Color tunability of NaLaMgWO₆:Mn⁴⁺, Tb³⁺ phosphor and its potential application in temperature sensor and wavelength detector

Abstract

Dual emission NaLaMgWO₆:Mn⁴⁺, Tb³⁺ phosphor has been synthesized via high temperature solid state reaction method. The phosphor shows both red Mn⁴⁺ emission and green Tb³⁺ emission and its emission color can change from red to green by adjusting the ratio of Mn⁴⁺/Tb³⁺ doping concentration. Moreover, the luminescence intensity ratio of Tb³⁺/Mn⁴⁺ exhibits a monotonic relationship with both temperature and excitation wavelength within certain range, meaning its possible application in optical temperature sensing and wavelength detection.

1 Introduction

Photoluminescent materials are widely applied in lasers, lightings, displays, biological molecular detection and imaging and so on [1,2,3,4,5]. Dual emission luminescent materials with suitable luminescent centers have potential application in WLEDs and sensor technologies. For example, Zhang et al. [6] designed a dual emission double-perovskite La₂MgTiO₆:Pr³⁺,Dy³⁺ phosphor for thermometer with a high relative sensitivity of 2.357%K⁻¹; Li et al. [7] reported that NaLa(MoO₄)₂:Ce³⁺,Tb³⁺ dual emission phosphors can be effectively excited by near ultraviolet to achieve different emission colors from indigo to yellow-greenish, which is suitable for use in white LEDs; The properties of the La₂ZnTiO₆:Mn⁴⁺,Cr³⁺ dual emission phosphor developed by Ou et al. [8] suggested that it can be applied in wavelength detection and indoor cultivation of LEDs.

Mn⁴⁺ as a luminescent center has been widely studied for its bright red emission and broad excitation band [9,10,11,12]. The doped Mn⁴⁺ ion acting as luminescent activator usually prefers to occupy the octahedral position in the matrix [13,14,15,16,17]. Mn⁴⁺ is very sensitive to the matrix environment in which it is located. As well as influencing emission band position, the local environment contributes to thermal quenching speed and luminescence intensity. Tb³⁺ is a widely used luminescent center in phosphors due to its ⁵D₄ → ⁷F₅ transition, which produces sharp green emission peaks near 545 nm [18,19,20]. The emission peaks of Tb³⁺ (green) and Mn⁴⁺ (red) are well separated, while their excitation spectra overlap. Thus, with suitable wavelength excitation, the luminescence of Tb³⁺ and Mn⁴⁺ can be simultaneously obtained. Changing the concentration of Tb³⁺ and Mn⁴⁺ can produce color-tunable luminescence, which can be applied to WLEDs. Moreover, the luminescence intensity ratio (green/red) can also be used in temperature measurement and light wavelength detection since the emission intensity of Tb³⁺ and Mn⁴⁺ depends on temperature and excitation wavelength in different ways.

Tungstate as a matrix of luminescent materials has been widely studied for its low cost, stable chemical properties, suitable local environment for luminescence center [21,22,23,24]. Especially, the double perovskite tungstate NaLaMgWO₆ with the layered Na⁺/La³⁺ cation ordering and rock salt Mg²⁺/W⁶⁺ cation ordering is an ideal matrix for both rare earth ions and transition metal ions [25]. The La³⁺ site in the matrix can be replaced by Tb³⁺ and other trivalent rare earth ions, and the [WO₆] octahedrons can provide octahedral sites suitable for Mn⁴⁺ ions. The Mn⁴⁺ doped NaLaMgWO₆ red phosphors and the Tb³⁺ doped NaLaMgWO₆ phosphors have been widely reported [26,27,28,29]. As far as we know, however, there are no reports so far about NaLaMgWO₆:Tb³⁺, Mn⁴⁺ co-doped dual-emitting phosphor.

In this work, a series of NaLaMgWO₆:Tb³⁺, Mn⁴⁺ phosphors have been synthesized. The Tb³⁺/Mn⁴⁺ doping concentration, temperature and excitation wavelength dependent luminescence properties have been studied, which suggests that NaLaMgWO₆:Tb³⁺, Mn⁴⁺ phosphor is a promising high-performance multifunctional material.

2 Experimental details

The NaLaMgWO₆:xTb³⁺, yMn⁴⁺ (x = 0–25%, y = 0–1.5%) phosphors were obtained from one-step high-temperature Solid-state method. The chemical raw materials were NaCO₃ (A.R.), La₂O₃ (99.99%), WO₃ (A.R.), 4MgCO₃⋅Mg(OH)₂⋅5H₂O (A.R.), MnCO₃ (A.R.) and Tb₄O₇ (99.99%). The electronic balance was utilized to weigh all the raw materials in stoichiometric ratios, which were subsequently ground in an agate mortar for a duration of 25 min. After that, the materials were calcined for eight hours at 1400 °C in a muffle furnace. Finally, the sample is ground into a powder and then characterized.

The phase purity of the NaLaMgWO₆:10 mol% Tb³⁺,0.6 mol% Mn⁴⁺ phosphor was determined by using X-ray diffraction (XRD) data acquired on a Bruker D8 Focus Diffractometer (Instrument model: PIGAKV Ultima IV) with Cu K radiation. With the help of a monochromator (Zolix Instrument, Omni-λ320i) outfitted with a photomultiplier tube (PMTH-S1-CR928), Data Acquisition System and a xenon lamp (LSP-X150), the photoluminescence excitation (PLE) and emission (PL) spectra were obtained. A steady-state/transient fluorescence spectrometer (Edinburgh Instruments, FLS980) was used to measure the PL spectra changing with temperature of NaLaMgWO₆:10%Tb³⁺, 0.3%Mn⁴⁺ phosphor.

3 Results and discussion

Figure 1a depicts XRD patterns of the NaLaMgWO₆:10 mol% Tb³⁺,0.6 mol% Mn⁴⁺ sample, confirming that all the diffraction peaks correspond to standard card (JCPDS # 37–0243). Since no other impurity phases are observed, Tb³⁺ and Mn⁴⁺ activators are successfully incorporated into the NaLaMgWO₆ matrix. As revealed in Fig. 1b, the crystal structure of NaLaMgWO₆ consists of anomalous polyhedrons with layered ordering of Na/La(the peaks (001)/(111)) and octahedrons with rock salt ordering of Mg/W(the peaks (101)/(121)) [30]. The crystal of NaLaMgWO₆ is the monoclinic double-perovskite with space group C2/m [31]. Mn⁴⁺ can occupy the [WO₆] octahedron because its radius (R = 0.53 Å, Z = 6) is similar to that of W⁶⁺ (R = 0. 62 Å, Z = 6), resulting in the 2E_g → 4A_2g transition and emitting red light. Because of the same ionic radius and valence state between Tb³⁺ (R = 0.923 Å, Z = 8) and La³⁺(R = 0.1032 Å, Z = 8) ions, Tb³⁺ ions tend to stay in the [LaO₈] anomalous polyhedrons.

Fig. 1

a XRD patterns of NaLaMgWO₆:10 mol% Tb³⁺,0.6 mol% Mn⁴⁺ phosphors; b The crystal structure of NaLaMgWO₆

Figure 2a depict the PL and PLE spectra of the NaLaMgWO₆: Mn⁴⁺ sample. The NaLaMgWO₆: Mn⁴⁺ PLE spectra is constituted of two broad bands at 347 nm and 466 nm (monitored at 700 nm).The broad bands can be attributed to the strong electron–phonon interaction [32]. The two excitation bands are well-fitted by three Gaussian peaks at 290 nm, 377 nm and 487 nm, corresponding to the transitions of Mn⁴⁺-O²⁻ charge-transfer band(CTB), ⁴A₂ → ⁴T₁ and ⁴A₂ → ⁴T₂(dotted lines in the figure), respectively[27]. Under 487 nm excitation, the PL spectra in Fig. 2a show a broad band with a center at 700 nm, which is ascribed to the spin-forbidden ²E_g → ⁴A_2g transition of Mn⁴⁺. Mn⁴⁺ ion is located in the [WO₆] octahedron and is susceptible to the crystal field environment. Figure 2b depicts the Tanabe-Sugano energy level diagram of Mn⁴⁺ with d³ electron configuration. When the value of the crystal field strength Dq/B is large (Dq/B > 2.2), Mn⁴⁺ is located in a strong crystal field environment, and the emission originates in the ²E_g → ⁴A_2g transition. On the contrary, Mn⁴⁺ is located in a weak crystal field environment, and the electrons returning from the ⁴T_2g state to the ⁴A_2g state will give the Mn⁴⁺ emission. The following formula can be used to compute Dq/B [33,34,35].

$$\frac{Dq}{B} = \frac{15(x - 8)}{{(x^2 - 10x)}}$$

(1)

$$Dq = \frac{{E({\,}^4T_2 - {\,}^4A_2 )}}{10}$$

(2)

$$x = \frac{{E({\,}^4A_2 - {\,}^4T_1 ) - E({\,}^4A_2 - {\,}^4T_2 )}}{Dq}$$

(3)

$$\frac{{E({\,}^2E_g - {\,}^4A_{2g} )}}{B} = \frac{3.05C}{B} + 7.9 - \frac{1.8B}{{Dq}}$$

(4)

where $Dq$ is the crystal field strength, $B$, $C$ is the Racah parameters. It can be obtained from Fig. 2a that the values of $E({\,}^4A_2 - {\,}^4T_1 )$, $E({\,}^4A_2 - {\,}^4T_2 )$ and $E({\,}^2E_g - {\,}^4A_{2g} )$ are 26,525.2 cm⁻¹, 20,533.9 cm⁻¹ and 14,285.7 cm⁻¹, respectively. According to calculations, $Dq$, $B$ and $C$ are 2053, 557 and 3332 cm⁻¹, respectively. Mn⁴⁺ is found to be located in a strong crystal field environment because Dq/B is 3.69. In addition, the illustration in Fig. 2a depict that the optimal concentration of Mn⁴⁺ is 0.6%mol.

Fig. 2

a Excitation and emission spectra of NaLaMgWO₆: Mn⁴⁺ phosphor (λ_em = 700 nm and λ_ex = 487 nm) and b Tanabe-Sugano energy level diagram of Mn⁴⁺ with d³ electron configuration c Excitation and emission spectra of NaLaMgWO₆: Tb³⁺ phosphor (λ_em = 545 nm and λ_ex = 487 nm). d the linear fitting relationship between lg(x) and lg(I/x). The insets in (a) and (b) show the relationship between single-doped Mn⁴⁺ and Tb³ concentrations and emission intensity, respectively

Figure 2c depicts the PLE spectra of the NaLaMgWO₆:Tb³⁺ phosphor monitoring at 545 nm. The excitation band within 250–380 nm is originated from CTB of W⁶⁺ → O²⁻ and 4f⁸ → 4f⁷5d¹ transition. The most intense excitation peak occurs at 487 nm, which corresponds to the ⁷F₆ → ⁵D₄ transition. The PL spectra under 487 nm excitation have three emission peaks at 545 nm, 594 nm and 623 nm, assigning to ⁵D₄ → ⁷F₅, ⁷F₄, ⁷F₃ transitions, respectively [36]. The intensity of Tb³⁺ emission increases as Tb³⁺ content increases. When the content of Tb³⁺ exceeds 10%, the average distance between Tb³⁺ ions become small, the energy transfer between Tb³⁺ become convenient and the excitation energy is easy to reach quenching centers. Therefore, the luminescence efficiency of the system is reduced and the luminescence intensity of Tb³⁺ is weakened. The critical distance ($R_c$) can be used to identify the type of energy transfer between Tb³⁺ ions. When $R_c$ is smaller than 5 Å, exchange interaction predominates in the energy transfer between Tb³⁺ ion. On the contrary, the electric multipole interaction rather than the exchange interaction is responsible for the energy transfer. The following formula can be used to compute $R_c$ [37].

$$R_c = 2\left( {\frac{3V}{{4\pi x_c N}}} \right)^{1/3}$$

(5)

where, $V$ is the crystal cell volume, $x_c$ is the critical concentration of Tb³⁺, and N is the amount of sites available for Tb³⁺ ions in NaLaMgWO₆ unit cell. For NaLaMgWO₆: Tb³⁺ phosphor, $N = 4$, $V = 477.83$${\text{\AA}}^3$ and $x_c = 0.1$. $R_c$ is determined to be 13.17 Å, which is more than 5 Å. Electric multipole interaction thus dominates the energy transfer mechanism between Tb³⁺ ion in the NaLaMgWO₆ matrix. According to Dexter theory it is possible to further determine which mechanism might be [38, 39].

$$\frac{I}{x} = \mu \left[ {1 + \varpi (x)^{\frac{\theta }{3}} } \right]^{ - 1}$$

(6)

where $I$ represents the luminous intensity, $\mu$ and $\varpi$ are constants, and $x$ represents the Tb³⁺ concentration. The values of $\theta$ = 6, 8 and 10 correspond to dipole–dipole, dipole-quadrupole and quadrupole–quadrupole interactions, respectively [40]. Figure 2d fits the slope ($- \frac{\theta }{3}$) to -1.53, so $\theta$ is 4.59(close to 6). Therefore, the mechanism of energy transfer between Tb³⁺ ion in NaLaMgWO₆ matrix is the dipole–dipole interaction.

Figure 3a depicts the normalized excitation spectra of the NaLaMgWO₆:10%Tb³⁺,0.6%Mn⁴⁺ sample, in which the monitored wavelength is 545 nm and 700 nm corresponding to the emission peak of Tb³⁺ and Mn⁴⁺ ions, respectively. Figure 3a shows that light within the range 272–400 nm and 468–508 nm can simultaneously excite Tb³⁺ and Mn⁴⁺ ion, with 487 nm being the optimal excitation wavelength. The emission spectra for the NaLaMgWO₆:x%Tb³⁺,0.6%Mn⁴⁺ and NaLaMgWO₆:10%Tb³⁺, y%Mn⁴⁺ samples under 487 nm excitation are depicted in Fig. 3b and c, respectively. The PL spectra of NaLaMgWO₆:Tb³⁺, Mn⁴⁺ phosphors are mainly composed of the ⁵D₄ → ⁷F₅ transition(545 nm) of Tb³⁺ ions and Mn⁴⁺²E_g → ⁴A_2g transition(700 nm). The inset of Fig. 3b depicts the variation of emission intensities of Tb³⁺ and Mn⁴⁺ with Tb³⁺ concentration under 487 nm excitation. Along the increasing content of Tb³⁺, the emission intensity of Mn⁴⁺ diminishes, suggesting that Tb³⁺ can quench the emission of Mn⁴⁺. Simultaneously, the Tb³⁺ ion concentration quenching is responsible for the subsequent decline in the intensity of Tb³⁺ emission at high Tb³⁺ concentration. Similarly, as depicted in Fig. 3c, when the concentration of Tb³⁺ is fixed and the concentration of Mn⁴⁺ increases (0.1%-1.5%mol), Tb³⁺ emission gradually weakens, and Mn⁴⁺ emission increases and then decreases because of concentration quenching. This means that Mn⁴⁺ can also quench the emission of Tb³⁺. Therefore, it is further proved that the interaction between Tb³⁺ and Mn⁴⁺ in NaLaMgWO₆ matrix may be a mutual quenching process [41]. Importantly, since Tb³⁺ dominates green emission and Mn⁴⁺ dominates red emission, a red-yellow-green tunable emission color can be achieved by altering the content ratio of Mn⁴⁺ and Tb³⁺ in the sample. Under 487 nm excitation, Fig. 3d displays the normalized relative emission intensity of Tb³⁺ and Mn⁴⁺ ions in NaLaMgWO₆:x%Tb³⁺,0.6% Mn⁴⁺ phosphors. The red emission of Mn⁴⁺ predominates when Tb³⁺ concentration is very low, as shown more clearly in Fig. 3d. The Mn⁴⁺ emission weakens gradually as Tb³⁺ concentration increases, whereas Tb³⁺ emission is intensified, resulting in yellow emission from the phosphor. When Tb³⁺ concentration increases to 25%, the green emission of Tb³⁺ dominates and Mn⁴⁺ emission is hardly observed. Figure 3e displays the CIE chromaticity coordinates diagram of NaLaMgWO₆:x%Tb³⁺,0.6%Mn⁴⁺ (x = 1%-25%) samples ($\lambda_{ex} = 487nm$). A tunable red-yellow-green emission is displayed in these samples with CIE chromaticity coordinates from A(0.7333,0.2667) to G(0.3606,0.6327), indicating the potential application in WLEDs.

Fig. 3

a Normalized excitation spectra of the NaLaMgWO₆:10%Tb³⁺,0.6%Mn⁴⁺; Emission spectra of b NaLaMgWO₆:x%Tb³⁺,0.6%Mn⁴⁺ and c NaLaMgWO₆:10%Tb³⁺,y%Mn⁴⁺ phosphors under 487 nm excitation; d The relative normalized emission intensity of Mn⁴⁺ and Tb³⁺ ions in NaLaMgWO₆:x%Tb³⁺,0.6%Mn⁴⁺ phosphors;e CIE chromaticity coordinates for NaLaMgWO₆:x%Tb³⁺,0.6%Mn⁴⁺ phosphors under 487 nm excitation

It is well known that rare earth ions (Tb³⁺) are less sensitive to temperature due to the weak electron-lattice interaction because of the 4f level structure shielded by external 5s²5p⁶ shells. However, transition metal ions (Mn⁴⁺) with 3dⁿ electron configurations exhibit a strong electron–phonon coupling and are more sensitive to temperature, so the Mn⁴⁺, Tb³⁺ co-doped phosphor can be used as temperature probes. The PL spectra changing with temperature of NaLaMgWO₆: 10%Tb³⁺,0.3%Mn⁴⁺ is investigated to explore its temperature sensing properties. Figure 4a displays the PL spectra changing with temperature of the NaLaMgWO₆:10%Tb³⁺,0.3%Mn⁴⁺ normalized at 545 nm under 487 nm excitation, and Fig. 4b displays the integrated intensity histogram for the Tb³⁺:⁵D₄ → ⁷F₅ and Mn⁴⁺:²E → ⁴A₂ transitions. In 330–500 K range, Tb³⁺ and Mn⁴⁺ both exhibit temperature increasing dependent emission intensity weakening, but the emission reduction of Tb³⁺ is slower than that of Mn⁴⁺. Figure 4c demonstrates the configurational coordinate diagram of Mn⁴⁺ and Tb³⁺. In Fig. 4c, A is the bottom of the ²E state, and B is the intersection of the ⁴A₂ state and the ²E state parabola. The thermal quench activation energy (∆E) between A and B represents the energy barrier for electron to overcome to realize the nonradiative decay [42]. As the temperature rises, the electron–phonon coupling provides enough thermal quenching activation energy to allow a large number of electrons to travel from A to B and then relax to the ground state via a non-radiative process [43]. This is the reason for the luminescence thermal quenching of Mn⁴⁺ ion. However, Tb³⁺ has a completely different thermal quenching process than Mn⁴⁺ because the excited and ground states of Tb³⁺ do not intersect. As depicted in Fig. 4c,the large energy gap (~ 14,000 cm⁻¹) between the ⁵D₄ and the next state ⁷F₀ of Tb³⁺ sufficiently reduces the possibility of depopulation of the ⁵D₄ through the multi-phonon relaxation process [44].

Fig. 4

a Temperature-dependent PL spectra normalized at 545 nm of NaLaMgWO₆:10%Tb³⁺,0.3% Mn⁴⁺(λ_ex = 487 nm); b Temperature-dependent luminescence intensity of Tb³⁺ and Mn⁴⁺; cConfigurational coordinate diagrams of Mn⁴⁺, Tb³⁺ ions in NaLaMgWO₆ matrix

The Struck and Fonger theory [45] is used to analyze the temperature dependence of Tb³⁺ and Mn⁴⁺ luminescence intensity to further evaluate the temperature sensing capability of sample. In terms of Tb³⁺ and Mn⁴⁺, temperature relates to emission intensity as follows [46]:

$$\frac{I(T)}{{I_0 }} = \frac{1}{{1 + A\exp \left( { - E/k_B T} \right)}}$$

(7)

where $I$ is the luminous intensity at the initial temperature and $I_0$ is the luminous intensity at different temperatures; $A$ represents a pre-exponential constant, $E$ represents the thermal quenching activate energy, $k_B$ is the Boltzmann constant.

Then according to Eq. (7), the luminescence intensity ratio($LIR$) between Tb³⁺ and Mn⁴⁺ can be deduced and expressed by the following formula[47]:

$$LIR = \frac{{I_{Tb} }}{{I_{Mn} }} = \frac{{I_{0,Tb} }}{{I_{0,Mn} }}\frac{{1 + A_{Mn} \exp ( - \frac{{E_{Mn} }}{k_B T})}}{{1 + A_{Tb} \exp ( - \frac{{E_{Tb} }}{k_B T})}} \approx u + v\exp \left( {\frac{{ - {\Delta }E}}{k_B T}} \right)$$

(8)

where $u$, $v$, and ${\Delta }E$ are parameters. $I_{Tb}$ is the integrated emission intensity of Tb³⁺ ions ranging from 532 to 555 nm, while $I_{Mn}$ is the integrated emission intensity of Mn⁴⁺ ions ranging from 650 to 800 nm. Figure 5a illustrates the relationship between temperature and measured $LIR$ of NaLaMgWO₆: 10%Tb³⁺,0.3%Mn⁴⁺ sample. The experimental data can be adequately fitted by Eq. (8). $LIR$ decreases monotonically with increasing temperature because Mn⁴⁺ emission thermally quenches more quickly than Tb³⁺ emission.

Fig. 5

Temperature-dependent of a LIR of I_Tb/I_Mn and b the S_A and S_R for the NaLaMgWO₆:10%Tb³⁺,0.3% Mn⁴⁺(λex = 487 nm) sample; c FIR values in heating and cooling

The absolute temperature sensitivity($S_A$) and the relative temperature sensitivity($S_R$) can be further deduced by the following equations[48]:

$$S_A = \left| {\frac{\partial LIR}{{\partial T}}} \right| = \left| {C\exp \left( { - \Delta E/k_B T} \right) \times \frac{{{\Delta }E}}{k_B T^2 }} \right|$$

(9)

$$S_R = 100\% \times \left| {\frac{1}{LIR}\frac{\partial LIR}{{\partial T}}} \right| = 100\% \times \frac{{C\exp \left( { - \Delta E/k_B T} \right)}}{{B + C\exp \left( { - \Delta E/k_B T} \right)}} \times \frac{{{\Delta }E}}{k_B T^2 }$$

(10)

Figure 5b show the relationship between $S_A$, $S_R$ and temperature for sample of NaLaMgWO₆: 10%Tb³⁺,0.3%Mn⁴⁺. It is observed that the values of the $S_A$ and $S_R$ in the temperature range 330-500 K are both increasing. The maximum $S_A$ and $S_R$ values for the NaLaMgWO₆:10%Tb³⁺, 0.3%Mn⁴⁺ phosphor are obtained at 500 K, which are 0.011 K⁻¹ and 2.35% K⁻¹ respectively. Finally, temperature repeatability is assessed by subjecting 5 heating–cooling cycles on the phosphor and calculating the intensity ratio at 330 and 498 K (Fig. 5c). Temperature repeatability($R$) can be calculated as follows[32]:

$$R = 100 \times \left[ {1 - \frac{{\max \left| {LIR_j (T) - \left\langle {LIR(T)} \right\rangle } \right|}}{{\left\langle {LIR(T)} \right\rangle }}} \right]$$

(11)

where $\left\langle {LIR(T)} \right\rangle$ is the average of the luminous intensity ratio in all cycles, and $LIR_j (T)$ is the luminescence intensity ratio in the j cycle. It can be calculated from Eq. (11) that the temperature repeatability rate of NaLaMgWO₆:10%Tb³⁺, 0.3%Mn⁴⁺ phosphor is up to 98%.

Properties of some temperature sensing materials combined with Tb³⁺ or Mn⁴⁺ are shown in Table 1. The comparison of S_R and temperature measurement range shows that the performance of NaLaMgWO₆:Mn⁴⁺, Tb³⁺ is better than that of most optical temperature measuring materials reported previously. In addition, the monitoring peaks of Tb³⁺ (≈545 nm) and Mn⁴⁺ (≈700 nm) are well separated, indicating that NaLaMgWO₆:Mn⁴⁺, Tb³⁺ temperature measuring materials have good signal recognition ability. Therefore, the high relative sensitivity, wide temperature measurement range and excellent signal resolution of NaLaMgWO₆:Mn⁴⁺, Tb³⁺ dual emission make this material possible to be an excellent temperature measurement material without any additional luminescence intensity calibration.

Table 1 Temperature sensing range and maximum S_r values of Tb³⁺ or Mn⁴⁺ ions doped luminescent materials

In addition, the excitation intensity of Tb³⁺ exhibits a consistent decrease as the wavelength increases, whereas the excitation intensity of Mn⁴⁺ displays a consistent increase within the wavelength range of 293-348 nm, as depicted in Fig. 3(a). Therefore, as the excitation wavelength progressively increases from 294 to 348 nm, the Tb³⁺ emission diminishes, while Mn⁴⁺ emission intensifies. As depicted in Fig. 6a, the PL spectra of the NaLaMgWO₆:10%Tb³⁺,0.6%Mn⁴⁺ phosphor excited at different wavelengths (293-348 nm) are measured. The luminescence intensity ratio of Mn⁴⁺ to Tb³⁺ in NaLaMgWO₆:10%Tb³⁺,0.6%Mn⁴⁺ phosphor may exhibit monotonic wavelength-dependent change in 293-348 nm region. The calculation of the integrated $LIR$ of Mn⁴⁺ to Tb³⁺ at various excitation wavelengths is depicted in Fig. 6b. $LIR$ increases monotonically as the excitation wavelength increasing from 293 to 348 nm. The wavelength measurement sensitivity S can be defined as follows:

$$S = \left| {\frac{1}{LIR} \times \frac{{d\left( {LIR} \right)}}{{d\lambda_{ex} }}} \right|$$

(12)

Fig. 6

a Emission spectra of NaLaMgWO₆:10%Tb³⁺,0.6%Mn⁴⁺ phosphor excited at different wavelength (295 nm–340 nm); b LIR (square) and S(triangle) as a function of excitation wavelength

Here $\lambda_{ex}$ is the excitation wavelength[8, 10]. Figure 6b illustrates the relationship between $S$ and $\lambda_{ex}$. A maximum $S$ of 6.67%nm⁻¹ is obtained at 340 nm. The results indicate that NaLaMgWO₆:Tb³⁺, Mn⁴⁺ phosphor can be used in wavelength detection when temperature is maintained, which provides a possible method for designing new compact photodetectors or spectrometers based on this phosphor in the future.

4 Conclusion

In summary, NaLaMgWO₆:Mn⁴⁺, Tb³⁺ phosphors have been synthesized by one-step high temperature solid method. Red emission of Mn⁴⁺ and green emission of Tb³⁺ can be observed by 487 nm excitation. At room temperature, the red-yellow-green emission color transition can be attained by altering the doping concentration ratio of Tb³⁺ and Mn⁴⁺. Owing to their distinct thermal quenching behaviors, the LIR of Mn⁴⁺ and Tb³⁺ in NaLaMgWO₆:Mn⁴⁺, Tb³⁺ phosphors are monotonically related to temperature in 330–500 K, and the S_R is up to 2.35% K⁻¹ at 500 K, showing excellent temperature sensing performance. At a constant temperature, the LIR of Mn⁴⁺ to Tb³⁺ also increases monotonically as the excitation wavelength increases from 293 to 348 nm, indicating that it can serve as a wavelength detector within 293-348 nm. The results demonstrate the possible wide range of applications for NaLaMgWO₆:Mn⁴⁺, Tb³⁺ phosphor, including illumination, wavelength detection and optical temperature measurement.