Whitlockite-Type Structure as a Matrix for Optical Materials: Synthesis and Characterization of Novel TM-SM Co-Doped Phosphate Ca₉Gd(PO₄)₇, a Single-Phase White Light Phosphors

Abstract

A series of novel phosphates with the general formulas Ca₉Gd_0.9−xTm_0.1Sm_x(PO₄)₇ and Ca₉Gd_0.9−yTm_ySm_0.1(PO₄)₇ were synthesized by solid-state method. As-obtained phosphates were characterized by powder X-ray diffraction and second harmonic generation analyses, dielectric measurements, luminescence spectroscopy. All samples were single phase and characterized by the whitlockite-type structure with space group R3c. An influence of admixture concentration of REE³⁺ ions in the initial host on dielectric properties was studied in details. Synthesized phosphates are characterized by intensive luminescence. The emission in the orange region of the visible spectrum is observed for Ca₉Gd_0.9Sm_0.1(PO₄)₇ with a maximum intensity band at 602 nm. The line in blue region at 455 nm, which corresponds to ¹D₂ → ³F₄ Tm³⁺ transition, is registered for Ca₉Gd_0.9Tm_0.1(PO₄)₇. Emission in the white region of CIE coordinates was registered for Tm-Sm co-doped compounds.

1. Introduction

Inorganic oxysalts with the whitlockite-type structure [in particular, beta-tricalcium phosphate, β-Ca₃(PO₄)₂] attract interest due to different physical properties [1]. In the crystal structure of pure β-Ca₃(PO₄)₂ (space group R3c, Z = 21) Ca²⁺ ions occupy five non-equivalent M1–M5 sites [2,3]. The different types of Ca²⁺ substitutions by mono-, di- or trivalent cations make a significant influence on the physical properties (especially optical) [4,5] due to the local distortion of coordination environment of the central cations. Moreover, the variable occupancy of the M4 site (it can be either fully vacant or partially occupied up to half occupancy) makes it possible to change the symmetry of the crystal structure with the breaking of the symmetry center, which is important for the non-linear optical properties. The impact of cationic and anionic substitutions on luminescence properties has been studied previously in details [4,6]. It was shown that homo- and heterovalent substitutions occur without the change of β-Ca₃(PO₄)₂-type structure. However, in some compounds with β-Ca₃(PO₄)₂-type structure which contain cations of different oxidation states, the co-doping of admixture quantity ions is necessary to charge balance of the structure. Such doping may change physical properties and crystal structure [7].

During the last years, a number of studies were focused on heterovalent substitutions with the doping of the initial host by rare-earth elements (REE). REE ions are characterized by emission in the visible region, arising due to 4f–4f or 5d–4f electron transitions. The main goal of such modifications is the crystal chemical design of novel inorganic phosphors for LED applications. The materials developed using the initial β-Ca₃(PO₄)₂ host doped with Tm³⁺ ions can be considered as potential blue phosphors excited by UV radiation [8,9], while the Sm³⁺-doped phosphates show orange-red emission [10]. Moreover, the luminescent properties can be significantly improved by co-doping with other REE, such as Gd³⁺ [11] due to the energy transfer processes. Previously it was shown that co-doping by Gd³⁺ ions results in energy pump processes [11], high quantum yield [6], color tuning [12], and improving luminescence characteristics [6].

Among vanadates with the β-Ca₃(PO₄)₂-type structure, Ca₉Gd(VO₄)₇ is a good candidate as a host [13], and the influence of the co-doping by a Tm³⁺–Sm³⁺ pair has been recently reported [14]. However, a similar study for the phosphate analog has not been published so far, and the role of Tm³⁺–Sm³⁺ co-doping admixture on the crystal structure and phase transitions of Ca₉Gd(PO₄)₇ remains unknown. Despite the isostructurality of Ca₃(PO₄)₂ and Ca₃(VO₄)₂ phosphates are characterized by better chemical and thermal stability and environmental friendliness compared to vanadates. So, phosphates are more promising compounds for phosphors than other host-matrix, as vanadate [15].

The current study represents a complex analysis of phosphates with the formulas Ca₉Gd_0.9−xTm_0.1Sm_x(PO₄)₇ and Ca₉Gd_0.9−yTm_ySm_0.1(PO₄)₇ based on the means X-ray powder diffraction, the second harmonic generation as well as dielectric and luminescence spectroscopy.

2. Materials and Methods

Series of phosphates Ca₉Gd_0.9−xTm_0.1Sm_x(PO₄)₇ (Gd_0.9−xTm_0.1Sm_x, x = 0; 0.1; 0.2; 0.3) and Ca₉Gd_0.9−yTm_ySm_0.1(PO₄)₇ (Gd_0.9−yTm_ySm_0.1, y = 0; 0.1; 0.2; 0.3) were synthesized by a solid-state method in air from stoichiometric mixtures of CaHPO₄·2H₂O (99.9%), CaCO₃ (99.9%), REE₂O₃ (REE–Gd³⁺, Tm³⁺, Sm³⁺) (99.99%) at 1150 °C for 100 h with several intermediate grindings. The precursors were controlled by the powder X-ray diffraction (PXRD) patterns, and no impurities were detected.

The PXRD patterns were collected on a Thermo ARL X’TRA powder diffractometer (Bragg–Brentano geometry, Scintillator detector, CuKα radiation, λ = 1.5418 Å, Thermo Fisher Scientific, Waltham MA, USA). The PXRD data were collected at room temperature in the 2θ range between 5° and 65° with a step interval of 0.02°. Crystallographic Search-Match and Jana2006 [16] programs were used to determine the unit cell parameters.

Photoluminescence emission (PL) and excitation (PLE) spectra were recorded using laboratory set-up based on a LOT-Oriel MS-257 spectrograph (L.O.T.-Oriel, Irvine, CA, USA) with a 75 kW xenon light source (pulse length τ = 2 μs, pulse frequency ν = 80 Hz, wavelength resolution 0.5 nm; photomultiplier tube (PMT) Hamamatsu R928). All measurements were performed at room temperature.

Dielectric permittivity ε and dielectric loss tangent tan δ in the air were measured by a double-contact method in the frequency range of 1–106 Hz at 300–1270 K (heating rate of 10 K/min), with the assistance of a Solartron 7081 precision voltmeter and a Solartron 1260 frequency response analyzer (Karpov Institute, Moscow, Russia). Ceramic pellets (1.5 mm-thick and 5–6 mm in diameter) were prepared by pressing and sintering at 1473 K for 12 h. Pt paste was applied to the flat surfaces of the pellets, and then, they were heated at 1023 K for 4 h to produce platinum electrodes.

The second harmonic generation (SHG) signal was measured with a Q-switched YAG: Nd laser at λ_ω =1064 nm in the reflection mode (Karpov Institute, Moscow, Russia) [17].

Scanning electron microscopy (SEM) (Tescan Analytics, Fuveau, France) observations were performed using a Tescan VEGA3 scanning electron microscope. SEM images were acquired using a secondary electron (SE) imaging and backscattered electron (BSE) imaging technique.

3. Results and Discussion

PXRD patterns of Gd_0.9−xTm_0.1Sm_x and Gd_0.9−yTm_ySm_0.1 are shown in Figure 1. The number and positions of peaks correspond to Ca₉Dy(PO₄)₇ (PDF Card No 49-1086) for all as-synthesized solid solutions. Thus, the samples were single-phase and characterized by the whitlockite-type structure with polar space group (SG) R3c (depending on the structural feature of compounds with β-Ca₃(PO₄)₂-type structure may also crystallize in non-polar SG R$\overline{3}$c [18]). PXRD patterns for both SGs are almost identical, and it is rather difficult to identify them based on the powder diffraction data only [19]. However, the noncentrosymmetric structure (described by the polar SG R3c) can be confirmed by SHG measurements. The presence of a weak SHG signal response (≈1–1.5 by quartz standard) for Gd_0.9−xTm_0.1Sm_x and Gd_0.9−yTm_ySm_0.1 confirms that these samples are characterized by polar SG R3c (in case of the absence of SHG response the SG should be R$\overline{3}$c [2]).

The calculated values of the unit cell volumes for Gd_0.9−xTm_0.1Sm_x and Gd_0.9−yTm_ySm_0.1 are given in Figure 2. In comparison with pure Ca₉Gd(PO₄)₇ [20] the linear decrease of the unit cell volume with Tm³⁺ doping is observed for Gd_0.9−yTm_ySm_0.1, while the increase of the unit cell volumes corresponds to Sm³⁺ doping of Gd_0.9−xTm_0.1Sm_x. Such changes of the unit cell volumes can be explained by the substitution of Gd³⁺ ion (r_VI = 0.94 Å) by Tm³⁺ (r_VI = 0.88 Å) and Sm³⁺ (r_VI = 0.96 Å) with smaller and bigger ionic radius, respectively [21].

The relative ionic radius difference (D_r) characterizes an effective substitution in the crystal structure of the initial host. The D_r must be less than 30%. The value of relative ionic radius difference can be calculated by the following equation [3]:

$$D_r=\left|\frac{r_s-r_d}{r_s}\right|\times 100\%,$$

where r_s and r_d are ionic radii for substituted and dopant ion, respectively, with the same coordination number (CN). The obtained D_r values are given in

Table 1. The ionic radii difference D_r between substituted and dopant ions.

Dopantion	Radius, Å/CN	Dr, %
		Gd3+		Ca2+
		1.05 Å/8	0.94 Å/6	1.12 Å/8	1.00 Å/6
Tm3+	0.88/6	−	6.8	−	13.6
	0.99/8	6.1	−	13.1	−
Sm3+	0.96/6	−	1.1	−	5.3
	1.08/8	2.7	−	3.7	−

and do not exceed 30%. Thus, Tm³⁺ and Sm³⁺ can substitute Gd³⁺ and Ca²⁺ in the initial crystal structure. These data are in good agreement with the occupations of the crystallographic sites. In particular, the octahedral M5 site is occupied by Tm³⁺ jointly with Ca²⁺, and the largest M1-M3 sites are statistically occupied by Gd³⁺, Tm³⁺, Sm³⁺, and Ca²⁺ [20]. The cationic distribution in Gd_0.9−xTm_0.1Sm_x and Gd_0.9−yTm_ySm_0.1 is based on D_r calculations. Despite the fact that the D_r value of Ca²⁺/Sm³⁺ in the octahedral M5 site is less due to larger similarity of the ionic radii, Tm³⁺ ions are preferably occupied it. The reason is the smaller difference in pair Ca²⁺/Sm³⁺ (D_r = 3.7) in comparison with Ca²⁺/Tm³⁺ (D_r = 13.1) in larger eight-coordinates sites M1-M3. So, the smallest size of Tm³⁺ ion is the argument to prefer occupation among other REE ions Gd³⁺ and Sm³⁺ in the synthesized compounds.

Figure 3 shows the SEM image for Gd_0.8Tm_0.1Sm_0.1. Observed phosphate, as well as all as-prepared samples, is characterized by narrow size distribution in the interval from 10 to 100 μm with a large number of large particles.

The temperature dependences of dielectric permittivity (ε) and dielectric loss tangent (tan δ) for pure Ca₉Gd(PO₄)₇ are shown in Figure 4. The maxima at the ε(T) curves may indicate the phase transition (PT). The presence of anomalies in the tan δ curves demonstrates the type of PT, which can be determined as ferroelectric type. So, the PT is observed with SG R3c → R$\overline{3}$c changing for Ca₉Gd(PO₄)₇. The temperature of PT, determined by maxima and minima in ε(T) and tan δ(T) curves, respectively, takes value of 850 K. The similar behaviors have been also found for Gd_0.9−xTm_0.1Sm_xand Gd_0.9−yTm_ySm_0.1 samples. The temperatures of PT vary between 850 K to 865 K for as-synthesized phosphates. The obtained data might be considered as equal due to instrumental error (20 K). The significant changes in PT were not detected with doping by admixture concentration of Tm³⁺ and Sm³⁺.

The PLE spectrum of Gd_0.9Sm_0.1, recorded at λ_em = 602 nm, is given in Figure 5a. The series of sharp lines in the region of 300–500 nm correspond to f–f transitions within Sm³⁺ ions. The bands at 346, 363, 375, 404, 416, 439, and 475 nm correspond to Sm³⁺ transitions from the ⁶H_5/2 ground level to the excited levels ⁴H_13/2, ⁴D_3/2, ⁶P_7/2, ⁴F_7/2, ⁶P_5/2, ⁴G_9/2, and ⁴I_11/2, respectively [22,23]. The most intensive ⁶H_5/2 → ⁴F_7/2 transition is located at 404 nm.

At PL spectrum of Gd_0.9Sm_0.1, registered at λ_ex = 404 nm, a standard set of emission lines, which arise due to transitions within Sm³⁺ ions is observed (Figure 5b). The emission bands in the visible orange-red spectral region peaking at 563, 602, 644 and 704 nm correspond to ⁴G_5/2 → ⁴H_J/2 (J = 5, 7, 9, 11) transitions within Sm³⁺ ions [22,23]. The dominant line was found at 602 nm (⁴G_5/2 → ⁴H_7/2). The 4f-4f transitions of REE³⁺ are almost insensitive to the changing of the crystal field strength, and the profile and relative intensities of the Sm³⁺ related emission bands were similar for all studied series. The main difference between the samples was in the integral emission intensity, which is ascribed to the energy transfer processes.

The PLE spectrum of Gd_0.9Tm_0.1, monitored at λ_em = 477 nm, is presented in Figure 6a. The spectrum demonstrates an expressed band at 356 nm, corresponding to ³H₆ → ¹D₂ transition within Tm³⁺. The onset observed below 300 nm might be attributed to the defect-related absorption. The characteristic emission line, corresponding to 4f–4f transition within Tm³⁺ is observed at PL spectrum (Figure 6b). This band is located at 455 nm and is due to ¹D₂ → ³F₄ transition [14,24,25].

The emission bands of Tm³⁺ and Sm³⁺ ions, are registered in PL spectrum of Gd_0.8Tm_0.1Sm_0.1 at λ_ex = 356 nm, which corresponds to intracenter excitation of Tm³⁺ ions (Figure 7). Thus, energy transfer from Tm³⁺ to Sm³⁺ occurs. In contrast, energy transfer from Sm³⁺ to Tm³⁺ is not registered, which is confirmed by the absence of Tm³⁺ emission lines under excitation at λ_ex = 404 nm (Sm³⁺ intracenter excitation).

The data on the intensity dependence of Tm³⁺ and Sm³⁺ emission on the REE concentrations are presented in Figure 8. The intensity of Sm³⁺ emission for Gd_0.9−xTm_0.1Sm_x samples increases with x. At the same time, the intensity of Tm³⁺ emission decreases while its concentration doesn’t change (Figure 8a). Such behavior indicates the possibility of energy transfer from Tm³⁺ to Sm³⁺ under at 356 nm, which correspond to Tm³⁺ intraceneter excitation. It is worth noting that the intensity of Sm³⁺ emission is nonzero (Figure 8b) at λ_ex =356 nm even for Gd_0.9Sm_0.1, (i.e. without Tm³⁺ ions in the sample) that is due to the presence of a Sm³⁺ excitation band at 356 nm. The increase of the Sm³⁺ emission is observed with the incorporation of Tm³⁺ into studied phosphates for the sample Gd_0.8Tm_0.1Sm_0.1. The observed intensity increase also indicates the existence of an energy transfer process from Tm³⁺ to Sm³⁺. However, the intensity of Sm³⁺ emission decreases with a further increase of the Tm³⁺ concentration in the host.

CIE coordinates (x, y, z) show the color of emission for phosphors, according to international standards. They were calculated using the following equation [26]:

$$x=\frac{X}{X+Y+Z};y=\frac{Y}{X+Y+Z};z=\frac{Z}{X+Y+Z};$$

where

$$X=\underset{{\lambda }_{inf}}{\overset{{\lambda }_{sup}}{{\displaystyle \int }}}S\left(\lambda \right)\overline{x}\left(\lambda \right)d\left(\lambda \right); Y=\underset{{\lambda }_{inf}}{\overset{{\lambda }_{sup}}{{\displaystyle \int }}}S\left(\lambda \right)\overline{y}\left(\lambda \right)d\left(\lambda \right); Z=\underset{{\lambda }_{inf}}{\overset{{\lambda }_{sup}}{{\displaystyle \int }}}S\left(\lambda \right)\overline{z}\left(\lambda \right)d\left(\lambda \right)$$

where S(λ) is the intensity at wavelength λ; $\overline{x}, \overline{y}, \overline{z}$ are color 1931CIE coordinates (CIE coordinates) at wavelength λ; d is integration stp.

The calculated color coordinates (x, y) are presented in Figure 9. The CIE value for Gd_0.9Tm_0.1 is (0.17; 0.07) and corresponds to the blue region of the color space, while (0.53; 0.37) for Gd_0.9Sm_0.1 — to the orange region. The adjusting of Sm³⁺ concentration shifts the color coordinates from a blue region, through a near white one, to an orange one for Gd_0.9−xTm_0.1Sm_x. This behavior can be explained by the redistribution of Sm³⁺ and Tm³⁺ emission bands intensity under λ_ex = 356 nm.

4. Conclusions

Novel Ca₉Gd_0.9−xTm_0.1Sm_x(PO₄)₇ (x = 0; 0.1; 0.2; 0.3) and Ca₉Gd_0.9−yTm_ySm_0.1(PO₄)₇ (y = 0; 0.1; 0.2; 0.3) phosphates with β-Ca₃(PO₄)₂-type structure were synthesized by a solid-state technique. It was found that the admixture concentration of REE ions in the initial host does not influence on the formation of the whitlockite-type structure and dielectric properties. The temperature of phase transitions is 850–865 K for as-prepared samples. The REE emitting-ions can be exctied by ultraviolet radiation being in admixture quantity in the whitlockite-type host. The typical emissions in the orange (the most intensive band at 602 nm) and the blue (the most intensive band at 455 nm) spectral regions were observed for Ca₉Gd_0.9Sm_0.1(PO₄)₇ and Ca₉Gd_0.9Tm_0.1(PO₄)₇, respectively. The energy transfer from Tm³⁺ to Sm³⁺ was detected for Ca₉Gd_0.9−yTm_ySm_0.1(PO₄)₇ solid solution. It is shown that the resulting emission in the near-white area of CIE coordinates can be achieved by tuning of Sm³⁺ concentration in the Ca₉Gd_0.9−xTm_0.1Sm_x(PO₄)₇ solid solutions.