of electric modulus spectra as a function of temperature of the MHyPbCl₃ pellet measured on heating. The representative curves are plotted in frequency decades between 10 Hz and 1 MHz. The changes of dielectric permittivity for 1 MHz were enlarged and are presented in the inset in (a). Dashed lines correspond to the structural phase-transition temperature.</figcaption></figure>

on heating, indicating a structural phase transition. In contrast to the temperature-resolved SHG results, in which the phase transition had a clear effect on the intensity of the produced second harmonic of radiation, the observed dielectric anomaly leads to a rather subtle change in the ε' value. For this reason, we have not investigated temperature-induced switching of the dielectric permittivity. The overall increase of ε* with increasing temperature is due to the electrical conductivity. To reduce the effect of the conductivity characteristic for semiconducting compounds at higher temperatures, the modulus representation (M* = 1/ε*) was used (see Figure 5c and d).⁴⁹ The complex modulus representation of data exhibits strong frequency-dependent changes, clearly enhanced at high temperatures, with
a small anomaly owing to the structural phase transition (Figure 5c). The peak maximum of M'' shifts toward high temperatures with increasing frequency, which indicates that the observed relaxation becomes thermally activated.

The temperature-dependent current of the MHyPbCl₃ pellet after poling the sample with 180 V/cm electric field is presented in Figure 6. The current increases rapidly with the increase of

<figure><img src="image_15.png" /><figcaption>**Figure 6.** Temperature dependence of the background-corrected pyroelectric current and electric polarization of the MHyPbCl₃ pellet measured after poling the sample with 180 V/cm electric field. The current prior to the background correction and the exponential background fit are presented in the inset.</figcaption></figure>

temperature, while a phase-transition anomaly is observed at ~339 K on heating. The pyrocurrent is obtained by subtracting

the transition-independent background (inset in Figure 6). The integration of the baseline-corrected pyrocurrent yields the temperature dependence of the electric polarization (Figure 6), which saturates to 1.22 µC/cm². Note that the obtained value of polarization is rather sensitive to the background correction, although it is clearly much higher than in the antipolar MAPbX₃ (X = Cl, Br, I) family.¹⁹

The acentricity of both phases and the observed pyroelectric response could indicate a proper ferroelectric behavior of MHyPbCl₃. This phenomenon is occasionally encountered in hybrid compounds,⁵⁰,⁵¹ although the majority of such compounds are less attractive from the applicational standpoint for improper ferroelectrics.⁵² To check for a proper ferroelectric response, we measured the electric-field dependence of the electric polarization at 323 K of a single-crystal sample (Figure S9). As can be seen, we did not observe any switching of the electric polarization up to an electric field of 55 kV/cm, demonstrating the absence of proper ferroelectric behavior in MHyPbCl₃. We stress that the obtained closed loop originates from a conductivity of the sample, and it is not the ferroelectric hysteresis loop because no switching of the current is observed.⁵¹,⁵³ The incapability of cation reversible switching by an external electric field in the host–guest system was also observed for metal–organic formate templated by MHy⁺.⁵⁴ Such results suggest that MHy⁺ hybrid systems may just exhibit improper ferroelectricity. The absence of proper ferroelectric behavior in MHyPbCl₃ is consistent with the X-ray diffraction data showing transition from the Pb2₁m to the P2₁ phase because the mm2F2 type transition is not a ferroelectric one.⁵⁵

**Optical Properties.** The diffuse reflectance spectrum, presented in Figure S10a, shows a band at 373 nm that can be

<figure><img src="image_16.png" /><figcaption>**Figure 7.** (a, b) Temperature-dependent PL spectra, (c) normalized RT absorption and 90 K PL spectra, and (d) temperature dependence of bands' intensity (contour map).</figcaption></figure>
attributed to excitonic absorption. The RT diffuse reflectance spectrum was used to determine the energy bandgap (E<sub>g</sub>) of MHyPbCl<sub>3</sub> with the Kubelka-Munk relation,<sup>56</sup>

$$F(R) = \frac{(1 - R)^2}{2R}$$

where R denotes reflectance. The inset to Figure S10b shows that the value of E<sub>g</sub> in MHyPbCl<sub>3</sub> is ~365 nm (3.4 eV). This value is larger than the E<sub>g</sub> values of MAPbCl<sub>3</sub> (experimental 2.88–2.94 eV, calculated 3.04 eV)<sup>10,57,58</sup> and FAPbCl<sub>3</sub> (calculated 3.02 eV, experimental 3.00 eV).<sup>58,59</sup> Similar behavior was observed also for MHyPbBr<sub>3</sub> and it can be attributed to much larger distortion of the PbCl<sub>3</sub><sup>-</sup> framework in MHyPbCl<sub>3</sub> than in MAPbCl<sub>3</sub> or FAPbCl<sub>3</sub>.

The low-temperature PL spectrum of MHyPbCl<sub>3</sub> shows two weak bands close to the excitonic absorption band: a very narrow P1 band at 362 nm (fwhm = 3.5 nm) and a broader P2 band at 369 nm (5.7 nm) (Figure 7). Although Figure 7 suggests that the 362 nm band corresponds to the E<sub>g</sub> and that at 369 nm corresponds to the excitonic absorption, allowing for assigning them to the interband and exciton recombination, respectively, it should be noticed that the PL band corresponds to 90 K and the excitonic absorption corresponds to RT. Former studies of isostructural MHyPbBr<sub>3</sub> also showed that the narrow PL band observed at 80 K is blue-shifted by ~11 nm compared to the RT excitonic absorption, but the RT PL band nearly overlaps with the RT excitonic absorption and can be attributed to a free exciton (FE).<sup>23</sup> We suppose, therefore, that the P1 band of MHyPbCl<sub>3</sub> can also be attributed to FE recombination, observed for isostructural MHyPbBr<sub>3</sub> at 457.9 nm (fwhm = 3.9 nm),<sup>23</sup> for MAPbCl<sub>3</sub> at 404 nm (3.07 eV, fwhm = 11 nm),<sup>51</sup> and for CsPbCl<sub>3</sub> single crystal near 416 nm (2.98 eV).<sup>60</sup> The second narrow band was not observed for MHyPbBr<sub>3</sub>, but it was observed for MAPbCl<sub>3</sub> as a weak band at 415 nm (2.98 eV, fwhm = 24 nm).<sup>57</sup> This band originates most likely from the recombination of photoexcited carriers in surface defects.<sup>57</sup>

In addition to the UV PL, MHyPbCl<sub>3</sub> shows the presence of a strong broadband emission at 512 nm (band P3, fwhm = 125 nm) (Figure 7). Time-resolved measurements at 80 K under 266 nm excitation line show that this PL can be best fitted by biexponential decay function with τ<sub>1</sub> = 0.065 ns and τ<sub>2</sub> = 0.306 ns (Figure S11). The lifetimes decrease with increasing temperature (Figure S11). Our recent studies of isostructural MHyPbBr<sub>3</sub> also revealed the presence of red-shifted PL that was attributed to bound exciton (BE) recombination.<sup>23</sup> However, the Stokes shifts of the two broad PL bands were much smaller (27.8 and 47.8 nm compared to 150 nm (~1 eV) in MHyPbCl<sub>3</sub>), and the bands were narrower (fwhm = 17.3 and 19.5 nm).<sup>23</sup> Thus, the P3 band of MHyPbCl<sub>3</sub> cannot be attributed to BE states, but the very large width and Stokes shift suggest that it originates from many energy levels within the bandgap. Because the PL measurements were performed on millimeter-sized single crystals, it is very likely that the broadband emission does not originate from radiative surface states but is an intrinsic property of MHyPbCl<sub>3</sub>. Therefore, we tentatively assign this broadband emission to self-trapped excitation (STEx) states. Weak emission attributed to STEx was observed for CsPbCl<sub>3</sub> below 150 K at 1.9 eV (653 nm),<sup>60</sup> and strong STE-related emission was observed near 500–600 nm for many two-dimensional lead chlorides with a strongly distorted lead chlorine framework.<sup>61,62</sup> Interestingly, very similar broadband emission was also reported for three-layered perovskite (CH<sub>3</sub>CH<sub>2</sub>NH<sub>2</sub>)<sub>4</sub>Pb<sub>3</sub>Cl<sub>10</sub>, which contains inorganic layers

composed of two types of PbCl<sub>6</sub> octahedra, one strongly distorted (the distortion parameter Δd = 45.09 × 10<sup>-4</sup>) and one less distorted (Δd = 13.15 × 10<sup>-4</sup>).<sup>63</sup> Although MHyPbCl<sub>3</sub> does not possess organic layers separating inorganic slabs, like (CH<sub>3</sub>CH<sub>2</sub>NH<sub>2</sub>)<sub>4</sub>Pb<sub>3</sub>Cl<sub>10</sub>, which contains layers of ethylammonium cations, it resembles this perovskite in a sense that its structure also consists of two inorganic layers, one weakly distorted (Δd = 2.2 × 10<sup>-4</sup>) and the other strongly deformed (Δd = 11.4 × 10<sup>-4</sup>). Thus, the presence of the broadband white-light emission in MHyPbCl<sub>3</sub> can most likely be attributed to the very large structural distortion and polar nature of this compound. The presence of less-broad and smaller Stokes-shifted bands for MHyPbBr<sub>3</sub><sup>23</sup> is consistent with smaller distortion of its inorganic layers (Table 1).

PL intensity decreases quickly on heating, and PL is hardly visible already at 180 K (Figures 7 and S11). On the basis of the collected spectra, CIE (Commission Internationale de l'Eclairage) chromaticity coordinates have been calculated, which allowed for the depiction in Figure 8 of temperature-dependent

<figure><img src="image_17.png" /><figcaption>Figure 8. CIE coordinates of MHyPbCl<sub>3</sub> at different temperatures.</figcaption></figure>

emission color changes. The color changes from yellowish-green at 80 K to white above 115 K. For instance, the chromaticity coordinates (x, y) of the white emission change from warmer (0.33, 0.40) at 120 K to colder (0.30, 0.33) at 150 K.

## CONCLUSIONS

We report the synthesis of a new all-noncentrosymmetric 3D perovskite, methylhydrazinium lead chloride. The energy bandgap of MHyPbCl<sub>3</sub> (365 nm, 3.4 eV) is larger than the bandgaps of other known 3D lead chloride perovskites comprising methylammonium (~2.88–2.94 eV) or formamidinium (~3.0 eV). Furthermore, contrary to FAPbCl<sub>3</sub>, MAPbCl<sub>3</sub>, and CsPbCl<sub>3</sub>, which exhibit narrow excitonic blue PL, MHyPbCl<sub>3</sub> shows weak UV and strong broadband white emission. It crystallizes in a polar P2<sub>1</sub> structure, while the earlier mentioned 3D lead chlorides all possess centrosymmetric structures. TR-SHG measurements also revealed that the second-harmonic response is enhanced for the high-temperature Pb<sub>2</sub>m phase compared to the RT P2<sub>1</sub> phase. This intriguing property of MHyPbCl<sub>3</sub> has been employed to demonstrate an unprecedented kind of quadratic nonlinear optical switching in which the second-harmonic response is switched between the RT low-SHG state and the HT high-SHG state.
Different optical and electrical characteristics of MHyPbCl₃,
compared to the known 3D analogues, can be attributed to the
presence of two perovskite layers, one weakly distorted and the
other strongly deformed. Our studies prove that exceptionally
large distortion of the Pb(2)Cl₆ octahedral layer is forced by the
large size of the methylhydrazinium cations and the ability of
NH₂⁺ terminal groups of methylhydrazinium cations to form
coordinate covalent bonds with Pb²⁺ metal centers.

We also show that MHyPbCl₃ undergoes a structural phase
transition at 342 K into another polar Pb21m phase, which
possesses even more distorted Pb(2)Cl6 octahedra but still
ordered MHy+ cations. The increase of SHG activity in the
Pb21m phase was attributed to the specific reorientation of
MHy+, resulting in more parallel setting of the dipoles in respect
to the polar b-direction. Electrical studies revealed a clear
pyroelectric anomaly. However, the switching of the electric
polarization could not be observed up to an electric field of 55
kV/cm, indicating the absence of proper ferroelectric behavior
in MHyPbCl3.

■ ASSOCIATED CONTENT

❶ Supporting Information

The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.chemmater.0c00973.

Crystal data, selected geometrical and HB parameters for
the studied compound at 300 and 360 K, atomic
displacements after the symmetry change from the
Pb21m to the P21 phase, powder XRD pattern, DSC
traces, TGA plot, histogram representing length of Pb-
NH2 coordinate bonds, crystal structures and HB
interactions, SHG traces, electric-field dependence of
the electric polarization and current, diffuse reflectance
spectrum, and temperature-dependent decay times
(PDF)

Structure at 300 K (CIF)

Structure at 360 K (CIF)

■ AUTHOR INFORMATION

**Corresponding Author**

Mirosław Mączka — Institute of Low Temperature and Structure
Research, Polish Academy of Sciences, 50-950 Wrocław 2,
Poland; ● orcid.org/0000-0003-2978-1093; Phone: +48-
713954161; Email: m.maczka@int.pan.wroc.pl; Fax: +48-
713441029

Authors

Anna Gagor – Institute of Low Temperature and Structure
Research, Polish Academy of Sciences, 50-950 Wrocław 2, Poland

Jan K. Zaręba – Advanced Materials Engineering and Modeling
Group, Faculty of Chemistry, Wrocław University of Science and
Technology, 50-370 Wrocław, Poland; ● orcid.org/0000-
0001-6117-6876

Dagmara Stefanska – Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wrocław 2, Poland

Marek Drozd – Institute of Low Temperature and Structure
Research, Polish Academy of Sciences, 50-950 Wrocław 2, Poland

Sergejus Balciunas – Faculty of Physics, Vilnius University, LT-
10222 Vilnius, Lithuania

Mantas Šimėnas – Faculty of Physics, Vilnius University, LT-
10222 Vilnius, Lithuania; ● orcid.org/0000-0002-2733-2270

Juras Banys – Faculty of Physics, Vilnius University, LT-10222
Vilnius, Lithuania

Adam Sieradzki – Department of Experimental Physics, Wrocław
University of Science and Technology, 50-370 Wrocław, Poland;
● orcid.org/0000-0003-4136-5754

Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.chemmater.0c00973

Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work has received funding from the Research Council of
Lithuania (LMTLT), Agreement no. S-MIP-19-4. J.K.Z.
acknowledges financial support from the Faculty of Chemistry,
Wroclaw University of Science and Technology. J.K.Z. is
supported by the Foundation for Polish Science (FNP).

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