Pressure-Induced Structural Phase Transition and Fluorescence Enhancement of Double Perovskite Material Cs₂NaHoCl₆

Abstract

Cs₂NaHoCl₆, a double perovskite material, has demonstrated extensive application potential in the fields of anti-counterfeiting and optoelectronics. The synthesis of Cs₂NaHoCl₆ crystals was achieved using a hydrothermal method, followed by the determination of their crystal structures through single crystal X-ray diffraction techniques. The material exhibits bright red fluorescence when exposed to ultraviolet light, confirming its excellent optical properties. An in situ high-pressure fluorescence experiment was conducted on Cs₂NaHoCl₆ up to 10 GPa at room temperature. The results indicate that the material possibly undergoes a structural phase transition within the pressure range of 6.9–7.9 GPa, which is accompanied by a significant enhancement in fluorescence. Geometric optimization based on density functional theory (DFT) revealed a significant decrease in the bond lengths and crystal volumes of Ho-Cl and Na-Cl across the predicted phase transition range. Furthermore, it was observed that the bond lengths of Na-Cl and Ho-Cl reach an equivalent state within this phase transition interval. The alteration in bond length may modify the local crystal field strength surrounding Ho³⁺, consequently affecting its electronic transition energy levels. This could be the primary factor contributing to the structural phase transition.

1. Introduction

Perovskite materials, a family of compounds characterized by an ABX₃-type structure, have been extensively studied and applied in various fields such as optoelectronics, magnetism, and diverse physical–chemical properties [1]. The perovskite structure, which originated from the mineral perovskite CaTiO₃, can be modified by altering the combination of A, B, and X to form a variety of perovskite structures, including two-dimensional perovskite and double perovskite.

Perovskite materials have demonstrated exceptional photoelectric conversion efficiency in applications such as photovoltaics, LEDs, and sensors, making them widely utilized in solar cells [2,3,4]. Additionally, certain perovskite materials exhibit unique magnetic and electrical properties, showing significant potential in magnetic materials and electronic devices. Cs₂NaHoCl₆, a typical double perovskite material, possesses excellent luminescent properties, making it a promising candidate for various applications [5,6,7,8,9]. Double perovskite materials, known for their high photoelectric conversion efficiency and long luminescence lifetime, are considered potential luminescent materials for LED devices [10,11,12]. By optimizing the structure and composition of double perovskite, efficient and stable LED devices can be fabricated for lighting and display applications, with the expectation of exhibiting unique properties.

High pressure, as an extreme condition, is widely applied in research fields, including superconducting materials, superhard materials, energetic materials, pharmaceutical materials, and perovskite materials [13,14,15,16,17]. From previous studies on the relationship between high-pressure phase transitions and fluorescence [18,19,20], the high-pressure PL spectra showed that HoF₃ micro/nanocrystals exhibited two structural phase transformations at 5 (6 GPa for NCs) and 12 GPa [9]. Gong et al. demonstrated pressure-induced phase transitions of YF₃ and YF₃:Eu³⁺ through changes in the fluorescence spectra [21]. In this paper, the structural change and phase transition process of Cs₂NaHoCl₆ in the range of 0 to 10 GPa were systematically studied by diamond anvil cell [22,23,24]. The investigation of the fluorescent properties of rare-earth-doped perovskites holds significant scientific importance [25,26,27,28]. Through Materials Studio simulations, the relationship between the bond length, bond angle, and volume shrinkage in the crystal structure and the fluorescence enhancement phenomenon was discussed. It will help us gain a comprehensive understanding of the perovskite structure and its phase transition behavior under high-pressure conditions, providing theoretical support for its practical applications. Through studying the changes in its fluorescence spectra, we aim to reveal the structural and performance variations of Cs₂NaHoCl₆ materials under different pressures, opening new avenues for the research and application of double perovskite materials.

2. Materials and Methods

2.1. Sample Preparation

Double perovskite material Cs₂NaHoCl₆ was synthesized by the hydrothermal method. The materials used in the experiment were all purchased from Shanghai Aladdin Biochemical Technology Corporation (Shanghai, China). Amounts of 3 mL HCl (37 wt% in water), 2 mmol CsCl (99.8%), 1 mmol NaCl (99.9%), and 1 mmol HoCl₃·6H₂O (99.9%) were mixed and put into a 25 mL beaker. The solution was stirred well with a glass rod for nearly two hours. Next, the solution was transferred to a stainless-steel Parr autoclave liner and heated at 200 °C for 12 h. Subsequently, the autoclave was taken out and cooled naturally. The samples were washed repeatedly with EtOH, and then sealed and dried at 60 °C. The crystals obtained exhibit yellow grains under natural light, while they emit bright red fluorescence when exposed under 365 nm ultraviolet light.

2.2. Experimental Setup

The XRD patterns of the samples were tested using a Japan Shimadzu, XRD-7000 X-ray diffractometer, using Cu Kα1 radiation (λ = 1.5406 Å). The crystal structure of the sample, recognized as Cs₂NaHoCl₆, was evaluated through a single crystal X-ray diffractometer (Bruker D8 Quest, Bruker (Beijing) Technology Co., Ltd., China, Beijing), while a Hitachi SU8000 scanning electron microscope (Hitachi High-Tech Corporation, Tokyo, Japan) provided insights into the morphology of the samples. High-pressure tests utilized a symmetrical diamond anvil cell with a diameter of 400 μm. The diamond anvil cell originates from Shanghai Ouluodia Superhard Materials Technology Co., Ltd. (Shanghai, China). The diamond anvil cell model SEDY17-001 was used, with two non-beveled diamonds with a height (H) of 2.31 mm. The diameter is 150 μm, the thickness is 60 μm, and the autoclave volume is 1.06 × 10⁶ μm³. Additionally, a T301 steel sheet was initially compressed to approximately 55 μm in thickness, and a central hole measuring 130 μm in diameter was created in the middle to fit the Cs₂NaHoCl₆ crystal. For pressure calibration, ruby spheres were utilized, employing the R1 ruby fluorescence method. The experiments relied on silicone oil to facilitate pressure transmission. In situ high-pressure fluorescence signals were recorded using a HORIBA XPLORA PLUS spectrometer (HORIBA, Kyoto, Japan), which was stimulated using a 638 nm diode laser. The fluorescence measurements were carried out at a spectral resolution of 1 nm, the focal diameter measures around 3 μm. Following this, the power of the laser was set at 10 mw. The fluorescence spectrum underwent decomposition and fitting using Gaussian and Lorentzian functions.

2.3. Experiments

We observed and focused on the sample under the microscope to ensure that the laser beam was accurately illuminated on the sample. We initiated the measurement of the fluorescence spectrum and meticulously recorded the spectral data. Pressure calibration was conducted based on the shift in the R1 ruby fluorescence line in response to pressure changes. The equation is as follows [29,30]:

$$\mathrm{P}={\displaystyle \frac{\mathrm{A}}{\mathrm{B}}}\left[{\left({\displaystyle \frac{\Delta \mathsf{\lambda }}{{\mathsf{\lambda }}_0}}+1\right)}^{\mathrm{B}}-1\right]$$

(1)

where A = 1904 GPa, B = 7.665, and λ₀ = 694.24 nm. During the pressurization process, the diamond anvil was gradually compressed by rotating the screw. A designated period was allowed after each pressurization step to ensure uniform stress distribution within the pressure chamber. Fluorescence spectra were repeatedly acquired at various pressure levels, and the spectral alterations were carefully documented.

Utilizing the CASTEP module in Materials Studio 7.0, we simulated the structural changes in the material under high-pressure conditions based on density functional theory (DFT). The norm-conserving pseudopotential plane-wave approach was employed within the framework of reciprocal space. The interactions associated with exchange and correlation were comprehensively tackled using the generalized gradient approximation (GGA) scheme, utilizing the PW91 parameterization to improve precision. BFGS algorithm was employed for calculating the unit cell volumes and lattice parameters. After performing convergence assessments, a kinetic energy cutoff of 830 eV was established, along with an FFT lattice density of 45 × 54 × 80 at a scaling factor of 1.0. The finite basis set correction was effectively utilized in automatic mode within the calculations, in addition to utilizing a three-point numerical differentiation method. The sampling of k-points was carried out with high precision, separated by a spacing of 0.05 Å⁻¹.

3. Results and Discussion

The experimental XRD pattern was refined using HighScore Plus (version 4.0). The Rietveld refinement results showed a high degree of correspondence with the standard pattern, indicating the successful synthesis of pure Cs₂NaHoCl₆, as shown in Figure 1. The goodness of fit was 1.545, with an R profile value of 5.97% and a weighted R profile value of 8.80%. The refined results exhibited a high level of reliability. Ultimately, we obtained precise unit cell parameters (a = b = c = 10.7255 Å, α = β = γ = 90°), which were compared with literature values to validate the reasonableness of the refinement [8]. The differences in diffraction peak intensities and the absence of (311) crystal face diffraction peaks revealed significant changes in crystal orientation. Notably, the marked increase in the intensity of the (222) crystal face diffraction peak clearly indicated the occurrence of preferred orientation, which may be attributed to variations in the crystal growth process.

Figure 2a displays the SEM image of Cs₂NaHoCl₆, with the scale bar being set at 1 mm. Meanwhile, Figure 2b presents the SEM image of it, where the scale bar is at 400 μm. Figure 2 reveals the granular morphology of the sample. The average particle size is about 0.5 mm. The observed particle size and smooth surface characteristics indicate relatively complete crystal growth. However, the irregularity in shape suggests the possibility of small defects or inhomogeneities during the growth process.

As shown in

Table 1. Crystal data and structure refinement for Cs₂NaHoCl₆.

Empirical Formula	Cl24 Cs8.10 Ho4 Na3.80
Formula weight	2674.45
Temperature	296(2) K
Wavelength	0.71073 Å
Crystal system	Cubic
Space group	Fm-3m
Unit cell dimensions	a = 10.7173(8) Å	a = 90°.
	b = 10.7173(8) Å	b = 90°.
	c = 10.7173(8) Å	g = 90°.
Volume	1231.0(3) Å3
Z	1
Density (calculated)	3.608 Mg/m3
Absorption coefficient	13.603 mm−1
F (000)	1163
Crystal size	0.120 × 0.100 × 0.080 mm3

, the size of a single unit cell is a = b = c = 10.7173(8) Å, and the angle between the axes is α = 90.00°, β = 90.00°, and γ = 90.00°, indicating its highly symmetrical cubic structure. In addition, the cell volume is 1231.0(3) ų, and the crystal size of the sample is 0.120 × 0.100 × 0.080 mm³. The crystal structure and cell parameters of Cs₂NaHoCl₆ have been successfully determined based on the data.

As shown in Figure 3a,b, The compound Cs₂NaHoCl₆ is characterized by its cubic Fm-3m crystalline structure, which is related to the perovskite type. The twelve Cs-Cl bonds are arranged around the central cesium, and all Cs-Cl bond lengths are 3.790 Å. Each cesium atom is surrounded by four sodium atoms and four holmium atoms in the center of the cube. The six Na-Cl bonds are arranged (cuboctahedra) around the central sodium. All Na-Cl bond lengths are 2.749 Å. The six Ho-Cl bonds are arranged (cuboctahedra) around the central holmium, and all Ho-Cl bonds are 2.610 Å.

The high-pressure fluorescence spectra of the Cs₂NaHoCl₆ crystal were measured in the pressure range of 0–10 GPa at room temperature (Figure 4). At a pressure of 6.9 GPa, two new fluorescence peaks appear at 657 nm and 669 nm in the fluorescence spectrum (as indicated by the red plum blossom marker in Figure 4a). As the pressure increases to 7.9 GPa, the two original peaks disappear (as shown by the red arrow marker in Figure 4a), while the new peaks persist (as demonstrated by the single fitting peak in Figure 4a). Upon depressurization, the fluorescence spectrum is restored to its state prior to the phase transition, and the phase change is reversible.

Figure 4b presents the fluorescence spectrum of Cs₂NaHoCl₆ in the range of 900 to 1100 nm. When the pressure rises to 6.9 GPa, the original fluorescence peak vanishes (as indicated by the red arrow marker in Figure 4b). A new fluorescence peak emerges at 999 nm at 7.9 GPa, and this new peak persists during subsequent pressure changes (as shown by the red plum blossom marker in Figure 4b). The fluorescence spectrum suggests that the Cs₂NaHoCl₆ material undergoes a structural phase transition within the pressure range of 6.9 GPa to 7.9 GPa. According to previous research reports, the electronic transitions of Ho³⁺ correspond to the ⁵F₅ → ⁵I₈ transition in the range of 640 nm to 740 nm and the ⁵F₅ → ⁵I₇ transition in the range of 900 nm to 1100 nm [31,32].

Figure 4c illustrates the increase in pressure, and the positions of the fluorescence peak gradually shift to the right, exhibiting a blue shift. At 6.9 GPa, the positions of some fluorescence peaks appear to reach an inflection point and begin to move to the left, resulting in a red shift [33,34]. Some peaks in the spectrum show discontinuous movement at 6.9 GPa. This phenomenon is further verified by the blue shift in the fluorescence peaks occurring before 6.9 GPa (as shown in Figure 4d). However, the inflection point appears in the shaded region (the speculated phase transition interval), and some of the fluorescence peaks begin to red shift, such as the 980 nm fluorescence peak (blue pentagonal star). The precise alteration in the fluorescence peak positions indicates the occurrence of a phase transition in the material.

Figure 5a–c depict the relationship between the fluorescence peak intensity of Cs₂NaHoCl₆ and pressure under different pressure conditions. When the pressure increases from 0 GPa to 6.2 GPa, the fluorescent peak intensity gradually weakens, as indicated by the arrows (Figure 5a). With increasing pressure, the fluorescence peak intensity significantly increases from 6.2 GPa to 7.4 GPa, with an increase reaching 154% (Figure 5b). When the pressure exceeds 7.4 GPa, the fluorescence peak intensity ceases to increase. As depicted in Figure 5c, the fluorescence peak intensity decreases again in the range of 7.9 GPa to 8.7 GPa. Figure 5d illustrates the change in fluorescence peak intensity at different positions with pressure. The fluorescent peak gradually decreases from 0 GPa. At 6.2 GPa, a notable increase in intensity is observed. Upon completion of the phase transition interval, the fluorescence peak intensity diminishes once more. These findings unveil the variations in the optical properties of the material subjected to diverse pressure conditions.

As shown in Figure 6a, the chromaticity coordinates are (x = 0.72, y = 0.28), the nearest spectral color coordinates are (x_s = 0.727, y_s = 0.273), and the white point is D65, i.e., (x_w = 0.3127, y_w = 0.3290).

$$P={\displaystyle \frac{\sqrt{{\left(x-x_w\right)}^2+{\left(y-y_w\right)}^2}}{\sqrt{{\left(x_s-x_w\right)}^2+{\left(y_s-y_w\right)}^2}}}$$

(2)

By taking the relevant values into the formula for calculation, the spectral purity of the color is determined to be 98.4%. This high spectral purity indicates that the color is close to a single wavelength of light, exhibiting extremely high color purity. The illustrations show the sample under natural light and 365 nm ultraviolet light, with the sample emitting bright red fluorescence at 365 nm. This shows the material has excellent optical properties. It is further shown that Cs₂NaHoCl₆ has great application potential in optical fields such as anti-counterfeiting labels and LED lamps.

The above in situ high-pressure PL experimental results show that the luminescence peak intensity of the Cs₂NaHoCl₆ crystal exhibits an abnormal pressure dependence during the phase transition. Figure 6b–d display our simulations of the pressure dependence of the normalized lattice parameters, lattice volume, and bond lengths at 0 K. At 0 GPa, the Na-Cl bond is longer than the Ho-Cl bond. As the pressure increases, both Ho-Cl and Na-Cl bonds gradually shorten. Between 5 and 6 GPa, Ho-Cl and Na-Cl bond lengths equalize, and this pressure range is accompanied by significant bond length and volume reduction, with the volume shrinkage rate reaching 10.9%. As shown in Figure 6b, at 5 GPa, the Cl-Cs-Cl bond angle controlled by the Ho-Cl bond is 59.92°, while the Cl-Cs-Cl bond angle controlled by the Na-Cl bond is 60.08°. Due to the unequal bond lengths of Ho-Cl and Na-Cl, the Cs ion bond angles are bent. As the pressure rises to 6 GPa, the bond length of Ho-Cl exceeds that of Na-Cl, resulting in the torsion of the bond angle of the Cs ions in the opposite direction, which may be the main reason for the red shift in the fluorescence peak. Simultaneously, the change in bond length between Ho³⁺ and Cl⁻ may alter the local crystal field strength around Ho³⁺ and affect the electron transition energy levels of Ho³⁺.

Through the combination of theoretical calculations and experimental results, it was found that with increasing pressure, all bond lengths begin to shorten, and the fluorescence peaks in the fluorescence spectrum exhibit a blue shift. When the bond lengths of Na-Cl and Ho-Cl achieve a state of equilibrium, a transformation in the crystal structure is triggered, leading to the emergence of additional fluorescence peaks. This phenomenon is corroborated by the substantial alterations in bond lengths and volume that have been identified in theoretical calculations (the pressure error is within an acceptable range). Subsequently, the Ho-Cl bond length becomes longer, which results in a change in the bond angle direction of the Cs ions and a red shift in the fluorescence spectrum. Through this series of theoretical and experimental analyses, it can be concluded that the material in question undergoes a structural phase transition when subjected to high pressure.

4. Conclusions

In summary, we successfully synthesized single-crystal Cs₂NaHoCl₆ with a unique morphology and a size of approximately 500 μm. We investigated the pressure-induced phase transition and photoluminescence properties of the Cs₂NaHoCl₆ crystal. The results show that Cs₂NaHoCl₆ undergoes a reversible high-pressure structural phase transition in the pressure range of approximately 6.9–7.9 GPa. In situ high-pressure fluorescence spectroscopy revealed a significant increase in fluorescence intensity in the phase transition region. It was discovered that the enhancement of luminescence intensity in Cs₂NaHoCl₆ was attributed to the decrease in bond length and the distortion of bond angles. The investigation of the structure and photoluminescence properties of Cs₂NaHoCl₆ under high-pressure conditions deepens our understanding of the relationship between its phase transition path and PL characteristics.